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Development of
Multicellular Organisms
Shu-Ping Lin, Ph.D.
Institute of Biomedical Engineering
E-mail: [email protected]
Website: http://web.nchu.edu.tw/pweb/users/splin/
Date: 01.03.2011
Chromosome Numbers
All are even numbers –
diploid (2n) sets of
homologous chromosomes!
-ploid = number of copies of
each chromosome.
Haploid
Diploid
In humans …
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23 chromosomes donated by each parent (total = 46 or 23 pairs).
Sex cells divide to produce gametes (sperm or egg), occurs only in gonads
(testes or ovaries). Male: spermatogenesis; Female: oogenesis:
 The form of cell division by which gametes, with half the number of
chromosomes, are produced.
 Contain 22 autosomes and 1 sex chromosome.
 Are haploid (haploid number “n” = 23 in humans).
Fertilization results in zygote with 2 haploid sets of chromosomes now diploid.
 Diploid cell; 2n = 46. (n=23 in humans)
Meiosis is similar to mitosis with some chromosomal differences (Most
cells in the body produced by mitosis).
Only gametes are produced by meiosis, is sexual reproduction. Two
divisions (meiosis I and meiosis II).
Meiosis Leading to Sperm
and Oocytes
From Unfertilized Egg to Zygote
# Meiotic cell division
produces gametes
1. During interphase parent cell has
2n chromosomes.
2. Chromosomes duplicate and form
sister chromatids.
3. Homolog pairs of sister
chromatids then synapse
and exchange genetic
material.
4. Sister chromatids remain
together, but homologs
separate to opposing poles.
5. Nuclear membrane forms around the
chromosomes, and equatorial plane
contracts and separates the newly
formed cells.
6. Next cell division occurs without
DNA synthesis.
7. Chromatid pairs align on the equatorial plane.
8. Chromatids of each pair separate and move to opposing poles, and from then on the
division proceeds as described for mitotic division.
Meiosis I
First division of meiosis
 Prophase 1: Each chromosome dupicates and
remains closely associated. These are called sister
chromatids. Crossing-over can occur during the latter
part of this stage.
 Metaphase 1: Homologous chromosomes align at
the equatorial plate.
 Anaphase 1: Homologous pairs separate with sister
chromatids remaining together.
 Telophase 1: Two daughter cells are formed with
each daughter containing only one chromosome of the
homologous pair.
Meiosis II
Second division of meiosis: Gamete
formation
 Prophase 2: DNA does not replicate.
 Metaphase 2: Chromosomes align at the
equatorial plate.
 Anaphase 2: Centromeres divide and sister
chromatids migrate separately to each pole.
 Telophase 2: Cell division is complete. Four
haploid daughter cells are obtained.
Meiosis – Key
Differences from Mitosis
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Meiosis reduces the number of chromosomes by half.
Daughter cells differ from parent, and each other.
Meiosis involves two divisions, Mitosis only one.
Meiosis I involves:
 Synapsis – homologous chromosomes pair up.
Chiasmata form (crossing over of non-sister
chromatids).
 In Metaphase I, homologous pairs line up at
metaphase plate.
 In Anaphase I, sister chromatids do NOT separate.
 Overall, separation of homologous pairs of
chromosomes, rather than sister chromatids of
individual chromosome.
Animation
DIFFERENCES BETWEEN MITOSIS AND MEIOSIS
EVENTS
Occurrence
Definition
Number of
daughter cells
Prophase
MITOSIS
In all the body cells including germ cells.
Only in the germ (reproductive) cells.
It is an equational division.
It is a reductional division.
Only two
Four
Involves relatively few changes.
Chromomeres Not visible in prophase.
Synapsis
Does not occur.
Crossing over Does not occur.
Metaphase
Centromeres
in Anaphase
Centromeres
in Metaphase
Telophase
Cytokinesis
MEIOSIS
Chromosomes arrange along the
equator.
Each centromere splits into two.
Orient towards the equator while
chromatids orient towards poles.
Involves a series of changes in
chromosomes distinguished into 5
substages.
Visible in the leptotene stage of prophase
-I
Occurs in zygotene of prophase-I.
Occurs in pachytene stage of prophase-I.
Chromosomes arrange equally on either
side of the equator in metaphase-I.
Centromeres do not split in metaphase-l.
Orient towards poles while chromatids
orient towards the equator in metaphaseI
Results in the formation of two daughter Telophase-II results in the formation of
nuclei having the same no. of
two daughter nuclei, each having half the
chromosomes as that of parent cell.
no. of chromosomes as that of parent cell.
Follows immediately after karyokinesis.
May or may not occur at the end of first
karyokinesis
Mitosis vs. Meiosis
Polarization of Fertilized Frog Egg
(a) Unfertilized egg has radial
symmetry around an axis that
passes through cell
centrosome and nucleus.
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Hemisphere that contains centrosome and nucleus is called animal
hemisphere, whereas lower hemisphere contains large numbers of yolk
platelets full of nutrients. Point of contact between egg and second polar
body coincides with animal pole of egg.
Entry of sperm into egg induces rotation of cell cortex around cell center(b).
This movement is initiated by molecular motors such as myosin and involves
relative sliding movement of cell cortex on underlying cytoskeleton. Cortical
rotation is resisted by viscous forces exerted by surrounding fluid(c).
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Resultant moment exerted by fluid is proportional to the rate of rotation of cortex as well
as cell radius squared.
This moment has opposite sense of direction to angular velocity of cortex.
Since this is the only external moment acting on the cell, Newton’s laws of motion dictate
that inner region of cell must move in opposite direction of cortex (c).
As a result of this complex movement, radial symmetry is destroyed.
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Bisection of Eight-cell Sea Urchin
Embryos in Two Different Midplanes
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Division of asymmetric fertilized egg results
in cells with differing cell contents
represented by different shading.
Horizontal plane in the figure separates
each cell into chemically distinct upper
hemisphere (animal hemisphere) and lower
hemisphere (vegetal hemisphere).
When embryo is bisected vertically,
resulting 4-cell clusters develop into small,
but normal, larva (a).
Bisection along middle horizontal plane
leads to different result (b).
Cells from animal pole remain embryonic,
whereas cells from vegetal plane develop
into small, but abnormal, embryo.
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Three cycles of divisions of a fertilized
egg give rise to 3 different cell types.
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DNA-binding proteins can block
parts of DNA from action of
transcription machinery.
Idealized DNA molecules shown
in figure belong to stem cells (top
row) and cells that differentiated
into cell types 1 and 2 (bottom
row).
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Cleavage refers to the first
stage of embryonic
development during which
cells divide rapidly without
significantly increasing
overall mass volume of
zygote.
Amphibian and mammary
embryos exhibit different
courses of development.
Frog zygote develops into a spherical shell of cells called blastula
at the late stage of cleavage (a), whereas human zygote develops
into what is called blastocyst (b).
Outer layer of cells is called trophoblast.
Embryonic cells massed at top of fluid-filled sack (blastocoel)
give rise to human embryo.
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Gastrulation phase of embryonic
development, during which cells change
their shape and move within the
embryo.
Cells move as individual cells or as a
sheet of cells.
Figure shows some of modes of cell
movement observed during gastrulation.
Invagination involves a layer of cells
moving inward toward the center of
blastula.
Inward migration of single cells from
outer layer of embryo is known as
ingression.
Term of delamination is used to
characterize inward movement of newly
formed daughter cells while keeping
shell of blastula intact.
Involution refers to folding of cell layers,
leading to new cell-cell contacts and
opportunities for induction.
Compact domain of embryo
extending to cover large surface
area through cell movements is
characterized as convergent
extension.
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Induction by direct
cell-cell contact (a)
and by diffusion of a
morphogen through
the embryo (b).
3 germ layers developed during
gastrulation: ectoderm,
mesoderm, and endoderm.
Through a series of cell movements
and inductive interactions, these 3
layers of cells give rise to all
the tissues in the body.
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Generation of
different cell types
through induction.
4 Essential Processes
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Cell proliferation: producing many cells from one
Cell specialization: creating cells with different characteristics
at different positions
Cell interaction: coordinating the behavior of one cell with
that of its neighbors
Cell movement: rearranging the cells to form structured
tissues and organs
Fruit Fly Embryo
Nuclear division is not accompanied
by cell division until about 20003000 nuclei form.
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Maternal mRNA of bicoid gene is
localized at anterior end, and mRNA
of nanos gene is concentrated at
posterior end
Products of these genes play roles in
the activation or repression of gene
transcription involved in development.
About 6 hours after fertilization,
midsection of fruit fly embryo exhibits
14 stripes, each a few cells thick, with
alternative expression of even-skipped
(Eve) and fushitarazu (Ftz) proteins,
both of which are involved in the
generation of development of smallerscale patterns.
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Activator and Repressor Sites
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Transcription factors that bind to activator sites are shown as
half circles, whereas those that bind to repressors are shown
as triangles.
High-affinity regulator sites are darker than low-affinity sites.
Capital letters B and H stand for bicoid and hunchback
proteins, respectively.
Head-to-tail Pattern
Formation in Fruit Fly
Figure shows spatial distribution of
transcription factors bicoid,
hunchback, giant, and krüpple
along axis of fruit fly larva between
segments 1 and 3.
Expression of even-skipped (Eve) protein in
stripe 2 is regulated by these transcription
factors.
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Hypothetical Homeobox
Gene Language
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Hox genes can be
either on (1) or off
(0).
Expression of Hox d2
and d3 in the context
of no expression of
other Hox genes
specifies that those
cells will go on to
form tissue in the tail
of mouse.
If other Hox genes are inappropriately expressed or timing of
expression is wrong, then ability of Hox d2 and Hox d3 to
specify a tail fate is lost.
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Body pattern
of mouse and
spatial range
of activation
of hox genes
that mediate
mouse
development.
Stem Cells and Tissue Engineering
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Tissue engineering is a field of biotechnology that aims to
generate living tissue from isolated cells, scaffolds, matrix
proteins, and growth factors.