Module 6.2

What is osteogenesis? What is another name for it?

(ossification)— bone tissue formation

During endochondral ossification where do you find cartilage?

-Cartilage limited to the articular cartilage covering each joint surface, and to the epiphyseal plate -a thin wall of cartilage separating the primary and secondary marrow cavities

During endochondral ossification where does the secondary ossification center form and how?

-It is created by chondrocyte enlargement and death in the epiphyses -They become hollowed out by the same process generating a secondary marrow cavity in epiphyses -The cavity expands outward from the center in all directions

How long does the epiphyseal plate last? What does it do?

-epiphyseal plate persists through childhood and adolescence serves as a growth zone for bone elongation

What happens as osteoblasts continue to deposit minerals?

-produce a honeycomb of bony trabeculae -spongy bone (diploe) -osteoclasts resorb and remodel others to form a cavity in the middle of bone

What are the five zones of the metaphysis?

1. Zone of reserve cartilage: Typical histology of resting hyaline cartilage 2. Zone of cell proliferation: Chondrocytes multiplying and lining up in rows of small flattened lacunae 3. Zone of cell hypertrophy: Cessation of mitosis; enlargement of chondrocytes and thinning of lacuna walls 4. Zone of calcification: Temporary calcification of cartilage matrix between columns of lacunae 5. Zone of bone deposition: Breakdown of lacuna walls, leaving open channels; death of chondrocytes; bone deposition by osteoblasts, forming trabeculae of spongy bone

Where do blood vessels penetrate and what do they form during endochondral ossification?

Blood vessels penetrate the bony collar and invade primary ossification center primary marrow cavity - forms from blood and stem cells filling hollow cavity

In endochondral ossification, what fills the epiphyses during infancy and childhood?

During infancy and childhood, the epiphyses fill with spongy bone

In intramembranous ossification what cells become osteogenic cells?

Mesenchymal cells differentiate into osteogenic cells

What is the region of transition from cartilage to bone at each end of primary marrow cavity called?

Metaphysis

What happens in the zone of calcification? What does it provide?

Mineral deposited in the matrix between columns of lacunae temporary support for cartilage

Where do ossification centers appear?

Ossification centers appear in the fibrous connective tissue membrane.

How does appositional growth occur?

Osteoblasts in osteogenic layer of periosteum (abutting the bone surface) deposit osteoid tissue Become trapped as tissue calcifies (osteocyte)

During endochondral ossficiation what lines the cavity and deposits osteoid tissue, calcifying it and forming temporary network of trabeculae?

Osteoblasts line the cavity and deposit osteoid tissue and calcify it forming temporary network of trabeculae

In intramembranous ossification osteogenic cells differentiate into what?

Osteogenic cells differentiate into osteoblasts these cells deposit organic matrix - osteoid tissue

Where is the primary ossification center during endochondral ossification? What occurs here (3 things)?

Primary ossification center - chondrocytes in the middle of the model enlarge 1. matrix between lacunae are reduced to thin walls 2. walls of this thin matrix ossify and block nutrients from reaching chondrocytes 3. they die and their lacunae merge into a single cavity in the middle of the model

Epiphyseal plate cartilage organizes into four important functional zones, which are?

Proliferation (growth) Hypertrophic Calcification Ossification (osteogenic)

What forms an ossification center?

Selected centrally located mesenchymal cells cluster and differentiate into osteoblasts, forming an ossification center.

Where does spongy bone persist? What becomes of its vascular tissue?

Spongy bone (diploë), consisting of distinct trabeculae, persists internally and its vascular tissue becomes red marrow.

During endochondral ossification stem cells give rise to what?

Stem cells give rise to osteoblasts and osteoclasts

During intramembranous ossification what are the trabeculae at the surface doing? What does this process give rise to?

Trabeculae at the surface continue to calcify until the spaces between them are filled in, converting spongy bone to compact bone; Gives rise to the sandwich-like arrangement of mature flat bone

What happens to the trabeculae just deep to the periosteum?

Trabeculae just deep to the periosteum thicken, and are later replaced with mature lamellar bone, forming compact bone plates.

True or False: Mesenchyme close to the outer surface of the trabeculae remains uncalcified, becomes denser and more fibrous, forming periosteum

True

True or False: Mesenchyme develops into a body of hyaline cartilage in location of future bone

True

what condenses on the external surface of woven bone?

Vascularized mesenchyme condenses on the external face of the woven bone and becomes the periosteum.

During endochondral ossification the wave of cartilage death progresses toward what? How are osteoclasts involved?

Wave of cartilage death progresses toward the bone ends osteoclasts follow the wave dissolving the cartilage remnants enlarging the marrow cavity

In intramembranous ossification, bones develop within what?

a fibrous sheet similar to epidermis of the skin (dermal bones)

What makes up the epiphyseal plate? What is on either side? What is the zone facing the marrow cavity called?

consists of typical hyaline cartilage in the middle with a transition zone on each side where cartilage is being replaced by bone metaphysis is the zone of transition facing the marrow cavity

Where does interstitial growth occur?

epiphyseal plate - a region of transition from cartilage to bone

Once a bone collar has formed around the hyaline cartilage during endochondral ossification, the former perichondrium is considered what?

former perichondrium is now considered to be periosteum

What is the function of the epiphyseal plate?

functions as growth zone where the bones elongate

What is interstitial growth?

growth in length

What are the two types of ossification?

intramembranous and endochondral

In intramembranous ossification what type of tissue type of tissue condenses into a soft tissue dense with blood capillaries?

mesenchyme - embryonic connective tissue condenses into a layer of soft tissue with dense supply of blood capillaries

What do trapped osteoblasts become?

osteocytes

what do osteoblasts secrete?

osteoid

During endochondral ossification perichondrium produces what initially and what later?

perichondrium produces chondrocytes initially, and later produces osteoblasts

In intramembranous ossification what eventually forms trabeculae?

regions of mesenchyme become a network of soft sheets - trabeculae

What is endochondral ossification?

theprocess in which bone develops from pre-existing cartilage model (hyaline cartilage) beginning the 6th to 9th fetal week and ending in early 20's most bones develop by this process

What is appositional growth?

thickness and remodeling of all bones by osteoblasts and osteoclasts on bone surfaces

Accumulating osteoid is laid down between embryonic blood vessels in a random manner. The result is a network (instead of lamellae) of trabeculae called what?

woven bone.

What occurs in intramembranous ossification?

Membrane bone develops from fibrous membrane Forms flat bones - clavicles and cranial bones

What are the steps endochondral ossification?

1. Bone collar forms around hyaline cartilage model. 2. Cartilage in the center of the diaphysis calcifies and then develops cavities. 3. The periosteal bud inavades the internal cavities and spongy bone begins to form. 4. The diaphysis elongates and a medullary cavity forms as ossification continues. Secondary ossification centers appear in the epiphyses in preparation for stage 5. 5. The epiphyses ossify. When completed, hyaline cartilage remains only in the epiphyseal plates and articular cartilages.

What are the stages of osteogenesis?

1. Bone formation—begins in the 2nd month of development 2. Postnatal bone growth until early adulthood 3. Bone remodeling and repair is lifelong

What are the steps of intramembranous ossification?

1. Ossification centers appear in the fibrous connective tissue membrane. 2. Bone matrix (osteoid) is secreted within the fibrous membrane and calcifies 3. Woven bone and periosteum form. 4. Lamellar bone replaces woven bone, just deep to the periosteum. Red marrow appears.

What happens in the zone of bone deposition (ossification)?

1. chondrocytes die, longitudinal columns fill with osteoblasts and blood vessels 2. osteoclasts dissolve the calcified cartilage 3. Osteons and spongy bone are created by osteoblasts

When is the cartilage in the epiphyseal plate consumed? What occurs when this happens?

By late teens to early twenties, all remaining cartilage in the epiphyseal plate is generally consumed; 1. gap between epiphyses and diaphysis closes 2. primary and secondary marrow cavities unite into a single cavity 3. bone can no longer grow in length

What occurs in endochondral ossification?

Cartilage (endochondral) bone forms by replacing hyaline cartilage Forms most of the rest of the skeleton

True or False: Appositional growth ends at puberty

False: Bones increase in width throughout life

Where is the Zone of reserve cartilage (resting zone)?

It is typical hyaline cartilage farthest from marrow cavity that shows no sign of transforming into bone

As appositional growth occurs, matrix is laid down parallel to surface forming the circumferential what?

Lay down matrix in layers parallel to surface forms circumferential lamellae over surface osteoclasts of endosteum enlarge marrow cavity

Where is the zone of proliferation?

They are the chondrocytes multiply forming columns of flat lacunae

Where is the Zone of hypertrophy, what happens here?

This is the zone of Chondrocyte enlargement, matrix between lacunae become very thin

What controls bone growth? What hormones are involved, what do they do, and when?

Thyroid hormone modulates activity of growth hormone Growth hormone stimulates epiphyseal plate activity Testosterone and estrogens (at puberty) Promote adolescent growth spurts End growth by inducing epiphyseal plate closure