Rapid Leydig Cell Proliferation and Luteinizing Hormone Receptor Replenishment in the Neonatal Rat Testis after a Single Injection of Human Chorionic Gonadotropin1

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Abstract
Two-day-old rats were stimulated with a single dose of human chorionic gonadotropin (hCG). Changes in the Leydig cell number, mitotic activity, cell size, and number of luteinizing hormone (LH) receptors were studied. The Leydig cell number of the hCG-treated animals was 1.8 times that of the control on Day 1 and remained elevated for the rest of the 5-day experiment (p less than 0.0001). On Day 1 the number of Leydig cell mitoses in the hCG group was greater (p less than 0.05) than in the controls. The Leydig cell size increased transiently to two times that of the control (p less than 0.01) within the first day after the treatment and returned to control size by Day 5. The number of LH receptors per testis decreased 81% in 1 day (p less than 0.01), but returned to control level by Day 3. Since Leydig cell numbers were constant after Day 1, the rapid receptor recovery was obviously due to restoration of the binding sites rather than increased cell number. The present results demonstrate a rapid proliferative response and rapid LH receptor replenishment in the fetal-neonatal Leydig cells after gonadotropic stimulation. These responses of fetal-type Leydig cells are in clear contrast to those observed in adult testes after a similar stimulation.