Sea urchins exhibit radial holoblastic cleavage. The first and second cleavages are both meridional and are perpendicular to each other. That is to say, the cleavage furrows pass through the animal and vegetal poles. The third cleavage is equatorial, perpendicular to the first two cleavage planes, and separates the animal and vegetal hemispheres from one another. The fourth cleavage, however, is very different from the first three.
The four cells of the animal tier divide meridionally into eight blastomeres, each with the same volume. These cells are called mesomeres. The vegetal tier, however, undergoes an unequal equatorial cleavage to produce four large cells, the macromeres, and four smaller micromeres at the vegetal pole. As the 16-cell embryo cleaves, the eight mesomeres divide to produce two "animal" tiers, an1 and an2, one staggered above the other. The macromeres divide meridionally, forming a tier of eight cells below an2. The micromeres also divide, albeit somewhat later, producing a small cluster beneath the larger tier. All the cleavage furrows of the sixth division are equatorial, and the seventh division is meridional, producing a
128-cell blastula.
96 3.2 Blastula formationin sea urchin
The blastula stage of sea urchin development begins at the 128-cell stage. Here the cells form a hollow sphere surrounding a central cavity, or blastocoel (Figure 8.11A).
By this time, all the cells are the same size, the micromeres having slowed down their cell division. Every cell is in contact with the proteinaceous fluid of the blastocoel on the inside and with the hyaline layer on the outside. At this time, tight junctions unite the once loosely connected blastomeres into a seamless epithelial sheet that completely encircles the blastocoel As the cells continue to divide, the blastula remains one cell layer thick, thinning out. as it expands. This is accomplished by the adhesion of the blastomeres to the hyaline layer and by an influx of water that expands the blastocoel.
These rapid and invariant cell cleavages last through the ninth or tenth cell division, depending upon the species. After that time, there is a mid-blastula transition, when the synchrony of cell division ends, new genes
become
expressed, and many of the nondividing cells develop cilia on their outer surfaces; The ciliated blastula begins to rotate within the fertilization envelope. Soon afterward, differences are seen in the cells. The cells at the vegetal pole of the blastula begin to thicken, forming a vegetal plate. The cells of the animal half synthesize and secrete a hatching enzyme that digests the fertilization envelope. The embryo is now a free-swimming hatched blastula.
3.2.1 Fate maps and the determination of sea urchin blastomeres
Cell Fate Determination
The fate map of the sea urchin embryo was originally created by observing each of the cell layers and what its descendants became. More recent investigations have refined these maps by following the fates of individual cells injected with fluorescent dyes such as dil. These studies have shown that by the 60-cell stage, most of the embryonic cell fates are specified, but that the cells are not irreversibly committed. In other words, particular blastomeres consistently produce the same cell types in each embryo, but these cells remain pluripotent and can give rise to other cell types if experimentally placed in a different part of the embryo.
A fate map of the 60-cell sea urchin embryo is shown below. The animal half of the embryo consistently gives rise to the ectoderm the larval skin and its neurons. The veg1 layer produces cells that can enter into either the ectodermal or endodermal organs. The veg2 layer gives rise to cells that can populate three different structures the
endoderm, the coelom (body wall), and secondary mesenchyme (pigment cells, immunocytes, and muscle cells). The first tier of micromeres produces the primary mesenchyme cells that form the larval skeleton, while the second tier of micromeres contributes cells to the coelom.
Although the early blastomeres have consistent fates in the larva, most of these fates are achieved by conditional specification. The only cells whose fates are determined autonomously are the
skeletogenic micromeres. If these micromeres are isolated from the embryo and placed in test tubes, they will still form skeletal spicules. Moreover, if these micromeres are transplanted into the animal region of the blastula,
not only will their descendants form skeletal spicules, but the transplanted micromeres will alter the fates of nearby cells by inducing a secondary site for gastrulation.
Cells that would normally have produced ectodermal skin cells will be respecified as endoderm and
will produce a secondary gut. The micromeres appear to produce a signal that tells the cells adjacent to them to become endoderm and induces them to invaginate into the embryo.
Their ability to reorganize the embryonic cells is so pronounced that if the isolated micromeres are recombined with an isolated animal cap (the top two animal tiers), the animal cap cells will generate endoderm, and a more or less normal larva will develop.
In a normal embryo, the veg2 cells become specified by the micromeres, and they, in turn, help specify the veg1 layer. Without the veg2 layer, the veg1 cells are able to produce endoderm, but the endoderm is not specified as foregut, midgut, or hindgut.
Thus, there appears to be a cascade wherein the vegetal pole micromeres induce the cells above them to become the veg2 cells, and the veg2 cells induce the cells above them to assume the veg1 fates.Thus, the micromeres undergo autonomous specification to become skeletogenic mesenchyme, and these micromeres produce the initial signals that specify the other tiers of cells.
The identities of the signaling molecules involved in this process are just now becoming known. The molecule responsible for specifying the micromeres (and their ability to induce the neighboring cells) appears to be β-catenin. As we saw in β-catenin is a transcription factor that is often activated by the Wnt pathway, and several pieces of evidence suggest it for this role. First, during normal sea urchin development, β-catenin accumulates in the nuclei of those cells fated to become endoderm and mesoderm accumulation is autonomous and can occur even if the micromere precursors are separated from the rest of the embryo. Second, this accumulation appears to be responsible for specifying the vegetal half of the embryo.
3.1.2 Axis specification
In the sea urchin blastula, the cell fates line up along the animal-vegetal axis established in the egg cytoplasm prior to fertilization. The animal-vegetal axis also appears to structure the future anterior-posterior axis, with the vegetal region sequestering those maternal components necessary for posterior development.
In most sea urchins, the dorsal-ventral and left-right axes are specified after fertilization, but the manner of their specification is not well understood. Since the first cleavage plane can be either parallel, perpendicular, or oblique with respect to the eventual dorsal-ventral axis, it is probable that the dorsal-ventral axis is not specified until the 8-cell stage, when there are cell boundaries that correspond to these positions.
Interestingly, in those sea urchins that bypass the larval stage to develop directly into juveniles, the dorsal- ventral axis is specified maternally in the egg cytoplasm.