The Post-Natal Development of the Ovary and Uterus of the Merino Lamb

Summary. The growth and development, between birth and 33 weeks of age, of the ovaries, uterus and pituitary of Merino lambs were studied at approximately monthly intervals. The ovarian weight at 4 and 8 weeks of age was 6·8 and 10·8 times, respectively, the ovarian weight at birth. At 12 weeks, it had declined significantly and then remained relatively constant to 33 weeks. The uterine weight at 4 weeks was approximately twice the weight at birth, but it did not increase significantly again until 33 weeks. There was a small increase in pituitary weight between 4 and 8 weeks but no significant change thereafter. The mean numbers of growing and vesicular follicles in the ovaries were 455±62 and 935±208, respectively, at birth. They increased significantly at 4 weeks, then declined significantly in older lambs. Mean diameters of the largest follicle per ovary and per animal increased steadily from birth to 33 weeks. Oestradiol-17α was detected, by thin-layer chromatography, in the urine of all newborn lambs and oestradiol-17β was detected in the urine of one newborn lamb. After birth, oestrogens were not detectable in urine until 33 weeks when oestradiol-17α was found in two lambs and oestradiol-17β in another.