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Bio 103
Winter Quarter
Lake Tahoe Community College
Instructor: Sue Kloss
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Ch. 47 - Animal Development
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I. Principles of embryonic development
A. Fertilization - union of a haploid sperm and egg to form a diploid zygote
1. initiates metabolic (chemical) processes in the egg to begin embryonic development
2. Micrograph of sperm
3. structure of sperm follows function
B. Sequence of events in fertilization (Sea urchins)
1. sperm approaches
2. acrosomal enzymes digest the eggs jelly coat
3. proteins on head bind to receptors spanning vitelline layer in egg, preventing fertilization by wrong
species; esp important in external fertilization. Sperm proceeds thru vitelline layer
4. sperm plasma membrane fuses w/ egg plasma membrane
5. sperm nucleus enters the egg
C. Chemical changes occur as sperm enters the egg, some prevent entrance of other sperm (too many
chromosomes)
1. less than 1 sec. after 1st sperm enters, entire egg plasma membrane becomes impenetrable to
other sperm
2. vitelline layer hardens and becomes fertilization membrane, another impenetrable barrier to sperm
3. either contact or fusion of plasma membranes (depends on species) causes depolarization of egg’s
plasma membrane, allowing sodium ions to flow into egg and change membrane potential.
4. sperm binding appears to activate a signal transduction pathway causing calcium ions to be release
from egg’s ER into cytosol
D. Activation of the egg
1. sharp rise in Ca2++ causes significant increase in metabolism - cell respiration and protein synthesis
2. this happens without any input from sperm or egg nucleus, so proteins and mRNAs pesent in egg
are sufficient for egg activation
3. nucleus of sperm merges with nucleus of egg about 20 min. later, producing a diploid, 1 celled
zygote
E. Fertilization in mammals
1. generally internal
2. secretions in female’s repro tract enhance sperm motility after they have been in there 6 hrs.
3. sperm cell must migrate thru follicle cell
4. receptors in zona pellucida have complement molecule
5. acrosomal reaction
6. cortical reaction triggered, causing changes to zona pellucida
7. whole sperm comes in, tail and all
8. basal body of sperm is centriole
F. Cleavage produces a ball of cells from the zygote
1. animals are made up of thousands, to billions to trillions of cells
2. order and precision are required at every step.
3. cleavage is the first major phase in embryonic development
4. cleavage is a rapid succession of cell divisions that produces a ball of cells
5. DNA replication, mitosis and cytokinesis occur rapidly,
6. nutrients stored in egg nourish rapidly dividing cells
7. morula is lobed surface structure present after 5-7 divisions
8. as cleavage continues, fluid filled cavity called blastocoel form
9. In most animals, possibly excepting mammals, specific mRNAs, proteins and yolk are concentrated
10. cleavage - 2 imp. contributions
G. Gastrulation produces a 3 layered embryo
1. gastrulation is the second phase of embryonic development
2. adds more cells to the embryo
3. sorts cells into distinct layers called germ layers
4. transforms blastula from 2 layered structure into a 3 layered structure called the gastrula
5. ectoderm forms the outer layer (skin) of the gastrula
6. Endoderm forms embryonic digestive tract
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7. mesoderm fills the space between them
8. Eventually, these 3 cell layers form all the organs and parts of the adult animal
9. nervous system and epidermis (outer layer) of skin come from ectoderm
10. innermost digestive tract lining comes from endoderm
11. most other organs such as kidney, heart, muscles, inner skin layer come from mesoderm
12. gastrulation occurs somewhat differently in various animal species;
13. in sea urchins, individual cells from blastula wall enter blastocoel
14. in sea urchin, gastrula eventually becomes a ciliated larva
H. Organs start to form after gastrulation
1. gastrulation sets the stage for shaping an animal by organizing the embryo into 3 layers
2. unique to vertebrate embryos, neural crest develops along border of neural tube
3. condensations occur in strips of mesoderm lateral to notochord3
4. as organogenesis proceeds, morphogenesis and cell differentiation continue to refine the organs 4
5. 4 embryonic sources of major organs and tissues in many vertebrates
I. Development adaptations of amniotes
1. all vertebrate embryos require an aqueous environment for development
Ch 47 Objectives
1. List the two functions of fertilization.
2. Describe or draw the acrosomal reaction and explain how it ensures that appropriate gametes are
conspecific.
3. Describe or draw the cortical reaction.
4. Explain how the fast and slow blocks to polyspermy function sequentially to prevent multiple sperm from
fertilizing the egg.
5. Describe the changes that occur in an activated egg and explain the importance of cytoplasmic materials to
egg activation.
6. Compare fertilization in a sea urchin and in a mammal.
7. Describe the general process of cleavage.
8. Explain the importance of embryo polarity during cleavage. Compare the characteristics of the animal
hemisphere, vegetal hemisphere, and gray crescent in amphibian embryos.
9. Describe the formation of a blastula in sea urchin
10. Describe the process of gastrulation and explain its importance. Explain how this process rearranges the
embryo. List adult structures derived from each of the primary germ layers.
11. Describe gastrulation in a sea urchin.
12. Describe the formation of the notochord, neural tube, and somites in a frog.
13. Describe the significance and fate of neural crest cells. Explain why neural crest cells have been called a
“fourth germ layer.”
14. List and explain the functions of the extraembryonic membranes in reptile eggs.
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