III. SPECIAL HISTOLOGY. 



BLOOD FORMING AND BLOOD DESTROYING ORGANS. 



BONE MARROW. 



Bone marrow is the soft tissue found within the central cavities of 

 bones. Its source in the embryo is the vascular mesenchyma invading a 

 cartilage which is being replaced by bone. Early in its development it 

 contains osteoblasts and osteoclasts, and these cells may be found in adult 

 marrow, where it is in contact with the bone. The greater part of the 

 mesenchyma becomes reticular tissue with fat cells intermingled. The 

 meshes of the reticular tissue are occupied by an extraordinary variety of 

 cells, most of which are called myelocytes (marrow cells). In ordinary 



sections the tissue of the marrow appears 

 to be riddled with large round holes. 

 Under high magnification the holes are 

 seen to be fat cells, the nuclei of which 

 are here and there included in the sec- 

 tion (Fig. 190.) The reticular tissue 

 framework of the marrow consists of flat- 

 tened cells, generally seen cut across; their 

 nuclei then appear slender and elongated. 

 The abundant meshwork of fibrils asso- 

 ciated with these cells is not apparent in 

 FIG. 190. HUMAN BONE MARROW. ordinary sections. In the meshes are 



e., Eosinophilic myelocyte; e-b., erythro- 



biast; e-c., erythrocyte; f. c., part of found giant cells; premyelocytcs; myelocytes 



the protoplasmic rim of a fat cell; g. c., 



giant cell ; my., neutrophylic myelocyte ; which are HeutrOpkilic, bdSOpkilic OT COSinO- 



n-b., normoblast; pm. f premyelocyte; 



r., reticular tissue ceii. philic; erythrocytes; lymphocytes; and ma- 



ture corpuscles both red and white. 



The giant cells of the marrow have a single polymorphous nucleus. 

 They have therefore been named "megakaryocytes," in distinction from 

 the multinucleate osteoclasts or "polykaryocytes." The nucleus is so 

 large that it may be cut into several slices, and by combining these it has 

 been found that the entire nucleus is a hollow sphere with perforated walls; 

 the nuclei, however, are very irregular, and some may be of other forms. 

 With Wright's stain the protoplasm clearly shows an outer hyaline exo- 

 plasm and an inner granular endoplasm. It has been said that the latter 

 is divisible into two concentric zones, which differ from the protoplasm 

 within the nuclear sphere. In ordinary preparations these details are 



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