Mast Cell Population and Histamine Content in Hypothyroid Rat Tissues

The morphological features and the relative number of mast cells (MCs) were studied in the skin and exorbital lacrimal gland of hypothyroid Wistar rats, Rattus norvegicus. Hypo-thyroidism significantly increased the number of MCs (up to 4.5-fold) and the histamine content (up to 50%) in the examined tissues. The magnitude of the increase in the number of MCs was greater in the cheek skin and the exorbital lacrimal gland than in the back skin. In the skin, the MCs were mainly located within the hypodermis and were closely associated with blood vessels, nerve fascicles, and adipocytes. In the exorbital lacrimal gland which is a seromucous gland located lateral to the cheek below the ear, the MCs were distributed in the connective tissue surrounding the acini. The secretory granules of MCs showed histochemical characteristics of connective tissue MCs. They were meta-chromatic with Toluidine blue and safranin positive with the Alcian Blue/safranin reac-tion. Finally, a significant increase in degranulating MCs was observed in hypothyroid tissues relative to euthyroid tissues. At the ultrastructural level, the MCs of euthyroid rats were predominantly non-degranulating (stage I). In hypothyroid animals, numerous MCs showed partial degranulation (stage II-III) or were in a stage of complete degranulation. Our results concerning the skin and the exorbital lacrimal gland suggested that the thyroid status might be involved in regulating the frequency and activation state of MCs.