Epsilon Archive for Student Projects

Abstract

Grey seals (Halichoerus grypus) are top predators in the Baltic Sea and important indicators of ecosystem health. In Sweden, their health status is investigated by long-term monitoring conducted by the Swedish Museum of Natural History (SMNH). Although there is some knowledge about female grey seal reproduction, male sexual maturity and seasonal activity are less studied. This is partly due to the reproductive seasonality, which reduces the availability of comparable samples throughout the year. Understanding these aspects is essential to adequately monitor reproductive health in male seals and detecting deviations from normal patterns over time. The aim of this master’s thesis was therefore to investigate whether histological criteria in testis sections, together with testis weight, could be used to indicate sexual maturity in male Baltic grey seals, including periods when spermatogenetic activity is absent.
From paraffin embedded tissue blocks, histological sections of testicle and epididymis were prepared at SMNH and studied semi-quantitatively under a light microscope. The tissue originated from three presumed juvenile seals and seven adult seals. In addition, sperm smears from nine grey seals were examined for presence of spermatids. Testis weights from 1,042 seals were obtained from the SMNH and analysed for their relation to age and seasonal variation.
The findings in this study indicate that both age and season strongly influences male reproductive physiology. Testis weight increased sharply from 4 to approximately 6 years of age, reflecting the onset of puberty, followed by a gradual plateau. In males older than 8 years sampled outside of the
breeding season, the testis weight was consistently above 50g, while peak testis weight was during January to April. Histological observations of juvenile seal testes included a high proportion of Sertoli cells, and a large amount of interstitial tissue. Adult testes showed thicker seminiferous epithelium, a higher number and variety of spermatogenic cells, and the presence of spermatozoa in the epididymal duct before and after the breeding season. During autumn, outside the breeding season, the seminiferous epithelium was thinner and spermatozoa and spermatids were absent, suggesting arrest at spermatocyte stage.
In order to assess sexual maturity, histological evaluation could be used as a complement to testis weight and macroscopic findings. During the seasonally active period, the finding of spermatids in the seminiferous epithelium, as well as spermatozoa on smears from the epididymis, is indicative of sexual maturity. Outside of the reproductive season, a small amount of interstitial tissue, accompanied by a low proportion of Sertoli cells may serve as an indicator of sexual maturity. Further studies are needed to confirm these findings, as the small sample size, variability in tissue preservation, and lack of tissue samples from certain months and ages limit the current conclusions.