Annual Research & Review in Biology

The theoretical formulation of cell differentiation forming the stem cells is used to explain the growth, metamorphosis, germ cell production and death in the higher animal, which develops many organs and tissues through multiple steps of cell differentiation.   Although the stem cells are mainly focused on inducing the desired differentiated cells in medical engineering, the present study indicates that the proliferation of stem cells plays a central role in the above processes after the development.   In a born larva of most insects, the stem cells in each lineage of somatic cells proliferate to increase the number of differentiated cells in the corresponding lineage under the material and energy acquired from the outside, leading to the enlargement of the body.   When the larva reaches a definite body size, it carries out the metamorphosis that some kinds of pre-existing differentiated cells are replaced by newly differentiated cells through the apoptosis of the former and the increase in the latter due to the proliferation of primordial stem cells, and then immediately produces germ cells by consuming the acquired energy. In the land vertebrates, on the contrary, the body size continues to enlarge after the metamorphosis and the production of germ cells occurs when the adult body reaches a definite size. Although both metamorphosis and germ cell production are known to be triggered by the respective hormones, it is proposed that their secretion is the reflection of biological activity raised by the suppression of body size to be definite, suggesting　the cellular dynamics that the proliferation of stem cells in each lineage is stimulated by other lineage(s) of differentiated cells and is suppresses by the differentiated cells in its own lineage.The longer lifetime and many times of breeding in most vertebrates are also explained by the duration of the cellular dynamics.