1. Hard Tissue Formation
Cell Types of Bone
• Osteoprogenitor cells ---- undifferentiated cells
• Can divide to replace themselves can become
osteoblasts
• Found in inner layer of periosteum and endosteum
• Osteoblasts--form matrix collagen fibers but
cant divide
• Osteocytes ---mature cells that no longer secrete
matrix
• Osteoclasts---- huge multinucleated cells, function in bone resorption at
surfaces such as
endosteum
Matrix of Bone
• Inorganic mineral salts provide bones hardness
• Hydroxyapatite (calcium phosphate) calcium
carbonate
• Organic collagen fibers provide bones
flexibility
• Their tensile strength resists being stretched or
torn
• Remove minerals with acid rubbery structure
results
• Calcification is hardening of tissue when mineral
crystals deposit around collagen fibers
• Bone is not completely solid since it has small
spaces for vessels and red bone marrow
• Spongy bone has many such spaces
• Compact bone has very few
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2. Bone Formation or Ossification
• Intramembranous bone formation, formation of
bone directly
• from mesenchymal cells.
• All embryonic connective tissue begins as
mesenchyme.
• Endochondral ossification formation of bone
from hyaline
• cartilage.
Some Important Points
• When the chondrocytes die during bone lengthening, their extracellular
matrix gets very tough. The reason is that it gets loaded with calcium
salts, which make it harder. This hardening process is called
calcification.
In order for the osteoblasts to be able to get into the region of dead chondrocytes,
they have to get through the calcified cartilaginous matrix. They cannot.
Therefore, yet another type of cell, the osteoclast, is enlisted to help. The osteoclast
is specialized in endocytosis-- taking in and chewing up material that is around it.
These cells are derived from the blood; a certain type of blood cell, the monocyte,
differentiates into osteoclasts in bone. Therefore, the osteoclasts clear the way for
the invasion of osteoblasts.
Secondary Bone Remodelling
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