Definition of Thymus

The Thymus is the site for development of thymic derived lymphocytes, or T cells.

What happen inside the Thymus 

Developing T cells, called thymocytes, differentiate from lymphoid stem cells derived from the marrow into functional, mature T cells. It is here that T cells acquire their repertoire of specific antigen receptors to cope with the antigenic challenges received throughout one’s life span. Once they have completed their maturation, the T cells leave the Thymus and circulate in the blood and through secondary lymphoid tissues.

Anatomy of the Thymus

The thymus is located in the superior mediastinum, overlying, in order, the left brachiocephalic (or innominate) vein, the innominate artery, the left common carotid artery, and the trachea. It overlaps the upper limit of the pericardial sac below and extends into the neck beneath the upper anterior ribs. It receives its blood supply from the internal thoracic arteries. Venous blood from the thymus drains into the brachiocephalic and internal thoracic veins, which communicate above with the inferior thyroid veins.

Embryology of the Thymus

Arising from the third and fourth brachial pouches as an epithelial organ populated by lymphoid cells and endoderm derived thymic epithelial cells, the thymus develops at about the
eighth week of gestation.

Thymus size thorough life

The thymus increases in size through fetal and postnatal life and remains ample into puberty, when it weighs approximately 40 g. Thereafter, the size progressively decreases with aging as a consequence of thymic involution. The volume of the thymus can be estimated by sonography.

Studies about Thymus

In one study of 149 healthy term infants within 1 week of birth, there was a significant correlation between the estimated thymic volume and the weight of the infant. However, no correlation was apparent between the estimated thymic volume and the infant’s sex, length, or gestational age. Also, there was no apparent correlation between estimated volume and the proportions of CD4+ T cells or CD8+ T cells found in the blood.

The estimated thymic volume of healthy infants increases from birth to 4 and 8 months of age and then decreases. Most of the individual variation at 4 and 10 months of age appears to correlate with breast feeding status, body size, and, to a lesser extent, illness. Breast fed infants at 4 months of age have significantly larger estimated thymic volumes than do age matched formula fed infants with similar thymic volumes at birth.

Structure of The Thymus

A longitudinal fissure divides the Thymus into two asymmetrical lobes, a larger right and a smaller left, that are derived from the right and left brachial pouches, respectively. These two developmentally separate parts of the Thymus are easily separated from each other by blunt dissection.

Each lobe of the thymus is divided into multiple lobules by fibrous septa. Each lobule consists of an outer cortex and an inner medulla. The cortex contains dense collections of thymocytes that appear as lymphocytes of slightly variable size with scattered, rare mitoses.

The lighter staining medulla is more sparsely populated with cells. It contains loosely arranged mature thymocytes and characteristic tightly packed whorls of squamous-appearing epithelial cells, called thymic or Hassall corpuscles. These appear to be remnants of degenerating cells and are rich in high molecular weight cytokeratins.

The thymus contains several other important cell types in addition to thymocytes. There are several types of specialized epithelial cells within the thymus. The three main categories of thymic epithelial cells are the medullary epithelial cells, which are organized into clusters, the cortical epithelial cells, which form an epithelial network, and the epithelial cells of the outer cortex.

The epithelial cells in the cortex and medulla often have a stellate shape, display desmosomal connections to one another, and may function as nurse cells to developing thymocytes. In addition, the thymus contains marrow derived antigen presenting cells, primarily in terdigitating dendritic cells and macrophages, particularly at the corticomedullary junction. 

After puberty, thymic involution begins within the cortex. This region may disappear completely with aging, while medullary remnants persist throughout life. Glucocorticoids also may induce atrophy of the cortex secondary to glucocorticoid induced apoptosis of cortical thymocytes. This also may be seen in conditions that are associated with increases in circulating glucocorticoid hormones, for example, pregnancy or stress.