OOCYTE STRUCTURE AND FOLLICULAR STEROID CONCENTRATIONS IN SUPEROVULATED VERSUS UNSTIMULATED HEIFERS

A highly variable yield of viable embryos in superovulated cattle is a major hindrance to the embryo transfer industry. To trace the cause o f this problem, investigations were carried out on the intrafollicular steroids and structure of oocytes originating from follicles of folli cular stimulating hormone (FSH)-stimulated (superovulated) and unstimu lated heifers. Unstimulated heifers were slaughtered at midcycle, or a dministered cloprostenol (PG) at midcycle and slaughtered after 24, 48 , or 72 hr, while superovulated heifers were administered 4 injections of pFSH (2 injections per day) and slaughtered 12 hr later, or admini stered 6, 7, or 8 injections of FSH in combination with PG at the 5th and 6th injection, and slaughtered 24, 36, or 60 hr, respectively, aft er the first PG injection. The follicular fluid from the largest (pres umptive dominant) follicle of the unstimulated heifers and from potent ially ovulatory follicles (greater than or equal to 8 mm in diameter) of the superovulated heifers were assayed for estradiol-17 beta (E2) a nd progesterone (P4), while the oocyte cumulus complexes from such fol licles were processed for transmission electron microscopy. The mean E 2 and especially P4 concentrations of the potentially ovulatory follic les of the superovulated heifers were lower than similar follicles of the unstimulated animals (83.7 +/- 76.7 ng/ml vs. 208.1 +/- 357.0 ng/m l, P > 0.05 and 31.1 +/- 38.7 ng/ml vs. 150.3 +/- 202, P < 0.05, respe ctively). The unstimulated oocytes had, in general, spherical oocyte n uclei and compact nucleoli before PG administration, while after PG, u ndulation of the nuclear envelope and nucleolus vacuolization was char acteristic. The superovulated oocytes, in comparison, displayed the fo llowing deviations: premature perivitelline space formation, lack of n ucleolar vacuolization, reduced amount of lipid droplets and lack of l ipid-mitochondria association, enlarged rough endoplasmic reticulum co mpartment, and increased condensation of chromatin and elongation, i.e ., expansion of some cumulus cells. Degenerative oocytes were only fou nd in the superovulated group. It is concluded that FSH-stimulation is associated with reduced intrafollicular E2 and P4 concentrations and subcellular deviations in the oocytes that are established early in th e superovulatory process. These deviations may contribute to the reduc ed developmental competence of superovulated oocytes. (C) 1994 Wiley-L iss, Inc.