These occur in the of lymphatics of lymph vessels. Lymph flows through them. These vary in size form those just visible to naked eye to those several centimeters in diameter.
They are flattened and been-shaped with a definite hilum. They have a connective tissue capsule containing a few elastic and smooth muscle fibres. Septa or trabeculae detached form the capsule pass towards the interior to form shallow compartments.
In the inner zone the tarabeculae anastomose ad pass into the hilus. This framework divides the node into compartments, which communicate and are filled with reticular tissue.
The reticular tissue of the node consists of reticular cells and fibres, which form a framework. The reticular fibres are continuous with the collagen fibres at the trabeculae.
The Reticular cells have an irregular shape with processes and pale staining nuclei. These are phagocytic reticular cells, which form part of the reticuloendothelial system. There are also non-phagocytic undifferentiated primitive reticular cells (which may give rise to the phagocytic reticular cells or lymphocytes).
The trabeculae are more or less perpendicular to the surface and divide the cortex into a number of compartments. The trabeculae pass deeper and become continuous with the irregularly arranged trabeculae of the medulla.
In the cortical compartments the lymphocytes are closely packed together to form cortical nodules. These are more or less spherical discrete masses of closely packed lymphocytes but in some nodes, the nodules may be irregular and ill defined becoming continuous laterally with adjacent nodules.
The cortical nodules often contain lighter staining central areas called germinal centers because lymphocytes are formed in them by proliferation. In these area, many medium sized and a few large lymphocytes have a greater amount of cytoplasm and the undifferentiated large lymphocytes have pale vesicular nuclei.
These lightly stained central areas are also referred to as secondary nodules. Active proliferation of cells in the germinal center pushes outward to the surrounding cell and thus the primary nodule contains peripheral darker zone consisting of closely packed small lymphocytes.
The cortical nodules are separated from the capsule and the trabeculae by channel-like spaces called lymph sinuses through which lymph circulates. The marginal or cortical sinus receives lymph from the afferent lymphatics. The lymph then flows down the trabecular sinuses to enter the sinuses in the medulla.
The trabeculae of connective tissue are irregularly arranged and anastomose freely.
The lymphocytes do not form nodules as in the cortex, but form annstomoising lymph cords.
The medullary sinuses are wider and passing between the lymph cords and the trabeculae.
Lymphatic vessels
The afferent lymphtics are numerous and pierce the capsule on the convex side of the node-and open into the marginal sinus.
The lymph flows through the trabecular and medullary sinuses and from here it is collected by a plexus of vessels, which penetrate through the capsule at the hilum, from where it flows through the efferent lymphatic vessels.
Structure similar to blood capillaries but their diameter is greater and they show dilations.
Lymphatcis
Structure similar to venules and veins; show a much thinner poorly defined three layered wall. T.intima lacks internal elastic membrane, T. media is made up of smooth muscle, elastic and collagenous tissue, T.Adventita is made up of collagenous tissue with few muscle fibres.
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