Microtubule-organizing centers in the mitotic melanophores of Xenopus laevis larvae in vivo: Ultrastructural study

Mitotic melanophores of Xenopus laevis larvae at 51-53 stages of developmen t were morphologically studied using light and electron microscopy, with sp ecial reference to their microtubule-organizing centers. These melanophores represented a highly branched cell shape in mitosis, each cell process is distributed with melanosomes without exhibiting any responsiveness to hormo nal (melatonin) stimulation, and upon completion of mitosis, recovered the ability to translocate these granules in response to such a stimulus, At th e metaphase, these cells contained bipolar or multipolar spindles, whose po les were composed of three zones: the centrosome with centrioles; the centr osphere; and an outlying radial arrangement of microtubules and their assoc iated inclusions. In these mitotic melanophores, a number of microtubules a re distributed within the radially stretching cell processes, whereas an ab undance of microtubules reside in the spindles, Possible origins of the mic rotubules observed in these cytoplasmic processes are discussed in relation to the loss of the ability of pigment translocation during mitosis.