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Introduction From the histological point of view, bone is a highly vascularized and innervated, mineralized conjunctive tissue, which is structured in lamellae of calcified osteoid matrix.
The arrangement of these lamellae determines whether the bone is cortical or cancellous. Both are composed of osteons.
Cortical or compact bone, containing osteocytes, is arranged concentrically around Haversian Canals. Cancellous or trabecular bone is formed by a network of bone lamellae, delimiting areolar cavities inside which the bone marrow is found 2.
Both cortical and trabecular bone contain specialized cells, organic matrix and mineral phase. Bone Cells Various cell types co-exist in bone table 1. Bone cells are found within the bone tissue itself or in the conjunctive stroma of the bone marrow, which is rich in mesenchymal stem cells. From the studies by Friedenstein in it is known that five distinct cell types can originate from these stem cells: fibroblasts, osteoblasts, chondroblasts, adipocytes and myoblasts 3 , in response to different molecular signals which initiate the activation cascade of different genes.
Also essential are the transcription factor Cbfa1 core-binding factor a-1, also known as Runx2 6,7,8,9 and the bone morphogenetic proteins BMPs , which constitute the most powerful regulators of osteoblastic differentiation from the mesenchymal stem cells 4. The expression of Cbfa1 is the first indication of osteogenic differentiation 4 , the maximum level of which is reached in the pre-osteoblasts.
Collagen I and osteopontin OPN are expressed early on in osteoprogenitor cells. In the same way alkaline phosphatase ALP is a surface protein that could participate in the regulation of proliferation, migration and differentiation of the osteoblastic cells.
Bone sialoprotein BSP and osteocalcin OCN , are pre-osteoblast to osteoblast differentiation markers and appear when mineralization begins. The expression of these proteins is especially useful as osteogenic markers in the final stages of osteoblastic differentiation. They originate from the mesenchymal stem cells of the bone marrow, endosteum, periosteum, and perivascular pericytes They emit cytoplasmic processes towards the matrix, which communicate with the osteocyte network and neighbouring osteoblasts.
The osteoblasts and the osteocytes communicate with each other by transmembrane proteins or integrins, which act as a link between cells or between a cell and the extracellular matrix, allowing the passage of messengers such as calcium, cytokines and prostaglandins. In these cells the intercellular connection is Connexin 43 It is now known that they: 1 - synthesize the collagen and non-collagen proteins of the organic bone matrix, 2 - direct the arrangement of the extracellular matrix fibrils, 3 - contribute to the mineralization of the osteoid material, due to the alkaline phosphatase, 4 - mediate in the resorption carried out by the osteoclasts, through the synthesis of specific cytokines 12 and 5 - synthesize growth factors.
Both types of cells represent more advanced stages of maturation. The bone lining cells are elongated and flat, with spindle shaped nuclei, and scarce organelles.
They can express the previously mentioned osteoblastic markers such as bone sialoprotein, osteopontin, osteonectin, and alkaline phosphatase as well as parathyroid hormone receptor PTH. They remain along the endosteal surface, forming with the endosteum a protective layer to the bone surface, which plays an important role in the activation of bone remodeling.
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The osteoblasts, osteoclasts and bone lining cells are found on the bone surface, while the osteocytes are on the interior. The osteocytes are the most abundant cells in bone 10 times the osteoblasts. They are stellate and are found in the interior of lacunae, the cytoplasmic processes communicate with each other through bone canaliculi filled with extracellular bone fluid.
In this way, the osteocytes organize themselves into a syncytium of interconnected cells which forms a single structure, with the advantage that there is a large contact surface both on the inside and towards the bone surface, assuring the supply of oxygen and nutrients.
When trauma occurs in the bone, the cessation in the blood supply causes hypoxia and necrosis of the osteocytes situated further than 0. The osteocytes also participate in the synthesis and mineralization of the osteoid matrix, but it is believed that their principal function is to control bone remodeling, detecting the mechanical variations of the loads, a phenomena known as mechanotransduction Osteocytes constitute the final stage of the osteoblastic line and are incapable of self-renewal.
They have the same markers as the osteoblasts, but also have a specific marker, CD44, a membrane receptor, strongly expressed in osteocytes and negative in osteoblasts and bone lining cells.
The osteoclasts contain tartrate-resistant acid phosphatase TRAP , which permits the dephosphorylation of the proteins, the activity of which is used for its identification both in vivo and in vitro.
In addition they possess receptors for calcitonin. Osteoclasts have two special features in the membrane: a ruffled border, where resorption takes place, and a clear area rich in microfilaments, with integrins that serve as an anchor to the matrix.
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To this end, the osteoclasts move towards the area to be resorbed and then immediately adhere to the mineralized bone surface with the ruffled border and sealing the edges of the area with the integrins.
With respect to osteoclastogenesis, it is currently known that the osteoblasts are fundamental to the formation of osteoclasts. Thus, the Macrophage Colony-Stimulating Factor M-CSF produced by the osteoblasts is required in the early phases of osteoclastogenesis for the formation of giant multinucleate cells.
Present knowledge on osteoclastogenesis regulation is based on the existence of 3 key molecules: OPG osteoprotegerin, a soluble protein synthesized by osteoblasts and pre-osteoblasts , RANKL a ligand situated on the surface of the osteoblasts and pre-osteoblasts , and RANK a receptor of the above, situated on the osteoclast and pre-osteoclast membranes.
Epithelial tissue: classification of epithelium. Covering and lining epithelium. Basement membrane, epithelial cell adhesion and cell surface modifications. Connective tissue: Classification; cells of the connective tissue; the extracellular matrix; study of the main types of connective tissues: proper conective tissues, adipose tissue, cartilage, bone.
Blood: peripheral blood, hematopoiesis. Muscle Tissue: classification, morphology and functional meaning.
Nervous tissue: origin and composition; the neurone and supportive cells. Special histology Study of the normal histology of the main systems of the body of domestic mammals and birds. Organization of organs: parenchyma and stroma, tubular organs, organization of solid organs, body cavities and membranes. Cardiovascular System: Histology of heart, elastic and muscular arteries, arterioles, capillaries, venioles, small, medium and large sized veins, vasa vasorum.
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Lymphatic system: Lymphoid tissue; histology of thymus, lymph nodes, spleen, and tonsils. Integumentary System: Histology of skin of animals; hair follicles, sweat and sebaceous glands; microscopic structure of mammary gland.
Respiratory System: Histology of nasal cavity, nasal sinuses, nasopharynx, larynx, trachea, lungs, bronchi, bronchiole, respiratory bronchiole, alveolar duct, alveolar sac and alveoli; blood air barrier. Urinary System: Histology and histogenesis of kidneys, ureters, urinary; bladder; maale and female urethrae. Digestive System: Histology of oral cavity, oropharynx, esophagus, rumen, reticulum, omasum, abomasum and simple stomach; small and large intestine, liver, pancreas and salivary glands, histogenesis of digestive system and accessory digestive glands.
Reproductive System: Histology of gonads and duct system of male and female reproductive systems. Endocrine System: Histology of pituitary, pineal, thyroid, parathyroid, pancreas and adrenal glands.
Nervous system: Histology of cerebrum, cerebellum and spinal cord. Special sense organs. Pratical Preparation of histological sections and staining for light microscopy. Components of light microscope. Units of measurement.
Study of electron micrographs of different components of animal cell. Microscopic examination and identification of cells of different types and sizes. Preparation of blood smear, staining and identification of cellular components and platelets.
Microscopic examination and identification of different types of epithelia, connective tissue, muscular tissue, and nervous tissue. Identification of various developmental stages of mammalian and fowl embryos.
Microscopic examination and identification of various organs of domestic mammals and birds. Blackwell Publishing. Azevedo, Carlos Coord. Bacha, J.
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Bancroft, J.Harvey J. These molecular signals promote the differentiation towards endothelial cells, fibroblasts, chondroblasts and osteoblasts, creating a new fibrovascular tissue which replaces the initial coagulate. In addition they possess receptors for calcitonin.
Why should be here? Interamericana-McGraw Hill.
Connective tissue: Classification; cells of the connective tissue; the extracellular matrix; study of the main types of connective tissues: proper conective tissues, adipose tissue, cartilage, bone. Editorial Acribia. Alkaline phosphatase is an enzyme that liberates inorganic phosphate from phosphoric esters, and is necessary for mineralization.