Lecture 4: Osteogenesis & ossification (Wronski) Flashcards Preview

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Flashcards in Lecture 4: Osteogenesis & ossification (Wronski) Deck (43):

define osteogenesis or ossification

the formation of bone


what are the 2 principal mechanisms for the formation of bone?

1. intramembranous ossification
2. endochondral ossification


define intramembranous ossification

bone that forms within embryonic connective tissue


define endochondral ossification

formation of bone within a hyaline cartilage model


what type of bone is the first bone formed in both intramembranous and endochondral ossification?

cancellous (primary spongiosa). it is composed of trabeculae that can later be transformed into compact bone


what are the 4 steps of intramembranous ossification?

1. osteoblasts differentiate within richly vascularized embryonic mesenchyme and produce thin strands of extracellular matrix - forming a network of osteoid trabeculae
2. osteoblasts and osteoprogenitor cells become aligned on the trabecular surfaces.
3. osteoblasts produce cytoplasmic processes that become embedded to form osteocytes within lacunae and canaliculi.
4. osteoprogenitor cells form new osteoblasts and additional matrix is laid down and calcified to form new lacunae and canaliculi


define appositional growth

process of adding new bone upon pre-existing bone surfaces


is interstitial growth of bone possible in intramembranous ossification? why or why not?

no; due to mineralization immediately following the deposition of bone matrix


describe the characteristics of woven bone

* collagen fibers are randomly oriented*
*non-uniform distribution of osteocytes*
- it is formed during development or fracture repair
- is immature bone with a lower mineral content
- is eventually replaced with lamellar bone


describe the characteristics of lamellar bone

*regularly arranged collagen fibers within lamellae*
- mature bone with high mineral content
- more uniform distribution of osteocytes
- structurally sound


main difference between woven bone and lamellar bone

woven bone has randomly oriented collagen fibers and non-uniform distribution of osteocytes. it is considered inferior and eventually transforms to lamellar bone which is high in mineral content


how can trabecular or cancellous bone be converted to compact bone?

by the deposition of repeating layers of new bone that encroach upon and largely fill the connective tissue spaces between trabeculae.


are primary osteons in newly compact bone that was originally cancellous bone lamellar or woven in configuration?

these primary osteons are considered woven bone despite having lamellar configuration because the collagen fibers are not highly oriented within each lamina.


how does the initial cartilage model develop?

from differentiation of mesenchymal cells into chondroblasts that produce matrix and perichondrium.


what type of growth is applied to the cartilage model?

both appositional and interstitial growth!


what are the 8 steps in forming the primary (diaphyseal) center of ossification?
(didn't know how to break this one up, sorry!)

1. the surrounding matrix of the hypertrophy of chondrocytes in the diaphysis becomes calcified,
2. chondrocytes and cartilage matrix degenerate so that large cavitations of confluent lacunae are formed.
3. perichondrium transforms to periosteum as mesenchymal cells of the diaphysis differentiate into osteoblasts and form a subperiosteal collar of bone around the diaphysis (this is a form of intramembranous ossification)
4. projections of tissue containing blood vessels, stem cells, and osteoprogenitor cells invade the cavitations of the cartilage model and form a primary center of ossification
5. osteoprogenitor cells differentiate into osteoblasts and produce bone matrix
6. trabeculae of cancellous bone is formed
7. osteoclasts remove cancellous bone within the center of ossification to form a medullary cavity
8. the periosteal bud provides osteoprogenitor cells and stem cells that form endosteum and myeloid tissue of bone marrow


how does developing bone lengthen via endochondral ossification?

via interstitial growth of hyaline cartilage at the epiphyses.


what does the growth plate leave behind as it continues to grow?

as it advances, it leaves behind cartilage cores and bone is deposited on the surface of cartilage cores, this is considered rudimentous or cancelous bone.
keep in mind there is also remodeling and resorption going on along the surface.


ossification of the epiphyses extends in all directions to form cancellous bone except for 2 regions. what are they?

1. articular cartilage - covering the articular surface
2. the epiphyseal growth plate - a plate of hyaline cartilage between the epiphysis and metaphysis


what is another term for the growth plate?

the physis


what is the metaphysis?

the enlarged portion of the diaphysis that abuts the epiphyseal plate


how many centers of ossification do most long bones develop?

2 :-)


how many centers of ossification do most short bones develop?

only one :-(


how is the principal increase in bone length achieved?

by elongation of the metaphysis and diaphysis at the epiphyseal growth plate


following the formation of primary and secondary centers of ossification, bone growth and elongation involves 2 main processes. what are they?

1. interstitial growth of epiphyseal plate cartilage along the epiphyseal margin
2. endochondral ossification of cartilage at the metaphyseal margin


there are 4 zones of the epiphyseal growth plate. what are they?

1. zone of resting cartilage
2. zone of proliferating cartilage
3. zone of maturing and hypertrophying cartilage
4. zone of calcifying cartilage


what is the zone of resting cartilage?

it is composed of chondrocytes nearest the epiphyseal bone tissue and serves to anchor the plate to the bony epiphysis


what is the zone of proliferating cartilage?

it is composed of dividing chondrocytes arranged in columns


what is the zone of maturing/hypertrophying cartilage?

it is composed of hypertrophied chondrocytes arranged in columns


what is the zone of calcifying cartilage?

it is composed of calcified cartilage :-P


as bone length increases, how does the bone maintain is proportion? length relative to width?

as new bone is formed at the metaphysis, the trabeculae at the periphery of the epiphyseal plate become added to the shaft as the bone grows in length.

the width of the added shaft at the metaphysis is maintained by resorption of bone by osteoclasts at the periphery thereby reducing the diameter to match the diaphysis


how does bone diameter increase? what is this an example of?

appositional bone growth occurs at the periosteal surface of the diaphysis, adding successive layers of new bone. simulatenously, bone resoprtion occurs at the endocortical surface to maintain and expand the medullary cavity.

this is a classic example of bone remodeling


how does bone growth stop?

cartilage cell proliferation eventually stops in the epiphyseal plate. the epiphyseal plate then disappears resulting in confluence of trabeculae and cavities of the epiphysis and metaphysis. following closure, longitudinal bone growth is no longer possible.


what two hormones affect bone growth? how?

estrogen and growth hormone

growth hormone stimulates chondrocyte proliferation within the growth plate
estrogen inhibits chondrocyte proliferation


define osteoporosis

low bone mass that results in bone fractures after only minimal trauma.


which sites are the most common for bone fractures?

hip (femoral neck), thoracic and lumbar vertebral bodies and the wrist


what is the most important factor influencing development of postmenopausal osteoporosis?

estrogen deficiency


describe the early stages of postmenopausal osteoporosis

- rapid bone loss (primarily cancellous bone)
- increased bone formation
- greater increase in bone resoprtion due to hyperactive osteoclasts


describe the late stages of osteoporosis

- slow bone loss
- decreased bone formation
- normal bone resorption


what are some (duh) factors affecting the development of osteoporosis

- estrogen deficiency
- genetic predisposition
- sedentary lifestyle
- smoking
- low dietary calcium


describe some treatements for postmenopausal osteoporosis

- anti-resorptive agents (like estrogen, calcitonin, biphosphates, etc.)
- anabolic agents to stimulate bone formation like sodium fluoride (it induces formation of woven bone and is not approved by the FDA)
- Parathyroid hormone


which animal can experience nutritional osteodystrophy (bone breakdown due to low dietary calcium)



what are some clinical signs of nutritional osteodystrophy

- muscle twitching a.k.a. tetany
- pathologic bone fractures
- diaphyseal enlargements of long bones
- widened growth plates in growing animals (inconstant)

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