Role of Luteinising Hormone in ovarian follicle development and maturation in the mare
View/ Open
Schauer2013.docx (16.06Mb)
Date
06/07/2013Author
Schauer, Stephanie Nicole
Metadata
Abstract
Luteinising hormone (LH) is a crucial regulator of ovarian follicle maturation,
ovulation and luteinisation. Development of healthy follicles and fertile ovulation
can only occur within a specific range of circulating LH concentrations, with
differing upper and lower limits depending on the stage of the oestrous cycle. The
objective of the three studies in this thesis was to investigate the effects of both
physiological and non-physiological circulating LH levels on equine follicular
maturity by examining ovulatory and steroidogenic capacity, gene expression
profiles and miRNA expression in ovulatory-size follicles at various stages of the
oestrous cycle and/or in response to supplementation with LH.
The aim of the first study was to investigate the hypothesis that deficient circulating
LH is a primary cause for the inability of equine follicles to ovulate during the
physiological anovulatory season. A LH-rich equine pituitary fraction (eLH) given
twice daily to early transitional mares did not restore steroidogenic capacity of the
ovulatory-size follicle or advance the onset of the natural breeding season; however,
it significantly stimulated follicular growth to a level similar to that occurring during
the normal oestrous cycle. The results demonstrated that a deficiency in LH is
critically involved in reduced follicle growth during the anovulatory season.
The second study examined the effects of elevated circulating LH levels early during
follicle development on follicle maturation and ovulatory ability in cycling mares,
with the hypothesis that excessive LH would disrupt ovulation and produce
haemorrhagic anovulatory follicles (HAFs). Treatment with eLH or a luteolytic dose
of prostaglandin F2α (to stimulate an increase in endogenous levels of LH) did not
have any effects on follicle growth or ovulation, but did impair follicular production
of androstenedione and insulin-like growth factor 1 (IGF1), suggesting a deleterious
effect of high LH on follicle and oocyte maturation.
The third study examined the expression of different follicular factors associated
with follicle maturation as well as microRNAs (miRNAs) in ovulatory-size follicles
naturally developing under different LH milieus (oestrus, dioestrus and spring
transitional period). Progesterone and IGF1 were significantly reduced in follicles
developing in a low LH environment (dioestrus and transition). All four miRNAs
measured, miR-378, miR-542, miR-202 and miR-21 were found at higher levels in
subordinate follicles than in preovulatory follicles during oestrus. In addition miR-
202 and miR-21 were significantly increased in transitional follicles relative to
oestrous follicles. The results of this study indicate that follicles developing during
both the spring transitional and dioestrous periods are developmentally immature and
suggested potential important roles of miRNAs in follicle maturation in the horse.
In summary, although LH is a key factor promoting follicular growth, it is by itself
not sufficient to restore steroidogenic activity in transitional follicles. Elevated LH
levels during follicle development do not disrupt ovulation, but induce changes in
follicular fluid factors related to follicle maturation and oocyte quality. Follicles
developing under different LH milieus show altered miRNA expression, suggesting
an important role of miRNAs in follicle maturation.