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I got a question from a textbook <<Seeley's Anatomy & Physiology Twelfth Edition>> that does not have an answer at the back of the book (only odd numbered questions do have an answer and this question is even):

Explain why bones cannot undergo interstitial growth as cartilage does.

i can only think of two reasons:

  1. In bones, calcification causes the chondrocytes to die, so cartilage cannot form in the bone, leading to no interstitial growth in the bone.

  2. Cartilages are usually only found at the tip of the bones, so its growth can cause the bone to lengthen, which is interstitial growth.

I got a nagging feeling that there is got to be more. Did i miss any other reason? (Also are my reasons correct?)

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While mature cartilage is solid, the extracellular matrix secreted by chondroblasts during cartilage formation is soft enough to allow for both cell division and tissue expansion. This is interstitial growth.

As the matrix hardens, secretion and proliferation slow down and then stop altogether. Isogenous groups are a testimony of the last cell divisions before the surrounding matrix turns solid. From this point on, further growth can only take place by apposition, i.e. new cartilage matrix is secreted by chondroblasts found in the perichondrium.

Bone matrix becomes mineralized (and therefore very hard) soon after its organic components are secreted by osteoblasts. Osteoblasts that get trapped in the matrix shrink and become quiescent. Thus, there can be no expansion from within, i.e. no interstitial growth.

Bone grows exclusively by apposition, where further osteoblasts differentiate from precursors and deposit new matrix on top of the existing bone.

While this is the answer to the original question, your tentative responses suggest that you need to do some further reading. You seem to refer to the interaction between the two tissues during endochondral ossification, which is a separate, more complex topic.

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