Lymphoid (or lymphatic) tissues

Lymphoid Tissues

Lymphoid (or lymphatic) tissues, which mainly consist of dense accumulations of lymphocytes, are widely distributed in the body. Lymphoid tissues are typically located at sites that provide a possible route of entry of pathogens and/or sites that are liable to infections. Epithelia delimit all other tissues from the "outside world", and it is not surprising that lymphoid tissues are often found near them. Such lymphoid tissues are grouped together as epithelium-associated lymphoid tissues. Depending on their precise location these lymphoid tissues may be referred to as e.g. mucosa-associated lymphoid tissue (MALT) or bronchus-associated lymphoid tissue (BALT). The tonsils or Peyer's patches are examples of mucosa-associated lymphoid tissues. Lymphoid tissues represent the sites of proliferation and differentiation of lymphocytes.

Lymphoid organs may be defined as anatomical "entities" which consists chiefly of lymphoid tissues. The thymus is a primary lymphoid organ in that it supplies other lymphoid organs and tissues with T-lymphocytes. Inserted into the blood and lymph vascular system, the spleen and lymph nodes (secondary lymphoid organs) monitor the internal environment of the body.

Thymus

The thymus is situated in the upper parts of the thorax, behind the sternum and the upper four costal cartilages, in the anterior and superior mediastina. The size of the thymus changes in the course of life. It weighs about 10-15 g at birth and reaches its top weight (about 30-40 g) at puberty. After puberty a progressive involution (see below) occurs, which leaves a middle-aged person with a thymus weighing about 10 g. The thymus consists of a right and left lobe which are joined by connective tissue.

The thymus is enclosed by a thin connective tissue capsule from which numerous septa extend into the thymus subdividing the two lobes into numerous lobules (about 0.5 -2 mm in diameter). Blood vessels enter and leave the thymus via the connective tissue septa. Each lobulus is divided into a darker peripheral zone, the cortex, and a lighter, central zone, the medulla. Medullary tissue is continuous from lobule to lobule throughout each lobe.

Reticular cells and macrophages are present in addition to the lymphocytes, which are the dominant cell type within the lobules.

Reticular cells

are quite abundant. Their cytoplasm is eosinophilic, and their large, ovoid and light nuclei may contain one or two nucleoli. The cells are branched, and their slender processes are connected with the processes of other reticular cells to form a cellular reticulum (or cellular network). This cellular network (reticular fibres are scant in the thymus) provides support for other cells of the thymus.

Reticular cells sheathe the cortical capillaries; they form an epitheloid layer which delimits the cortical tissue from the connective tissue and secrete substances (hormones and other factors) important for thymic function. Thereby they create and maintain the microenvironment necessary for the development of T-lymphocytes in the cortex. Their functions thus go beyond those of "typical" reticular cells and, to reflect this, they are also referred to as thymic epitheliocytes.

Macrophages

occur in both cortex and medulla. They are difficult to distinguish from the reticular cells in H&E stained sections.

Lymphocytes

are present in both cortex and medulla, but are more numerous (denser) in the cortex. Their sizes are variable (5 - 15 µm) in the cortex but generally small in the medulla. The vast majority of them will be developing T-lymphocytes. They are also called thymic lymphocytes or thymocytes.

Thymus, foetal human - H&E Identify the connective tissue capsule and septa, a lobule and its cortex and medulla at low magnification. Identify lymphocytes and thymic corpuscles. They look pretty much like a sliced (very, very small) onion. Take a close look at the medulla and try to find some cells which contain large and light nuclei. They will be either macrophages or reticular cells. Sketch part of the thymus at low magnification. Identify medulla and cortex. Draw a segment of a lobule at high magnification and identify lymphocytes and nuclei of reticular cells/macrophages and, if possible a Hassall corpuscle.

Title: Lymphoid (or lymphatic) tissues
by:om at 2013-02-15T03:24:00+07:00
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Lymphoid (or lymphatic) tissues