THYMUS

The thymus is a primary  lymphatic organ, that enables the maturation of T-lymphocytes. According to the histological structure, the thymus can be characterized as a lymphoepithelial organ – its parenchyma consists of reticular epithelium and lymphocytes. Each component has a different embryological origin: the reticular epithelium develops from the 3rd pharyngeal endodermal pouch whereas the lymphocytes originate from the mesenchyme. The  organ is covered by a dense connective tissue capsule from which septa emanate. The parenchyma of the organ consists of cortex and medulla (Fig. 1A). The cortex represents superficial areas that are dark due to dense accumulation of small maturing T-lymphocytes. The medulla includes central pale areas with less lymphocytes. The border between cortex and medulla is called the corticomedullary zone. Here the lymphocytes are evenly distributed. Septa divide the parenchyma into false lobules which enter the cortex but not the medulla. That is why the lobules are known as pseudolobules.

Fig. 1A (left). Light microscopic structure of the thymus in children. Parenchyma is divided in the cortex (darker areas) and the medulla (paler areas), which contains Hassal's corpuscles (orange). Capsule and septa are labelled in brown. Fig. 1B (right). Reticular epithelium of the thymus. In a 3D network of  reticular epithelial cells (yellow), lymphocytes (thymocytes - white) accumulate. Reticular epithelial cells tend to form closed complexes: tehy flatten and become apposed to a surface and give rise to Hassal's corpuscles (orange). Similar to stratified epithelium the cells are connected by desmosomes. Their cytoplasm contains granules that look like keratohyalin granules. A centre of the corpuscle is quite far from blood capillaries (red) and degenerates. Macrophages are labelled in green colour. Author of the image: R. Krstic; colourized by: V. Vrbacká and J. Mokrý.

The reticular epithelium within the thymus is arrangend like a three-dimensional network and therefore it is referred to as the epithelial cytoreticulum (Fig. 1B). Inside the medulla, epithelial cells in some positions aggregate to form Hassal's corpuscles (Fig. 1B). The first corpuscles are formed during the first half of the fetal life. By the apposition of reticular epithelial cells the Hassal's corpuscles gradually increase in size and form eosinophilic spherical formations formed by concentrical arrangement of flattened cells of the reticular epithelium. Hassal's corpuscles may be larger than 200 μm but are not vascularized. As a result, central cells undergo hyaline degeneration and disintegrate. In addition to lymphocytes, granulocytes, macrophages, mast cells and others may be present in the parenchyma. The thymus is fully developed in newborns, reaching roughly the size of the heart. In puberty, the thymus gains its largest absolute size (30-40 g) and then becomes subject to involution. After puberty the thymus gradually transforms into the corpus adiposum thymi, which contains only isolated islets of functional parenchyma. This transformation is characterized by the disappearance of boundaries between cortex and medulla (shrinkage of the parenchyma), thickened septa, more Hassal's corpuscles and increased sizes as well as a decrease in the production of T-Lymphocytes. Nevertheless the involuted thymus remains a functional lymphatic organ, i.e. it is still capable of producing T-lymphocytes.