Ultrastructural studies were undertaken to investigate structural manifestations of this genetic infertility. A prominent feature of the maturing (35-60 days of age) H^re testis was the presence of vacuoles within the cytoplasm of Sertoli cells. Inclusions were often observed within the nuclei of spermatids. The cytoplasm of these cells contained numerous vacuoles and malformed flagella.

The morphology of seminiferous tubules from mature (over 60 days of age) H^re rat testis was variable. Some tubules displayed only the alterations observed in the maturing testis. Others were characterized by atrophic Sertoli cells and reduced numbers of germ cells. The presence of abnormal sperm heads and flagellar elements in Sertoli cell cytoplasm suggested that degenerating germ cells had been phagocytosed by those Sertoli cells. Nuclei of Sertoli cells often contained membrane-bounded inclusions, and, in older animals, numerous small lipid droplets and vacuoles of varying sizes were distributed in the cytoplasm. Degenerating Sertoli cells were observed occasionally near the lumen of the tubule.

Although there was an apparent increase in the incidence of Leydig cells, their absolute numbers were likely unchanged from normal controls. Leydig cell fine structure was similar to that observed in control rats.

The possibility of a defective response of the seminiferous tubules to FSH is discussed in the light of the observed structural alterations in the H^re rat.