Microtubules and pigment migration in the melanophores ofFundulus heteroclitus L.

Abstract

The dermal melanophores ofFundulus heteroclitus L. have been investigated by light and electron microscopy with the purpose of revealing the mechanisms controlling pigment migration. As predicted by earlier studies, the nerve endings of a double innervation were found adjacent to and in synaptic relation to the melanophore surface. Not expected were the large number of small pits or invaginations present in the cell surface. These appear identical to the so-called micropinocytotic vesicles found generally in cells of the vascular endothalium and smooth muscle. In chromatophores they are more reasonably interpreted as receptor sites for neurohormones than as uptake and transport mechanisms. Observations made on the kinetics of pigment migration within the processes of these melanophores indicate that the granules move along relatively fixed channels arranged parallel to the long axes of the processes. Examined at fine structure levels, the zones of cytoplasm around these channels are found to be populated by microtubules about 225 A in diameter aligned parallel to the direction of pigment movement. These long slender elements are present in the processes regardless of whether the melanin is concentrated in the cell center or dispersed. It is reasoned from these and other observations that the microtubules function as cytoskeletal elements which help maintain the extended form of the melanophore arms and at the same time define the channels in which the pigment moves. The possible role of the tubule in generating the motive force for pigment migration is discussed.