REGENERATION 691 



as normal growth and those which are regenerative in nature. The 

 ability of various tissues to regenerate will be described briefly. 



Epithelial Tissue. — Cuts and other wounds of the body surface 

 are healed by the migration of cells from the old epithelium over a 

 substrate formed by the underlying vascular and connective tissues. 

 Proliferation of the epithelial cells by mitosis follows. 



Glands, such as the mammary and salivary glands, are capable 

 of considerable regeneration by proliferation of the small cells of 

 their secretory ducts. The liver is capable of regeneration by pro- 

 liferation of cells of the small bile ducts. The relation of the re- 

 generated tissue to the old tissue is usually atypical. If a lobe of 

 the liver is removed, the other lobes increase in mass, but the lost 

 lobe is not replaced by a new lobe. This process is called compensa- 

 tory hypertrophy. The kidney has a very limited capacity for regen- 

 eration, but if one kidney is removed, the remaining one does undergo 

 considerable compensatory hj^pertrophy. 



Connective Tissues. — Blood cells of all sorts are formed contin- 

 ually in the mammalian body by special groups of cells. White 

 blood cells are formed largely in patches of lymphoid and myeloid 

 tissue scattered throughout the body. Red cells are formed largely 

 in the red bone marrow. When considerable amounts of blood are 

 lost, regeneration takes place through increased proliferation of 

 these special groups of cells. 



Small blood vessels are regenerated by sprouting from pre-existing 

 capillaries. These sprourts may unite distally. The sprouts thicken 

 and become channeled to complete the vascularization of the injured 

 area. 



Cartilage is regenerated largely from cells of the perichondrium. 

 Bone is not regenerated primarily from pre-existing bone cells. The 

 cells of the periosteum or marrow fonn a cellular connective tissue or 

 the perichondrium forms hyaline cartilage to form a mass called a 

 callus. Osteoblasts then invade this tissue to form new bone and 

 complete the repair. 



The fibrous connective tissues regenerate by the division of undif- 

 ferentiated cells. A homogeneous ground substance is formed be- 

 tween the cells. Fibers appear in the ground substance and complete 

 the regeneration. 



