ELECTRON MICROSCOPY 



elastic, structure, which gives the cells of the organ and is probably dependent on the age 



epidermis a certain elasticity and firmness, and the function of the fibril. Each collagen 



Neurofibrils. In the axoplasm of nerve cells fibril is in turn composed of small protofibrils 



still another fibrillar cytoplasmic structure or tropocollagcn units with a diameter of loA 



can be found. The neurotibrils of classical and a length of 2G00A. The tropocollagcn 



histology represent aggregates of axon fila- units are tied up in a staggered fashion to 



ments large enough to be resolved in the form the collagen fil)ril. The bands of the 



light microscope. Deposition of heavy met- collagen fibril represent discontinuities in the 



als, as with the histological silver and gold staggered arrangement of the protofibrils 



technics, favors the detection of the very and stand out in the electron micrograph 



thin fibrous structures. In the early days of because heavy metals have a greater affinity 



electron microscopy, these structures were for this irregularity. The formation of the 



mistaken for tubules, hence the first name to immature collagen seems to occur at the sur- 



be coined was neurotubules. Presently, there face of the fibroblasts, the main cell type of 



seem to be two kinds of fibrils in the axo- all connective tissues. Small fibrils without 



plasm — the protoneurofibrils and the neuro- periodicity appear at the surface of the 



filaments. The proto7ieuro fibrils appear as fibroblast. These unit fibrils organize out of 



smooth threads with a thickness of about 60 or polymerize from material present at the 



to 80A. Their length ranges between 0.5 to cell surface, and from here the fibrils, in 



1 micron. They have an irregular course and many cases already in bundles, are shed into 



are seen to branch and interconnect. The the intercellular space. The fibrils will first 



neurofilaments have a thickness of about appear with an axial periodicity of 210A, but 



150A with indefinite length. They are some- as they increase in size they also change into 



times of a double-edged appearance. Their a 640A periodicity. The subsequent fibril 



surface is smooth and they do not seem to growth apparently occurs by accretion of 



constitute any part of the endoplasmic materials from the general environment of 



reticulum. The function of either type of the intercellular spaces and not by fusion of 



fibrillar structure is unknown. smaller fibrils as believed earlier. Subsequent 



Extracellular. Collagen. The main fine layers of collagenous material are deposited 



component of the connective tissue is the upon the core, represented by the tropocoi- 



collagen fibers. The width of the collagen lagen unit fibril. The origin of the tropocol- 



fiber is about one micron and its length is lagen fibril is not settled, but considering the 



indefinite. Each fiber is in turn built up of presence of abundant rough-surfaced endo- 



numerous collagen fibrils which also may be plasmic reticulum in the cytoplasm of the 



seen single or in groups of two or more fibrils fibroblast which presumably is involved in 



scattered in the interstitial tissue (Fig. 15). the protein synthesis, it seems justifiable to 



Within each group of collagen fibrils the suggest the following mechanism as being 



fibrils are usually parallel with each other, the most likely regarding the role of the 



The collagen fibril has a thickness which fibroblast in the formation of the collagen 



varies between 400 and 2400A depending on fibrils. From the cisternae of the rough-sur- 



the age (Fig. 10) ; its length is indefinite. The faced endoplasmic reticulum of the fibroblast 



fibril has an axial periodicity of light and the monomeric form of the tropocoUagen 



dense bands with a length of each period of unit fibril is discharged to the environment of 



approximately 640A. In the light and dense the cell and is here ciuickly induced to 



segments can be seen several smaller bands polymerize by enzymes resident in tem- 



of varying density and thickness. The length plates at the cell surface or in the unit fibrils 



and number of bands varies from organ to themselves. 



108 



