Abstract
To study the development of the reproductive tract in heifers, the ovaries, uterus, cervix and vagina were examined by transrectal ultrasonography every 2 weeks, from 2 to 60 weeks after birth. First ovulation occurred at 63.7±1.1 weeks of age. Ovarian dimensions increased rapidly from 2 to 14 weeks of age, and increased again after 34 weeks of age ( P<0.05). The size of the largest ovarian follicles increased from 8 to 14 weeks of age, from 38 to 42 weeks of age, and finally from 52 to 60 weeks of age ( P<0.05). The number of follicles ≥3 mm in diameter tended to increase from 6 to 14 weeks of age ( P<0.10) and increased significantly from 6 to 60 weeks of age ( P<0.05). Mean numerical pixel values of the ovarian images decreased from 4 to 26 weeks of age, and then rose to 44 weeks of age ( P<0.05). Diameter of the uterine body, cervix and vagina increased from 2 to 20–24 weeks of age, and again after 32 weeks of age ( P<0.05). Mean numerical pixel values for the uterus and vagina decreased initially (uterus: 4–8 weeks and vagina: 6–22 weeks of age) and then increased (uterus: 14–42 weeks and vagina: 22–32 weeks of age; P<0.05). Pixel heterogeneity showed a consistent peak at 20–22 weeks of age for the uterus, cervix and vagina ( P<0.05). In summary, in the heifer calf, the marked growth of the reproductive tract in the first few months of age, and prior to first ovulation, reflects phases of increased ovarian follicle (≥3 mm in diameter) numbers and size. Ultrasonographic image analysis revealed patterns of numerical pixel values and heterogeneity that may be useful in determining important stages of growth and differentiation of the reproductive system.
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