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chapter 23 summary Junqueira's Basic Histology
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EYE DEVELOPMENT The first indication of the eyes is the optic grooves in the neural folds at the cranial end of the embryo.
The grooves form at the beginning of the fourth week and deepen to form hollow optic vesicles that
project from the forebrain.
he optic vesicles contact the surface ectoderm and induce development of the lens placodes.
As the lens placode thickens to form a lens pit and lens vesicle, the optic vesicle invaginates to form the
optic cup. The retina forms from the two layers of the optic cup.
he retina, optic nerve fibers, muscles of the iris, and epithelium of the iris and ciliary body are derived
from the neuroectoderm of the forebrain. The sphincter and dilator muscles of the iris develop from
the ectoderm at the rim of the optic cup.
The surface ectoderm gives rise to the lens and the epithelium of the lacrimal glands, eyelids,
conjunctiva, and cornea. The mesenchyme gives rise to the eye muscles, except those of the iris, and to
all connective and vascular tissues of the cornea, iris, ciliary body, choroid, and sclera.
The eyes are sensitive to the teratogenic effects of infectious agents (e.g., cytomegalovirus). Defects of
sight may result from infection of tissues and organs by certain microorganisms during the fetal period
(e.g.,rubella virus, Treponema pallidum [causes syphilis]).
Most ocular defects are caused by defective closure of the retinal fissure during the sixth week (e.g.,
coloboma of the iris).
Congenital cataract and glaucoma may result from intrauterine infections, but most congenital
cataracts are inherited.
EAR DEVELOPMENT
The otic vesicle develops from the surface ectoderm during the fourth week. The vesicle develops in to
the membranous labyrinth of the internal ear.
The otic vesicle divides into a dorsal utricular part, which gives rise to the utricle, semicircular ducts,
and endolymphatic duct, and a ventral saccular part, which gives rise to the saccule and cochlear duct.
The cochlear duct gives rise to the spiral organ.
The bony labyrinth develops from the mesenchyme adjacent to the membranous labyrinth. The
epithelium lining the tympanic cavity, mastoid antrum, and pharyngotympanic tube is derived from
the endoderm of the tubotympanic recess, which develops from the first pharyngeal pouch.
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