Ovarian activity during sexual maturation and following introduction of the ram to ewe lambs

Abstract

The aim of this study was to assess the applicability of transrectal ovarian ultrasonography in quantifying ovarian antral follicles during the sexual development of ewe lambs, from 4 to 31 weeks of age. Also to record how the introduction of a ram at 25 and 29 weeks after birth affected ovarian function in peripubertal ewe lambs. Suffolk×Western White Face ewe lambs born in April underwent ultrasonography every 2 weeks (from 4 to 24 weeks of age; n=27) or weekly (from 26 to 30 weeks of age; n=14). When the ewes were 25 weeks old, a vasectomised ram was placed in the pen next to the ewes for 1 week, and at 29 weeks of age, the ram was placed with the ewes for another week. One week after second contact with the ram, estrus was synchronised by a 12-day treatment with intravaginal sponges containing medroxyprogesterone acetate (MAP). Ovarian ultrasonography was done every 6 h, from 0 to 96 h after sponge removal, and then daily for three more days, to determine the number of ovulations and the time at which they occurred. The mean number and total diameter of all follicles ≥3 mm increased from 14 to 16 weeks, declined between 16 and 18 weeks, and then increased between 22 and 24 weeks after birth. All these changes in total follicular number and diameter were significant. Maximum follicular diameter increased from 8 to 16 weeks of age (P<0.05). The first fence-line contact with a ram resulted in increases (P<0.05) in follicular diameters (from 25 to 26 weeks of age), the number of luteal structures per ewe, the percentage of ewe lambs with luteal structures, and mean serum progesterone (P₄) concentrations (from 1 to 2 weeks after the contact began). The second, full ram contact increased (P<0.05) the number of luteal structures per ewe, the proportion of ewe lambs with luteal structures, and serum P₄ concentrations, from 29 to 30 weeks of age. Subsequent treatment with MAP sponges was followed by estrus, ovulations and full-length luteal phases in all ewe lambs studied. In summary, there were two phases of increased antral follicle development during the post-natal period in ewe lambs, with peak follicle numbers at 16 and 24 weeks of age. Fence-line contact with the ram at 25 weeks of age resulted in increased follicle size and stimulated the formation of some luteal structures, and subsequent full ram contact at 29 weeks of age markedly enhanced the number of luteal structures. Transrectal ovarian ultrasonography has proven to be a valuable tool in the study of ovarian function in ewe lambs from 4 weeks of age onwards.

Introduction

Post-natal ovarian follicular development has been previously studied, in ewe lambs of different genotypes and ages, but only with infrequent, single-time observations post-mortem (Mansour, 1959, Land and McGovern, 1968, Land, 1970, Kennedy et al., 1974, Foster et al., 1975, Foster and Ryan, 1979). Large numbers of vesicular follicles were found in the ovaries of new-born lambs (Mansour, 1959, Land and McGovern, 1968). Based on observations every 4 weeks, from 4 to 24 weeks, and at 33 weeks of age, numbers of vesicular follicles were greatest at 4 weeks after birth and then declined in older lambs (Kennedy et al., 1974). Diameters of the largest antral follicles increased from birth to12 weeks of age, but there was no significant change until 33 weeks of age, in fall-born Merino ewe lambs (Kennedy et al., 1974). Transrectal ultrasonography allows for the repeated, non-stressful monitoring of ovarian follicular dynamics in the ewe (Ravindra, 1993, Schrick et al., 1993, Ravindra et al., 1994; Bartlewski et al., 1998, Bartlewski et al., 1999a, Bartlewski et al., 1999b), but this technique has not been used to study ovarian antral follicular development in individual ewe lambs during sexual maturation. This approach would enable accurate enumeration of all ovarian follicles ≥3 mm in size in the same individual animals, over this period of development.

The sudden introduction of novel rams into a flock of seasonally anestrous sheep stimulates follicular development, steroidogenesis and ovulation (Riches and Watson, 1954, Schincel, 1954, Radford and Watson, 1957, Martin and Scaramuzzi, 1983, Martin et al., 1986, Pearce and Oldham, 1988, Cushwa et al., 1992). Although well-documented in adult anestrous sheep, evidence that the “ram effect” may be used in induction of puberty in ewe lambs is scant. In a previous study, introduction of rams to ewe lambs approaching puberty resulted in a high degree of estrous synchrony at the first mating (Dyrmundsson and Lees, 1972). Introduction of rams from mid-September to mid-October significantly increased pulsatile secretion of LH and advanced first estrus by approximately 2 weeks (O’Riordan and Hanrahan, 1989), but exposure of ewe lambs to rams in late October was less effective (Al-Mauly et al., 1991). Ovarian changes in ewe lambs after ram introduction have not been studied to date. The aim of this study was to see if transrectal ovarian ultrasonography could be used as a method of collecting morphological and physiological data describing ovarian changes in ewe lambs as young as 4 weeks of age. This technique was then used to quantify antral follicular populations in ewe lambs from 4 to 24 weeks of age, and to detect antral follicles and luteal structures from 26 to 31 weeks of age after exposure to rams and estrous synchronisation.