Ovarian Response, Follicular Function and Oocyte Developmental Competence in Gonadotropin Treated Prepubertal Calves
Date
2019-12-18
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ORCID
Type
Thesis
Degree Level
Doctoral
Abstract
Sexual immaturity during the prepubertal period in cattle is characterized by low pulse frequency of LH, anovulatory waves of follicle development, absence of corpora lutea (i.e., progesterone) and oocytes of reduced developmental capacity in vitro when compared to oocytes of sexually mature animals. However, calf ovaries are responsive to exogenous gonadotropin treatment early in life, and the use of prepubertal animals as a source of oocytes for in vitro embryo production may have significant potential to decrease generation interval and accelerate the rate of genetic gain. The overall objective of this thesis was to investigate the effect of exogenous FSH treatment on the ovarian response, hormonal profiles, ovulation and oocyte developmental competence in prepubertal calves.
In the first study (Chapter 3) the effect of cumulative dose (200 mg vs. 350 mg) and duration (4 vs. 7 days) of FSH treatment on the ovarian response and the number of spontaneous and induced ovulations in 5-month-old calves were compared. Calves (n=24) were selected for gonadotropin treatment from a group of spring-born calves (n=51) based on the antral follicle counts (AFC) at the time of wave emergence. Calves were classified in low, medium and high AFC, and the ones in the medium classification (250.8, range of 20 to 32 follicles) were used in this study. At the end of the FSH treatment and 24 hours after treatment with pLH, the number of follicles 9 mm was greater in the 7-day than in the 4-day treatment group and in calves given a cumulative dose of 350 mg of FSH compared to those given 200 mg. Spontaneous ovulations were observed in 14 calves between Day 4 of FSH treatment and 12-hours post-LH treatment. The number of total and spontaneous ovulations was higher in the 7-day treatment groups than in the 4-day groups, and the number of spontaneous ovulations was higher in calves given a cumulative dose of 200 mg FSH than 350 mg. Numbers of ovulations in response to exogenous LH did not differ among groups.
In the second study (Chapter 4), data showed in prepubertal calves (n=46) that the number of follicles at the beginning of a wave was predictive of the number recruited into subsequent waves and that after FSH treatment, the number of medium and large sized follicles available for follicular aspiration was positively associated with the number of follicles 1 mm at the time of wave emergence. In calves with low (n=12) and high (n=10) AFC at wave emergence, 7 days of FSH treatment resulted in a higher number of large than small size follicles than the 4 days of FSH treatment. High AFC at wave emergence resulted in a greater number of follicles 6 mm available for aspiration and a greater number of cumulus oocyte complexes (COC) collected than low AFC.
The third study (Chapter 5) was designed to investigate the relationship between the antral follicular counts and plasma concentrations of AMH and FSH at the time of wave emergence in prepubertal calves and to compare the effects of age and duration of gonadotropin treatment on ovarian response of prepubertal and pubertal cattle. The AFC and the plasma concentrations of AMH at the time of wave emergence were positively correlated and both were positively correlated with the number of follicles 6 mm at the time of oocyte collection. Ovarian response was greater in calves at 4 months of age than at 7 months and was characterized by a higher number of medium to large (i.e., 6mm) sized follicles and higher number of recovered oocytes following transvaginal ultrasound-guided aspiration. Prolonged follicular growth by the 7-day treatment resulted in a greater number of large follicles at the end of the treatment and a greater degree of follicular maturation, characterized by lower intrafollicular E2:P4 ratio and higher proportion of fully expanded COC after LH treatment.
In the fourth study (Chapter 6), different methods to control endogenous LH release and prevent ovulations during 7 days of exogenous FSH treatment were investigated. A long-acting progesterone (Long-acting P4) treatment at the time of follicular ablation and the GnRH antagonist Cetrorelix given at 48 hours intervals during FSH treatment were effective in preventing endogenous LH release and spontaneous ovulations during 7 days of exogenous FSH treatment. Ovulations were observed after FSH treatment in the Long-acting P4 treatment groups, while ovulations were prevented in calves of the Cetrorelix group. Luteal structures formed due to spontaneous ovulations were functional and a positive correlation was observed between the number of structures and the plasma concentrations of progesterone.
In Chapter 7, oocyte developmental competence following in vitro vs. in vivo maturation of oocytes from prepubertal calves and in vivo maturation of oocytes from calves and pubertal heifers given 4 or 7 days of exogenous FSH treatment were evaluated in Study I. Spontaneous ovulations before oocyte collection were observed in calves and prevented meaningful comparisons among groups, resulting in lower rates of cleavage and blastocyst in the 7-day in vivo group. Blastocyst rates did not differ between 4-day in vivo vs. 4- and 7-day in vitro. In Study II, 4, 6 and 7 days of exogenous FSH treatment under controlled endogenous LH release were compared and six days of exogenous FSH support was associated with the greatest developmental competence of oocytes collected from 5-month-old calves.
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Keywords
follicular growth, superstimulation, puberty, oocyte competence, cattle, gonadotropin
Citation
Degree
Doctor of Philosophy (Ph.D.)
Department
Veterinary Biomedical Sciences
Program
Veterinary Biomedical Sciences