Testicular development and testosterone concentrations in the testis and plasma of young male shrews ( Suncus murinus)

Testicular development and testosterone concentrations in the peripheral plasma and testes of young, adult, and castrated male musk shrews ( Suncus murinus) were examined. Fifty immature, 24 adult, and eight castrated males were used. Immature males ranged from 5–45 days of age. Plasma and testis homogenates were assayed for testosterone by radioimmunoassay. Testes were examined histologically for spermatogenic development. Plasma testosterone concentrations increased gradually from Day 5 to Day 24, then rapidly to Day 30. There was a sharp decrease between Day 30 and 33, then a gradual increase to Day 45. Testis testosterone concentrations followed the same pattern, except that on Day 5, the concentration was significantly higher than on Day 10. Adult males had higher testis testosterone and lower plasma testosterone concentrations than Day 45 males. Castrated males had barely detectable plasma testosterone levels. Testis weights increased linearly with age, except for a slight decrease between Day 30 and 33. Spermatogenic development was already initiated by Day 5. Cap-phase spermatids were present on Day 23, acrosome-phase spermatids on Day 30, and mature spermatozoa on Day 31. Epididymal sperm were present by Day 45.