Anatomy and Physiology of Children and Adolescents - M. R. Sapin 2007

Organs of Hematopoiesis and the Immune System. The Lymphatic System
Central Organs of the Immune System
Thymus

The thymus, much like the Bone Marrow, is a primary organ of The Immune System where stem Cells originating from the bone marrow via the bloodstream mature and differentiate into T-lymphocytes, which are responsible primarily for Cell-mediated immune responses.

The thymus is located behind the manubrium and the upper body of the Sternum, nestled between the right and left mediastinal pleurae. It consists of two elongated, asymmetrical lobes—right and left—which are fused together in their mid-section or lie in close contact at that level. The lower, expanded, and rounded part of the thymus reaches the level of the fourth costal cartilages. Each lobe typically has a cone-like shape. Narrower at the top, both lobes extend upward into the neck region as a two-pronged fork, which is why the thymus was formerly known as the toothed gland (or sweetbread).

The thymus is encased in a delicate Connective Tissue capsule, from which interlobular septa extend deep into the organ, dividing it into lobules ranging from 1 to 10 mm in size. The thymic parenchyma consists of a darker peripheral zone known as the cortex, and a lighter central area known as the medulla.

The stroma of the thymus is formed by a network of reticular cells and fibers, along with star-shaped epithelial cells called epithelio-reticular cells, which interconnect via cytoplasmic processes. The meshes of this network harbor thymic lymphocytes (thymocytes), alongside small numbers of plasma cells, macrophages, and leukocytes. Lymphocytes are packed more densely in the cortex than in the medulla, giving the cortical region a more compact and darker appearance under stains.

The medulla of the thymus contains large, multi-process epithelial cells (epithelio-reticular cells). It also houses concentric lamellar bodies known as thymic corpuscles (Hassall's corpuscles), which are formed by concentrically arranged, modified, and highly flattened epithelial cells.

Age-related Features of the thymus. The thymus reaches its maximum size during childhood and adolescence. In newborns, it is well developed with a mass of 13 g, increasing to 30 g by age 6.5, and reaching 31 g between 10 and 15 years. The upper border of the thymus in newborns lies 2–2.5 cm above the manubrium of the sternum. The right lobe typically sits slightly higher than the left; the lower border of the left lobe is found at the level of the 2nd–3rd costal cartilages, while the right lobe reaches the level of the 4th–5th cartilages. The cortical zone predominates over the medullary zone. A high Abundance of Hassall's corpuscles indicates tissue maturity at birth. After the age of 16, the mass of the thymus gradually declines, dropping to 13.4 g by age 50–90. However, thymic lymphoid tissue does not disappear entirely, even in advanced age.

Alongside structural remodeling and a reduction in both cortical and medullary tissue, adipose tissue appears early within the thymic parenchyma. Isolated fat cells can be detected in the thymus of children aged 2–3 years. Subsequently, the connective tissue stroma proliferates and adipose tissue accumulates. By age 30–50, fat tissue replaces the majority of the organ's parenchyma.

While connective tissue accounts for only 7% of the thymic mass in newborns, it increases to as much as 90% in individuals over the age of 50.