| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
|
|
|
|||||||
|
|||||||||
Biology of Reproduction, Vol 25, 85-92, Copyright © 1981 by Society for the Study of Reproduction
1 Reproductive Endocrinology Program,
Department of Obstetrics and Gynecology and
Division of Biological Sciences,
The University of Michigan,
Ann Arbor, Michigan 48109 The influence of season of birth and photoperiod on the mechanism governing initiation of
ovulation in lambs was studied. The ages at the decrease in responsiveness to estradiol-feedback
inhibition of tonic LH secretion and first ovulation in lambs born out of their normal birth season
(fall) was compared with those in lambs born during the normal season of birth (spring). The
change in feedback responsiveness was determined from the pattern of circulating LH in ovariectomized lambs treated chronically with estradiol (Silastic implant) from 20 weeks of age. First
ovulation was determined from the pattern of serum progesterone in intact lambs. In control lambs
born in spring (March) and raised in natural photoperiod, first ovulation occurred at 31 ± 1 (mean
± SEM) weeks of age, about 4-8 weeks after the onset of the breeding season of adults. Lambs
born in the fall (October) and reared in natural environment exhibited no evidence of ovulation at
26-35 weeks, ages which in fall-born females are attained during the anestrous season; rather,
ovulations were delayed until shortly after the onset of the breeding season when they were 49 ± 1
weeks old. Rearing fall-born lambs in an annual photoperiod similar to that which they would have
experienced had they been born in the spring, markedly reduced the delay in onset of cycles. In
these females, ovulations were initiated at 35 ± 0.5 weeks. In each of the three groups, the decrease
in responsiveness to negative feedback, as reflected by the increase in serum LH in estradiol-treated
lambs, occurred at the time ovulations began in intact females. Relative to spring-born lambs, it
was delayed in fall-born lambs under natural environment and was restored to near normal in
fall-born lambs under reversed photoperiod. The results lead to the hypothesis that the delay in onset of ovulation in lambs born out of
season (fall) is due to a photoperiod-induced delay of the decrease in responsiveness to estradiol
inhibition of tonic LH secretion. When viewed in the context of mechanisms governing sexual
maturation in the spring-born lamb and seasonal breeding in the adult, two conceptually different
explanations emerge to account for the delay in fall-born lambs: 1) development is retarded by
photoperiodic cues, and delayed onset of cyclicity reflects late sexual maturity; 2) development
is not retarded; instead photoperiod induces seasonal anestrus at sexual maturity and the onset of
cyclicity simply reflects the onset of a breeding season.
Note:
ACKNOWLEDGMENTS
The assistance of several colleagues is gratefully
acknowledged, especially that of Drs. Kathleen D.
Ryan, Fred J. Karsch, Robert L. Goodman, and Eric
L. Bittman for their constructive criticism of the
design of the study and interpretation of results and
Drs. Gordon D. Niswender and Leo E. Reichert for
providing reagents used for radioimmunoassay. Valuable technical assistance was given by Mr. Douglas D.
Doop and Ms. Karen S. Bergman in surgery, construction of estradiol capsules, and collection of blood
samples, and by Ms. Erica K. Paslick in radioimmunoassays.
This article has been cited by other articles:
|
|
 |
|
  H. Tucker and R. Ringer Controlled photoperiodic environments for food animals Science, June 25, 1982; 216(4553): 1381 - 1386. [PDF]
|
|
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
