Developmental clock

Temple S, Raff MC 1986 Clonal analysis of oligodendrocyte development in culture evidence for a developmental clock that counts cell divisions. Cell 44 773-779 Walters SN, Morell P 1981 Effects of altered thyroid states on myelinogenesis. J Neurochem 36 1792-1801... 

If methylation and possibly other covalent modifications of DNA occur, how can one explain the toti-potency observed for some nuclei of differentiafed cells During development of the ovum and the sperm there appears to be a "resetting" of the developmental clocks that led to differentiation. At this time all of the methyl groups on the CpG pairs of imprinted genes are removed (Section The mechanism is... 

The above observations indicate that in the evolution of the neopterygians there has been a posterior shift of the point of caudal ray initiation relative to the hypural/ parhypural boundary, and that that shift was probably coincident with the advent of a new symmetry in the developing caudal fin exoskeleton. Some differences between the ancestral and derived tail morphologies can probably be attributed to heterochronic changes, such as the earlier and more complete flexion of the teleost notochord. But it is clear from the preceding that crucial elements of morphological evolution require more than adjustments of the developmental clock. 

At every clock tick, the developmental program of each cell (in the graph of interconnected cells) computes its next state and whether it will produce a new cell or not. Division produces a new cell with exactly the same (unchangeable) developmental program as the mother cell, but with a new location = location of the mother cell + 1. Because CGP and by extension DCGP only allow feed-forward graphs, the inputs of all cells will come from external inputs and/or the outputs of other cells, which are directly/indi-rectly connected to the external inputs. Hence, if the external inputs stay stable, then so will the outputs of all the cells in the graph. 

What is special about the testis that requires the absence of the circadian clock so pervasive in other tissues The testis has a number of characteristics, which make it quite different from other tissues. The testis contains spermatogenic cells that perform a constant and complex cell differentiation program where reductive cell divisions occur. It may be that the complex pattern of gene expression engendered by the circadian clock leads to unfavourable interactions with the developmental process of paramount importance to the testis. Alternatively, the normal oscillation of clock gene expression may be distorted by other transcriptional regulators or co-activators which are only present in seminiferous tubules (Sassone-Corsi 2002). 

A number of different lithium secondary systems serve for very low energy demands of electronic modules as memories and clocks. The batteries for these fields of application are manufactured as button cells. These cells are described first. Then for medium and high energy requirements lithium-ion batteries are explained. Today they are widely applied for portable electronic devices such as cellular phones and notebooks, which need much more energy than the aforementioned components for as many hours as possible (practically today up to 4 hours in notebooks). On the developmental stage it has been attempted to apply the lithium-ion technology also for much bigger accumulators, e.g. for so-called hybrid vehicles with a combined combustion and electric propulsion system. 

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