Ovulation occurs in Sminthopsis macroura approximately 160 hr after administration of 1.3 IU PMSG, and yields significantly more oocytes than does spontaneous ovulation (P = 0.001). Germinal vesicle (GV)-stage oocytes have a thin cortical rim of microfilaments, which is disrupted by exposure to cytochalasin D. After GV breakdown, the first meiotic spindle forms subcortically and parallel to the oolemma. It rotates during anaphase and telophase to extrude the first polar body. This rotation is associated with a local cortical concentration of microfilaments, which is extruded in the first polar body. The second meiotic spindle is orthogonal to the surface, and extrusion of the second polar body is not associated with obvious local changes in cortical actin, resulting in a polar body containing little polymerized actin. The sites of second polar body emission and sperm entry are always in the half of the oocyte opposite the concentrating yolk mass, and are within 60 degrees of each other in most oocytes. During the concentration and eccentric movement of the yolk, microfilaments condense around it. During yolk expulsion, these microfilaments become continuous with those located subcortically. During early cleavage, the cytocortex of the zygote, but not of the extruded yolk mass, stains heavily for polymerised actin. Multiple sites of pericentriolar material are detectable in the cytoplasm of some secondary unfertilized oocytes which, in the presence of taxol, generate large cytasters and pseudospindle structures. After fertilization, a large aster is formed in association with the sperm entry point and serves as the center of an extensive cytoplasmic network of microtubules which surrounds but does not enter the yolk mass. Taxol treatment generates small cytasters within this meshwork and promotes selective stabilization of some periyolk microtubules opposite to the sperm aster.

The timetable of oogenesis in Sminthopsis macroura is accelerated like in other marsupials showing relatively early maturation of the female. On the day of parturition (day 0) migration of primordial germ cells to the indifferent gonads has been completed. Follicular growth seems not to correspond to the biphasic pattern, in which oocyte and follicle grow synchronously until antral stages when only the follicle increases in size, but shows a continuous growth of the oocyte and the follicle up to the time of ovulation. During primordial and early primary follicle stage a paranuclear complex is present in the oocyte, consisting mainly of smooth tubules of endoplasmic reticulum. Cortical granules appear early in oocytes in secondary follicles. The conspicuous inclusions in the antral follicle are the clusters of electron-lucent vesicles in the oocyte. These inclusions grow from multivesicular bodies (MVB), which are formed from Golgi and endoplasmic reticulum vesicles. Further increase in the size of MVB involves the incorporation of endocytic vesicles and the coalescence of larger vesicles. The polarized nature of the oocyte at ovulation is due in part to the to accumulation of these vesicles in the cytoplasm opposite the eccentrically placed nuclear material.

Induced ovulation resulting in normal embryos is rare in marsupials. In this study natural and induced ovulations were compared in mature Sminthopsis macroura (n = 122). Comparison of maturation of preovulatory oocytes by ovarian histology and examination of oocytes removed from developing follicles in 12 ovaries of 23 animals receiving 0.058 iu equine serum gonadotrophin (eSG) g(-1) with ovaries of 12 animals undergoing natural cycles showed that oocyte maturation was significantly more irregular when it was induced (P < 0.001). Postovulatory stages were examined by estimating the number of eggs ovulated from ovarian histology, and by counting oviduct and uterine contents recovered after ovulation. S. macroura receiving 0.087 iu eSG g(-1) (n = 34), administered as one (n = 17) or two (n = 17) injections, were significantly (P < 0.05) more likely to ovulate (74%), mate (80%) and have conceptuses (66%) than were animals receiving 0.058 iu eSG g(-1) (12, 53 and 0%, respectively) (n = 17), and the values were similar to those in animals (n = 36) undergoing natural cycles (100, 81 and 56%, respectively). Induced ovulation using 0.087 iu eSG g(-1) yielded significantly (P < 0.05) more oocytes per ovary (20.8 +/- 8.5; combined data) than did ovulation in animals undergoing natural cycles (13.7 +/- 3.2) (ANOVA, t test). The responses of animals induced in different phases of the oestrous cycle with 0.087 iu eSG g(-1) were not significantly different (ANOVA) with respect to the number of corpora lutea per ovary, conceptuses per animal or days to ovulation after injection. However, the proportion of females that responded after receiving 0.058 iu eSG g(-1) in the luteal phase was significantly different from that in animals treated with the same dose in the intermediate phase (P < 0.01) and in non-cyclic females treated with 0.058 iu eSG g(-1) (P < 0.02). The main benefits of the treatment were that normal embryos resulted and that 70-78% of non-cyclic animals could be induced to ovulate.