ELECTRON ^IICROSCOPY 



stance of cartilage do not always show the 

 cross striatioiis (Fig. 10), and it may, there- 

 fore, be assnmed that not all reticular fibers 

 can be transformed into collagenous ones. 



Elastin. Elastic fibers are found in loose 

 connective tissue (Fig. 15). They have a 

 thickness of about one micron and seem to 

 have an indefinite length. They are not 

 fibrillar but have a homogeneous appearance 

 in the light microscope. Elastic fibers at 

 most show only a weak positive birefrin- 

 gence, but become strongl}^ birefringent on 

 stretching. This is caused by an orientation 

 of the submicroscopic components in the 

 direction of the fiber axis. These ultrastruc- 

 tural components are difficult to demonstrate 

 in osmium-fixed specimens, but it has been 

 possible to distinguish thin filaments with 

 a thickness of about 70A at the periphery 

 of the elastic fiber. Hence, it seems that the 

 elastic fiber has two main components. The 

 dominating structure is a non-fibrous dense 

 cement substance, presumably an albumi- 

 noid which embeds the less apparent elastic 

 filaments. 



Crystals 



There are only a few examples of where 

 crystals may be found in normal mammalian 

 tissues. 



Intracellular. Intracellularly located are 

 the so called crystalloids of the interstitial 

 (Sertoli) cells of the human testis. These 

 crystals are probably of protein natvn-e. The 

 crystals can be seen in the light microscope. 

 They are usually elongated structures with 

 rounded or pointed ends. Each cr3^stalloid 

 body is made up of numerous dense granules 

 with a diameter of about 150A. They are 

 spaced about 190A apart along two axes 

 which are approximately at right angles to 

 each other. This pattern is thought to 

 represent the arrangement of macromole- 

 cules in the lattice of a protein crystal. The 

 function of the crj^stals and the reason for 

 their presence in the Sertoh cells of the testis 

 is unknown. It has been suggested that these 



cells have an endocrine glandular function 

 and, therefore, the crystals may be involved 

 in the production of the hormone. 



Extracellular. Extracellularly located 

 crystals are encountered in the bone tissue 

 where they make up the strong and resistant 

 component of the skeletal system. The crys- 

 tals have a width of about 35A. They are scat- 

 tered in the extracellular matrix of the bone 

 tissue but they are also lined up within the 

 collagen fibrils with their long axes oriented 

 parallel to the long axis of the collagen fibril. 

 Selected-area electron-diffraction of these 

 structures has revealed that they are crys- 

 tals of apatite, more specifically hydroxy- 

 apatite [Caio(P04)6(OH)2]. Similar crystals 

 appear under pathologic conditions in areas 

 undergoing calcification like in aging car- 

 tilage, and they also form the main com- 

 ponent of concretions precipitated through- 

 out the urinary system (Fig. 16). 



Secretion 



As already mentioned, most of the large 

 granules encountered in secretory cells are 

 associated with secretory processes and 

 have, therefore, been called secretory granules. 

 Although formed within the cell and from 

 the beginning being a part of the cytoplasm, 

 they become discharged and perform their 

 action outside the cell territory. There are 

 two types of secretory cells, namely the 

 exocrine and the endocrine cells. 



Exocrine Secretion. The exocrine secre- 

 tory granules appear within the Golgi zone, 

 but are evidently preformed in association 

 with the ergastoplasmic sacs (or the cis- 

 ternae of the rough-surfaced endoplasmic 

 reticulum). The fii'st indication of secretory 

 granules within the Golgi zone is a condensa- 

 tion of the Golgi vacuoles or a swelling of the 

 Golgi vesicles and small granules. 



Sei'ous secretion. In secretory cells with a 

 serous production, the indi^-idual secretory 

 granules are usually surrounded by a single 

 membrane and there is no indication of a 

 coalescence of granules. The granules mi- 



110 



