The effect of hydrocortisone on morphological changes in developing chick embryo was observed by scanning electron microscopy and light microscopy in relation to changes in enzyme activities.
The morphological changes caused by administration of hydrocortisone were remarkable. In the hydrocortisone-dosed 19th day embryonic duodenum, this hormone caused increases in the height and microvillus length ef absorptive epithelial cells, and in the ratio of microvillus length to cell height, up to the normal just hatched 1st day hatched duodenal level. Injections of this hormone also induced precocious increases in maltase and alkaline phosphatase activities.
On the basis of these results, it is considered that hydrocortisone may be one of the important factors regulating normal development in chick embryonic duodenum and may exert a major influence on the growth and maturation of absorptive epithelial cells.

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Using short-term cultures of rabbit thymocytes, the effect of nonradioactive (cold) deoxycytidine on ³H-thymidine incorporation by these cells in vitro was studied, together with the effect of cold thymidine on the uptake of ³-deoxycytidine, chiefly by autoradiographic techniques.
Without addition of cold deoxycytidine to the culture medium, a fairly high proportion of the thymocytes was heavily labeled with ³H-thymidine. However, the extent of this reaction was strikingly decreased (to 1/5-1/20 of the control levels)by the addition of cold deoxycytidine at oncentrations ranging from 10 to 80μg/ml. On the other hand, no remarkable reduction in rate of ³H-deoxycytidine incorporation by the thymocytes was caused by the addition of cold thymidine to the medium in sufficient amounts (10-80μg/ml). Among the thymocytes, a minor population was identified which was characterized by intense labeling with ³H-thymidine in spite of the addition of cold deoxycytidine to the medium in sufficient amounts. Such thymocytes are considered to correspond to those which have recently been shown to be cortisone-resistant and immunologically active cells by immunological studies

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The nature of the migratory process and the role of the mesenchymal cells (primarily lymphocytes) have been studied by light and electron microscopy in the small and large intestines of the urodele amphibian, Ambystoma mexicanum, and in the mouse (C57 Black) and rat (Wistar Albino). In all three species the migration is intense and in all the large intestine shows a much higher proportion of mesenchymal cells in the apical position in the epithelium between the epithelial nuclei and the lumen.
The mouse intestine was chosen for quantitative studies because of the genetic homogeneity of the material and the availability of animals of known age. A marked decrease in numbers of mesenchymal cells in epithelium is seen with advaning age in our specimens. The total numbers. of migrating cells in the large intestine of the mouse do not seem to be greater than in the small intestine but the number in the apical position in the large intestine is approximately nine times that in the villi. It is in this apical position in all three species that degenerative change in the mesenchymal cells usually is seen.
Degenerating mesenchymal cells (lymphocytes) are within epithelial cytoplasm. Condensation of nuclear chromatin is followed by nuclear breakdown (karyorrhexis)and solution of nuclear contents (karyolysis). The material of the lymphocyte nucleus seems either to be absorbed in the eyithelial cytoplasm or discharged to the intestinal lumen.
The appearance of mesenchymal cells in the basal portion of the epithelium suggests strongly that some at least of them are undergoing a transformation, in all three species, in very much the way that Mori (1937) suggested as occurring in the intestine of the amphibian Megalobatrachus japonicus. Such transformation would appear to be occurring frequently enough to play a very significant role in replacement of epithelial cells.

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Ultrastructural comparisons were made by transmission and scanning electron microscopy between Dunn and Potter's mastocytoma cells and normal mast cells of CDf/1 mice which were carriers of Dunn and Potter's mastocytoma cells or Wistar rat normal mast cells.
The mastocytoma cell was found to be round to oval with many irregular short microvilli. The nucleus of the mastocytoma cell had a few prominent nucleoli. It consisted mainly of euchromatin. Medium sized mitochondria were observed in moderate quantities in the cytoplasm. The mastocytoma cell had abundant ribosomes and several areas of tubular endoplasmic reticulum. It had a relatively well developed Golgi apparatus. The centrioles and small specific granules of the mastocytoma cells were localized in the neighborhood of the Golgi apparatus. Normal mast cells showed various cell profiles. The nucleus of the normal mast cell had no nucleolus. The chromatin consisted of almost equal amounts of euchromatin and heterochromatin. Several elongated mitochondria were found in the normal mast cell. There were a few areas of slightly dilated rough surfaced endoplasmic reticulum and a small number of ribosomes. The normal mast cell had a small Golgi apparatus and its cytoplasm was extensively occupied by large granules.
Scanning electron microscopy also revealed certain differences between these two types of cells. The mastocytoma cell was found to be generally much more villated than the normal mast cell. The surface architecture of the mastocytoma cell resembled that of other neoplastic cells. The use of the scanning electron microscope alone was insufficient to differentiate between the mastocytoma cell and mast cell.
The nucleus to cytoplasmic ratio of the mastocytoma cell was larger than that of the mast cell. The granule to cytoplasmic ratio of the mast cell was apparently larger than that of the mastocytoma cell. Quantitative analysis of these cells at the cell organella level was able to clarify the difference between the tumor cells and normal cells to some extent.

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