The development of the hypothalamic-pituitary-gonadal axis in juvenile great tits

The development of the hypolamic-pituitary-gonadal axis in free-living nestling great tits (Parus major) was studied from the day of hatching until about 40 days of age. The hypothalamic content of GnRH was low during the entire nestling period, although a significant increase was noted between 6 and 9 days of age. Thereafter there was no further increase. The pituitary content of LH was low in 1- to 6-day-old nestlings, but increased significantly at 9 days of age; this increase was only temporary, and levels decreased in 12-day-old nestlings. Injections of chicken GnRH increased plasma levels of LH in newly hatched great tits. The high plasma levels of LH in newly hatched great tits of both sexes decreased to basal by 9 days of age. Circulating levels of testosterone and estradiol showed more or less the same pattern as LH. Both steroids showed very high levels in newly hatched birds of both sexes. Testosterone levels were down to basal in 3-day-old nestlings, and estradiol levels were basal in 2-day-old nestlings. The only sex difference was observed in 1- and 2-day-old nestlings. During this period males had significantly higher plasma levels of testosterone than females. Gonads showed a more or less linear growth during the entire nestling period, and did not increase further after fledging. Histological and ultrastructural examinations of the testis showed that mature Leydig cells appeared for the first time in 6-day-old birds and become common in 9-day-old nestlings. The development of the seminiferous tubules, the interstitial tissue, and the tunica albuginea reached a "plateau" when the nestlings had reached an age of 9 days. At no age had the germinal epithelium developed beyond the stage of spermatogonia. GnRH treatment did not change male testosterone or female estradiol levels until the nestlings were 9 days of age. The response decreased in older nestlings. Castration of 9-day-old male nestlings showed that neither was there a negative feedback of testosterone on LH in older nestlings.