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

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REPRODUCTIVE BIOLOGY AND EMBRYOLOGY OF CROCODILIANS
indicates that the paired maxillary processes each develop two intra-oral
migratory fronts of mesenchymal cells, namely the tectoseptal processes
and the palatal shelves.
The bilateral tectoseptal processes migrate superiorly between the floor
of the brain and the roof of the oronasal cavity; eventually merging with
one another along the midline in a progressive anteroposterior direction
(Figs. 25A-O and 28B). Anteriorly, the merged tectoseptal processes grow
downward to form the secondary nasal septum (Fig. 28A).
The bilateral palatal shelves arise from the maxillary processes as thick,
blunt-ended structures (Figs. 25A-O, 28A and B), which grow horizontally
from their first appearance (Fig. 28A), except in the posterior one-fifth of
the palate where they are more vertically oriented (Fig. 28B). With later
development, these posterior shelves gradually flow over the tongue to
become truly horizontal (Figs. 28C and 0). The reorientation involves the
migration of mesenchymal and epithelial cells, the hydration of palatal
glycosaminoglycans, the contraction of microfilaments, and differential
mesenchymal proliferation. The shelves first approximate each other anteriorly
behind the primary palate and from there closure spreads backward
(Figs. 25A-O, 28C, and 0). Palatal closure occurs principally during Stage
18 and is characterized macroscopically by a V-shaped gap along which the
posterior margins of the opposing shelves approximate each other (Figs.
25B and C, 28E). Palatal closure has been studied in normal cross-sectional
sequences and also in ovo in the same embryo (longitudinal sequence)
developing in a semi-sheIl-less culture (Ferguson, 1981a, 1982b, 1984).
The process of palatal closure involves contact and adherence of the
epithelial cells of the two shelves beginning on the oral aspect of the shelf
margins and spreading nasally (Figs. 28C and 0). Cell death is limited to
the small area of initial contact, after which the epithelial cells of the two
shelves migrate nasally and posteriorly out of the region of closure (Figs.
28C, 0, and E). There is never an extensive epithelial seam (Fig. 28C) and
epithelial remnants cannot be detected following closure. Numerous small
blood vessels in the shelf mesenchyme adjacent to the area of closure
represent the earliest development of the palatal vascular plexus that
characterizes late embryos (Figs. 24F and G) and adults. Mesenchymal
continuity is usually established on the oral edges of the palatal shelves
before the nasal edges are in contact (Fig. 28C). Anteriorly, the shelves
Fig. 28. Alligator mississippiensis. (A) Stage 17. Mallory stain. Transverse section through
head. Note the horizontal palatal shelves, bulge from the nasal septum, tongue, Meckel's
cartilages, and intermandibularis muscle. (B) Stage 17. More posterior section of H & E and
Alcian Blue stained specimen. Note the vertical secondary palatal shelves, closing tectoseptal
processes, anlage of the anterior pterygoid muscle, Meckel's cartilages, anlagen for the intermandibularis,
genioglossus, and hyoglossus muscles. (C) Stage 18. Transverse section
through the closing secondary palatal shelves (Mallory stained). Shelf contact and mesenchymal
continuity are established on the oral edges of the shelves, before the nasal edges have
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even contacted each other. Epithelial seam absent. The matrix stains positively for glycosaminoglycans
and numerous blood vessels are present. (D) Stage 18. Scanning electron
micrograph of palate. Note the closing palatal shelves, midline palatal elevation, primary
palatal bulge, denticles, caruncle, and eyes. (E) SEM of the medial edge epithelial cells in the
region of palatal closure seen in (D). Note the markedly cobblestoned appearance and numerous
microvilli, both characteristic of cell migration. (F) Stage 18. SEM of the developing tongue
and lower jaw. Note the paired lingual swellings, tuberculum impar, hypobranchial eminences,
developing larynx and epiglottis, and denticles. The elevated hypobranchial eminences
later form from the inferior flaps of the basihyal valve-see Figs. 25H and I. A,
Anterior pterygoid muscle; B, bulge from the nasal septum; C, caruncle; 0, denticles; E, eye;
G, genioglossus muscle; H, hyoglossus muscle; HB, hypobranchial eminences; I, intermandibularis
muscle; L, lingual swellings; LA, larynx; M, Meckel's cartilages; MP, midline palatal
elevation; P, palatal shelves; PP, primary palatal bulge; T, tongue; II, tuberculum impar; TS,
tectoseptal process.