Chapter 47 Reading Quiz

1. Chapter 47 Reading Quiz • Which reaction acts as a “fast block” to polyspermy? • Which reaction acts as a “slow block” to polyspermy? • Name the series of cell divisions that transforms a zygote into a blastula. • Which vertebrate germ layer becomes the skin? • Which extraembryonic avian membrane acts as a disposal sac?

2. 1. Contrast the theories of preformation and epigenesis. • Preformation  suggests that the embryo contains all of its descendants as a series of successively smaller embryos within embryos (popular until the 18th century) • Epigenesis  proposes that the form of an embryo gradually emerges from a formless egg - organism’s development is determined by the zygote’s genome and cytoplasmic determinants 

3. 2. Describe the acrosomal reaction. • Acrosomal reaction  the discharge of hydrolytic enzymes from a vesicle in the acrosome of a sperm cell - upon contacting the egg’s jelly coat, the enzymes enable the sperm’s membrane to reach the egg’s membrane and cause depolarization of the plasma membrane to prevent polyspermy 

5. 3. Describe the cortical reaction. • The fusion of the egg and sperm membranes stimulates a series of changes in the egg’s cortex known as a cortical reaction - stimulates transduction pathway that releases Ca+ from egg ER - eventually through the pathway a hardened “fertilization membrane” prevents the entry of more sperm by a slow block to polyspermy 

7. 4. What happens when the egg is activated? • The sharp rise in cytoplasmic Ca2+ concentration incites metabolic changes and activates the egg cell • Cellular respiration & protein synthesis rates increase • Cytoplasmic pH changes from slightly acidic to mildly alkaline due to H+ extrusion • Activation can be artificially induced by injection of Ca2+ • The sperm nucleus fuses with the egg nucleus to form zygote • DNA replication begins and the first division occurs in about 90 minutes 

10. 5. Define cleavage, and explain what happens as differentiation takes place. • Cleavage  a succession of rapid mitotic cell divisions following fertilization that produce a multicellular embryo, the blastula • Zygote is composed of two hemispheres, the vegetal and animal • The first two cleavage divisions are vertical and divide the embryo into four cells • The third cleavage plane is horizontal & produces an 8 cell embryo 

12. 6. Distinguish between meroblastic and holoblastic cleavage. • Meroblastic  an incomplete division of the yolk-rich egg; cleavage is restricted to a small disc of yolk-free cytoplasm at the animal pole of the egg cell (birds, reptiles) • Holoblastic  the complete division of eggs having a moderate amount or a little yolk (sea urchins, frogs) 

13. 7. Describe gastrulation and the process, including the layers which result. • Gastrulation  a dramatic process that rearranges the blastula (a hollow ball of cells) into a gastrula (a three-layered embryo) • Ectoderm • Endoderm • Mesoderm • Where the opening occurs determines if the animal is a protostome or deuterostome 

14. Sea Urchin gastrulation

15. Frog gastrulation

16. 8. Define organogenesis, and describe what changes occur in the developing animal. • Organogenesis the development of the organs • Folds, splits, and condensation of cells are the first steps to building organs • The neural tube and notochord are the first organs to develop in chordates • Ectoderm  skin, inner ear, eye lens • Mesoderm  notochord, coelom lining, muscles, skeleton, gonads, kidneys, circulatory system • Endoderm  digestive tract linings, liver, pancreas, and lungs 

19. Chicken organogenesis

20. 9. Briefly overview avian development, including key terminology. • The yellow yolk is a large food reserve for the embryo • The egg white is protein-rich and provides additional nutrients during development • After fertilization, meroblastic cleavage occurs at the animal pole, which creates the blastodisc • Similar to the blastula, the blastomeres sort into upper and lower layers, with a blastocoel in between • Gastrulation occurs and the three-layered embryo is formed • The primary germ layers also form 4 extraembryonic membranes  yolk sac, amnion, chorion, and allantois 

23. 10. Briefly overview mammalian development. • Fertilization occurs in oviducts, early development occurs as the zygote travels to the uterus • Egg stores little nutrients and shows holoblastic cleavage, which is slow • At 7 days, the embryo consists of 100 cells forming the blastocyst • The inner cell mass protrudes into one end of the cavity and will develop into the embryo and some extraembryonic membranes • The trophoblast is the outer epithelium surrounding the cavity which will form the fetal part of the placenta 

24. 10 continued… • The blastocyst reaches the uterus and begins to implant (enzymes, projections) • Chorion  forms the trophoblast and surrounds embryo and all extraembryonic membranes • Amnion  forms as a dome and encloses the embryo in a fluid-filled cavity • Yolk sac  encloses a fluid-filled cavity but no yolk, its membrane is the site of early blood cell formation • Allantois  develops from an outpocketing of the rudimentary gut and is incorporated into the umbilical cord (nutrients, gases, wastes) 

26. 11. Describe what morphogenesis entails. • Morphogenesis in animals involves specific changes in cell shape, position, and adhesion • Cell extension, contraction, and adhesion are all involved in these movements • Changes in shape involves the reorganization of the cytoskeleton • Cell adhesion molecules (CAMs) exist on the cell surface and contribute to the selective association of certain cells with each other 

28. 12. Describe what effects cytoplasmic determinants have on the fate of cells. • The heterogeneous distribution of cytoplasmic determinants leads to regional differences in early embryo development • Different substances (mRNA, proteins) are partitioned differently among the cells • The local differences in distribution of these cytoplasmic determinants influences gene expression and the developmental fate of the cells 

30. 13. Briefly describe how polarity is determined. • The eggs of most vertebrates have cytoplasmic determinants that help establish the body axes • The first step to development is determining the anterior-posterior, dorsal-ventral, and right-left axes • Polarity is not determined until after cleavage (mammals) • Some species the polarity is determined much earlier (frogs) with the locations of the vegetal and animal hemispheres (determines anterior-posterior) 

31. 14. Describe how inductive signals drive differentiation and pattern formation in vertebrates. • Induction  the ability of one cell group to influence the development of another - usually the switching on of a set of genes that makes the receiving cells differentiate into a specific tissue - one part of the early gastrula plays a role in where the neural tube and notochord develop 

32. 15. Define pattern formation and how positional information is relayed. • Pattern formation  the development of an animal’s spatial organization with organs and tissues in their characteristic places in the three dimensions of the animal • Is controlled by positional information, which is a set of molecular cues that indicate a cell’s location relative to other cells in an embryonic structure and that help to determine how the cell and its descendants respond to future molecular signals The End!