BONE TISSUE.= (BT). OSTEOHISTOGENESIS.
BT is composed of 2 components:1) CELLs and 2)
CALCIFIed EXTRACELLULAR MATRIX.
Extra
cellular
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) – Osteo
Blasts – Osteo
cytes
2nd originates from
HEMOPOIETIC cs: PHSC – MONOCYTE – Osteo
Clast.
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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.
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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.
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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).
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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, 2) COMPACT bone, 3) ENDOSTEUM
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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.
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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.
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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.