HISTOLOGY 95 



The myeloblasts are large cells with large, round, vesicular nuclei, contain- 

 ing coarse chromatin granules, surrounded by a small amount of non-granu- 

 lar cytoplasm. They undergo proliferation and give rise to myelocytes 

 which have indented bean-shapped nuclei and slightly granular cytoplasm. 

 The myelocytes divide by mitosis and give rise to metamyelocytes or pro- 

 leukocytes which have ring-shaped nuclei and cytoplasm containing some- 

 what coarser granules. These cells do not proliferate, but their nuclei 

 change into irregular lobulated shapes, typical of mature polymorphonuclear 

 leukocytes (Fig. ri8). The cytoplasmic granules of the myelocytes and 

 metamyelocytes may be eosinophilic or neutrophilic. BasophiHc myelo- 

 cytes are not found in mouse bone marrow (Haam, in Jaffe, 56). The 

 neutrophilic granules are fine and stain faintly, while the eosinophihc 

 granules are larger and stain intensely. The megakaryocyte is a giant, 

 irregular shaped cell which has a single lobulated nucleus. It undergoes 

 degeneration within the marrow. The theory that small processes of the 

 cytoplasm of megakaryocytes are pinched ofT and enter the circulation as 

 platelets is still under discussion. In addition to the cells described, the 

 presence of fat cells, large mononuclear cells and lymphocytes is constant in 

 the marrow. 



Petri (77) gives the following average differential count based on the cells 

 of the femoral marrow of 14 white mice: nucleated red blood cells 23%, 

 myeloblasts 4.7%, myelocytes 9.2%, proleukocytes 6.5%, leukocytes 34.3%, 

 non-identified 18.7%, large mononuclear cells 0.1%, reticulo-endothelial 

 elements ^.2)%, megakaryocytes 0.2%. 



The mature myeloid cells enter the circulation by passing through the 

 thin wall of the venous sinusoids. Arteries and veins are numerous in the 

 bone marrow. 



The femur or sternum are suitable for obtaining marrow smears for 

 histological examination. According to Jaffe (56) the marrow of the long 

 bones is functional throughout the life of the mouse, and is not replaced by 

 fatty, yellow marrow. 



Lymph nodes. — ^Lymph nodes are small, bean-shaped organs composed of 

 lymphatic tissue and located in the course of lymph vessels. At the 

 indented area, which forms the hilus, blood vessels enter and leave the node. 

 Intercommunicating large lymph spaces, the lymph sinuses, are present 

 throughout the organ. Each node is surrounded by a thin connective tissue 

 capsule which is especially well developed at the hilus, where it may project 

 for a distance into the medullary area. Trabeculae which are continuous 

 with the capsule divide the cortical part into alveolar areas and the medul- 



