The thymus as haematopoietic tissue of non-lymphoid cells.

Non-lymphoid haematopoiesis in the thymus was studied in 33 cases ranging from 14 weeks gestation up to 15 months postnatally. All thymuses displayed focally granulopoiesis in the connective tissue septa and in the lymphoid tissue, where it was localized in the peripheral cortex and at the cortico-medullary junction. Within the lymphoid tissue the granulocytic series showed many precursor cells with large rounded nuclei. Their origin could only be identified by the naphtol AS D chloroacetate esterase stain. In cases with thymic atrophy these precursor cells were present in relatively large numbers. Within the connective tissue septa many granulocytes with nuclear segmentation were intermingled with precursor cells. The erythroid series were detected with an immunohistochemical method for demonstration of haemoglobin. The positively stained nucleated cells occurred mainly in nests mostly localized in the peripheral cortex. These precursor cells were usually found in different places from the granulopoietic cells. No megakaryocytes were found. With appropriate techniques non-lymphoid haematopoiesis can be demonstrated in the developing thymus and must be considered as an expression of normal growth.