S377 Chapter 15

The numbers of [blank_start]differentiated cells[blank_end] of each type is determined by the [blank_start]requirements[blank_end] of the organism, and is affected by both the [blank_start]rate[blank_end] of cell production and the rate of cell [blank_start]death[blank_end].

Cell division occurs during [blank_start]development[blank_end], and for many cells [blank_start]replacement[blank_end] (as a result of division of precursor or [blank_start]stem cells[blank_end]) occurs during adult life when necessary.

Growth factors such as [blank_start]erythropoietin[blank_end] can signal division and/or [blank_start]differentiation[blank_end] and therefore affect [blank_start]cell numbers[blank_end].

Cell positioning is signalled by direct [blank_start]cell–cell contacts[blank_end], and via [blank_start]soluble factors[blank_end]. During [blank_start]development[blank_end], differentiating cells find their correct position in the [blank_start]embryo[blank_end] and divide appropriately. In the adult, most cell types are [blank_start]fixed[blank_end] and only divide if [blank_start]extracellular signals[blank_end] are present.

Failure to respond appropriately to signals controlling cell division and cell position can lead to [blank_start]tumour[blank_end] development and progression.

Changes in cell function involve changes in [blank_start]protein activity[blank_end], localization and/or expression. The speed of a physiological response to an [blank_start]extracellular[blank_end] signalling molecule is related to the activated network of [blank_start]signal transduction[blank_end] pathways leading to effects [blank_start]specific[blank_end] for an individual cell type.