Reproductive Biology and Embryology of the ... - Seaturtle.org

426 REPRODUCTIVE BIOLOGY AND EMBRYOLOGY OF CROCODILIANS
their medial edges keratinize, the palatal shelves fail to fuse, and thus
produce physiological cleft palate (Sippel, 1907; Barge, 1937; Pasteels, 1950;
Shah and Crawford, 1980; Koch and Smiley, 1981).
The differing characteristics of the medial edge epithelial cells in the
alligator (cobblestones, villous, migrating), chicken (keratinized stratified
squamous) and mouse (cell death) suggest that palatal differentiation could
be regulated by epithelial-mesenchymal interactions. To test this hypothesis,
palatal shelves of alligators, chicks, and mice were separated into
epithelia and mesenchyme, recombined in various heterotypic, homotypic,
isochronic, and heterochronic combinations, and cultured. Additionally,
palatal epithelia and mesenchyme were cultured in isolation and in
combination with mandibular epithelia and mesenchyme, respectively
(Ferguson and Honig, 1984).
If palatal epithelium from alligator is cultured in isolation, it disintegrates
after approximately three days, whereas palatal mesenchyme cultured
in isolation differentiates into bony, cartilaginous, and muscular anlagen;
control recombinations differentiate normally (Fig. 29E). Palatal
shelf epithelium recombined with mandibular mesenchyme differentiates
into typical stratified squamous mandibular epithelium. Conversely, mandibular
epithelium recombined with palatal shelf mesenchyme differentiates
into a typical palatal epithelium (Fig. 29F). These results suggest
that, in the alligator, differentiation of palatal epithelium is regulated by
an instructive epithelial-mesenchymal interaction. This interpretation is
confirmed in recombinations between vertebrate species. Thus, alligator
palatal epithelium recombined with a chick palatal mesenchyme exhibits
the typical avian stratified squamous pattern of medial edge epithelial differentiation.
Equally, alligator palatal epithelium recombined with mouse
palatal mesenchyme exhibits massive medial edge epithelial cell death (Fig.
29H). Contrariwise, either chick or mouse palatal epithelium recombined
with alligator palatal mesenchyme shows differentiation characteristic of
the alligator (Fig. 29G). These results show that the pattern of epithelial
differentiation is regulated by the source of the mesenchyme (Ferguson
and Honig, 1984).
The alligator oronasal cavity is not as restricted as that of mammals,
which may explain why alligator palatal shelves grow horizontally (Ferguson,
1981a, b, 1982b). Further development of the palate involves the appearance
and growth of osseous, muscular, and cartilaginous blastemata,
the expansion of the palatal plexus of blood vessels, and the development
of numerous domed tactile receptors (Figs. 3A and B) from subepithelial
condensations of mesenchymal (Merkel) cells (Fig. 24]). Small outpouchings
of the nasal cavity expand to form the maxillary sinuses, which
enlarge laterally and medially to invade the palatal processes of the maxillae
after hatching.
The fibrous superior flap of the basihyal valve arises by a posteroinferior
extension of palatal shelf closure (Figs. 25D-F). Crocodilians possess no
true soft palate; the superior flap of the basihyal valve descends from
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the palate to seal behind a lower, more rigid flap, that lies posterior to the
tongue and is supported by the hyoid cartilage (Wood Jones, 1940). The
superior valve flap is attached anteriorly to the posterior nasal choanae (in
the pterygoid bone), runs parallel to the pterygoid palate for a short distance,
and so forms a small area of nasopharynx between the pterygOid
bony palate and the upper mucosa of the superior basihyal valve flap
(Ferguson, 1981a). A failure to recognize the structure of the basihyal valve
led Muller (1967) to misinterpret this space as a division of the nasopharyngeal
duct (by a process of the pterygoid bone) into a "cavum
ventrale" and a "cavum dorsale" (see her Figs. 9 and 10). Muller (1967) also
confused the near simultaneous closure of the tectoseptal processes and
secondary palatal shelves in crocodilians (see Ferguson, 1981a, 1984a).
Very little is known about the development of maxillary, palatal and
salivary glands (Rose, 1893b; Woerdeman, 1920; Reese, 1925; Barge, 1937;
Fahrenholz, 1937; Kochva, 1978). The maxillary glands may arise from
invaginations of the oral epithelium, closely related to the invaginating
dental epithelium (Woerdeman, 1920). A review of the development of oral
glands in Reptilia includes some data on crocodilians (Kochva, 1978).
D. Tongue
The tongue develops from three principal anlagen on the pharyngeal aspect
of the branchial arches. The paired lingual swellings of the first arch
form the anterior two-thirds of the adult tongue, whereas the midline
tuberculum impar of the second and the paired hypobranchial eminences
of the third form the posterior one-third (Ferguson, 1982b, 1984a; Figs. 25H
and I, 27F). The epithelia of these anlagen come together and merge (Fig.
27F). Behind the tongue lie the developing epiglottis, larynx, and pharynx
(Fig. 27F). The inferior flap of the basihyal valve arises from a superior
outgrowth of the paired hypobranchial eminences (Figs. 25H and I, 27F).
Crocodilian lingual development resembles that of mammals and birds
(portmann, 1950; Scott and Symons, 1974; Sperber, 1981).
During palatogenesis, as in the adult, the tongue lies low in the oronasal
cavity (Ferguson, 1981a,b, 1982b). The body of the tongue contains fibrous
tissue anteriorly and lipid posteriorly, but intrinsic lingual musculature is
absent (Ferguson, 1981a, b, 1982b, 1984a). Anlagen for the Mm. genioglossus,
hyoglossus, geniohyoid, and intermandibularis develop, but their origins
are poorly known. It has been suggested that all the lingual musculature
develops from the geniohyoid anlage, which itself has split off from
the ventral longitudinal muscle anlage formed by the 4th to 8th trunk
myotomes (Edgeworth, 1907). Later stages of development of the lingual
musculature have been illustrated by Humboldt (1807), Rathke (1866),
Voeltzkow (1899), Gappert (1903), Taguchi (1920), Sewertzoff (1929),
Wettstein (1954), and Ferguson (1981a, b, 1982b, 1984a).
The lingual glands arise from thickened epithelia on the dorsum of the
tongue. These glands secrete salt in the estuarine crocodile, Crocodylus