Abstract
Preantral follicles are the majority of the ovarian follicle population and their use as a source of homogeneous oocytes for bovine reproductive biotechnologies could result in a substantial advance in this field. However, while in other species embryos and offspring have been produced, in bovine species the results have been limited to the follicular activation of small (primordial) preantral follicles and formation of early antral follicles from large (secondary) preantral follicles after in vitro culture. Therefore, this review will highlight the basic aspects of bovine folliculogenesis by focusing on preantral follicles, the methods of harvesting preantral follicles, the main results from in vitro follicular culture during the last 20 years, and the potential candidate substances (basic supplements, growth factors, and hormones) for improving the efficiency of in vitro follicle growth.
Highlights
The mammalian ovary is responsible for the development, maturation, and release of mature oocytes for fertilization, as well as for the synthesis and secretion of hormones that are essential for follicular development, menstrual/estrous cyclicity, and maintenance of the reproductive tract and its function
Considering that primordial follicles constitute the supreme starting material for in vitro culture due to their large number when compared with mature follicles [2], it would be of remarkable help to possess a renewable source of primordial follicles from high-yielding animals for culture in order to maximize offspring from these animals [3]
In vitro culture systems for bovine preantral follicles For bovine and other farm domestic species, the development of culture systems capable of supporting the growth of immature follicles to a stage where they could be matured and the oocyte fertilized would ensure a large supply of oocytes for manipulation
Summary
The mammalian ovary is responsible for the development, maturation, and release of mature oocytes for fertilization, as well as for the synthesis and secretion of hormones that are essential for follicular development, menstrual/estrous cyclicity, and maintenance of the reproductive tract and its function. Maintaining communication among the oocyte, the surrounding somatic cells, and the extracellular matrix is vital to the achievement of normal folliculogenesis, and to sustain follicular growth and viability [58] This technique allows the isolation of several morphologically normal and intact follicles from ovarian tissue. In vitro culture systems for bovine preantral follicles For bovine and other farm domestic species, the development of culture systems capable of supporting the growth of immature follicles to a stage where they could be matured and the oocyte fertilized would ensure a large supply of oocytes for manipulation These oocytes could potentially be used to shorten the generation interval of selected animals and, to increase the number of offspring born per animal. In both species, only a few antral follicles were obtained after 4 [81] or 15 days [79] of culture of secondary follicles
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