108. Ectoderm and its derivatives.

ectoderm

the outermost germ layer of the embryo. It gives rise to various structures that maintain contact with the outside world and contributes to several key organ systems:

• Nervous System: Includes the central nervous system (CNS), which is derived from the neural tube, and the peripheral nervous system (PNS), which originates from the neural crest. The ectoderm also contributes to the retina and the neurohypophysis (posterior pituitary gland).

• Integumentary System:

  • Epidermis: The outer layer of skin, including hair and nails.

  • Pigmentation: Melanocytes, which produce pigment.

• Eye Structures: The lens and cornea.

• Glands:

  • Sweat Glands: Involved in temperature regulation.

  • Sebaceous Glands: Produce oil to lubricate the skin.

  • Mammary Glands: Produce milk.

• Epithelium:

  • Oral Cavity and Anus: Lining of the mouth and anal canal.

  • Nasal Cavity and Auditory Canal: Lining of the nasal passages and ear canal.

neuroectoderm

Neuroectoderm is a specialized portion of the ectoderm that forms the neural structures:

• Neurulation: The process of forming the neural tube from the neuroectoderm, beginning around the 17th embryonic day.

  • Neural Plate: Induced by signaling molecules from the notochord.

  • Neural Groove: Formed by the invagination of the neural plate.

  • Neural Folds: These folds come together to form the neural tube.

  • Neural Crest: Cells that migrate from the edges of the neural tube and contribute to the formation of the PNS and other structures.

Neural Tube Formation:

Closure of Neural Tube: Regulated by bone morphogenetic proteins (BMPs).

• Cranial Neuropore: Closes around embryonic day 25.

• Caudal Neuropore: Closes around embryonic day 27.

Neural Crest Cells:

Migrate to the mesoderm and differentiate into:

• Melanocytes: Found in skin and hair follicles.

• Sensory Ganglia: Clusters of nerve cells that transmit sensory information.

• Schwann Cells: Myelinate peripheral nerves.

• Enteric Neurons: Part of the enteric nervous system, which controls the gastrointestinal tract.

• Craniofacial Skeleton: Contributes to the bones and cartilage of the head and face.

Otic and Lens Placodes:

• Otic Vesicles: Develop into structures needed for hearing.

• Lens Placodes: Form the lenses of the eyes.

• Neural Crest Cells are sometimes referred to as a "fourth germ layer" due to their extensive contributions to various structures.

• Abnormalities in ectoderm development can lead to congenital defects and certain cancers.