Perfiles de pozos

The ME is an air-filled cavity lined with a mucous membrane, which mechanically links the air- filled OE and the fluid-filled IE via the auditory ossicles. Because of the difference in impedance of air and water, the ME acts as an acoustical transformer by compensating for much of the impedance mismatch that would result if airborne sound waves impinged directly on the fluid-filled cochlea. Thus, the areal ratio of the tympanic membrane to the stapes footplate and the lever system that exists within the ossicles significantly reduces the reflection of sound energy that would otherwise occur when sound waves from the OE are transmitted from the stapes footplate in the oval window to the sensory epithelium in the cochlea.

The earliest embryological sign of the development of the ME is the primitive formation of the tympanic cavity and the eustachian tube (i.e., turbotympanic recess) from the dorsal recess of the first (and possibly the second) endodermal pharyngeal pouch (Frazer, 1914; Hammer, 1902; Wong, 1983). The turbotympanic recess starts its development in the third to fourth week of fetal development. The first and second branchial arches constitute the anlage for the auditory ossicles, muscles, tendons and connective tissues. Moreover, at this early developmental stage the nerves of the first and second branchial arches--the trigeminal and facial nerves respectively- -are connected through the chorda tympani (Altmann, 1950). The auditory ossicles appear in the sixth week gestational age (GA), with the malleus and incus visible within the mesenchyme of the first and second branchial arches, and the stapes at the end of the second visceral bar. In the eighth week GA, the endoderm of the turbotympanic recess comes in contact with the

ectodermal membrane of the first branchial groove. The positioning of these epithelia is brief, since connective tissue grows between them which later condense to form the trilaminar tympanic membrane. Within this connective tissue is a mass of condensed mesenchyme which is the primordia of the manubrium of the malleus (Anson & Donaldson, 1981; Bast & Anson, 1949; Pearson, Jacobson, Van Calcar, & Sauter, 1973). Shortly after the eighth week GA, the proximal part of the pharyngeal pouch constricts to form the eustachian tube, and the distal end widens to form the tympanic cavity proper. In the 10th week GA, the future of the external auditory meatus becomes apparent when the terminal end of the entodermal pouch flattens against the ectoderm of the invaginating first branchial groove (Altmann, 1950; Anson & Donaldson, 1981; Pearson et al, 1973). In the same week the pars tensa of the tympanic membrane becomes visible (Altmann, 1950).

As the growth and transformation of the tympanic cavity continues, the formation of the tegman tympani occurs from the petrous portion of the temporal bone. During the following week the anlage of the stapedius muscle becomes defined from the dense mesenchyme lateral to the cochlea, while the tensor tympani muscle is actualized slightly later in the 13th week GA (Altmann, 1950). Although just a sulcate impression on the wall of the otic otocyst (cf. Subsection 3.2.2), the facial canal which already houses the facial nerve, the stapedius muscle and blood vessels that supply the tympanic cavity is realized in the 12th week GA (Anson & Donaldson, 1981; Bast & Anson, 1949; Pearson et al., 1973). In the same week, the incus approaches the stapes, and the stapes applies itself closely to the otic otocyst. During this time, the malleus and incus as well as the otic otocyst begin to change to precartilage. In the 13th _week

GA, the stapes is pressed into the lateral wall of the otic otocyst. Shortly after, the stapes and the otic otocyst begin to undergo a similar change to true cartilage (Bast & Anson, 1949). By the

16th week GA, all the ossicles have attained maximum size and have clear adult morphology. At this time, bone formation is initiated in the malleus and incus, and three weeks later, by the 19th week GA the initial step in ossification begins in the stapes (Altmann, 1950; Anson & Donaldson, 1981; Pearson et al., 1973). Shortly after, just one week later the pars flaccida of the tympanic membrane is visible (Altmann, 1950). During this same time, the insertion of the tendons of the tensor tympani muscle (onto the malleus) and stapedius muscle (onto the stapes) occur (Saunders, Kaltenbach, & Relkin, 1983). From the 18th to the 21st week GA, mesenchyme tissue in the tympanic cavity transforms to myxomatous (i.e., mucoid) tissue, which extends into the epitympanic recess. By the 23rd week GA, the anterior and part of the lateral wall of the tympanic cavity develop from the squamous portion of the temporal bone. Moreover, the pneumatic space behind the epitympanic recess opens to form the tympanic antrum, and, the ossification of the tegman tympani begins (Bast & Anson, 1949; Pearson et al., 1973; Wong, 1983).

As development proceeds, a posterior mesentery-like fold is realized in the medial wall of the tympanic cavity, which forms the round window, tympanic sinus, and a large part of the oval window. Concurrently, the tympanic cavity widens and its mucoid lining envelops the ossicles, tensor tympani, and the chorda tympani in the 25th week GA (Pearson et al., 1973). By the 28th week GA the mucoid tissue of the tympanic cavity is fully absorbed. Shortly after, pneumatization of the temporal bone occurs, and the deposition of the petrous and mastoid bones continues through childhood (Anson & Donaldson, 1981; Bast & Forester, 1939; Pearson et al., 1973). In the 29th _{week GA, pneumatization of the epitympanic recess lags behind that of the}

tympanic cavity; however, both are virtually complete by the 36th week GA (Bast & Anson, 1949). The tympanic space, albeit continues to enlarge in postnatal life until adolescence (Eby &

Nadol, 1986; Pearson, 1973). Ossification of the malleus, incus, and stapes are complete by the 32nd week GA; however, pneumatization of the ossicles may continue until adulthood (Anson & Donaldson, 1981; Eby & Nadol, 1986; Wong, 1983).

By the 40th week GA, the formation of the tympanic membrane is fully developed and lies nearly parallel to the walls of the external auditory canal. After birth, the lower wall of the canal comes in contact and begins to fuse with the adjacent walls of the canal (Pearson, 1973). After the second year of life, the bony portions of the external auditory canal become completely osseous (Pearson, 1973). It is during this same time, the tympanic membrane assumes are more conical shape and its plane changes its relative position to take a more vertical orientation which becomes adult-like by three years of age (Ikui, Sando, Sudo, & Fujita, 1997; Pearson, 1973).