Bone Formation and Development

2 Osteogenic pathways

intramembranous and endochondral

ossification (osteogenesis)

The process of bone development

Cartilage

model tissue on which the bone lays down its mineral matrix

Intramembranous ossification

compact and spongy bone develops directly from sheets of mesenchymal (undifferentiated) connective tissue.

mesenchymal cells

multipotent stromal cell found in connective tissues throughout the body. They have the ability to differentiate into a variety of cell types

ossification center

early osteoblasts appear in a cluster called

osteoid

osteoblasts secrete _____, uncalcified matrix, which calcifies (hardens) within a few days as mineral salts are deposited on it, thereby entrapping the osteoblasts within.

Chondrocytes

Cartilage cells

perichondrium

a membrane that covers the cartilage

primary ossification center

a region deep in the periosteal collar where ossification begins

secondary ossification center

After birth, this same sequence of events (matrix mineralization, death of chondrocytes, invasion of blood vessels from the periosteum, and seeding with osteogenic cells that become osteoblasts) occurs in the epiphyseal regions, and each of these centers of activity is referred to as a

reserve zone

is the region closest to the epiphyseal end of the plate and contains small chondrocytes within the matrix.

proliferative zone

is the next layer toward the diaphysis and contains stacks of slightly larger chondrocytes. It makes new chondrocytes (via mitosis) to replace those that die at the diaphyseal end of the plate.

 zone of maturation and hypertrophy

older and larger cells, they sit closer to the diaphysis. More mature cells than that of prolifation zone.

zone of calcified matrix

zone closest to the diaphysis, are dead because the matrix around them has calcified.

appositional growth

the process by which bones increase in diameter (thickness) or width, rather than in length.

modeling

Osteoclasts resorb old bone that lines the medullary cavity, while osteoblasts, via intramembranous ossification, produce new bone tissue beneath the periosteum.

remodeling

in which resorption of old or damaged bone takes place on the same surface where osteoblasts lay new bone to replace that which is resorbed.