1. Ch. 42 - Animal Development

2. I. Fertilization B. Provides stimulus for egg to begin development
A. Union of sperm & egg to form diploid zygote
B. Provides stimulus for egg to begin development
C. Egg
1. Has 3 barriers to entry of sperm
a. Corona radiata - follicle cells
b. Zona pellucida - extracellular matrix
c. Plasma membrane
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3. Fertilization

4. 1. Shape adapted to swimming 2. Flagellum for movement
D. Sperm
1. Shape adapted to swimming
2. Flagellum for movement
3. Head contains:
a. Haploid nucleus
b. Acrosome
 Membrane enclosed-sac that contains enzymes that help it penetrate egg
4. Has middle piece with many mitochondria to help move flagellum
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5. 1. Sperm squeeze through corona radiata
E. Sperm Entry into Egg
1. Sperm squeeze through corona radiata
2. Species-specific proteins on sperm bind with receptors on zona pellucida
3. Sperm acrosome releases digestive enzymes
4. Sperm binds to egg plasma membrane
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6. 5. This triggers changes in egg:
a. Membrane depolarizes (fast block); becomes impermeable to other sperm. This prevents polyspermy
b. Vesicles in egg, cortical granules, release enzymes that turn zona pellucida into an impenetrable fertilization membrane (slow block)
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7. 6. Formation of diploid zygote:
a. Microvilli extending from egg plasma membrane bring entire sperm into the egg
b. Nucleus releases its chromatin & forms it into a sperm pronucleus
c. Egg chromosomes are in egg pronucleus
d. Nuclear envelope envelopes both pronuclei.
e. NOW there is a fertilized egg, or a zygote.
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8. II. Cleavage B. Produces a cluster of cells still size of original egg
A. Rapid series of mitotic divisions
B. Produces a cluster of cells still size of original egg
a. This is called a
morula
C. Nuclei cycle rapidly between DNA replication, mitosis & cytokinesis
1. Transcription does not occur
a. Early mRNA comes from egg
2. Little protein synthesis occurs
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9. III. Blastulation
A. Newly formed cells (blastomere) pump sodium ions into extracellular spaces
1. Water follows by osmosis
2. Cells become arranged in a sphere surrounding a fluid-filled cavity called a blastocoel.
3. The embryo is now called a blastula.
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10. Lancelet Early Development (little yolk)

11. B. Blastulation Depends on Yolk
1. Egg with Moderate Yolk (Frogs)
a. Cleavage is not equal because of the presence of yolk, a dense nutrient material. This affects cleavage.
b. When yolk is present, the zygote & embryo exhibit polarity. The animal has two poles:
 Animal pole - small cells - gray
 Vegetal pole - larger cells due to presence of more yolk - yellow
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12. Sea Urchin & Frog Cleavage
Little Yolk Moderate Yolk

13. 2. Egg with Much Yolk (Birds)
a. Yolk-filled eggs don’t particpate in cleavage
b. The blastula is a layer of cells that spreads out over the yolk.
c. The blastocoel is a space that separates these cells from the yolk.

14. IV.Gastrulation A. Sorts cells into distinct layers
1. Ectoderm - outer layer
a. Epidermis (skin); sense organs; eye; nervous system
2. Endoderm - inner layer
a. Lining of digestive & other major systems; some endocrine glands
3. Mesoderm - middle layer
a. Notochord; skeleton; muscles, circulatory system; reproductive system; dermis
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15. B. Steps of Gastrulation:
1. Begins when small groove appears on one side of blastula
2. Invagination begins where cells are somewhat smaller
3. Continues until there is a second layer of cells inside; almost destroying the blastocoel.
4. Early gastrula has two layers: ectoderm & endoderm
a. Endoderm borders a new cavity called the archenteron
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16. Gastrulation
Blastocoel
Archenteron
Endoderm
Ectoderm

17. a. Ectoderm covers all of surface except for the yolk plug
4. Gastrulation is not complete until all three germ layers are produced.
a. Ectoderm covers all of surface except for the yolk plug
b. Endoderm surrounds the new cavity, the archenteron, which will become the digestive tract
c. Mesoderm has now formed in between these two layers
5. The opening of the cavity to the outside is the blastopore
a. This will become the anus in human
6. The embryo is now called a gastrula
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18. Comparative Development of Mesoderm
Mesoderm forms by outpocketing of archenteron
Mesoderm forms as cells migrate between ectoderm & endoderm
Mesoderm forms by invagination of cells between upper ectoderm & lower endoderm

19. V. Organ Formation
A. Tissues & organs must appear quickly (bones, heart, muscles)
B. After gastrulation, a structure called the notochord appears
1. It forms from mesoderm just above the archenteron
2. It is made of cartilage
3. It extends the length of the embryo
4. Provides support for other developing tissues
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20. Early Frog Embryo

21. a. This is a thickened region of the ectoderm above the notochord
C. Neurulation
1. The neural plate forms
a. This is a thickened region of the ectoderm above the notochord
2. From it arises a pair of ridges called neural folds
3. These folds roll up and form a tube which sinks below the surface
a. This new tube is called the neural tube
b. It will become the brain and spinal cord
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22. Development of Neural Tube in a Frog Embryo

23. Development of Neural Tube and Coelom in a Frog Embryo

24. D. Other Organ Formation
1. Somites appear
a. These are blocks of mesoderm that will give rise to vertebrae & muscles of backbone
2. The notochord will eventually be replaced by the backbone which will surround the spinal cord
3. Limb buds begin to form as well as a tail
4. The eyes begin to appear
5. A hollow space, called the coelom,
forms. This will be the body cavity.
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25. Vertebrate Embryo, Cross Section

26. VI. Developmental Processes
A. Development requires:
1. Growth
2. Cellular differentiation
a. When cells become specialized
3. Morphogenesis
a. Produces shape & form of body
b. Includes pattern formation –
♣How tissues & organs are arranged in body
♣Apoptosis – cell death
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27. a. Used to destroy cells between developing fingers and toes in humans
B. Apoptosis
1. Animals have suicide genes that code for proteins that will kill the cell that produces them
a. Used to destroy cells between developing fingers and toes in humans
b. Also used in development of nervous & immune systems
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28. C. Embryonic Polarity (frogs)
1. Dorsal-ventral
a. Determined by yolk
2. Site of sperm fusion (ANTERIOR)
a. Crescent shaped gray area forms on egg opposite of sperm entry. Called the gray crescent.
● First cleavage should pass through this dividing it equally
♣ If it does and then the cells are separated, both will develop into tadpoles
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29. Polarity of Embryos

30. ● If first cleavage does NOT pass through crescent:
 the cell without it will not grow
3. Totipotent
a. When first 2 cells have the full potential to develop normally
b. Occurs in sea stars & most vertebrates
♣ Works up to 8-cell stage in mammals
c. In mollusks & segmented worms after 3rd cleavage, not totipotent
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31. Polarity of Embryos

32. VII. Embryonic Induction
A. Process by which one embryonic tissue can influence the development of another
1. Frog gastrula
a. Gray crescent becomes dorsal lip of blastopore
b. Moves inward to form mesoderm in roof of archenteron
c. Detaches & forms notochord
d. Induces ectoderm lying above to fold inward & form the nervous system
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33. 2. Hans Spemann’s experiment
a. Transplanted dorsal lip from normal position in blastula of a salamander to the belly-region of another blastula
 Gastrulation occurs in 2 places
 2 nervous systems formed
 2 nearly complete embryos were created; joined ventrally
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34. Hans Spemann’s experiment

35. Hans Spemann’s experimental results

36. 3. Lens Induction
a. Eyes form as lateral outpockets (optic vesicles) from brain tissue
 When optic vesicles contact epidermis on head, epidermal cells form a thick plate of cells
 Plate of cells sinks inward
 Plate later detaches & becomes a lens
 Optic vesicle transforms into the optic cup, will eventually becomes the retina
 Lens induces formation of cornea
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37. Eye - Lens Induction

38. b. Warren Lewis’ experiment:
 Removed optic vesicle from head of an embryo & transplanted it to the trunk
 Vesicle grew and developed
 It induced the formation of a lens in the trunk
 No lens appeared in the head
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39. VIII. Human Embryonic Development
A. Pregnancy = gestation
1. Time from implantation to birth (parturition) is 266 days (approximately 9 months)
2. Divided into two periods:
a. Embryonic development
First 2 months (8-9 weeks)
Major organs are formed
b. Fetal development
Months 3 through 9
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40. B. Extraembryonic Membranes in Humans
1. Develop from extensions of the germ layers
a. Chorion
Forms fetal half of placenta
b. Amnion
Sac around embryo & fetus; contains fluid to cushion & protect
c. Allantois
Blood vessels become umbilical cord
d. Yolk sac
Lacks yolk; first site of blood formation
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41. Extraembryonic Membranes
Collects nitrogen wastes
Contains amniotic fluid to bathe & cushion embryo
Provides nourishment
Carries on gas exchange

42. Human Development Before Implantation
Fertilization occurs in upper 1/3 of oviduct
Trophoblast begins to secrete HCG which helps to maintain corpus luteum & the endometrium
~ Day 5: blastocyst formed. The outer trophoblast will help embryo to implant in uterus. Inner cell mass becomes embryo.

43. Human Embryonic Development

44. Human Embryo at Fifth Week

45. Human Embryo at Eight Weeks
By the 8th week of development, the embryo is recognizable as human. About 38 mm long at this point.

46. Anatomy of Placenta
Human placenta functions in gas, nutrient & waste exchange. It is fully formed by the 10th week of development.
It then starts to produce estrogen & progesterone which help to maintain pregnancy.
Maternal & fetal blood do not mix. Just an exchange of materials occurs. Harmful chemicals in mom can enter the fetus.
Umbilical cord stretches between placenta & the fetus.

47. Fetal Development By end of third month, genitals appear.
By about six months a fine, downy hair called lanugo covers the fetus. Skin is wrinkled making fetus look “old”.
By 24 weeks (~6 months) the fetus has a chance of surviving if born early.
Dramatic weight gain during the last three months increases chances of survival.

48. Stages of Birth
When fetal brain is sufficiently mature, the hypothalamus starts a series of events that lead to the production of oxytocin.
This helps the uterus to contract and expel the fetus.
Birth involves three stages:
Cervix dilates & the amnion ruptures
Baby is born
Placenta is delivered

49. Stages of Birth