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NAME: GLORY P.A. BRIGGS MATRICULATION NUMER: 14/MHS06/017 DEPARTMENT: MEDICINE AND SURGERY COURSE: GENERAL HISTOLOGY COURSE CODE: ANA 203 Histology of Haversian System Histology according to the Medical dictionary is the science concerned with the minute structure of cells, tissues and organs in relation to their function. Hence from the definition above, we can infer two things: Firstly, that in simple terms, histology of haversian system refers to the microscopic study of the haversian system and secondly, the haversian system comprises of cells and tissues, which brings us to the question, “What is haversian system?” Haversian system also known as osteon refers to the chief basic unit of structure of each compact bone (found in the shafts of long bones) in humans consisting of a small hollow passageway called HAVERSIAN CANAL (named for Clopton Havers, a 17th-century English physician), surrounded by a series of concentric bone layers called LAMELLAE of which there may be 4 to 20, each 3 to 7 microns thick, in a single haversian system. Osteons are several milimetres long and about 0.2 milimetre (0.008 inch) in diameter; they tend to run parallel to the long axis of a bone and are not present in many vertebrate animals. It is also important to note that this microscopic canal system was first noticed by ANTON VON LEEUWENHOEK in 1978 but later in 1691, an English physician, Clopton Havers published and lectured about the microscopic makeup of canals in the bone, hence these canals were generally referred to as CANALS OF HAVERS. His theory was that they provided the oils necessary to harden the bone around it. As discovered during studies by the above researchers, down the middle of each of these systems (i.e running longitudinally in the centre) is a hollow tube-THE HAVERSIAN CANAL, a freely anastomosing channel containing blood vessels, nerves, loose connective tissue and endosteum. The blood vessels provide nutrients to the living bone tissue while the nerves and lymph vessels help with immune response in the bones and also pick up signals from various stimuli. The haversian canal also absorb necrotic or dying bone tissue which leave empty lacunae so that they can be replaced by new osteocytes. Transverse vessels called VOLKMANN CANALS run perpendicular to the long axis of the cortex. These Volkmann canals connect adjacent osteons and also connect the blood vessels of the haversian canals with the PERIOSTEUM, the tissue covering the bone’s outer surface. Also, they consist of multiple EXTERNAL CIRCUMFERENTIAL LAMELLAE (around the marrow cavity) and often some INNER CIRCUMFERENTIAL LAMELLAE (immediately beneath the periosteum) as stated earlier. Between successive lamellae are series of spaces called LACUNAE, each with ONE OSTEOCYTE OR BONE CELL, interconnected by CANALICULI containing the cells’ dendritic processes. Processes of adjacent cells are in contact via GAP JUNCTIONS. The osteocytes give rise to the OSTEOBLASTS and also work along with collagen and calcium phosphate that make up the matrix of the lamellae to ensure that the compact bone is very strong. Scattered among the intact osteons are numerous irregular shaped groups of parallel lamellae called INTERSTITIAL LAMELLAE, which are lamellae remaining from osteons partially destroyed by OSTEOCLASTS during growth and remodeling of bone. The outer boundary of each osteon is a more-collagen rich layer called the CEMENT LINE. Having known the histology of the haversian system, it is important to know its function. The exact function of this system is not known but some theories state that it repairs damaged bone tissue, reduce stress on the compact bone and give muscle a place to anchor.