Reconstituting human somitogenesis in vitro.

The segmented body plan of vertebrates is established during somitogenesis, a well-studied process in model organisms; however, the details of this process in humans remain largely unknown owing to ethical and technical limitations. Despite recent advances with pluripotent stem cell-based approaches, models that robustly recapitulate human somitogenesis in both space and time remain scarce. Here we introduce a pluripotent stem cell-derived mesoderm-based 3D model of human segmentation and somitogenesis-which we termed 'axioloid'-that captures accurately the oscillatory dynamics of the segmentation clock and the morphological and molecular characteristics of sequential somite formation in vitro.

Imaging and manipulating the segmentation clock.

During embryogenesis, the processes that control how cells differentiate and interact to form particular tissues and organs with precise timing and shape are of fundamental importance. One prominent example of such processes is vertebrate somitogenesis, which is governed by a molecular oscillator called the segmentation clock. The segmentation clock system is initiated in the presomitic mesoderm in which a set of genes and signaling pathways exhibit coordinated spatiotemporal dynamics to establish regularly spaced boundaries along the body axis; these boundaries provide a blueprint for the development of segment-like structures such as spines and skeletal muscles.

Species-specific segmentation clock periods are due to differential biochemical reaction speeds.

Although mechanisms of embryonic development are similar between mice and humans, the time scale is generally slower in humans. To investigate these interspecies differences in development, we recapitulate murine and human segmentation clocks that display 2- to 3-hour and 5- to 6-hour oscillation periods, respectively. Our interspecies genome-swapping analyses indicate that the period difference is not due to sequence differences in the locus, the core gene of the segmentation clock.

Publisher Correction: Methylation deficiency disrupts biological rhythms from bacteria to humans.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Methylation deficiency disrupts biological rhythms from bacteria to humans.

The methyl cycle is a universal metabolic pathway providing methyl groups for the methylation of nuclei acids and proteins, regulating all aspects of cellular physiology. We have previously shown that methyl cycle inhibition in mammals strongly affects circadian rhythms. Since the methyl cycle and circadian clocks have evolved early during evolution and operate in organisms across the tree of life, we sought to determine whether the link between the two is also conserved.

Coupling delay controls synchronized oscillation in the segmentation clock.

Individual cellular activities fluctuate but are constantly coordinated at the population level via cell-cell coupling. A notable example is the somite segmentation clock, in which the expression of clock genes (such as Hes7) oscillates in synchrony between the cells that comprise the presomitic mesoderm (PSM). This synchronization depends on the Notch signalling pathway; inhibiting this pathway desynchronizes oscillations, leading to somite fusion.

Dynamic Delta-like1 expression in presomitic mesoderm cells during somite segmentation.

During somite segmentation, the expression of clock genes such as Hes7 oscillates synchronously in the presomitic mesoderm (PSM). This synchronous oscillation slows down in the anterior PSM, leading to wave-like propagating patterns from the posterior to anterior PSM. Such dynamic expression depends on Notch signaling and is critical for somite formation.

ES cell-derived presomitic mesoderm-like tissues for analysis of synchronized oscillations in the segmentation clock.

Somites are periodically formed by segmentation of the anterior parts of the presomitic mesoderm (PSM). In the mouse embryo, this periodicity is controlled by the segmentation clock gene , which exhibits wave-like oscillatory expression in the PSM. Despite intensive studies, the exact mechanism of such synchronous oscillatory dynamics of expression still remains to be analyzed.