III. SPECIAL HISTOLOGY. 



'- rny 



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 marrow appears as a com- 

 pact tissue of small cells riddled with 

 large round holes. Under high magnifi- 

 cation the holes are seen to be fat cells 

 the nuclei of which are here and there 

 included in the section (Fig. 174). The 

 reticular framework of the marrow con- 

 sists of flattened cells generally seen cut 

 across; their nuclei then appear slender and 

 elongated. The abundant meshwork of 

 fibrils associated with these cells is not ap- 

 parent in ordinary sections. In the meshes 

 are found giant cells; premyelocytes; my- 

 elocytes which are neutrophilic, basophilic 

 or eosinophilic; erythrocytes; lymphocytes; 

 and mature corpuscles both red and white. 



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

 They have been named therefore '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 



152 . 



FIG. 174. HUMAN BONE MARROW. 

 Eosinophilic myelocyte; e-b., erythro- 

 blast; e-c., erythrocyte; f. c., part of 

 the protoplasmic rim of a fat cell; g. 

 c., giant cell; my., neutrophylic 

 myelocyte; n-b., normoblast; pm., 

 premyelocyte; r., reticular tissue cell. 



