Migration efficiency of paired sperm in the tract of the peri-ovulatory female grey short-tailed opossum (Monodelphis domestica)

American marsupials are the only mammals known to ejaculate paired spermatozoa, which confer a motility advantage in vitro over single spermatozoa in viscous environments. In the only American marsupial examined, the Virginian opossum (Didelphidae), relatively low numbers of spermatozoa are ejaculated (106), but transport is extremely efficient with ~1 in 20 spermatozoa reaching the site of fertilization compared to ~1 in 10,000 in the rabbit. This study examined the post-copulatory distribution and state (paired or single) of spermatozoa at various times in the female reproductive tract of another didelphid, the polyovular grey short-tailed opossum (Monodelphis domestica). After a single mating, the reproductive tracts of 19 females were dissected at 0.5 (n=4), 6 (n=4), 12 (n=3), 18 (n=3) and 24h (n=5) post coitum (p.c.). Each tract was dissected into 8 major anatomical sections and spermatozoa were recovered by flushing. Mating occurred 5.4 ± 0.4d (mean ± SEM; n=19) after pairing, copulation lasted 4.4 ± 0.2 min (n=18) and ovulation occurred 18.0 - 24.1h p.c. (n=5). Shortly after mating (0.5h p.c.) the tract contained 1.2 ± 0.2g of seminal gel (n=2) and 2.0 ± 1.3 x106 spermatozoa (n=3; 38% of which were paired) found predominantly in the anterior vaginal culs de sac. A uterine sperm reservoir was never observed, but spermatozoa reached the isthmus and ampulla within 6 and 18h p.c. respectively. Paired spermatozoa localized almost exclusively in the isthmus from 6h p.c., and pairing decreased to only 4% of the total sperm population in the tract by the start of ovulation. In total ~1 in 300 ejaculated spermatozoa (~6.5 x103; n=9) reached the oviduct. In conclusion, sperm pairing appears to confer effective colonization of the isthmus in M. domestica and, like the Virginian opossum, transport of spermatozoa is relatively efficient

Kidney development in the Brazilian grey short-tailed opossum (Monodelphis Domestica)

Kidney development was examined by histological methodology in the opossum Monodelphis domestica from birth through to 21 days of age. The pronephros and metanephros are rudimentary in the neonate, the well-developed mesonephros being the functional kidney. Regression of the latter has commenced by day 10, at which time renal corpuscles and tubules are detectable in the metanephros. By day 16 both kidneys are well-formed and by day 21 the mesonephros has degenerated and the metanephros, composed of glomeruli, Bowman’s capsules and tubules, has taken over as the functional kidney. No evidence of a mesonephric contribution to the gonads was found

The cremasteric neuromuscular complex in male and female grey short-tailed opossums (Monodelphis domestica)

In marsupials, both sexes possess a cremaster muscle, that of the female being attached to the mammary gland and surrounding subcutaneous tissues. In the grey short-tailed opossum, the muscle in both sexes has a distinct proximal origin from the anterior superior iliac spine. The number of muscle fibres is higher in male than in female opossums on average, but the difference is not statistically significant. Retrograde tracing, achieved by injecting B-cholera toxin into the muscle, showed that labelled neurons occupied several spinal cord segments, the bulk being in L2–L4. There was no overall sex difference in the number of labelled neurons. It is concluded that cremaster muscles are present in both sexes of the grey short-tailed opossum Monodelphis domestica , contain approximately equal numbers of fibres, and differ only in their insertion target. Since the motor innervation also shows no dimorphisms in this species, it is concluded that the whole neuromuscular complex is homologous in the two sexes. This is in marked contrast to the situation in adult eutherian mammals where the muscle is possessed by males only, and where the cremasteric nucleus is highly dimorphic in terms of motor neuron numbers