Chapter 47 Reading Quiz 1 Which reaction acts

Chapter 47 Reading Quiz 1. Which reaction acts as a “fast block” to polyspermy? 2. Which reaction acts as a “slow block” to polyspermy? 3. Name the series of cell divisions that transforms a zygote into a blastula. 4. Which vertebrate germ layer becomes the skin? 5. Which extraembryonic avian membrane acts as a disposal sac?

1. Contrast 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 18 th 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

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

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

4. What happens when the egg is activated? • The sharp rise in cytoplasmic Ca 2+ concentration incites metabolic changes and activates the egg cell • Cellular respiration & protein synthesis rates increase • Cytoplasmic p. H changes from slightly acidic to mildly alkaline due to H+ extrusion • Activation can be artificially induced by injection of Ca 2+ • The sperm nucleus fuses with the egg nucleus to form zygote • DNA replication begins and the first division occurs in about 90 minutes

5. Define cleavage, and explain what happens as differentiation takes place. • 1. 2. 3. 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

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)

7. Describe gastrulation and the process, including the layers which result. • 1. 2. 3. • 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

Sea Urchin gastrulation

Frog gastrulation

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

Chicken organogenesis

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

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

10 continued… • 1. 2. 3. 4. 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)

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

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 (m. RNA, 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

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 anteriorposterior)

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

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!