Growth and mitotic rate of the proximal tibial epiphyseal plate in hypophysectomized rats given estradiol and human growth hormone.

Hypophysectomized female immature albino rats were injected with HGH, HGH and estradiol, or estradiol solely. Some animals were left as controls. All animals were killed on day 3 or 5 of the experiment. One hour before sacrifice, the animals were flash labelled with tritiated thymidine. The following parameters were registered: labelling index, thickness of the growth plate, weight gain, and size of the uterus. The following conclusions were drawn. 1. Estradiol decreases the mitotic rate and retards growth of the epiphyseal cartilage. Its effect on the mitotic rate is more pronounced than its effect on the thickness of the tibial epiphyseal plate. 2. HGH increases the mitotic rate and promotes growth of the epiphyseal cartilage. Its effect on the thickness of the epiphyseal plate is more pronounced than its effect on the mitotic rate. 3. Estradiol decreases the mitotic rate even in the presence of HGH. 4. Estradiol seems to have a direct effect on mitosis of the chondrocytes.