polarity in embryogenesis and yin and yang theory

A number of morphogens have been proved to participate in the development of the neural tube. The formation of the vertebrate neural tube, the embryonic precursor to the central nervous system, is quite an elaborate process highly requested for the correct specification and positional information. During the past 50 years, quantities of morphogens are identified to not only specify the cell fate but also provide the positional information to the organ or the whole embryo to form a complicated 3D organization. Cells respond to these signals for further fate and positional determination by inducing different target genes at different exposing duration and amounts. These morphogens are thought to be emanated from a localized source and can diffuse to establish a concentration gradient and create a biochemical boundary for controlling cell proliferation, differentiation and migration. In this model, an idealized morphogen signal is raised to interpret the cells’ position and further spatial patterns that drive larger-scale morphogenesis from a molecular perspective. However, for the first time that positional information (PI) emerged, it should be traced back to the “French Flag” model presented by Lewis Wolpert in 1969 (Wolpert 1969). In this case, establishing a more complete organoid culture system with positional information might be one main direction in organoid research.Įarly in 1885, Hans Driesch had already proposed a coordinate system that specified the position of each developmental cell, based on his experiments on sea urchin embryo: separated two cells of the embryo can develop into a complete sea urchin in a correct manner, although with a smaller size (Wolpert 2016). each cell type and extracellular matrices are located in their corresponding sites along body axes) has become an intractable challenge for the development of the whole organoid technique.

However, although the current organoid can recapitulate most of the molecular and cellular features, lacking a reproducible topographic organization (i.e.

This fancy technique provides an important supplement to investigate those events that only occur in humans. In addition, it transformed the traditional 2D tissue culture techniques to more complicated 3D cell culture systems which can better mimic in vivo environments. These systems well reproduce the developmental processes and organization of the developing human brain. For brain organoids, the intact systems of hPSC-derived neural organoids have been built since the early 2010s (Lancaster et al. Existing types including cerebral, gut, hepatic, pancreatic, epithelial organoid and so on are widely used in researching both developmental biology and diseases (Lancaster et al. Currently, organoids are often derived from embryonic stem cells, organ progenitor cells or induced pluripotent stem cells (iPSCs). It is a self-organized structure with multiple organ-specific cell types that is able to recapitulate the specific function of the corresponding organs (Lancaster and Knoblich 2014). The organoid is a novel technique with rapid development in this century. Therefore, a better model system that can recapitulate the features and functions of the human brain is urgently required for promoting research in this field. These obstacles make it hard to dissect and comprehend the developmental process of the human brain as well as the relevant neurodevelopmental and neurodegenerative diseases detailly. Besides, the division patterns of neural stem cells varied and become more complicated in humans, rodents are born with a relatively intact brain with most of its neurons in it, whereas human children do not develop large numbers of neurons in the cerebral cortex until after birth (Homem et al. For example, the surface of the mouse brain is smoother, while the neocortex is largely expanded and highly folded in humans due to the contribution of ventricular radial glial cells (vRGCs) and outer radial glial cells (oRGCs) (Lui et al.

However, there are a lot of nonnegligible differences between the brain of animal models and humans. The brain specimens taken from the corpses cannot be sufficient to examine the dynamic development and function of the brain, therefore, animal models such as mice are widely used. Considering human ethics, it is difficult to directly work on the live human brains. Understanding human brain development and relevant neurological diseases have been one of the most important challenges in current biology.

Polarity in embryogenesis and yin and yang theory