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4. Bone Development.pdf

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Anatomy & Cell Biology
ANAT 261
Craig Mandato

0:// :@由 0 : // : @ 由Naveen Sooknanan McGill Fall 2011 Bone Development: Intramembranous ossification is typical of flat bones (part of mandible, maxilla, occipital, temporal and parietal bones). It takes place on a mesenchymal sheet  These don’t look like typical “bones”  This occurs during development  This takes place within plates of mesenchymal cells o First, groups of mesenchymal cells differentiate into osteoblasts. This is known as the primary ossification center o These osteoblasts secrete ECM, followed by encapsulation of osteocytes and the formation of spicules (pieces of bone) o From these spicules, secondary ossification centers arise o On the periphery of the plate, mesenchymal cells begins to differentiate to form the periosteum o Ossification grows radially and eventually the spicules fuse together, forming spongy bone Endochondral ossification makes long bones in the body. Typically, this takes place on hyaline cartilage with a similar shape of the bone to be formed. Cartilage is surrounded by the perichondrium everywhere but articulated surfaces (the future heads of the bones)  Hyaline cartilage will turn into bone  Formation of primary ossification center: o The primary ossification center forms at the level of the diaphysis (middle of the bone) o Here, the perichondrium is transformed into periosteum, which deposits bone  The bone deposit, as it builds up, is called the bone collar o The bone collar prevents nutrients from entering the cartilage, which causes the hypertrophy (swelling) and death of the cartilage in the area of the bone collar o Bone ECM is then produced and dying cartilage tissue becomes calcified o Blood vessels invade the spaces left by dying chondrocytes and carry osteoprogenitor cells  These blood vessels come from the perichondrium, which is turning into the periosteum  Remember, the periosteum has osteoprogenitor cells  This invasion pushes the bone collar inwards o The osteoprogenitor cells become osteoblasts, which then divide and secrete ECM  Formation of secondary ossification center: o This takes place at the epiphysis of the future bone 1Naveen Sooknanan McGill Fall 2011  Formation of the secondary ossification center also produces the epiphyseal plate o Nutrients are blocked from getting to the perichondrium, causing the degeneration of cartilage o Blood vessels from the perichondrium (which is becoming the periosteum) then invade the area and bring in osteoblasts  Just like in primary, they take the space of dead chondrocytes o Cartilage persists in articulate surfaces, like the head of the bone, and at the epiphyseal plate  The osteoclasts are constantly active in re-absorption of woven bone and calcified cartilage to remodel the bone and produce a hollow marrow The epiphyseal plate is the area between the primary and secondary ossification centers. It is made up of multiple layers. From the top down:  Zone of resting cartilage: o Chondrocytes, usually 2 per lacuna  Zone of proliferation o Tons of chondroblasts, which are dividing. They look like a stack of coins  Zone of hypertrophy: o Chondrocytes swell and absorb liquid  Zone of cell death: o Chondrocytes begin to die here  Zone of mixed spicules: o There are no more chondrocytes here o It is made up of:  Spicules of bone  Osteoblasts, which make the spicules. Came in through blood vessels  Calcified cartilage, which comes from dead cartilage from formation of the bone collar  Cartilage becomes calcified because osteoblasts secrete calcium in the ECM, which invade the cartilage and make it calcified  Osteoclasts, which break down calcified cartilage  Blood cells, because as the zone of mixed spicules is broken down, it is replaced by bone marrow, which contains blood
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