Programmed Endocytosis During Epiboly ofFundulus heteroclitus

Abstract

A major question in the analysis of teleost epiboly is the fate of the yolk cytoplasmic layer. It diminishes during epiboly and eventually disappears at the completion of epiboly. This paper is concerned with the fate of the surface of the yolk cytoplasmic layer during epiboly. When gastrulae during epiboly are bathed in lucifer yellow (CH) and then observed with fluorescent microscopy or bathed in ferritin and then fixed and observed with TEM, a thin circumferential ring of endocytic vesicles is observed, confined to the external yolk syncytial layer just peripheral to the advancing margin of the blastoderm. Even though the entire egg is immersed in the marker, endocytosis is confined to this limited region. More precisely, this endocytosis occurs only within the region of the external yolk syncytial layer, where the surface is most folded. The endocytic vesicles thus formed move downward and settle on the surface of the membrane separating the yolk from the cytoplasm in the yolk syncytial layer. They do not join the surface of the internal yolk syncytial layer; hence they do not contribute to its expansion. Prior to the onset of epiboly there is no such endocytosis at the surface of the egg. Since this endocytosis occurs only during epiboly and only at the surface of the external yolk syncytial layer just peripheral to the advancing margin of the blastoderm, and in the absence of large molecules in the medium, we conclude that it is programmed. We, therefore, present this as a case of programmed internalization of cell surface serving as the morphogenetic mechanism responsible for the disappearance of the surface of the yolk cytoplasmic layer during gastrulation of the teleost Fundulus heteroclitus