Background: The in vitro culture of preantral follicles is an important tool to elucidate the mechanisms involved in controlling
early folliculogenesis as well as to improve reproductive efficiency by providing a large number of oocytes for in vitro
production of embryos. To this end, improvements in the performance of current culture systems are needed, including
changes in the composition of the medium. Supplementation of culture medium with proteins is a great strategy to improve
survival and in vitro development of preantral follicles. This review highlights the importance of in vitro culture of preantral
follicles with emphasis on the main protein sources used to supplement the culture media.
Review: Throughout the study were discussed issues related to the use of protein substances to culture medium of preantral
follicles, highlighting the fetal bovine serum, bovine serum albumin and follicular fluid, and demonstrating the importance of
each these substances to follicular development. The fetal bovine serum (FBS) is a protein supplement widely used in in vitro
culture of preantral follicles, composed of various substances, including proteins, carbohydrates, amino acids, growth factors
and hormones which maintain the viability and promote the growth of these follicles. Although several studies have found that
serum promotes beneficial effects such as maintaining the viability and promotion of follicular growth, other studies showed
that serum decreased membrane integrity, increased extrusion rates and induced luteolysis. These contradictory results show
that it is still difficult to determine which substances in the serum are necessary for development of follicles, oocytes or even
embryos. Moreover, it is noteworthy that the results repeatability when using the serum may vary. In semi-defined culture
media, bovine serum albumin (BSA) is a protein supplement used in replacing the serum, being a protein of high biological
value and their various properties. BSA may act as a surfactant, heavy metals chelator or also as a scavenger of reactive oxygen
species. These properties contribute to maintain follicles structure favoring their survival. Nevertheless, the role of albumin
during culture is not well understood, since the BSA is a highly variable substance. Another protein supplement widely used
is the follicular fluid (FF), whose composition has been studied recently in an attempt to increase knowledge about the
follicular development and atresia. Studies show that FF may acts in maintaining the follicular survival and growth similar to
serum. On the other hand, studies have shown an inhibitory effect of this substance on the follicles growth. Similar to SFB, the
beneficial effect of supplementation with FF is still not well understood because of the great variability of results, which occurs
due to the difference in the composition of the samples collected.
Conclusion: Although great progresses have been obtained in adapting the culture medium of preantral follicles in different
species, including domestic animals, the results are still unsatisfactory. Therefore, there is the necessity to optimize the existent
culture systems, which can be obtained with protein supplementation, the major component of in vitro culture of this follicular
category. However, further studies are still needed in order to provide more information about the mechanism of proteins action
on follicular cells.
