Control of follicular growth: Local interactions and nutritional influences

ABSTRACT Regulation of ovarian activity is an integrated process encompassing both extraovarian signals and intrafollicular factors. Initiation of primordial follicle growth and the early stages of folliculogenesis can occur without gonadotropins, but FSH may affect the rate of preantral follicle growth. Antral follicle development from 1 to 4 mm in sheep and cattle is completely gonadotropin dependent. These recruited follicles express a range of mRNA encoding steroidogenic enzymes, gonadotropin receptors, and local regulatory factors and their receptors. As follicles continue to mature, there is a transfer of dependency from FSH to LH, which may be part of the mechanism involved in selection of follicles for continued growth. Locally produced growth factors, such as the IGF and members of the transforming growth factor-β superfamily, work in concert with gonadotropins throughout the follicular growth continuum and can have significant effects on follicle selection. Environmental influences, such as changes in nutrition, also have an effect on ovarian activity. This can occur without significant variation in circulating gonadotropin concentrations and can be correlated with changes in circulating concentrations of metabolic hormones, including insulin, IGF-I, GH, and leptin. Nutrition can also affect the expression of mRNA encoding components of the ovarian IGF system to regulate the sensitivity/response of follicles toward gonadotropins. Hence, the roles of growth factors in follicular development and survival depend on gonadotropin status and differentiation state of the follicle. In conclusion, it is the integration of these extraovarian signals and intrafollicular factors that determine whether a follicle will continue to develop or be diverted into atretic pathways.

Key Words: Bovine, Follicle, Gonadotropins, Growth Factors, Nutrition

© 2004, by the American Society of Animal Science. All rights reserved.

J. Anim. Sci. 2004. 82(E. Suppl.):E63-E74

Implications

Despite recent progress, the precise mechanisms underlying the follicular growth continuum have yet to be fully elucidated. Gonadotropins are certainly the main driving force for antral follicle development, but also interact with a range of local growth factor systems. Extraovarian factors, such as nutritionally mediated changes in metabolic hormones, also directly affect follicle development and oocyte quality. Diets that are optimal for follicle growth may not necessarily be optimal for oocyte quality. Hence, diets are required that both optimize oocyte quality while maintaining follicle development. This is of key importance because oocyte quality will impinge on subsequent embryo survival. All these factors must be considered when developing approaches to first halt and then reverse the downward trend in dairy cow fertility.