ENDOCHONDRAL BONE. 



97 



Fig. 127. 



Osteoblast 

 becoming 

 a bone-cell 



Osteoblasts 



Bone-cell 



Section of a portion of osseous tra- 

 becula and foetal marrow. X 375. 



finally, they lie isolated within the newly formed bone as its cells ; the bone-cells are 

 therefore imprisoned osteoblasts, which, in turn, are specialized connective-tissue 

 elements. The bone-cells occupy minute lenticular spaces, the primary lacuncs, at 

 this immature stage the canaliculi being still unformed. The early bone-matrix is at 

 first soft, since the deposition of the calcareous materials takes place subsequently. 



The increase in the thickness of the new bone is attended by the gradual disap- 

 pearance of the enclosed remains of the calcified cartilage, the last traces of which, 

 however, can be seen for some considerable time as 

 irregular patches within the osseous trabeculae (Fig. 

 131), somewhat removed from the zone of calcification. 

 The cartilage and the bone of the trabeculae stand, 

 therefore, in inverse relations, since the stratum of bone 

 is thinnest where the cartilage is thickest, and, con- 

 versely, the calcified matrix disappears within the robust 

 bony trabeculae. A number of the latter, together with 

 the enclosed remains of the calcified cartilage, soon 

 undergo absorption, with a corresponding enlargement 

 of the intervening marrow-spaces. The remaining tra- 

 beculae increase by the addition of new lamellae on the 

 Surface covered by the osteoblasts, and at some distance 

 from the zone of calcification form a trabecular reticulum, 

 the primary central spongy bone. In the case of the 

 irregular bones, the central spongy bone is represented 

 by the cancellated substance forming the internal frame- 

 work ; in the long bones, on the contrary, the primary 



cancellated tissue undergoes further absorption within the middle of the shaft simul- 

 taneously with its continued development at the ends of the diaphysis from the car- 

 tilage. As the result of this absorption, a large space — the central marrow- cavity — is 

 formed (Fig. 129), the growth of which keeps pace with the general expansion of 

 the bone. 



The absorption of the young osseous tissue to which reference has been made 

 is effected through the agency of large polymorphonucleated elements, the osteo- 

 clasts. These are specialized marrow- 

 ^ cells whose particular role is the break- 



ing up and absorption of bone-matrix. 

 They are relatively very large, their 

 irregularly oval bodies measuring from 

 .050 to .100 millimetre in length and 

 from .030 to .040 millimetre in breadth. 

 The osteoclasts (Fig. 128), singly or 

 in groups, lie in close relation to the 

 surface of the bone which they are at- 

 tacking within depressions, or How- 

 ship' slaciaicB, produced in consequence 

 of the erosion and absorption of the 

 osseous matrix which they effect. 

 When not engaged in the destruction of 

 bone, these cells occupy the more central 

 portions of the marrow-tissue, where, 

 in the later stages, they are probably 

 identical with the myeloplaxes or giant 

 cells encountered in the red marrow. 

 The only part of the central spongy bone which persists after the completed 

 development and growth of the long bones is that constituting the cancellated tissue 

 occupying their ends. It will be seen, therefore, in many cases, that the product of 

 the endochondral bone-formation, the primary central osseous tissue, is to a large 

 extent absorbed, and constitutes only a small part of the mature skeleton. The 

 early marrow-cavity, as well as all its ramifications between the trabeculae, is filled 

 with the young marrow-tissue ; the latter gives rise to the permanent red marrow 



Howship's- 

 lacuna 



Osteoclast 



Bone-cell 

 within 

 lacuna 



Osteoblasts 



Portion of trahecula of spongy bone undergoing absorp 



tion by osteoclast. X 500. 



