Growth and death of cells of the mesencephalic fifth nucleus in Xenopus laevis larvae.

Positions, numbers, and cell and nuclear sizes of mesencephalic fifth nucleus (M-V) cells were determined in Xenopus laevis larvae in stages 47 through the end of metamorphosis at stage 66. M-V cells may be found at every tectal level, from the rostralmost section almost to the caudal pole, and in the anterior medullary velum. A large majority of the cells lie between 15 and 65% caudad of the rostral tip of the tectum. At anterior and middle tectal levels the cells lie lateral to but mainly above the optic ventricle. At posterior levels, to which the ventricle does not extend, a few cells may be seen at middle and lower tectal levels, as if in transition to the anterior medullary velum. At stage 47 fewer than ten cells are seen in each animal. The numbers rise to 20-40 by stage 50, and are uniformly above 100 after stage 51. Initially many M-V cells were small, i.e., 6-7 microns in diameter, but grew to a mean diameter of about 19 microns at stage 59, with a maximum value of 29 microns. A single individual at stage 57 had 581 cells. The peak of mean cell numbers, 387, occurred at stage 59, which was also the stage with the highest mean values for nuclear and cell sizes. Pyknotic M-V cells at low frequency were seen at stages 55 and 57, and at all stages thereafter. Cell death frequency peaked at stage 62, but continued through stage 66. By stage 66 mean cell numbers had been reduced to about 240, indicating survival of about 60% of cells present at stage 59.