24 The Lymphocyte and Lymphocytic Tissue 



and, of course, in the bone marrow. As we shall see, myelocytes are common 

 in the developing lymph nodes of human embryos; erythroblasts also occur, 

 but megakaryocytes are rare. 



In all these hematopoietic foci, the process begins with the contraction of 

 outstretched mesenchymal cells into spherical, ameboid cells of various sizes 

 characterized by relatively little cytoplasm, large nucleoli, and — in fixed and 

 stained preparations — an intense cytoplasmic basophilia, more marked in the 

 larger than in the smaller forms. These free mesenchymal cells (stem cells 

 or "blasts") give rise to all types of blood cells in all of the successive sites of 

 blood cell formation. Occasionally an outstretched mesenchymal cell seems 

 to round up and turn directly into a granulocyte without passing through a 

 free, basophil, stem cell stage. 



EMBRYOGENESIS OF HUMAN LYMPHOCYTIC TISSUE 



In human embryos of 30 mm., the first lymph nodes begin to develop in 

 the walls of the cervical lymph sacs, later in the other lymph sacs, and still 

 later along peripheral lymphatics. In all the sacs, the process is much the 

 same: cords of mesenchyme bulge into the lumen of the sacs and give rise to 

 some free macrophages and foci of intensely proliferating stem cells. These 

 are of all sizes, although the smaller forms predominate. Because of their 

 location these free stem cells must be called lymphocytes (Slide 8) . From the 

 stem cells (i.e., lymphocytes) there develop eosinophil and neutrophil 

 myelocytes, although in smaller numbers than in the embryonic bone mar- 

 row (Slides 9-12) . Sometimes these small myelocytes show mitoses. These 

 myelocytes have nuclei indistinguishable from those of the surrounding 

 lymphocytes. Occasional megakaryocytes and small groups of erythroblasts 

 also occur. 



It is rare to find the specific leukocyte granules in the larger lymphoid 

 cells. Very occasionally, eosinophil granules are found in cells with exceed- 

 ingly pale cytoplasm and with large nuclei lacking the large nucleoli charac- 

 teristic of stem cells. They are probably mesenchymal cells giving rise di- 

 rectly to myelocytes. 



As the embryo grows, the lymphatic tissue, except in some of the labora- 

 tory mammals, normally ceases to produce myeloid elements, although in 

 ectopic myelopoiesis this process may become active again (Slide 13) . 



In the embryonic thymus eosinophil myelocytes are found in small 

 numbers; their nuclei are identical with those of the surrounding smaller 

 lymphocytes (Slide 14) . A few developing erythroblasts and megakaryo- 

 cytes have also been reported. 



The primordium of the spleen begins primarily as a vascular meshwork of 

 mesenchyme which gives rise to free stem cells. These may persist as such, 



