Morphometric and autoradiographic analysis of frontonasal development in the chick embryo.

Dimensional changes in the nasal processes were measured in chick embryos from Hamburger and Hamilton (1951) stages 20 through 27.5. Transverse measurements in the frontonasal region of freshly fixed embryos were compared to frontal sections of the nasal region of comparably staged embryos. These observations were correlated with autoradiographic studies of cell movement employing an implant labeling technique. Morphometric analysis indicated that between stages 20 and 25 the separation of the nasal pit orifices increased coincidentally with rapid forebrain enlargement. Since the separation of the nasal pit fundi increased more rapidly, the orientation of the nasal pits changed. Autoradiographic studies indicated that lateral movement of medial nasal process mesenchyme into the base of the nasal groove and medial area at the base of the lateral nasal process had occurred. After stage 25, the separation of the nasal orifices declined dramatically, coincidental with rapid orbital enlargement. In contrast, the separation of the nasal pit fundi was maintained. It is proposed that nasal development of the chick embryo may be governed initially by forebrain enlargement and associated lateral movements of mesenchyme in the medial nasal processes, resulting in reorientation of the invaginating nasal placodes; subsequently, orbital enlargement and an associated medial redirection of growth of the lateral nasal processes assumes greater significance to the continued development of the frontonasal region.