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Main page | Structure | Departments | Department of Histology and Embryology | Materials for students | Modul 1 | BONE TISSUE.

BT is composed of  2 components:1) CELLs and 2) CALCIFIed EXTRACELLULAR MATRIX.
Extracellular matrix = ECM.
ECM is composed of CALCIFIED collagen fibres. It means: Fibres  are infiltrated by crystals of
HYDROXYAPATITes (=HA) : Ca10 (PO4)6 (OH)2. It is main INorganic component of BT. Crystals of HA constitute 65% in BT and give for BT property of STABILITY, hardness.
CELLs of BT belong to TWO   differons:
1st originates from mesenchyme of SCLEROTOM of SOMITE:
SCLEROTOM – Osteoprogenitor cs (=PREosteoblasts) – OsteoBlasts – Osteocytes
2nd originates from HEMOPOIETIC cs: PHSC – MONOCYTE – OsteoClast.
  1. OSTEOBLASTs (=OBL) – are cells, forming CALCIFIed ECM. On the 1st step of osteohystogenesis OBL  secrete  protein collagen. It forms COLLAGEN FIBRES. Fibres form OSTEOID, or ORGANIC MATRYX of BT. On the 2nd step OBLs provide CALCIFICATION of osteoid. For this, OBLs  infiltrate collagen fibres by crystals of HA,  resulting formation of CALCIFIed  ECM. During this blood calcium level is  reduced.
  2. OSTEOCYTEs (=OC) – are STAR-shaped cs - are composed of cell body and many cytoplasmic PROCESSes. Cell body locates in LACUNA, processes pass in CANALICULI.I. Lacunae and canaliculi – are cavities in calcified ECM.
  3. OSTEOCLASTs (=OCL). It are : a)contain 5-40 NUCLEI; b)plasma membrane forms  compact invaginations, named RUFFLED BORDER; c) many LYSOSOMEs.  Funct.: It provide DEcalcification, or RESORPTION, of ECM. It mean PARTIAL lysis of ECM. During resorption calcium ions LEAVE bone and enter into blood, resulting in increasing calcium level in blood. Location: OBLs, OCs and OCLs locate in ECM of bone LAMELLA (lamella= plate).
  4. OSTEOPROGENITOR cells (=OPC) locate in inner cellular layer of periosteum. Funct.:a) it provide APPOSITIONAL growth of long bone (bone becomes THICKER); b)provide REGENERATION of diaphysis after it fracture. Zone of fracture is replaced by PRIMARY, or woven, bone.
Microscopically BT is classified on 2 types:1) Primary, or woven, bone; 2) LAMELLAR bone.
Primary  bone locates in: a) fetal skeleton, b) zone of HEALed FRACTURE.
LAMELLAR bone (=LB) is MAIN type of BT in adults; it locates in  middle layer of DIAPHYSIS, named COMPACT bone.
LB is composed of bone LAMELLAE (or plates). Lamella contains:a) bone cells+ b) ECM.
Layers in  DIAPHYSIS on the cross section (from externally to deeper):
  1. Periosteum has 2 layers: a) outer  FIBROUS layer. It contains collagen fibres+blood VESSELs. Funct.: TROPHIC funct. for compact bone. b)Inner CELLULAR layer. It contains OSTEOPROGENITOR cs. Funct.: - appositional growth; - regeneration of diaphysis after fracture.
  2. Compact bone. It is composed of 3 layers of LAMELLAR bone. Middle –is layer of OSTEONs. OSTEON (or, HAVERSIAN system) - is structural and functional unit of compact bone. Struct. of OSTEON: is system of CYLINDERS of BONE LAMELLAE concentrically arranged around HAVERSIAN canal (or, osteon canal). Canal contains blood VESSEL. Adjacent osteons are connected to each other by vessel - containing VOLKMANNs canals. It are oriented perpendicular to Haversian cnals. Haversian canal is iriented LONGITUDINAL to diaphysis.
  3. Endosteum – is thin layer of connective tissue.
Since birth and until 20 years zone between epiphysis and diaphysis is NOT calcified and is
composed of HYALINE CARTILAGE. It is EPIPHYSEAL PLATE. Funct.: It provides long bone GROWTH in LENGTH.
Osteohistogenesis (=OHG). – is embryonic development of BT:
Development of LONG bones happens by ENDOCHONDRAL OHG. Mesenchyme of SOMITE diff. into CARTILAGE MODEL (hyaline cart.)  Mesenchimal cs  in perichondrium diff into OPSs and OBLs and perichondrium is replaced by PERIOSTEUM. Below it  OBLs form subperiostal bone COLLAR. In midriff of diaphysis OCLs resorb carilage model and carilage dies. OBLs produce osteoid and calcify it. All these events –is PRIMARY center of ossification. SECONARY center  of ossification – is replacement of cartilage by bone in future EPIPHYSIS. Only EPIPHYSEAL plate is not calcified.
Составитель – доцент В.В. Бондаренко.
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