47 Chapter

Question 1

Model organisms

Answer 1

Species chosen for the ease with which they can be studied in the laboratory.

Question 2

Fertilization

Answer 2

The formation of a diploid zygote from a haploid egg and sperm.

Question 3

General events that occur during fertilization

Answer 3

First, sperm dissolve or penetrate any protective layer surrounding the egg to reach the plasma membrane. Next, molecules on the sperm surface bind to receptors on the egg surface, helping ensure that fertilization involves a sperm and egg of the same species. Finally, changes at the surface of the egg prevent polyspermy, a condition in which multiple sperm nuclei enter the egg, fatally disrupting development.

Question 4

The cell surface events that take place during fertilization have been studied most extensively in sea urchins.

Answer 4

True

Question 5

Acrosome

Answer 5

A vesicle in the tip of a sperm containing hydrolytic enzymes and other proteins that help the sperm reach the egg.

Question 6

Acrosomal reaction

Answer 6

The discharge of hydrolytic enzymes from the acrosome when the sperm approaches or contacts an egg.

Question 7

Fast block to polyspermy

Answer 7

The depolarization of the egg in plasma membrane that begins within 1-3 seconds after a sperm binds to an egg membrane protein. The depolarization lasts about 1 minute and prevents additional sperm from fusing with the egg during that time.

Question 8

Slow block to polyspermy

Answer 8

The formation of the fertilization envelope and other changes in an egg’s surface that prevent fusion of the egg with more than one sperm. The slow block begins about 1 minute after fertilization.

Question 9

Cortical reaction

Answer 9

Cortical granules, in the cortex of the egg cytoplasm, lifts the vitelline layer away from the egg and hardens the layer into a protective fertilization envelope. Additional enzymes clip off and release the external portions of the remaining receptor proteins, along with any attached sperm.

Question 10

Formation of the fertilization envelope requires a high concentration of Ca 2+ in the egg. Why?

Answer 10

First, the binding of the sperm to the egg activates a signal transduction pathway that triggers release of Ca2+ into the cytosol from the endoplasmic reticulum. The resulting increase in Ca2+ levels causes cortical granules to fuse with the plasma membrane.

Question 11

Fertilization initiates and speeds up metabolic reactions that trigger the onset of embryonic development, “activating” the egg. There is, for example, a marked increase in the rates of cellular respiration and protein synthesis in the egg following fertilization.

Answer 11

True

Question 12

What marks the end of the fertilization stage?

Answer 12

The first cell division

Question 13

Zona pellucida

Answer 13

The extracellular matrix surrounding a mammalian egg.

Question 14

No fast block to polyspermy has been identified in mammals.

Answer 14

True

Question 15

Overall, the process of fertilization is much slower in mammals than in sea urchins: the first cell division occurs within 12-36 hours after sperm binding in mammals, compared with about 90 minutes in sea urchins. This cell division marks the end of fertilization and the beginning of the next stage of development, cleavage.

Answer 15

True

Question 16

Cleavage

Answer 16

The succession of rapid cell divisions without significant growth during early embryonic development that converts the zygote to a ball of cells.

Question 17

Blastomeres

Answer 17

An early embryonic cell arising during the cleavage stage of an early embryo.

Question 18

Blastula

Answer 18

A hollow ball of cells that marks the end of the cleavage stage during early embryonic development in animals. Consists of the first five to seven cleavage divisions.

Question 19

Blastocoel

Answer 19

The fluid-filled cavity that forms in the center of a blastula.

Question 20

Yolk

Answer 20

Nutrients stored in an egg.

Question 21

In frogs (and many other animals), cleavage is asymmetric, reflecting the asymmetric distribution of yolk across the egg.

Answer 21

True

Question 22

Vegetal pole

Answer 22

The point at the end of an egg in the hemisphere where most yolk is concentrated; the opposite of animal pole.

Question 23

Animal pole

Answer 23

The point at the end of an egg in the hemisphere where the least yolk is concentrated; opposite of vegetal pole.

Question 24

Cleavage furrow

Answer 24

An indentation that forms in the cell surface as cytokinesis divides the cell in half.

Question 25

Holoblastic

Answer 25

Referring to a type of cleavage in which there is complete division of the egg; occurs in eggs that have little yolk or a moderate amount of yolk (such as those of the frog)

Question 26

Meroblastic

Answer 26

Referring to a type of cleavage in which there is incomplete division of a yolk-rich egg, characteristic of avian development.

Question 27

Morphogensis

Answer 27

The development of the form of an organism and its structures.

Question 28

Gastrulation

Answer 28

In animal development, a series of cell and tissue movements in which the blastula-stage embryo folds inward, producing a three-layered embryo, the gastrula.

Question 29

Organogenesis

Answer 29

The formation of organs.

Question 30

Gastrula

Answer 30

An embryonic stage in animal development encompassing the formation of three layers: ectoderm, mesodermal, and endoderm.

Question 31

The cell layers produced in gastrulation are collectively called the ______________________.

Answer 31

Embryonic germ layers

Question 32

Ectoderm

Answer 32

The outermost of the three primary germ layers in animal embryos; gives rise to the outer covering and, in some phyla, the nervous system, inner ear, and lens of the eye.

Question 33

Endoderm

Answer 33

The innermost of the three primary germ layers in animal embryos; lines the archenteron and gives rise to the liver, pancreas, lungs, and the lining of the digestive tract in species that have these structures.

Question 34

Diploblasts

Answer 34

Are organisms that only develop an ectoderm and an endoderm during gastrulation. -Include cnidarians

Question 35

Mesoderm

Answer 35

The middle primary germ layer in a triphoblastic animal embryo; develops into the notochord, the lining of the coelom, muscles, skeleton, gonads, kidneys, and most of the circulatory system in species that have these structures.

Question 36

Invagination

Answer 36

The infolding of a sheet of cells into the embryo.

Question 37

Archenteron

Answer 37

The endoderm-lined cavity, formed during gastrulation, that develops into the digestive tract of an animal.

Question 38

Blastospore

Answer 38

In a gastrula, the opening of the archenteron that typically develops into the anus in deuterostomes and the mouth in protostomes.

Question 39

Deuterostomes

Answer 39

Are animals in which the mouth develops from the second opening of the embryo, not the blastospore.

Question 40

Protostomes

Answer 40

Are animals in which the mouth develops from the first opening formed during gastrulation.

Question 41

Dorsal lip

Answer 41

The region above the blastospore on the dorsal side of the amphibian embryo.

Question 42

Primitive streak

Answer 42

A thickening along the future posterior-anterior axis on the surface of an early avian or mammalian embryo, caused by a piling up of cells as they congregate at the midline before moving into the embryo.

Question 43

The starting point for gastrulation in chicks is an embryo consisting of upper and lower layers— known as the ____________________—lying atop a yolk mass

Answer 43

Epiblast and hypoblast

Question 44

All the cells that will form the chick embryo come from the __________

Answer 44

Epiblast

Question 45

Blastocyst

Answer 45

The blastula stage of mammalian embryonic development, consisting of an inner cell mass, a cavity, and an outer layer, the trophoblast. In humans, blastocyst forms 1 week after fertilization.

Question 46

Inner cell mass

Answer 46

An inner cluster of cells at one end of a mammalian blastocyst that subsequently develops into the embryo proper and some of the extraembryonic membranes.

Question 47

Trophoblast

Answer 47

The outer epithelium of the blastocyst

Question 48

Embryo Implantation

Answer 48

The stage in embryo development when the fertilized egg attaches to the uterine wall.

Question 49

Embryo implantation is initiated by _______________. How?

Answer 49

The trophoblast secretes enzymes during implantation which breaks down molecules of the endometrium, allowing invasion by the blastocyst.

Question 50

Around the time the embryo undergoes implantation, the inner cell mass of the blastocyst…

Answer 50

Forms a flat disk with an inner layer of cells, the epiblast, and an outer layer, the hypoblast.

Question 51

As is true for a bird embryo, the human embryo develops almost entirely from epiblast cells.

Answer 51

True

Question 52

Extraembryonic membrane

Answer 52

One of four membranes located outside the embryo that support the developing embryo in reptiles and mammals.

Question 53

Four embryonic membranes

Answer 53

Yolk sac Amnion Chorion Allantois

Question 54

As implantation is completed, gastrulation begins.

Answer 54

True

Question 55

Cells that contribute to the formation of the placenta

Answer 55

Cells of the invading trophoblast, the epiblast, and the adjacent endometrial tissue all contribute to the formation of the placenta.

Question 56

Amniote

Answer 56

A member of a clade of tetrapods named for a key derived character, the amniotic egg, which contains specialized membranes, including the fluid-filled amnion, that protects the embryo. Amniotes include mammals as well as birds and other reptiles.