Thymus – Tutorial

Please read Unit 8 – Introduction to Lymphatic System Tissues prior to completing the activities in this chapter.

Introduction to the Thymus

The thymus is a primary organ of the lymphoid system as it serves as a primary site for T-lymphocyte production and maturation during childhood.  The thymus is most active early in life and it is relatively large in infants and young children and it extends from the superior border of the heart to the base of the neck in the anterior mediastinum.  After puberty, the thymus becomes less active as an organ of lymphocyte production and decreases in size and its lymphoid tissue is gradually replaced by fibrous connective tissue and adipose.

The thymus is surrounded by a connective tissue capsule and some connective tissue extends from the outer capsule into the interior of the thymus forming septa that separate thymic tissue into hundreds of small lobules that measure approximately 2mm in diameter.  Each thymic lobule consists of an outer cortex that is packed full of clusters of T-lymphocytes surrounded by epithelial reticular cells which help form the blood-thymus barrier and secrete hormones that stimulate lymphocyte division and maturation.

Throughout childhood, large numbers of lymphocytes migrate via the bloodstream from red bone marrow to the cortex of thymic lobules where they continue to divide and produce more T-lymphocytes.  As lymphocytes mature in the cortex of lobules, they gradually migrate toward the medulla of lobules where they eventually leave the thymus via lymphatic vessels or medullary blood vessels which are not separated from the general circulation by a blood-thymus barrier.  After exiting the thymus, T-lymphocytes take up transient residence in lymph tissues and organs all over the body where they continue immune surveillance for foreign antigen and abnormal cells.

Tutorial:  Use the image slider below to learn more about the structure and characteristics of the thymus. 

Microscopy:  Use the image slider below to learn how to use a microscope to study the thymus on a microscope slide.

Tutorial:  Use the hotspot image below to learn more about the structure and characteristics of the thymus.   

Tutorial:  Use the image slider below to study numerous examples of tissue from the thymus.