Nature back-to-back . . . Milestone! In vitro reveals the characteristics of the "section clock" during the occurrence of human body sections.

The spatial patterns of basic morphogenesis and cell differentiation of vertebrate early embryos can be divided into two categories: one is non repetitive pattern, including heart, kidney, limbs, tail, etc.; the other is repetitive pattern, including body segment, feather, etc., among which somite is the most representative transitional structure of vertebrate.somite is a kind of mesoderm in the early embryonic development of all vertebrates, With the continuous development of the embryo, these somites gradually differentiate into bone, skin and muscle areas, and then form bone, endothelium, dermis, muscle, tendon and cartilage tissues [2].somitogenesis refers to the biological process of somitogenesis (Fig. 1). In the late projejunum embryo, the embryo begins to elongate, the back begins to form nerve plate, further forms nerve groove and nerve fold, and finally forms neural tube.the neural tube and the notochord below constitute the dorsal axis of the embryo body, and the embryo body slowly forms a cylindrical shape.on both sides of the neural tube, there are two pieces of tissue called quasi somite mesoderm (PSM). With the development of embryo, PSM and neural tube will grow and extend along the tail of embryo.at the same time, the cells in the two PSM begin to aggregate at the same time in the direction of the head of the embryo. When the two cell clusters are separated from the PSM tissue, a pair of somites are formed.after the formation of a pair of somites, the cells in the adjacent PSM begin to aggregate again, thus starting the formation of the next pair of somites.in such a cycle, the somites are connected in such a way as to form a pair of pairs from the beginning to the end [2].this process is strictly regulated in time and space, so as to ensure that somites are generated in the right position at a fixed time interval.if the process of somite occurrence is disturbed, the formed somites will have defects, which will lead to abnormal physical development and various congenital diseases.Fig.1 schematic diagram of somite occurrence (picture from Wikipedia entry: somite) the rhythmicity of somite is regulated by a periodic oscillation molecular oscillator in PSM, i.e. segmentation clock.during the formation of each pair of somites, the segmented clock sweeps through the PMS from the tail at a fixed time, and then regulates the generation of somites at the head position.therefore, the segmented clock plays an important role in regulating the periodic formation of somites.in the past 40 years, a large number of studies have shown that the segmented clock is mainly composed of the genes of notch, Wnt and fibroblast growth factor (FGF) pathway, and these genes present a periodic oscillatory expression pattern, and their oscillation cycle is consistent with the cycle of somite occurrence [3].although the oscillator has been well described in many model organisms, whether there is a similar molecular clock in the development of human embryos has always been unknown.on January 8, 2020, Professor Olivier pourqui é of Harvard Medical School published the title "in vitro characterization of the human segmentation clock" on nature In this paper, the oscillatory effect of segmented clock was reproduced by using human and mouse PSM cells cultured in vitro. Single cell sequencing revealed the development trajectory and molecular oscillation characteristics of mouse and human PSM cells in vitro. It was pointed out that the oscillation cycle of human PSM cells was twice that of mouse cells, and was highly conserved. It was also regulated by FGF, Wnt, notch and Yap signals Developmental biology provides a milestone in research.first of all, the researchers used activin A and FGF to induce ectoderm formation of mouse embryonic stem cells, and then cultured in medium containing Wnt agonist chiron99021 (chir) and BMP inhibitor ldn193189 (LDN) (CL medium), so as to reproduce the early stage of mouse paraxial mesoderm development in vitro (Fig. 2, left). by using single cell RNA sequencing (scrna SEQ), the transcriptome of mouse embryonic (ES) cells cultured in vitro and embryonic cells of 9.5 days in vivo were analyzed and compared. The results showed that there were extensive transcriptional similarities between the paraxial mesoderm cells extracted from mouse embryonic stem cells and their corresponding cells in vivo. furthermore, a reporter system was constructed in mouse ES cells. Achilles, an unstable yellow fluorescent protein variant, was knocked into the 3 'end of Hes7 gene to realize two-dimensional visualization of segmented clock oscillation. Then, the segmented clock of PSM cells differentiated from mouse ES cells in vitro was successfully simulated with the oscillation frequency of 2.5 ± 0.4 hours. then, similar in vitro culture strategies were used to simulate the human segmented clock. the researchers first differentiated human induced pluripotent stem (IPS) cells in CL medium to obtain neural mesodermal progenitor cells or pre primordial stripes (Fig. 2, right). by comparing the status of 14750 differentiated human iPS cells with those of mouse ES cells in vivo and in vitro, the results showed that human iPS cells differentiated into PSM fate in CL medium in vitro, which reproduced the development sequence similar to mouse embryos, resulting in the generation of mesoderm cells in the paraaxial of trunk. subsequently, the Hes7 Achilles iPS cell reporting system cell line successfully confirmed the existence of a human segmented clock with a cycle of about 5 hours (Fig. 3). one of the characteristics of the segmented clock oscillations of differentiated mouse embryonic cells (left) and human pluripotent stem cells (right) in vivo is their high local synchronization. The researchers found that this oscillation synchronization was verified in human PSM cells cultured in vitro, but not in mouse PSM cells, and the synchronization gradually weakened with time. further experiments showed that the molecular oscillation of human PSM cells depended on the Notch signaling pathway. in addition, similar to mice, the human segmented clock can be regarded as an activated system, which is stimulated by Notch signal and controlled by Yap signal. we know that in vivo, the maturation of PSM cells is controlled by FGF and Wnt signals. In vitro, our researchers evaluated the effect of FGF and Wnt signal down-regulation before maturation on segmented clock oscillation, and found that FGF signal can also regulate the characteristics of oscillators, including not only controlling the oscillation stagnation of wavefront, but also regulating the periodic gene oscillation The dynamics of (period, phase and amplitude). Fig. 3 oscillation frequency of segmented clock in human PSM cells. In conclusion, the study in this paper provides strong evidence for the existence of human segmented clock, and proves that the oscillator is highly conservative from fish to human. at the same time, it is confirmed that the cycle of human segmented clock is about 2 times slower than that of mice, which is about 5 hours, which is consistent with the known difference in the development time of mouse and human embryos. in addition, the culture conditions provided in this paper enable cells to produce unlimited number of human like PSM cells in a specific culture medium, which provides an ideal research system for studying the dynamic characteristics of segmented clock oscillation and its role in pathological segmental defects (such as congenital scoliosis) and human development Advances in biology have provided a huge boost. it is worth mentioning that in the same journal, the team of Professor ryoichiro Kageyama of Tokyo University published the article coupling delay controls synchronized surgery in the segmentation The same Hes7 Achilles report system was used to observe the PSM of mice at the single cell level, and the lfng mouse model was used to prove that the coupling delay regulates the synchronous oscillation of segmented clock. original link: plate maker: Ke reference 1. Jonathan cook. Control of somite number during morphogenesis of a vertebrate, Xenopus laevis. Nature. 1975, 254 (5497), 196-9.2. M J McGrew, O pourqui é. Somitogenesis: segmentation a vertebrate. Curr opin gene dev. 1998, 8 (4), 487-93.3. Aulehla a, Pourquie O. Oscillating signaling pathways during embryonic development[J]. Current Opinion in Cell Biology. 2008, 20: 632-637.