Structural changes in the pars intermedia of the cichlid teleost Sarotherodon mossambicus as a result of background adaptation and illumination.

The pars intermedia of Sarotherodon mossambicus (Tilapia mossambica) contains two cell types which can be differentiated at both the light and electron microscopic level. The predominant cell type is lead haematoxyline positive, and has been shown to be the MSH producing cell type by means of immunocytochemical staining at the ultrastructural level. The changes in cellular and nuclear volume, as well as 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, whereas the activity is low when the blinded fish are kept in total darkness. From the observed differences in activity of the MSH cells between the experimental groups, it is concluded that the MSH cells are not activated by the absence of reflected light, but by a high ratio between direct and reflected light. A second light-sensitive organ, supposedly the pineal gland, is also involved in the background response of the MSH producing cells.