84
D.R. Ketten
deep Azan staining in the basal region of Monodon monoceros (narwhal)
that they attributed to tonofibrils of the pillar cells, which would act as membrane stiffeners. This is consistent with Wever et al.'s observation that the
pillar cells are exceptionally thick in the lower basal turn and Reysenbach
de Haan's (1956) earlier description of "short ... compact" pillar cells in P.
phocoena.
An important functional feature of all odontocete ears is an exceptionally dense stria vascularis and a spiral ligament with a tightly woven collagen infrastructure (Fig. 2.6A). Stria vascularis, or specifically its marginal
cell layer, is considered to be the source of high potassium ion concentrations in the endolymph that control endocochlear potentials (see Wangemann and Schacht 1996). Recent transmission electron microscope images
show that odontocetes have up to five layers of marginal cells in the basal
stria vascularis (Burgess and Ketten, in preparation). The spiral ligament
has the conventional five divisions of cellular types, but again, like other
cochlear duct elements, cells are heavily packed. In particular, the collagen
fiber density is two- to fivefold that of most mammals with only moderate
decreases in the cell packing density in the most apical regions. The ligament's marginal region, which contains fibroblasts that anchor and add
tension to the basilar membrane, has dense cellular packing throughout the
cochlea.
The significance, if any, of the size of each cochlear canal is not yet known
for any mammal, but there are such dramatic differences between land and
cetacean ears in some canals that they are worth noting. In cetaceans, there
are large changes in the cross-sectional area of scala tympani and of scala
vestibuli from base to apex. Scala tympani in all cetaceans has a large area
at the basal end of the cochlea that tapers to an apical area that is approximately half that of the base. The large basal scala tympani area is coincident with the entrance of the cochlear aqueduct, which is also exceptionally
wide in all cetaceans (up to 5 mm versus 0.2 mm in humans) (Ketten
1998a,b; Schuknecht 1993). Scala vestibuli tapers as well, but more slowly
and, interestingly, can have a smaller cross-section in mysticetes than in
odontocetes.
Further, because the periotic is disjunct from the skull, the cochlear aqueduct ends at the medial edge of the periotic in the peribullar plexus, not the
subarachnoid space. This does not preclude the perilymphatic duct traversing the retro-bullar space to the skull, but it has not been shown specifically
to do so. Whether it does or not has implications about the origin, flow, and
contents of the perilymph. Although it was originally assumed that some
filter mechanism was in place in the perilymphatic duct, it is now clear that
in most mammals there is a free communication with the subarachnoid
space through which a variety of cells pass (Schuknecht 1993). In fact, in
cases of subarachnoid hemorrhage, large deposits of blood in scala tympani
enter the ear by the cochlear aqueduct and/or the internal auditory canal.
The fact that cetaceans with concussive injuries have the same phenomenon of blood deposits in scala tympani (Ketten 1995) begs the question of
D.R. Ketten
deep Azan staining in the basal region of Monodon monoceros (narwhal)
that they attributed to tonofibrils of the pillar cells, which would act as membrane stiffeners. This is consistent with Wever et al.'s observation that the
pillar cells are exceptionally thick in the lower basal turn and Reysenbach
de Haan's (1956) earlier description of "short ... compact" pillar cells in P.
phocoena.
An important functional feature of all odontocete ears is an exceptionally dense stria vascularis and a spiral ligament with a tightly woven collagen infrastructure (Fig. 2.6A). Stria vascularis, or specifically its marginal
cell layer, is considered to be the source of high potassium ion concentrations in the endolymph that control endocochlear potentials (see Wangemann and Schacht 1996). Recent transmission electron microscope images
show that odontocetes have up to five layers of marginal cells in the basal
stria vascularis (Burgess and Ketten, in preparation). The spiral ligament
has the conventional five divisions of cellular types, but again, like other
cochlear duct elements, cells are heavily packed. In particular, the collagen
fiber density is two- to fivefold that of most mammals with only moderate
decreases in the cell packing density in the most apical regions. The ligament's marginal region, which contains fibroblasts that anchor and add
tension to the basilar membrane, has dense cellular packing throughout the
cochlea.
The significance, if any, of the size of each cochlear canal is not yet known
for any mammal, but there are such dramatic differences between land and
cetacean ears in some canals that they are worth noting. In cetaceans, there
are large changes in the cross-sectional area of scala tympani and of scala
vestibuli from base to apex. Scala tympani in all cetaceans has a large area
at the basal end of the cochlea that tapers to an apical area that is approximately half that of the base. The large basal scala tympani area is coincident with the entrance of the cochlear aqueduct, which is also exceptionally
wide in all cetaceans (up to 5 mm versus 0.2 mm in humans) (Ketten
1998a,b; Schuknecht 1993). Scala vestibuli tapers as well, but more slowly
and, interestingly, can have a smaller cross-section in mysticetes than in
odontocetes.
Further, because the periotic is disjunct from the skull, the cochlear aqueduct ends at the medial edge of the periotic in the peribullar plexus, not the
subarachnoid space. This does not preclude the perilymphatic duct traversing the retro-bullar space to the skull, but it has not been shown specifically
to do so. Whether it does or not has implications about the origin, flow, and
contents of the perilymph. Although it was originally assumed that some
filter mechanism was in place in the perilymphatic duct, it is now clear that
in most mammals there is a free communication with the subarachnoid
space through which a variety of cells pass (Schuknecht 1993). In fact, in
cases of subarachnoid hemorrhage, large deposits of blood in scala tympani
enter the ear by the cochlear aqueduct and/or the internal auditory canal.
The fact that cetaceans with concussive injuries have the same phenomenon of blood deposits in scala tympani (Ketten 1995) begs the question of