A cell is not a bag of proteins, billions, and organelles, millions of ribosomes, thousands of mitochondria…: a careful order is imposed. Multicellular organisms are composed by many cells, trillions in Vertebrates. Metazoa develop large organs and organisms, able to run, fly, swim, well and fast, making use of huge numbers of small replicating units, the cells; from one cell, the fertilized oocyte, organisms develop made up of billions of cells: millions of different species, each one showing its own characteristic shape, which is achieved through a precise stereotypical positioning of cells. Indeed Vertebrates are able to build extremely sophisticated structures: our external ears (pinna), middle (little bones) and internal ear (cochlea and semicircular canals), besides bilaterally symmetric, are good examples. To assemble so complex organs, cells must know their position and operational directions: cells must know the real location of "up", "down", "front", "rear" and these points of reference must be fine-tuned and shared with the neighboring cells. The biological mechanisms involved in metazoan tissue and organ development are in fact highly directional: division plane orientations, cell movements, stretching and bending of cylindrical structures, adhesions between cells, gradients of morphogens: what organizes "cell geometry" in Metazoa? The spatial resolution of diffusible molecules gradients in very small environments like cells is limited by the ability of receptors to discriminate small differences in ligand concentration: sensitivity to concentration changes in one part of the gradient comes at the cost of saturation in the rest of the gradient and global positional information might provide at most a low-resolution map of position within the cell: a more refined map must then exist What is the structural and molecular basis of a high-resolution map? Indeed microtubules (MTs) appear to be universally utilized to shape cells and organs: the synthesis of microfibrils of cellulose and
In zebrafish inner ear, hair cell orientation in anterior and posterior maculae of the embryonic otic vesicle is different (about 30-40 degrees): this is rather unusual in planar polarity mechanism of action, instead suggests that kinocilia may be rotationally polarized. In mice node, the innermost monociliated cells generate a left-ward fluid flow sensed by the immotile primary cilia of Left peri-nodal cells: the Nodal signaling pathway is then expressed asymmetrically, in the Left lateral plate mesoderm, breaking symmetry in visceral organs (situs solitus); however, Right peri-nodal cells also, if artificially excited by a right-ward flow, break symmetry and activate the Nodal cascade, though inverting visceral organ asymmetry (situs inversus); surprisingly, peri-nodal cells prove to be adept at distinguishing flow directionality. Recently, in the Kupffer vesicle (the zebrafish laterality organ), chiral primary cilia orientation has been described: primary cilia, in the left and right side, are symmetrically oriented, showing a mirror average divergence of about 15-20 degrees from the midline. This finding, taken together with the mirror behavior of mouse perinodal cells and zebrafish hair cells, champions the idea of primary cilia enantiomerism.
Every adult male of the little roundworm Caenorhabditis elegans is always and invariably comprised of exactly 1031 somatic cells, not one more, not one less; and so it is for the adult hermaphrodite (959 somatic cells); its intestine founder cell (the ‘E’ blastomere), if isolated and cultured, undergoes the same number of divisions as in the whole embryo (Robertson et al., 2014); the zygote of Drosophila melanogaster executes 13 cycles of asynchronous cell divisions without cellularization: how are these numbers counted? Artificial Intelligence (First and Second Order Logic, Knowledge graph Engineering) infers that, to perform precise stereotypical numbers of asynchronous cell divisions, a nucleic (genomic) counter is indispensable. Made up of tandemly repeated similar monomers, satellite DNA (satDNA) corresponds to iterable objects used in programming. The purpose of this article is to show how satDNA sequences can be iterated over to count a deterministic number of cell divisions: computational models (attached for free download) are introduced that handle DNA repeated sequences as iterable counters and simulate their use in cells through an epigenetic marker (cytosine methylation) as an iterator. SatDNA, because of its propensity to remodel its structure, can also operate as a strong accelerator in the evolution of complex organs and provides a basis to control interspecific variability of shapes.
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