THE THYMUS 



567 



its circumference, the medulla reaches the surface of the lobule, and 

 at such locations a narrow column of medullary lymphoid tissue connects 

 it with the adjoining lobule. 



A close-meshed reticulum, within the narrow meshes of which are 

 closely packed lymphoid cells, composes the lymphoid tissue of the lobule. 

 That of the cortex and the medulla is alike, except for the fact that 

 the meshes of the reticulum in the cortex are much more crowded 

 with 'lymphocytes' than are those of the medulla. The medulla of 

 each lobule is also charac- 



^_ . , - /-^V.^ 



terized by the presence of 

 several groups of concen- 

 trically arranged epitheli- 

 oid cells, the tJiymic cor- 

 puscles (concentric corpus- 

 cles of Hassall). 



Each thymic corpuscle 

 consists of a large central 

 cell or group of cells, which 

 is surrounded by two to five 

 layers of concentrically ar- 

 ranged flat epithelioid cells. 



These groups or cell nests FJG 48g _ A THYMIC CoRPUSCLE FROM THE 

 are strongly acidophil in THYMUS OF AN INFANT. 



their staining reactions, Hematein and eosin. X 665. 



and therefore stand out in 



marked contrast to the basophilic nuclei of the surrounding lymphoid 

 tissue. A substance simulating the colloid of the thyroid may be present 

 among the cells. Since no similar structure occurs elsewhere in the body, 

 the concentric corpuscles are absolutely characteristic of the thymus 

 lobule. 



The nature of the thymic corpuscles is not satisfactorily understood. 

 According to one hypothesis they represent blood-vessels whose lumen 

 has been obliterated by proliferation of its endothelial cells. Another 

 theory regards them as remains of the epithelial columns from which 

 the organ arose in the embryo. They are also interpreted as masses 

 of hypertrophied cells of the reticulum. 



The tij/H'x of lymphoid corpuxrlc which are found in the thymus 



;uv similar to those of Hie lymph nodes, though polymorphonuclcnr 



leukocytes are rather more frequent here, and giant cells, mononuclear 



or multinuclear in form, may be readily found in the medulla of this 



36 



