The Cochlear Nuclei of Lizards MALCOLM R. MILLER D e p a r t m e n t of A n a t o m y , Uniuersity of Culifornici, S o n Francisco, Ctrlfornitr 94143

ABSTRACT The cochlear nuclei of 14 lizard species (eight families) were studied in normal animals and in a small series of animals with lesions of the posterior division of the eighth nerve. The development of the cochlear nuclei is directly related to the length and complexity of the papilla basilaris. The best development of basilar papillae and cochlear nuclei is found in teiid and gekkonid lizards, and an intermediate grade of development in scincid, lacertid, and anguid lizards. A lesser degree of development occurs in the iguanids, and practically no cochlear nuclei differentiation is observed in Chameleo. Two well defined cochlear nuclei are found in most lizard families: nucleus angularis (NA) and nucleus magnocellularis medialis (NMM). NA is located in the cephalic third of the acoustic tubercle and contains variably sized darkly staining cells. NMM is the most caudally located nuclear group and is characterized by regularly round to ovoid cells. The development of NMM is more closely related to papilla basilaris length and complexity than is NA. Two less well defined cochlear nuclei are also thought to be present. In the region between NA and NMM is a sparsely cell-populated area, nucleus magnocellularis lateralis (NML), which consists of a variety of small, darkly staining cells and large, pale staining cells which are usually laterally located in the nucleus. Like NMM, NML is better developed in lizards with more complex papillae basilares. Nucleus laminaris may be represented by a few fusiform cells in the ventral portion of the NML region.

In the past 20 years the reptilian ear has been the object of ever increasing attention. Excellent summaries of our knowledge in this area have been presented by Baird ('70, '75). More recently, scanning electron microscope studies of the lizard papilla basilaris (Miller, '73a,b, '74, '75) have extended earlier studies of the gross morphology of the lizard cochlear duct (Miller, '66) and have further demonstrated the striking diversity of the structure of the lizard cochlea. Physiological studies by Wever ('71), Manley ('72), Mulroy et al. ('74) and Weiss et al. ('74) have revealed a corresponding functional diversity in the lizard ear. Knowledge of the reptilian cochlear nuclei, on the other hand, is very limited. Up until very recently, the best reported studies on the cochlear nuclei of reptiles were those by Holmes ('03), Beccari ('11) and Weston ('33, '36). Recently, Leake ('75) has described the cochlear nuclei of the caiman. This is the first report in a reptile where the cochlear J. COMP. NEWR., 159. 3 7 5 4 0 6

nuclei were defined as a result of degeneration studies subsequent to a variety of peripheral labyrinthine lesions. The present report is based in large part on normal lizard material, although one series of experimental lesions was carried out that has served to corroborate the description of the cochlear nuclei based on normal material. While I have also studied a number of snake and turtle brains, this paper is limited to the description and illustration of the cochlear nuclei in lizards. MATERIALS AND METHODS

Lizards were obtained from a number of reptile supply houses and from The California Academy of Sciences. The following species were studied: Iguanidae Iguana iguana Basiliscus basiliscus Dipsosaurus dorsalis Sceloporus occidentalis Anolis carolinensis

375

3 76

MALCOLM R. MILLER

with cresylecht violet, and the other half with Bodian's protargol method. Of particular value was a large collecScincidae tion of serially sectioned and stained series Eumeces skiltoneanus of embryos and fetuses of the lizard, XanLacertidae tusia vigilis. The cochlear nuclei were Lacerta viridis particularly well defined in the late fetal stages of this species. Anguidae Gerrhonotus multicarinatus An attempt was made to produce cochlear lesions in 12 specimens of Ameiva Chameleonidae Chameleo jacksoni ameiva, a teiid lizard, by inserting a curved Xantusiidae needle through either the foramen ovale Xantusia vigilis or the foramen rotundum and into the Gekkonidae cochlear duct or the vicinity of the ganGekko gecko glion of the posterior division of the eighth Hemidactylus flavimaculatus nerve. Either one or both these structures Animals were killed by a n intraperito- was damaged by rotation of the curved neal injection of sodium pentabarbital. needle. In several specimens, the cochlear The abdomen and thorax were opened and duct, but not the posterior ganglion, was a cannula tied into the carotico-systemic damaged. In these cases there was no arterial trunk from the heart. The abdom- evidence of transneuronal degeneration. inal aorta was tied off to prevent perfusate In three animals the entire posterior ganfrom filling the lower part of the body. The glion was specifically damaged with exright atrium was nicked to permit outflow cellent subsequent degeneration. In a few of perfusate. The animal was first per- specimens, hemorrhage occurred with fused with a small amount of saline to widespread damage to the medulla. wash out as much blood as possible. ExNine days after treatment these animals periment with a number of fixatives proved were perfused with 10% neutral formalin that Bouin's fixative was superior to acetic and the heads fixed €or several days in the acid-formalin or to 10% neutral formalin same fixative. Eventually the brains were to achieve fixation most suitable for both removed from the skulls, washed, soaked nerve cell body and nerve fiber staining. in 30% sucrose, infiltrated with 10% and Usually about 2 ml/gm of body weight of then 20% gelatin, then sectioned at 15 p fixative was perfused through the animal. on a freezing microtome. Sections were After perfusion the skull was opened dor- stained by the uranyl nitrate method of sally to check the success of the perfusion, Ebbesson and Nauta ('70), by the Finkand then the entire head was placed in Heimer silver technique ('67), and with fixative for at least three days. After re- cresylecht violet. moving the brains from the skull, the OBSERVATIONS brains were stored in 50% alcohol. Brains For orientation of the reader, the entire were dehydrated and embedded in paraffin and serially sectioned in transverse, fron- membranous labyrinth of a lizard is shown tal, or sagittal planes. Usually a minimum in figure 1. In figure 2, the location of the of three brains of each species were pre- cochlear duct sensory end organs, namely, pared so that sections in all three planes the papilla basilaris and the macula lagenae are depicted. In figure 3 , the brainwould be available for study. In sectioning of specimens, the frontal stem with the still attached cochlear ducts plane was parallel to the surface of the shows the relationship of the peripheral medulla, and the transverse plane was receptor to the posterior division of the made at a right angle to the frontal plane eighth nerve and the medulla. Sections were cut at 10 p and arranged The statoacoustic nerve (eighth cranial on slides so that every four sections (40 p ) nerve) characteristically has two primary would be stained for cellular details, and divisions. The posterior division of this the next four sections placed on another nerve in lizards, a s in most reptiles, transslide that would be stained for fiber tract mits fibers to the medulla from part of the detail. Half of the sections were stained sacculus, the macula neglecta, the posTeiidae Ameiva ameiva Cnemidophorus tigris

377

THE COCHLEAR NUCLEI OF LIZARDS Crus Commune

Post. Semicircular

Cochlear Duct

culo- Cochlear LATERAL VIEW

Crus Commune Utriculus

, .

,R% EndolvmDhatic Duct

2 :&-'I!.

Ant. Semicircular

Post. A&pullo MLDIAL VIEW

VJ

ANTERIOR

macula lage

VENp& macula l a g e n a e

membrane

scala vertibu

cochlear d u

triangular limbur

Fig. 1 Left membranous labyrinth of the lizard, Xantusia vigilis. (Miller, '66). Fig. 2 Cochlear duct of Gekko gecko showing the location of the two regions of sensory epithelium, the macula lagenae and the papilla basilaris. The papilla basilaris in this species is 2.0 mm long. (Miller, '73a).

378

MALCOLM R . MILLER

Among reptiles, the best developed cochterior ampullary crista, and from the macula lagenae and papilla basilaris of the lear nuclei are found in the Crocodilia. A cochlear duct (figs. 4, 5). Nerve fibers from brief description of the cochlear nuclei in the posterior ampullary crista and from the alligator and caiman serves as a usethe macula lagenae course laterally, ven- ful and necessary point of reference for trally, and anteriorly in relation to the descriptions of the cochlear nuclei in nerve fibers from the papilla basilaris, other reptiles. Weston ('33, '36) recognized three cochwhich remain 'the most medial, and ultimately enter the dorsolateral aspect of lear nuclei in Alligator mississipiensis: the medulla, dorsal to other statoacoustic nucleus angularis, laminaris, and dorsalis fibers. The relationships of the nerve from magnocellularis. Leake's ('75) studies on the posterior ampullary crista, the macula Caiman crocodilus agree with those of neglecta, macula lagenae, and papilla Weston in large part, but are based on basilaris are shown stereographically in experimental work and are more concise. figures 4 and 5. In histologic sections the fibers of the papillary nerve are relatively Abbreviations small and uniform in diameter. Other ar, anterior division of t h e eighth nerve posterior division fibers, as well a s anterior c, cerebellum cd, cochlear duct division fibers, are variable in size. These e, elevated portion of t h e acoustic tubercle fiber size differences, especially in relag, ganglion of the posterior division of the tion to the lagenar and papillary nerves, eighth nerve aid in determining their nuclear termiIII,, oculomotor nerve, motor nucleus IV, trochlear nerve nations. IV,, trochlear nerve, motor nucleus While most all the fibers of the papillary lg, nerve from the macula lagenae nerve enter the medulla dorsal to, and MB, midbrain separately from other posterior division mn, macula neglecta NA, nucleus angularis fibers, some of the lagenar fibers apparNML, nucleus magnocellularis lateralis ently remain associated with the papillary NMM, nucleus magnocellularis medialis fibers and enter into some parts of the N V , nucleus vestibularis (specific portion not cochlear nuclei (also see below, Teiidae). indicated) p, nerve from papilla basilaris While Hamilton ('63) states that lagenar pa, nerve from the posterior ampulla fibers in Lacerta vivipara terminate in pb, papilla basilaris cochlear nuclei, my lizard material shows pr, posterior division of the eighth nerve that most of the lagenar fibers terminate R, rostra1 in vestibular nuclei. Beccari ('11) likeV, trigeminal nerve V, trigeminal nerve, motor nucleus wise found in Lacerta muralis that most VIII, statoacoustic nerve lagenar fibers end in the vestibular nuclei, and only a few fibers end in the cochFig. 3 Gerrhonotus multicarinatus (Anguidae). lear nuclei. Stereophotograph of the caudal part of the brain The fibers of the statoacoustic nerve showing both cochlear ducts still attached to the enter the medulla dorsolaterally near its posterior divisions of the statoacoustic nerves. x 10. cephalic end (figs. G 8 ) . Posterior division Fig. 4 Gerrhonotus multicarinatus. Stereophofibers enter the medullary substance not tograph of the medial side of the left cochlear duct only more caudally, but also more dorsally showing the relationship of the posterior ampullary (from the crista of the posterior semicircular than the anterior division fibers (figs. 15canal), papillary (from the papilla basilaris) and 18). The accumulation of cell bodies of lagenar (from the macula lagenae) nerves. Note the secondary neurones upon which the the nerve from t h e macula neglecta joining the statoacoustic nerve fibers terminate are posterior ampullary nerve. The papillary nerve responsible for the grossly apparent dorso- becomes medial, caudal, and dorsal in relation to posterior ampullary and lagenar nerves. T h e lateral alar elevations of the medulla (figs. the anteroventral tip of the cochlear duct faces the 6-43, 12-14). Since the approximate ce- right side of the photograph. X 18. phalic half of each of the medial portions Fig. 5 Cnemidophorus tigris (Teiidae). Stereoof these elevations is immediately related photograph of medial side of left cochlear duct. t h e papillary, lagenar and posterior ampullary to the cell bodies of the cochlear nuclei, As nerves converge, the papillary nerve remains they have been termed the acoustic emi- more medial and dorsal in position. The anteronence (Holmes, '03) or acoustic tubercles. ventral tip of the duct faces downwards. X 18.

THE COCHLEAR NUCLEI OF LIZARDS

379

380

MALCOLM R. MILLER

Nucleus angularis (NA) occupies the oral one-fourth of the acoustic tubercle and lies just anterior to the posterior root of the eighth nerve. This mixed group of large and small ovoid cells is shaped

roughly like a pyramid with its apex directed toward the ventricle. Nucleus laminaris (NL) is a thin, curved sheet or lamina of darkly staining spindle-shaped cells stretching across the anterior half

Fig. 6 Gerrhonotus multicarinatus (Anguidae). Right oblique view of c a u d a l portion of brain. The left cochlear duct remains attached to the eighth nerve. Note the slight bulge (arrow) on the medial edge of the alar elevation of the medulla. This slightly elevated area is associated with a portion of the underlying cells of the cochlear nuclei. I n large part, the cochlear nuclei occupy the medial portion of the alar ridge or elevation and extend from the junction of the cerebellar peduncle a n d the medulla to a point approximately 0.4 m m caudal to the point of entry of the posterior division of the eighth nerve. Compare with figures 2529. X 9. Fig. 7 Gekko gecko (Gekkonidae). T h e cochlear nuclei underlie the medial portion of the alar elevation and extend from the cerebellar-medullary junction caudally for a distance of approximately 1.5 mm. X 6. Fig. 8 Sceloporus occidentalis (Iguanidae). The cerebellum and most of the midbrain have been removed to expose the medullary surface. T h e entire acoustic and vestibular regions form distinct elevations. X 10. Fig. 9 Chameleo jacksoni (Chameleonidae). Note the marked lack of development of the medial portion of the alar elevation of the medulla. This condition is probably related to the greatly reduced size of the papilla basilaris and macula lagenae in the chameleons. x 10.

T H E COCHLEAR NUCLEI OF LIZARDS

of the acoustic tubercle, arching ventrally as it approaches the ventricle. A s a result of experimental studies, Leake (’75) has shown that this nucleus probably does not receive primary cochlear fibers. Nucleus magnocellularis lateralis (NML) overlies NL but extends further laterally. The large cells of this nucleus stain quite palely, and, after degeneration studies, have been shown to be primary cochlear nuclei cells. Nucleus magnocellularis medialis (NMM) is situated medially and caudally to NML and is characterized by large round, moderately staining cells which in the medial portion of the nucleus are arranged into distinct vertical columns or rows. To clarify Leake’s nomenclature, it should be noted that Weston (’33) based his description of nucleus laminaris of the alligator on that given by Holmes (’03). Weston noted, however, that Holmes did not describe a second group of large cells overlying the small spindle cells comprising nucleus laminaris. In Weston’s figures (‘33, figs. 14, 15) he labels this second nucleus, nucleus laminaris, pars dorsalis. While Weston mentions that the cells are lighter staining, he confuses the issue by stating that they are similar in size and type to the cells of nucleus laminaris, pars ventralis. Leake (‘75) and I (unpublished) also found in the caiman, large pale staining cells overlying the small fusiform cells of NL. As the large pale cells, both in location and cytology appear to be homologous with the avian NML (Boord and Rasmussen, ’63), they may better be termed NML. Weston (‘33) also points out that Holmes’ (’03) nucleus laminaris, pars posterior is really his (Weston’s) nucleus dorsalis magnocellularis. Again, because of the obvious homology of this nuclear group with the avian nucleus magnocellularis medialis, Leake (’75) terms this nucleus in the caiman, NMM. Thus, in the Crocodilia, three primary cochlear nuclei (angularis, magnocellularis lateralis, and magnocellularis medialis) and one possibly secondary cochlear nucleus (laminaris) have been well localized and described. In saurians (lizards and snakes), Holmes (’03) describes a nucleus dorsalis of small size lying on the dorsomedial border of the acoustic eminence a short distance caudally from the junction of the cerebel-

38 1

lum and medulla and ending cephalic to the entrance of the acoustic nerve. This nucleus (nucleus dorsalis) is probably homologous with the nucleus angularis of birds and crocodiles and is so termed in the present account. Holmes (’03, p. 118) stated that caudal to nucleus dorsalis (nucleus angularis), another nucleus corresponding to nucleus laminaris of the Crocodilia appears. In Varanus, this nucleus is broken up by fiber masses passing between its constituent parts, and a large number of the constituent cells are described as spindleshaped. Thus, Holmes implies the presence of other cell types but does not describe them. Weston (’33, p. 48) was unable to identify a nucleus laminaris in small lizards such as Anolis and horned toads, but he thought that in larger lizards such as Varanus and Heloderma, a cell mass similar to that of the turtle, Chrysemys, could be found. Also, Weston does not mention a large-celled dorsal portion of nucleus laminaris as he described in the alligator. Neither Weston’s descriptions nor figures of his nucleus laminaris in either the turtle or in lizards is adequate to serve as a basis for comparison with my material. As mentioned above, Holmes described in the alligator a group of large ovoid cells in the caudal portion of his nucleus laminaris which was subsequently renamed nucleus dorsalis magnocellularis by Weston, and in turn became Leake’s nucleus magnocellularis medialis. Holmes did not mention this group of cells in the lizards he studied, and Weston only mentions it in relation to the iguanids where he found it poorly developed. Beccari’s (’11) Golgi studies of embryos of the lizard, Lacerta muralis, clearly demonstrated two cochlear nuclei which he termed anterior and posterior cochlear nuclei. While it is difficult to compare Beccari’s material with mine, his excellent figure 11 on page 673 indicates that his anterior cochlear nucleus is probably the equivalent of my nucleus angularis, and his posterior cochlear nucleus, the equivalent of my nucleus magnocellularis medialis. (Vide infra). As a result of study of 14 species of lizards from 8 different families, it can be stated that there are usually two well defined cochlear nuclei: nucleus angularis

382

MALCOLM R . M I L L E R

and nucleus magnocellularis medialis; and two poorly defined nuclei: nucleus magnocellularis lateralis and nucleus laminaris. On the basis of topography and cytological characteristics, I feel that the lizard cochlear nuclear groups are probably homologous with those in the Crocodilia, and thus, I have applied the terminology used by Leake ('75) in the caiman to the lizard. Nucleus angularis (NA), as in the caiman, occupies the anterolateral part of the cephalic end of the acoustic tubercle and is made up of a mixed group of small and large cells. For the most part, NA is related to the transverse anterolaterally curving part of the tubercle. Nucleus magnocellularis medialis (NMM) is located along the dorsomedial edge of the longitudinally running posterior part of the acoustic tubercle, and its caudal extent varies from species to species. The cells of this nucleus are very characteristically round to ovoid, moderately well stained, numerous, and are often aligned in dorsoventrally oriented columns. Nucleus magnocellularis lateralis (NML) which is so distinct in the caiman is variable in development and indistinct, but is probably represented in lizards by a region of sparse cell body density interposed between NA and NMM. In good part, the cells of this region are separated into smaller or larger groups by the incoming posterior division fibers. Some cells in this region are large and pale staining and are very reminiscent of the cells in the caiman NML. Often, however, the sparse cells in this region are darkly staining, irregular in size and shape, and look more like posteriorly displaced cells of NA. A thin lamina of small fusiform cells is only occasionally observed lying ventrally or medially to the larger and more pale staining cells of NML. These cells may represent an incipient or poorly developed nucleus laminaris (NL). In agreement with Beccari ('ll), however, I have never been able to define a n unequivocal NL in lizards. As might be anticipated, and as was also suggested by both Holmes ('03) and Weston ('33), the extent and development of the cochlear nuclei of lizards is directly related to the size and complexity of the papilla basilaris. Thus, those lizards with larger and more complex papillae basilares have better developed cochlear nuclei. The

basic morphology of the lizard cochlear duct and papilla basilaris has been described (Miller, '66, '73a,b, '74a,b) so that the developmental correlation between the peripheral end organ and the central sensory nuclei is easily made. A s will be seen in the detailed account to follow, N A is fairly well developed in most lizard families and does not show a great degree of variation. Both NMM and NML, however, vary in the degree of development in direct relationship to the size and complexity of the papilla basilaris. Since it is much easier to describe species with well developed cochlear nuclei, I will treat these first, and those species with less well developed nuclei, last. The following descriptive material is summarized in table 1.

Xantusiidae Xantusia vigilis. The papilla basilaris of this lizard is relatively long and moderately complex (Miller, unpublished), and correspondingly, the cochlear nuclei are well developed and easily defined. Xantusia vigilis is a very small lizard; an adult animal attains a snout-vent length of approximately 40 mm and a total body weight of 1.5 gms. Related to the small body size is a correspondingly small brain. The moderately large papilla basilaris (0.7 mm in length), however, gives rise to a proportionately large number of nerve fibers which, in turn, is related to a relatively large population of cochlear nuclei cells that are confined within a small space. Thus, the cochlear nuclei of this species appears to be densely packed with cells. By contrast, in Gekko gecko, a large animal with a correspondingly larger brain, the cochlear nuclei cell population appears sparse although the papilla basilaris is relatively very large (2.0 mm in length). From study of late fetal brains of X a n tusia vigilis, the outline of three separate cochlear nuclei are very clear (figs. 10, 11). Lying most anteriorly in the acoustic tubercle is a group of cells (nucleus angularis) of varying diameter (6-12 p ) . Immediately caudal to N A is a smaller group of larger (14-16 p ) , paler staining cells (nucleus magnocellularis lateralis). Immediately caudal to NML is a group of smaller (8 p ) , more densely packed but regularly round and evenly sized cells

383

THE COCHLEAR NUCLEI OF LIZARDS TABLE 1

Szimmciry of b o d y size, p n p i l l n basiloris d e v e l o p m e n t , rind s o m e cochleclr nuclei drtciils Papilla basilaris

Body size 1

Length (in mm)

Total no. of hair cells

vs

0.7

300

Family species

Xantusiidae Xantusia vigilis

Nucleus an gu 1 aris (Always present and well defined)

(estimate) Teiidae Ameiva ameiva Cnemidophorus tigris Gekkonidae Gekko gecko Scincidae Eumeces skiltoneanus

Nucleus magnocellularis 1 aterali s : :4:

6-12 p 2 0.11 m m

i?z

Nucleus magnocellularis medialis

.._ ...... .,.

3

14-16 p

0.07 m m

0.09 mm ........ ........

.,. "._.. "' 3

M M

1.1

0.8

700

lizccrds

6-12 p 0.3 m m

6-12 and 1&18 0.3 mm ....... ......

600

6-12 p

(estimate)

0.3 m m

6-12 and 1&18 p 0.4 m m

p

12-14 p

0.3 mm I.:>

>:

4

>,'Z

L

2.0

2100

6-12 p 0.4 m m

12-15 p 0.5 mm >:

S

0.8 (estimate)

600

(estimate)

&12p 0.2 m m

4

12-14 p 0.4 mm

04mm

3

4

12-14p 0.1 m m

8P 0 2 mm

3

1

." ,.'. "

Anguidae Gerrhonotus multicarinatus

M

0.4

162

10-16 p 0.2 m m

6-12 and 1&20 p 0.4 mm

Lac ertid ae Lacerta viridis

::i 3

...... ......

M

0.6

150

6-12 p 0.2 m m

6-12 and 12-14 p 0.25 m m

8-10 p 0.2 mm

::i 3

>:: 4

6-12 and 12-14 p 0.2 m m

8P 0.14.2 m m

Iguanidae (average of 5 species studied)

M

Chameleonidae Chameleo jacksoni

M

0.34.6

0.15

100-300

6-12 p 0.2 m m

12-14 p 0.3 mm

No differentiation of cochlear nuclei

60

(estimate) I

VS, very small; S, small; M, medium; L, large. Size range of constituent cells are given in micra, Cephalocaudal extent of nucleus is given in mm. , well defined; :.',notwell defined. very well developed; .:.::', well developed; . , poorly developed.

3 -:::: 4

(nucleus magnocellularis medialis). In no other lizard species have I observed NML as well defined. It is possible that the dense packing of the cochlear nuclear areas makes the recognition of the separate cellular groups easier, but it is also possible that the groups are easier to recognize in the fetal as compared with the adult condition. It would be valuable to look at fetal specimens of species in which delineation of NML is difficult in the adult animal.

Teiidae Both Ameiva ameiva and Cnemidophorus tigris have fairly well defined cochlear

nuclei. As in Xantusia, the relative distinctness of three cochlear nuclei in these species is probably related to the fact that these medium to small sized animals have relatively large and complex papillae basilares. As described in MATERIALS A N D METHODS, attempts were made to damage the posterior division of the eighth nerve in 12 specimens of Ameiva. In three specimens, only the posterior ganglion and nerve root were severely damaged. The great majority of fibers in this root come from the papilla basilaris and the macula lagenae. While fibers from the posterior ampulla, macula neglecta, and a very small part of

384

MALCOLM R . MILLER

the macula of the sacculus were also involved, there was no difficulty in distinguishing cochlear from vestibular nerve projections. All cochlear nuclei areas were clearly defined by almost complete degeneration of the incoming fibers and endings. Most of these fibers arise from the papillary nerve, but an undetermined portion come from the lagenar nerve. Unfortunately no lesions of the lagenar or the papillary nerve alone were effected so that the proportion of lagenar fibers terminating on cochlear nuclei cells was not determined. Figures 15 and 16 show the extensive degeneration of both fibers and terminals in the cochlear nuclei subsequent to damage of the posterior division of the eighth nerve. Ameiva ameiva. (Figs. 12-16). Nucleus angularis occupies the approximate oral one-third of the acoustic tubercle and extends medially from near the cerebellarmedullary junction almost to the acoustic tubercle angle (the point at which the medial edge of the acoustic tubercle bends laterally near its cephalic end). The cells of this nucleus tend to be darkly stained and variable in size ( 6 1 2 p ) and shape. The dimensions of this nucleus are approximately 0.3 X 0.3 X 0.3 mm. Caudal to NA, and occupying the region between NA and NMM, is a sparsely populated region in which the cells are variable in size, shape and disposition. Large (16-18 p ) , pale staining cells are most numerous in the lateral part of the region between N A and NMM. Because the location, size and staining characteristics of these large pale cells is so similar to the NML of the caiman, I believe that this nuclear group in lizards is probably homologous with the caiman and avian NML. Mixed in with the Iarge pale celIs of NML are small, darkly staining cells. Often, but not always, the smaller and darker cells are more ventral and medial in location. They do not, however, form a definite lamina and are not cytologically identical to the small fusiform cells of NL of the caiman. Unfortunately, my degeneration studies did not enable me to determine whether these small cells were related to degenerating endings after posterior root section. While it is likely that NL is represented in lizards, I am not yet able to clearly define its location or cytological characteristics.

Nucleus magnocellularis medialis occupies the caudal one-third of the acoustic tubercle and is easily delineated by the densely packed, regularly round (12-14 p), moderately staining cells. This nucleus is approximately 0.3 mm long, 0.2 mm wide, and 0.2 mm deep. Cnemidophorus tigris. As in Ameiva, the cochlear nuclei in Cnemidophorus are fairly well circumscribed (figs. 17-21). The most easily defined nuclear group is NMM. The cell bodies in this nucleus are regularly round, 12-14 p in diameter, moderately well stained, densely packed and arranged in vertical columns. This group of cells lies mostly caudal to the site of entry of the posterior eighth nerve root and occupies the caudal third of the acoustic tubercle. The cephalic extremity of NMM is approximately 0.4 mm from the point at which the acoustic tubercle bends laterally near its cephalic end (acoustic tubercle angle) and has a cephalocaudal extent of 0.4 mm. While the overall average depth of this nucleus is 0.2 mm, i t is deeper on its very medial edge where it is approximately 0.3 mm thick. The average thickness of the nucleus is 0.15 mm. Nucleus magnocellularis lateralis occupies the approximate middle third of the acoustic tubercle and is much less densely populated than either NMM or NA. While the cells in this nucleus are variable in size (6-12 p ) and shape, there are numerous large (1&18 p ) , quite pale staining cells that are found mostly in the lateral part of the region between NA and NMM. These large pale cells are similar to the NMM cells of the caiman. The approximate dimensions of NML are 0.4 X 0.3 X 0.3 mm. Nucleus angdaris is the most cephalic and lateral of the three nuclei. Its medial edge is approximately 0.1 mm from the acoustic tubercle angle and the nucleus extends 0.3 mm toward the cerebellar stalk. Cell bodies are numerous and include an array of darkly stained cells variable in size and shape. The nucleus measures 0.3 X 0.3 X 0 . 3 mm.

Gekkonidae Gekho gecko. (Figs. 22-24). The papilla basilaris of Gekko gecko is one of the larger and more complex in lizards (Miller, '73a, '74) and contains 2100-2200

T H E COCHLEAR NUCLEI OF LIZARDS

hair cells. Gekko gecko is a large lizard with a n adult snout-vent lenpth of approximately 15 cm and a body weight of 150 gms. This larger lizard has a larger brain than the smaller teiid species described above; this may account for the apparently sparsely populated cochlear nuclei as compared with the densely packed teiid nuclei. The cochlear nuclei are, nevertheless, well defined in Gekko gecko. Nucleus magnocellularis medialis is caudally located along the medial edge of the acoustic tubercle and is easily defined by regularly round, similarly sized, moderately staining cells. NMM measures 0.4 mm in length, 0.25 mm in width and 0.20.25 mm in depth. Nucleus magnocellularis lateralis is less well defined but occupies the region between NMM and NA and consists of a mixture of small, darkly staining cells and larger, lightly staining cells. As in Cnemidophorus and Ameiva, the large pale cells are most abundant in the lateral portion of the nucleus. NML measures approximately 0.5 X 0.3 X 0.3 mm. Nucleus angularis occupies the cephalic one-fourth to one-third of the acoustic tubercle and, as is observed in many lizards, is related to a slight dorsal elevation of the tubercle along its cephalic edge as it curves laterally toward the cerebellar peduncle. NA has the usual complement of variably sized, darkly stained cells and measures 0.4 mm in cephalocaudal extent, 0.4 mm in width, and 0.3 mm in depth. Hemidactylusflauimaculutus. Thecochlear nuclei are similar to those of Gekko gecko except that the nerve cell bodies are more densely packed in this smaller species that has a correspondingly smaller brain.

Scincidae Eumeces skiltoneanus. The papilla basilaris of skinks, like that of gekkonids and teiids, is among the larger in lizards (Miller, '66, '74) and consequently the papillary nerve is large. Like Xantusia vigilis, Eumeces skiltoneanus is a small species of lizard, small brained, and the cochlear nuclei appear densely packed with cells. Even though the cochlear nuclei are well developed, the entire length of the three nuclei is not greater than 0.5 mm. Nucleus angularis occupies the cephalic

385

tip of the acoustic tubercle and the densely packed, variously sized, darkly staining cells are related to the lateral part of a slight elevation of the cephalic end of the acoustic tubercle. N A is approximately 0.2 mm in cephalocaudal extent, 0.2 mm wide, and 0.15 mm deep. Nucleus magnocellularis lateralis is relatively small in extent and forms the anteromedial aspect of the acoustic tubercle. NMM is also small and compact, and the cephalic portion underlies NML. The cells of NMM are regularly round but are small (8 p ) , as are the cells of NMM in Xantusia vigilis. Also, as in Xantusia, the cells of NML are moderately large (12-14 p ) and lightly staining and are much less variable in size than the NML of the teiids and gekkonids.

Lacertidae Lacerta viridis. The papilla basilaris of Lacerta viridis is less well developed than that of species of the previously described families (Miller, '75). Likewise, the papillary nerve is not large. Nevertheless, three cochlear nuclei are present. Nucleus magnocellularis medialis is easily recognized by the regularly round, well stained cells lying along the medial edge of the acoustic tubercle. In this species, NMM is close to the ventricular surface and measures approximately 0.2 mm in length, 0.2 mm in width, and 0.15 mm in depth. Nucleus magnocellularis lateralis is difficult to differentiate from NA, as there are but few large, pale staining cells amidst the more common variably sized cells, but cell density is not a s great a s in NA. NML measures approximately 0.25 X 0.2 X 0.15 mm. NA occupies the anterolateral cephalic portion of the acoustic tubercle and extends from near the cerebellar peduncle approximately 0.2 mm medially. This nucleus measures approximately 0.2 x 0.2 x 0.2 mm. Anguidae Gerrhonotus multicarinatus. The papilla basilaris of this species is one of the smaller among lizards (0.4 mm long) and contains only 162 hair cells (Miller, '73b). Figure 4 illustrates the size of the papillary nerve in relation to that of the lagenar and posterior ampullary nerves. Figures

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MALCOLM R. MILLER

3, 6 and 7 illustrate the dorsal surface of the medulla and show the region of the acoustic tubercle. Despite the relatively few hair cells contained in the papilla basilaris of Gerrhonotus multicarinatus, the cochlear nuclei are fairly well developed (figs. 25-29). NMM is very distinct and occupies the medial edge of the tubercle posterior to the entrance of the posterior division of the eighth nerve. While the number of cell bodies in NMM is not large, this nucleus is elongate and is approximately 0.3 mm in cephalocaudal extent. The maximum width and depth is not much greater than 0.15 mm. The cell bodies are round to ovoid in shape, 12-14 p in diameter, and are darkly staining. Nucleus magnocellularis lateralis extends from NMM almost to the acoustic tubercle angle, a distance of approximately 0.4 mm. As in the other lizards so far described, this nucleus is made up of darkly stained cells of mixed size together with cells that are quite large (16-20 p ) and more lightly staining. Nucleus angularis lies in the usual anterolateral position in the acoustic tubercle and consists of variously sized (10-16 p in diameter), darkly stained cells. Its dimensions are approximately 0.2 x 0.2 x 0.2 mm.

lguanidae Five species of iguanid lizards were studied, namely, Iguana iguana, Basiliscus basiliscus, Dipsosaurus dorsalis, Sceloporus occidentalis, and Anolis carolinensis. Since the findings were so similar in all these species, I shall describe the group as a whole. The papillae basilares of iguanids are relatively short (0.3-0.6 m m long) and contain approximately 100-300 hair cells (Miller, '73b). Nucleus magnocellularis medialis, which is so well developed in the lizards with larger papillae basilares (Gekkonids, teiids, and skinks), is poorly developed in the iguanids (fig. 31). This nucleus, as in other lizards, lies mostly caudal to the entrance of the posterior division of the eighth nerve along the medial edge of the acoustic tubercle. The entire nucleus is only 0.1 x 0.1 X 0.1 m m in its .dimensions. The cell bodies are characteristic

of NMM of other lizards, are quite regularly round in shape, darkly staining, but are small (8 p in diameter). The total number of cells in NMM is probably less than in any of the other lizard families reported here with the exception of the chameleons. Nucleus magnocellularis lateralis occupies the region between NMM and NA and, as in other lizards, consists of smaller, darkly staining cells and occasional larger, lighter staining cells. Nucleus angularis (fig. 30) is responsible for the anterolateral portion of the acoustic tubercular bulge and is made up of the usual array of mixed cell types.

Chameleonidae Chameleo jacksoni. The chameleons have lost their middle and external ears, and, as a probable consequence, the papilla basilaris is a very small circular patch approximately 0.15 mm in diameter (Miller, '66). The very small papilla basilaris is directly reflected in the morphology of the medulla in that the acoustic tubercle is practically non-existent (figs. 9, 32). While Shanklin ('30) described a nucleus dorsalis and laminaris in Chameleo vulgaris, I found no differentiation of cochlear cell groups in the anterior dorsal medullary ridge that is related to the incoming fibers from the posterior eighth nerve root in Chameleo jacksoni. A small group of variously sized cell bodies (fig. 32) seem to represent the cochlear nucleus of this species. The extent of this nucleus is probably not greater than 0.15 X 0.15 X 0.15 mm. DISCUSSION

Holmes ('03) studied the cochlear nuclei of species of all four reptilian orders and, in agreement with my observations, found that the acoustic nuclei of lizards were variable in the degree of development, and while they were better developed than in turtles, they were less well developed than in the crocodiles. Weston ('33, '36) also found the cochlear nuclei of the Crocodilia much better developed than either in the Chelonia or the Sauria. While variability in the development of the lizard cochlear nuclei was mentioned by both Holmes and Weston, the only comment concerning which portion of the re-

THE COCHLEAR NUCLEI OF LIZARDS

gion was variant was that made by Weston concerning the NMM (his nucleus dorsalis magnocellularis), which he found poorly developed in the iguanid lizards. This observation is verified in the present study in that the NMM which is the most variably developed of the lizard cochlear nuclei is not well developed in the iguanids, but is moderately developed in the skinks and anguids, and well developed in the gekkonids and teiids. Nucleus magnocellularis lateralis corresponds in its degree of development with NMM, and it is thus probable that these nuclear regions are functionally closely related. By contrast, NA is fairly constant in the degree of development throughout the lizards. The most problematical cochlear nucleus in lizards is NL. Some of the cells of the NML region are small and fusiform, and it was apparently these cells that Holmes (’03) and Weston (’33) took note of and referred to as nucleus laminaris. For the most part, these workers did not describe any larger cells in this region. However, neither Holmes’ nor Weston’s studies were primarily concerned with lizards, and both their descriptions and illustrations are lacking in detail. Beccari (‘11) was unable to find any evidence of nucleus laminaris in Lacerta muralis. Further degeneration studies may help to clarify the status of this nucleus in lizards. While both Holmes (’03) and Weston (‘33) looked at a few snakes, they did not describe the cochlear nuclei. Weston merely identifies a dorsal cochlear mass. So far, I have looked at the cochlear nuclei of several primitive burrowing snakes (Eryx, Epicrates, and Xenopeltis) and two colubrids (Natrix and Pituophis). The cochlear nuclei are much better developed in the burrowing snakes than in the nonburrowing colubrids, but a s yet, I have not determined the homologies of the snake cochlear nuclei. Neither Weston’s (‘33) nor my observations on turtle cochlear nuclei are sufficiently complete to warrant comparison with the cochlear nuclei of the Crocodilia or the lizards. While a great deal is known about the functional capacities of the reptilian cochlear duct, very little is known concerning the physiology of the cochlear nuclei. Recently, Manley (’72) studied frequency

387

responses in single auditory neurons in the cochlear nuclei of Gekko gecko. He did not find the cochlear nuclei of this species to be functionally divided into two as it is in the caiman. However, his studies indicated a rough organization of the cochlear nuclei consistent with some spatial distribution of frequency response on the basilar membrane. As yet, both our structural and functional knowledge of the reptilian cochlear nuclei is very incomplete, and much more structural, functional, and behavioral studies need to be undertaken. ACKNOWLEDGMENTS

Extensive technical assistance was rendered in this study by Ms. Michiko Kasahara and Ms. Gail Smith, and photographic aid was given by Mr. David Akers. This work was supported by United States Public Health Service Grant NS-09231. LITERATURE CITED Baird, I. L. 1970 The anatomy of the reptilian ear. I n : Biology of the Reptilia. Vol. 11. C. Gans and T. Parsons, eds. Academic Press, New York, pp. 193-275. 1975 Anatomical features of the inner ear in submammalian .vertebrates. In: Handbook of Sensory Physiology. Auditory System. Vol. V. W. D. Keidel and W. D. Neff, eds. Springer-Verlag, Berlin, New York, in press. Beccari, N. 1911 La costituzione, i nuclei terminali e l e vie di connessione del nervo acustico nella Lacerta muralis Merr. Arch. ital. di anat. e d i embriol., 10: 646-698. Boord, R. L., and G. L. Rasmussen 1963 Projection of the cochlear and lagenar nerves on the cochlear nuclei of the pigeon. J. Comp. Neur., 120: 463-476. d e Burlet, H. M. 1934 Vergleichende Anatomie des Stato-akustischen Organs. In: Handbuch der vergleichende Anatomie der Wirbeltiere. Vol. 11, Part 11. L. Bolk, E. Goppert, E. Kallius and W. Lubosch, eds. Urban und Schwartzenberg, Berlin, pp. 1293-1432. Ebbesson, S. 0. E., and W. J. H. Nauta 1970 Contemporary Research Methods in Neuroanatomy. Springer-Verlag, New York. Fink, R. P., and L. Heimer 1967 Two methods for selective silver impregnation of degenerating axons and their synaptic endings in the central nervous system. Brain Res., 4: 369-374. Hamilton, D. 1963 Posterior division of the eighth cranial nerve in Lacerta vivipara. Nature, 200: 705-706. Holmes, G. 1903 On the comparative anatomy of the nervus acusticus. Trans. Royal Irish Acad., 32: 101-144. Leake, P. A. 1975 Primary afferent projections of the VIIIth cranial nerve in Caiman crocodilus. Brain, Behav. Evol., i n press.

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Manley, G . A. 1972 Frequency response of the ear of the Tokay gecko. J. Exp. Zool., 181 : 159168. Miller, M. R. 1966 The cochlear duct of lizards. Proc. Calif. Acad. Scic, 33: 255-359. 1973a A scanning electron microscope study of the papilla basilaris of Gekko gecko. Zeit. f. Zellforsch., 136: 307-328. 1973b Scanning electron microscope studies of some lizard basilar papillae, Am. J. Anat., 138: 301-330. 1974 Scanning electron microscope studies of some skink papillae basilares. Cell Tiss. Res., 150: 125-141. 1975 Scanning electron microscopy of the lizard papilla basilaris. Brain, Behav. Evol., in press. Mulroy, M., D. W. Altmann, T. F. Weiss and W. T. Peake 1974 Intracellular electric re-

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sponses to sound in a vertebrate cochlea. Nature, 249: 482-485. Shanklin, W. M. 1930 The central nervous system of Chameleon vulgaris. Acta zoologica, 11 : 425490. Weiss, T. F., M. J. Mulroy and D. W. Altmann 1974 Intracellular responses to acoustic clicks in the inner ear of the alligator lizard. J. Acoust. SOC.Am., 55. 606-619. Weston, J . K. 1933 The reptilian vestibular and cerebellar gray with fiber connections. Doctoral thesis, Univ. of Michigan, Ann Arbor, Michigan. 1936 T h e reptilian vestibular and cerebellar gray with fiber connections. J. Comp. Neur., 65: 93-199. Wever, E. G. 1971 The mechanics of hair-cell stimulation. Trans. Amer. Otol. SOC.,59: 89107.

PLATE 1 EXPLANATION

OF FIGURES

10

Xantusia vigilis (Xantusiidae). Sagittal section of entire brain of a near term fetus. The region demarcated by the square is shown at higher magnification below. X 30.

11

Xantusia vigilis. (See outlined area above). Sagittal section through the acoustic tubercle region. Cephalically located is nucleus angularis made u p of cells of variable size and shape. Between NA and NMM is nucleus magnocellularis lateralis which is characterized by many large cells. Caudally located is nucleus magnocellularis medialis made up of regularly round cells. X 520.

T H E COCHLEAR N U C L E I OF LIZARDS Malcolm R . Miller

PLATE 1

389

PLATE 2 EXPLANATION O F F I G U R E S

390

12

Ameiva ameiva (Teiidae). Transverse section near the cephalic end of t h e acoustic tubercle. X 30. Inset is from another specimen showing more cellular detail of nucleus angularis. X 100.

13

Ameiva ameiva. Transverse section through the midportion of the acoustic tubercle. Note the more lateral distribution of the cells in this location. x 30. Inset is from another specimen showing the larger paler cells characteristic of NML. X 100.

14

Ameiva ameiva. Transverse section from the caudal portion of the acoustic tubercle. Inset shows the characteristic, regularly round cells of NMM from another specimen. X 100.

T H E COCHLEAR N U C L E I OF LIZARDS Malcolm R . Miller

PLATE 2

39 1

PLATE 3 EXPLANATION OF FIGURES

15

Ameiva ameiva. Fink-Heimer preparation subsequent to destruction of the posterior eighth nerve ganglion on the right side. Transverse section near the anterior end of the tubercle. Presumably most of the cochlear cells are NA, although some NML could b e included in this plane. x 30. Inset is a higher power magnification of t h e area outlined above showing degenerating fibers and terminals. X 100.

16

Same specimen as in figure 15. Transverse section near the junction of NML and NMM. The more dorsolaterally placed cells a r e probably NML and the ventromedial cells are NMM. X 30. Inset shows at higher magnification the outlined area above with many degenerating fibers and terminals associated with the cells of both NML and NMM.

x 100.

392

T H E COCHLEAR NUCLEI OF LIZARDS Malcolm R . Miller

PLATE 3

393

PLATE 4 EXPLANATION OF FIGURES

394

17

Cnemidophorus tigris (Teiidae). Frontal (horizontal) section showing the extent of the cochlear nuclei which occupy the medial portion of the outlined area. X 30.

18

Same as 17. Higher magnification of cochlear nuclei from the area outlined above. Usually the cells of NML a r e much more lightly stained than shown here. x 100.

THE COCHLEAR NUCLEI OF LIZARDS Malcolm R. Miller

PLATE 4

395

PLATE 5 EXPLANATION O F FIGURES

396

19

Cnemidophorus tigris. Sagittal section through the medial portion of the acoustic tubercle. The cochlear nuclei are confined within the upper portion of the outlined area. X 30.

20

Higher magnification of figure 19. Note that the regularly round cells of NMM tend to be aligned in rows. T h e cells of NML are usually lighter stained than shown here. x 100.

21

Same specimen a s the two figures above, but cut more laterally in order to show NA. X 100.

T H E COCHLEAR NUCLEI OF LIZARDS Malcolm R . Miller

PLATE 5

397

Gekko gecko (Gekkonidae). Frontal section through the cephalic end of the medulla. Note the more regularly round to oval nature of the cells of NMM and the larger paler aspect of the cells of NML. The cells of NA are better seen in figure 23. T h e bar in this and all remaining figures = 100 k . X 100.

Gekko gecko. Transverse section through the cephalic end of the acoustic tubercle. Note the variability of cell size in the nucleus angularis. X 100.

Gekko gecko. Transverse section through the caudal part of the acoustic tubercle. X 100

23

24

O F FIGURES

22

EXPLANATION

PLATE 6

(0

w I-

d

el

a

399

PLATE 7 EXPLANATION

400

OF FIGURES

25

Gerrhonotus multicarinatus (Anguidae). Frontal section through the cephalic part of the medulla. Note the somewhat small but regularly round cells of NMM, the larger lighter staining cells of N M L , and t h e variably staining cells of NA. X 95.

26

Gerrhonotus multicarinatus. Transverse section through the cephalic portion of the acoustic tubercle. X 100.

27

Same specimen as in figure 25. Transverse section near the caudal end of the tubercle. X 95.

THE COCHLEAR NUCLEI OF LIZARDS Malcolm R . Miller

PLATE 7

40 1

PLATE 8 E X P L A N A T I O N OF F I G U R E S

28

Gerrhonotus multicarinatus. Sagittal section through the lateral aspect

of the acoustic tubercle showing mixed sized cells in NA and a few large cells in NML.

29

402

X

100.

Same specimen a s in figure 28, but cut more medially showing the regularly round to oval cells of the more caudally located NMM. x 100.

T H E COCHLEAR NUCLEI OF LIZARDS Malcolm R . Miller

PLATE 8

403

PLATE 9 E X P L A N A T I O N OF F I G U R E S

404

30

Basiliscus basiliscus (Iguanidae). Transverse section through the cephalic end of t h e tubercle showing numerous mixed cells of NA. x 100.

31

Same specimen as figure 30 n e a r caudal part of acoustic area showing small round cells of N M M (arrows). X 100.

T H E COCHLEAR NUCLEI O F LIZARDS Malcolm R . Miller

PLATE 9

405

PLATE 10

T H E COCHLEAR N U C L E I OF LIZARDS Malcolm R. Miller

EXPLANATION

32

406

O F FIGURE

Chameleo jacksoni (Chameleonidae). Transverse section through the acoustic region near the point of entry of the posterior eighth nerve division. The presumably cochlear nuclei cells (arrows) do not form recognizably different nuclear groups. Note lack of development of the medial portion of the alar eminence. x 100.