The effects of insulin-glucose on microappendages of the plasmalemma in the early chick embryo.

Chick embryos incubated for 18-24 hours (stages 4-6) were immersed for 3-5 minutes in Hanks' Balanced Salt Solution containing either insulin (10(-9) M) or D-glucose (10(-1) M) or a mixture of both. The ventral and dorsal surfaces of the entoderm and the ventral aspect of the mesoblast were examined in a scanning electron microscope. D-glucose alone or insulin alone produced negligible change. When combined these ingredients generated highly pleomorphic microappendages of the plasmalemma. The ventral surface of the entoderm was characterized by a large number of microvilli, both long and short. Concentrations of microvilli and blebs appeared in recesses at cell margins. Some of these represented mesoblastic protrusions through intercellular apertures in the entoderm. The dorsal entodermal surface developed stellate clusters of microvilli and blebs. The ventral aspect of the mesoblast became the most pleomorphic area of all. It possessed a variety of microappendages including large populations of microvilli and blebs that resembled the protrusions observed from the subblastodermic cavity. There were also stellate clusters similar to those on the dorsal entodermal surface. The same mesoblastic area in controls was virtually free of microappendages. The mesoblastic protrusions, occurring as they do prior to an established circulation, are interpreted as a "feeding" phenomenon.