Chapter 21- Development and Gene
Expression
Central questions:
1) How are cells in various locations of the body different -is
the variation in their genome or in the proteins they
express?
2) Can differentiated cells be coaxed to retrace their steps and
become de-differentiated?
3) How do cells become different from one another to form
different body parts when they all develop from ONE cell the zygote?
4) Are there difference or similarities in the way genes behave
during development between various species?
Types of stem
cells:
Embryonic
totipotent
Embryonic
pluripotent
Adult stem cell
(misnomer)
Embryonic
development—
single-celled zygotes
2 cell stage
4 cell stage
A cell upto the 8 cell
stage is an embryonic
stem cell - it is able to
differentiate into a full
organism - totipotent!
Blastula (hollow ball
with cells on the
outside). Humans - this
is called blastocyst
A cell from the
blastula/blastocyst is
also an embryonic stem
cell - it is able to
differentiate into many
types of cells but not a
full organism pluripotent!
Embryonic blastocyst stem cells
in animals are pluripotent
because epigenetic modifications
are minimal- that is = DNA
methylation/histone acetylation
(turning off its genes) is minimal
Embryonic development—
Blastula
Gastrula
Adults/babies (humans )
(Ectoderm, Mesoderm,
have adult stem cells Endoderm) in special places like the
Tissues
bone marrow which are
also pluripotent but to a
lesser degree than
embryonic stem cells!
Organs
Organ Systems
Once the cell is
committed to its fate
during gastrula
formation, they become
differentiated - they lose
their pluripotency
(genes turned off)!
Stem Cells In The Human Adult
Bone Marrow Cells – make
blood cells all through life
Brain Stem Cells – can make
neurons and glial cells
Skin stem cells –
keratinocytes, hair follicles,
epidermis
Are human stem cells
PLURIPOTENT? (-can
differentiate into multiple cell
types)
Yes, but to a limited extent
Stem Cells - another property
Stem Cells have
telomerase
(immortal) - capable
of self-renewal
Cell division (mitosis) of the zygote
increases the number of cells in an
organism.
Differentiation is when each cell becomes
specialized in both structure and function
(genes turned off by epigenetic processes).
Morphogenesis is when the eventual
shape (body plan) of the organism forms head-tail axes; top-down axes….
Plants
N/A
Animals
Movement
cells
andsome
tissue
If pluripotentof
- this
means
genes areto
stilltransform
off (epigenetics)!
needed
embryo
Continuous
Differentiation only during
differentiation, and
embryonic development and
morphogenesis
in some adult stem cells
throughout life
like bone marrow cells
Any cell in a plant can be Totipotent - only upto 8 cell
a stem cell at any stage - embryonic cell stage;
‘totipotent’. Meristems Pluripotent - blastocyst,
- regions of growth and bone marrow adult stem cells
differentiation.
(can only make certain type
of cells like blood cells)
Carrot cells are totipotent.
In plants, cells remain totipotent—
all genes can be activated, and any cell
can form any part of the organism
Review Questions:
Are there differences in gene
number or type between
undifferentiated (stem) cells
and differentiated
(mature/adult) cells?
Are there stem cells in an
adult human body? Where?
Can adult differentiated cells
be induced to make any (and
all) types of body cells retrace and become ‘embryo
like’?
NO! And Yes! And Yes!
Are there differences in genes between
different cells in the body?
No = Genomic Equivalence
Are there differences in genes expression
between undifferentiated (stem) cells and
differentiated (mature/adult) cells?
Yes. Genes get inactivated/activated
through processes like methylation
(epigenetics)
When cells differentiate, are genes
inactivated irreversibly? Can they retrace to
become de-differentiated?
Well, it depends on the organism! Dec 2007 scientists de-differentiated the human skin
Can adult skin cells retrace steps to become dedifferentiated?
Somatic Cell Nuclear transfer (SCNT)
1) Take egg cell and remove nucleus
(haploid) - throw it away
2) Take skin cell and remove nucleus
(diploid) - save this
3) Insert skin cell nucleus (somatic cell)
into egg cell cytoplasm. No need for
sperm! Why?
4) Allow egg cell to divide and become
blastocyst
5) Now you can extract stem cells
(THERAPEUTIC CLONING) OR carry
out REPRODUCTIVE CLONING implant blastocyst in a surrogate mom
and grow a clone!
Dolly and Bonny!
Reproductive
Cloning is an
offshoot of stem cell
research
SCNT made Dolly
the sheep!
-Mammary gland
cells from donor
arrested in G0
phase apparently
“dedifferentiated.”
Review: Dolly’s mitochondrial
DNA is from the
egg donor sheep.
Remember in cloning - an egg nucleus is
replaced with a nucleus of a differentiated
cell. Ability of differentiated nucleus to
support normal development is related to its
age - Dolly may have died prematurely and
developed arthritis at a young age!
(epigenetics controls this)
Therefore… in animals - review
 Nuclei change as cells differentiate
 The DNA sequence usually doesn’t change,
but chromatin structure may be altered
 Nuclear “potency” is restricted as cells
develop and become more differentiated.
Ques. 3) How do cells become different from one another to form
different body parts when they all develop from ONE cell - the
zygote?
How does a stem cell make a differentiated cell?
Determination & differentiation of muscle cells
What turns on the Master control gene?
Master control gene => codes for transcription factors =>
turned on (determination) => transcription factors => turs on
other genes=> more transcription factors => muscle protein
genes turned on => muscle protein (myosin) made => cell has
differentiated (These are INTERNAL SIGNALS)
Figure 21.9 Determination and differentiation of muscle cells (Layer 1)
Figure 21.9 Determination and differentiation of muscle cells (Layer 2)
Figure 21.9 Determination and differentiation of muscle cells (Layer 3)
Model organisms for development studies—
 observable embryos
 short generation times
 relatively small genomes
 knowledge about the organism and its genes
* Drosophila, C. elegans, mouse, zebrafish,
Arabidopsis
What tells a cell (and triggers the master
gene) what its fate will be?
 Cytoplasmic determinants (from the mom internal signals in egg)
 Induction—signal molecules from cells
nearby (neighbors)
1) Cytoplasmic
determinants
include mRNA, proteins,
chemicals, and organelles
and how they are
distributed in the egg.
They are distributed
unevenly - and this can set
up gradients that says
‘head’ side, ‘tail’ side , etc.
Cytoplasmic determinants are coded for by
maternal effect genes (or egg-polarity genes)
Example: --Bicoid
mRNA is present
at the anterior end
of the egg
-Bicoid protein is
essential for head
formation.
Background on Drosophila:
Cytokinesis does not occur in
the early Drosophila embryo.
Nuclei migrate to the periphery
in the blastula.
Bicoid is a morphogen—a
substance that
establishes an organism’s
axis or other 3D
features.
Bicoid helps create the
anterior/posterior
axis.
It’s a transcription factor
that activates expression of
segmentation genes
3 types of segmentation genes:
 Gap genes map out basic subdivisions along
anterior/posterior axis
 Pair-rule genes define smaller regions
 Segment-polarity genes determine the
anterior/posterior axis of each specific
segment.
2) Induction =
signals impinging on
an embryonic cell
from other nearby
embryonic cells.
These can be
transcription
factors remember cell
communication?
Our friend Drosophila !
Has 3 parts—head, thorax, and abdomen
has an anterior/posterior axis and a dorsal/ventral axis
Cytoplasmic determinants and induction together
lead to PATTERN FORMATION
Dorsal
Anterior
Posterior
Ventral
Pattern formation is the development of the
spatial organization of an organism.
Molecular clues (positional information) tell
cells
 where they’ll be located in the body
 who their neighbors will be
 how to respond to other molecular signals
Cell Lineage of all 959 C. elegans cells!
Homeotic genes determine the segment on
which appendages or other structures will
form
The expression of these genes is activated by
Transcription factors coded by segmentation
genes.
All homeotic genes contain a homeobox
domain.
Homeobox domains have been found in many
other animals besides flies, and most genes
with a homeobox are related to
development.
The homeobox domain is actually a DNAbinding domain!
So proteins containing it are likely to be
transcription factors!!
Flies and mice
have homologous
genes coding for
proteins involved
in development.
Induction is when cells signal other cells to
change in a specific way—mostly activating
or inactivating transcription.
Induction has been studied most in the
nematode, C. elegans.
Vulva precursor cells
can develop into 3
different types of cells.
Signals from the
anchor cell induce the
determination of each
cell.
Effects of inducers
can vary depending
on concentration.
Apoptosis is programmed cell death—occurs at
various stages of development. Suicide proteins
are activated - cell blebs (becomes multilobed),
nucleus condenses, and then slowly degrades due to
nucleases and proteases….how painful! It is then
eaten by neighboring cells.
Apoptosis is programmed cell death—occurs
at various stages of development ex: ‘web
retraction’ between digits/fingers (textbook
activity)
MUTANT MICE GALLERY!
In the name of Science…..
Are you ready for the gore?
Figure 21.x2a Laboratory mice: brachyury mutant
Figure 21.x2b Laboratory mice: eye-bleb mutant
Figure 21.x2c Laboratory mice: Hfh11 mutant
Figure 21.x2d Laboratory mice: Lama2 mutant
Figure 21.x2e Laboratory mice: Lepr mutant
Figure 21.x2f1 Laboratory mice: Mgf mutant
Figure 21.x2f2 Laboratory mice: Pax3 mutant
Figure 21.x2g Laboratory mice: Otc mutant
Figure 21.x2h Laboratory mice: Pax6 mutant
Figure 21.x2i Laboratory mice: Pit1 mutant
Figure 21.x2j Laboratory mice: pudgy mutant
Figure 21.x2k Laboratory mice: ruby-eye mutant
Figure 21.x2l Laboratory mice: stargazer mutant
Figure 21.x2m1 Laboratory mice: ulnaless mutant
Figure 21.x3 Nude mouse
Figure 21.x4 Normal and double winged Drosophila