Establishment and movement of egg regions revealed by the size class of yolk platelets in Xenopus laevis

Abstract

Sizes of yolk platelets were measured in sections of oocytes and embryos in Xenopus. It was found that the average size of the largest group of platelets in cells differed between germ layers of neurulae. It was small (3 to 5 μm) in the ectoderm, medium-sized (5 to 8 µm) in the mesoderm, and large (over 8 μm) in the endoderm. Platelets of these size classes formed layers in egg, the yolk gradient, by the end of oocyte maturation. The yolk gradient contained products of the mitochondrial cloud and a part of the germinal vesicle material at certain positions. The layers of small, medium and large platelets in the egg changed their locations to distribute to the ectoderm, mesoderm and endoderm of neurulae, respectively. The yolk layers in the egg thus represented different prospective fates, and a figure describing the locations of these layers could be regarded as a fate map for the one-cell stage. Most of the marginal blastomeres of embryos at cleavage stages consisted of a few parts with different prospective fates. Results were discussed with reference to available fate maps for cleavage stage embryos.

Regional Differences in Yolk Platelet Degradation Activity and in Types of Yolk Platelets Degraded during Early Amphibian Embryogenesis

“…One type is formed in the animal region and accumulates there; the second is formed in the animal region but is then translocated to the vegetal region, while the third is formed and accumulates within the vegetal region. The YPs that accumulate in this region-specific manner within the egg cytoplasm have different prospective fates as development proceeds [Imoh, 1995]. The present results highlight the possibility that the type of YP being degraded differs between the animal and dorsal regions.…”

“…This segregation of YPs within the amphibian egg is established while the YPs are being formed during oogenesis [Danilchik and Gerhart, 1987] and during the rearrangement of the cytoplasm just after fertilization [Brown et al, 1993]. This distribution by size of YPs establishes yolk compartments in the developing embryo and these have different prospective fates [Imoh, 1995]. However, the developmental significance of the compartmentalization of YPs is not yet fully understood.…”

Novel functions of the ubiquitin-independent proteasome system in regulatingXenopusgermline development

“…Nevertheless, it seems unlikely that this reorganization event is specific to proteasome RNAs. The vegetal-toanimal translocation of the proteasome is reminiscent of the massive rearrangement of yolk platelets and deep cytoplasmic components during the oocyte-to-embryo transition (Danilchik and Denegre, 1991;Imoh, 1995). In 1991, Danilchik and Denegre hypothesized that this dramatic intracellular rearrangement during the oocyte-toembryo transition might represent an important mechanism to regulate the functions of some maternal factors (Danilchik and Denegre, 1991).…”

Yolk platelets impede nuclear expansion inXenopusembryos

“…4B). The yolk platelets are concentrated in the vegetal hemispheres of the eggs and segregate to greater degree into the vegetal blastomeres through embryonic cleavages, which is thought to contribute to cell fate decisions during later development (Danilchik & Gerhart, 1987; Imoh, 1995). In terms of intracellular functions, the biasedly located yolk platelets are known to impede the formation of cleavage furrows during embryonic cleavages (Neff et al, 1984; Newport & Kirschner, 1982).…”

“…In particular, heavy yolk platelets in the cytoplasm and large nucleoli in the nucleus sediment towards the vegetal side by gravitational forces (Neff et al, 1984; Feric & Brangwynne, 2013). Conversely, the yolk-free cytoplasm is located on the side of the animal hemispheres, around the germinal vesicle (GV) (Imoh, 1995). Thus, various determinants such as yolk platelets (Danilchik & Gerhart, 1987), transcription factor VegT (Fukuda et al, 1990), and aPKC (Chalmers et al, 2003) are distributed differently inside the oocytes and segregate to cleaved blastomeres in different amounts.…”