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EYE

OPTIC STALK

The nerve fibers coursing through the optic stalk increase in number and eventually completely obliterate its lumen thereby converting it into the optic nerve. The hyaloid artery passing through the stalk will give rise to the central artery of the retina.

The same protective membranes that form around the brain (viz. dura mater, arachnoid and pia mater) also develop around the optic nerve. The subarachnoid space containing cerebrospinal fluid extends peripherally to the retina around the optic nerve.

LENS

Secondary lens fibers are continually added to the nucleus of the lens throughout prenatal and much of postnatal life.

CAPSULAR MESENCHYME

Two layers begin to differentiate in the capsular mesenchyme: a) an inner, thin, vascular, pigmented layer called the choroid, and b) an outer, thick, white fibrous layer called the sclera.

ACCESSORY STRUCTURES

The upper and lower eyelid folds become thin, grow toward each other and begin to fuse. When fusion is complete (ninth week), a space is located between the surface of the eye and the eyelids called the conjunctival sac. The eyelids remain fused until about the seventh month of prenatal life.

The nasolacrimal duct courses from the medial aspect of the conjunctival sac to the nasal cavity. Its orifice in the nasal cavity is located below the inferior nasal concha in a space called the inferior meatus.

Small, solid epithelial buds begin to sprout from the upper, lateral part of the conjunctival sac. These buds are the lacrimal gland primordium and will increase in number, branch and canalize to form the ducts and ductules of the lacrimal gland. The lacrimal gland secretes fluid into the conjunctival sac for protection of the eye surface. Excessive fluid production is channeled to the nasal cavity by way of the nasocrimal duct.

EAR

INTERNAL EAR

The membranous labyrinth attains, for the most part, its definitive configuration. Two minor changes occur subsequent to the eighth week: a) the formation of the Y-shaped connection between the endolymphatic duct, utricle and saccule and b) the elongation and narrowing of the communication between the saccule and cochlear duct forming the ductus reunions.

Vestibular Pouch

One end of each semicircular canal enlarges to produce an ampulla at its junction with the utricle. A crest or crista will differentiate later within each ampulla that functions as a receptor for the maintenance of equilibrium. Similar receptor areas called maculae will also form in the walls of the utricle and saccule. Nerve fibers from both types of receptors have their cell bodies in the vestibular ganglion.

Cochlear Pouch

The cochlear duct completes the two and one half turn spiral (definitive number of turns). The receptor area for sound (spiral organ of Corti) will differentiate later from a ridge of modified epithelial cells in the cochlear duct. As the cochlear duct spirals, so follows the cochlear ganglion thereby producing the spiral ganglion. Nerve fibers from the receptor area for sound have their cell bodies in the spiral ganglion.

Otic Capsule

The membranous labyrinth is encased in cartilage. Perilymphatic spaces containing fluid develop between the membranous labyrinth and the cartilage. The cartilage will subsequently transform into bone forming the bony labyrinth.

MIDDLE EAR

The mesenchymal condensations representing the middle ear ossicles transform into cartilage thereby producing a physical connection between the tympanic membrane and the perilymphatic space around the cochlear duct.

The endodermal lining of the primitive tympanic cavity expands subsequently to form the definitive tympanic cavity. As it expands, it envelops the ossicles and chorda tympani nerve. Expansion dorsally produces the tympanic antrum and mastoid air cells.

EXTERNAL EAR

The auricular cartilages appear as cellular condensations when most of the definitive external features of the auricle become evident.

Epithelial cells lining the external acoustic meatus begin to form a meatal plug that eventually occludes the entire meatus. The plug dissolves around the seventh month of prenatal life.

The tympanic membrane separates the external acoustic meatus from the tympanic cavity. It is composed of three layers, ectoderm on the exterior, endoderm on the interior and an intermediate layer of mesenchymal cells in which the handle of the malleus is enmeshed.

Source: Atlas of Human Embryos.