TISSUES (CONNECTIVE) 



Growth Cartilage. In regions where 

 forces are predominantly unidirectional the 

 third type of cartilage is found. This growth 

 cartilage is mainly found at the growing ends 

 of long bones. Instead of the cells being in 

 groups of two or four, as in the primitive 

 cartilage, a whole column of cells is seen as 

 a single unit. The cells in the column are 

 flattened, with evidence of frequent division, 

 and the direction of the columns is along the 

 lines of force. Between the columns of cells 

 is collagen, oriented in the same direction 

 (Fig. 16). 



At the metaphyseal end of the cartilage 

 the cells in the columns expand, to form the 

 hypertrophic cells, which are of the order of 

 40 jLt across. At the same time that expansion 

 occurs there is normally a change in the ma- 

 trix between the cells. This shows up in the 

 electron microscope as a change of texture, 

 the matrix assuming a more compact appear- 

 ance. It is this altered matrix which can 





Fig. 16. Section from epiphyseal growth car- 

 tilage of rabbit. The collagen fibrils are parallel to 

 the columns of cells. Formalin fixed. Embedding 

 medium removed. X4000. 



Fig. 17. Metaphj-seal side of growth cartilage 

 in rabbit. A vessel containing red cells has invaded 

 the last hypertrophic cell space. The vessel wall is 

 not yet complete. The first osteoblasts can be seen 

 between the vessel walls and calcified cartilage. 

 Formalin fixed. Embedding medium removed. 

 X2000. 



calcify, when the necessary vessels, calciun\ 

 phosphate and vitamins are present. The 

 cells, thus removed from their source of 

 nourishment, die and are replaced by the 

 encroaching vessel, as shown in Fig. 17. 

 Here red cells are within the capsule of a 

 hypertrophic cell, with calcified cartilage on 

 either side. It may be seen that the capillary 

 wall is incomplete — a very common observ'a- 

 tion. (J. Trueta and K. Little, /. Bone and 

 Joint Surg 42B: 367, 1960). 



Bone. At different stages of its develop- 

 ment bone contains primitive cartilage, ar- 

 ticular cartilage, growth cartilage and calci- 

 fied cartilage; but the greater part of the 

 bone matrix, for most of the life of a man or 

 animal, is the osteoid matrix. The osteoid is 

 laid down by cells which differ in form from 

 fibroblasts or chondrocytes. In Fig. 17 are 

 seen the first osteoblasts, which appear to 

 be on or near the capillary walls, and are 

 sending out processes toward the calcified 

 cartilage. Previously existing calcified tissue 

 appears to be necessary for the complete 

 development of osteoblasts. When mature, 

 these lay do^vn a collagenous matrix. As 

 with fibroblasts, the collagen of bone matrix 

 appears to originate at the cell wall of the 

 osteoblast. The cell processes remain, and lie 



287 



