BIO132 Lecture 10: Fertilization (PDF)

Dr. Ahmed Deghidy PhD Molecular Biology Alex U, Alexandria, Egypt Field of Basic Sciences ‫الدولية‬ ‫جامعة العلمين‬ 1  “BIO132” Lecture 10 Dr. Ahmed Deghidy PhD Molecular Biology Alex U, Alexandria, E...

Dr. Ahmed Deghidy PhD Molecular Biology Alex U, Alexandria, Egypt Field of Basic Sciences ‫الدولية‬ ‫جامعة العلمين‬ 1  “BIO132” Lecture 10 Dr. Ahmed Deghidy PhD Molecular Biology Alex U, Alexandria, Egypt  Fertilization Beginning a New Organism Fertilization accomplishes two separate activities: 1- Sex (the combining of genes derived from two parents) 2- Reproduction (the creation of a new organism). The events of fertilization usually include: (1) contact and recognition between sperm and egg (2) regulation of sperm entry into the egg (3) fusion of genetic material from the two gamete (4) activation of egg metabolism to start development. Structure of the Gamete (Sperm) The sperm head consists of a haploid nucleus and an acrosome. The acrosome is derived from the Golgi apparatus and contains enzymes needed to digest extracellular coats surrounding the egg. The midpiece of the sperm contain mitochondria and the centriole that generates the microtubules of the flagellum. Energy for flagellar motion comes from mitochondrial ATP  Structure of the Gamete (egg) The female gamete can be an egg (with a haploid nucleus, as in sea urchins) or an oocyte (in an earlier stage of development, as in mammals). The egg (or oocyte) has a large mass of cytoplasm storing ribosomes nutritive proteins morphogenetic factors. Many eggs also contain protective agents needed for survival in their particular environment. Surrounding the egg cell membrane is an extracellular layer often used in sperm recognition. In most animals, this extracellular layer is the vitelline envelope. In mammals, it is the much thicker zona pellucida. Cortical granules lie beneath the egg’s cell membrane.  Recognition of egg and sperm The interaction of sperm and egg generally proceeds according to five steps: 1. Chemoattraction of the sperm to the egg by soluble molecules secreted by the egg 2. Exocytosis of the sperm acrosomal vesicle and release of its enzymes 3. Binding of the sperm to the extracellular matrix (vitelline envelope or zona pellucida) of the egg 4. Passage of the sperm through this extracellular matrix 5. Fusion of the egg and sperm cell membranes External Fertilization in Sea urchins  Sperm attraction: Action at a distance In many species, sperm are attracted toward eggs of their species by chemotaxis—that is, by following a gradient of a chemical secreted by the egg In sea urchins, the chemotactic peptides (sperm-activating peptides (SAPs) such as resact and speract have been shown to increase sperm motility and provide direction toward an egg of the correct species. Resact from egg jelly binds to its receptor on the sperm. This activates the receptor(RGC) activity, leading to opens calcium channels in the sperm cell membrane, allowing Ca2+ to enter the sperm. The influx of Ca2+ activates sperm motility, and the sperm swims up the resact gradient toward the egg  The acrosome reaction A second interaction between sperm and egg jelly results in the acrosome reaction. The acrosome reaction has two components: 1- the fusion of the acrosomal vesicle with the sperm cell membrane (an exocytosis that results in the release of the contents of the acrosomal vesicle) 2- the extension of the acrosomal process The acrosome reaction releases proteolytic enzymes that digest the egg’s protective coating, allowing the sperm to reach and fuse with the egg cell membrane.  The acrosome reaction Globular actin polymerizes to extend the acrosomal process. Bindin on the acrosomal process is recognized by a protein complex on the sea urchin egg surface. In sea urchins, bindin may mediate gamete recognition and fusion  One egg, one sperm In the normal case—monospermy—only one sperm enters the egg Polyspermy results when two or more sperm fertilize an egg. It is usually lethal, since it results in blastomeres with different numbers and types of chromosomes. sea urchin egg blocks polyspermy through two mechanism: 1- The fast block to polyspermy a change in the electric potential of the egg cell membrane by the influx of sodium ions causes the egg membrane resting potential to rise. Sperm can no longer fuse with the egg. takes 1-3 seconds. remains for only about a minute 2- The slow block to polyspermy The slow block, or cortical granule reaction, is physical and is mediated by calcium ions. A wave of Ca2+ propagates from the point of sperm entry, causing the cortical granules to fuse with the egg cell membrane. The released contents of these granules cause the vitelline envelope to rise and harden into the fertilization envelope. starts about 20 seconds after sperm attachment and is complete by the end of the first minute of fertilization  Fusion of Genetic Material in Sea urchins Genetic material is carried in a male and a female pronucleus, which migrate toward each other. In sea urchins, the male and female pronuclei merge and a diploid zygote nucleus is formed. DNA replication occurs after pronuclear fusion