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EYE

OPTIC CUP

The three zones of the visual layer of the retina are limited to the deep four-fifths of the optic cup where the layer is thick and is called the pars optica retinae. The superficial one-fifth is thin and is referred to as the pars caeca retinae.

The three zones of the pars optica retinae will differentiate the following manner.

The ependymal zone located next to the intraretinal space will become the photoreceptor layer of rods and cones.

The mantle zone will produce the ganglion and nuclear layers.

The marginal zone next to the optic cup cavity will form the nerve fiber layer.

The pars caeca retinae will differentiate into the iris and ciliary body, both of which contain smooth muscle.

Hyaloid blood vessels differentiate within the mesenchyme that passes through the choroid fissure into the optic cup cavity. These vessels supply the visual part of the retina and initially the lens. A delicate network of fibers, the primordial vitreous body, develops between the lens and the retina.

OPTIC STALK

The relative diameter of the optic stalk is reduced as it increases in length.

The optic stalk lumen that connects the intraretinal space with the third ventricle of the brain is reduced.

LENS

The lens cavity disappears as the deep lens epithelium becomes contiguous with the superficial lens epithelium. The deep layer forms the primary lens fibers which collectively make up the nucleus of the lens.

MESENCHYME AROUND THE OPTIC CUP

A space develops in the mesenchyme between the brim of the optic cup and the overlying ectoderm. This space becomes the anterior chamber, which separates the mesenchyme into two layers; a thin inner layer next to the lens called the iridopupillary membrane and a thick outer layer next to the ectoderm, which together give rise to the cornea.

The loose mesenchyme adjacent to the surface of the optic cup condenses to form a capsule next to the pigmented layer of the retina. At the brim of the optic cup the capsule is continuous with the cornea.

ACCESSORY STRUCTURES

Thick flaps of ectoderm, called upper and lower eyelid folds, contain mesenchymal cells and begin to grow over the surface of the eye.

The walls of the maxillonasal groove fuse to form the nasolacrimal duct that courses from the medial aspect of the eye to the primitive nasal cavity.

The extraocular muscles appear as distinct masses each receiving an innervation from either cranial nerve III, IV or VI. The ophthalmic division of cranial nerve V and the autonomic ciliary ganglion are evident in the vicinity of the eye.

EAR

INTERNAL EAR

Vestibular Pouch

Three separate semicircular canals (anterior, posterior and lateral) develop from three flat outpouchings of the upper part of the vestibular pouch. The central area of the wall of each outpouching fuses and disappears forming a curved duct. The anterior and posterior canals join to form a common crus so that there is a total of only five crura.

The five crura are received into a large sac called the utricle that develops from the lower part of the vestibular pouch.

The endolymphatic diverticulum arises from the lower part of the utricle. Its proximal narrow segment becomes the endolymphatic duct. Its distal end near the roof of the fourth ventricle dilates to form the endolymphatic sac.

Cochlear Pouch

A small sac known as the saccule differentiates from the upper part of the cochlear pouch.

The cochlear duct develops from the lower part of the pouch but remains in communication with the saccule. The duct begins to spiral.

Otic Capsule

Cartilage appears in the condensation around the derivatives of the vestibular and cochlear pouches.

MIDDLE EAR

As the tubotympanic recess elongates in a dorsolateral direction, its narrow proximal part becomes the auditory tube and its wide distal part forms the primitive tympanic cavity.

Mesenchymal condensations at the dorsal end of the first and second arch cartilages represent the middle ear ossicles. The condensations initially lie dorsal to the primitive tympanic cavity. The malleus and incus differentiate at the dorsal end of the first arch cartilage, the stapes differentiates at the dorsal end of the second arch cartilage.

A remnant of the second aortic arch, the stapedial artery, passes through stapes condensation accounting for the opening in its center. The artery is a transient structure.

EXTERNAL EAR

The auricle begins to take shape as the auricular hillocks blend around the first branchial groove. The groove gives rise to the external acoustic meatus.

Source: Atlas of Human Embryos.