Scanning electron microscope studies on blastodisc formation in the zebrafish, Brachydanio rerio

Abstract

Upon fertilization, the zebrafish egg undergoes marked physiological and structural changes, one of which involves blastodisc formation. Before fertilization, yolk globules are rounded and the endoplasm extends throughout the oocyte. During blastodisc formation, the yolk globules become angular and the endoplasm is restricted to streamers among the yolk globules. The streamers are oriented in an anterior-posterior axis of the egg. During blastodisc formation the cytoskeleton consists of an extensive array of filamentous structures of variable width in both the cortex as well as within elongate endoplasmic streamers. Although the filamentous components in the cortex and endoplasmic streamers probably include both microfilaments and microtubules, frequently they are somewhat wider than the usual dimensions, and possible reasons for this are suggested. From their arrangement in both the cortex and endoplasm, it seems likely that the components of the cytoskeleton (e.g., microfilaments and microtubules) may provide, through contraction, the major force responsible for the streaming of the endoplasm into the forming blastodisc. It is assumed that the surface tension of the vegetal hemisphere exceeds that of the animal hemisphere, thus forcing, through differential contraction, the endoplasm to flow in the direction of the forming blastodisc. No distinct barrier between the yolk and forming blastodisc was observed. The compressed condition of the larger and many-sided yolk globules could prevent their movement into the blastodisc. Scanning electron microscopy is limited in the resolution with which it can depict the cytoskeleton, but nonetheless it provides useful information about structural interrelationships.

Calcium signalling during zebrafish embryonic development

“…This is achieved via a process known as ooplasmic segregation, where ooplasm is separated from the yolk and accumulates at the animal pole (AP) to form a blastodisc. (26) Via subsequent meroblastic cleavages, the blastodisc divides atop of the non-dividing yolk cell, and it is from these cells that the embryo is derived. Thus, ooplasmic segregation is the first major pattern-forming process following egg activation.…”

Identification of recessive maternal‐effect mutations in the zebrafish using a gynogenesis‐based method

“…One of the major events during egg activation is ooplasmic streaming, which causes the segregation of the ooplasm away from the vegetal yolk mass and toward the animal pole of the egg (Hisaoka and Firlit, 1960;Beams et al, 1985). This movement results in the lifting of the first cell, or blastodisc, at the animal pole (Fig.…”

Analysis of Early Development in the Zebrafish Embryo

“…The development of the blastula has been documented by scanning electron microscopy (Beams et al 1985). The timing of developmental stages noted in this review will be for embryos grown at 28.5°C (Westerfield 1989).…”