The nucleolus is visible in 2 of the 3 cells shown 

 in figure 320. Since primordial osteogenic cells 

 are relatively rare, it has not been possible to 

 study them in as wide a variety of circumstances 

 as would be desirable. At one time it was thought 

 that they stained lightly because they were 

 partially squashed, and the nuclear staining in 

 figure 325 and in cell 31 of figure 321 may be 

 evidence for this; but the idea has been given up 

 because, if squashed, the chromatin should show 

 some liquefaction. It is agreed that they are 

 probably delicate and easily broken, but they 

 have been seen intact in sufficient numbers to 

 warrant the belief that they make up a distinct 

 cell type. 



Cell 2 of figure 319 shows cytoplasmic dif- 

 ferentiation leading toward the osteoblast or the 

 granulocyte. Had a nucleolus been visible, the 

 possibility of development toward the granu- 

 locyte would have been excluded. Cell 2 of 

 figure 320 shows differentiation toward the osteo- 

 clast. It is believed that both osteoblasts and 

 osteoclasts of these cell lines arise from a com- 

 mon stem cell, called the primordial osteogenic 

 cell, which, as shown in these figures, is a large 

 cell with abundant pale-staining cytoplasm and 

 a definite nucleolus. At tlie same time osteo- 

 genic activity is taking place, myelocytes also are 

 making their first appearance. This cell has 

 many features in common with the primordial 

 osteogenic cell. The nuclear stiiicture of cell 2 

 of figure 319 resembles that of a myelocyte, but 

 the strong mitochondrial spaces, the intense blue 

 staining, and especially the clear area beside the 

 nucleus indicate that it is an early osteoblast. 

 The magenta granules are not so large as they 

 typically are for myelocytes and there may be 

 some question of whether they are on or in the 

 cell ; if they were in it, the fact would add weight 

 to the idea that this was a metagranuloblast or an 

 early promyelocyte. In figure 320 somewhat 

 similar cells {11 and 12) have been identified as 

 metagranuloblasts and probably cells 13 and 14 

 are the same. 



Dantschakoff (1908b) states that the bone 

 marrow begins its blood-forming function on the 

 14th day of incubation. 



Large young osteoblasts (cell 5 of fig. 319 

 and cells 5—9 of fig. 320) show their immaturity 

 by their large size, lightly stained cytoplasm, 

 open reticulate chromatin pattern, and clearly 

 visible nucleoli. Vacuoles are present in the 



cytosome and some of these are pushed up into 

 the nuclear substance but they are not so abun- 

 dant as in the osteoclasts. 



Small mature osteoblasts (cells 3, 4, and 6 of 

 fig. 319) are more differentiated than their pre- 

 cursors, tlie large young osteoblasts. The cyto- 

 plasm stains more intensely than any other cell 

 of the marrow. Mitochondrial spaces are not so 

 distinct as they were previously and the clear 

 area adjacent to the nucleus is often masked. 

 The nucleus retains the eccentric position it pre- 

 viously had but now it is smaller, and the chro- 

 matin pattern is condensed, which causes it to 

 stain more intensely. All these features are 

 shown in the high-power drawing of a cell from 

 an embryo incubated 14 days 11 hours (fig. 

 323). Osteoblasts may clump as in cell 6 of 

 figure 319 but this apparently binucleated con- 

 dition does not make this an osteoclast like cell 

 9. 



These small cells are called mature because 

 they are morphologically identical with cells ob- 

 served adjacent to bone in sections. They have 

 a darkly stained cytoplasm and a distinct para- 

 nuclear area. The texts on general histology 

 call these blast cells, but, functionally, they are 

 mature.^ They form a row along the bone 

 spicule and produce a bone matrix. Eventually 

 they will be surrounded by this matrix and will 

 rest within a lacuna; they will then be called 

 osteocytes. 



Cells 7 and 8 of figure 319 are mononuclear 

 osteoclasts. The appearance of the cytoplasm 

 of one of these cells is quite different from the 

 cytoplasm of the other; cell 8 has many orange- 

 colored granules to the left of the nucleus and no 

 vacuoles; cell 7 has only a few orange-colored 

 bodies but has many vacuoles. The binucleate 

 cell, 9, also has numerous vacuoles but no orange 



" This Atlas may not be a suitable work in which to propose 

 changing the name of the functional bone-producing cell from 

 osteoblast to something more appropriate, but it does seem 

 somewhat confusing to retain the suffix "blast" through all 

 the developmental stages of this cell. It is recognized that 

 "large young osteoblast" is not an adequate substitute term 

 for the immature stage. Any attempt to standardize hema- 

 tologic nomenilature should include all the connective-tissue 

 derivatives, the phagocytic cells, and the cells of the circulating 

 blood and their developmental stages, and this must come from 

 the united efforts of many scientists. The same confusion 

 exists in assigning terms to the developmental stages of the 

 osteoclast. They could be called osteoclastblasts and osteo- 

 clastcytes, but these terms are awkward. For the present the 

 stages are separated on the basis of the number of nuclei, on 

 the assumption that the number is a fair measure of the stage 

 of differentiation. 



152 



