Embryos are composed of a monolayered epithelium and spherical in shape. Processes of gastrulation in typical sea urchins. Concomitantly with these changes, PMC become rounded and slip from the vegetal plate into the blastocoele (for migration and patterning of PMC in the blastocoele, see Ettensohn 1999). Retraction of cilium and destruction of the microtubules that had retained cells in columnar shape also occur during the ingression ( Tilney & Gibbins 1969a). The ingression of PMC is accompanied by retraction of the anchoring microvilli from the hyaline layer, loss of the cell adhesion with neighboring cells, and disruption of the basal lamina that coats the inner surface of PMC ( Fink & McClay 1985 Amemiya 1989). After hatching, descendants of large micromeres, primary mesenchyme cells (PMC), begin to migrate into the blastocoele ( Fig. 1B). Step 1: ingression of primary mesenchyme cells and formation of the vegetal plateĪ fertilized sea urchin egg undergoes 10 rounds of cleavages ( Endo 1966), and develops into a spherical and hollow blastula, which is composed of a monolayered epithelium ( Fig. 1A). This late recruitment of presumptive endodermal cells will be called tertiary invagination (Step 5). In recent years, it has been elucidated that the recruitment of the presumptive endodermal cells lasts up to the mid- or late prism stage ( Martins et al. After such a pause of archenteron elongation, the gut rudiment rapidly elongates until its tip reaches the inner surface of the apical plate (Step 4, secondary invagination). Meanwhile, another population of mesodermal cells, secondary mesenchyme cells (SMC), appears at the archenteron tip (Step 3). During a couple of hours after primary invagination, the gut rudiment scarcely elongates. Then the thickened vegetal plate bends inwardly and gives rise to a short stub-like gut rudiment (Step 2, primary invagination). Preceding the occurrence of invagination, the cells around the vegetal pole become elongated and give rise to a thickened vegetal plate (Step 1). In this review, the processes are divided into five steps for convenience. Processes of sea urchin gastrulation have been conventionally divided into two distinct phases, primary and secondary invagination ( Dan & Okazaki 1956 Gustafson & Kinnander 1956 Okazaki 1956 Kinnander & Gustafson 1960 Okazaki 1975). Here, we mainly focus on the cellular and mechanical basis of gastrulation in the sea urchin embryo. However, it is not our aim to review a large number of recent studies that deal the meso- and endoderm patterning along the animal–vegetal axis. During gastrulation, presumptive meso- and endodermal cells, which have been already specified to each lineage, move into the internal cavity of embryos, hence the preceding specification of blastodermal cells should deeply affect gastrulation. For analyzing the mechanism of gastrulation, sea urchin embryos have long been used as a model system owing to their transparency, simple organization and small number of constituent cells. Gastrulation is the most prominent morphogenetic event in early development, resulting in the formation of three germ layers, ecto-, meso- and endoderm. Lastly, we will discuss how behavior of pigment cells defines the manner of gastrulation, because pigment cells recently turned out to be the bottle cells that trigger the initial inward bending of the vegetal plate. These factors, in spite of their significance, have been neglected in the analysis of sea urchin gastrulation. Attention will be also paid to some other factors, such as the turgor pressure of blastocoele and the force generated by blastocoele wall. In those embryos, bottle cells are scarcely observed, and the archenteron cells are not rearranged during invagination unlike in typical sea urchins. Third, differences in the manner of gastrulation among sea urchin species will be described in some species, the archenteron does not elongate stepwise but continuously. Second, several factors, such as cytoskeletons, cell contact and extracellular matrix, will be discussed in relation to the cellular and mechanical basis of gastrulation. In this review, we first illustrate the current outline of sea urchin gastrulation. However, recent studies have shown that the recruitment of the archenteron cells lasts as late as the late prism stage, and some descendants of veg1 blastomeres are also recruited into the archenteron.
It is widely accepted that the invagination proceeds in two steps (primary and secondary invagination) until the archenteron reaches the apical plate, and that the constituent cells of the resulting archenteron are exclusively derived from the veg2 tier of blastomeres formed at the 60-cell stage. Processes of gastrulation in the sea urchin embryo have been intensively studied to reveal the mechanisms involved in the invagination of a monolayered epithelium.
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