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Chapter 38 Reproduction and Development (Sections 38.9 - 38.11). 38.9 Overview of Animal Development. All sexually reproducing animals begin life as a zygote, the diploid cell that forms at fertilization - PowerPoint PPT Presentation
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Albia Dugger • Miami Dade College
Cecie StarrChristine EversLisa Starr
www.cengage.com/biology/starr
Chapter 38 Reproduction and Development
(Sections 38.9 - 38.11)
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
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
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
Overview: Frog Development
Fig. 38.13, p. 642
eggs and sperm
larva (tadpole)
adult, three years old
transformation to adult nearly complete
Sexual reproduction (gamete formation, external fertilization)
tadpole
zygote
organ formation cleavage
adult, three years old
transformation to adult nearly complete
tadpole
larva (tadpole)
Sexual reproduction (gamete formation, external fertilization)
eggs and sperm
organ formation cleavage
zygoteFig. 38.13, p. 642
Stepped Art
Overview: Frog Development
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
Details of Frog Development (1)
Fig. 38.13.1, p. 643
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.
gray crescent
1
Details of Frog Development (1)
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
Details of Frog Development (2)
Fig. 38.13.2, p. 643
Cleavage is over when the blastula forms.
blastocoel
blastula
2
Details of Frog Development (2)
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
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
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
Details of Frog Development (3)
Fig. 38.13.3, p. 643
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.
yolk plug
neural plate
ectodermdorsal lipfuture gut cavity
ectoderm
mesodermendoderm
3
Details of Frog Development (3)
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
Details of Frog Development (4)
Fig. 38.13.4, p. 643
neural tube
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.
gut cavity
notochord
4
Details of Frog Development (4)
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
Details of Frog Development (5)
Tadpole Sexually mature, four-legged adult frog
Metamorphosis
ANIMATION: Leopard frog life cycle
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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
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
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
Fig. 38.14a, p. 644
graycrescent
sperm penetrating egg
vegetal pole
yolk-rich cytoplasm
cortex
animal pole pigmented
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.
fertilized egg
Experiment: Cytoplasmic Localization
ANIMATION: Cytoplasmic localization
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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
Fig. 38.14b, p. 644
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.
Two normal larvae develop from the two blastomeres.
gray crescent of salamander zygote
First cleavage plane; gray crescent split equally. The blastomeres are separated experimentally.
Experiment: Cytoplasmic Localization
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
Fig. 38.14c, p. 644
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.
A ball of undifferentiated cells forms.
Only one normal larva develops.
First cleavage plane; gray crescent missed entirely. The blastomeres are separated experimentally.
gray crescent of salamander zygote
Experiment: Cytoplasmic Localization
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
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
Gastrulation in a Fruit Fly
• The opening cells move in through will become the fly’s mouth; descendants of stained cells will form mesoderm
Fig. 38.15a, p. 645
Gastrulation in a Fruit Fly
Fig. 38.15b, p. 645
Gastrulation in a Fruit Fly
Fig. 38.15c, p. 645
Gastrulation in a Fruit Fly
Fig. 38.15d, p. 645
Gastrulation in a Fruit Fly
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
Fig. 38.16a, p. 645
A Dorsal lip excised from donor embryo, grafted to novel site in another embryo.
Experiment: Dorsal Lip Transplant
Fig. 38.16b, p. 645
B Graft induces a second site of inward migration.
Experiment: Dorsal Lip Transplant
Dorsal Lip Transplant (cont.)• The embryo develops into a “double” larva, with two heads,
two tails, and two bodies joined at the belly
Fig. 38.16c, p. 645
C The embryo develops into a “double” larva, with two heads, two tails, and two bodies joined at the belly.
Dorsal Lip Transplant (cont.)
ANIMATION: Embryonic induction
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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
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
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
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
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
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
Fig. 38.17a, p. 646
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.
Cell Movements in the Brain
Neural Tube Formation
• Sheets of cells expand and fold to form the neural tube
1. Gastrulation produces a sheet of ectodermal cells
2. Cells at the embryo’s midline elongate and neighboring cells become wedge-shaped, forming a neural groove
3. 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
Neural Tube Formation
Fig. 38.17b, p. 646
Gastrulation produces a sheet of ectodermal cells.
A neural groove forms as microtubules constrict or lengthen in different cells, making the cells change shape.
Edges of the groove meet and detach from the main sheet, forming the neural tube.
neural tube
neural groove
B Cells change shape. Here, shape changes in ectodermal cells form a neural tube.
1
2
3
Neural Tube Formation
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
Apoptosis
• As a human hand develops, cells in the webs of skin between digits die
ANIMATION: Formation of human fingers
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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
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
Experiment: AER and Limb Formation
Fig. 38.18, p. 647
B Experiment 2: Graft a bit of leg mesoderm under the AER of a wing leg forms
mesoderm of chick embryo forelimb
A Experiment 1: Remove wing bud’s AER
AER removed
no limb forms
AER (region of signal-sending ectoderm)
mesoderm from leg
wing
Experiment: AER and Limb Formation
Fig. 38.18, p. 647
no limb forms
leg forms
wing
Stepped Art
A Experiment 1: Remove wing bud’s AER
AER removed
B Experiment 2: Graft a bit of leg mesoderm under the AER of a wing
mesoderm from leg
mesoderm of chick embryo forelimb
AER (region of signal-sending ectoderm)
Experiment: AER and Limb Formation
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
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
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
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
ANIMATION: Early frog development
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