The Ovulated Ovum of the Horse: Cytology of Nonfertilized Ova to Pronuclear Stage Ova1

Abstract

Fertilization and early development in the horse were studied by recovering oviductal ova at various times after postovulatory mating. Ova collected between 7 and 22 h post coitum (pc) were examined for evidence of fertilizing sperm, cellular changes accompanying fertilization, and pronuclear development. Five ova collected between 7 and 9 h pc contained a marginal metaphase plate, but had no indication of sperm components; three of these, however, showed reduced numbers of cortical granules. Two activated ova (10 and 14 h pc) were in telophase of the second meiotic division, following incorporation of the fertilizing sperm. The fertilizing sperm was situated in a slight elevation; the nucleus was expanding but lacked a nuclear envelope. The pronuclear stage in the horse began as early as 12 h pc, and lasted at least until 21 h pc. Sperm tail remnants were seen in 5 of 7 pronuclear-stage ova, although the crowding of the cytoplasm with clusters of lipid and vacuoles made discerning sperm tail remnants difficult. The spindles of the metaphase stage of the second meiotic division were oriented radially, that is, at right angles to the cell surface, in all but one ovum, so this orientation is not a response to fertilization.

Fine structure of equine oocytes matured in vitro for 15 hours

“…Overall, the similarity to in vivo matured equine oocytes Vogelsang et al, 1987) indicates that equine oocytes matured in vitro under these conditions are ultrastructurally normal. Further descriptions of equine oocytes in comparison to oocytes of other species can found in Enders et al (1987) and Vogelsang et al (1987). These results are further evidence that equine oocytes recovered and cultured under these conditions would be appropriate candidates for equine in vitro fertilization studies.…”

Use of Confocal Microscopy to Evaluate Equine Zygote Development After Sperm Injection of Oocytes MaturedIn VivoorIn Vitro

“…The equine conceptus will remain in the oviduct for 5 – 6 days before entering the uterus as a morula or early blastocyst (Freeman, et al, 1991). Initial observations of equine zygotes were done using transmission electron microscopy, with limited numbers of samples collected from oviducts based on timing of breeding or administration of ovulation inducing compounds (Enders, et al, 1987; Grondahl & Hyttel, 1996). Later, ICSI was used as a method to fertilize equine oocytes matured in vitro , prior to analysis by electron microscopy (Grondahl, et al, 1997) or confocal microscopy (Tremoleda, et al, 2003).…”

The effect of co‐culture on the development ofin vitromatured equine oocytes after intracytoplastic sperm injection

“…However, in vitro freezing studies report that, after freezing/thawing procedures and 36 h of culture, some blastocysts collapsed and the degenerated cells formed a dark mass of nucleated cells (Young et al 1997). A marked polarity in the distribution of organelles has been found in horse oocytes and zygotes (Enders et al 1987;Bézard et al 1989;Grøndahl et al 1995a); and a clear oocyte polarity in the distribution of microvilli in the mouse and of cortical actin in both the mouse and hamster has been observed (Longo 1991). It is probable that the allocation of blastomeres in particular sites on the inside of the zona pellucida is very much affected by the locations at which yolk is extruded from the cleaving zygote (Betteridge 1995).…”