Development of the secondary palate in chick embryo: a light and electron microscopic and histochemical study.

Development of the secondary palate was studied in the chick embryo using light and electron microscopic and histochemical techniques. The palatal shelves develop as horizontal outgrowths of the maxilla on day 6 of incubation (HH stage 29). During the next 2 days (HH stages 30-33) the shelves continue their growth toward the midline, and on day 9 (HH stage 34) they approximate. At no time do the approximating shelves form direct contacts. Ultrastructural and histochemical observations indicate that the midline epithelia of the opposite shelves differentiate into a stratified squamous pattern. Unlike rodents and humans, the midline epithelial cells of the chick embryonic palate are not programmed to die. Other features of cellular differentiation such as the appearance of intracytoplasmic tonofilaments and glycogen, seen in species where palatal fusion occurs, were absent during palatogenesis in the chick. It is suggested that morphogenesis of the palate in chick embryos is different from that in the rodents, primates and humans. These differences are attributed to the pattern of subcellular differentiation in the palatal tissues.