The thymus varies in size and development with the age of the individuals.
Thymus begins to decrease in size and undergone involution with advancing age and gets gradually replaced by fat and connective tissue.
The thymus is covered by a connective tissue capsule, which gives off numerous septa, which divide the gland into a number of polygonal lobules.
Each lobule consists of a cortex and medulla.
Often the medullary substance is continuous through several lobules.
Cortex
It consists of a compact, dense lymphoid tissue (reticular tissue and lymphocytes). The cells often called as thymocytes are closely packed but do not form nodules. The reticular cells are often obscured by the loosely packed thymocytes of lymphocytes.
Medulla
The thymocytes are less numerous and are not so closely packed together and hence the medulla presents a lightly stained appearance in sections in contrast to the darkly stained cortex. The Reticular tissue is more distinct.
The medulla also contains spherical or oval bodies known as Thymic or Hassall’s corpuscles. These are characteristic of thymus.
Each corpuscle consists of a central eosinophilic hyaline mass surrounded by concentrically arranged flattened reticular cells which become continuous with the cells of the surrounding reticulum.
The reticular tissue in thymus is different from the reticular tissue in other organs, in that it is derived from the endoderm, where as the reticular tissue in other organs is derived from mesoderm.
A number of views have been held regarding the functions of thymus and it is now considered that thymus serves as a lymphocytes producing center during foetal and early postnatal life and the lymphocytes produced are distributed to other lymphatic organs, where they establish colonies and multiply.
The colonization of the various lymphatic tissue of the body with lymphocytes of thymic origin begins at different times in different species. It has also been suggested that thymus produces a Lymphocyte stimulating factor, which is responsible for proliferation of lymphocytes in other parts of the body. The thymus has some interrelationship with the gonads, adrenal and thyroids.
