Chronobiology, growth hormone and healthy and malignant growth

Chronobiology, the computer-aided science (logos) of life (bios) in time (chronos), provides novel concepts, tools and facts for those concerned with mitosis, growth and growth hormone (GH). GH concentrations in human plasma demonstrate a statistically significant circadian rhythm on a 6h as well as on a 24h rest-activity cycle. On a 24h routine of light (L) and darkness (D), alternating at 12h intervals, and in continuous D, circadian mitotic rhythms in mice persist as a feature of growth or regeneration. A circadian cell cycle commences with an increase in phospholipid labeling, followed by an increase in cytoplasmic RNA formation, preceding, in regular sequences, an increase in nuclear DNA formation and the next mitotic peak in those cells that are dividing in a growing or regenerating (reversibly "post-mitotic') rodent liver. About 5-day (presumably estral) and about 7-day (circaseptan) components as well as circadians are resolved as a spectrum of mitotic rhythms in rodent cornea. The effects of hormones such as GH or a synthetic ACTH analogue, ACTH 1-17, depend upon the circadian cell cycle stages when the agent is administered. No effect or statistically significant effect can be the result only of 1) the timing of a fixed dose of GH or 2) of the timing of the samples taken to investigate any effect. For both the timing of administration and the assessment of effects, a multifrequency spectrum of rhythms, if taken into account, can provide (in lieu of a considerable and often formidable source of variation) a new critical dimension of growth and development.