Download Biology 211 Intro Molecular and Cell Biology

Biology 211 Intro Molecular and Cell Biology
Lectures 22 and 23 Animal Development
Reading: Campbell et al., Chap. 47 to p. 946
Outline:
1. Fertilization
2. Cleavage and early development
3. Gastrula
4. Organogenesis
Lecture:
In these final series of lectures, we will examine how cells are organized into
tissues and organs in the process of development.
Development: Transformation from a single cell to a multicellular organism.
Involves cell division and differentiation.
1. Fertilization: The union of egg and sperm to form a zygote, which goes on to
form a new individual.
Model organism: sea urchin (invertebrate animal)
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The "Acrosomal Reaction" is described in Fig. 47.2. This sequence of
events ensures that only one sperm fertilizes a single egg and that the
sperm is from the correct species.
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The "Cortical Reaction" is a series of changes in the outer zone of the
egg cytoplasm. The fusion of sperm and egg triggers a pathway that
leads to the egg's release of calcium ions from the endoplasmic reticulum
into the cytosol. This reaction leads to changes in the membrane of the
egg (vitelline membrane) to produce the fertilization membrane that
resists entry of additional sperm.
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Activation of the egg. Calcium ion release and pH changes (increased
intracellular pH) leads to metabolic changes in the egg. This enables the
egg to begin making proteins.

Fusion of the egg and sperm nuclei creates the diploid nucleus of the
zygote. DNA synthesis begins, then the cell divides.
2. Cleavage and early development
Fertilization is followed by 3 stages that build the animal's body:
A. Cleavage creates a multicellular embryo (blastula) from the zygote
B. Gastrulation produces a three-layered embryo
C. During organogenesis, organs begin to develop
Cleavage--Series of mitotic cell divisions that partition the zygote into many
smaller cells.
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Most eggs (except mammalian eggs) have polarity.
Animal pole--forms head
Vegetal pole--where yolk is concentrated; forms tail
Cell division is assymetrical
Early events of development determined by asymmetric distribution of
molecules in egg and early embryo.
Figure 47.8 Cleavage in a frog embryo
Cleavage stages:

Zygote--fertilized egg

2-cell stage

4-cell stage

8-cell stage
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16-cell stage (morula--solid ball of cells)

64-cell stage (morula)

128-cell stage (blastula--hollow ball of cells)
3. Gastrula
Gastrulation--Rearrangement of cells of the blastula to form the three main cell
layers.
Three layers formed:
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Ectoderm: Forms outer layer of gastrula; become our nervous system
and the skin.

Endoderm: Lines the digestive tract; the lining of the digestive tract and
associated organs such as liver and pancreas come from this layer.

Mesoderm: Partly fills the space between the ectoderm and the
endoderm; kidney, heart, muscles, skeleton, and the inner layer of skin
(dermis) develop from this layer.
Observe process of gastrulation in sea urchin (Fig. 47.9) and frog embryo (Fig.
47.10).
Key elements of process:
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Inward migration (involution) of cells.
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Formation of primitive gut (archenteron), lined with endoderm).
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Outer cell layer forms ectoderm.
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Layer of cells between archenteron and ectoderm forms the mesoderm.
4. Organogenesis--formation of organs and organ systems.
First organs formed in chordates:

Neural tube--forms brain and spinal cord.
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Notochord--forms support structure for skeleton.
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Somites--repeating blocks of mesoderm that form muscles.
Figure 47.11 Frog early organogenesis.
How does organogenesis take place?
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Contact between cell layers "induces" structures to form.
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Molecules in these cells have been found to be the "inducers".