Analysis of Polarity of Bovine and Rabbit Embryos by Scanning Electron Microscopy1

Abstract

Cellular polarization during preimplantation development of the embryo is believed to be a crucial event in the transition of a zygote to a blastocyst stage embryo with morphologically and functionally differentiated cell types. While extensive studies have been done on polarity development in mouse embryos, little information is available in other species, particularly in cattle. The objective of this study was to examine the initiation of polarity by microvilli distribution on blastomeres of cattle and rabbit embryos using scanning electron microscopy (SEM). Bovine embryos were obtained by in vitro fertilization of in vitro-matured follicular oocytes. Rabbit embryos of various stages were collected from superovulated rabbits. Intact embryos and isolated blastomeres were examined in both species. Blastomeres from 1- to 8-cell embryos in both cattle and rabbits showed no polarity. The onset of transitional polarization of microvillous distribution occurred in some blastomeres of cattle embryos at the 9- to 15-cell stage; but typical, distinct polarity was not manifested until after the 16-cell stage with approximately 40% polar cells per embryo. In the rabbit blastomere polarity occurred one cell cycle later, with 46% polar cells per embryo after the 32-cell stage. The difference in cell numbers at the time polarity is evident is probably related to the different cell stages for embryo compaction and blastocyst formation in the two species.

Trophoblast development

“…In cattle, polarisation, as determined by the distribution of surface microvilli using scanning electron microscopy, occurs one cell division later than in mice, that is at the 16-cell (early morula) stage (Koyama et al 1994). Importantly, compaction -concomitant with E-cadherin depletion in the apical regions of outer cells (Barcroft et al 1998) occurs only at the 32-cell stage (Betteridge & Flechon 1988, Van Soom et al 1997.…”

Observation of human embryonic behavior in vitro by high‐resolution time‐lapse cinematography

“…The onset of compaction varies among species. It is the most prominent in the mouse, appearing as cell-to-cell attachment at the 8-cell stage, whereas in bovines it begins at the 16-to 32-cell stage, and in rabbit it commences at the 32-to 64-cell stage [47,54,55]. Compaction is less marked in the rhesus monkey and baboon, appearing at the 16-to 32-cell stage, and in pig embryos compaction is not established until shortly before blastocyst formation [52,[56][57][58].…”

Effect of Brefeldin-A on the Development of Mouse 4-Cell Embryos In Vitro.

“…Newly formed 4-cell embryos were cultured with BFA for 15 h, and they were fixed with 3% glutaraldehyde and 0.5% paraformaldehyde in Hanks' balanced salt solution (HBSS) containing 0.1% polyvinyl alcohol (HBSS-PVA) [13] for 1 h. They were then placed on coverslips coated with poly-L-lysine (Sigma). After specimens were fixed with 1% osmium tetroxide in HBSS-PVA for 1 h, they were stained with 1% tannic acid aqueous solution for 2 h and further stained with 1% osmium tetroxide aqueous solution for 1 h. Samples were dehydrated through a graded alcohol series and they were critical-point dried using liquid CO 2 , mounted, and coated with gold.…”