Creation of Bone Tissue
Mesenchymal stem cells within mesenchyme or the medullary cavity of a bone fracture initiate the process of intramembranous ossification. A mesenchymal stem cell, or MSC, is an unspecialized cell whose morphology undergoes characteristic changes as it develops into an osteoblast. Before it begins to develop, the morphological characteristics of a MSC are: a small cell body with a few cell processes that are long and thin; a large, round nucleus with a prominent nucleolus that is surrounded by finely dispersed chromatin particles, giving the nucleus a clear appearance; a small amount of Golgi apparatus, rough endoplasmic reticulum, mitochondria, and polyribosomes; and the cells are widely dispersed within an extracellular matrix that is devoid of every type of collagen except for a few reticular fibrils.
Then a small group of adjacent MSCs begin to replicate until they have formed a small, dense aggregation of cells, a nodule. Once a nodule has been formed the MSCs within it stop replicating. At this point, changes in the morphology of the MSCs occur: the cell body becomes larger and rounder; the long, thin cell processes are no longer present; and the amount of Golgi apparatus and rough endoplasmic reticulum increases. Eventually, all of the cells within the aggregate display the morphologic characteristics of an osteoprogenitor cell.
At this stage of development, changes in the morphology of the osteoprogenitor cells occur: their shape becomes more columnar; the amount of Golgi apparatus and rough endoplasmic reticulum increases; and the cells begin to create an extracellular matrix consisting of Type-I collagen fibrils. This matrix is osteoid and the cells that created it are osteoblasts. The osteoblasts, while lining the periphery of the nodule, continue to form osteoid at its center and some of them become incorporated within it to become osteocytes.
At this point, the osteoid becomes mineralized resulting in a nidus consisting of mineralized osteoid that contains osteocytes and is lined by active osteoblasts. The nidus, that began as a diffuse collection of MSCs, has become rudimentary bone tissue.
Read more about this topic: Intramembranous Ossification
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