TISSUES (CONNECTIVE) 



Ligaments subjected to lateral stresses are 

 usually found to contain elastin. Thus, while 

 the human ligamentum nuchac has been ob- 

 sei'\'ed to contain predominantly collagen 

 and ground substance, specimens of the 

 ligamentum nuchae from goats, sheep and 

 similar animals with the head held in a for- 

 ward direction are frequently seen to contain 

 more elastin than collagen. In these elastic 

 ligaments, the appearance of the elastin is 

 more genuinely fiber-like than in any other 

 site, excepting the vocal cords. It is also 

 noticeable that, whereas in most tendons 

 collagen fibrils lie practically parallel within 

 their bundles, in elastic ligaments the colla- 

 gen fibrils between the elastic fibers present 

 a wavy appearance — no doubt to allow for a 

 greater extension. 



Fibrocartilage. Tissues such as the 

 meniscus of the knee and the intervertebral 

 discs are compact connective tissues, with 

 tightly interwoven bundles of collagen pro- 

 duced by fibroblasts. In each bundle, the 

 individual fibrils are approximately parallel, 

 but there is not the same over- all Hnear 

 orientation as is found in tendons. The direc- 

 tions of orientation are related to the forces 

 to which the tissue is subjected. When these 

 are variable, as in the meniscus, the average 

 orientation is very low, whereas in the disc 

 it tends to be parallel to the circumference. 

 Such obsei'vations are best made using X-ray 

 diffraction (W. G. Horton, Thesis, London, 

 1956). When specimens of more Umited size 

 are examined in the electron microscope the 

 finer details of structure are more apparent. 

 In the bundles there is often seen a distinct 

 tendency for fibrils to be arranged in sheets 

 (Fig. 13), while bundles of fibrils with differ- 

 ent orientation can be glued together by 

 ground substance almost as efficiently as the 

 individual sheets within the bundles. In sites 

 subjected to deformation, the collagenous 

 matrix is usually reinforced by elastic tissue 

 (Fig. 8). The proportion in adult epiglottis 

 is often high. 



Primitive Cartilage. Cartilage proper — 

 the tissue produced by chondrocytes — is of 

 three main types. Articular cartilage and 

 growth cartilage both develop from a primi- 

 tive cartilage which has cells of the type 

 shown in V\g. 1. Frequently these are in 

 groups of two or four, with adjacent faces 

 flattened against one another. They do not 

 have the prominent cell walls of the fibro- 

 blasts, but have ones which are finer than 

 the nuclear membrane. Most cells or groups 

 of cells are in a capsule appearing, when 

 dried, as a very fine network. The matrix 

 between cells, or groups of cells, in their cap- 

 sules appears as a dried gell containing only 

 very fine fibers. There is no indication that 

 any collagen fibrils are formed at the cell 

 wall, but the appearances are consistent with 

 them having been precipitated in situ. 



Articular Cartilage. Bones develop from 

 a model, or anlage, of primitive cartilage. In 



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Fig. 13. Section of fibrocartilage from menis- 

 cus of the knee. Embedding medium removed. 

 Metal shadowed. X 10,000. 



285 



