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EMBRIOLOGY MUSKULOSKELETAL

Dr.ELLI KUSMAYATI,Sp.A

Embryonic Development of Bone

In the initial stages of developmental, the tube-

shape embryo contains three primary germ celllayers :

- Ectoderm or covering layer

- Endoderm or lining layer

- Mesoderm or middle layer

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From mesodermcomes themesenchyme, a diffuse

cellular tissue thatexhibitspluripotentially in thesense that its

undifferentiated cellsare capable ofdifferentiating into anyone of several type of

connective tissue suchas bone, cartilage,ligament, muscle,tendon and fascia.

During the fifth week ofembryonic development,the limb bud, covered byectoderm, appear.

In the central axis of eachlimb bud, themesenchymal cell becomecondensed into the shortcylinder.

The cylinder is segmentedby less densely areas at thesites of future joint andeach segment represents atiny mesenchymal modelof the future long bonethat will develop from it.

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By the sixth embryonic week, the undifferentiated

mesenchymal cells of each model begin to differentiate by

manufacturing cartilage matrix and thereby forming a

cartilaginous modelof the future bone The cartilaginous models grows partly through the

apposition of new cells on its surface (appositional growth)

from the deeper layers of the perichondrium.

After the seventh weeks of embryogenesis, the cartilage

cells in the center of the model hyperthrophy and form

longitudinal rows, after which the intercellular substance,

or matrix, calcifies, resulting in cell death. Vascular connective tissue then growth into the central

area of dead cartilage bringing osteoblast that secrete

collagen and proteoglycan into matrix.

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Matrix is then impregnated with calcium salt and

become immature bone on the calcified cartilage

matrix, thereby forming theprimary center of

ossification.

This process of replacement of cartilage by bone is

called endochondral ossificationand it occur only in

the presence of capillaries

The perichondrium has by this time become

periosteum, and in its deeper layer, the

mesenchymal cells, which have differentiated intoosteoblast, lay down bone directly by the process of

intramembranous ossification.

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By the sixth month of embryonic development, the

resorption of the central part of long bone results in

the formation of a medullary cavitythe process of

tubulation.

At the time birth, the largest epiphysis in the body

(distal femoral epiphysis) has develop a secondary

center of ossification by the process ofendochondral ossification within it.

Each such center, or ossific nucleus, is separated

from the metaphysis by a special plate of growingcartilagethe epiphyseal plate, orphysis, which

provides growth in the length of the bone through

the interstitial growth of cartilage cells.

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The short bones (e.g., the carpal bones)are develop by endochondrall ossification

in the same manner as the epiphysis. By contrast, the clavicle and most of the

skull develop bone directly in the

mesenchymal model by the process ofintramembranous ossification from theperiosteum without going through a

cartilagenous phase.

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JOINTS AND ARTICULAR CARTILAGE

Classification of the type of the joints

1. Syndesmosis

2. Synchrondosis

3. Synostosis

4. Symphysis

5. Synovial joint.

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Embryonic Development of Synovial

joints

By the seventh or eighth week ofembryonic life, clefts of space, which arefilled with tissue fluid, appear in the

primitive joint plate (cavitation) andgradually coalesce to form a single jointcavity.

The synovial fluid may be considered amucin (hyaluronic acid) diluted by tissuefluid

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The outer layer of the joint capsule differentiates

into fibrous tissue, whereas the inner layer

becomes specialized to form the synovialmembrane.

From sixth week embryonic life, active intrauterine

movement of the limbs is essential to the normalembryonic development of synovial joints (this just

one example of the critical importance of motion in

maintaining healthy joints)

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