Intra-follicular and peripheral steroid characteristics during vernal transition in the pony mare.

This experiment investigated steroid production by ovarian tissues, in vitro, of pony mares during vernal transition from anoestrus to the breeding season. Follicular dynamics were monitored to detect the first, second, third or fourth transition follicle, greater than or equal to 30 mm diameter or the first large post luteal follicle of the breeding season. Twenty-four hours after a large follicle was detected, theca (T) and granulosa (G) tissues were harvested. Separate and co-incubations of these tissues were conducted to determine steroid production in early transition (ET), late transition (LT) and pre-ovulatory (OV) follicles. Peripheral plasma and follicular fluid steroids and gonadotrophins also were assayed. Peripheral plasma oestradiol concentrations increased from ET to LT and again from LT to OV in parallel with tissue production and follicular fluid content. Androgen production increased from LT to OV whereas progesterone production showed no change, thereby indicating a possible failure of 17-alpha steroid hydroxylase in ET follicles. Examination of tissue steroid secretion rates revealed that granulosa was the major site of oestrogen production, whereas theca secreted greater amounts of androgen.