Endochondral ossification vs intramembranous ossification

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.

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Long story short:

Intramembranous ossification is the direct laying down of bone into the primitive connective tissue (mesenchyme), while endochondral ossification involves cartilage as a precursor.

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