Was den Menschen zum Menschen macht

Gastrulation

5 Min. Lesezeit

🇩🇪 Deutsche Version: Gastrulation

Gastrulation is the phase of human embryonic development in which the three germ layers — ectoderm, mesoderm, and endoderm — form out of the bilaminar germ disc. The morphological marker of its beginning is the appearance of the primitive streak (CS 6, about day 17 post-fertilization); the phase extends to the notochordal process (CS 7, about day 19).

Gastrulation is not a single process, but a choreographed wave of cell migrations in which the basic axes of the body (cranio-caudal, dorso-ventral, left-right) are laid down. With the beginning of gastrulation, monozygotic twinning is no longer possible — from here on the embryo is, in a strict morphological sense, indivisible.

Tyser, Srinivas, and colleagues (Nature 600:285–289, 2021) were the first to map human gastrulation (CS 7) at single-cell resolution, thereby molecularly distinguishing epiblast, primitive-streak cells, hemogenic endothelium, and primordial germ cells from one another. The study is part of the HuDeCA Cell Atlas; it shows that gastrulation is not a leap, but a trajectory of overlapping cell-fate decisions.

Embryological Course

Formation of the three germ layers (ectoderm, mesoderm, endoderm) via the primitive streak from about day 14-17 (CS 6-7). End of the possibility of monozygotic twinning. The classical fourteen-day-rule limit.

The primitive streak is the axis of symmetry whose appearance marks the beginning:

Axis of symmetry whose appearance marks the beginning of gastrulation. CS 6 (~day 17). Smith & Brogaard 2003 see here the moment of individuation; Damschen/Schönecker 2006 dissent.

The corresponding Carnegie stages:

  • CS 6 (day 17) — appearance of the primitive streak; beginning of gastrulation.
  • CS 7 (day 19) — formation of the notochordal process; gastrulation advances.

Bioethical Significance

Gastrulation is the developmental-biological threshold to which the most important research-ethical conventions have anchored themselves since the 1980s. Three central points:

  1. End of monozygotic twinning. The possibility that an embryo might still split into two genetically identical embryos closes with gastrulation. See:

    Splitting of an embryo into two genetically identical embryos, possible until about day 14 (the close of gastrulation). Condic, Untangling Twinning (2020), defends the budding model, which is compatible with the beginning of the person at fertilization.

  2. Classical limit of the fourteen-day rule. The international research convention sets its threshold exactly at this point — not by chance, but because here the individuation arguments before and after gastrulation diverge.

  3. Pivot point of the individuation debate. Smith and Brogaard (Sixteen Days, 2003) date the beginning of human individuality to gastrulation. Damschen, Gómez-Lobo, and Schönecker (2006) dissent: the possibility of splitting does not exclude an individual substance, but only demands a different model of substance. Condic (Untangling Twinning, Notre Dame UP 2020) develops this position in developmental-biological terms.

    The open philosophical debate: does personhood begin with fertilization (CS 1) or only with the close of the possibility of twinning (CS 6, about day 14-17)? The ontology represents the question without deciding it — see ps:OntologischUnklarerStatus for the methodological stance.

Conversely there exists the phenomenon of tetragametic chimerism:

Fusion of two zygotic embryos into one organism with two genomes. The converse case of twinning; relevant for arguments concerning numerical identity.

Personal-Ontological Assessment

In personal-ontological terms, gastrulation is not the beginning of personhood. The substance-ontological position (Spaemann, Seifert, Wojtyła) dates personhood to fertilization (CS 1). Gastrulation is not the threshold at which personhood arises, but a threshold at which the morphological character of the person changes: before gastrulation the person is present as a still-undirected axial anlage, after gastrulation as a bodily structured organism.

Whoever nonetheless sees gastrulation as the beginning of the person rejects the substance-ontological thesis in favor of an organismic-structural or neurobiological threshold theory. The personalist norm holds — understood in substance-ontological terms — already before gastrulation; every practical conclusion drawn from gastrulation as the beginning of the person (e.g., that the use of pre-gastrulation embryos for research purposes is unproblematic) contradicts it.

Sources: Generated by querying the Personhood ontology.

Further sources:

  • O’Rahilly, R. & Müller, F. (2010): Developmental Stages in Human Embryos: Revised and New Measurements. Cells Tissues Organs 192(2): 73–84. DOI: 10.1159/000289817.
  • Sadler, T. W. (2023): Langman’s Medical Embryology. 15th ed. Wolters Kluwer (ch. 5: Third Week of Development – Gastrulation).
  • Carlson, B. M. (2018): Human Embryology and Developmental Biology. 6th ed. Elsevier.
  • Smith, Barry & Brogaard, Berit (2003): Sixteen Days. Journal of Medicine and Philosophy 28(1): 45–78.
  • Damschen, Gregor; Gómez-Lobo, Alfonso & Schönecker, Dieter (2006): Sixteen Days? A Reply to B. Smith and B. Brogaard on the Beginning of Human Individuals. Journal of Medicine and Philosophy 31(2): 165–175.
  • Condic, Maureen L. (2020): Untangling Twinning. What Science Tells Us About the Nature of Human Embryos. Notre Dame Studies in Medical Ethics and Bioethics. Notre Dame, IN: University of Notre Dame Press.
  • Spaemann, Robert: Persons. The Difference between ‘Someone’ and ‘Something’, transl. Oliver O’Donovan. Oxford: Oxford University Press, 2006.
  • Tyser, R. C. V.; Mahammadov, E.; Nakanoh, S.; Vallier, L.; Scialdone, A. & Srinivas, S. (2021): Single-cell transcriptomic characterization of a gastrulating human embryo. Nature 600(7888): 285–289. DOI: 10.1038/s41586-021-04158-y. (The first single-cell-resolved map of human gastrulation, CS 7.)

See also

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