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Flashcards in Connective Tissue III Deck (23):

2 types of bone



Bone components

Diaphysis (central shaft) and epiphysis (2 expanded ends).
Longitudinal section:
outer region is solid (compact bone)inner part is spongy, has thin trabeculae (trabecular bone)..
In trabeculae spaces is bone marrow (made of hematopoietic tissue or adipose cells, red and white marrow). surrounded by loose CT with blood vessels.

Periosteum: outer surface of bone. fibroblasts, bone precursors, bone cells. dense CT.
Endosteum: inner surface. most ca mobilization and storage.


Compact vs. trabecular bone

compact/trabecular have same types of cells/same matrix but diff activities of cells... so get diff structure.
Compact gives strength, trabeculae gives SA for metabolism.


Bone cell types

-Osteoprogenitor: stem cells
-Osteoblasts: secrete un-mineralized extracell matrix, initiate mineralization
-Osteocytes: bone maintenance
-Osteoclasts: resorb bone/matrix



Stem cells capable of cell division to make osteoblasts and osteocytes.
In periosteal and endosteal surfaces. In soft connective tissue of channels.



Line inner layers of periosteal/endosteal surfaces (where growth/remodeling occurs). Secretory engines.
Actively secrete un-mineralized matrix (osteoid). Pinch of matrix vesicles (enzymes that initiate calcification).
Connected via gap junctions.
Capable of cell division.



Derived from osteoblasts!
Form as get surrounded by bone matrix in lacuna.
DO NOT divide (stay in G0)
extend processes (canaliculi) that form gap junctions w/ canaliculi from other osteocytes.
Communication via this lattice.
Have ability to modify bone matrix, don't secrete much matrix.
May send signals that help with bone maintenance.



Not related in lineage to other bone cells.
Unique fx.
Derived form monocytes in blood (hematopoietic origin).
Like macrophages. Degrade matrix.
Allow inward growth of blood vessels during bone formation, resorb already made bone for remodeling, resorb for Ca mobilization.


Bone matrix

Mostly calcified, packed with dense parallel collagen fibers.
Has hydroxyapatite (crystallized ca and po4).
Has proteoglycans and glycoproteins, but some are specific to bone.


Is bone innervated/vascularized ?

yes, nerves/vessels thread through channels in bone. So lots of diffusion happening.
Long bones, channels that are length-wise through compact bone are Haversian canals (bone lamellae form concentric rings around these... this canal + ring unit is an osteon)
Volkmann's canals (horizontal) link Haversian canals and to periosteum.


2 general ways bone is formed?

Intramembranous ossification
Endochondral ossification


Intramembranous ossification

Absence of pre-made cartilage. (all flat bones form this way)
Mesenchymal cells gather (condensation) --> transform into osteoprogenitors --> differentiate into osteoblasts (islands) --> secrete osteoid --> get trabecular network of bone! (blood vessels grown in, get delayed calcification).

Remodeling (osteoclasts) and bone deposition (osteoblasts) --> compact lamellar bone.


Endochondral ossification

Previously made cartilage.
Some cells in perichondrium turned into osteoprogenitors... sets in motion cartilage replacement with bone.
mesenchymal cells differentiate into chondrocytes --> secrete matrix of hyaline cartilage --> chondrocytes get encased in lacuna ---> get elongate structure of long bone (but it's cartilage), which continues to grow two ways:
1.) Appositional growth
2.) Interstitial growth


Appositional and interstitial growth

appositional: growth at surface. in perichondrium at cartilage surface, mesenchymal and/or fibroblast-like cells proliferate and differentiate into more chondrocytes, secrete more hyaline matrix.
Interstitial: growth from within. Chondrocytes proliferate in lacuna and secrete ECM. (groups of chondrocytes in a lacunae are isogenous groups).


Cartilage replacement by bone

In perichondrium form a collar of bone, becomes periosteum.
mesenchyme --> osteoprogenitors --> osteoblasts (in this region perichondrium becomes periosteum).
Nearby: chondrocytes enlarge --> cartilage matrix calcifies --> cartilage matrix calcifies --> oseteoclasts recruited, degrade calcified cartilage matrix --> blood vesselvs grown in here and bring osteoprogenitors on their CT sheath

Some osteoclasts penetrate deep to form center of ossification (primary or diaphysial center). This zone of ossification expands out...


Epiphyseal plate

At birth only region of proliferative cartilage that remains is beneath epiphyseal ossification regions.


Long bone growth

Epiphyseal plate stays until length maintained. (continued endochondral ossificaiton) ensured by continued interstitial growth of chondrocytes in their lacunae in the direction of long axis of the bone. new chondrocytes put down new hyaline matrix... gets made into bone.
wave of bone growth.
Bone growth STOPS when proliferation of cartilage stops.
after growth only articular cartilage remains.


Diameter growth

By appositional growth (on periosteum).
Requires osteoprogenitor proliferation... differentiate into osteoblasts and etc.
Net bone growth occurs only as long as resorption is less than deposition.
bone can't grow by interstitial growth (expand from within) like cartilage.. too rigid


Bone remodeling

Osteoclasts activated, begin resorbing older calcified bone and signal osteoblasts (initiate mineralization) and stimulate osteoprogenitors to make new osteoblasts.

Reshaping in growth and development, continual turn-over or bone matrix in mature/fully grown bones.



By osteoclasts. At endosteal surface. MUST be directly/carefully coupled to formation. Formation must also be coupled to calcification.
osteoporosis (issues in coupling resorption/formation)
osteopetrosis (defective resorption, increased bone mass)
rickets (increase in uncalcified osteoid)



Osteoblasts initiate via matrix vesicles.
They have lots of Ca, phosphate, alkaline phosphatase enzymes (generate free phosphate which makes precipitates with Ca).
Precipitates grow and rupture the vesicles.


Bone and cartilage regulation?

Short range signals in local enviro: Bone morphogenetic proteins (BMPs) secreted by cells bind receptors and trigger intracell protein phosphorylation to alter gene expression (control bone development, remodeling, turn over)... can stimulate chondrocyte differentiation and cartilage formation OR osteogenesis (osteoblasts make bone)...
Long range: from endocrine (hormones)
Mechanical stress
Neuronal stimulation


Calcium homeostasis

Regulated by endocrine hormones and dietary Ca intake.
Parathyroid: Ca liberation (resorption)
Calcitonin: Ca uptake into bone
They control activity of osteoclasts and osteoblasts.
Vit D imp for intestinal uptake.