Developing testicular microvasculature in the golden hamster, Mesocricetus auratus: a model for angiogenesis under physiological conditions.

The ultrastructure of the developing testicular microvasculature in the testes of immature (3, 5, 8, 10, 12, 16, 20, 25, 30 and 35 days old) golden hamsters was examined and compared to the testicular microvasculature of adult (3 months old) hamsters. In addition, in 16- to 35-day-old hamsters vascular permeability was studied after localization of injected horseradish peroxidase (HRP). Angiogenic processes were present in the testes of all examined immature hamsters and were most conspicuous between 8 and 25 days of age. These processes were absent in the testes of 3-month-old hamsters. On days 3 and 5, few undifferentiated blood vessels with activated endothelium were present in the interstitial spaces. Endothelial cell migration started from these 'mother vessels' and led to invasion of intertubular spaces by vascular sprouts, before vascularization of peritubular spaces occurred (after day 12). Sprouting endothelial cells were identified by the presence of a basal lamina and characterized by abundant cytoplasm and cell organelles. HRP-positive slits were seen in developing vessels, which opened to form the vascular lumen. HRP exited the vascular lumen through unspecialized endothelial contacts and micropinocytotic vesicles. By day 16, the blood-testis barrier prevented HRP from entering the seminiferous tubules beyond the basal compartment. By days 30 and 35 most testicular microvessels and at the age of 3 months all testicular microvessels were of the mature type, with narrow inactive endothelium and specialized cell contacts (including tight junctions). These results demonstrate that the postnatal vascularization of the testis in the golden hamster is a timed complex process. Due to high permeability, vascular sprouts are likely to influence the metabolic situation and thus the maturation processes of the testis. Angiogenesis in the golden hamster testis shares typical morphological features with angiogenic processes in other organs and species under various pathological and physiological conditions. We therefore conclude that the postnatal testis can be viewed as a physiological model of angiogenesis.