Ectoderm Derivatives Notes

Derivatives Of The Ectoderm

Neurulation:

Notochord induces the overlying ectoderm to thicken and form the neural plate. These cells of the neural plate are termed as neuroectoderm.

• The neural plate gradually extends towards the primitive streak.
• By the end of the 3rd week, lateral edges of neural plate become elevated to form neural folds and depressed median area is called neural groove.
• The neural folds fuse in the midline. This fusion of neural fold begins in the cervical region and proceeds cranially and caudally. This leads to the formation of the neural tube and the process is called neurulation.

Fate of neural tube:

• The narrow caudal part of neural tube develops into the spinal cord.
• The broader cephalic part of the neural tube with a number of swellings Develops into the brain.

Neural crest formation:

As the neural folds elevate and fuse, the cells at the lateral border or crest of neuroectoderm begin to dissociate from the neural fold as neural crest cells.

Derivatives of the cells of the neural crest:

• Neural crest cells which migrate towards the skin give rise to
  • Melanocytes in the skin
  • Hair follicles
• Some of the neural crest cells which migrate ventrally become
  • Sensory ganglia, sympathetic and enteric neurons, schwann cells and Cells of the medulla of the adrenal (suprarenal) gland.
• Neural crest cells migrating from cranial neural folds contribute to the Craniofacial skeleton as well as neurons for cranial ganglia, glial cells and melanocytes.

Neural crest derivatives:

Skeletal elements of pharyngeal arches which form craniofacial bones c

• Cells of the thyroid gland
• Odontoblasts
• Cranial nerve ganglia
• Spinal (dorsal root) ganglia
• Dermis in face and neck
• Adrenal medulla
• Schwann cells
• Melanocytes
• Arachnoid and pia mater (leptomeninges)

Other ectodermal derivatives:

By the time neural tube is closed, two bilateral ectodermal thickenings, the otic placodes and lens placodes can be seen in the cephalic region of the embryo.

• Otic placodes form otic vesicles, which develop into different structures involved in hearing and equilibrium.
• Lens placodes develop into lenses of eyeballs.

Derivatives of intraembryonic mesoderm:

To begin with, intraembryonic mesodermal cells form a thin sheet of loosely arranged tissue on either side of the midline. At a later stage, longitudinal grooves appear and subdivide the intraembryonic mesoderm into three parts, namely paraxial mesoderm, intermediate mesoderm and lateral plate mesoderm.

Paraxial mesoderm and formation of somites:

• Paraxial mesoderm is a thickened plate of tissue close to the midline formed due to cell proliferation.
• During earlier days of 3rd week, paraxial mesoderm gets organized into small segments called somites.
• Somites are initially formed in the cephalic region of the embryo, and their formation then proceeds cephalocaudally.
• In the head region, somites are formed in association with segmentation of neural plate and contribute to the mesenchyme of the head.

Fate of somites:

• The cells of the somites get organized in three layers, namely sclerotome, myotome and dermatome.
• Sclerotome consists of loosely arranged mesenchymal tissue forming the Ventromedial part of the somite. The cells of sclerotome become polymorphous and migrate to surround the notochord and developing spinal cord to form vertebral column.
• Myotome consists of cells of dorsolateral portion of the somite. The cells of myaotome migrate to their corresponding locations as precursors of limb and body wall musculature. Each segmentally arranged myotome contributes to the muscles of back.
• Dermatome is the remaining dorsal epithelium of the somite. The cells of the dermatome disperse to form the dermis and subcutaneous tissue of the skin.

Derivatives of somites:

Intermediate mesoderm:

Intermediate mesoderm is the part of intraembryonic mesoderm which lies lateral to the paraxial mesoderm. For a short period, it connects the paraxial mesoderm with the lateral plate mesoderm.

Fate of intermediate mesoderm:

Major derivatives of intermediate mesoderm are kidney and gonads (ovary or testis). In cervical and upper thoracic regions, it shows segmentations which later develop into nephrotomes. More caudally, it forms unsegmented mass of tissue called nephrogenic cord from which excretory units of urinary system and gonads develop.

Lateral plate mesoderm:

Lateral plate mesoderm is the lateralmost part of intraembryonic mesoderm.

Unlike other two parts of intraembryonic mesoderm it remains thin. Soon, intercellular cavity appears within the lateral plate mesoderm which divides it into two layers: A somatopleuric or parietal layer which is continuous with mesoderm covering the amnion and a splanchnopleuric or visceral layer which is continuous with mesoderm covering the yolk sac.

Together, these two layers line a newly formed cavity:

The intraembryonic coelom (cavity), which is continuous with extraembryonic coelom (cavity) on either side of the embryo.

Fate of lateral plate mesoderm:

Somatopleuric or parietal layer of lateral plate mesoderm lines intraembryonic cavity and along with overlying ectoderm it forms lateral and ventral body wall. It also forms the parietal layer of peritoneal, pleural and pericardial cavities.

Splanchnopleuric or visceral layer of lateral plate mesoderm surrounds the organs and forms the connective tissue and musculature of gut wall. It also forms the visceral layer of peritoneal, pleural and pericardial cavities.

Intraembryonic cavity develops into the serous sacs of the body, namely peritoneal cavity (in abdomen and pelvis), pleural cavity (in thorax surrounding the lungs) and pericardial cavity (in thorax surrounding the heart).

Endoderm:

Endoderm covers the ventral surface of the embryo and forms roof of yolk sac.

Folding of embryo:

The trilaminar embryonic disc is a pear-shaped flattened structure.

• With the development and growth of brain vesicles, embryonic disc begins to bulge into the amniotic cavity.
• At the end of the 3rd week, embryonic disc begins to fold cephalocaudally. This folding is most significant in head and tail regions which are called head and tail folds, respectively.
• Due to rapid growth, the initial flat embryonic disc also folds laterally to obtain a rounded shape. During the process of folding, growth of the yolk sac is slowed down. But, the amniotic cavity enlarges rapidly and surrounds the embryo and embryo now starts floating in the amniotic fluid.
• Simultaneously, the ventral body wall of the embryo is established except for a small part in the ventral abdominal region where the duct of the yolk sac and connecting stalk are attached.

Derivatives of endoderm:

• Due to the formation of the head and tail folds, a large portion of endoderm lined yolk sac is incorporated into the body of embryo leaving a small part of it outside.
• Both the parts will remain communicated for some time through vitelline duct which passes through the connecting stalk.

The part of the yolk sac which is incorporated into the embryo forms the primitive gut which develops into

• Foregut
• Midgut
• Hindgut

Foregut:

• Foregut is the cranialmost part of the primitive gut.
• Its cephalic end is temporarily bounded by the buccopharyngeal membrane that later ruptures and establishes an open connection between the amniotic cavity and primitive gut.
  • Cephalic part of the foregut is called primitive pharynx in which later pharyngeal arches appear giving rise to a number of structures in the head and neck regions.
  • Caudal part of foregut gives rise to epithelial lining of oesophagus, Stomach and proximal part of duodenum.

Midgut:

• Midgut is the part of primitive gut between the foregut and hindgut.
• It develops into the distal part of the duodenum, jejunum, ileum, caecum, appendix, ascending colon and right two-thirds of transverse colon.

Hindgut:

• The part of the primitive gut which is caudal to the midgut in the tail region of the embryo is called the hindgut.
• Distally, it terminates temporarily at the cloacal membrane which disintegrates in the 7th week of intrauterine life to create the anal opening.
• Hindgut later develops into lining epithelium of the left one-third of transverse colon, descending colon and part of rectum.