Bone Development, Growth And Structure

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Ossification:

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-The process of bone formation.

-appears in the 6-week-old embryo and continues till about 25 years old.

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There are 2 types of ossification: "IE"

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1. Intramembranous ossification

2. Endochondral Ossification

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Intramembranous ossification:

("growth inside membranes")

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where bone develops from mesenchyme or fibrous connective tissue

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Endochondral Ossification

("growth inside cartilage")

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where bone develops from a pre-exisintg cartilage model.

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Process of intramembranous ossification

STEP 1: The people migrate and form aggregations...

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-Mesenchymal cells migrate and form aggregations called OSSIFICATION CENTERS.

-The mesenchymal cells differentiate into OSTEOBLASTS

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The osteoblasts produce....

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bone matrix which undergoes CALCIFICATION.

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As the osteoblasts become encases in bone matrix they become ....

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osteocytes.

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STEP 2: then the people extend their arms to their neighbors...

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- ossification centers extend outward called SPICULES "pointy-things; sticking out".

- BVs grow and branch around spicules to support the bone tissue.

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STEP 3: The people and the neighbors begin to hug.

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-continued ossification;

-SPICULES FUSE together creating TRABECULAR NETWORKS. AKA SPONGY BONE.

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Further remodeling produces....

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the spongy bone, marrow cavities and COMPACT BONE- characteristic of mature bone.

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Process of endochondrial ossification:

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-occurs in a cartilage pre-model of the bone that appears first in the embryo.

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The cartilage model approximates the shape of the future bone and grow by both....

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INTERSTITIAL and APPOSITIONAL growth.

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Step 1: The Smith family gets larger so they sell their home..

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-Chondrocytes in the center BECOME LARGER and the surrounding cartilage matrix becomes CALCIFIED.

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These chondrocytes undergo...

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programmed death leaving behind CAVITIES within the cartilage matrix.

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Step 2: They purchase a new home on Periosteum Way....

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- the PERICHONDRIUM of the cartilage model becomes transformed into a PERIOSTEUM("around the bone") as an OSTEOGENIC LAYER forms.

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Periosteum--->

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produces a THIN COLLAR OF BONE AROUND THE DIAPHYSIS ("SHAFT")

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Step 3: Their smaller home did not sell, so squatters quickly came to stake their claim...

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-BVs & OSTEOPROGENITOR CELLS penetrate in the calcified cartilage matrix. (center of diaphysis)

-inside becomes OSSIFICATION CENTER

-OSTEOBLASTS can produce SPONGY BONE

-this becomes the SECONDARY OSSIFICATION CENTER.

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Step 4: The Smiths remodel the old home...

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-remodling occurs: OSTEOGENESIS

-a cavity results as the bone is remolded = MARROW CAVITY.

-increase in LENGTH and DIAMETER

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Increasing the length of the developing bone:

diaphysis

epiphysis (epiphyses)

metaphysics

epiphyseal plate

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- the shaft of the long bone

-two expanded ends

-the flared portion between the epiphysis and the diaphysis

-a piece of cartilage in the epiphysis where growth in the length of the bone occurs

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Step 5: The second story remodel is now finished...

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SECONDARY OSSIFICATION CENTERS appear in the epiphyses in a manner similar to that described above for the primary ossi. center.

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Step 6: Articular Cartilage and Zones

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-Ossi. proceeds, the epiphyses become filled w/ spongy bone and only a thin piece of hyaline cartilage left = ARTICULAR CARTILAGE.

-thin plate of cartilage, EPIPHYSEAL PLATE, now separate the bone of the epiphyses from that of the diaphysis.

-growth in length of the bone occurs at the epiphyseal plate as is evidenced by the appearance of zones within the cartilage.

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1. Zone of reserve or resting cartilage:

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-chondrocytes here exhibit no division or active matrix production

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2. Zone of proliferation

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-chondrocytes undergo CELL DIVISION and arrange themselves in distinct columns that are parallel to the direction of growth.

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3. Zone of Hypertrophy

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-chondrocytes ENLARGE and compress the cartilage matrix surrounding them.

**not good for the chondrocytes**

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4. Zone of Calcification

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enlarged chondrocytes undergo programmed cell death (lysis) and die; become calcified.

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5. Zone of resorption (a.k.a) ossification

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-the cavities left by dead chondrocytes are invaded by blood vessels that bring osteoprogenitor cells with them.

-Bone is produced on the surface of calcified cartilage as it undergoes RESORPTION.

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Step 7:

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-bone increases in length as long at the rate of cartilage production equals that rate by which it is replaced by bone.

-as bone matures the rate of bone production "catches up" with the rate of cartilage production and the cartilage that was the epiphyseal plate is replaced by an EPIPHYSEAL LINE.

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Increasing the diameter of the developing bone:

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-the diameter increases by APPOSITIONAL GROWTH.

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-Osteoblasts in the cellular layer of the periosteum add success layers of bone called...

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CIRCUMFERENTIAL LAMELLAE ("layers")

-at the same time, osteoclasts on the interior surface resorb bone and widen the marrow cavity.

-the final result is an increase in both the diameter and marrow cavity of the bone.