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PRACTICAL HISTOLOGY LAB.5 ----------------------------------------------------------------------------- Bone Bone is a strong, flexible and semi-rigid supporting tissue. It can withstand compression forces, and yet it can bend. Like cartilage, and other types of connective tissue, bone is made up of Cells and Extracellular matrix: Cells - which in bone are called osteoblasts and osteocytes, (osteo - bone). There are also two other cell types: osteoprogenitor cells and osteoclasts. What is bone for? Support - bones make up a structural framework for the .1 body, and provide attachment sites for muscles. Protection - protection of internal organs - i.e. brain, heart .2 and lungs, Assisting movement. .3 Mineral homeostasis - the bone is a store for calcium and .4 phosphorus Blood cell production - takes place in the bone marrow. .5 Types of bone: There are two types of mature bone: 1. Compact - which is found in the shafts of long bones (in the diaphyses). This makes up 80% of all bone. 2. Spongy (cancellous) bone - which is found at the ends of long bones (in the epiphysis). This makes up 20% of all bone. This type of bone contains red bone marrow and a network of bony trabeculae. A 'periosteum' is found on the outside of bone. This is a dense fibrous layer, where muscles insert. It contains bone forming cells. It is not found in the regions of bone covered by articular cartilage. The endosteum is the name given to the tissue that lines the inner surfaces of bones. Lymphoid Tissue Lymphoid tissue covers all of the various tissues that are important in mounting an immune response. This includes discrete organs such as the spleen, thymus and lymph nodes, as well as more diffuse aggregations of lymphocytes. A fluid called lymph, (lymph = clear fluid) flows in lymphatic vessels, lymphatic tissue and red bone marrow. Fluid filters out of capillaries and drains into lymphatic vessels to become lymph. The content of lymph is the same as interstitial fluid, the fluid around tissue cells. Lymph eventually drains into venous blood. Lymph drains interstitial fluid, transports dietary lipids and facilitates immune responses. Primary and Secondary Lymphoid Tissues Primary lymphatic organs are where lymphocytes are formed and mature. They provide an environment for stem cells to divide and mature into B- and T- cells: There are two primary lymphatic organs: the red bone marrow and the thymus gland. Both T-cell and B-cells are 'born' in the bone marrow. However, whereas B cells also mature in the bone marrow, Tcells have to migrate to the thymus, which is where they mature in the thymus. Secondary lymphoid tissues are arranged as a series of filters monitoring the contents of the extracellular fluids, i.e. lymph, tissue fluid and blood. The lymphoid tissue filtering each of these fluids is arranged in different ways. Secondary lymphoid tissues are also where lymphocytes are activated. These include: lymph nodes, tonsils, spleen, Peyer's patches and mucosa associated lymphoid tissue (MALT). Tonsils are large partially-encapsulated masses of lymphoid tissue, found in the walls of the pharynx and nasopharynx, and at the base of the tongue. They form an incomplete ring around the gastrointestinal and respiratory tracts, where they cross over. Peyer's patches are large masses of confluent lymphoid follicles, found in the walls of the ileum, part of the small intestine. The spleen is a large encapsulated organ in left upper part of abdomen, the outer capsule is fibro-elastic.