Cell Structure Cytoplasm Physiology Notes

Cell Introduction

All the living things are composed of cells. A single cell is the smallest unit that has all the characteristics of life. The cell is defined as the structural and functional unit of the living body.

The general characteristics of a cell:

1. Needs nutrition and oxygen
2. Produces its own energy necessary for its growth, repair, and other activities
3. Eliminates carbon dioxide and other metabolic wastes
4. Maintains the medium, i.e. the environment for its survival
5. Shows immediate response to the entry of invaders like bacteria or toxic substances into the body
6. Reproduces by division. There are some exceptions like neurons, which do not reproduce.

Read And Learn More: Medical Physiology Notes

Tissues: Tissue is defined as a group of cells having similar functions. There are many types of tissues in the body. All the tissues are classified into four major types which are called the primary tissues. The primary tissues include:

1. Muscle tissue- Skeletal muscle, smooth muscle, and cardiac muscle
2. Nervous tissue- Neurons and supporting cells
3. Epithelial tissue- Squamous, columnar, and cuboidal epithelial cells
4. Connective tissue-Connective tissue proper, cartilage, bone, and blood.

Organs: An organ is defined as a structure that is formed by two or more primary types of tissues, which execute the functions of the organ. Some organs are composed of all four types of primary tissues. The organs are of two types namely tubular or hollow organs and compact or parenchymal organs. Some of the organs in the body are the brain, heart, lungs, stomach, intestine, liver, gallbladder, pancreas, kidneys, endocrine glands, etc.

Systems:

• The organ system is defined as a group of organs that work together to carry out specific functions of the body. Each system performs a specific function. The digestive system is concerned with the digestion of food particles. The excretory system eliminates unwanted substances. The cardiovascular system is responsible for the transport of substances between the organs.
• The respiratory system is concerned with the supply of oxygen and the removal of carbon dioxide. The reproductive system is involved in the reproduction of species. The endocrine system is concerned with the growth of the body and the regulation and maintenance of normal life. The musculoskeletal system is responsible for the stability and movements of the body. The nervous system controls locomotion and other activities including intellectual functions.

Structure Of The Cell

Each cell is formed by a cell body and a membrane covering the cell body called the cell membrane. The cell membrane separates the cell body from the fluid sur- rounding the cell. The cell body has two parts namely the nucleus and the cytoplasm surrounding the nucleus.

Thus, the structure of the cell is studied under three headings:

1. Cell membrane
2. Cytoplasm
3. Nucleus.

Cell Membrane

The cell membrane is a protective sheath, enveloping the cell body. It is also known as the plasma membrane or plasmalemma. This membrane separates the fluid outside the cell called extracellular fluid (ECF) and the fluid inside the cell called intracellular fluid (ICF). The cell membrane is a semipermeable membrane. So, there is the free exchange of certain substances between the ECF and ICF. The thickness of the cell membrane varies from 75 to 111Å.

Composition Of Cell Membrane: The cell membrane is composed of three types of substances:

1. Proteins (55%)
2. Lipids (40%)
3. Carbohydrates (5%).

Structure Of Cell Membrane:

• On the basis of structure, the cell membrane is called a unit membrane or a three-layered membrane. The electron microscopic study reveals three layers of cell membrane namely, one central electron-lucent layer and two electron-dense layers. The two electron-dense layers are placed on either side of the central layer.
• The central layer layers are protein layers formed by proteins. The cell is a lipid layer formed by lipid substances. The outer two membrane contains some carbohydrate molecules also.

Structural Model of the Cell Membrane:

1. Danielli-Davson model: ‘Danielli-Davson model’ was the first proposed basic model of membrane structure. It was proposed by James F. Danielli and Hugh Davidson in 1935. And it was accepted by scientists for many years. This model was basically a ‘sandwich of lipids’ covered by proteins on both sides.
2. Unit membrane model: In 1957, JD Robertson replaced the ‘Danielli-Davson model’ by ‘Unit membrane model’ on the basis of electron microscopic studies.
3. The fluid mosaic model: Later in 1972, SJ Singer and GL Nicholson proposed ‘The fluid mosaic model’. According to them, the membrane is a fluid with a mosaic of proteins (mosaic means pattern formed by the arrangement of different colored pieces of stone, tile, glass or other such materials). This model is accepted by scientists till now. In this model, the proteins are found to float in the lipid layer instead of forming the layers of the sandwich-type model.

Lipid Layer of the Cell Membrane:

The central lipid layer is a bi-layered structure. This is formed by a thin film of lipids. The characteristic feature of the lipid layer is that it is fluid in nature and not a solid structure. So, the portions of the membrane move from one point to another point along the surface of the cell. The materials dissolved in the lipid layer also move to all the areas of the cell membrane.

The major lipids are:

1. Phospholipids
2. Cholesterol.

1. Phospholipids:

• Phospholipids are the lipid substances containing phosphorus and fatty acids. The phospholipids of the lipid layer are amino phospholipids, sphingomyelins, phosphatidyl choline, phosphatidyl ethanolamine, phosphatidyl glycerol, phosphatidyl serine, and phosphatidyl inositol.
• The phospholipid molecules are arranged in two layers. Each phospholipid molecule resembles the headed pin in shape. The outer part of the phospholipid molecule is called the head portion and the inner portion is called the tail portion.
• The head portion is the polar end and it is soluble in water and has a strong affinity for water (hydrophilic). The tail portion is the nonpolar end. It is insoluble in water and repelled by water (hydrophobic).
• The two layers of phospholipids are arranged in such a way that the hydrophobic tail portions meet in the center of the membrane. The hydrophilic head portions of the outer layer face the ECF and those of the inner layer face the ICF (cytoplasm).

2. Cholesterol: The cholesterol molecules are arranged in between the phospholipid molecules. The phospholipids are soft and oily structures, and cholesterol helps to “pack” the phospholipids in the membrane. So, cholesterol is responsible for the structural integrity of the lipid layer of the cell membrane.

Functions of Lipid Layer:

The lipid layer of the cell membrane forms a semi-permeable membrane. It allows only fat-soluble substances to pass through it. Thus, only the substances like oxygen, carbon dioxide, and alcohol can pass through the lipid layer. And, this layer forms a barrier to water-soluble materials such as glucose, urea, and electrolytes.

Protein Layers of the Cell Membrane:

The protein layers of the cell membrane are the electron-dense layers. These layers cover the two surfaces of the central lipid layer. The protein layers give protection to the central lipid layer. The protein substances present in these layers are mostly glycoproteins. These protein molecules are classified into two categories:

1. Integral proteins or transmembrane proteins
2. Peripheral proteins or peripheral membrane proteins.

1. Integral proteins:

The integral or transmembrane proteins are the proteins that pass through the entire thickness of the cell membrane from one side to the other side. These proteins are tightly bound with the cell membrane.
Examples of integral proteins: Cell adhesion proteins, cell junction proteins, some carrier (transport) proteins, channel proteins, some hormone receptors, antigens, and some enzymes.

2. Peripheral proteins:

The peripheral proteins or peripheral membrane proteins are the proteins which are partially embedded in the outer and inner surfaces of the cell membrane and do not penetrate the cell membrane. The peripheral proteins are loosely bound with integral proteins or the lipid layer of the cell membrane. So, these protein molecules dissociate readily from the cell membrane.
Examples of peripheral proteins: Proteins of the cytoskeleton, some carrier (transport) proteins, and some enzymes.

Functions of Proteins in Cell Membrane:

1. Integral proteins: Integral protein molecules provide the structural integrity of the cell membrane.
2. Channel proteins: Some integral protein molecules function as channels for the diffusion of water-soluble substances like glucose and electrolytes. So, these proteins are called channel proteins.
3. Carrier or transport proteins: Some protein molecules help in the transport of substances across the cell membrane by means of active or passive transport. These proteins are called carrier proteins.
4. Pumps: Some carrier proteins act as pumps by which ions are transported actively across the cell membrane.
5. Receptor proteins: Some protein molecules serve as receptor sites for hormones and neurotransmitters. Such proteins are known as receptor proteins.
6. Enzymes: Some of the protein molecules form the enzymes which control chemical (metabolic) reactions within the cell membrane.
7. Antigens: Some proteins act as antigens and induce the process of antibody formation.
8. Cell adhesion molecules or proteins: Some integral proteins are responsible for the attachment of cells to their neighbors or to the basal lamina.

Carbohydrates of the Cell Membrane:

Throughout the surface of cell membrane, there are some carbohydrates, which are attached to either the proteins or the lipids. The carbohydrates attached to proteins form glycoproteins (proteoglycans) and those attached to lipids form the glycolipids. All these carbohydrate molecules form a thin loose covering over the entire surface of the cell membrane called glycocalyx.

Functions of Carbohydrates:

1. The carbohydrate molecules are negatively charged and do not permit the negatively charged substances to move in and out of the cell.
2. The glycocalyx from the neighboring cells helps in the tight fixation of cells with one another. 3. Some of the carbohydrate molecules function as the receptors for some hormones.

Functions Of Cell Membrane

1. Protective function: The cell membrane protects the cytoplasm and the organelles present in the cytoplasm.
2. Selective permeability: The cell membrane acts as a semipermeable membrane which allows only some substances to pass through it and acts as a barrier for other substances.
3. Absorptive function: The nutrients are absorbed into the cell through the cell membrane.
4. Excretory function: The metabolites and other waste products from the cell are excreted out through the cell membrane.
5. Exchange of gases: Oxygen enters the cell from the blood and carbon dioxide leaves the cell and enters the blood through the cell membrane.
6. Maintenance of shape and size of the cell: The cell membrane is responsible for the maintenance of the shape and size of the cell.

2. Golgi Apparatus:

• Golgi apparatus or Golgi body or Golgi complex is a membrane-bound organelle involved in the processing of proteins. It is present in all the cells except red blood cells. It is named after the discoverer Camillo Golgi. Usually, each cell has one Golgi apparatus. Some of the cells may have more than one Golgi apparatus. Each Golgi apparatus consists of 5 to 8 membranous sacs. The sacs are usually flattened and are called the cisternae.
• The Golgi apparatus is situated near the nucleus. It has two ends or faces namely, the cis face and the trans face. The cis face is positioned near the endoplasmic reticulum. The reticular vesicles from the endoplasmic reticulum enter the Golgi apparatus through the cis face. The trans face is situated near the cell membrane. The processed sub-stances make their exit from the Golgi apparatus through the trans face.

 

 

Functions of Golgi Apparatus: The major functions of the Golgi apparatus are the processing, packing, labeling, and delivery of proteins and other molecules like lipids to different parts of the cell.

• Processing of materials: The vesicles containing glycoproteins and lipids are transported into the Golgi apparatus. Here, the glycoproteins and lipids are modified and processed.
• Packaging of materials: All the processed materials are packed in the form of secretory granules, secretory vesicles, and lysosomes which are transported either out of the cell or to another part of the cell. Because of this, the Golgi apparatus is called the post office of the cell.
• Labeling and delivery of materials: Finally, the Golgi apparatus sorts out the processed and packed materials and labels them (such as the phosphate group depending on the chemical content) for delivery (distribution) to their proper destinations. Hence, the Golgi apparatus is also called the shipping department of the cell.

3. Lysosomes:

• Lysosomes are membrane-bound vesicular organelles found throughout the cytoplasm. The lysosomes are formed by the Golgi apparatus. The enzymes synthesized in the rough endoplasmic reticulum are processed and packed in the form of small vesicles in the Golgi apparatus. Then, these vesicles are pinched off from the Golgi apparatus and become the lysosomes.
• Among the organelles of the cytoplasm, the lysosomes have the thickest covering membrane. The membrane is formed by bilayered lipid material. Many small granules are present in the lysosome. The granules contain hydrolytic enzymes.

Types of Lysosomes:  Lysosomes are of two types:

1. Primary lysosome: It is the one that is pinched off from the Golgi apparatus. In spite of having the hydrolytic enzymes, the primary lysosome is inactive.
2. Secondary lysosome: It is the active lysosome that is formed by the fusion of a primary lysosome with a phagosome or endosome (see below).

Functions of Lysosomes: Lysosomes are often called the ‘garbage system’ of the cell because of their degradation activity. About 50 different hydrolytic enzymes, known as acid hydroxylases are present in the lysosomes. Lysosomes execute their functions through these enzymes.

Important lysosomal enzymes:

• Proteases that hydrolyze the proteins into amino acids
• Lipases which hydrolyze the lipids into fatty acids and glycerides
• Amylases which hydrolyze the polysaccharides into glucose
• Nucleases hydrolyze the nucleic acids into mononucleotides.

Mechanism of lysosomal function: Two mechanisms are involved in the lysosomal functions:

• Heterophagy: By this mechanism the extracellular materials which are engulfed by the cell via endocytosis are digested.
• Autophagy: By this mechanism, intracellular materials such as worn-out cytoplasmic organelles are digested.

Specific functions of lysosomes:

1. Degradation of macromolecules:
   • The macromolecules are engulfed by the cell by means of endocytosis (phagocytosis, pinocytosis, or receptor-mediated endocytosis- Chapter 3). The macromolecules such as bacteria engulfed by the cell via phagocytosis are called phagosomes or vacuoles. The other macro-molecules taken inside via pinocytosis or receptor-mediated endocytosis are called endosomes.
   • The primary lysosome fuses with the phagosome or endosome to form the secondary lysosome. The pH in the secondary lysosome becomes acidic and the lysosomal enzymes are activated. The bacteria and the other macromolecules are digested and degraded by these enzymes. The secondary lysosome containing these degraded waste products moves through the cytoplasm and fuses with the cell membrane. Now the waste products are eliminated by exocytosis.
2. Degradation of worn-out organelles:
3. The rough endoplasmic reticulum wraps itself around the worn-out organelles like mitochondria and forms the vacuoles called autophagosomes. One primary lysosome fuses with one autophagosome to form the secondary lysosome. The enzymes in the secondary lysosome are activated. Now, these enzymes digest the contents of autophagosomes.
4. Removal of excess secretory products in the cells Lysosomes in the cells of the secretory glands play an important role in the removal of excess secretory products by degrading the secretory granules.
5. Secretory function – Secretory lysosomes Recently, lysosomes having a secretory function called secretory lysosomes are found in some of the cells, particularly in the cells of the immune system. The conventional lysosomes are modified into secretory lysosomes by combining with secretory granules (which contain the particular secretory product of the cell).

Examples of secretory lysosomes:

• Lysosomes in the cytotoxic T lymphocytes and natural killer (NK) cells secrete perforin and granzymes which destroy both viral-infected cells and tumor cells. Perforin is a pore-forming protein that initiates cell death. Granzymes that belong to the family of serine proteases (enzymes that dislodge the peptide bonds of the proteins) cause cell death by apoptosis.
• Secretory lysosomes of melanocytes secrete melanin.
• The secretory lysosome of mast cells secretes serotonin which is a vasoconstrictor substance and an inflammatory mediator.

4. Peroxisomes: Peroxisomes or microbodies are membrane-limited vesicles like the lysosomes. Unlike lysosomes, peroxisomes are pinched off from the endoplasmic reticulum and not from the Golgi apparatus. Peroxisomes contain some oxidative enzymes such as catalase, urate oxidase, and D-amino acid oxidase.

Functions of Peroxisomes:

• Peroxisomes:
  • Breakdown the fatty acids by means of a process called beta-oxidation: This is the major function of peroxisomes
  • Degrade toxic substances such as hydrogen peroxide and other metabolic products by means of detoxification. A large number of peroxisomes are present in the cells of the liver which is the major organ for detoxification. Hydrogen peroxide is formed from poisons or alcohol, which enter the cell. Whenever hydrogen peroxide is produced in the cell, the peroxisomes are ruptured and the oxidative enzymes are released. These oxidases destroy hydrogen peroxide and the enzymes, which are necessary for the production of hydrogen peroxide
  • Form the major site of oxygen utilization in the cells
  • Accelerate gluconeogenesis from fats
  • Degrade purine to uric acid
  • Participate in the formation of myelin
  • Play a role in the formation of bile acids.

5. Centrosome And Centrioles: The centrosome is the membrane-bound cellular organelle situated almost in the center of the cell close to the nucleus. It consists of two structures called centrioles. These structures are cylindrical in shape and are made up of proteins. Centrioles are responsible for the movement of chromosomes during cell division.

6. Secretory Vesicles: The secretory vesicles are organelles with limiting membranes and contain secretory substances. These vesicles are formed in the endoplasmic reticulum and are processed and packed in the Golgi apparatus. The vesicles are present throughout the cytoplasm. When necessary, the secretory vesicles are ruptured and the secretory substances are released into the cytoplasm.

7. Mitochondrion:

• The mitochondrion (pleural = mitochondria) is a membrane-bound cytoplasmic organelle concerned with the production of energy. It is a rod-shaped or oval-shaped structure with a diameter of 0.5 to 1 u. It is covered by a bilayered membrane. The outer membrane is smooth and encloses the contents of the mitochondrion. This membrane contains various enzymes such as acetyl-CoA synthetase and glycerolphosphate acetyl-transferase.
• The inner membrane is folded in the form of shelf-like inward projections called cristae and it covers the inner matrix space. The cristae contain many enzymes and other protein molecules that are involved in respiration and synthesis of adenosine triphosphate (ATP). Because of these functions, the enzymes and other protein molecules in cristae are collectively known as the respiratory chain or electron transport system.

The respiratory chain includes:

• Succinic dehydrogenase
• Dihydronicotinamide adenine dinucleotide (NADH) dehydrogenase
• Cytochrome oxidase
• Cytochrome C
• ATP synthase.

The inner cavity of the mitochondrion is filled with a matrix that contains many enzymes. The mitochondrion moves freely in the cytoplasm of the cell. It is capable of reproducing itself. The mitochondrion contains its own deoxyribonucleic acid (DNA) which is responsible for many enzymatic actions. In fact, the mitochondrion is the only organelle other than the nucleus that has its own DNA.

Functions of Mitochondrion:

• Production of energy: The mitochondrion is called the ‘ powerhouse’ or power plant’ of the cell because it produces the energy required for cellular functions. The energy is produced during the oxidation of digested food particles like proteins, carbohydrates, and lipids by the oxidative enzymes in cristae. During the oxidative process water and carbon dioxide are produced with the release of energy. The released energy is stored in mitochondria and used later for the synthesis of ATP.
• Synthesis of ATP: The components of the respiratory chain in the mitochondrion are responsible for the synthesis of ATP by utilizing the energy of oxidative phosphorylation. The ATP molecules diffuse throughout the cell from the mitochondrion. Whenever energy is needed for cellular activity, the ATP molecules are broken down.
• Apoptosis: Cytochrome C and the second mitochondria-derived activator of caspases (SMAC)/diablo secreted in mitochondria are involved in apoptosis (refer to Apoptosis below).
• Other functions: Other functions of mitochondria include the storage of calcium and detoxification of ammonia in the liver.

8. Ribosomes: The ribosomes are the organelles without a limiting membrane. These organelles are granular and small dot-like structures with a diameter of 15 nm. The ribosomes are made up of proteins (35%) and ribonucleic acid (RNA – 65%). The RNA present in ribosomes is called ribosomal RNA (rRNA). Ribosomes are concerned with protein synthesis in the cell.

Types of Ribosomes:

• Ribosomes are of two types:
  1. Ribosomes that are attached to rough endoplasmic reticulum
  2. Free ribosomes that are distributed in the cytoplasm

Functions of Ribosomes:

• Ribosomes are called protein factories because of their role in the synthesis of proteins. Messenger RNA carries the genetic code for protein synthesis from the nucleus to the ribosomes. The ribosomes, in turn, arrange the amino acids into small units of proteins.
• The ribosomes attached to the rough endoplasmic reticulum are involved in the synthesis of proteins such as enzymatic proteins, hormonal proteins, lysosomal proteins, and the proteins of the cell membrane. The free ribosomes are responsible for the synthesis of proteins of hemoglobin, peroxisome, and mitochondria.

9. Cytoskeleton:

• The cytoskeleton is the cellular organelle that determines the shape of the cell and gives support to the cell. It is a complex network of structures of various sizes present throughout the cytoplasm. In addition to determining the shape of the cell, it is also essential for the cellular movements and the response of the cell to external stimuli.
• It acts both as muscle and skeleton for the stability and movements of the cell. The cytoskeleton consists of three major protein components, viz.
  1. Microtubules
  2. Intermediate filaments
  3. Microfilaments.

1. Microtubules:

• Microtubules are the straight, hollow, and tubular structures of the cytoskeleton. These organelles without the limiting membrane are arranged in different bundles. Each tubule has a diameter of 20 to 30 nm. The length of the microtubule varies and it may be 1000 times more than the thickness.
• Structurally, the microtubules are formed by bundles of globular proteins called tubulin. Tubulin has two subunits namely a subunit and ẞ subunit.

Functions of microtubules: Microtubules may function alone or join with other proteins to form more complex structures like cilia, flagella, or centrioles and perform various functions.

• Microtubules:
  • Determine the shape of the cell
  • Give structural strength to the cell
  • Act like conveyor belts which allow the movement of granules, vesicles, protein molecules, and some intermediate filaments help to maintain the shape of organelles like mitochondria to different parts of the cell
  • Form the spindle fibers which separate the chromosomes during mitosis
  • Are responsible for the movements of centrioles and complex cellular structures like cilia.

2. Intermediate Filaments: The intermediate filaments are the structures that form a network around the nucleus and extend to the periphery of the cell. The diameter of each filament is about 10 nm. The intermediate filaments are formed by rope-like polymers which are made up of fibrous proteins.

These filaments are divided into five subclasses:

1. Keratins – In epithelial cells
2. Glial filaments – In astrocytes
3. Neurofilaments – In nerve cells
4. Vimentin – In many types of cells
5. Desmin – In muscle fibers

Functions of intermediate filaments: The intermediate filaments Help To Maintain The Shape of the cell. These filaments also connect the adjacent cells through desmosomes.

3. Microfilaments:

• Microfilaments are long and fine thread-like structures with a diameter of about 3 to 6 nm. These filaments are made up of nontubular contractile proteins called actin and myosin. Actin is more abundant than myosin.
• The microfilaments are present throughout the cytoplasm. The microfilaments present in ectoplasm contain only actin molecules and those present in endoplasm contain both actin and myosin molecules.

Functions of microfilaments:

• Microfilaments:
  • Give structural strength to the cell
  • Provide resistance to the cell against the pulling forces
  • Are responsible for cellular movements like contraction, gliding, and cytokinesis (partition of cytoplasm during cell division).

Nucleus

• The nucleus is the most prominent and the largest cellular organelle. It has a diameter of 10 to 22 and occupies about 10% of the total volume of the cell. The nucleus is present in all the cells in the body except red blood cells. The cells with a nucleus are called eukaryotes and those without a nucleus are known as prokaryotes. The presence of the nucleus is necessary for cell division.
• Most of the cells are uninucleated, i.e. have only one nucleus. A few types of cells like skeletal muscle cells have many nuclei and are called multinucleated cells. Generally, the nucleus is located in the center of the cell. It is mostly spherical in shape. However, the shape and situation of the nucleus vary in some cells.

Structure Of Nucleus: The nucleus is covered by a membrane called a nuclear membrane and contains many components. The major components of the nucleus are nucleoplasm, chromatin, and nucleolus.

Nuclear Membrane:

• The nuclear membrane is double-layered and porous in nature. This allows the nucleoplasm to communicate with the cytoplasm. The outer layer of the nuclear membrane is continuous with the membrane of the endoplasmic reticulum. The space between the two layers of the nuclear membrane is continuous with the lumen of the endoplasmic reticulum.
• The pores of the nuclear membrane are guarded (lined) by protein molecules. The diameter of the pores is about 80 to 100 nm. However, it is decreased to about 7 to 9 nm because of the attachment of protein molecules with the periphery of the pores. The exchange of materials between nucleoplasm and cytoplasm occurs through these pores.

Nucleoplasm:

• Nucleoplasm is a highly viscous fluid that forms the ground substance of the nucleus. It is similar to the cytoplasm present outside the nucleus. The nucleoplasm surrounds chromatin and nucleolus. It contains a dense fibrillar network of proteins called a nuclear matrix and many substances such as nucleotides and enzymes.
• The nuclear matrix forms the structural framework for organizing chromatin. The soluble liquid part of nucleoplasm is known as nuclear hyaloplasm.

Chromatin:

• Chromatin is a thread-like material made up of large molecules of DNA. The DNA molecules are compactly packed with the help of a specialized basic protein called histone. So chromatin is referred to as the DNA-histone complex. It forms the major bulk of nuclear material.
• DNA is a double helix that wraps around a central core of eight histone molecules to form the fundamental packing unit of chromatin called a nucleosome. The nucleosomes are packed together tightly with the help of a histone molecule to form a chromatin fiber. Just before cell division, the chromatin condenses to form chromosomes.

Chromosomes:

• A chromosome is a rod-shaped nuclear structure that carries a complete blueprint of all the hereditary characteristics of that species. A chromosome is formed from a single DNA molecule coiled around histone molecules. Each DNA contains many genes.
  Normally, the chromosomes are not visible in the nucleus under a microscope. Only during cell division the chromosomes are visible under a microscope. This is because DNA becomes more tightly packed just before cell division which makes the chromosome visible during cell division.
• All the dividing cells of the body except reproductive cells contain 23 pairs of chromosomes. Each pair consists of one chromosome inherited from the mother and one from the father. The cells with 23 pairs of chromosomes are called diploid cells. The reproductive cells called gametes or sex cells contain only 23 single chromo- somes. These cells are called haploid cells.

Nucleolus:

• The nucleolus is a small round granular structure of the nucleus. Each nucleus contains one or more nucleoli.
  The nucleolus contains RNA and some proteins, which are similar to those found in ribosomes. The RNA is synthesized by five different pairs of chromosomes and stored in the nucleolus. Later, it is condensed to form.
• The subunits of ribosomes. All the subunits formed in the nucleolus are transported to the cytoplasm through the pores of the nuclear membrane. In the cytoplasm, these subunits fuse to form ribosomes which play an essential role in the formation of proteins.

Functions Of Nucleus: The major functions of the nucleus are the control of cellular activities and the storage of hereditary material. Several processes are involved in nuclear functions.

• The functions of the nucleus are:
  • Control of all the cell activities that include metabolism, protein synthesis, growth, and reproduction genes. (cell division).
  • Synthesis of RNA
  • Formation of subunits of ribosomes
  • Sending genetic instruction to the cytoplasm for protein synthesis through messenger RNA (mRNA)
  • Control of the cell division through genes
  • Storage of hereditary information (in genes) and transformation of this information from one generation of the species to the next.

Deoxyribonucleic Acid

• Deoxyribonucleic acid (DNA) is a nucleic acid that carries genetic information to the offspring of an organism. DNA forms the chemical basis of hereditary characteristics. It contains the instructions for the synthesis of proteins in the ribosomes. Gene is part of a DNA molecule.
• DNA is present in the nucleus (chromosome) and mitochondria of the cell. The DNA present in the nucleus is responsible for the formation of RNA. RNA regulates the synthesis of proteins by ribosomes. DNA in mitochondria is called nonchromosomal DNA.

Structure Of DNA:

DNA is a double-stranded complex nucleic acid. It is formed by deoxyribose, phosphoric acid, and four types of bases. Each DNA molecule consists of two polynucleotide chains, which are twisted around one another in the form of a double helix. The two chains are formed by the sugar deoxyribose and phosphate. These two substances form the backbone of a DNA molecule. Both chains of DNA are connected with each other by some organic bases.

Each chain of DNA molecules consists of many nucleotides. Each nucleotide is formed by:

• Deoxyribose – Sugar
• Phosphate
• One of the following organic (nitrogenous) bases:
  • Purines 
    • Adenine (A)
    • Guanine (G)
  • Pyrimidines
    • Thymine (T)
    • Cytosine (C)
• The strands of DNA are arranged in such a way that both are bound by specific pairs of bases. The adenine of one strand binds specifically with the thymine of the opposite strand. Similarly, the cytosine of one strand binds with the guanine of the other strand.
• DNA forms the component of chromosomes, which carries hereditary information. The hereditary information that is encoded in DNA is called the genome. Each DNA molecule is divided into discrete units called genes.

Gene

• Gene is a portion of a DNA molecule that contains the message or code for the synthesis of a specific protein from amino acids. It is like a book that contains the information necessary for protein synthesis. Gene is considered as the basic hereditary unit of the cell.
• In the nucleotide of DNA, three of the successive base pairs are together called a triplet or codon. Each codon codes or forms a code word (information) for one amino acid. There are 20 amino acids and there is a separate code for each amino acid. For example, the triplet CCA is the code for glycine and GGC is the code for proline.
• Thus, each gene forms the code word for a particular protein to be synthesized in the ribosome (outside the nucleus) from amino acids.

Genetic Disorders:

A genetic disorder is a disorder that occurs because of abnormalities in an individual’s genetic material (genome). Genetic disorders are either hereditary disorders or due to defects in genes. An inherited genetic disorder is one that is caused by defective genes inherited from parents.

Gene defects may be due to two causes:

1. Genetic variation: The presence of a different form of a gene is called genetic variation.
2. Genetic mutation: Generally mutation refers to an alteration or a change in nature, form, or quality. In genetics, mutation means a change of the DNA sequence within a gene or chromosome of an organism which results in the creation of a new character. Genetic disorders are classified into four types:
   1. Single gene disorders
   2. Multifactorial genetic disorders
   3. Chromosomal disorders
   4. Mitochondrial DNA disorders

1. Single Gene Disorders: Single gene disorders or Mendelian or monogenic disorders occur because of variation or mutation in one single gene. Examples include sickle cell anemia and Huntington’s disease.

2. Multifactorial Genetic Disorders: Multifactorial genetic disorders or polygenic disorders are caused by a combination of environmental factors and mutations in multiple genes. Examples are coronary heart disease, Alzheimer’s disease, arthritis, and diabetes.

3. Chromosomal Disorders: Chromosomal disorder is a genetic disorder caused by abnormalities in chromosomes. It is also called chromo-somal abnormality, anomaly, or aberration. It often results in genetic disorders which involve physical or mental abnormalities. The chromosomal disorder is caused by a numerical abnormality or structural abnormality.

Numerical abnormality of chromosomes is of two types:

1. Monosomy: Chromosomal abnormality due to the absence of one chromosome from a normal diploid number. An example of a disorder due to monosomy is Turner’s syndrome which is characterized by physical disabilities.

2. Trisomy: Chromosomal abnormality due to the presence of one extra chromosome along with a normal pair of chromosomes in the cells. The best example of disorders due to trisomy is Down syndrome which is characterized by physical disabilities and mental
retardation.

Structural abnormality (alteration) of chromosomes leads to many disorders. Examples are chromosome instability syndromes which are a group of inherited diseases that may cause malignancies.

4. Mitochondrial DNA Disorders: Mitochondrial DNA disorders are genetic disorders caused by mutations in the DNA of mitochondria (nonchromosomal DNA). Examples are Kearns-Sayre syndrome (a neuromuscular disorder characterized by loss of myopathy, cardiomyopathy, and paralysis of ocular muscles) and Leber’s hereditary optic neuropathy (a disease characterized by degeneration of the retina and loss of vision).

Ribonucleic Acid

Ribonucleic acid (RNA) is a nucleic acid that contains a long chain of nucleotide units. It is similar to DNA but contains ribose instead of deoxyribose. Various functions coded in the genes are carried out in the cytoplasm of the cell by RNA. RNA is formed from DNA.

Structure Of RNA:

Each RNA molecule consists of a single strand of polynucleotide, unlike the double-stranded DNA. Each nucleotide in RNA is formed by:

• Ribose sugar
• Phosphate
• One of the following organic bases:
  • Purines     
    • Adenine (A)
    • Guanine (G)
  • Pyrimidines   
    • Uracil (U)
    • Cytosine (C)

Uracil replaces the thymine of DNA and it has a similar structure to thymine.

Types Of RNA:

RNA is of three types. Each type of RNA plays a specific role in protein synthesis. The three types of RNA are:

1. Messenger RNA (mRNA): It carries the genetic code of the amino acid sequence for the synthesis of protein from the DNA to the cytoplasm.
2. Transfer RNA (tRNA): This RNA is responsible for decoding the genetic message present in mRNA.
3. Ribosomal RNA (rRNA): It is present within the ribosome and forms a part of the structure of the ribosome. It is responsible for the assembly of protein from amino acids in the ribosome.

Gene Expression

Gene expression is the process by which the information (code word) encoded in the gene is converted into a functional gene product or document of instruction (RNA) that is used for protein synthesis. Gene expression involves two steps, transcription, and translation.

Transcription Of Genetic Code:

• The word transcription means copying. It indicates the copying of genetic code from DNA to RNA. The proteins are synthesized in the ribosomes which are present in the cytoplasm. However, the synthesis of different proteins depends upon the information (sequence of codon) encoded in the genes of the DNA which is present in the nucleus. Since DNA is a macromolecule, it cannot pass through the pores of the nuclear membrane and enter the cytoplasm. But, the information from DNA must be sent to the ribosome. So, the gene has to be transcribed (copied) into mRNA which is developed from DNA.
• Thus, the first stage in protein synthesis is the transcription of genetic code, which occurs within the nucleus. It involves the formation of mRNA and the simultaneous copying or transfer of information from DNA to mRNA. The mRNA enters the cytoplasm from the nucleus and activates the ribosome resulting in protein synthesis. The formation of mRNA from DNA is facilitated by the enzyme RNA polymerase.

Translation:

• Translation is the process by which protein synthesis occurs in the ribosome of the cell under the direction of genetic instruction carried by mRNA from DNA. Or, it is the process by which the mRNA is read by ribosomes to produce a protein. This involves the role of other two types of RNA namely tRNA and rRNA.
• The mRNA moves out of the nucleus into the cytoplasm. Now, a group of ribosomes called polysome gets attached to mRNA. The sequence of codons in mRNA is exposed and recognized by the complementary sequence of the base in tRNA. The complementary sequence of the base is called the anticodon.
• According to the sequence of bases in anticodon, different amino acids are transported from the cytoplasm into the ribosome by tRNA which acts as a carrier. With the help of rRNA, the protein molecules are assembled from amino acids. The protein synthesis occurs in the ribosomes which are attached to rough endoplasmic reticulum.

Growth Factors

• Growth factors are proteins that act as cell signaling molecules like cytokines (Chapter 17) and hormones
  (Chapter 65). These factors bind with specific surface receptors of the target cell and activate the proliferation, differentiation, and/or maturation of these cells.
• Often the term growth factor is interchangeably used with the term cytokine. However, growth factors are distinct from cytokines. Growth factors act on the cells of the growing tissues. But cytokines are concerned with the cells of the immune system and hematopoietic cells.

Many growth factors are identified. The known growth factors are:

1. Platelet-derived growth factor-PDGF
2. Colony stimulating factors CSF
3. Nerve growth factors – NGF
4. Neurotrophins
5. Erythropoietin
6. Thrombopoietin
7. Insulin-like growth factors – IGF
8. Epidermal growth factor-present in keratinocytes and fibroblasts. It inhibits the growth of hair follicles and cancer cells
9. The basic fibroblast growth factor is present in blood vessels. It is concerned with the formation of new blood vessels
10. Myostatin is present in skeletal muscle fibers. It controls skeletal muscle growth
11. Transforming growth factors (TGF) – are present in transforming cells (cells undergoing differentiation) and in large quantities in tumors and cancerous tissue. TGF is of two types:
    • TGF-α secreted in the brain, keratinocytes, and macrophages. It is concerned with the growth of epithelial cells and wound healing.
    • TGF-β secreted by hepatic cells, T lymphocytes, B lymphocytes, macrophages, and mast cells. When the liver attains the maximum size in adults it controls liver growth by inhibiting the proliferation of hepatic cells. TGF- β also causes immunosuppression.

Cell Death

Cell death occurs by two distinct processes:

1. Necrosis
2. Apoptosis.

1. Necrosis: Necrosis (which means ‘dead’ in Greek) is the uncontrolled and unprogrammed death of cells due to unexpected and accidental damage. It is also called ‘cell murder’ because the cell is killed by extracellular or external events. After necrosis, the harmful chemical substances released from the dead cells cause damage and inflammation of neighboring tissues.

Causes for Necrosis: Common causes of necrosis are injury, infection, inflammation, infarction, and cancer. Necrosis is induced by both physical and chemical events such as heat, radiation, trauma, hypoxia due to lack of blood flow, and exposure to toxins.

Necrotic Process: Necrosis results in lethal disruption of cell structure and activity. The cell undergoes a series of characteristic changes during the necrotic process, viz.

• The cell swells causing damage to the cell membrane and the appearance of many holes in the membrane
• The intracellular contents leak out into the surrounding environment
• The intracellular environment is altered
• Simultaneously, large amounts of calcium ions are released by the damaged mitochondria and other organelles
• The presence of calcium ions drastically affects the organization and activities of proteins in the intracellular components
• Calcium ions also induce the release of toxic materials that activate the lysosomal enzymes
• Lysosomal enzymes cause the degradation of cellular components and, the cell is totally disassembled resulting in death
• The products broken down from the dissembled cell are ingested by neighboring cells.

The Reaction of Neighboring Tissues after Necrosis:

The tissues surrounding the necrotic cells react to the breakdown products of the dead cells particularly the derivatives of membrane phospholipids like arachidonic acid. Along with other materials, arachidonic acid causes the following inflammatory reactions in the surrounding tissues:

• Dilatation of capillaries in the region thereby increasing local blood flow
• An increase in the temperature leads to the reddening of the tissues
• Release of histamine from these tissues which induces pain in the affected area
• Migration of leukocytes and macrophages from blood to the affected area because of increased capillary permeability
• Movement of water from the blood into the tissues causes local edema
• Engulfing and digestion of cellular debris and foreign materials like bacteria by the leukocytes and macrophages
• Activation of the immune system resulting in the removal of foreign materials
• Formation of pus by the dead leukocytes during this process
• Finally, tissue growth in the area and wound healing. Activation of Apoptosis

2. Apoptosis:

• Apoptosis is defined as the natural or programmed death of the cell under genetic control. Originally apoptosis refers to the process by which the leaves fall from trees in autumn (In Greek apoptosis means ‘falling leaves’). It is also called ‘cell suicide’ since the genes of the cell play a major role in death.
• This type of programmed cell death is a normal phenomenon and it is essential for the normal development of the body. In contrast to necrosis, apoptosis usually does not produce inflammatory reactions in the neighboring tissues.

Functional Significance of Apoptosis: The purpose of apoptosis is to remove unwanted cells without causing any stress or damage to the neighboring cells.

The functional significance of apoptosis:

• Plays a vital role in cellular homeostasis. About 10 million cells are produced every day in the human body by mitosis. An equal number of cells die by apoptosis. This helps in cellular homeostasis
• Useful for the removal of a cell that is damaged beyond repair by a virus or a toxin
• An essential event during development and in the adult stage.

Significance of Apoptosis Examples:

1. A large number of neurons are produced during the development of the central nervous system. But up to 50% of the neurons are removed by apoptosis during the formation of synapses between neurons
2. Apoptosis is responsible for the removal of tissues of webs between fingers and toes during a developmental stage in the fetus
3. It is necessary for the regression and disappearance of duct systems during sex differentiation in the fetus.
4. The cell that loses contact with neighboring cells or basal lamina in the epithelial tissue dies by apoptosis. This is essential for the death of old enterocytes shed into the lumen of intestinal glands.
5. It plays an important role in the cyclic sloughing of the inner layer of endometrium resulting in menstruation
6. Apoptosis removes the autoaggressive T cells and prevents autoimmune diseases.

Activation Of Apoptosis: Apoptosis is activated by either the withdrawal of positive signals (survival factors) or the arrival of negative signals.

Withdrawal of positive signals: Positive signals are the signals that are necessary for the long-time survival of most of the cells. The positive signals are continuously produced by other cells or some chemical stimulants. Best examples of chemical stimulants are:

• Nerve growth factors (for neurons)
• Interleukin-2 (for cells like lymphocytes). The absence or withdrawal of the positive signals activates apoptosis.

The arrival of negative signals: Negative signals are the external or internal stimuli that initiate apoptosis. The negative signals are produced during various events like:

• Normal developmental procedures
• Cellular stress
• Increase in the concentration of intracellular oxidants
• Viral infection
• Damage of DNA
• Exposure to agents like chemotherapeutic drugs, X-rays, ultraviolet rays, and death receptor ligands.

Death receptor ligands and death receptors:

• Death receptor ligands are the substances that bind with specific cell membrane receptors and initiate the process of apoptosis. The common death receptor ligands are tumor necrosis factors (TNF-a, TNF- B) and Fas ligand (which binds to the receptor called Fas).
• Death receptors are the cell membrane receptors that receive the death receptor ligands. Well-characterized death receptors are TNF receptor-1 (TNFR1) and TNF-related inducing ligand (TRAIL) receptors called DR4 and DR5.

Role of mitochondria in apoptosis:

• The external or internal stimuli initiate apoptosis by activating the proteases called caspases (cysteinyl-dependent aspartate-specific proteases). Normally, caspases are suppressed by the inhibitor protein called apoptosis inhibiting factor (AIF).
• When the cells receive the apoptotic stimulus, mitochondria release two protein materials. The first one is Cytochrome C and the second protein is called the second mitochondria-derived activator of caspases (SMAC) or its homolog Diablo. SMAC/diablo inactivates AIF so that the inhibitor is inhibited. During this process SMAC/diablo and AIF
• aggregate to form apoptosome which activates caspases. Cytochrome C also facilitates caspase activation.

Apoptotic Process: The cell shows the sequence of characteristic morphological destruction and death of the cell. changes during apoptosis, viz.:

• Activated caspases digest the proteins of the cytoskeleton and the cell shrinks and becomes round
• Because of shrinkage, the cell loses contact with neighboring cells or the surrounding matrix Cellular adaptation occurs by any of the following mechanisms.
• Chromatin in the nucleus undergoes degradation and D. Dysplasia condensation
• The nuclear membrane becomes discontinuous and the DNA inside the nucleus is cleaved into small fragments
• Following the degradation of DNA, the nucleus breaks into many discrete nucleosomal units which are also called chromatin bodies
• The cell membrane breaks and shows a bubbled appearance
• Finally, the cell breaks into several fragments containing intracellular materials including chromatin bodies and organelles of the cell. Such cellular fragments are called vesicles or apoptotic bodies 8. The apoptotic bodies are engulfed by phagocytes and dendritic cells.

Abnormal Apoptosis: Apoptosis within normal limits is beneficial for the body. However, too much or too little apoptosis leads to abnormal conditions.

Common abnormalities due to too much apoptosis:

• Ischemic related injuries
• Autoimmune diseases like
  • Hemolytic anemia
  • Thrombocytopenia
  • Acquired immunodeficiency syndrome (AIDS)
• Neurodegenerative diseases like Alzheimer’s disease.

Common abnormalities due to too little apoptosis:

• Cancer
• Autoimmune lymphoproliferative syndrome (ALPS).

Cell Adaptation

Cell adaptation refers to the changes taking place in a cell in response to environmental changes. The normal functioning of the cell is always threatened by various factors such as stress, chemical agents, diseases, and environmental hazards. Yet, the cell survives and continues its function by means of adaptation. Only during extreme conditions, the cell fail to withstand the hazardous factors which results in the Destruction and death of the cell

Cellular adaptation occurs by any of the following mechanisms

1. Atrophy
2. Hypertrophy
3. Hyperplasia
4. Metaplasia.

1. Atrophy: Atrophy means a decrease in the size of a cell. Atrophy of more cells results in decreased size or wasting of the concerned tissue, organ, or part of the body.

Causes for Atrophy:

• Atrophy is due to one or more causes such as:
  • Poor nourishment
  • Decreased blood supply
  • Lack of workload or exercise
  • Loss of control by nerves or hormones
  • Intrinsic disease of the tissue or organ.

Types of Atrophy: Atrophy is of two types, physiological atrophy and pathological atrophy. Examples of physiological atrophy are the atrophy of the thymus in childhood and tonsils in adolescence. The pathological atrophy is common in skeletal muscle, cardiac muscle, sex organs, and the brain.

2. Hypertrophy: It is the increase in the size of a cell. Hypertrophy of many cells results in enlargement or overgrowth of an organ or a part of the body. Hypertrophy is of three types.

• Physiological Hypertrophy: It is the increase in size due to increased workload or exercise. The common physiological hypertrophy includes:
  • Muscular hypertrophy: Increase in the bulk of skeletal muscles that occurs in response to a strength training exercise
  • Ventricular hypertrophy: Increase in size of ventricular muscles of the heart which is advantageous only if it occurs in response to exercise.
• Pathological Hypertrophy: An increase in cell size in response to pathological changes is called pathological hypertrophy. An example is ventricular hypertrophy which occurs due to pathological conditions such as high blood pressure where the workload of ventricles increases.
• Compensatory Hypertrophy: It is the increase in the size of the cells of an organ that occurs in order to compensate for the loss or dysfunction of another organ of the same type. Examples are the hypertrophy of one kidney when the other kidney stops functioning; and the increase in muscular strength of an arm when the other arm is dysfunctional or lost.

3. Hyperplasia:

Hyperplasia is the increase in the number of cells due to increased cell division (mitosis). It is also defined as abnormal or unusual proliferation (multiplication) of cells due to constant cell division. Hyperplasia results in gross enlargement of the organ. Hyperplasia involves constant cell division of the normal cells only. Hyperplasia is of three types.

• Physiological Hyperplasia: It is the momentary adaptive response to routine physiological changes in the body. For example, during the proliferative phase of each menstrual cycle, the endometrial cells in the uterus increase in number.
• Compensatory Hyperplasia: It is the increase in the number of cells in order to replace the damaged cells of an organ or the cells removed from the organ. Compensatory hyperplasia helps the tissues and organs in regeneration. It is common in the liver. After the surgical removal of the damaged part of the liver, there is an increase in the number of liver cells resulting in regeneration. Compensatory hyperplasia is also common in epithelial cells of the intestine and epidermis.
• Pathological Hyperplasia: Pathological hyperplasia is the increase in the number of cells due to an abnormal increase in hormone secretion. It is also called hormonal hyperplasia. For example, in gigantism, hypersecretion of growth hormone induces hyperplasia that results in overgrowth of the body.

4. Dysplasia:

It is a condition characterized by abnormal changes in the size, shape, and organization of the cell. Dysplasia is not considered a true adaptation and it is suggested as related to hyperplasia. It is common in epithelial cells of the cervix and respiratory tract.

5. Metaplasia: Metaplasia is the condition that involves the replacement of one type of cell with another type of cell.

It is of two types.

1.  Physiological Metaplasia: The replacement of cells in normal conditions is called physiological metaplasia. Examples are the transformation of cartilage into bone and the transformation of monocytes into macrophages.
2. Pathological Metaplasia: It is the irreversible replacement of cells due to constant exposure to harmful stimuli. For example, chronic smoking results in the transformation of normal mucus-secreting ciliated columnar epithelial cells into nonciliated squamous epithelial cells which are incapable of secreting mucus. These transformed cells may become cancerous cells if the stimulus (smoking) is prolonged.

Cell Degeneration

Cell degeneration is a process characterized by damage to the cells at the cytoplasmic level without affecting the nucleus. Degeneration may result in functional impairment or deterioration of a tissue or an organ. It is common in metabolically active organs like the liver, heart, and kidney. Degenerative changes are reversible in most of the cells.

Common causes for cell degeneration are:

• Atrophy, hypertrophy, hyperplasia, and/or dysplasia of the cell
• Fluid accumulation in the cell
• Fat infiltration into the cell
• Calcification of cellular organelles.

Cell Aging

• Cell aging is the gradual structural and functional changes in the cells that occur over the passage of time. It is now suggested that cell aging is due to damage to cellular substances like DNA, RNA, proteins, lipids, etc. when the cell becomes old. When more cellular substances are damaged, the cellular function decreases.
• This causes the deterioration of tissues, organs, or parts of the body. Finally, the health of the body starts declining and this leads to death. So, cell aging determines the health and life span of the body.

Stem Cells

Stem cells are the primary cells capable of reforming themselves through mitotic division and differentiating into specialized cells. These cells serve as a repair system of the body and are present in all multicellular organisms.

Types Of Stem Cells:

• Stem cells are of two types:

1. 1. Embryonic stem cells derived from the embryo
   2. Adult stem cells derived from adults

1. Embryonic Stem Cells: Embryonic stem cells are derived from the inner cell mass of a blastocyst which is an early stage of the embryo. It takes about 4 to 5 days after fertilization to reach the blastocyst stage and it has about 30 to 50 cells. Embryonic stem cells have two important qualities:

1. The self-renewal capacity
2. Pluripotent nature, i.e. these cells are capable of differentiating into all types of cells in ectodermal, endodermal, and mesodermal layers.

Because of these two qualities, embryonic stem cells can be used therapeutically for regeneration or replacement of diseased or destroyed tissues. In fact, embryonic pluripotent stem cells are now cultured and a lot of research is going on to explore the possibility of using these cells in curing disorders like diabetes mellitus by cell replacement technique. However ethical issues arise because the embryo has to be destroyed to collect the stem cells.

Stem cells from umbilical cord blood:

• Stem cells in umbilical cord blood are collected from the placenta or umbilical cord. The use of these stem cells for research and therapeutic purposes does not create any ethical issues because it does not endanger the life of the fetus or newborn.
• Because of vitality and easy availability, the umbilical cord blood stem cells are becoming a potent resource for transplant therapies. Nowadays these stem cells are used to treat about 70 diseases and are used in many transplants worldwide.

2. Adult Stem Cells:

• Embryonic stem cells do not disappear after birth. But remain in the body as adult stem cells and play a role in the repair of damaged tissues. However, their number becomes less. Adult stem cells are the undifferentiated multipotent progenitor cells found in growing children and adults.
• These are also known as somatic stem cells and are found everywhere in the body. These cells are capable of dividing and reforming the dying cells and regenerating the damaged tissues. So these stem cells can also be used for research and therapeutic purposes.

Adult stem cells are collected from bone marrow. Two types of stem cells are present in bone marrow:

1. Hemopoietic stem cells which give rise to blood. cells.
2. Bone marrow stromal cells can differentiate into cardiac and skeletal muscle cells.

Advantages Of Stem Cells:

• Adult stem cells from bone marrow are used in bone marrow transplants to treat leukemia and other blood disorders for 30 years. Recently, it is known that these stem cells can develop into nerve cells, liver cells, skeletal muscle cells, and cardiac muscle cells.
• Recent discoveries also reveal that stem cells are present in several tissues which include blood, blood vessels, skeletal muscle, liver, skin, and brain. It is also found that these cells are capable of differentiating into multiple cell types. So, cell-based therapy using stem cells may be possible to treat many diseases such as heart disease, diabetes, Parkinson’s disease, Alzheimer’s disease, spinal cord injury, stroke, and rheumatoid arthritis.