Is gastrulation the most important time in your life?

Gastrulation is considered a fundamental process of metazoan embryogenesis, as the layers that give place to all body parts are laid down by extensive cell movements and cell differentiation. It is commonly thought that gastrulation establishes the entire body axis. The discovery of neuromesodermal progenitors is challenging this view. These cells keep their pluripotency after gastrulation and they are able to differentiate into ectoderm (neural tube) and mesoderm derivatives (paraxial mesoderm, notochord) directly, without passing through a germ layer intermediate. They contribute to the elongation of the body axis and could play a key role in the evolution of the chordate body plan.

The term gastrulation was coined by Haeckel to refer to the transformation of an early embryo from a homogeneous, single-layered, hollow sphere (i.e., the blastula) to a heterogeneous, two-layered structure (i.e., the gastrula) (Fig. 1). He proposed that the gastrula was originated by the invagination of the blastula's outer layer-as when one squeezes a deinflated ball with one finger -, and the differentiation of the resulting two layers (outer layer: ectoderm; inner layer: endoderm). This invagination gave place to a cavity lined by the endoderm, the primary intestine (Fig. 1). The term "gastrula" derives from the Greek root "γαστήρ" = gaster, which means "gut", plus the Latin diminutive suffix -"ula". Translated as "little gut", Haeckel coined it to name the developmental stage at which this structure is formed (Haeckel 1872 support of Darwin's theory of common descent. The gastrula would correspond to their common ancestor, a hypothetical animal he called Gastraea (Levit et al. 2022). Contrary to Haeckel's expectations, however, gastrulation is highly variable among metazoans, being tissue invagination one of several mechanisms capable of generating tissue layers. The author also considered cell ingression as a mechanism of gastrulation (Levit et al. 2022), but the important point in support of his theory was not a specific mechanism, but its conservation across the animal kingdom. Furthermore, mechanisms of gastrulation are not mutually exclusive (i.e., they can conjointly contribute to the formation of a gastrula), and they do not reflect phylogenetic relationships (closely related taxa can display different gastrulation modes, and vice versa). This has led to re-thinking how embryonic development could have evolved (Steventon et al. 2021;Uesaka et al. 2022) According to Haeckel's Gastraea theory, all metazoans would pass through a two-layered gastrula stage, which means that the third layer of triploblastic metazoans (i.e., the mesoderm), would develop the last. However, in many metazoans the mesoderm derives from the endomesoderm, or mesendoderm, a cell/tissue capable of differentiating into endoderm and mesoderm, i.e. the two layers differentiate simultaneously (McClay et al. 2021). Finally, the primary intestine, if vestigially present, does not give place to the gut in some animals (e.g., chicken), i.e., their gastrula does not present the structure that gave it its name.
Although the gastrula does not develop as expected by Haeckel, the term has prevailed to define the stage at which the germ layers develop. This includes both their morphogenesis and their differentiation (Stern 1992), and it is independent of: 1) the temporal sequence of these two events, 2) the specific morphogenetic mechanism, 3) the number of germ layers, and 4) its relationship with gut morphogenesis. According to it, gastrulation is a common characteristic of all metazoans, except sponges. The latter develop two tissue layers by the morphogenetic mechanisms characteristic of gastrulation, but they lack the fate determination and stability that define them (Nakanishi et al. 2014). Based on this observation, some authors have stressed that the common feature of all metazoans is not gastrulation, but its morphogenetic component only (Ereskovsky and Dondua 2006).
It is commonly thought that the formation of the germ layers is complete after gastrulation, i.e., that the whole body axis is laid down during this process. This led Wolpert to state "it is not birth, marriage, or death, but gastrulation which is truly the most important time in your life" (Hopwood 2022). In Xenopus, the whole body axis is established during gastrulation, except the tail. This has led to a long-standing controversy as to whether the tail forms by the growth of established structures or anew from undifferentiated tissue, independently from gastrulation (Handrigan 2003). This incompleteness is more remarkable in amniotes, as only the head and the most anterior structures have been specified at the gastrula stage (Stern et al. 2006). This is because the process of gastrulation, i.e., the internalization of cells to form different germ layers, continues after the gastrula stage in this group (Stern et al. 2006). However, it is not the only process that contributes to the elongation of their body axis. In addition, an organizer appears at the anterior region of the primitive streak, called the node (Fig. 2). After the gastrula stage, the node -conjointly with the primitive streak -regresses in a head-to-tail progression laying down part of the body axis, specifically: the notochord, the floor plate and the medial somites (Solovieva et al. 2022a;Solovieva et al. 2022b). When most of the trunk structures have been established, the node forms part of the tailbud, which continues elongating the body (Guillot et al. 2021;Solovieva et al. 2022b).
Qeios, CC-BY 4.0 · Article, June 7, 2023 Qeios ID: AMS5J1 · https://doi.org/10.32388/AMS5J1 4/7 Recent studies have shown the existence of pluripotent cells capable of differentiating into neural tube and paraxial mesoderm located at the node in chickens, and the node-streak border in mouse (Guillot et al. 2021;Solovieva et al. 2022a). A recent study has shown they can also give place to the notochord in zebrafish (Lange et al. 2023). Therefore, tissues that generally derive from different germ layers (ectoderm and mesoderm), can also be derived from these pluripotent cells, called neuromesodermal progenitors (NMPs), without passing through a germ layer intermediate, i.e., independently from gastrulation. Located at the interface between the ectoderm and the mesoderm (Wood et al. 2019), they may help to zip the body axis, assuring its functionality. The zipping of the ectoderm and the mesoderm could play a key role in the evolution the chordate body plan (Sato et al. 2023 (Guillot et al. 2021;Solovieva et al. 2022a). This answer is based on the most common definition of gastrulation (see previous paragraphs), but if redefined as "a process whereby Qeios, CC-BY 4.0 · Article, June 7, 2023 Qeios ID: AMS5J1 · https://doi.org/10.32388/AMS5J1 5/7 the embryo acquires a system of coordinates to organize and position the primordia for the different tissues and organs" as recently suggested (Steventon et al. 2021), gastrulation would be the most important time to your entire body.
Like in the case of sponges, a definition of gastrulation that leaves out some animal taxa may be seen as "undesired" by some researchers, however, stretching the term to include all them, would hide key embryological differences that could help to better understand the evolution of their body plans.