Source of Circulating Progesterone in Prepubertal Ewes

In ewe lambs peripheral progesterone increases for a 1–4 day period prior to puberty. The source of this increase was investigated in 15 spring-born ewe lambs. Ewes were checked twice daily with a teaser ram throughout the experiment but no signs of behavioral estrus were observed. Daily samples of jugular plasma were assayed for progesterone on the day of collection. Basal concentrations of progesterone (ng/ml; x ± SEM) averaged 0.3 ± 0.1 for all ewes and increased to 1.1 ± 0.1 during the first rise. The duration of the first rise in 5 ewes was 2.5 ± 0.5 days. Progesterone then decreased to 0.4 ± 0.1 for 4 ± 1 days and rose again to 1.9 ± 0.3 ng/ml. Six to 8 h after progesterone had been elevated in 2 consecutive daily samples during the first (5 ewes) and second (5 ewes) rises, ewes were laparotomized, bilateral ovarian venous samples were drawn and ovariectomy was performed. Five ewes that had not exhibited any rise in progesterone were sampled and ovariectomized at the same time as ewes ovariectomized during the first rise. Concentrations of progesterone were higher (150 ± 18 ng/ml) in venous plasma from ovaries containing luteal tissue than from ovaries that did not contain luteal tissue (1 ± 0.8 ng/ml; P<0.05). Luteal tissue was observed grossly and microscopically in ovaries removed during a rise in progesterone. No ovulation papillae (stigmata) were apparent on luteal structures (7.0 ± 1.7 mm in diameter) in ovaries removed during the first rise in progesterone and no corpora albicantia were observed. Definitive stigmata were observed on corpora lutea (8.2 ± 1.5 mm in diameter) in ovaries removed during the second rise in progesterone and corpora albicantia were present. After ovariectomy, progesterone decreased to basal concentrations in ewes that had shown either the first or second rise in progesterone, but remained unchanged in those ewes that had not shown a rise. Neither rise in progesterone was preceded by behavioral estrus. It is concluded that the increase in circulating progesterone prior to puberty in ewe lambs is produced by luteal tissue in the ovary.