ELECTRON MICROSCOPY 



the epithelium. The nucleus of the goblet 

 cell is pushed to the base of the cell because 

 of the heavy accumulation of mucous gran- 

 ules. After the discharge of the mucin, the 

 cell resumes its resting columnar shape and a 

 new production cycle of mucin starts again 

 (Figs. 1,2). 



Serous cell. The structure of the serous 

 cells is quite variable, depending on what 

 organ the cells are located in. In the trachea 

 only few serous cells have been recognized, 

 but in the fine bronchi and in the hronchioles 

 they become quite abundant. Their cyto- 

 plasm has an abundance of RNA-particles, 

 presumably related to the ability of the cells 

 to secrete enzymes. In addition, a large 

 number of cytoplasmic vesicles indicate that 

 the cells are turning out fluid or substances 

 dissolved and transported within the mem- 

 brane-bound vesicles. It is believed that 

 these cells play an important role in the 

 mechanism involved in pulmonary edema. 

 In the oviduct of some mammals (man not 

 included), the serous cells display a large 



Fig. 5. A schematic representation of the fine 

 structure of the mammalian cilium. (After Rhodin 

 and Dalhamn, 1956). 



mmiber of secretion granules. They appear 

 first within the Golgi zone and migrate from 

 here to the cell surface where they are dis- 

 charged without previous fusion with one 

 another. The granules seem to have some 

 nutritional or enzymatic relation with the 

 ovum when it passes along the oviduct to the 

 uterus. The surface of the serous cells is 

 characterized by a number of microvilli, the 

 size of which is smaller than either the cilia of 

 neighboring cells or the brush border ex- 

 tensions seen on the cells of the intestine or 

 the proximal convolution of the nephron in 

 the kidney. The nucleus is located in the 

 center of the cell and is not dislocated during 

 the secretory cycle, the latter being struc- 

 turally less obvious than what is noticed in 

 the mucous cells. 



Basal cell. The basal cells are always lo- 

 cated near the basement membrane upon 

 which most of the cells of the ciliated epi- 

 thelia rest. The basal cell is round or slightly 

 elongated and does not reach the surface of 

 the epithelium. Its cytoplasm contains small 

 fibrils of unknown function. Eventually, the 

 cell migrates to the upper part of the epi- 

 thelium where it gains contact with the sur- 

 face and starts to differentiate into a cell 

 type which is ready to replace any of the 

 two kinds of cells that dominates the epi- 

 thelium, the ciliated and the mucous cell 

 (Fig. 6). 



Brush cell. It is probable that the brush 

 cell represents a basal cell which has recently 

 moved to the surface and started to differen- 

 tiate into a ciliated cell. This is quite obvious 

 in the trachea, where several features of the 

 brush cell are identical with the basal cell. 

 The surface of the brush cell is covered with 

 a large number of brush border-like exten- 

 sions. In man, a dense plate is found at a 

 distance of about half a micron beneath the 

 surface of the brush cell, presumably the 

 site of differentiation of the ciliary basal 

 bodies. Another typical feature of the brush 

 cell is the clustering of small mitochondria 

 beneath this plate in the brush cell of the 



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