Comparative ultrastructural analysis of diestrous and anestrous canine Grade 1 cumulus-oocyte complexes

The objective of the present study was to characterize, by means of transmission electron microscopy, immature Grade 1 cumulus-oocyte complexes (COCs) obtained from ovaries collected from bitches at diestrus and anestrus, after routine ovariohysterectomy. Cumulus-oocyte complexes were recovered after slicing the ovarian cortex and Grade 1 COCs were selected and prepared for transmission electron microscopy. All oocytes were at germinal vesicle stage in two different configurations. In 29 out of 37 COCs, oocytes presented a central or eccentric located nucleus (GVc), frequently containing a reticulated nucleolus, with a predominance of profound follicular cell processes (FCP), abundant endoplasmic reticulum (ER) and a moderated number of lipid droplets. In eight out of 37 COCs, oocytes had a peripheral located nucleus (GVp) containing small compact nucleolus, a thin perivitelline space, both superficial and profound FCP, moderate ER content and abundant lipid droplets. A higher (p < 0.05) proportion of GVp was found in oocytes collected in diestrus (36.8% – 7 out of 19) than in anestrus (5.6% – 1 out of 18). In addition, distinct ultrastructure characteristics among GVc associated with estrous phase were noted. Furthermore, a number of structures, some of which had not been reported before, were present in canine prophase I oocytes at variable frequencies. The evident variation in the presence, quantity and distribution of cell organelles in canine immature Grade 1 oocytes is discussed in relation to the biological phase of the oocyte and the reproductive stage of the donor bitch.