Chapter 18: The Nervous System: General and Special Senses
Chapter 18: Nervous System – Development of the Special Sense Organsby John F. Neas
All special sense organs develop mostly from ectoderm. The sensory organs develop concurrently beginning early in the fourth week, and their development continues through the second trimester. The sensory receptors in the skin, visceral organs, and muscles and tendons develop with the organs that they serve.
Optic Vesicle, Stalk, and Cup
The development of the eye is a complex and rapid process that involves a precise interaction of neuroectoderm, surface ectoderm, and mesoderm. These germ layers have differentiated by early in the fourth week. The sensory components of the eye develop by twenty weeks and the accessory structures by thirty-two weeks.
The initial differentiation involves neuroectoderm that forms a lateral optic diverticulum on each side of the prosencephalon (forebrain). As the diverticula increase in size, the distal portions dilate to become bulges in the lateral walls of the prosencephalon called optic vesicles that extend to either side. The proximal portions constrict to become the optic stalks. The optic vesicles contain a cavity that is continuous with the neurocoel. Concurrent with the formation of the lens vesicle, the optic vesicles invaginate and differentiate into two-layered optic cups that remain connected to the diencephalon by the optic stalks.
Lens Placode, Pit, and Vesicle
Once the optic cup is formed, the overlying epidermis responds by thickening and forming an invagination. The thickened portion is the lens placode; the invagination is the lens pit. During the fifth week, the lens placode becomes depressed and eventually separates from the surface ectoderm forming a lens vesicle. A proliferation of cells in the lens vesicle leads to formation of the lens.
Besides the lens placode, other derivatives of surface ectoderm include the lacrimal apparatus (lacrimal gland, sac, nasolacrimal duct), eyelids, eyelashes and associated glands, and conjunctiva of the cornea.
A groove called the optic fissure that develops along the inferior surface of the optic cup is continuous with a depression along the optic stalk. The hyaloid artery and hyaloid vein traverse the optic fissure longitudinally to supply the developing eyeball. The walls of the optic fissure eventually close so that the hyaloid vessels become confined within the tissue of the optic stalk. The hyaloid vessels become the central vessels of the retina of the mature eye. The optic stalk eventually becomes the optic nerve that consists of sensory axons from the retina.
By the end of the sixth week and the early part of the seventh week, the optic cup differentiates into two epithelial sheets that become the sensory and pigmented layers of the retina. These layers also line the entire vascular coat, including the ciliary body, iris, and the choroid. Thus, neuroectoderm (neural plate ectoderm) produces the optic cups diencephalic derivatives (retina, iris, and ciliary body) and the optic stalk that forms the optic nerve and optic chiasma. The optic lobes and optic tectum also develop from neural plate ectoderm (mesencephalon).
Differentiation of Surrounding Mesoderm
The tissue mass that surrounds the differentiating ectoderm is thus far undifferentiated mesoderm. After the primordial ectodermal structures develop, however, the mesodermal tissue begins to differentiate. The lens capsule develops from the mesoderm that surrounds the lens. The mesoderm between the lens and retina through which the hyaloid vessels pass develops into the vitreous humor. The head mesoderm that surrounds the optic cup differentiates into (1) an inner layer that becomes the vascular choroid (possibly from neural crest), and (2) an outer layer that becomes the toughened sclera posteriorly and the transparent cornea, anteriorly. The anterior and posterior chambers are the fluid-filled spaces that develop as cavities within the mesoderm between the cornea and the iris and between the iris and the lens, respectively. After the cornea develops, additional surface ectoderm produces the thin conjunctiva that covers the anterior surface of the eyeball. The epithelium of the eyelids and the lacrimal glands and ducts develop from surface ectoderm, but the extrinsic, extraocular (intraorbital) muscles and all connective tissue associated with the eye develop from head mesoderm or a preotic myotome derived from epimere (paraxial mesoderm). These accessory structures of the eye gradually develop during the embryonic period and into the fifth month of the fetal period.
The least specialized of the special sensory organs are the gustatory, or taste, receptors. Sensory fibers grow into the developing mouth and pharynx to produce the taste buds. The epithelial cells differentiate into gustatory cells when the nerve endings contact epithelial cells.
The nasal and olfactory sensory epithelium and the olfactory nerve begin to develop during the fifth week as a pair of thickened areas of surface ectoderm called the nasal placodes (later the olfactory pit) in front of the prosencephalon. The olfactory bulb and tract develop from neural plate ectoderm associated with the telencephalon. The nasal placodes gradually become enfolded and protected by developing facial structures.
The ear begins its development concurrently with the eye early during the fourth week. The development of the ear involves ectoderm (surface ectoderm and neuroectoderm), mesoderm, and endoderm. The sensory components of the ear develop by twenty weeks and the accessory structures by thirty-two weeks.
The ear of an adult structurally and functionally consists of an external ear, a middle ear, and an inner ear. Each of these has a separate embryonic origin.
The inner ear begins to develop early in the third week when a plate of surface ectoderm called the otic placode appears on the lateral side of the rhombencephalon. The otic placode soon invaginates to form an otic pit. Toward the end of the fourth week, the outer edges of the invaginated otic pit converge and fuse to form an otocyst (otic vesicle). The otocyst soon separates from the surface ectoderm and then differentiates to form a dorsal utricular portion and a ventral saccular portion.
Three diverticula extend outward from the utricular portion and develop into the semicircular canals that later function in balance and equilibrium. A tubular diverticulum, called the cochlear duct, extends in a spiral fashion from the saccular portion and becomes the membranous portion of the cochlea. The organ of Corti, the functional portion of the cochlea, differentiates from cells along the wall of the cochlear duct. During the sixth week, the neuroectoderm of the developing brain differentiates into the spinal and vestibular ganglia, and their sensory nerves that innervate the inner ear extend toward the developing air cells.
The mesodermal tissue (mesenchyme) that surrounds the differentiating otocyst soon forms a cartilaginous otic capsule. As the otocyst and surrounding otic capsule grow, vacuoles containing the fluid perilymph develop within the otic capsule. The vacuoles soon enlarge and unite to form the perilymphatic space that divides into the scala tympani and the scala vestibuli. The cartilaginous otic capsule eventually ossifies to form the bony (osseous) labyrinth of the inner ear.
Thus, the sacculus, utriculus, cochlea, semicircular canals, and the auditory nerve and ganglion develop from the otic placode and vesicle (surface ectoderm). Secondary supporting investments of the otic vesicle come from mesoderm.
The lining of the middle ear chamber, or tympanic cavity, develops from foregut endoderm associated with the first pharyngeal pouch. The auditory ossicles that amplify incoming sound waves develop from the cartilages of the first and second pharyngeal arch. The tympanic cavity enlarges, surrounds, and encloses the developing ossicles. The connection of the tympanic cavity to the pharynx gradually elongates into the auditory (eustachian) tube. The auditory tube remains patent throughout life and is important in maintaining equilibrium of air pressure between the pharyngeal and tympanic cavities.
The external ear includes the fleshy auricle (pinna) attached to the side of the head and the tubular external auditory meatus that continues into the temporal bone of the skull. The external auditory meatus develops from the surface ectoderm that covers the dorsal end of the first pharyngeal grove. A solid epithelial plate meatal plug soon develops at the bottom of the funnel-shaped pharyngeal groove. The meatal plug contributes to formation of the inner wall of the external auditory meatus and to the tympanic membrane (eardrum). The inner surface of the eardrum (tympanum) develops from foregut endoderm associated with the first pharyngeal pouch. The outer surface of the eardrum develops from surface ectoderm. Mesoderm separates the two layers of this first branchial membrane.
The auricle develops from six separate swellings of surface ectoderm called auricular hillocks that develop around the first branchial groove. The auricular hillocks enlarge, unite, and become supported by mesodermal mesenchyme that differentiates into fibrous tissue. The auricles migrate superiorly as the face of the embryo and fetus develops. The auricles have ascended to the dorsoventral level of the eyes by thirty-two weeks.
Lens Placode, Pit, and Vesicle
Differentiation of Surrounding Mesoderm