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Lecture 16

GENERAL BIOLOGY II Lecture 16 & 17 Notes

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Biological Sciences
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HermoniseAuguste Section: W1 Group: 6 March 30, 2014 Development I I. Fertilization and Cleavage A. Fertilization 1. Formation of a diploid zygote from a haploid egg and sperm 2. Fertilization a. Molecules and events at the egg surface play a crucial role in each step of fertilization i. Sperm penetrate the protective layer around the egg ii. Receptors on the egg surface bind to molecules on the sperm surface iii. Changes at the egg surface prevent polyspermy, the entry of multiple sperm nuclei into the egg 3. Acrosomal Reaction a. The acrosomal reaction is triggered when the sperm meets the egg b. The acrosome at the tip of the sperm releases hydrolytic enzymes that digest material surrounding the egg c. Gamete contact and/or fusion depolarizes the egg cell membrane and sets up a fast block to polyspermy 4. Cortical Reaction a. Fusion of egg and sperm also initiates the cortical reaction b. Seconds after the sperm binds to the egg, vesicles just beneath the egg plasma membrane release their contents and form a fertilization envelope c. The fertilization envelope acts as the slow block to polyspermy d. Reaction requires high concentration of Ca2 ions in the egg i. Reaction is triggered by a change in Ca2 concentration (1) Increase in Ca2 levels causes cortical granules to fuse with the plasma membrane ii. Ca2 spread across the egg correlates with the appearance of the fertilization envelope e. Acrosomal and Cortical Reaction i. Contact (1) The sperm contacts the egg’s jelly coat, triggering exocytosis of the sperm’s acrosome ii. Acrosomal reaction HermoniseAuguste Section: W1 Group: 6 March 30, 2014 (1) Hydrolytic enzymes released form the acrosome make a hole in the jelly coat (2) Growing actin filaments form the acrosomal process ~ protrudes form the sperm head and penetrates the jelly coat (3) Proteins on the surface of the acrosomal process bind to receptors in the egg plasma membrane iii. Contact and fusion of sperm and egg membranes (1) Fusion triggers depolarization of the membrane which acts as a fast block to polyspermy iv. Cortical reaction (1) Cortical granules in the egg fuse with the plasma membrane (2) Secreted contents clop of sperm-binding receptors and cause the fertilization envelope to form. This acts as a slow block to polyspermy v. Entry of the sperm nucleus 5. EggActivation a. The rise in Ca2+ in the cytosol increases the rates of cellular respiration and protein synthesis by the egg cell b. With these rapid changes in metabolism, the egg is said to be activated c. The proteins and mRNAs needed for activation are already present in the egg d. The sperm nucleus merges with the egg nucleus and cell division begins 6. Fertilization in Mammals a. Fertilization in mammals and other terrestrial animals is internal b. Secretions in the mammalian female reproductive tract alter sperm motility and structure c. This is called capacitation, and must occur before sperm are able to fertilize an egg d. Sperm travel through an outer layer of cells to reach the zona pellucida, the extracellular matrix of the egg e. When the sperm binds a receptor in the zona pellucida, it triggers a slow block to polyspermy f. No fast block to polyspermy has been identified in mammals i. In mammals the first cell division occurs 12-36 hours sperm binding ii. The diploid nucleus forms after this first division of the zygote 7. Cleavage HermoniseAuguste Section: W1 Group: 6 March 30, 2014 a. Fertilization is followed by cleavage i. Aperiod of rapid cell division without growth b. Cleavage partitions the cytoplasm of one large cell into many smaller cells called blastomeres c. The blastula is a ball of cells with a fluid-filled cavity called a blastocoel d. Cleavage in embryo i. Fertilized egg ii. Four-cell stage (1) Remnants of the mitotic spindle can be seen (2) Completion of second cleavage division iii. Early blastula (1) after further cleavage divisions, embryo is a multicellular ball sill surrounded by fertilization envelope (2) blastocoel begins to form in the center iv. Later blastula (1) Single layer of cells surrounds a large blastocoel (2) Fertilization envelope is still present 8. Cleavage Patterns a. Distribution of yolk i. Key factor influencing the patter of cleavage b. Vegetal pole has more yolk: the animal pole has less yolk c. The difference in yolk distribution results in animal and vegetal hemispheres that differ in appearance d. The first two cleavage furrows in the frog form four equally sized blastomeres e. The third cleavage is asymmetric, forming unequally sized blastomeres f. Holoblastic cleavage i. Complete division of the egg ~ Occurs in species whose eggs have little or moderate amounts of yolk ii. Meroblastic cleavage i. Incomplete division of the egg ~ Occurs in species with yolk-rich eggs HermoniseAuguste Section: W1 Group: 6 March 30, 2014 9. Regulation of Cleavage a. Animal embryos complete cleavage when the ratio of material in the nucleus relative to the cytoplasm is sufficiently large II. Morphogenesis in animals A. Overview 1. After cleavage, the rate of cell division slows and the normal cell cycle is restored 2. Morphogenesis a. The process by which cells occupy their appropriate location, which involves i. Gastrulation- the movement of cells from the blastula surface to the interior of the embryo ~ Organogenesis, the formation of organs B. Gastrulation 1. Rearranges the cells of a blastula into a three-layered embryo, called a gastrula 2. 3. Three layers produces are embryonic germ layers a. The ectoderm- forms the outer layer b. The endoderm-lines the digestive tract c. The mesoderm- partly fills the space between the endoderm and ectoderm 4. Each germ layer contributes to specific structures in the adult animal C. Gastrulation in sea urchins 1. Gastrulation begins at the vegetal pole of the blastula 2. Mesenchyme cells migrate into the blastocoel 3. The vegetal plate forms from the remaining cells of the vegetal pole and buckles inward through invagination 4. The newly formed cavity is called the archenteron a. Opens through the blastopore, which will become the anus 5. Gastrulation in sea urchin embryo a. Once the blastula is formed, gastrulation begins with the migration of mesenchyme cells from the vegetal pole into the blastocoel b. The vegetal plat invaginates. Mesenchyme cells migrate throughout the blastocoel HermoniseAuguste Section: W1 Group: 6 March 30, 2014 c. Endoderm cells form the archenteron (future digestive tube). New mesenchyme cells at the tip of the tube send out thin extension (filopodia) towards the blastocoel wall d. The filopodia then, contract, dragging the archenteron across the blastocoel e. Fusion of the archenteron with the blastocoel wall forms the digestive tube, which now has a mouth and an anus. The gastrula has three germ layers and is covered with cilia, which will function later in feeding and movement D. Gastrulation if Frogs 1. Frog gastrulation begins when a group of cells on the dorsal side of the blastula begins to invaginate 2. This forms a crease along the region where the gray crescent formed 3. The part above the crease is called the dorsal lip of the blastopore 4. Cells continue to move from the embryo surface into the embryo by involution 5. These cells become the endoderm and mesoderm 6. Cells on the embryo surface will form the ectoderm E. Gastrulation in Chicks 1. Prior to gastrulation a. The embryo is composed of an upper (epiblast) and lower layer (hypoblast), respectively 2. During gastrulation a. Epiblast cells move toward the midline of the blastoderm b. Then moves into the embryo toward the yolk c. The midline thickens and is called the primitive streak d. The hypoblast cells contribute to the sac that surrounds the yolk and a connection between the yolk and the embryo, but do not contribute to the embryo itself F. Gastrulation in Humans 1. Human eggs have very little yolk 2. Anatomy a. Blastocyst b. Inner cell mass- cluster of cells at one end of the blastocyst c. Trophoblast- outer epithelial layer of blastocyst, initiates implantation 3. Following implantation, the trophoblast continues to expand a set of extraembryonic membranes is formed a. Enclose specialized structures outside of the embryo HermoniseAuguste Section: W1 Group: 6 March 30, 2014 b. Gastrulation involves the inward movement from the epiblast, through a primitive streak, similar to the chick embryo 4. Process a. Blastocyst reaches uterus b. Blastocyst implants (7 days after fertilization) c. Extraembryonic membranes start to form (10-11 days) and gastrulation begins (13 days) d. Gastrulation has produced a three-layered embryo with four extraembryonic membranes G. DevelopmentalAdaptations ofAmniotes 1. The colonization of land by vertebrates was made possible only after the evolution of a. The shelled egg of birds and other reptiles as well as Monotremes (egg-laying mammals) b. The uterus of marsupial and eutherian mammals 2. In both adaptation, embryos are surrounded by fluid in a sac called the amnion a. Protects the embryo from desiccation and allows reproduction on dry land b. Mammals and reptiles including birds are called amniotes for this reason 3. The four extraembryonic membranes that form around the embryo a. The chorion functions in gas exchange b. The amnion encloses the amniotic fluid
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