University:
Kentucky State University
Course:
BIO 111 | Principles of Biology
Academic year:

2024
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223

Pages:

22
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lankstoetp17

Chapter 10 Sea Urchin and Tunicates Early cleavage in sea urchin • Sea urchin exhibit radial holoblastic cleavage. • The first seven cleavage divisions are stereotypic: meaning that the same pattern is followed in every individual of the same species • By the end of the seventh cleavage division, the embryo is 120 cell blastula in which the cells form hollow sphere surrounding a central cavity called a blastocoel. blastocoel blastocoel • By the blastula stage, all the cells are of the same size, as the micromeres slowed down their cell division. • Every cell is in contact with proteinaceous fluid of the blastocoel on the inside and hyaline layer on the outside. Blastocoel Hyaline layer • The rapid and invariant cell cleavages that occur in the blastula last through the ninth and tenth divisions, depending on the species. • The ciliated blastula begins to rotate within the fertilization envelope. • The cells of the animal sphere synthesize and secrete a hatching enzyme that digests the fertilization envelope. Free swimming hatched blastula Fate maps and the determination of sea urchin blastomeres • Researchers followed the fate of individual cells by injecting them with fluorescent dyes. • By the 60-cell stage, the fates of embryonic cells are specified. • However, these cells remain pluripotent and can give rise to other cell types if experimentally placed in a different part of the embryo. • The veg2 layer gives rise to cells that can populate three different structures: the endoderm, the coelom (internal mesodermal body wall), and the non-skeletogenic mesoderm (mesenchyme). • The non-skeletogenic mesenchyme gives rise to pigment cells, immunocytes and musculature around the gut and also contributes to the coelomic pouches (Larval skin and its neurons) (Secondary mesenchyme cells) i (Skeletogenic mesenchyme) , larval coelom The fate of different cell layers is determined in two-step processes: 1. The large micromeres are autonomously specified: They inherit maternal determinants that were deposited at the vegetal pole of the egg and become incorporated into the large micromeres at fourth cleavage. The determinants regulate gene expression to direct the cells into a particular path of development. Maternal determinants The fate of different cell layers is determined in two-step processes: 2. The autonomously specified large micromeres are able to produce paracrine and juxtacrine factors that conditionally specify the fates of their neighbors. • These experiments showed that the large micromeres produce signals that tell the cells above them to become endomesoderm (endoderm and secondary mesenchyme cells). • These experiments showed that the micromeres produce signals that induce the cells above them to invaginate into the embryo Sea Urchin Gastrulation Ingression of the skeletogenic mesenchyme • Epithelial- to mesenchymal transition (EMT), where cells that had formerly been part of an epithelium lose their attachment and become individual, migrating cells. There are five distinct processes in EMT: 1. Apical-basal polarity. • The vegetal cells of the blastula elongate to form a thickened “vegetal plate” epithelium. There are five distinct processes in EMT: 2. Apical constriction of the micromeres. • The cells alter shape, wherein the apical end becomes constricted. 3. Basement membrane remodeling. • The micromeres secrete proteases that digest a hole in laminin-containing basement membrane above the vegetal cells. There are five distinct processes in EMT: 4. De-adhesion. • Cadherins that couple epithelial cells together are degraded, thereby allowing cells to become free from their neighbors. 5. Cell motility. • The cells bind to and travel on extracellular matrix fibrils (proteins) within the blastocoel. • Many skeletogenic mesenchymal cells cluster together and fuse with one another to form the syncytial cables that will form the calcium carbonate axis of the larval skeletal rods. Invagination of the archenteron First stage of archenteron invagination • The vegetal plate cells remain bound to one another and to the hyaline layer of the egg, and they move to fill the gaps caused by the ingression of the skeletogenic mesenchyme. Invagination of the archenteron First stage of archenteron invagination • The movement of the vegetal plate into the blastocoel appears to be initiated by changes in the shape of the vegetal plate cells and in the extracellular matrix underlying them. Bottle shaped vegetal cells Hyaline layer Invagination of the archenteron First stage of archenteron invagination • The invaginated region is called the archenteron (primitive gut) and the opening of the archenteron at the vegetal pole is the blastopore. Invagination of the archenteron The second and third stage of archenteron invagination • After the pause of the initial invagination, the second phase of archenteron formation begins. • In the process of archenteron extension, the wide short gut rudiment is transformed into a thin, long tube. • The phenomenon, where cells intercalate to narrow the tissue and at the same time lengthen it, is called convergent extension. Invagination of the archenteron The second and third stage of archenteron invagination • The final (third) phase of archenteron elongation is initiated by the tension provided by the nonskeletogenic mesenchyme cells, which form at the tip of the archenteron and remain there. • Filopodia: long, thin processes containing microfilaments; cells can move by extending, attaching, and then contracting filopodia • The filopodia attach to the wall at the junction between the blastomeres and then shorten, pulling up the archenteron. Invagination of the archenteron The second and third stage of archenteron invagination • The final (third) phase of archenteron elongation is initiated by the tension provided by the nonskeletogenic mesenchyme cells, which form at the tip of the archenteron and remain there. • Filopodia: long, thin processes containing microfilaments; cells can move by extending, attaching, and then contracting filopodia • The filopodia attach to the wall at the junction between the blastomeres and then shorten, pulling up the archenteron. Invagination of the archenteron The second and third stage of archenteron invagination • As the top of the archenteron meets the blastocoel wall in the target region, many of the nonskeletogenic mesenchyme cells disperse into the blastocoel, where they proliferate to form mesenchyme organs. • Where the archenteron contacts the wall, a mouth eventually forms at the ventral side of the pluteus larva. • The blastopore marks the position of the anus.

Chapter 10 Sea Urchin and Tunicates

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