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Flashcards in Bone development Deck (16):

Most bones form in a cartilage frame. The process is called ______ ossification.


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Some bones (especially those of the skull) develop in sheets of connective tissue. This process is called _______ ossification. 


This is also the method by which most bones heal

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Cells that deposit bony matrix and are responsible for ossification?


Osteoblasts are cells which originate in the bone marrow and contribute to the production of new bone. These cells build up the matrix of the bone structure and also play a role in the mineralization of the bone matrix. Bone is constantly being built up and broken down by the body, making osteoblasts rather critical. The counterpart to the osteoblast is the osteoclast, a cell which is responsible for breaking down bone.

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Cells that dissolve bone mineral and matrix. Can absorb 8xthe amount of bone produced by an osteoblast. 


are very large (up to 100 µm), multi-nucleated (about 5-10 visible in a histological section, but up to 50 in the actual cell) bone-resorbing cells. They arise by the fusion of monocytes (macrophage precursors in the blood) or macrophages. Osteoclasts attach themselves to the bone matrix and form a tight seal at the rim of the attachment site. The cell membrane opposite the matrix has deep invaginations forming a ruffled border. Osteoclasts empty the contents of lysosomes into the extracellular space between the ruffled border and the bone matrix. The released enzymes break down the collagen fibres of the matrix. Osteoclasts are stimulated by parathyroid hormone (produced by the parathyroid gland) and inhibited by calcitonin (produced by specialised cells of the thyroid gland). Osteoclasts are often seen within the indentations of the bone matrix that are formed by their activity (resorption bays or Howship's lacunae).

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Cells that hel to maintain calcium homeostasis and mechanical properties of bone. 


Osteocytes contain less endoplasmatic reticulum and are somewhat smaller than osteoblasts.

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Long bones grow _______ by producing subperiosteal new bone and _______ by mineralizing cartilage at centers of ossification. 



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Primary cneters of ossification develop in diaphyses, growing and lengthening as cartilage _____. 


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Secondary centers of ossification develop in _____ (contributing to length of bone) and ______ (Contributing to shape of bone). 



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Accesory centers of ossification form _____ ____ and create foci of mineralization near joints. 

seasamoid bones

NOTE- accessory centers can be mistaken for chip fractures or osteophytes. 

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The area where wider metaphyseal cancellous bone is actively remodeling to thinner diaphyseal cortical bone is called?

Cut Back Zone

NOTE- areas of actively remodeling bone normally appear irregular and hazy and may be mistaken for pathology such as hypertophic osteodystrophy, osteomyelitis, or trauma, however, soft tissue swelling is absent durin normal growth.

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Physes are highly susceptible to injury and when damaged can lead to limb deformity. ____ ____ physis is especially vulnerable to injury due to its unique conical shape. 

Distal Ulnar

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Long bones cease growing when _____ stops growth. Ossification from the metaphysis and ______ gradually closes the physis, leaving a thin sclerotic line (physeal scar). The physeal scar later disappears as the _____ and metaphysis continue to remodel and eventually blend together. Closure of physes occur at predictable times by vary b/t breed and ____. 





(Harris lines, also known as growth arrest lines, are lines of increased bone density that represent the position of the growth plate at the time of insult to the organism and formed on long bones due to growth arrest. They are only visible by radiograph or in cross-section.)

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Fetal ossification becomes visible on survey rads at predictable times. What is it for dogs/cats? Identifying boney structures in developing fetus aids in estimating time to parturition. 

Dogs- 42-45 days

Cats- 35-39 days

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Dog body references

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bone development

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