Chapter 7 - Lecture 9.pdf

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Cell and Systems Biology
Ashley Bruce

Chapter 7 – Sea Urchins and Tunicates: Deuterostome Invertebrates Early Development in Sea Urchins Sea Urchin Cleavage  Sea urchins exhibit radial holoblastic cleavage  This type of cleavage occurs in eggs with sparse yolk, and that holoblastic cleavage furrows extend through the entire egg  In sea urchins, the first 7 cleavage divisions are stereotypic in that the same pattern is followed in every individual of the same species st nd  The 1 and 2 cleavages are both meridonal and perpendicular to each other o That cleavage furrows pass through the animal and vegetal poles rd  3 cleavage is equatorial, perpendicular to the first 2 cleavage planes, and separates the animal and vegetal hemispheres from each other th  4 cleavage is very different o The four cells of the animal tier divide meridonally into 8 blastomeres, each with the same volume o These 8 are called mesomeres  The vegetal tier undergoes an unequal equatorial cleavage to produce 4 large cells – macromeres – and 4 smaller micromeres at the vegetal pole  As the 16-cell embryo cleaves, the 8 “animal” mesomeres divide equatorially to produce 2 tiers, an₁ and an₂, one staggered above the other  The macromeres divide meridonally, forming a tier of 8 cells below an₂  Somewhat later, the micromeres divide unequally, producing a cluster of 4 small micromeres at the tip of the vegetal pole, beneath a tier of 4 large micromeres  The small micromeres divide once more, then stop until larval stage  At the 6 division, the animal hemisphere cells divide meridonally while the vegetal cells divide equatorially th  This pattern is reversed in the 7 division  At that time, the embryo is a 120 cell blastula in which the cell form a blastocoel: a hollow sphere surrounding a central cavity  From here on, the pattern of divisions becomes less regular Blastula formation  By the blastula stage, all the cells of the developing sea urchin are the same size, the micromeres having slowed down their cell divisions  Every cell is in contact with the proteinaceous fluid of the blasocoel on the inside and with the hyaline layer on the outside  TJs unite the 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 o This is accomplished by the adhesion of the blastomeres to the hyaline layer and by an influx of water that expands the blastocoel th th  These rapid and invariant cleavages last through the 9 or 10 division, depending on the species  The fates of the cells have become specified and each cell becomes ciliated on the region of the cell membrane farthest from the blastocoel  Thus, there is apical-basal polarity in each of the embryonic cells, and there is evidence that PAR proteins are involved in distinguishing the basal cell membranes  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  The cells of the animal hemisphere synthesize and secrete a hatching enzyme that digests the fertilization envelope  The embryo is not a free swimming hatched blastula Fate maps and the determination of sea urchin blastomeres  The first fate maps of the sea urchin embryo followed the descendants of each of the 16-cell-stage blastomerse  Studies following the fates of individual cells that have been injected with fluorescent dyes that glow in the injected cells progeny for many divisions show that by the 60 cell stage, most of the embryonic cell fates are specified but the cells are not irreversibly committed o 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  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 larval ectodermal or the endodermal organs  The veg₂ layer gives rise to cells that can populate 3 different structures – the endoderm, the coelom (internal mesodermal body wall) , and the non- skeletogenic mesenchyme (secondary mes), which generates pigment cells, immunocytes, and muscle cells  The first tier of micromeres (large micromeres) produces the skeletogenic mesenchyme (primary mes), which forms the larval skeleton  The second tier micromerse (small micromeres) play no role in embryonic development o They contribute cells to the larval coelom from which the tissues of the adult are derived during metamorphosis o Also contribute to producing the germline cells  The fates of the different cell layers are determined in a 2 step process 1. The large micromeres are autonomously specified o They inherit maternal determinants that had been deposited at the vegetal pole of the egg and that become incorporated into the 4 micromeres at 4 cleavage o Thus, they’re determined to become skeletogenic mesenchyme cells that will leave the blastula epithelium to enter the blastocoel, migrate to particular positions along the blastocoel wall, and then differentiate into the larval skeleton 2. The autonomously specified large micromeres are now able to produce paracrine and juxtacrine factors that conditionally specify the fates of their neighbours o The micromeres produce a signal that tells the cells above them to become endoderm and induces them to invaginate into the embryo  The ability of the micromeres to produce signals that change the fates of the neigbouring cells is so pronounced that if micromeres are removed from the embryo and placed on top of an isolated animal cap – the AC cells will generate endoderm and more or less normal larva will develop  These skeletogenic micromeres are the first cells whose fates are specified autonomously  If micromeres are isolated from the 16-cell embryo and placed in petri dishes, they will divide the appropriate number of times and produce the skeletal spicules  Thus, isolated micromeres do not need any other signals to generate their skeletal fates  If skeletogenic micromeres are transplanted into the animal region of the blastula, their descendants will form skeletal spicules and the transplanted micromeres will alter the fates of nea
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