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Biol168_10F_Lecture 12_20Oct2010

Biol168_10F_Lecture 12_20Oct2010 - Dr Morris Maduro UC...

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Dr. Morris Maduro, UC Riverside Biology 168 – 10F – Lecture 12, page 1 Lecture #12: Fertilization Textbook: 2nd Ed.: pp. 438-44. 3rd Ed.: 432-436. Some figures in these notes are redrawn from Wolpert et al. (2nd edition). In this lecture we will discuss the very early development of sea urchins , beginning with fertilization. Sea urchin fertilization has been studied in the most detail, because of the ease of obtaining large amounts of eggs and sperm, and the more important fact that fertilization followed by development can be observed directly because these occur externally. The basic problems of fertilization and the mechanisms that overcome these are conserved in most animals. Fertilization: Creation of the zygote Fertilization is the fusion of the haploid egg and sperm to form a diploid zygote, a new individual containing genetic material from both parents. Fertilization is the trigger that starts development, as the egg is dormant (metabolically inactive) beforehand. Here are the essential steps: 1. Contact and recognition of sperm and egg. Problems: Species specificity (for external fertilization), migration of sperm to egg. 2. Fusion of the sperm and egg, allowing the sperm and egg pronuclei to fuse, forming a diploid zygote nucleus. Problem: Restricting fusion to only a single sperm to prevent polyspermy . 3. Re-activation of the cell cycle. At the time of sperm-egg fusion, the eggs of many species are arrested at some stage of meiosis. For example, most mammal eggs are arrested at second metaphase. Once meiosis is complete and the zygote nucleus is formed, mitosis must start. 4. Metabolic activation. The egg must resume respiration and protein synthesis which will be required for embryonic development. Recognition of sperm and egg Both human and sea urchin eggs are approximately 100 μ m in diameter, while sperm are approximately 40 μ m long in urchins and 50 μ m long in humans (see structure diagram at right). Hence the egg is tens of thousands of times larger in volume than the sperm. The region containing mitochondria is called the midpiece . The acrosome is a vesicle containing enzymes that will be used during fertilization. The nucleus carries the paternal chromosomes and the mitochondria provide energy for motility. The motile flagellum consists of a ‘9+2’ arrangement of microtubules: transmission electron micrograph of flagellum, approx. 150nm in diameter Æ In sea urchin eggs, as in most invertebrates, the egg cell membrane is surrounded by an extracellular matrix called the vitelline envelope that is essential for sperm-egg recognition (see diagram below). In mammals, this is a separate, thicker structure called the zona pellucida , and this layer is surrounded by a layer of cells called the cumulus . In urchins, the vitelline envelope is surrounded by a layer of egg jelly , which is important for sperm attraction. Just beneath the egg cell membrane is a thin shell called the
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