Oocyte morphology and estrogen concentrations following a reduction in progesterone in beef cattle

Abstract

Low dosages of progestogens promote persistent follicles, high systemic estrogen and low fertility. The objectives of this study were to determine effects of a reduction in progesterone on (1) morphology of oocytes and intrafollicular concentrations of estradiol. Cows on low progesterone (n=12) received used intravaginal progesterone inserts on d 4 after estrus and prostaglandin (PG) F 2α (25 mg, i.m.) on d 6. Control animals (n=12) received saline on d6. The oocyte and follicular fluid were recovered from the largest follicle on d 8 or d 10. Serum estradiol was lower during d 4-6 but greater (P < .01) during d 7-10 in cows treated with progesterone inserts and PGF 2α while the largest follicle was larger in treated cows on day 10 only (14 vs. 12 mm; P < .05). Intrafollicular concentrations of estrogen were greater in treated than in control cows (990± 87 vs 191±106; P < .01). Progesterone in follicular fluid (mean = 42 ng/ml) did not differ. Oocytes were observed in oocyte nuclear stage I in the control group on d 8. All other oocytes were in nuclear stage II. In addition, the degree of clumping of mitochondria, the percentage of intact cumulus cell processes and percentage of normally shaped mitochondria was greater in oocytes from d 8 control cows than in all other groups. Changes in concentrations of estradiol and oocyte morphology typically associated with the preovulatory period had occurred within 2 d after a reduction in progesterone, even when low peripheral concentrations of progesterone were maintained. These earliest stages of oocyte maturation occurred in response to a reduction in progesterone. Similar changes in oocyte morphology were observed in control animals by d 10 of the estrous cycle, probably representing the onset of atresia. iii ACKNOWLEDGEMENTS I would like to thank my mentor, Dr. Keith Inskeep, for his guidance, wisdom and support that have helped me complete my program and grow as a scientist. I would also like to thank the other members of my committee Drs.

Effect of concentrations of progesterone on follicular development and fertility in beef cows

“…It has been proposed that a period of 10 days of low progesterone is required to significantly decrease fertility (Mihm et al, 1994;Austin et al, 1999). Many studies have shown that events associated with a decrease in fertility occur after 9 days of low progesterone treatment (Ahmad et al, 1995;NE-161, 1996;Revah and Butler, 1996;Taft et al, 1996;Taft, 1999). Because the period of lowered progesterone in the present study was approximately 4 days, it may have occurred too early in the life of the follicle or may have been too short in duration to reduce fertility.…”

“…The authors proposed that oocytes from follicles with a duration of dominance greater than ten days resumed meiosis. However, Taft (1999) observed changes in the oocyte associated with the resumption of meiosis after only two days of low progesterone.…”

“…Mihm et al (1999) proposed that a period of 10 days of dominance was required to initiate oocyte maturation in heifers. However, Taft (1999) observed changes in the oocyte associated with the resumption of meiosis after concentrations of progesterone were lowered for two days. Concentrations of progesterone in the study by Taft (1999) increased until day 6, when the injection of PGF 2 α was given.…”

“…Oocyte maturation and the factors that regulate this process in cattle have been reviewed in depth (Bevers et al, 1997;Taft, 1999). Oocytes are arrested in prophase of the first meiotic division until just before ovulation in ruminants.…”

Effects of ovulation of aged follicles, pregnancy diagnosis by ultrasonography, and treatments during lactation on reproduction in ewes

“…Thus, lower progesterone concentrations at the time of growth of the ovulatory follicle are associated with the ovulation of older follicles and decreased fertility. Taft (1999) reduced circulating progesterone on day 6 of the estrous cycle in cattle and obtained evidence that changes in oocyte maturation began within 48 hours of lowered progesterone (day 8). Those characteristics included advancement of stage of maturity, irregularity of the nuclear membrane, degeneration of cumulus cell processes, and changes in shape and clumping of the mitochondria.…”