Structural Changes of Yolk Platelets and Related Organelles during Development of the Newt Embryo: (amphibia/early development/electron microscopy/yolk platelet/cytoplasmic organelle).

Abstract

Structural changes in yolk platelets and related organelles in the cytoplasm of the presumptive ectodermal region up to the stage of gastrulation were studied by light and electron microscopies using full-grown oocytes, mature eggs descending the oviduct and embryos of the newt, Cynops pyrrhogaster. Yolk platelets with a superficial layer are first observed in mature eggs descending the oviduct. During the cleavage and early morula stages, the superficial layer increases in thickness and the main bodies become more slender. The superficial layer decreases in thickness in the blastula stage, and many yolk platelets lose this layer in the gastrula stage. The amount of rough-surfaced endoplasmic reticulum (r-ER) increases rapidly in the morula stage, while Golgi complexes gradually increase in number between the cleavage and gastrula stages. In the cleavage and early morula stages, most of the r-ER is closely adherent to yolk platelets and is associated with several mitochondria. Two types of free vesicles, large (0.5-4.0 μm diameter) and small (0.15-0.3 μm diameter), were seen in abundance from the early morula stage to the early gastrula stages. Changes in the structure of yolk platelets are discussed in relation to changes in other cytoplsmic organelles.