The relation of sex-associated facial profile reversal and stages of human palatal closure.

This report is concerned primarily with the chronological relations among stages of palatal closure, changes in facial profile, and the sex of a select group of human embryos representing the critical period of palate development. Histological serial sections of 40 human embryos ranging in age from 6 to 9 weeks and free from gross defects were studied using light microscopy and graphic reconstructions. Maxillary prognathism usually occurred concomitantly with vertically oriented palatal shelves. As the shelves progressively assumed a horizontal position prior to palatal closure, a profile reversal occurred, i.e., the lower jaw became prognathic. Both profile reversal and reorientation of the palatal shelves occurred earlier in males (seventh week) than females (eighth week). These patterns suggest a sexual dimorphism in which the palatal shelves in females are apart and vertical for a relatively longer time than in males. Also, the concordance between palatal-shelf elevation and mandibular prognathism suggests that the forward repositioning of the lower jaw may contribute to the removal of the tongue from between the palatal shelves. The role of genetic factors has become an increasingly important consideration in the understanding of normal and abnormal facial development. It has been demonstrated that the frequency of human cleft palate is greater in females than males (Ivy, '57; Greene, '63; Milham, '63; Meskin et al., '65). Few attempts, if any, have been made to relate this sex difference with the possible occurrence of sex-associated prenatal variations. Recently Meskin et al. ('68) addressed themselves to this question and designed a hypothetical model that attempted to explain why females have a greater incidence and severity of palatal clefting. Their model assumed that the palatal shelves of human female embryos are apart for a longer time than those of males. This assumption was subsequently supported by a study (Burdi and Silvey, '69) that showed that females lag behind males in palatal closure. This finding implies (1) that sex-associated developmental patterning may operate in prenatal human facial development, and (2) that female embryos experience a longer period during which the normal sequence of palatal closure can be interrupted by teratogenic or other intrauterine disturbances. TERATOLOGY, 2: 297-304. These observations (Burdi and Silvey, '69) raise the question as to whether sexual variations can be observed in other facial regions before birth. For this purpose sexual variations in the embryonic human facial profile were sought during the period of palatal formation. Before the present study profile changes in the embryonic human face received only cursory attention, without specific reference either to sample size or sex. What is known about prenatal facial profiles stems primarily from Retzius ('04) who showed that before the seventh week the upper jaw is prognathic relative to the lower jaw but that this relation is temporarily reversed at about the ninth week. Later work (Schultz, '25; Schwarz, '31 ) supplemented the earlier study by showing that the anteroposterior profile of the lower jaw was generally recessive from the end of the second month until birth. Rabkin ('52) subsequently related variations in jaw profdes with ossification of the embryonic facial bones. These studies of prenatal 1This study was supported in part by National Faculty Research Grant (345081, ProJect 21 from the Horace H. Rackham Graduate School, the University of Michigan, and Pubhc Health Service Research grant No. 5 (SO-I FR5321) from the General Research Support Branch, Division of Research Facilities and Resources, to R.G.S.