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During growth of an individual animal some components of the body grow in size but not in number (type 1) while some others increase in number but not in size (type 2). Which of the following is correct? (A) type 1: bones and muscle cells; type 2: hair follicles, red blood cells and epithelial cells. (B) type 1: bones and red blood cells; type 2: hair follicles, muscle cells and epithelial cells. (C) type 1: hair follicles and muscle cells; type 2: bones, red blood cells and epithelial cells. (D) type 1: epithelial cells and bones; type 2: hair follicles, red blood cells and muscle cells.

I know muscle cells only grow in size. I am unsure about hair follicles, which is why I am confused between options A and C. I am also confused about bone growth. Does it ever increase in number? It would be nice if someone could clear that up. enter image description here

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  • $\begingroup$ Welcome to Biology.SE! Did you ever see giant hair follicles in adults? :) $\endgroup$ – LinuxBlanket Jun 21 '18 at 9:25
  • $\begingroup$ I'll admit I didn't think of it that way. So do they increase in number then? And what about bones? $\endgroup$ – Fenil Jun 21 '18 at 11:56
  • $\begingroup$ Well, either they grow in number, or, if they stay the same size, they become less and less dense when we grow up. Which option do you think is the correct one? Also, have a look at this page. Can you tell something about bone cell growth? $\endgroup$ – LinuxBlanket Jun 21 '18 at 12:40
  • $\begingroup$ (It's not that I don't want to give you the correct answer straight away; it's just, you know: "Give a man a fish, and you feed him for a day. Teach a man to fish, and you feed him for a lifetime.") $\endgroup$ – LinuxBlanket Jun 21 '18 at 12:42
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    $\begingroup$ You're right. I got confused. I'm not thinking straight. They grow in number. I think that solved my question. Answer should be A. Thank you for being so patient. You're a good teacher. :) $\endgroup$ – Fenil Jun 21 '18 at 16:30
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We navigated through this question with Fenil in comments, I'll resume the answer here:

Osteocytes

We know from this page that "osteocytes have an average half life of 25 years, they do not divide" and that "when osteoblasts become trapped in the matrix that they secrete, they become osteocytes. Osteocytes are networked to each other via long cytoplasmic extensions that occupy tiny canals called canaliculi".

We can thus conclude that bone cells don't grow in numbers, but in size by their cytoplasmic extensions.

Hair follicles

Well, we draw the conclusion that hair follicles grow in number and not in size based on the following common sense observations:

  • We don't get to see tiny hair follicles in babies and giant ones in adults;
  • If they don't grow in size, either they grow in number or they become less and less dense as the skin surface grows with age. If the latter, we should see the same hair number in babies and in adults, whichever their height is, and this is obviously not the case.

So, they grow in number.

The answer is $A$.

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