Blastula formation developmental Biology

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Blastula formation

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Blastula formation
• The blastula stage of sea urchin development begins at the 128-cell stage (7c). Here the cells
form a hollow sphere surrounding a central cavity, or blastocoel (see Figure 8.7F).
• By this time, all the cells are the same size, the micromeres having slowed down their cell
divisions.
• Every cell is in contact with the proteinaceous fluid of the blastocoel on the inside and with the
hyaline layer on the outside.
• 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.

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These rapid and invariant cell cleavages last through the ninth or tenth division, depending on the
species.
By this time,
1. The fates of the cells have become specified (discussed in the next section) and
2. Each cell gets ciliated on the region of the cell membrane farthest from the blastocoel.
• This 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 (see Figure
8.7F).
• The cells of the animal hemisphere synthesize and secrete a hatching enzyme that digests the
fertilization envelope.
• The embryo is now a free-swimming hatched blastula .

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Fate maps and the determination of sea urchin blastomeres
Cell fate determination
• 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.

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• A fate map of the 60-cell sea urchin embryo
• 4.The animal half of the embryo consistently gives rise to the ectoderm—1) the larval skin
and its 2) neurons.
• 3.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
• 2.1) endoderm, the 2) coelom (body wall), and 3) secondary mesenchyme (pigment cells,
immunocytes, and muscle cells).
• 1.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.

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• Conditional Specification
• Although the early blastomeres have consistent fates in the larva, most of these fates are
achieved by conditional specification.
• That is, a cell's fate depends on its position relative to its neighboring cells.
• The micromeres are able to produce a signal that tells the cells adjacent to them to become
endoderm and induces them to invaginate into the embryo.

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Experimental demonstration
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 (Figure 8.9).

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• 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 only cells whose fates are determined autonomously are the skeletogenic micromeres.

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Experimental Demonstration
• Condition 1: If these micromeres are isolated from the embryo and placed in test tubes, they will
still form skeletal spicules.
• Condition 2: 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 (Figure 8.13).

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β-catenin
• The molecule responsible for specifying the micromeres (and their ability to induce the
neighboring cells) appears to be β-catenin.
• β catenin is a transcription factor that is often activated by the Wnt signaling pathway.
1. First, during normal sea urchin development, β-catenin accumulates in the nuclei of those cells
fated to become endoderm and mesoderm (Figure 8.llA). This accumulation is autonomous and
can occur even if the micromere precursors are separated from the rest of the embryo.
2. Second, this nuclear accumulation appears to be responsible for specifying the vegetal half of
the embryo. It is possible that the levels of nuclear β-catenin accumulation help to determine the
mesodermal and endodermal fates of the vegetal cells.
3. Third, β-catenin is essential for giving the micromeres their inductive ability.*

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β-catenin
Experimental Demonstration
1. Overexpression
• Treating sea urchin embryos with lithium chloride causes the accumulation of β-catenin in
every cell and transforms the presumptive ectoderm into endoderm.
2. Inhibited Expression
• Conversely, experimental procedures that inhibit β-catenin accumulation in the vegetal cell
nuclei prevent the formation of endoderm and mesoderm (Figure 8.llB,C;).

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Presumptive Ectoderm--Endoderm
No Endoderm & Mesoderm formed
here

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• Experiments described above demonstrated that the micromeres were able to induce a
second embryonic axis when transplanted to the animal hemisphere. However, micromeres
from embryos in which β-catenin was prevented from entering the nucleus were unable to
induce the animal hemisphere cells to form endoderm, and a second axis was not formed.
• β-Catenin is critical for the specification of the micromeres and for empowering them with
the ability to induce the veg, cells above them.
• The specification of the micromeres by β-catenin is mediated by the Pmarl gene product, a
homeodomain transcription factor that acts as a transcriptional repressor. The Pmarl‘
protein represses an as yet unidentified gene whose product is a general repressor of
several genes that characterize primary mesenchyme