The segmentation clock, cellular oscillator, determination front and period differences (continued). Introduction to timer Hox 

This fourth lesson begins with a review of the essential elements discussed in the previous lecture : from the origin of mesoderm to the progressive condensation of somites along the anterior-posterior axis. The mechanisms of the cellular oscillator underlying the segmentation clock are briefly reviewed, along with the signalling pathways involved. Next, a model is presented that explains the determination front, with a source of Fgf8 in the most posterior part. RNAs are produced in these posterior cells and then progressively degraded, building up a gradient of Fgf8 whose precise concentration could establish the determination front. Then the question of the different clock periods in different vertebrate animals is discussed, with the general conclusion that this period is a function of a metabolic activity specific to each species and in principle linked to the mass of the individuals (in a non-linear way, according to Kleiber's law). Finally, the Hox gene system is presented as an introduction to the discussion of the " timer Hox ", which will be the subject of the last lecture of this year 2022-2023.