Brains Emerging: On Modularity and Self-organisation of Neural Development In Vivo and In Vitro

Abstract

Molecular developmental biology has expanded our conceptions of gene actions, underpinning that embryonic development is not only governed by a set of specific genes, but as much by space–time conditions of its developing modules (determinate vs. regulative development; or, nature vs. nurture discussion). Typically, formation of cellular spheres, their transformation into planar epithelia, followed by tube formations and laminations are modular steps leading to the development of nervous tissues. Thereby, actions of organising centres, morphogenetic movements (in- and evaginations), inductive events between epithelia, tissue polarity reversal, widening of epithelia, and all these occurring orderly in space and time, are driving forces of emergent laminar neural tissues, e.g. the vertebrate retina. Analyses of self-organisational formation of retina-like 3D structures from dispersed cells (so-called retinal spheroids, also called retinal organoids) under defined cell culture conditions (in vitro) demonstrate that not only particular genetic networks, but—at least as important—the applied culture conditions (in vitro constraints) define phenotypes of emergent tissues. Such in vitro approaches allow assigning emerging tissue formation to ground-laying genetic networks separately from contributions by conditional constraints.

Glossary and Abbreviations

• Blastocoel—fluid-filled hollow space of blastula;

• Blastula—cell ball (sphere) formed through cleavage divisions;

• Cleavage—rapid cell divisions after fertilisation;

• Coelom—fluid-filled space surrounded by mesodermal epithelium;

• Constraints—limitations of development through environmental (non-genetic) conditions;

• Differential adhesion hypothesis, see sorting-out;

• Ectoderm—outer germ layer;

• Endothelium—epithelium forming blood vessels;

• Endoderm (entoderm)—inner germ layer;

• Epithelium—planar tissue covering internal and external surfaces, e.g., skin, gut, etc.;

• fire-and-wire mechanism—refinement and stabilisation of neuronal connectivities by their repeated usage;

• Gastrulation—proces by which three germ layers are established in animals;

• Growth factors (cytokines):

  • FGF, fibroblast growth factor;

  • PEDF, pigment epithelium-derived factor;

  • GDNF, glial derived neurotrophic factor;

• Lamination, see stratification;

• Mesoderm—middle germ layer in between ecto- and entoderm;

• Morphogenetic movements—classification of cell migratory mechanisms, e.g., during development, such as e- and invagination, ingression, epiboly, etc.;

• Müller glial cell—radial glial cell of retina, spanning its entire width;

• Neural crest—cell population in most vertebrates emigrating dorsally from closing neural tube, which will found peripheral nervous system (and more);

• Neuromeres—early regional subdivisions of frontal neural tube;

• Ontogeny—course/process of development of an individual organism;

• Organising centre—cells or tissue parts, from which particular steps of development are initiated;

• Organoid—from stem cells in vitro regenerated organ-like tissue;

• Phylogeny—course/process of appearance of all phyla (stems) of organisms (phylogenetic tree) over the entire evolutionary period;

• Primitive streak—tissue structure in developing birds and mammals indicating the onset/course of gastrulation;

• Pseudostratified neuroepithelium—monolayered cellular status of neural tube, which due to its width appears to be stratified, but it is not;

• Retinal cell layers:

  • GCL, ganglion cell layer;

  • INL, ONL, inner and outer nuclear layer;

  • IPL, OPL, inner and outer plexiform layer;

• Retinal cell types:

  • AC, amacrine cell—large axon-less cell positioned at inner border of INL, connecting BPs and GCs in IPL;

  • BP, bipolar cell—interneuron in INL, connecting PRs and HCs in OPL, and with ACs and GCs in IPL;

  • HC, horizontal cell—large cell positioned at outer border of INL, connecting PRs with BPs;

  • PR, photoreceptor cell; comes either as rod or several types of cones;

• Rhombomeres—segmental subdivisions of hindbrain;

• Reaggregate—ball (sphere) of adhering cells formed by reaggregation from dispersed cells;

• RPE—retinal pigmented epithelium;

• Sheefs—“synthetic human entities with embryo-like features”: a human organoid made from hiPSCs which presents a primitive streak (see, gastrulation);

• Sorting-out—process by which different reaggregating cells kept under rotation/in motion associate with similar, and separate from different partner cells; see, differential adhesion hypothesis;

• Spheroids, reaggregated from embryonic chicken retinae,

  • rosetted retinal spheroid—reaggregated cell sphere from dispersed embryonic chicken retinal cells, spatially organised by internal cell rosettes;

  • stratospheroid—dto., achieving a (nearly) complete retina-specific lamination (retinal organoid);

• Stem cells—cell with inherent proliferative ability, which in vitro can be amplified and then directed into one or more distinct differentiated cell type(s);

  • ESCs—embryonic stem cell;

  • iPSCs—induced pluripotent stem cell;

  • hiPSCs—human iPSCs;

• Stratification—arrangement of distinct cell types within cell layers, e.g., in brain and retina;

• Tissue Engineering—artificial (in vitro) reconstruction of tissues from stem cells applying engineering technologies;

• Wnt protein—cell-external ligand protein for the Wnt signalling pathway, a major communication pathway between cells during development and disease (Wnt stands for “wingless-related integration site”).