Cells of bone tissue

Osteogenic cell location

periosteum and endosteum

Osteogenic cell stimuli

Bone morphogenetic proteins and mechanical stress / fractures

Osteogenic cell function

Differentiate into osteoblasts, also can multiply to reinforce or rebuild

Osteoblast location

Compact and spongy bone of all bones, periosteum and endosteum

Osteoblast stimuli

Hormonal (growth hormone, thyroid hormone, testosterone, estrogen)

Osteoblast function

Actively miotic and differentiate into osteocytes.
- Hormones stimulate proliferation and activity / promoting bone formation.
- Secrete organic/unmineralized bone matrix called osteoid, which is made up of collagen and calcium binding proteins.

Osteocyte location

Compact and spongy bone of all bones

Osteocyte stimuli

Mechanical signals, increased concentration of calcium and phosphate ions, matrix proteins that bind and concentrate calcium, appropriate amount of enzyme alkaline phosphatase for mineralization.

Osteocyte function

Maintain bone matrix and act as mechanical stress / strain sensor and respond to mechanical stimuli such as increased force on bone or weightlessness.
- Communicate with osteoblast and osteoclasts so bone modelling can occur
- Secrete calcified matrix
- Connected by gap junctions and canaliculi.

Osteoclast location

Periosteum, endosteum and sites of old or injured bone

Osteoclast stimuli

Osteoblasts

Osteoclast function

Has ruffled borders to increase surface area for enzyme degradation of bone. Break down bone for resorption.

What is secreted when there are low levels of blood calcium ?

Parathyroid hormone, secreted by parathyroid glands in response to the stimulus.
- Prevents osteoblasts from differentiating into osteocytes, which would secrete calcified matrix and further decease blood calcium levels.
- Stimulates secretion of activation/survival factors by osteoblasts to stimulate osteoclasts to resorb bone. This releases calcium ions into blood and raises blood calcium levels.

What is secreted when there high levels of blood calcium?

Calcitonin, which is produced by parafollicular cells of the thyroid gland in response to the stimulus.
- Stimulates osteoblasts to differentiate into osteocytes, which secrete excess calcium ions into osteoid, thus lowering blood calcium levels.
- Calcitonin secretion stops when homeostatic calcium levels are reached.