Sense Organs: Embryonic Development
The human senses, which include sight, smell, hearing, and balance, develop primarily from the outermost tissue layer (called the ectoderm) during embryogenesis as highly specialized organs. These organs (the eyes, nose, and ear) appear as regions on the surface of the developing embryo called placodes, which are connected to the central nervous system. Placodes can be subdivided into two major groups. Each group has a pre-determined fate in terms of the specific tissues it gives rise to. For example, one group develops into organs related to the inner ear, the lens of the eye, and the olfactory sensory epithelium (tissues related to the sense of smell). The other group develops into a complex array of nerve cells and tissues that are wired to the brain.
The eye is one of the most complex structures of the sensory organs. Various signaling molecules or cues stimulate cells to differentiate into various cell types that lead to different structural components of the eye. A coordinated, natural developmental progression must take place to ensure proper development. For example, the cornea must be properly aligned with respect to the lens to allow light to reach the retina. Similarly, the retina must also develop properly in terms of structure and location to allow the reception of visual images and transmission of these signals through the optic nerve to specific parts of the brain.
Primitive eye structures are called diverticula and begin developing by 22 days of gestation. The retina, iris, and optic nerve develop from an ectodermal outgrowth of the developing brain. The optic grooves appear as the neural tube (the progenitor to the central nervous system) closes and these grooves enlarge to form optic vesicles. The optic vesicles are connected to the ectoderm. The connection is necessary in order for inductive processes that result in thickening of the surface of the ectoderm, which forms the lens of the eye. Failure for these inductive processes to take place results in loss of proper eye development.
By six weeks, the optic vesicles become the optic cup, an area that invaginates to encompass the developing eyes. A blood vessel in the groove of the optic cup (called the choroid fissure) develops. The choroid fissure closes a week later, and the vessel becomes the major artery of the retina. The retina is the receptive area of the eye and forms light-sensitive elements within specialized cells from the optic cup. The eyelid and iris develop after eight weeks. The choroid and sclera (other components that make up the eye) develop from tissue surrounding the optic cup. These structures along with the cornea and retina provide additional inductive signals for development of the lens.
Shortly after development of the eye, the ear begins to develop from an area of ectodermal tissue called the auditory plate. The ear can be divided into three anatomical parts: the external ear, the middle ear or tympanic cavity, and the internal ear or labyrinth. The inner ear is the first of the three to develop and serves as a sensory apparatus that controls both hearing and balance through structures called the cochlea and the vestibular apparatus, respectively. During the fourth week of embryogenesis, the otic placode (a thickened region on the surface of the ectoderm) appears on each side of an area of the brain called the hindbrain. An otic pit is formed as the otic placode invaginates and it sinks into the ectodermal tissue. The auditory plate becomes the auditory vesicle, from which an epithelial membrane of the labyrinth develops. The end of the membranous labyrinth elongates and forms a coiled tube called the cochlear duct. The vestibular apparatus develops from the central portion of the labyrinth.
The middle ear primarily functions to convert sound pressure waves into mechanical waves in the middle ear ossicles that connect to the tympanic membrane. It continues to grow through puberty. The auditory tube (also called the Eustachian tube), important for the equalization of pressure, and the tympanic cavity together make up the middle ear. The typmpanic cavity is derived from the first pharyngeal pouch, a region near the wall of the hindbrain, which becomes the tubotympanic recess as the first sign of cartilage develops. The tympanic cavity expands and encloses on the ossicles, tendons, ligaments, and nerves.
The external ear begins developing during the fifth week from the opposite end of the first pharyngeal groove that the middle ear develops from. The structures that develop include the auricle (pinna), the external auditory meatus (ear canal), and the tympanic membrane (eardrum). The external auditory meatus, which guides sound to the eardrum, develops from ectodermal tissue when a funnel-like tube called the meatal plug forms into a cavity. It is small at birth increasing the risk of damage to the tympanic membrane causing permanent hearing loss in the event of an injury until it fully elongates.
The sense of smell relies on the olfactory nerves, which develop between 11 and 15 weeks. They project into the upper part of the nose and are distributed in the mucous membranes of the nasal cavity. The nose, however, begins to develop during the fourth week of embryogenesis, from a pair of thickened areas of the ectoderm called olfactory placodes. The placodes form nasal pits that are deepened by rapid growth of the surrounding tissue and converge toward the midline of the face. They progressively billow into the oral cavity, separated by tissue that thins into a membrane called the osonasal membrane, which eventually breaks. Olfactory receptors project through the tissue surfaces of the nasal cavity and form nerve-like connections through an area called the olfactory bulb. The olfactory bulb consists of mitral cells, which relays messages to the olfactory centers of the brain, producing a sense of smell. The number and location of these sensory receptors determines the level of sensory perception. Bloodhounds, for example, develop a keen sense of smell due to an extremely large number of these receptors in the nose compared to humans.
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