Structure Of Skin Epidermis Physiology Notes

Structure Of Skin Introduction

Skin is the largest organ of the body. It is not uniformly thick. In some places, it is thick and in some places, It is thin. The average thickness of the skin is about 1-2 mm. In the sole of the foot, the palm of the hand, and in the interscapular region, it is considerably thick, measuring about 5 mm. In other areas of the body, the skin is thin. It is thinnest over eyelids and penis measuring about 0.5 mm only.

Skin is made up of two layers:

1. Outer epidermis
2. Inner dermis.

Read And Learn More: Medical Physiology Notes

Epidermis

The epidermis is the outer layer of skin. It is formed by stratified epithelium, which consists of 5 layers:

1. Stratum corneum
2. Stratum lucidum
3. Stratum granulosum
4. Stratum spinosum
5. Stratum germinativum

The important feature of the epidermis is that it does not have blood vessels. Nutrition is provided to the epidermis by the capillaries of the dermis.

1. Stratum Corneum

• It is also known as a horny layer. It is the outermost layer and consists of dead cells, which are called corneocytes.
• These cells lose their nucleus due to pressure and become dead cells. The cytoplasm is flattened with a fibrous protein known as keratin.
• Apart from this, these cells also contain phospholipids and glycogen.

2. Stratum Lucidum

• H is made up of flattened epithelial cells. Many cells have degenerated nuclei and, in some cells, the nucleus is absent.
• As these cells exhibit a shiny character, the layer looks like a homogeneous translucent zone. So, this layer is called stratum lucidum (Lucid = clear).

3. Stratum Granulosum: It is a thin layer with 2-5 rows of flattened rhomboid cells. The cytoplasm contains granules of a protein called keratohyalin. The keratohyalin is the precursor of keratin.

4. Stratum Spinosum

• Stratum spinosum is also known as the prickle cell layer because the cells of this layer possess some spine-like protoplasmic projections.
• By these projections, the cells are connected to one another.

5. Stratum Germinativum

• It is a thick layer made up of polygonal cells superficially and columnar or cuboidal epithelial cells in the deeper parts.
• Here, new cells are constantly formed by mitotic division. The newly formed cells move continuously toward the stratum corneum.
• The stem cells, which give rise to new cells, are known as keratinocytes. Another type of cells called melanocytes are scattered between the keratinocytes.
• The melanocytes produce the pigment called melanin. The color of the skin depends upon melanin.
• From this layer, some projections called rete ridges extend down up to the dermis. These projections provide anchoring and nutritional function.

Dermis

• Dermis is the inner layer of the skin. It is a connective tissue layer made up of dense and stout collagen fibers, fibroblasts and histiocytes.
• The collagen fibers exhibit elastic properties and are capable of storing or holding water.
• The collagen fibers contain the enzyme collagenase, which is responsible for wound healing.

Dermis is made up of 2 layers:

1. Superficial papillary layer
2. Deeper reticular layer.

1. Superficial Papillary Layer

1. This layer projects into the epidermis. It contains blood vessels, lymphatics, and nerve fibers.
2. This layer also has some pigment-containing cells known as chromatophores.
3. Dermal papillae are finger-like projections arising from the superficial papillary dermis.
4. Each papilla contains a plexus of capillaries and lymphatics which are oriented perpendicular to the skin surface.
5. The papillae are surrounded by rete ridges extending from the epidermis.

2. Reticular Layer

• The reticular layer is made up of reticular and elastic fibers. These fibers are found around hair bulbs, sweat glands, and sebaceous glands.
• The reticular layer also contains mast cells, nerve endings, lymphatics, epidermal appendages, and fibroblasts.
• Immediately below the dermis, subcutaneous tissue is present. It is a loose connective tissue, which connects the skin with the internal structures of the body.
• It serves as an insulator to protect the body from excessive heat division.
• The newly formed cells move continuously toward the stratum corneum. The stem cells, which give rise to new cells, are known as keratinocytes.
• Another type of cells called melanocytes are scattered between the keratinocytes and the cold of the environment. A lot of smooth muscles called arrector pili are also found in the skin around the hair follicles.

Appendages Of Skin

The hair follicles with hairs, nails, sweat glands, sebaceous glands, and mammary glands are considered appendages of the skin

1. Melanin

• Melanin is the skin pigment and it forms the major color determinant of human skin.
• Skin becomes dark when melanin content increases. It is a protein in nature and it is synthesized from the amino acid tyrosine via dihydroxyphenylalanine (DOPA).
• A deficiency of melanin leads to albinism (hypo-pigmentary congenital disorder).

2. Hemoglobin In The Blood

• The amount and the nature of hemoglobin that circulates in the cutaneous blood vessels play an important role in the coloration of the skin:
• When hemoglobin content decreases skin becomes pale
• When blood rushes to the skin due to cutaneous vasodilatation (blushing), the color of the skin becomes pink
• During cyanosis which is caused by an excess amount of reduced hemoglobin, the skin becomes bluish