Abstract
The ultrastructural analysis of oocytes and ovarian follicles has been used to evaluate the effects of assisted reproductive techniques, such as cryopreservation or in vitro oocyte maturation. It also benefits the understanding of such complex mechanisms that occur during folliculogenesis. From the beginning of primordial follicles growth until oocyte maturation in preovulatory follicles oocyte cytoplasmic organelles undergo dynamic alterations that reflect physiological changes and development. This review aims to make a retrospective survey of the relevant features of follicles and oocytes ultrastructure, highlighting the differences between mammalian species, specially the domestic ones.Electronic supplementary materialThe online version of this article (doi:10.1186/s13048-014-0102-6) contains supplementary material, which is available to authorized users.
Highlights
Female mammals have hundreds of thousands of oocytes already at the time of birth
Preantral follicles are usually classified in three stages: primordial, primary or secondary follicles [3]
Many studies have focused on the hormonal regulation of the development of large antral follicles, few studies have focused on follicle development at the early stages [7,8,9]
Summary
Female mammals have hundreds of thousands of oocytes already at the time of birth. The ovarian cortex contains follicles at different developmental stages [1,2]; these can be classified according to size, type and number of granulosa cells, or if they are dependent or not on gonadotrophic hormones. The initiation of primordial follicles growth starts a series of morphological changes leading to subsequent stages of follicular development - the primary and secondary follicles (preantral), tertiary and, the preovulatory follicles (antral) [43] These changes can be observed in follicular and oocyte diameter and the number of granulosa cells (Table 2). Mural granulosa cells of antral follicles are rich in Golgi complex, rough and smooth endoplasmic reticulum and small vesicles, as well as round and elongated mitochondria and lipid droplets [80]. In tertiary follicles, all the oocytes are completely surrounded by the zona pellucida, which is crossed by projections of the granulosa cells that form indentations in the oolemma [57] At this time, the organelles have achieved a more even distribution throughout the ooplasm, and elongated mitochondria, lipid droplets and vesicles increase in numbers [66] (Figure 7A). All those morphological changes happen concomitantly with biochemical and molecular modifications (for details see [114,116]), which lead the oocytes to nuclear and cytoplasmic maturation and guarantee their competence to be fertilized
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