Intramembranous ossification [flat bones] & endochondral ossification [long bones].The essential between them is the presence or absence of cartilaginous phase.Intramembranous ossification occurs when mesenchymal precursor cells proliferate & subsequently differentiate directly into osteoblasts w/c mineralize an immature bone tissue called woven bone,characterize by irregular bundles of randomly oriented collagen fibers & an abundance of partially calcified immature new bone called asteoid.At later stages this woven bone is progressively remodeled to mature,lamellar bone. Endochondral ossification entails the conversion of a cartilaginous template into bone.Mesenchymal cells condense & differentiate into chondrocytes w/c secrete the cartilaginous matrix.This embryonic cartilage is avascular,& during its early development,a ring of woven bone is formed by intramembranous ossification in the future mid shaft area.This calcified woven bone is then invaded by vascular tissue,& osteoclasts & osteoblasts are recruited to replace the cartilage scaffold w/ bone matrix & excavate the hematopoietic bone marrow cavity.
The plates start out at their thickest when you are young. As you age and your bones elongate the plates get thinner and thinner until they close at around age 19 or so. The growth you speak of are is a special growth of cartilage cells that stack up like coins. The divided cells will then become ossified and become bone. Chondrocytes divide rapidly and form columns of stacked cells at the epiphyseal cartilage of short and long bones. During the growth towards the ends of bones, the chondrocytes that were originally dividing in closest proximity to the center of the bone (still within the epiphyseal cartilage), die and become resorbed matrix. This matrix soon becomes calcified. Because the proliferation and destruction of chondrocytes occur at the same rate, the epiphyseal plate does not change thickness. It does, however, grow away from the middle of the...
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