Structural changes in the pars intermedia of the cichlid teleost Sarotherodon mossambicus as a result of background adaptation and illumination. 1. The MSH-producing cells

van Eys, GJJM.

Cell and Tissue Research 2081: 99-110


ISSN/ISBN: 0302-766X
Accession: 021806901

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The pars intermedia of S. mossambicus (Tilapia mossambica) contains 2 cell types which can be differentiated at the light microscopic and EM levels. The predominant cell type is lead hematoxylin positive and was the MSH melanocyte stimulating hormone -producing cell type by immunocytochemical staining at the ultrastructural level. The changes in cellular and nuclear volume, and the results of stereological measurements on the cytoplasmic organelles, show that the activity of MSH cells is high on a black background and low on a white background or in total darkness. In blinded fish under a normal day-night regime, the activity of the MSH cell is as high as that in black adapted fish; the activity is low when the blinded fish are kept in total darkness. The MSH cells apparently are not activated by the absence of reflected light but by a high ratio between direct and reflected light. A 2nd light-sensitive organ, supposedly the pineal gland, is involved in the background response of the MSH-producing cells.