The fine structure of the thymocytes of young rats

Abstract

Thymocytes of young rats were studied with the electron microscope to provide a base line for a comparative study of the fine structure of various types of lymphocytes. Fixation with Os01 or glutaraldehyde, Epon embedding, and heavy metal staining were the principal techniques employed. Small thymocytes, with nuclei from 3.5 to 4.5 . u in diameter are in the vast majority. Our results, which in general confirm previous reports, show the thymocytes to be primitive, or undifferentiated in structure. Certain exceptions and additions were noted, however. There are no protoplasmic bridges between thymocytes and epithelial cells and no clear evidence of epithelial-thymocyte transformations. The nucleus contains typical pores and in many cases a clearly defined finely granular nucleolus, although the latter is not evident in most of the smallest cells. Cytoplasmic fragments appear to be separating from thymocytes by a process similar to the separation of platelets from megakaryocytes. The existence of a special band of cytoplasm beneath the plasma membrane devoid of formed elements is described for the thymocytes. The possibility of the common occurrence of this zone in other cells, and its possible significance, are discussed.The past few years have seen a revival of interest in the problem of the nature and fate of the small cells of the thymus, and particularly as they may relate to the lymphocytes of the blood and hemopoietic tissues (Arnason, '62; Miller, '63). Part of the difficulty arises from the fact that the lymphocyte is not one clearly defined cell, but a family or species, varying from small cells composed almost entirely of dense nuclei, at one extreme, to large cells with abundant cytoplasm and relatively open nuclei at the other. These latter cells have been called by a variety of names (stem cell, lymphoblast, hemocytoblast, etc.) indicating a variety of opinions regarding their relation to the lymphocyte family and to other blood cells. In addition, the lymphocytes of the blood are connected to the monocytes by an unbroken series of transitional forms (Bloom and Fawcett, '62), and in the spleen and elsewhere the lymphocytes form a series with the plasma cells (Nossal and Makela, '62). There is evidence that under certain conditions the lymphocyte can act as a stem cell for granular leukocytes (Bloom, '37; Farr, '51) and possibly also erythrocytes (Yoffee, '60), and that thymocytes may have similar potential (Murray and Woods, '64). In the thymus the free cells present a series which is morphologically identical with the range seen in lymphatic tis-ANAT. REC., 1 5 1 : 17-40. sues, but there is very little evidence of in situ transformation to other cell types. Another factor is present here, however, which adds to the uncertainty. It is not clear whether the thymocytes originate from the local stroma of epithelial nature, or from mesenchymal elements which have invaded this stroma. The weight of evidence has favored the latter hypothesis (Maximow, 'Og), but recent data seem to point to the...

Anatomy and cytology of the thymus in juvenile Australian lungfish, Neoceratodus forsteri

“…The ultrastructural characteristics of thymocytes observed in N. forsteri , i.e. thin rim cytoplasm and few mitochondria, are similar to those observed in many other species (Murray et al. 1965; Bockman & Winborn, 1967; Kendall, 1980; Chilmonczyk, 1992).…”

“…2007). Other thymic cellular components are macrophages, myoid cells and mast cells (Murray et al. 1965; Fange & Sundell, 1969; Gorgollon, 1983; Chilmonczyk, 1992).…”

The association of lymphocytes with larger motile cells in cultures of mammalian thymus

“…Occasionally, more extensive areas of cytoplasm were seen extending out from the main body of the cell between the contiguous lymphocytes. These cells were identified as belonging to the epithelial component of the thymus, since they were similar to those described in the mouse by Weiss (1963), in the rat by Murray, Murray and Pizzo (1965) and in the guinea-pig by Izard (1966).…”

“…An alternative explanation for the presence of LMC in vitro would imply that it is a normal inhabitant of the intact thymus in vivo, but becomes more easily detectable in culture when the close packing of the cells in the intact thymus gives way to the wider separation resulting from emigration on the coverslip. If this is the case, the weight of the evidence discussed above would favour the possibility that the LMC is a modified form of epithelial cell; this evidence is entirely circumstantial, but it may be relevant to note here the observation of Murray (1947), who recorded two instances in which epithelial cells in cultures of rabbit thymus underwent a differentiating mitosis, producing in each case one daughter cell resembling the parent epithelial cell and another that was ''somewhat smaller . .…”

The Fine Structure of Mitosis in Rat Thymic Lymphocytes

“…The latter molecules are believed to be so oriented as to permit great expansion in area of the membrane if the molecular orientation is disrupted. We have commented previously (27) on the existence of a layer beneath the conventional plasma membrane of thymocytes which, although never so broad as 0.5 #, in other respects might qualify as the oriented protein layer Mitchison postulates. This is an essentially amorphous zone with a viscosity apparently much higher than that of the cytoplasm in general, as indicated by the absence of ribosomes and by the fact that mitochondria seem unable to compress the material of this zone sufficiently to come into direct contact with the osmiophilic plasma membrane.…”