DEPRESSION OF THE SOMATOTROPIC CELLS IN THE BOVINE PITUITARY-GLAND DUE TO APPLICATION OF RECOMBINANT BOVINE SOMATOTROPIN - AN IMMUNOHISTOCHEMICAL AND MORPHOMETRIC STUDY

The morphological alterations in the bovine pituitary gland resulting from the administration of recombinant bovine somatotropin (rBST) are described. Three groups of German Simmental heifers (12 animals for ea ch group) were treated. Group 1 received a placebo (vehicle), group 23 20 mg and group 3640 mg of a prolonged-release formulation of rBST at 2 week intervals. Treatments were started with an average body weight of 286 +/- 2.5 kg and continued until one week before the animals were slaughtered with an average body weight of 520 +/- 3.2 kg. Double imm unohistochemical labelling using rabbit anti-prolactin (PRL) and chick en anti-BST antibodies was performed on paraffin serial sections of th e pituitary glands. Morphometric analyses revealed a dose-dependent de crease of the volume fraction of the somatotropic cells due to a reduc tion in number and a diminution of the mean cellular volume. No signif icant differences in the volume fraction of the mammotropic cells were seen. The recorded reduction of the somatotropic cells is thought to be the result of a negative feedback upon the secretory activity as a result of long-term rBST treatment.