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BIO380H5F Lec 14,17 & 18 Notes.docx

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University of Toronto Mississauga
Kathleen Wong

BIO380H5F READINGS Lecture 14 p. 83 – 93 Cleavage is the first event that occurs after fertilization. It converts the unicellular egg into a multicellular embryo. There are 4 central developmental processes that occur in human embryogenesis: 1. Cell Division (Cleavage): involves initial cleavage followed by regular mitotic cell divisions. 2. Cell Differentiation: Formation of different, specialized cell types. 3. Morphogenesis: the generation of shape; results in embryonic organization and development of the pattern and polarity of organs and tissues. Cell differentiation is one component of morphogenesis. Cell differentiation and morphogenesis are not synonymous. 4. Growth: Increases the size of a cell, tissue, organ or organism. Functions and events of cleavage:  Initial division of zygote to multicellular embryo.  Mitotic divisions maintain the diploid (2N) complement of the genome.  Rapid cell division, no growth  Blastomeres get smaller with each division  Holoblastic cleavage; complete separation of blastomeres Cleavage has a shortened cell cycle. The normal cell cycle has four phases; growth phase 1, (G1), DNA Synthesis (S), growth phase 2 (G2) and mitosis (M). There is no G1 and G2 phase during cleavage and hence cleavage cells become smaller in size. All resulting blastomeres will be genetically identical. Since each blastomere has a diploid nucleus, the amount of nuclear DNA in the embryo also doubles. The zygote is surrounded by the zona pellucida which is separated from the cell itself by the perivitelline space. The first cleavage/ division gives a two-celled embryo i.e. two blastomeres. Now the embryo has double the DNA content of the zygote and it has made membrane to surround both cells. No growth has occurred so the two blastomeres together are approximate equal to the size of the original zygote. After 2 cleavage the embryo has four cells, and four times the DNA content of the zygote. Membrane now surrounds all four cells and the four blastomeres together = size of original zygote. After third cleavage the embryo has 8 cells and 8 times the DNA content of the zygote. Membrane surrounds the eight cells and still no growth so the eight blastomeres = the size of the orginial zygote. Then embryo undergoes compaction leading to the 16 cell stage. After compaction fluid appears between the cells after the morula stage resulting in the early blastocyst. The fluid accumulates in an increasingly larger cavity called the blastocoel. Compaction occurs when cells adhere together more tightly at the 8-cell stage. The embryo becomes more compact, but the cells remain separate from each other. E-cadherin causes the cells to adhere more tightly during compaction. E-cadherin is a cell adhesion molecule that is involved in calcium-dependent binding. N-cadherin, involves the adhesion of nerve cells. E – cadherins mediate epithelial cell adhesion. P-cadherins localize the placenta. E-cadherin molecules from one blastomere homophilically associate with cadherin molecules on adjacent blastomeres leading to their tight adhesion. E-cadherin, present during cleavage, will be replaced by N-cadherin when multicellular tissue formation begins (gastrulation). The presence of E-cadherin can be detected at the surface of cells in compacted embryos using immunofluorescence microscopy. If cells are treated with anti-cadherin at the time of compaction, they do not undergo compaction. Anti-cadherin binds to other cadherin molecules. Epithin is a mammalian transmembrane serine protease that has been found to be associated with E- cadherin in a diversity of tissues. Epithin and e-cadherin co-localize at the 8-cell and morula stages. Cavitation is the appearance of fluid between blastomeres in the embryo as cell numbers increase. It begins approximately four days after fertilization and leads to the formation of the blastocoele. Human eggs hatch prior to implantation. During hatching the embryo breaks through the zona pellucida due to proteases that have been secreted by the blastocyst. Inability to hatch causes infertility and it could be due to the presence of an altered zona pellucida or absence of proteases for zona pellucida digestion.Assisted hatching is done in vitro during ART procedures. In mutant mouse embryos lacking ZP1 protein, the zona pellucida is weak and the embryos hatch prematurely which may lead to ectopic pregnancy. Lecture 15 p. 94 – 101 Implantation of the embryo occurs approximately 6 – 7 days after fertilization. During implantation, the embryo will bind to the uterine epithelium then penetrate it transforming the embryo into a fetus. LECTURE 17 & 18 Chapter 10 p. 102 - 109 Gastrulation is the first phase of development in the fetus. It defines the primary germ layers (ectoderm, mesoderm and endoderm) and sets up new cellular and tissue relationships essential for development. Gastrulation will occur in the epiblast. Delamination occurs before gastrulation. It separates the inner cell mass , ICM, into the epiblast and hypoblast just before implantation and gastrulation. After cleavage, morphogenic movements will reorganize the embryo into distinct layers. The epiblast becomes a 2- layered (bilaminate) disc of approximately cuboidal cells that will form the embryo proper. The flatter layer of hypoblast cells lies below the epiblast will continue to the yolk sac. Gastrulation will convert the bliaminate epiblast into the three primary embryonic germ layers: 1. Ectoderm – outside; surrounds the other germ layers 2. Mesoderm – middle; lays between the ectoderm and endoderm 3. Endoderm – inside; lies at the most interior of the embryo No experimental work is being done on human gastrulation because is not ethical or legal. This makes it difficult to get specific stages or to define when and how specific events occur. Recently researchers have been able to fluorescently label cells and follow their movement using confocal microscopy to obtain sharper resolution of stained material and its precise location in a time- dependent manner. Confocal microscopy is a special laser-based microscopy which allows researchers to take optical sections through tissues to construct 3D images. At the start of gastrulation the human epiblast is bilaminate. Initially cells move along the surface and once at the centre line (primitive streak) they enter the embryo at this point. The cells then turn the corner and move internally. As they enter the moving surface cells first pile up to form a bump known as the primitive node. It is called the Hensen’s node in other animals. The pile up occurs because the cells move along the top faster than they can separate off and move internally. The cells that enter through the primitive node will become the notochord.
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