Histopathology and ultrastructure of the thymus in OP/OP rats with special focus on thymic macrophages. Effect of bone marrow transplantation.

The thymi of op/op rats were investigated before and after bone marrow graft. Normal littermates were used as controls. The thymic involution and atrophy in non-treated op/op rats was characterized by the presence of pyknotic thymocytes (apoptosis) and numerous lipid-laden macrophages containing pleiomorphic lysosomes. This thymic atrophy is histologically very similar to the thymic involution induced by glucocorticoids or by stress in normal rats. In treated new born op/op rats, the osteopetrotic process began to be reversed 2 weeks after the graft. The thymus of treated op/op recovered a normal weight and its histology and ultrastructure were quite similar to that of normal littermates. These findings are discussed in relation to the defective macrophage function and cellular immunity impairment reported in osteopetrosis.