Ossification, bone remodeling, and repair

Ossification/osteogenesis

bone formation that begins during the embryonic period and is completed by age 7

Primary bone

• first bone formed by both types of ossification

• immature bone made of irregular collagen, abundant osteocytes, and little organic ECM

Secondary Bone/lamellar bone

• Stronger bone that replaces primary bone resorbed by osteoclasts

• made of fully formed lamella, regular collagen with parallel bundles, more inorganic material for strength

Intramembranous ossification

• built with embryonic CT within mesenchymal membrane

• inner spongy bone forms before outer compact bone

• begins at primary ossification center

Intramembranous ossification forms

many flat bones like the skull and clavicle during fetal development

mesenchymal membrane

Embryonic CT (mesenchyme) rich with blood vessels and mesenchymal cells

Intramembranous ossification step 1

• osteoblasts develop in the primary ossification center from mesenchymal cells

• osteogenic cells → osteoblasts

Intramembranous ossification step 2

osteoblasts secrete organic matrix, it calcifies (depositing calcium in primary ossification center), and trapped osteoblasts become osteocytes

Intramembranous ossification step 3

• osteoblasts lay down new bone resulting in the formation of trabeculae of early spongy bone

• trabeculae enlarge and merge

• some mesenchyme differentiates into the periosteum, some vascular tissue in spongy bone becomes bone marrow

intramembranous ossification step 4

osteoblasts in periosteum continue to secrete organic bone matrix

they become heavily calcified

structure is remodeled to become mature compact bone

Intramembranous ossification step 5

large bones fuse

endochondral ossification

• most bones form from hyaline cartilage model

• Makes the outside of the bone first

what bones are formed from endochondral ossification?

all bones in the body below the head, excluding the clavicles

hyaline cartilage model

chondrocytes, cartilage ECM, surrounded by chondroblasts (immature cartilage)

Endochondral ossification step 1, after the hyaline cartilage model is formed

• perichondrium fills with blood vessels and chemical signals cause chondroblasts to turn into osteoblasts (chondroblasts → osteogenic → osteoblasts)

• periochondrium is now periostenum

Endochondral ossification step 2a

Osteoblasts secrete organic bone ECM to build the bone collar (early compact bone) on the bone’s external surface as the bone begins to ossify from the outside

Endochondria ossification step 2b

Simultaneously, the internal cartilage begins to calcify, chondrocytes are cut off from the blood supply and die

Endochondral ossification step 3

In the primary ossification center, osteoblasts replace the calcified cartilage with early spongy bone, secondary ossification centers and medullary cavity develop

endochondral ossification step 4

osteoclasts degrade spongy bone causing the medullary cavity to enlarge, calcified cartilage is replaced by bone, epiphyses finish ossifying

In endochondral ossification cartilage only remains on

epiphyseal plates and articular surfaces where bones interact at joints (articular cartilage)

Articular cartilage vs epiphyseal plates

articular cartilage persists for life, epiphyseal plates are replaced with bone once longitudinal growth stops