Facial development in normal and mutant chick embryos. I. Scanning electron microscopy of primary palate formation

Abstract

Early facial development in normal chick embryos was studied by scanning electron microscopy, and compared to the abnormal facial development of a mutant in which primary palate formation does not occur, thus resulting in bilateral cleft lip. In both normal and "cleft primary palate" mutant embryos, subsequent to the appearance of the nasal placodes, the surrounding tissues elevate to give rise to the presumptive facial primordia. As the facial primordia grow forward, they gradually assume the configuration of a square which is most pronounced at five days development. In normal embryos, the square configuration is then lost as the facial primordia become aligned in preparation for primary palate formation. The primary palate is formed at six days development by fusion of the "free-ended" medial nasal processes with the combined lateral nasal and maxillary processes across the nasal grooves. Just prior to fusion, long, slender filaments extend from the apposing surfaces of the facial primordia in the regions of prefusion contact. It is speculated that these "prefusion filaments" may function in alignment or adhesion of the facial primordia. In "cleft primary palate" embryos, facial morphogenesis appears to arrest at five days development, so that the square configuration persists. The medial nasal processes never contact the lateral nasal and maxillary processes, but instead remain separated from them by wide nasal grooves. Furthermore, facial primordia of mutant embryos do not exhibit the "prefusion filaments" characteristic of normal embryos.

Development of the upper lip: Morphogenetic and molecular mechanisms

“…Recently, Senders et al (2003) presented high resolution SEM pictures of developing cynomolgus monkey embryonic faces. Comparing these with other histological and SEM studies of facial development in mouse and chick (Trasler, 1968;Gaare and Langman, 1977a,b;Yee and Abbott, 1978;Millicovsky and Johnston, 1981;Millicovsky et al, 1982;Trasler and Ohannessian, 1983;Cox, 2004) provides an accurate understanding of the morphological processes involved in facial development.…”

“…Filopodial attachments are greatly reduced in A/WySn and CL/Fr mouse embryos, two strains with high frequency of spontaneous CLP (Millicovsky et al, 1982;Forbes et al, 1989). Similarly, filamentous projections have been observed in chick embryos between the fusing facial prominences and are notably missing from the cleft primary palate chick mutant embryos (Yee andAbbott, 1978, Cox, 2004). These observations, therefore, correlate the presence of filopodial processes spanning the prefusion primordia with an ability to fuse.…”

“…Facial morphogenesis in chick is slightly different from that in mammals, because the medial nasal process appears as a single entity sometimes referred as the frontal or frontonasal process, and the entire embryonic chick face appears in a square configuration before lip fusion (Yee and Abbott, 1978;Young et al, 2000;Cox, 2004). Despite the differences, close examination of SEM micrographs of the early fusion stage chick face showed that the initial contact and initiation of active cellular processes of fusion also begins between the lateral and medial nasal processes (Fig.…”

Utilizing the chicken as an animal model for human craniofacial ciliopathies

“…Despite the dramatic phenotype, there has been surprisingly little study of this mutant, save the original characterization by Ursula Abbott and one paper examining mechanism of beak outgrowth (MacDonald et al, 2004; Yee and Abbott, 1978). While a gene has yet to be identified for cpp , the mode of inheritance is autosomal recessive, with embryonic lethality, similar to that of ta 2 and ta 3 .…”

Parallels between tissue repair and embryo morphogenesis

“…Indeed, there are several earlier fusion events that occur as the vertebrate face is built that appear to use an almost identical bonding strategy (reviewed by Cox, 2004). Classic studies of the fusions between the medial nasal prominence and the right and left maxillary prominences (primary palate fusion) provide clear evidence of filopodia from both nasal and maxillary epithelial faces; and in the classic CPP (cleft primary palate) chick mutant, they are absent (Yee and Abbott, 1978). Similarly, as the two secondary palatal shelves flip up and over the tongue to make contact with one another, they also express…”