University:
Western Sydney University
Course:
MEDI 2003 | Clinical Sciences 2
Academic year:

2024
Views:

188

Pages:

4
Author:

Mariam A. A.

Describe the process of ossification ● ● ● Bones are derived from the mesoderm Bones are not completely developed when the baby is born, continues to develop and grow until the epiphyseal plates are gone Types of ossification: endochondral and intramembranous Endochondral ossification ● Forms long, short and irregular bones ● Bone begins from hyaline cartilage block that resembles a bone ● Primary ossification centre = shaft ● Secondary ossification centre = epiphysis 1. Cartilage cells (chondroblasts) in the external surface of the cartilage bone model (perichondrium) differentiate into bone cells (osteoblasts) ● Chondroblast → osteogenic cell → osteoblast ● Begin with a block of hyaline cartilage filled with chondrocytes ● When blood vessels get close to the cartilage block, there's a chemical signal that tells the chondroblasts (directly beneath the perichondrium) to differentiate into precursor bone cells (osteogenic cells) ● Osteogenic cells develop into osteoblasts which are bone forming cells ● External layer of connective tissue has a terminology change from perichondrium to periosteum 2. Formation of the collar bone, calcification and cell death of internal cartilage cells ● Bone collar (external parts around the diaphysis/shaft) begin to form ● Chondrocytes inside begin to die and calcify (harden) because they're not being replaced 3. Nutrient artery burrows into primary ossification centre, osteoblasts enter through new foramen and build new bone from the inside ● Arteries (blood vessels) that set the alarm off are able to penetrate into the bone model itself ● Osteoclasts make space to create a nutrient foramen ● Osteoblasts follow the blood vessel into the cartilage block in the centre, replace the calcified matrix into bone cells ○ Organic matrix hardens overtime and the osteoblasts differentiate into osteocytes as they spread into the area ● After birth, osteoclast goes into the epiphyseal area to make way for the arteries → blood vessels come in and it all happens again (at both epiphyseal ends) = secondary ossification centre 4. Ossification continues, osteoclasts begin to digest newly formed bone to form the medullary cavity ● Osteoblasts continue to make more bone cells ● Osteoclasts hollow out the medullary cavity ● Eventually the bone will completely replace the cartilage (epiphyseal growth plates) so the bone stops growing in length ● Articular cartilage (cap) remains present until adulthood → only cartilage left Intramembranous ossification ● Forms flat bones ● Bone begins from within the membrane of embryonic connective tissue and expands out ● Similarities: primary centres ● Differences: starts with mesenchymal cells (endochondral begins with cartilage cells), there are no osteoclasts ● ● ● ● Mesenchymal cells break off and differentiate into osteogenic cells and then osteoblasts Deposit matrix into the flat sheets and matrix hardens (calcifies) ○ Osteoblasts will create ring around the tissue area ○ Osteoblasts trapped inside become osteocytes There are multiple separate primary ossification centres and they start joining up in different directions = spongy bone Osteoblasts keep on moving out from the centre and secrete matrix that will harden = layers of compact bone around the spongy bone