1. Chapter 38 Reproduction and Development (Sections 38.9 - 38.11)

2. 38.9 Overview of Animal Development
All sexually reproducing animals begin life as a zygote, the diploid cell that forms at fertilization
The same development steps and processes occur in all vertebrates – evidence of their common ancestry

3. 5 Stages of Vertebrate Development
Fertilization
Sperm penetrates an egg, the egg and sperm nuclei fuse, and a zygote forms
Cleavage
Mitotic cell divisions yield a ball of cells (blastula); each cell gets a different bit of the egg cytoplasm
Gastrulation
Cell rearrangements and migrations form a gastrula, an early embryo that has primary tissue layers

4. 5 Stages of Vertebrate Development
Organ formation
Organs form as the result of tissue interactions that cause cells to move, change shape, and commit suicide
Growth and tissue specialization
Organs grow in size, take on mature form, and gradually assume specialized functions

5. Overview: Frog Development

6. external fertilization)
transformation to
adult nearly complete
adult, three
years old
Sexual reproduction
(gamete formation,
external fertilization)
Figure Above, overview of reproduction and development in the leopard frog. Opposite, a closer look at some stages.
organ formation
tadpole
cleavage
eggs and sperm
larva (tadpole)
zygote
Fig , p. 642

7. Overview: Frog Development
adult, three years old
transformation to adult nearly complete
Sexual reproduction (gamete formation, external fertilization)
eggs and sperm
tadpole
larva (tadpole)
Figure Above, overview of reproduction and development in the leopard frog. Opposite, a closer look at some stages.
organ formation
cleavage
zygote
Stepped Art
Fig , p. 642

8. Details of Frog Development (1)
Cleavage divides a zygote’s cytoplasm into smaller blastomeres
Number of cells increases, but the zygote’s original volume remains unchanged
cleavage
Mitotic division of an animal cell

9. Details of Frog Development (1)

10. Details of Frog Development (1)
gray crescent
Figure Above, overview of reproduction and development in the leopard frog. Opposite, a closer look at some stages.
Here we show the first three divisions of cleavage, a process that carves up a zygote’s cytoplasm. In this species, cleavage results in a blastula, a ball of cells with a fluid-filled cavity. 1
Fig , p. 643

11. Details of Frog Development (2)
In this species, cleavage results in a blastula, a ball of cells with a fluid-filled cavity (blastocoel)
Tight junctions hold cells of the blastula together
blastula
Hollow ball of cells that forms as a result of cleavage

12. Details of Frog Development (2)

13. Details of Frog Development (2)
blastocoel
Figure Above, overview of reproduction and development in the leopard frog. Opposite, a closer look at some stages.
blastula
Cleavage is over when the blastula forms. 2
Fig , p. 643

14. Details of Frog Development (3)
The blastula becomes a three-layered gastrula by the process of gastrulation: Cells at the dorsal lip migrate inward and start rearranging themselves
gastrula
Three-layered developmental stage formed by gastrulation
gastrulation
Cell movements that produce a three-layered gastrula

15. Germ Layers
A gastrula consists of three primary tissue layers (germ layers)
Three germ layers give rise to the same types of tissues and organs in all vertebrates – evidence of a shared ancestry
germ layer
One of three primary layers in an early embryo

16. Three Embryonic Germ Layers
ectoderm
Outermost tissue layer of an animal embryo
endoderm
Innermost tissue layer of an animal embryo
mesoderm
Middle tissue layer of a three-layered animal embryo

17. Details of Frog Development (3)

18. Details of Frog Development (3)
ectoderm
ectoderm
yolk plug
neural plate
dorsal lip
mesoderm
future gut cavity
endoderm
Figure Above, overview of reproduction and development in the leopard frog. Opposite, a closer look at some stages.
The blastula becomes a three-layered gastrula—a process called gastrulation.
At the dorsal lip (a fold of ectoderm above the first opening that appears in the blastula) cells migrate inward and start rearranging themselves. 3
Fig , p. 643

19. Details of Frog Development (4)
Organs begin to form as a primitive gut cavity opens up
A neural tube, then a notochord and other organs, form from the primary tissue layers
Many organs incorporate tissues derived from more than one germ layer

20. Details of Frog Development (4)

21. Details of Frog Development (4)
neural tube
notochord
Figure Above, overview of reproduction and development in the leopard frog. Opposite, a closer look at some stages.
gut cavity
Organs begin to form as a primitive gut cavity opens up. A neural tube, then a notochord and other organs, form from the primary tissue layers. 4
Fig , p. 643

22. Details of Frog Development (5)
In frogs, and some other animals, a larva undergoes metamorphosis: a remodeling of tissues into an adult form
The tadpole is a swimming larva with segmented muscles and notochord extending into a tail
During metamorphosis, the frog grows limbs, and the tadpole tail is absorbed

23. Details of Frog Development (5)
Tadpole
Metamorphosis
Sexually mature, four-legged adult frog
Figure Above, overview of reproduction and development in the leopard frog. Opposite, a closer look at some stages.

24. ANIMATION: Leopard frog life cycle
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25. 38.10 Early Marching Orders
Egg cytoplasm includes yolk proteins, mRNA transcripts, tRNAs and ribosomes, and other proteins
Some cytoplasmic components are not distributed evenly, but localized in one particular region or another
cytoplasmic localization
Accumulation of different materials in different regions of the egg cytoplasm

26. Cytoplasmic Localization
In a yolk-rich egg, the vegetal pole has most of the yolk and the animal pole has little
In some amphibian eggs, pigment molecules accumulate in the cell cortex, close to the animal pole
After fertilization, a gray crescent forms, where substances essential to development are localized

27. Experiment: Cytoplasmic Localization
At fertilization, cytoplasm shifts, and exposes a gray crescent opposite the sperm’s entry point
First cleavage normally distributes half of the gray crescent to each descendant cell

28. Experiment: Cytoplasmic Localization
animal pole pigmented
cortex
yolk-rich cytoplasm
vegetal pole
sperm penetrating egg
gray
crescent
Figure Experimental evidence of cytoplasmic localization in an amphibian oocyte.
fertilized egg
A Many amphibian eggs have a dark pigment concentrated in cytoplasm near the animal pole. At fertilization, the cytoplasm shifts, and exposes a gray crescent-shaped region just opposite the sperm’s entry point. The first cleavage normally distributes half of the gray crescent to each descendant cell.
Fig a, p. 644

29. ANIMATION: Cytoplasmic localization
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30. Experiment: Cytoplasmic Localization
In one experiment, the first two cells formed by normal cleavage were physically separated from each other
Each cell developed into a normal larva

31. Experiment: Cytoplasmic Localization
gray crescent of salamander zygote
First cleavage plane; gray crescent split equally. The blastomeres are separated experimentally.
Figure Experimental evidence of cytoplasmic localization in an amphibian oocyte.
Two normal larvae develop from the two blastomeres.
B In one experiment, the first two cells formed by normal cleavage were physi-cally separated from each other. Each cell developed into a normal larva.
Fig b, p. 644

32. Experiment: Cytoplasmic Localization
In another experiment, one descendant cell received all the gray crescent, and developed normally
The other gave rise to an undifferentiated ball of cells

33. Experiment: Cytoplasmic Localization
gray crescent of salamander zygote
First cleavage plane; gray crescent missed entirely. The blastomeres
are separated experimentally.
Figure Experimental evidence of cytoplasmic localization in an amphibian oocyte.
A ball of undifferentiated cells forms.
Only one normal larva develops.
C In another experiment, a zygote was manipulated so one descendant cell received all the gray crescent. This cell developed normally. The other gave rise to an undifferentiated ball of cells.
Fig c, p. 644

34. Cleavage: The Start of Multicellularity
During cleavage, a furrow appears on the cell surface and defines the plane of the cut
The plane of division is not random – it dictates what types and proportions of materials a blastomere will get
Each species has a characteristic cleavage pattern

35. From Blastula to Gastrula
At gastrulation, certain cells at the embryo’s surface move inward through an opening on the surface
Cells in the dorsal (upper) lip of the opening are descended from a zygote’s gray crescent
Gastrulation is caused by signals from dorsal lip cells

36. Gastrulation in a Fruit Fly
The opening cells move in through will become the fly’s mouth; descendants of stained cells will form mesoderm

37. Gastrulation in a Fruit Fly
Figure Gastrulation in a fruit fly (Drosophila). In fruit flies, cleavage is restricted to the outermost region of cytoplasm; the interior is filled with yolk. The series of photographs, all cross-sections, shows sixteen cells (stained gold) migrating inward. The opening the cells move in through will become the fly’s mouth. Descendants of the stained cells will form mesoderm. Movements shown in these photos occur during a period of less than 20 minutes.
Fig a, p. 645

38. Gastrulation in a Fruit Fly
Figure Gastrulation in a fruit fly (Drosophila). In fruit flies, cleavage is restricted to the outermost region of cytoplasm; the interior is filled with yolk. The series of photographs, all cross-sections, shows sixteen cells (stained gold) migrating inward. The opening the cells move in through will become the fly’s mouth. Descendants of the stained cells will form mesoderm. Movements shown in these photos occur during a period of less than 20 minutes.
Fig b, p. 645

39. Gastrulation in a Fruit Fly
Figure Gastrulation in a fruit fly (Drosophila). In fruit flies, cleavage is restricted to the outermost region of cytoplasm; the interior is filled with yolk. The series of photographs, all cross-sections, shows sixteen cells (stained gold) migrating inward. The opening the cells move in through will become the fly’s mouth. Descendants of the stained cells will form mesoderm. Movements shown in these photos occur during a period of less than 20 minutes.
Fig c, p. 645

40. Gastrulation in a Fruit Fly
Figure Gastrulation in a fruit fly (Drosophila). In fruit flies, cleavage is restricted to the outermost region of cytoplasm; the interior is filled with yolk. The series of photographs, all cross-sections, shows sixteen cells (stained gold) migrating inward. The opening the cells move in through will become the fly’s mouth. Descendants of the stained cells will form mesoderm. Movements shown in these photos occur during a period of less than 20 minutes.
Fig d, p. 645

41. Experiment: Dorsal Lip Transplant
Dorsal lip of a salamander embryo was transplanted to a different site in another embryo – a second set of body parts started to form

42. Experiment: Dorsal Lip Transplant
Figure Experimental evidence that signals from dorsal lip cells start amphibian gastrulation. A dorsal lip region of a salamander embryo was transplanted to a different site in another embryo. A second set of body parts started to form.
A Dorsal lip excised from donor embryo, grafted to novel site in another embryo.
Fig a, p. 645

43. Experiment: Dorsal Lip Transplant
Figure Experimental evidence that signals from dorsal lip cells start amphibian gastrulation. A dorsal lip region of a salamander embryo was transplanted to a different site in another embryo. A second set of body parts started to form.
B Graft induces a second site of inward migration.
Fig b, p. 645

44. Dorsal Lip Transplant (cont.)
The embryo develops into a “double” larva, with two heads, two tails, and two bodies joined at the belly

45. Dorsal Lip Transplant (cont.)
C The embryo develops into a “double” larva, with two heads, two tails, and two bodies joined at the belly.
Figure Experimental evidence that signals from dorsal lip cells start amphibian gastrulation. A dorsal lip region of a salamander embryo was transplanted to a different site in another embryo. A second set of body parts started to form.
Fig c, p. 645

46. ANIMATION: Embryonic induction
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47. Specialized Cells, Tissues, and Organs
All cells in an embryo have the same genes
Selective gene expression causes different cell lineages in the embryo to express different subsets of genes
Selective gene expression is the key to cell differentiation – the process by which cell lineages become specialized in composition, structure, and function

48. Cell Differentiation
An adult human has about 200 differentiated cell types
Example: Cells of one lineage turn on genes for crystallin, a transparent protein that forms the lens of the eye – no other cells in the body make crystallin
A differentiated cell still retains the entire genome
It is possible to clone an adult animal (a genetic copy) from one of its differentiated cells

49. Cell Communication in Development
Long-range intercellular signals (morphogens) diffuse out from certain embryonic cells and form a concentration gradient in the embryo that controls differentiation
morphogen
Chemical encoded by a master gene; diffuses out from its source and affects development
Effects on target cells depend on its concentration

50. Cell Communication in Development
Other chemical signals only operate at close range
Example: Cells of a salamander gastrula’s dorsal lip cause adjacent cells to migrate inward and become mesoderm
embryonic induction
Embryonic cells produce signals that alter the behavior of neighboring cells

51. Cell Movements and Apoptosis
Long-range and short-range signals regulate development of tissues and organs
Organs begin to form as cells migrate, entire sheets of tissue fold and bend, and specific cells die on cue

52. Cell Movements in the Brain
Neurons form in the center of the brain, then creep along extensions of glial cells or axons of other neurons until they reach their final position
Once in place, they send out axons

53. Cell Movements in the Brain
Figure How body takes shape.
A Cells migrate. This graphic shows one embryonic neuron (orange) at successive times as it migrates along a glial cell (yellow). Its adhesion proteins stick to glial cell proteins.
Fig a, p. 646

54. Neural Tube Formation
Sheets of cells expand and fold to form the neural tube
Gastrulation produces a sheet of ectodermal cells
Cells at the embryo’s midline elongate and neighboring cells become wedge-shaped, forming a neural groove
Edges of the groove move inward, and flaps of tissue fold and meet at the midline, forming the neural tube
The neural tube later develops into the brain and spinal cord

55. Neural Tube Formation

56. Neural Tube Formation
Gastrulation produces a sheet of ectodermal cells. 1
A neural groove forms as microtubules constrict or lengthen in different cells, making the cells change shape.
neural groove 2
Edges of the groove meet and detach from the main sheet, forming the neural tube. 3
Figure How body takes shape.
neural tube
B Cells change shape. Here, shape changes in ectodermal cells form a neural tube.
Fig b, p. 646

57. Apoptosis
Signals from certain cells activate self-destruction in target cells (apoptosis), which helps sculpt body parts
Apoptosis causes a tadpole to lose its tail and it separates the digits of the developing human hand
apoptosis
Mechanism of cell suicide

58. Apoptosis
As a human hand develops, cells in the webs of skin between digits die

59. ANIMATION: Formation of human fingers
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60. Pattern Formation
Pattern formation is the process by which certain body parts form in a specific place
Example: Signals from AER (apical ectodermal ridge) at the tips of limb buds induce mesoderm beneath it to form a limb
pattern formation
Formation of body parts in specific locations

61. Experiment: AER and Limb Formation
AER of a limb bud tells mesoderm under it to form a limb
If AER from a chick’s wing bud is removed, wing development stops
Earlier positional cues determined what the mesoderm will become – mesoderm from a chick’s hindlimb implanted under wing AER forms a leg

62. Experiment: AER and Limb Formation

63. Experiment: AER and Limb Formation
mesoderm of chick embryo forelimb
A Experiment 1: Remove wing bud’s AER
AER removed
no limb forms
B Experiment 2: Graft a bit of leg mesoderm under the AER of a wing
mesoderm from leg
wing
AER (region of signal-sending ectoderm)
leg forms
Figure Experiments demonstrating interactions between AER (ectoderm) and mesoderm in chick wing development. AER at the tip of a limb bud tells mesoderm under it to form a limb. Whether that limb becomes a wing or a leg depends on positional signals that the mesoderm received earlier.
Fig , p. 647

64. Experiment: AER and Limb Formation
mesoderm of chick embryo forelimb
AER (region of signal-sending ectoderm)
A Experiment 1: Remove wing bud’s AER
AER removed
no limb forms
B Experiment 2: Graft a bit of leg mesoderm under the AER of a wing
mesoderm from leg
leg forms
wing
Figure Experiments demonstrating interactions between AER (ectoderm) and mesoderm in chick wing development. AER at the tip of a limb bud tells mesoderm under it to form a limb. Whether that limb becomes a wing or a leg depends on positional signals that the mesoderm received earlier.
Stepped Art
Fig , p. 647

65. Evolution and Development
Where and when particular genes are expressed determines how an animal body develops:
Localized molecules in an unfertilized egg induce expression of master genes in the zygote
Products of master genes form gradients in the embryo
Depending on where they fall within these gradients, cells activate or suppress other genes

66. Evolution and Development (cont.)
Positional information set up by concentration gradients of products of master genes affects expression of homeotic genes, which regulate development of specific body parts
All animals have similar homeotic genes
homeotic gene
Type of master gene; its expression controls formation of specific body parts during development

67. Evolution and Development (cont.)
Evolution of body plans are influenced by physical constraints (such as surface-to-volume ratio) and existing body framework (such as four limbs)
Interactions among master genes also restrain evolution, since a major change in any one probably would be lethal
Mutations led to a variety of forms among animal lineages by modifying existing developmental pathways, rather than entirely new genetic innovations

68. Key Concepts Principles of Development
The same processes regulate development of all animals
Division of the single-celled zygote distributes different materials to different cells
These cells go on to express different master genes that regulate formation of body parts in particular places

69. ANIMATION: Early frog development
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