Lecture 9: Bone Cont. Flashcards
Osteoblast shape
- Cuboidal to polygonal with basophilic cytoplasm
- Aggregate in a single layer
Osteocytes
Mature bone cells enclosed in the bone matrix within lacunae
Osteocytes maintain contact with adjacent osteocytes via
cytoplasmic branching processes (filopodia) that connect to each other via small tunnels or canaliculi
How do osteocytes transfer nutritional needs to the next cell?
gap junction
Are osteocytes capable of undergoing mitosis?
No, because they are mature cells
How do osteocytes maintain the homeostasis of Ca?
Synthesize and to a lesser extent absorb matrix
Osteoclasts
multinucleated cells closely related to macrophages
Osteoclast cytoplasm (baso or acido)
acidophilic
Osteoclasts - bone marrow origin (from moncytes), formed via
DNA replication without division of the cytoplasm
Where are osteocytes observed?
In indentations (bays) called Howship’s lacunae, which are reabsorbed from bone surface
Osteoclasts induce
osteolysis to maintain Ca homeostasis and to remodel in response to growth or changing mechanical stress
When activated and in contact with a bony surface, osteoclasts
release acid and lysosomal enzymes to decalcify bone
Circumferential zone of osteoclast
Integrins tightly bind the matrix and surround a ruffled border of the cytoplasmic projections close to the matrix
(Osteoclast) The sealed space between the cell and the matrix is acidified by
A proton pump localized in the osteoclast membrane and receives hydrolytic enzymes secreted by the cell
Two ways of bone formation in fetal life
- Intramembrous ossification
- Endochondrial ossification
Intramembrous ossification
- Occurs within membranes of condensed primitive mesenchymal tissue
- Flat bones of the skull
Endochondrial ossification
- Bone develops from cartilaginous model (hyaline cartilage) that is subsequently replaced by osseous tissue at the ossification centers
- Occurs in the majority of bones of the skeleton.
Where does endochondrial ossification occur
Ossification centers of immature bones and in the growth plates (epiphyseal plates) of developing bones
Once growth plates are closed (mature animal),
no further longitudinal bone growth can occur
Two types of bone based on degree of maturity
Woven bone
Lamellar bone
Woven bone
- Immature bone present during fetal development and in the early stages of bone repair.
- Collagen fibers in woven bone are randomly arranged adopting a crisscross (woven pattern) microscopically
Lamellar bone
- Mature bone
- Collagen fibers are arranged in a parallel pattern
- Composed of successive concentric layers of bone (in laminar formation) around a central canal containing blood vessels and nerves
Osteons
functional unit of mature bone
5 factors that influence bone formation and resorption
- Parathyroid hormone (can increase recruitment and activity of osteoclasts)
- Vitamin D deficiency (May lead to bone disease)
- Hormones (Estrogen inhibits bone resorption)
- Mechanical constraints
- Genetic background
