Earlier studies on fetal thymus suggested that certain of the large pyroninophilic cells found there might have a hemopoietic role, and it was decided to determine the nature of these cells using histochemical and immunohistochemical methods. Thymic tissue from aborted fetuses, stillbirths, and neonatal deaths was examined histochemically using methods for the detection of chloroacetate esterase, peroxidase, and pseudoperoxidase, and by staining techniques for mast cells and eosinophils. Tissue was also examined using the indirect immunoperoxidase method for the presence of hemoglobin A (HbA) and F (HbF), for lysozyme (muramidase) and immunoglobins alpha, mu, gamma, kappa, lambda. Positive staining to some degree was seen in cells in the connective tissue stroma using all methods, and the cells stained corresponded to one or another of the types of pyroninophilic cells present. The finding of large cells with positive chloroacetate esterase and antilysozyme indicates the presence of granulopoiesis. Similarly, the presence of large nucleated cells with pseudoperoxidase and anti-hemoglobin (A and F) staining indicates the presence of erythropoiesis. Plasma cells were present in small numbers.

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