* Your assessment is very important for improving the work of artificial intelligence, which forms the content of this project
Doug Peterson Content Narrative #3 Embryonic Development Fertilization Although it is crucial in the process of embryonic development, we are going to skip over gametogenesis. It is essential, but it would add another twenty minutes to my presentation, so for the sake of brevity we are going to skip over it and proceed directly to fertilization. Fertilization is the combination of haploid sets of chromosomes from two individual organisms (in our case animals) into a single diploid cell, called a zygote. Fertilization also, functions in activating the egg, which initiates metabolic reactions that trigger the onset of embryonic development. In mammals, secretions in the female reproductive tract alter molecules on the surface of sperm cells and increase its motility. The egg is surrounded by follicle cells, released during ovulation, which the sperm must travel through before reaching the extracellular matrix of the egg (zona pellucida). The zona pellucida acts as a sperm receptor, and binds to a complementary molecule in the sperm head. This ensures that the sperm is off the same species. Binding of the sperm to the egg induces the acrosomal reaction. In this process, the sperm releases hydrolytic enzymes that penetrate the zona pellucida and reach the plasma membrane of the egg. The release of these enzymes exposes a protein in the sperm membrane that binds with the egg’s plasma membrane. The binding of sperm and egg triggers changes in the egg which leads to a cortical reaction, in which cortical granules are released to the outside of the cell via exocytosis. The cortical granules catalyze the alteration of the zona pellucida that prevents the entry of additional sperm. After the sperm and egg membranes fuse, the entire sperm is taken into the egg. The egg lacks a centrosome, but a centrosome forms around the centriole that was the basal body of the sperm’s flagellum. The centrosome then duplicates to form two centrosomes, which will generate the mitotic spindle in the first mitotic division of the zygote. The haploid nuclei from the sperm and egg do not fuse, but their nuclear envelopes disperse. The two sets of chromosomes will share a spindle apparatus during the first division of the zygote, so the chromosomes will not come together in the same nucleus until after the first division. Cleavage Once fertilization is complete, a succession of rapid cell divisions occurs. This period is called cleavage. The cells undergo S and M phase of the cell cycle, but virtually skip G1 and G2, which results in the production of a very small amount of protein. The embryo does not grow because cleavage simple partitions the cytoplasm of the large zygote cell into smaller cells called blastomeres, each of which have their own nucleus. The first five to seven divisions form a dense ball of cells known as the morula. As cell division continues, liquid fills the morula and pushes the cells out to form a circular cavity surrounded by a single layer of cells. The hollow sphere is known as the blastula and the cavity is the blastocoel. With the exception of mammals, zygotes of other animals have a polarity. This polarity causes the divisions of the zygote to have a specific pattern. The polarity is defined by the uneven distribution of substances in the cytoplasm (mRNA, proteins, and yolk). In many animals, the distribution of yolk is the key factor that influences the pattern of cleavage. Yolk is more concentrated towards one pole known as the vegetal pole, the opposite pole is known as the animal pole. In some organisms, early cleavages are polar, dividing the egg into segments that stretch from pole to pole. Other cleavages are parallel with the equator. In deuterostomes, early cleavages are radial, forming cells at the animal and vegetal poles that are aligned together, the top cells directly above the bottom cells. In protostomes, cleavages are spiral, forming cells on top that are shifted with respect to those below them. Radial cleavages are usually indeterminate, producing blastomeres that can independently complete normal development. Spiral cleavages are often determinate; producing blastomeres that cannot develop into a complete embryo if separated from other blastomeres. Gastrulation Gastrulation is a dramatic rearrangement of the cells of the blastula to form a threelayered embryo with a primitive gut. In all species, gastrulation occurs when some cells at or near the surface of the blastula invaginates, establishing three germ layers. The three-layered embryo is called the gastrula. The ectoderm forms the outer layer; the endoderm lines the embryonic digestive tract; and the mesoderm partly fills the space between the two. The positioning of these layers allows cells to interact with each other in new ways. Eventually these three cell layers develop into all the tissues and organs of the adult animal. The center cavity formed by gastrulation is called the archenteron and is surrounded by the endoderm cells. The opening into the archenteron is the blastopore. This will become the mouth in protostomes or the anus in deuterostomes. Extraembryoinic Membrane In the amniotes (birds, reptiles, and humans) extraembryonic membrane develops which allows the embryo to survive in its developmental environment. The chorion is the outer membrane, which acts in gas exchange in birds and reptiles. The chorion implants into the endometrium in mammals, and eventually forms the placenta (which allows for gas, nutrient, and waste exchange). From the archenteron, the allontois buds off. This eventually encircles the embryo forming a layer below the chorion. In animals and reptiles, it initially stores waste, but later fuses with the chorion and acts as a membrane for gas exchange with blood vessels below. In mammals, the allantois transports waste to the placenta, and eventually becomes the umbilical cord and transports gasses, nutrients, and wastes between the embryo and placenta. Enclosing the amniotic cavity is the amnion, a fluid filled cavity that cushions the embryo. In birds and reptiles, the yolk sac membrane digests the enclosed yolk and blood vessels transfer nutrients to the embryo. In mammals, the yolk sac is empty and nutrition is obtained through the placenta. Organogenesis During the process or organogenesis, the three germ layers develop into the rudiments of organs. Cell differentiation is responsible for the cells taking on the characteristics of specific tissues and organs. The first organs that begin to take form are the neural tube and the notochord. The notochord is formed from the dorsal surface of the mesoderm that condenses above the archenteron. The notochord is a still rod that provides support in lower chordates and the vertebrae of higher chordates. Signals sent from the notochord to the ectoderm above it cause that region to become the neural plate. The neural plate curves inward, rolling itself into the neural tube, which runs along the anterior-posterior axis of the embryo. The neural tube will later become the central nervous system. Along the border where the neural tube pinches off from the ectoderm, is a band of cells called the neural crest. Cells from the neural crest migrate to various parts of the embryo, forming nerves, teeth, skull bones, and many other different cell types. As organogenesis progresses, morphogenesis and cell differentiation refines the organs that arise from the germ layers. Presentation Similar to my mitosis/ meiosis presentation, I will post a set of PowerPoint slides with mostly pictures and maybe a few notes. The majority of the notes will be given in a hand out that my peers will fill out as I explain development. I will post pictures that depict the major components of development (fertilization, cleavage, gastrulation, and organogenesis). References Campbell, N. A., & Reece, J. B. (2005). Biology. (7th ed.). Benjamin-Cummings Pub Co. Pack, P. E. (2008). Cliffsap biology. Cliffs Notes.