298
the hair cells in the sensory epithelium (Tanimoto et al. 2009 ). Afferent innervation
does not appear to be necessary for hair cell differentiation, but it likely plays a role
for long-term maintenance of individual hair cells (Sokolowski et al. 1993 ; Fritzsch
et al. 2004 ). In the toadfi sh, Opsanus tau , cells located within the developing sensory epithelium that differentiate into hair cells have a layer of microvilli along the
luminal surfaces, which elongate to form the stereocilia as the kinocilia begin to
elongate (Lewis and Li 1973 ; Sokolowski and Popper 1988 ). Also, embryonic differentiation and hair cell addition in O. tau occurs throughout the saccule simultaneously and not only on the edges of the growing saccular macula (Sokolowski and
Popper 1988 ). Once the processes of innervation, differentiation, and hair cell maturation have taken place in the auditory end organs, all of the structural components
necessary for auditory perception are in place and transduction of acoustic stimuli
can begin, however the auditory system continues to develop after fi sh hatch.
2.3 Postembryonic Development of the Peripheral
Auditory System
Postembryonic sensory hair cell addition has been demonstrated in elasmobranch
(Corwin 1981 , 1983 ) and teleost fi shes (Platt 1977 ; Popper and Hoxter 1984 ; Coffi n
et al. 2012 ). The size and shape of the sensory epithelia also change during ontogeny (Corwin 1983 ; Popper and Hoxter 1984 ; Lombarte and Popper 1994 ). Additional
ontogenetic changes may include the density of sensory hair cells (Popper and
Hoxter 1984 ; Lombarte and Popper 1994 ; Lu and DeSmidt 2013 ), and number of
auditory nerve ganglion cells as well as the innervation patterns of the eighth nerve
(Corwin 1983 ; Popper and Hoxter 1984 ), but at least in the European hake,
Merluccius merluccius , it does not include orientation of the hair cells (Lombarte
and Popper 1994 ).
Because the saccule is the main end organ of hearing in most teleost fi shes, it has
been the most extensively studied auditory end organ, however some data exists
which suggests that the macula neglecta in elasmobranchs and other otolithic end
organs such as the lagena and utricle in teleosts may also serve an auditory function. Popper and Hoxter ( 1990 ) found that sensory hair cells are added throughout
the sensory macula of the saccule during normal development and not in a pattern
similar to the annular growth rings of the sagitta found in Astronotus ocellatus . Hair
cell addition was also observed throughout the lagenar and utricular sensory epithelia in M. merluccius (Lombarte and Popper 1994 ). This is in contrast to elasmobranchs, which have been shown to primarily add proliferating sensory hair cells to
the margins of the sensory epithelium (Corwin 1981 , 1983 ). Although hair cell
proliferation appears to occur throughout the saccule in teleost fi shes, in the
European hake, M. merluccius , the caudal region of the saccule undergoes more
hair cell proliferation than the rostral region of the saccule (Lombarte and Popper
R.O. Vasconcelos et al.
the hair cells in the sensory epithelium (Tanimoto et al. 2009 ). Afferent innervation
does not appear to be necessary for hair cell differentiation, but it likely plays a role
for long-term maintenance of individual hair cells (Sokolowski et al. 1993 ; Fritzsch
et al. 2004 ). In the toadfi sh, Opsanus tau , cells located within the developing sensory epithelium that differentiate into hair cells have a layer of microvilli along the
luminal surfaces, which elongate to form the stereocilia as the kinocilia begin to
elongate (Lewis and Li 1973 ; Sokolowski and Popper 1988 ). Also, embryonic differentiation and hair cell addition in O. tau occurs throughout the saccule simultaneously and not only on the edges of the growing saccular macula (Sokolowski and
Popper 1988 ). Once the processes of innervation, differentiation, and hair cell maturation have taken place in the auditory end organs, all of the structural components
necessary for auditory perception are in place and transduction of acoustic stimuli
can begin, however the auditory system continues to develop after fi sh hatch.
2.3 Postembryonic Development of the Peripheral
Auditory System
Postembryonic sensory hair cell addition has been demonstrated in elasmobranch
(Corwin 1981 , 1983 ) and teleost fi shes (Platt 1977 ; Popper and Hoxter 1984 ; Coffi n
et al. 2012 ). The size and shape of the sensory epithelia also change during ontogeny (Corwin 1983 ; Popper and Hoxter 1984 ; Lombarte and Popper 1994 ). Additional
ontogenetic changes may include the density of sensory hair cells (Popper and
Hoxter 1984 ; Lombarte and Popper 1994 ; Lu and DeSmidt 2013 ), and number of
auditory nerve ganglion cells as well as the innervation patterns of the eighth nerve
(Corwin 1983 ; Popper and Hoxter 1984 ), but at least in the European hake,
Merluccius merluccius , it does not include orientation of the hair cells (Lombarte
and Popper 1994 ).
Because the saccule is the main end organ of hearing in most teleost fi shes, it has
been the most extensively studied auditory end organ, however some data exists
which suggests that the macula neglecta in elasmobranchs and other otolithic end
organs such as the lagena and utricle in teleosts may also serve an auditory function. Popper and Hoxter ( 1990 ) found that sensory hair cells are added throughout
the sensory macula of the saccule during normal development and not in a pattern
similar to the annular growth rings of the sagitta found in Astronotus ocellatus . Hair
cell addition was also observed throughout the lagenar and utricular sensory epithelia in M. merluccius (Lombarte and Popper 1994 ). This is in contrast to elasmobranchs, which have been shown to primarily add proliferating sensory hair cells to
the margins of the sensory epithelium (Corwin 1981 , 1983 ). Although hair cell
proliferation appears to occur throughout the saccule in teleost fi shes, in the
European hake, M. merluccius , the caudal region of the saccule undergoes more
hair cell proliferation than the rostral region of the saccule (Lombarte and Popper
R.O. Vasconcelos et al.