Download Slides - Workforce Development in Stem Cell Research

image of human eggs by S. Schuh-Huerta
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Dr. Sonya M. Schuh-Huerta
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Dr. Shawn L. Chavez
Lecture 1, Part B: Developmental
Biology Review –
The Gametes, Fertilization &
Early Embryo
Developmental Biology
Embedded Assessment
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Draw a four-day old human embryo
1) Note the approximate size or scale
2) Include as much detail as you can in 5 minutes
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The fascinating process of sexual reproduction
begins with 1 egg and 1 sperm
What are germ cells?
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1. Germ cells become…?
2. Early in embryo development
primordial germ cells are set aside
from the embryo
3. Then they migrate back into the
embryo & settle into the genital
ridge (early gonad)
(Langman’s Medical Embryology, 2007)
4. They proliferate & give rise to
millions of early gametes
migrating
germ cells
(Castrillon et al. PNAS, 2000)
The beginning of human
development
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1. The male gamete  sperm
2. The female gamete  egg (oocyte)
3. Fertilization  fusion of sperm & egg
A sperm cell attempts to penetrate
the ovum’s coat in order to fertilize it
4. The beginning of a new individual!
testis
ovary
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(Human Anatomy, Marieb, Mallatt, & Wilhelm, 2010)
And how does it all begin?...
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vs.
Leonardo Da Vinci
“spermists”
“ovists”
-Which is more important?
 both contribute to new individual
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The beginning of human
development
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1. Gametogenesis  formation of eggs & sperm
• Oocytes & spermatocytes (23 chromosomes)
• Chromosomes in gametes are reduced by half
• The story of sperm
• The story of eggs
2. Fertilization
• One sperm + one egg = chromosome number
restored
• The genes from each are required for development
3. Embryogenesis  Formation of the embryo
4. Zygote  the earliest form of a human embryo (1
cell)
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The Sperm: Programmed to reach &
fertilize the egg
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1. Terminally differentiated cell
2. Highly specialized, with unique morphology &
physiology
3. Flagellum (tail), midpiece (packed with mitochondria
for power), and head (packed with DNA – to deliver its
genetic material)
4. Can live in female reproductive tract for 3–5 days or
longer…
Human sperm alive &
swimming after 30 hrs
at room temp!
5. Respond to chemical cues & signaling molecules within
the female reproductive tract
6. Average man can produce sperm his entire life
7. Takes ~75 days for mature sperm to develop
8. 4,000 sperm made per heartbeat!
cross section of testis
(images by S. Schuh-Huerta)
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Meiosis gives rise to haploid gametes
First meiotic division (MI)
(reduction division)
Paternal
homolog
Parental
(2n)
Maternal
homolog
Prophase 1 (4n)
Crossing over
Metaphase 1
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Second meiotic division (MII)
(mitosis with DNA replication)
Two parental cells (2n)
Prophase 2
Metaphase 2
Anaphase 1
Telophase 1 (2n)
Anaphase 2
Telophase 2
Four daughter cells (n)
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Spermatogenesis: Generation of
sperm
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1. Meiosis produces 4 sperm from 1 germ cell (spermatogonium).
2. 1st division is a reduction division (separates homologous
chromosomes that have been duplicated prior to meiosis; DNA content
reduced from 4n to 2n).
3. 2nd division is a mitosis without DNA replication, generating haploid
cells (= n chromosomes)  What does this mean?
4. Spermatogenesis occurs throughout an adult male’s life  How is this
possible?...
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Spermatogenesis: Generation of
sperm
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lumen
lumen
Seminiferous tubule
of the testis
2N
spermatogonial stem cell
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(image by S. Schuh-Huerta)
Oogenesis: generation of oocytes
Embryo
Meiosis I
Meiosis I
Puberty
Polar body (2n)
Meiosis II
(after fertilization)
Egg (n)
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1. Oogenesis: meiosis that produces 1 egg
and 3 polar bodies
2. 1st meiotic division begins in the female
fetus but stops before homologous
chromosomes are separated
Primary
oocyte
Secondary
oocyte (2n)
References
3. 1st meiotic division resumes at puberty
(ovulation)
4. 2nd meiotic division occurs after
fertilization (sperm & egg fusion), and
before sperm & egg pronuclei fuse
Polar bodies (n)
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The Egg: Programmed to give rise to
the embryo
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1. Oocytes develop within follicles in the ovary
2. Arrested in meiosis I until puberty or later
 some eggs can be arrested for 51 years+
3. Packed full of proteins & mRNA to sustain
early development
4. Females lose many oocytes/follicles over the
course of their lifetime, until they are
depleted at menopause
Oocytes within the human ovary
(images by S. Schuh-Huerta)
The Oocyte Life Span
References
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No oogonial stem cells
Fetal Life
Conception 12 wks 20 wks
Birth
Menarche
Age ~12
Menopause
Age ~51
Age ~30s
oocytes/follicles
~1 million
Germ
Cells
~1,000
oogonia enter
meiosis & differentiate
Ovulation begins
~500,000 oocytes
Fertility declines
~25,000 oocytes
Ovulation ends
<1,000 oocytes
Germ cells migrate
& proliferate
~5-7 million
(Langman’s Medical Embryology, 2007)
(S. Schuh-Huerta)
www.istockphoto.com
Fertilization
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1.
Fusion of sperm & egg to create a new individual
2.
The new diploid cell  zygote
3.
Restores the DNA content & combines genes
from both parents (sexual reproduction)
4.
Major events of fertilization:
•
•
•
•
•
Sperm & egg recognize, contact, & fuse with
each other
Block to polyspermy
2nd meiotic division of secondary oocyte
(2n) to produce egg (n)
Fusion of female & male pronuclei
Stimulation of zygotic metabolism & cell
cleavage
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(Dick Bowen, Colorado State)
References
The cell cycle and mitosis
The cell cycle
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Mitosis (M)
(parental cell)
Prophase
Mitosis (M)
Prometaphase
Interphase
Resting
phase
(daughter cell)
Metaphase
Telophase
Anaphase
DNA synthesis (S)
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References
Cleavage (days 1-6)
2 cell stage
4 cell
stage
8 cell stage
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Morula
1. Zygote divides into two cells
2. Day two: morula (Latin for mulberry)
3. Cell signaling begins
Blastocyst
Trophectoderm
4. Embryo begins to organize
5. Blastocyst forms on days 4-6
Inner cell mass
6. Two parts of blastocyst
• Trophectoderm  placenta, amnion
• Inner cell mass  embryo
7. Size is 0.1 mm
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Time-lapse imaging of human
pre-implantation development
Wong et al., Nature Biotech 2010
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The origin of embryonic stem
cells (ES cells)
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1. ES cells can be derived
from the morula
2. ES cells are normally
derived from the inner
cell mass of the blastocyst
3. ES cells can be derived
from primordial germ
cells
4. ES cells can be derived
from adult somatic cells
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[Figure modified from Gilbert’s “Developmental biology”, 8th edition, Sinauer]
The great potential of iPS Cells
Stem cells
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Neurons
growth
factors
Culture stem cells
in vitro
References
transplantation
into patient
Grow & characterize cells
But now adult cells can be
transformed into embryonic-like
cells  Induced Pluripotent Stem
Cells (iPSCs)
•Could prevent immune rejection
•Eliminates need for using embryos
(images by S. Schuh-Huerta)
Embryogenesis (week 3):
Formation of germ layers
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Amnion
Implantation
Uterus
Blastocyst
Ectoderm
Epithelial skin cells, inner ear, eye,
Yolk sac
mammary glands, nails, teeth,
nervous system (spine and brain)
Endoderm
Stomach, gut, liver, pancreas, lungs,
tonsils, pharynx, thyroid glands
Mesoderm
Blood, muscle, bones, heart,
urinary system, spleen, fat
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Lineage restriction: differentiation
into specialized cells
Totipotent
Pluripotent
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Multipotent
brain
skin
Ectodermal cell
Zygote
bone
marrow
ES cell
Mesodermal cell
heart
gut
Endodermal cell
progenitor cells
differentiated cells
The hematopoietic system as an
example of lineage restriction
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Multipotent stem cell
Progenitor cell
Differentiated cell
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[Image taken from Gilbert’s “Developmental biology”, 8th edition, Sinauer].
Summary
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• Immature (undifferentiated) cells
• Mature (differentiated) cells
• Differential gene expression
• Differential cell signaling pathways
• Gametogenesis
• Fertilization
• Early embryogenesis
• Origin of ES cells and iPS cells
• Lineage restriction & differentiation
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Review of Developmental Bio
Concept Mapping Terms
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Create a concept map using the key concepts from today’s
lecture. You should include (but are not limited to) the
following terms/concepts. Due by ___date_____:
• Stem cells
• Transcription
• Translation
• Chromosome
• Gene
• Cell signaling
• Signal transduction
• Differentiation
• Gametogenesis
• Germ cells & gametes
• Fertilization
• Zygote
• Blastocyst
• Inner Cell Mass
• Germ layers
• Ectoderm
• Mesoderm
• Endoderm
• iPS Cells
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