Growth and death of cells of the mesencephalic fifth nucleus in Rana pipiens larvae.

Positions, numbers, and nuclear sizes of mesencephalic fifth nucleus M-V) cells were determined in Rana pipiens larvae in stages XIII through the end of metamorphosis at stage XXV, in a few just postmetamorphic juveniles, and in adults. M-V cells are found throughout the tectum, with the greatest concentrations just anterior to each optic ventricle, and along the medioanterior aspect of each ventricle. Some cells lie below these levels, adjacent to the ependyma of the expanded aqueduct, the mesencephalic ventricle; a very few cells occur in the anterior medullary velum and in the adjacent anterior cerebellum. Up to 879 M-V cells were seen in single animals in stages XIV-XVIII, with mean values near 650 cells. Pyknotic M-V cells appear in all stages from XVII through XXV, and possibly in the youngest juveniles. Peak rates of cell loss occur at stage XXI, some 4-5 days after the forelimbs have emerged. Mean postmetamorphic cell numbers are about 350, suggesting an average M-V cell loss of 300 per animal. Nuclear cross-sectional areas are 72 microns2 at stage XIII, slightly above the size of such cells in hypophysectomized tadpoles. Nuclear growth is progressive to 102-111 microns2 at stages XXI-XXV. Adult sizes increase to a mean of 121 microns2. Hypophysectomized animals display pyknotic cells only after the tadpoles have been strongly stimulated by thyroid hormone, as demonstrated by significant growth of the legs and of the M-V cells themselves.