Embryonic Developmenf} Differentiation of Tissues - 285 



HISTOLOGY: ORIGIN AND NATURE OF 

 THE BODY TISSUES 



All the cells of the developing embryo pos- 

 sess equivalent chromosomes, derived by mi- 

 tosis from the zygote, and in young embryos, 

 the cells display little or no obvious differ- 

 entiation. But sooner or later the cells in 

 different regions of the embryo become dif- 

 ferent; some become muscles, some nerve, 

 etc., until finally all the tissues of the adult 

 become recognizable. 



Initially, the particular destiny of an em- 

 bryonic cell is not determined, and many 

 factors, both physical and chemical, cooper- 

 ate in determining the eventual fate. In the 

 frog embryo, for example, the ectoderm cells 

 that lie in the mid-dorsal region of the 

 embryo would normally give rise to nerve 

 cells. But these same cells can be trans- 

 planted to the side of another embryo — and 

 in this case they become skin cells. In other 

 words, the cells develop differently according 

 to their respective positions in the embryonic 

 body. The physical and chemical forces that 

 act upon the surface cells of an embryo 

 differ from those that play upon the deeper 

 cells; and there are many less obvious differ- 

 ences that affect development in the differ- 

 ent embryonic regions. Moreover, cells that 

 have become differentiated sometimes exert 

 a differentiating influence upon other cells, 

 especially in nearby regions of the embryo 

 (p. 534). But the problems of differentiation 

 will be considered later (Chap. 27), and at 

 present we will be concerned only with the 

 end results of differentiation — the tissues of 

 the adult animal. 



Many specific differences are found among 

 the tissues of various animals, although the 

 same four general types can always be recog- 

 nized. These fundamental tissues are: (1) 

 epithelium, (2) muscle, (3) nerve, and (4) 

 connective (sustentive) tissue. Each of these 

 fundamental tissues will be described in 

 terms of the vertebrate animal. 



Epithelial Tissues. The epithelial tissues, 

 which cover the various external and internal 



surfaces of the body, are composed of closely 

 fitting cells, with only a minimum of inter- 

 cellular material binding the tissue into a 

 continuous membrane. 



Epithelial cells are variously modified at 

 different surfaces, and accordingly, several 

 kinds of epithelia can be recognized. In squa- 

 mous epithelium (Fig. 2-8) the cells are flat 

 and scalelike; whereas in columnar epithe- 

 lium the cells are cylinders, or bricks, of 

 greater or lesser depth (Fig. 2-8). In simple 

 epithelia there is only a single tier of cells, 

 but in stratified epithelia there are several 

 or many tiers. Moreover, some epithelia are 

 ciliated (Fig. 15-12), but others are non- 

 ciliated. 



CILIATED 

 EPITHELIUM 



MUCUS 



GOBLET CELL 

 NUCLEUS 



CYTOPLASM 



Fig. 15-12. Epithelial cells in the mucosal lining of the 

 human trachea. 



Some epithelia are composed entirely of 

 glandular cells, which secrete their products 

 at the epithelial surface. This is true of the 

 lining of the coelomic cavity and all its 

 derivatives. In the abdominal cavity, where 

 the epithelium is called the peritoneum; in 

 the thoracic cavity, where it is called the 

 pleura; and in the pericardial cavity, where 

 it is called the pericardium — all the cells 

 produce a watery, or serous, fluid, which 

 lubricates the internal surfaces of the body. 

 But in the mucosa, which lines the digestive 

 tract and most of its offshoots, only some of 

 the cells are gland cells (Fig. 15-12). All multi- 

 cellular glands in the body are derived from 



