A preprint combines four cellular age “clocks” into a map of early development
A preprint combines four cellular age “clocks” into a map of early development
On August 24, physician Jaba Tkemaladze published a preprint on Research Square. He organized 19 quantitative studies across seven developmental transitions, from primordial germ cells to the blastocyst, the embryo before it attaches to the uterine wall.
Several systems undergo reorganization in the early embryo. Tkemaladze treats these systems as traces of cellular age. They include epigenetic marks, which are chemical modifications to DNA that affect gene activity; mitochondrial DNA in the organelles that supply the cell with energy; telomeres, which protect the ends of chromosomes; and centrioles, around which the cell division machinery assembles. These processes are usually studied separately. The preprint places them on a single timeline for the first time to test whether their trajectories coincide at the same developmental transitions.
Primordial germ cells are the precursors of eggs and sperm. In mice, researchers observed a two-stage reorganization of chromatin, the structure that packages DNA, together with widespread removal of methylation marks. In another study of mice carrying different mitochondrial DNA variants, researchers linked the proportion of these variants in offspring to how the molecules were distributed between cells and then replicated in primordial germ cells. On the map, telomeres follow their own trajectory in the germline and pluripotent lineage.
Centrioles raise a separate question in this framework. In an experiment with pig embryos, centrin, a protein associated with centrioles, disappeared from the one-cell embryo after fertilization and remained undetectable until the late blastocyst stage. The authors of the original study attributed its later appearance to new synthesis in pluripotent cells, which can still develop into different tissues. Tkemaladze places this transition alongside the restoration of DNA methylation and the increase in mitochondrial DNA copy number.
This part of the map differs among species. In mice, centrioles form anew, whereas in humans and cattle, the sperm centriole participates in the first cell divisions. The experiment proposed in the preprint should therefore track all four trajectories in one species and across the same developmental transitions. This series of measurements would test whether they form a coordinated sequence.
For cellular rejuvenation research, this framework provides a basis for comparison. During partial reprogramming, a cell is briefly shifted toward an earlier state. A change in a single gene does not show whether the age markers shifted together or whether the cell retained its identity. Comparing all four trajectories with the embryonic sequence would show which features change together.