When organic 'tissue' grows (either during embryonic development, post-natal growth, or tissue regeneration) there are two processes in operation which determine the final size of the tissue or organ. The first is the size of the cells themselves - which is governed by an as yet unknown process (see: Cell Sizeplugin-autotooltip__plain plugin-autotooltip_bigCell size

A detailed explanation of the factors which regulate cell size (i.e. the size to which cells grow) has not yet been found. Since (most) cells stop growing when they reach a specific size, it's suggested that there must be some feedback mechanism to restrict growth beyond that point.) The second factor is the number of cells, in other words the number of times which cell division occurs. Together, these two processes are known as 'cell proliferation'.

Since it's known that in almost all cases 'tissue' and organs stop growing at some point, there must be a regulatory mechanism which 'tells' the tissue to stop growing.

That mechanism is as yet unknown.

How cell proliferation is controlled during tissue growth to determine final tissue size is an open question in biology."

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Note: Understanding cell proliferation is of profound relevance in cancer research - where one of the major factors is the unconstrained proliferation of cancerous cells.

Also see : Cell Differentiationplugin-autotooltip__plain plugin-autotooltip_bigCell Differentiation

""The mechanisms that link cellular differentiation programs and dynamic gene regulation in complex eukaryotic systems remain mysterious. Such programs drive diverse and central biological processes including organismal development, immune function, disease progression, and meiosis."

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