Phytohemagglutinin-P (PHA-P) exhibited inhibitory action on the growth of two human epithelioid cell lines, HeLa and L-132, and on the G929 strain of mouse fibroblasts. Growth of stationary cultures was measured spectrophotometrically by DNA production after 96 hours incubation. The mitotic indices of the three cell lines were reduced significantly by PHA-P after 72 hours incubation.Histochemical studies demonstrate depressed production of cytoplasmic RNA in 72-hour HeLa cultures after treatment with PHA-P, with the appearance of prominent, PAS-positive, diastase-fast granules in the cytoplasm. Bound lipid aggregates increased in amount in the cytoplasm of HeLa cells after PHA-P treatment. The PAS-positive, diastase-fast granules were also found in the cytoplasm of the L-132 and L-929 cell lines after treatment, but cytoplasmic RNA remained unchanged qualitatively. Alkaline phosphatase was slight in amount in HeLa cells and was reduced to trace amounts after PHA-P treatment. L-132 cells showed moderate to heavy concentration of alkaline phosphatase, which remained unchanged qualitatively after PHA-P treatment.The enzyme could not be demonstrated in the L-929 fibroblasts.

Summary. Keratin and prekeratin have been visualized histochemically by testing the reactivity of disulfide groups in the thymic corpuscles of the sheep. Rhesus monkey, rabbit and dog, and the reactivity of sulfhydryl groups in the sheep. Rhesus monkey, rabbit and rat. Ten to 50% of the thymic corpuscles showed keratinization by this method. Acidic mucosubstances were visualized histochemically in the Rhesus monkey, sheep and rat but not in the rabbit. Staining for vic-glycol groups was prominent in all species tested.Neuraminidase treatment with subsequent Alcian blue-periodic acid-Schiff (PAS) staining indicated the presence of sialomucins in the thymic corpuscles of the monkey and rat, but sialomucins were not demonstrated in the rabbit. INTRODUCTIONThymic corpuscles have been studied histochemically in an effort to evaluate their possible relationship to secretory activity of the thymic medulla. The experiments of Metcalf and WakonigVaartaja (1964) have shown that the thymic medulla is necessary for the survival of thymic grafts, and the '^S uptake studies of Clark (1968) strongly indicate the elaboration by medullary reticular epithelial cells ofa macromolecule which may be a mucoid secretory product. Torok (1966) showed PAS-positive staining in the thymic corpuscles of the guinea-pig. Wekerle, Cohen and Feldman (1973) found both PAS-positive and negative cells among the thymus reticulum cells in tissue culture, and the E/M studies of Waksal and co-workers (1975) indicated cells ofa highly active, secretory type in the thymus reticulum. The work of Seligman (1954), Torok (1966) and Blau (1973) has demonstrated keratin and prekeratin in the thymic corpuscles.The present study has investigated the histochemical reactivity of the thymic corpuscle in two general areas. Structural proteins in the form of keratin and prekeratin were tested by the reactivity of their disulfide and sulfhydryl groups, respectively. Epithelial secretory products were investigated by testing for mucosubstances which have reactive sulfate and carboxyl groups, and the neutral mucosubstances which have an abundance of vic-glycol groups.

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Receptor-destroying enzyme (RDE) and purified neuraminidase (V. cholerae) were added in varying concentrations to cultures of L929 fibroblasts, HeLa cells and FL amnion cells. After 96 hours incubation at 36"C, RDE was found to enhance the growth of L929 fibroblasts and FL amnion cells, as measured spectrophotometrically by total DNA synthesis. Neuraminidase was found to stimulate growth in the three cell lines, using the same method of DNA determination. Neuraminidase, moreover, significantly increased the mitotic indices of the three cell lines after 72 hours incubation.