ELECTRON MICROSCOPY 



Nerve cells. The extremely elongated cyto- 

 plasmic extension of a nerve cell is called the 

 axon. The axon is covered by a number of 

 Schwann cells along its course. It is the 

 cytoplasm of the Schwann cell which wraps 

 itself around the axon to form the myelin 

 sheath. The axon and the myelin sheath to- 

 gether represent the nerve fiber. The main 

 component of the myelin sheath is the 

 plasma membrane of the Schwann cell which 

 builds up the myelin sheath in a varying 

 number of layers. It is, therefore, justifiable 

 to consider the myelin sheath as being a 

 specialized infolding of the plasma membrane 

 of the Schwann cell. 



Basement IVIembrane 



The basement membrane forms the struc- 

 ture upon wliich most cells rest. It is a 

 structureless layer with a thickness varying 

 between 400 and 1000 A (Fig. 13). Accord- 

 ing to histochemical tests, it does contam 

 mucopolysaccharides but so far no peculiar 

 ultrastructure has been detected in its 

 homogeneous layer. The old conception of 

 the basement membrane being a layer with 

 a thickness of several microns in some tissues 

 has been ruled out with the aid of the elec- 

 tron microscope. The thick basement mem- 

 branes seen in the light microscope contain, 

 in addition to the just described homogene- 

 ous layer, a number of fibrillar structures 

 most of which are reticular fibers (Fig. 10) 

 with some additional collagenous ones. It is 

 not quite clear what kind of cell is responsible 

 for the formation of the homogeneous base- 

 ment membrane seen in the electron micro- 

 scope. In some instances, it is surely laid 

 down by fibroblasts and it should, therefore, 

 be looked upon as being part of the connec- 

 tive tissue. However, sometimes no fibro- 

 blasts can be detected in adult tissue in 

 connection with the basement membrane and 

 it is, therefore, beheved that other cells may 

 have the ability of laying down this struc- 

 ture during their differentiation. 



Cell Organelles 



In the cell is recorded a number of or- 

 ganelles some of which are large and have a 

 definite form— rmioc/iondna, microhodies, 

 large granules, and pigments. Other or- 

 ganelles have more flexible form— Go/gi 

 apparatus, vesicles, and ergastoplasm (rough 

 surfaced endoplasmic reticulum). Within 

 the homogeneous ground substance of the 

 cytoplasm, as it was looked upon by means 

 of light microscopy, structures have been 

 detected with the electron microscope which 

 are of rather small diameters, but which 

 definitely should be listed among the cell 

 organelles— /?A^A-6franw?es and glycogen gran- 



^Jl PS 



Mitochondria. The mitochondria are dis- 

 crete bodies within the cell. They may vary 

 in number, size, and shape, but their ultra- 

 structure is remarkably unchanged from 

 cell to cell and from tissue to tissue. In the 

 fight microscope, they can be selectively 

 stained by Janus Green B, but even so, it is 

 sometimes difficult to distinguish them 

 from other granular structures in the cell. 

 The mitochondria are surrounded by a 

 double-contoured membrane, the thickness 

 of which is in the neighborhood of 180 A. 

 The matrix of each mitochondrion has a 

 higher density than the surrounding cyto- 

 plasm. The matrix itself is homogeneous or 

 slightly granulated. It is traversed by a vary- 

 ing number of double-contoured membranes 

 (or cristae) which mostly are arranged 

 parallel to each other. The inner mito- 

 chondrial membranes (or plates) with a 

 thickness of roughly 150A are always con- 

 nected with the mitochondrial capsule for 

 some distance, but there is no open connec- 

 tion between the cytoplasm of the cell and 

 the mitochondrial matrix (Fig. 3). Extremely 

 electron-dense spherical bodies of different 

 sizes are sometimes seen embedded in the 

 mitochondrial matrix between the mem- 

 branes. The mitochondria are the carriers of 



100 



