Kinetics of sperm penetration and fertilization in vitro in hamster follicular and oviductal ova

New Insights into the Mammalian Egg Zona Pellucida

Mammalian oocytes are surrounded by an extracellular coat called the zona pellucida (ZP), which, from an evolutionary point of view, is the most ancient of the coats that envelope vertebrate oocytes and conceptuses. This matrix separates the oocyte from cumulus cells and is responsible for species-specific recognition between gametes, preventing polyspermy and protecting the preimplantation embryo. The ZP is a dynamic structure that shows different properties before and after fertilization. Until very recently, mammalian ZP was believed to be composed of only three glycoproteins, ZP1, ZP2 and ZP3, as first described in mouse. However, studies have revealed that this composition is not necessarily applicable to other mammals. Such differences can be explained by an analysis of the molecular evolution of the ZP gene family, during which ZP genes have suffered pseudogenization and duplication events that have resulted in differing models of ZP protein composition. The many discoveries made in recent years related to ZP composition and evolution suggest that a compilation would be useful. Moreover, this review analyses ZP biosynthesis, the role of each ZP protein in different mammalian species and how these proteins may interact among themselves and with other proteins present in the oviductal lumen.

Oviductal secretion and gamete interaction

Experimental evidence from the last 30 years supports the fact that the oviduct is involved in the modulation of the reproductive process in eutherian mammals. Oviductal secretion contains molecules that contribute to regulation of gamete function, gamete interaction, and the early stages of embryo development. The oviductal environment would act as a sperm reservoir, maintaining sperm viability, and modulating the subpopulation of spermatozoa that initiates the capacitation process. It could also contribute to prevent the premature acrosome reaction and to reduce polyspermy. Many studies have reported the beneficial effects of the oviductal environment on fertilization and on the first stages of embryo development. Some oviductal factors have been identified in different mammalian species. The effects of oviductal secretion on the reproductive process could be thought to result from the dynamic combined action (inhibitory or stimulatory) of multiple factors present in the oviductal lumen at different stages of the ovulatory cycle and in the presence of gametes or embryos. It could be hypothesized that the absence of a given molecule would not affect fertility as its action could be compensated by another factor with similar functions. However, any alteration in this balance could affect certain events of the reproductive process and could perhaps impair fertility. Thus, the complexity of the reproductive process warrants a continuous research effort to unveil the mechanisms and factors behind its regulation in the oviductal microenvironment.

Maturation of hamster oocytes under chemically defined conditions and sperm penetration through the zona pellucida

This study aimed to achieve high frequencies of nuclear maturation and penetrability through the zona pellucida of hamster oocytes cultured under protein-free conditions. Completion of nuclear maturation by cumulus-intact, immature oocytes (79% metaphase II stage) was depressed (37%; p < 0.05) by adding four amino acids (glutamine, isoleucine, methionine and phenylalanine) reported necessary for nuclear maturation of cumulus-free oocytes. Following in vitro maturation, cumulus cells were removed and oocytes were inseminated with capacitated sperm, but after 6 h sperm:egg co-incubation, only 24% of in vitro matured oocytes were penetrated compared with 60% of in vivo matured oocytes (p < 0.05). Time required for zona lysis by alpha-chymotrypsin was not significantly different among in vitro and in vivo matured oocytes and 1-cell embryos. Addition to the maturation medium of soybean trypsin inhibitor or fetuin, both known to inhibit the zona reaction in vitro, did not improve penetrability of in vitro matured oocytes, implying that in hamsters, unlike other rodent species, a premature zona reaction is unlikely to be responsible for inhibiting sperm penetration. When oocytes were incubated with 20% periovulatory oviductal fluid (OF) for another 3 h after maturation, penetration was significantly improved (60% vs 37% with and without OF, respectively; p < 0.05), but was not equivalent to penetration of in vivo matured follicular oocytes similarly treated with OF (84%, p < 0.05). However, zona penetration was further improved by increasing sperm concentration from 1.0 x 10(4) (66%) to 5.0 or 10.0 x 10(4) sperm/ml (89%, p < 0.05). This study shows that nuclear maturation of hamster oocytes can occur in chemically defined medium, and indicates that a deficiency in the zona of in vitro matured oocytes can be overcome by preincubation with OF and insemination at high sperm concentration.