Prospective ultrasonographic and endocrine predictors of spermatogenic onset in ram lambs

- The age at puberty is a marker of future reproductive health in mammalian species.
- Testicular ultrasonographic characteristics were determined in prepubescent ram lambs.
- Testicular volume had the most stable relationship with pubertal timing.
- Spermatogenic onset also correlated with serum concentrations of FSH/thyroid hormones.

The objective of this study was to examine testicular ultrasonographic characteristics and endocrine profiles in prepubescent ram lambs for correlations with the age at first detection of elongated spermatids (ESt age). Bi-weekly ultrasound examinations and weekly testicular biopsies began at 10 weeks of age or at the time that testicular volume reached 15 cm3, and continued until 1-2 weeks after Est's were first detected by histological examination of testicular biopsies in twenty-two spring-born Rideau Arcott × Polled Dorset lambs. Computer-assisted analysis of testicular ultrasonograms was performed using commercially available image analytical software. Blood samples were drawn before each ultrasonographic examination and were used for measurements of free triiodothyronine (fT3) and thyroxine (fT4), and follicle-stimulating hormone (FSH) concentrations. The mean (±SEM) age at first detection of ESts was 15.9 ± 0.5 weeks. Testicular volumes recorded between 10 and 12 weeks of age correlated inversely with the ESt age (r = −0.44 to −0.50, P ≤ 0.05). Statistically significant correlations were recorded between the ESt age and numerical pixel values of testicular parenchyma at 10 (r = −0.48, P = 0.05) and 15 (r = 0.52, P = 0.05) weeks of age, and between the ESt age and testicular pixel heterogeneity in ram lambs aged 14.5 weeks (r = 0.60, P = 0.007). Lastly, circulating FSH concentrations at 10 weeks (r = −0.43, P = 0.05), serum fT3 concentrations at 13 weeks (r = 0.44, P = 0.04) and fT4 concentrations at 11.5 weeks of age (r = 0.48, P = 0.03) were all correlated with the ESt age. The present results show that testicular volume has the most stable relationship with pubertal onset; however, testicular echotexture as well as circulating concentrations of FSH and free fractions of thyroid hormones at specific ages may be indicative of more intricate developmental events heralding puberty.