SEM_16_Central nervous

• Midbrain (Mesencephalon) development: The neural cavity of the midbrain forms the mesencephalic aqueduct, which lacks a choroid plexus and is not a ventricle. Alar plates form the rostral and caudal colliculi, associated with visual and auditory reflexes, respectively. The basal plate gives rise to the oculomotor (III) and trochlear (IV) nerves, which innervate eye muscles.

• Forebrain (Prosencephalon) development: Derived entirely from the alar plate, comprising the diencephalon and telencephalon. Diencephalon: The neural cavity expands to the third ventricle, choroid plexuses develop in its roof, and the floor gives rise to the neurohypophysis. The thalamus, hypothalamus, and epithalamus develop from the mantle layer, obliterating the centre of the third ventricle. The optic nerve develops from the wall of the diencephalon. Telencephalon (cerebrum): Comprises bilateral hollow outgrowths that become hemispheres, each with a lateral ventricle communicating with the third ventricle via an interventricular foramen. The mantle layer gives rise to basal nuclei and cerebral cortex.

• Peripheral Nervous System (PNS) development: Comprises cranial and spinal nerves and their ganglia. Classified as the somatic (innervating skin and skeletal muscles) and autonomic (innervating viscera) nervous systems. Somatic efferent fibres arise from motor neurons in the spinal cord's ventral horns. Visceral efferent pathway involves two neurons: preganglionic neurons from the basal plate and postganglionic neurons from neural crests. Afferent fibres arise from neural crest cells in sensory ganglia and enter the spinal cord through the dorsal root, including special senses. Neurolemmocytes (Schwann cells) are glial cells derived from neural crests that form myelin sheaths and satellite cells.

• Meninges development: Three membranes surrounding the CNS and spinal and cranial nerve roots. Dura mater forms from the outer embryonic layer, arachnoid and pia mater form from the inner layer, and both layers coalesce into a single layer, which eventually forms the subarachnoid space.

• Eye development: Both eyes derived from a single optic field in the cranial neural plate. The optic vesicle forms as a lateral diverticulum, becomes the optic nerve, and has an optic fissure for blood vessel access. A lens placode forms on the surface ectoderm and induces the optic vesicle to invaginate, leading to the formation of the optic cup, retina, and eye structures.

• Eye development (continued): The optic stalk becomes the optic nerve and the optic vesicle and stalk have an inferior groove, the optic or choroidal fissure, which eventually fuses and forms a blood vessel canal. The lens placode invaginates to form the lens vesicle, which becomes the lens. The retina forms from the optic cup and has outer and inner layers.

• Eye development (continued): The optic nerve is myelinated by Schwann cells, and the retina's outer layer has a pigmented epithelium. The iris and ciliary body develop from the optic vesicle's periphery, and the choroid layer develops from the optic vesicle's wall, providing nutrients to the retina. The optic nerve and retina are connected by the optic chiasm, which separates the optic nerves into the left and right optic tracts.

• The vitreous body forms in the centre of the optic cup, comprised of vitreous humour derived from mesenchymal cells and neuroectoderm.

• A hyaloid canal runs through the vitreous body, containing the hyaloid artery in fetal life which supplies blood to the developing lens.

• The ectomesenchyme surrounding the optic cup condenses to form the choroid and sclera.

• The ciliary body and iris develop at the anterior rim of the optic cup, with the iris having non-pigmented and pigmented layers continuous with the retina.

• The sphincter pupillae and dilator pupillae muscles develop from optic cup neuroectoderm within the iris stroma.

• The ciliary muscle is derived from overlying mesenchyme and is responsible for changing the lens shape.

• The colour of the eye is determined by the amount of melanin in the iris and stroma, and a pupillary membrane develops as a blood supply source for the lens in fetal life.

• The anterior and posterior chambers develop as clefts in the mesenchyme between the cornea and lens.

• Eyelids form from ectoderm folds, which separate prenatally in ungulates or postnatally in carnivores, and conjunctiva develops from the ectoderm lining the inner surfaces.

• Skeletal extraocular muscles are derived from rostral somitomeres and are innervated by cranial nerves III, IV, and VI.

• Ear development includes an otic placode invaginating to form an auditory vesicle which becomes the inner ear, the middle ear forming from the first pharyngeal pouch, and the outer ear developing from the first pharyngeal groove.

• The inner ear contains the cochlea and vestibular apparatus for hearing and balance, the middle ear has the ossicles for vibration conduction, and the outer ear channels sound waves to the tympanic membrane.

Test your knowledge about neurogenesis in the adult brain, including the areas where new neurons can be formed and the limitations of mature neurons in cell division.