Ultrastructural analysis of the apical ectodermal ridge during vertebrate limb morphogenesis: I. The human forelimb with special reference to gap junctions

The fine structure of the human forelimb apical ectodermal ridge of stages 12–19 was examined using techniques of transmission electron microscopy, freeze fracture, and scanning electron microscopy. This paper reports the presence of subcellular structures that distinguish the inductively active apical ectoderm from adjacent dorsal and ventral ectoderms.The apex of the human forelimb begins development with an epithelium of two cell layers (stage 12) which thickens at the distal tip during stages 13 and 14 into a multilayered apical ectodermal ridge. During this transition we have observed that the basal lamina differentiates from a bilayered structure to the definitive single lamina. Some cells in the ectoderm become detached from the basal lamina as stratification begins. At the same time these cells show increased mitotic activity and the developing ridge cells acquire gap junctions. Annular gap junctions are also observed. Gap junctions are not observed in adjacent, presumably noninductive, epithelia. Finally, the ridge cells next to the basal lamina acquire bundles of microfilaments that are oriented in the dorsal-ventral plane in the basal cytoplasm of the cells.The apical ridge reaches its greatest dimensions during stage 15. The number and peripheral extent of gap junctions also appear to be greatest at this same time. At stage 17, cells within the ridge begin to die, and other ridge cells engulf them. By stage 19, gap junctions in the apical epithelium are sparse and are of lesser diameter than in the definitive ridge. In addition, the oriented bundles of microfilaments present at stages 14–17 are absent. Thus, at stage 19 a morphologically distinct apical ectodermal ridge is no longer present. The apex of the limb is covered by two cell layers typical of human embryonic epidermis.