L-2 Histo Platelets & Bone Marrow PDF

L -2 HISTO platelets & bone marrow Blood platelets : Platelets are not cells, they are non-nucleated bodies. Number of blood platelets varies from 200,000 to 400,000 per cubic millimeter, and...

L -2 HISTO platelets & bone marrow Blood platelets : Platelets are not cells, they are non-nucleated bodies. Number of blood platelets varies from 200,000 to 400,000 per cubic millimeter, and is counted by hemocytometer. Diameter: 2-4 microns and they have no nuclei. LM: each blood platelet is oval or rounded in shape formed of a peripheral pale clear part called Hyalomere, and a central granular basophilic part called Granulomere which is rich in specific granules. EM: the cell membrane of the blood platelet is irregular and is covered with a cell coat formed of glycoprotein. The hyalomere or the peripheral area of the blood platelets contains bundles of microtubules and microfilaments which are present mainly at the periphery. They have a contractile function; they can change the shape of platelets during retraction of blood clot. The granulomere or the central area of the blood platelets contains alpha granules which are numerous and are considered as lysosomes, mitochondria (beta granules) which are few in number and small in size, delta granules which are rounded vesicles with clear contents, very dense granules which are rich in serotonin substances, ATP, ADP and Ca, glycogen granules and system of tubules and vesicles. Functions: blood agglutination and coagulation Bone Marrow : Inactive yellow bone marrow: It is present in the cavities of long bones in adult. It is formed of many fat cells, reticular cells and some mesenchymal cells which may change into myeloid cells in certain conditions. The yellow bone marrow acts as storage area for fat cells, it also represents the reserve areas for hemopoietin tissue. It may change into active red bone marrow in cases of excessive loss or destruction of RBCs. 1|Page  L -2 HISTO Active red bone marrow: I. In the fetus, red bone marrow is present in most of the bones. II. In adult man, red bone marrow is found in the sternum, vertebrae, ribs, dipole of the skull, clavicle and in the pelvic bones. Structure The red bone marrow is formed of : A) Stroma of bone marrow: The stroma is formed of the following fixed cells: 1. Reticular cells: they are branched cells with large pale nuclei. They may be connected with each other by desmosomes. They help the B-Lymphocytes to be activated by the specific antigens. 2. Osteogenic cells: which are the immature bone cells, and some mature bone cells as osteoblast and osteoclast cells. 3. Fat cells and fibroblast. 4. Endothelial cells: which are the lining cells of the blood capillaries, blood sinusoids and blood arterioles. 5. Perivascular primitive mesenchymal pericyte cells: They can differentiate into fat cells, reticular cells and smooth muscles around blood vessels. B) Blood sinusoids: these are wide irregular blood channels lined with squamous endothelial cells. They are surrounded with thin and non- continuous basement membrane. Through this basement membrane the macrophages extend their pseudopodia between the endothelial lining cells of the sinusoids into their lumen where they phagocytose the foreign bodies from the blood stream. C) Free bells: they are the developing blood cells which are in various stages of formation and maturation. Functions: is the site of blood cells development, helps destroy old blood cells and iron is stored in bone marrow macrophages as ferritin. 2|Page