The organelle of differentiation in embryos: the cell state splitter

Abstract

The cell state splitter is a membraneless organelle at the apical end of each epithelial cell in a developing embryo. It consists of a microfilament ring and an intermediate filament ring subtending a microtubule mat. The microtubules and microfilament ring are in mechanical opposition as in a tensegrity structure. The cell state splitter is bistable, perturbations causing it to contract or expand radially. The intermediate filament ring provides metastability against small perturbations. Once this snap-through organelle is triggered, it initiates signal transduction to the nucleus, which changes gene expression in one of two readied manners, causing its cell to undergo a step of determination and subsequent differentiation. The cell state splitter also triggers the cell state splitters of adjacent cells to respond, resulting in a differentiation wave. Embryogenesis may be represented then as a bifurcating differentiation tree, each edge representing one cell type. In combination with the differentiation waves they propagate, cell state splitters explain the spatiotemporal course of differentiation in the developing embryo. This review is excerpted from and elaborates on "Embryogenesis Explained" (World Scientific Publishing, Singapore, 2016).

The cell state splitter:Embryogenesis Explained: A review by David Miller

“…If the UK law is ever changed to permit development in vitro beyond formation of the primitive streak, the sophisticated embryo recording equipment now installed in many of our IVF labs could be gainfully employed to confirm the sweep of developmental furrows in the embryos of our own species. An excerpted and distilled summary of the concepts and ideas so broadly described in the book is available to anyone wishing to learn a bit more about the Gordons' work and the cell state splitter model [Gordon and Gordon 2016b].…”

Quantifying Mosaic Development: Towards an Evo-Devo Postmodern Synthesis of the Evolution of Development via Differentiation Trees of Embryos

“…Based on the presence of volume differences between sibling cells in a binary division, we can reorganize the lineage tree into a differentiation tree. A differentiation tree for a mosaic organism is organized on the basis of smaller daughter cells being represented by leftward branches and larger daughter cells represented by rightward branches [10,13,14]. This rearrangement of the lineage tree by an alternative criterion than cell position provides comparative information between sub-lineages, and perhaps clues as to the role of cell volume asymmetry in causing and propagating regional differentiation processes in the embryo.…”

“…They have been observed as early as the Stage 8 blastula [49] in the chordate axolotl (Ambystoma mexicanum) regulating embryo [35,50,51]. There are two general types of differentiation waves: contraction and expansion, involving contraction or expansion of the cell state splitter, a cytoskeletal apparatus in epithelia that both triggers a step of differentiation and propagates as a wave from cell to neighboring cell [14] (there are six variants on these waves [10]). The bridge to mosaic organisms is made in theory by assuming that the trajectory of a differentiation wave involves many cells in regulating embryos, but only one cell in mosaic organisms [13].…”

“…Here we only apply CASTing to differentiation codes. For the differentiation code, the binary digit b i = 0 for the small cell in an asymmetric division or for a contraction wave in a regulating embryo [10,13,14], or b i = 1 for the larger cell or an expansion wave. When it is easier to think in terms of letters rather than binary numbers, one may substitute C = 0 and E = 1.…”

Quantifying Mosaic Development: Towards an Evo-Devo Postmodern Synthesis of the Evolution of Development via Differentiation Trees of Embryos

“…As an alternative to the lineage tree, we can use additional information from the developing embryo to construct a differentiation tree [ 10 , 13 , 14 ]. For strictly mosaic organisms, the differentiation tree is just a rearrangement of the branches of the lineage tree, changing their order left to right across the page.…”