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Relationship Between LH and Testicular Development in Progesterone-Implanted Prepubertal Ram Lambs¹

US Department of Agriculture², Clay Center, NE 68933

Abstract

The relationship between systemic luteinizing hormone (LH) and testicular development was investigated in Finn and Suffolk ram lambs treated at 2 wk of age with sc progesterone implants for 0, 4, 8 or 12 wk. Serial samples of blood (30-min intervals for 3 h) were collected from five rams·breed^–1·treatment^–1 at 6, 8, 10, 12, 14, 18 and 22 wk of age plus 1 and 2 wk after implant removal. Scrotal circumference was measured at 10, 14, 18 and 22 wk of age. A testicular biopsy was obtained at 14, 18 and 22 wk for microscopic evaluation of testicular development and spermatogenesis. Pulsatile LH releases occurred 60 to 180 min apart in control lambs between 6 and 22 wk of age; LH secretion was not affected by age except for a decrease (P<.01) in mean LH at 22 wk. Transient increases in testosterone were found subsequent to LH pulses. Systemic testosterone increased (P<.01) progressively with age, was higher (P<.05) in Finn than in Suffolk rams at 18 and 22 wk and was correlated positively (P<.01) within breeds with seminiferous tubule diameter at 14 wk and with scrotal circumference at 10 and 14 wk. Tubule diameter was larger (P<.01) but testes size and weight were smaller (P<.01) in Finn than in Suffolk rams. Progesterone implants decreased (P<.01) LH secretion at 4, 6, 8, 10 and 12, but not at 14 wk of age; decreased systemic testosterone between 4 and 14 wk; decreased (P<.01) seminiferous tubule diameter at 14 wk, which was related inversely to duration of progesterone treatment; and delayed puberty from 18 to 22 or more wk in rams implanted from 2 to 10 or 14 wk of age. Both control and treated rams with elongated spermatids at 18 vs 22 or more wk had increased LH secretion, higher systemic testosterone and larger tubule diameter and testes size at a younger age. These results suggest that rate of sexual maturation in ram lambs is related to level of postnatal LH stimulation and to the prepubertal age when increased LH stimulation occurs.

¹ The authors gratefully acknowledge the laboratory expertise of Ms. Shari Ellis and Mr. William McKay.

² Agricultural Research Service, Roman L. Hruska U.S. Meat Animal Research Center.