Physiology Topics
Bone is formed by either intramembranous ossification or endochondrial ossification. Fibrous membranes form the straigth portions of bones, like the flat bones of the skull and mandible, by intramembranous ossification. Most long boes are formed by endochondrial ossification: the replacement of cartilage by bone.
Intramembranous ossification starts when mesechymal cells in a fibrous membrane differentiate into osteoblasts, which then secrete the organic matrix of bone (mostly collagen) until they are surrounded by it. The enclosed osteoblasts become osteocytes, which extend cytoplasmic processes into caniculy radiating in all directions and touching other osteocytes. Calcium, phosphate and other minerals are then deposited and the matrix hardens (calcification).
This new bone still has some empty spaces between calcified areas, or trabeculae and is known as spongy bone. Blood vessels grow between the trabeculae and connective tissue associated with blood vessels differentiate into red blood marrow. On the outside of the bone, the mesenchyme condenses and develops into the periosteum. Eventually, most superficial layers are replaced by compact bone.
Endochondrial ossification starts when mesenchymal cells cluster into the shape of the future bone and differentiate into chondroblasts (hyaline cartilage). The cartilage grows in length by continual cell division and secretion of cartiage matrix. Chondrocytes in the midregion hypertrophy and burst, releasing their content. This changes the pH of the matrix and trtiggers calcification. Lacunae form, and eventyally merge into small cavities. A nutrient artery then enters the calcifying cartilage and stimulates formation of the periosteum, more capillaries and a primary ossification center. Osteoblast beguin to deposit bone matrix over the calcified cartilage, forming spomgy bone.
As the ossification center enlarges towards the ends of the bone, osteoclasts break down spongy bone forming the medullary cavity. The cavity fills with red bone marrow. Eventually, blood vessels enter the ends of the bones, and secondary ossification centers develop, but this time no medullary cavity is formed (spongy bone remains). Pior to adulthood, hyaline cartilage remains between the shaft and the ends of the bone as an epiphyseal plate responsible for length growth.
Growth is controlled by steriod hormones and growth factors like IGF-1. Growth hormone stimulates production of IGF-1. Excess growth hormone produces giantism, while deficiency in growth hormone or IGF-1 leads to stunt growth. Thyroid hormone and insulin are also needed for normal bone growth. Sex steroids cause the sudden groth that occurs at puberty. As adulthood is reached, chondrocytes at the epiphyseal plate stop responding to growth signals.
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Need more practice? Answer the review questions below.
1- List the two ways of bone formation and their main characteristics.
2- Describe the starting tissue for intramembranous ossification and how is the initial bone matrix formed.
3- What is spongy bone?
4- Describe the vascularization of bone during intramembranous osssification.
5- How is the outside of the bone formed during intramembranous ossification?
6- What are Osteogenic cells?
7- What are osteoblasts?
8- What are osteocytes?
9- What are osteoclasts?