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Periarteriolar lymphoid sheaths (or periarterial lymphatic sheaths, or PALS) are a portion of the white pulp of the spleen.They are populated largely by T cells and surround central arteries within the spleen; the PALS T-cells are presented with blood borne antigens via myeloid dendritic cells.
An easy way to remember the anatomy of the spleen is the 1×3×5×7×9×10×11 rule. The spleen is 1 by 3 by 5 inches (3 by 8 by 13 cm), weighs approximately 7 oz (200 g), and lies between the ninth and eleventh ribs on the left-hand side and along the axis of the tenth rib.
The fibroelastic coat of the spleen invests the organ, and at the hilum is reflected inward upon the vessels in the form of sheaths. From these sheaths, as well as from the inner surface of the fibroelastic coat, numerous small fibrous bands, the trabeculae of the spleen (or splenic trabeculae), emerge from all directions; these uniting, constitute the frame-work of the spleen.
White pulp is a histological designation for regions of the spleen (named because it appears whiter than the surrounding red pulp on cross section), that encompasses approximately 25% of splenic tissue. White pulp consists entirely of lymphoid tissue. Specifically, the white pulp encompasses several areas with distinct functions:
The cords of Billroth (also known as splenic cords or red pulp cords) are found in the red pulp of the spleen between the sinusoids, consisting of fibrils and connective tissue cells with a large population of monocytes and macrophages.
Inside of a bone showing the trabecular structure A typical carcinoid tumor of the lung showing a trabecular pattern of elongated groups of cells.. A trabecula (pl.: trabeculae, from Latin for 'small beam') is a small, often microscopic, tissue element in the form of a small beam, strut or rod that supports or anchors a framework of parts within a body or organ.
The red pulp of the spleen is composed of connective tissue known also as the cords of Billroth and many splenic sinusoids that are engorged with blood, giving it a red color. [ 1 ] [ 2 ] Its primary function is to filter the blood of antigens , microorganisms, and defective or worn-out red blood cells.
The cells are primarily monocytes and macrophages, and they accumulate in lymph nodes and the spleen. The Kupffer cells of the liver and tissue histiocytes are also part of the MPS. The mononuclear phagocyte system and the monocyte macrophage system refer to two different entities, often mistakenly understood as one. [citation needed]