THE INFLUENCE OF INCUBATION-TEMPERATURE ON OOCYTE MATURATION, PARTHENOGENETIC AND EMBRYONIC-DEVELOPMENT INVITRO OF THE MARSUPIAL MONODELPHIS-DOMESTICA

Forty-nine oocytes and 111 embryos from 34 female Monodelphis domestica were used to examine the effect of incubation temperature on oocyte maturation and embryonic development in vitro. An additional 19 females were not used because they were not cycling (7), had unfertilised eggs (7) or had retarded embryos (5). The incubation medium was Dulbecco's modified Eagle's medium with high glucose (4.5 g l-1) and 10% fetal calf serum held at 5% CO2 in air. The temperatures used for incubation were 32.6-degrees-C, the body temperature of M. domestica, or 37-degrees-C, a commonly used temperature for incubation of marsupial cells. At 37-degrees-C, fewer oocytes successfully eliminated the first and second polar bodies (63% and 17%, respectively) and successfully underwent the polarisation of yolky cytoplasm characteristic of activation of oocytes ( 17%), than at 32.6-degrees-C. At 32.6-degrees-C, 80% eliminated the first polar body, 28% eliminated the second polar body and 60% underwent polarisation of cytoplasm. Two oocytes (8%) developed parthenogenetically to the two-cell stage at 32.6-degrees-C. Embryos incubated at 37-degrees-C completed fewer divisions, had a slightly increased rate of cleavage and showed an increased tendency to degenerate than embryos cultured at 32.6-degrees-C. The zygote had markedly polarised cytoplasm; yellow yolky cytoplasm lav in one hemisphere and darker cytoplasm in the other. The first and second divisions were associated with extrusion of yellow cytoplasm as yolk vesicles and the dark cytoplasm as amorphous material into the perivitelline space. The first division was meridional. The cleavage planes of the second and third divisions varied according to the amount of yolk, from meridional to latitudinal. The fourth and fifth cleavage planes were latitudinal to obliquely latitudinal.