Development
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Big Questions:
1. How does a multicellular organism
2.
3.
4.
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develop from a zygote?
How is the position of the parts of an
organism determined?
How does differentiation of cell type
occur in organisms?
How are genetics and development
connected? Environment?
Development is Regulated
 The development of

an organism is
coordinated by
sequential changes in
gene expression.
Multicellular
organisms must
regulate:
Cell division
 Differentiation
 Morphogenesis

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Early Developmental Pathways
 Cytoplasmic
Determinants

unequal distribution of
mRNA and proteins
from mother
 Embryonic Induction:

Local signaling by
nearby cells send
cells down a specific
developmental path.
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Pattern Formation
 Cells need to establish their position in

the developing organism. (head/tail, right
/left etc.)
Cytoplasmic determinants and induction
lead to gradient morphogens.
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Cellular Differentiation
 Germ lines established
 Cell type already


determined
Differentiation
accomplished by the
expression of cell typespecific proteins.
There is normally no
turning back.
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Differentiation Control
 Genes must be both
turned on and turned off
at appropriate times and
locations.
 Regulated by internal
and external cues
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Internal Cues
 DNA packing, micro RNA’s and
epigenetics can switch genes ON or
OFF.
 Transcription factors must be present
and can be either stimulatory or
inhibitory.
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External Cues (outside the cell)
 External cues signal to cells, causing
cells to alter their gene expression.
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Environmental Cues
 The presence of particular molecules
and conditions in the local environment
is required for development to proceed
properly.

Ex. The role of temperature and
moisture in seed development.
Why do you water a seed
after you plant it? Hmm…
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Environmental Cues
 Changing the environment around the
cells can alter gene expression.
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Apoptosis
 “Programmed cell death”: Important
role in defining borders and openings
in the developing
organism.
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Homeobox (HOX) Genes
 A family of
related genes
that serve as
master
regulators of
animal
development
in all animals
on the planet.
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Experimental Evidence
 Mutations in normal developmental
pathways lead to malformations in
embryonic development
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Experimental Evidence
 Transplantation Experiments: Moving
regions of a developing embryo affects
normal pattern formation.
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