The histogenesis and cell structure progression of Hassall's bodies (HB) was observed employing light and electronmicroscopic histochemical methods in 212 human and 100 dog fetuses at various stages of ontogenesis. Several human pre- and postnatal thymuses were investigated immunohistochemically, employing a library of over 50 poly- and monoclonal antibodies, seeking to determine the cell origin of HBs. The organization of optimal microenvironmental conditions in the thymus requires the added participation of the neural crest. Spontaneous or experimental ablation of the neural crest during early ontogenesis results in abnormal thymic organogenesis. The reticulo-epithelial (RE) cells of 25 postnatal thymuses were also observed under various tissue culture conditions. The development of the first HB was detected on the 38th day of gestation in dogs and during the second part of the third intrauterine lunar month in human fetuses. The greatest developmental progression and main cell-tissue organization of the HBs was observed between 45 and 54 days of gestation in dogs and between 6 and 10 lunar months in humans. During thymic ontogenesis, the HBs appear when lymphopoiesis is already established and the cortex, medulla and the cortico-medullary junction are capable of conducting the positive and negative selection of T lymphocytes undergoing progressive maturation. The HBs are structurally organized from medullary RE cells, which usually undergo hypertrophy prior to their inclusion in the outer cell layer of the corpuscles. The cellular microenvironment of the thymic medulla is composed of networks of cell types, of a variety of origins, and all of them may participate in the construction of growing, progressive HBs. Histochemically, we detected a rich content of basic non-histone proteins, PAS positive substance (glycogen and acid mucopolysaccharides within the bodies. Employing the histological stain of Pasini and immunocytochemical methods with monoclonal antidodies (MoABs) AE2 and AE3, high molecular weight (56.5 to 67 kD) basic keratins were defined in human HBs. Employing a panel of MoABs developed against epithelial cell surface antigens (provided kindly by Dr. Haynes), we observed immunoreactivity localized to the outer cell layer of the HBs with MoABs TE8, TE16 and TE19, while the centrally located cells reacted positively with TE15 and TE19. Immunoreactivity employing the TE8, TE16 and TE19 MoABs was also observed in the epidermal granulosa cell layer, while TE15 reacted with cells of the stratum corneum. The presence of endocrine, peptide secreting RE cells within the HBs was defined with the use of MoAB A2B5, which binds to the GQ ganglioside. The hypertrophied, physiologically active RE cells of the peripheral cell layer of the HBs reacted positively with medium to strong intensity when stained with MoABs UJ127.11, J1153, A2B5, 215.D11, and 275.G7. These results further suggest that HBs are not exclusively degenerative structures. Our transmission electronmicroscopical (TEM) studies on HBs determined the existence of groups of RE cells connected to one another by desmosomes. We also observed long cytoplasmic processes originating from medullary RE cells and directly contacting thymic T lymphocytes and accessory antigen presenting cells (macrophages, dendritic cells, interdigitating cells, Langerhans cells, etc.) by the use of scanning electronmicroscopy (SEM). During our observations on human thymic organ and tissue cultures, no development of HBs could be registered. It is possible that additional cell to cell interactions with cells of different derivations may be necessary for the development of HBs. Thus, our results indicate that the HBs are unique, antigenically distinct, functionally active, multicellular components of the nonlymphocytic, cellular microenvironment of the thymic medulla, and participate in the physiological activities of the prenatal and adult thymus.(ABSTRACT TRUNCATED)