Download The Saga of the Germ Line

Bio 127 - Section III
Late Development
Germ Line Development
Gilbert 9e – Chapter 16
Section 4 Encompasses :
•
•
•
•
Development of the Tetrapod Limb
Sex Determination
The Saga of the Germ Line
Post-Embryonic Development
Student Learning Objectives
1. You should understand that sexual reproduction requiring the fusion of
gametes from male and female gonads occurs in specific organisms.
2. You should understand that the primordial germ cells that give rise to
gametes arise outside of the gonads and must migrate to them.
3. You should understand that in most organisms the primordial germ
cells are specified conditionally, while in some they are specified
autonomously by cytoplasmic determinants in the egg.
4. You should understand that migration of the germ cells from their site
of origin to the gonads is an essential part of reproductive success .
• In all plants and some animals, somatic
cells can readily form new organisms
– Cnidarians, flatworms, tunicates
• In many animals, there is an early division
between somatic and germ cells
– Insects, roundworms, vertebrates
• Two step process;
– Primordial germ cells (PGCs) are determined
in a specific location in the embryo
– PGCs migrate to the gonad and become the
progenitor population for eggs and sperm
Two Methods of Germ Cell Determination
• Autonomous Specification
– Egg cytoplasmic determinants
– Called ‘Germ Plasm’
– Nematodes, flies, frogs
• Conditional Specification
– Signals from surrounding cells
– Majority of sexually reproducing organisms
– Including mammals
The nematode Caenorhabditis elegans
Remember cleavage and gastrulation:
Asymmetrical divisions produce a
stem cell (P-lineage), “founder” cell.
Stem cell divisions are meridional
Founder cell divisions are equatorial
Gastrulation in C. elegans
P-granules hold cytoplasmic determinants in C. elegans
PIE-1: Blocks all transcription, thus all differentiation
Germ plasm also has blocks to translation, stem cell factors,
controls for asymmetric divisions and meiosis inducing agents.
P4
Blue is DNA marker, Green is P-granule marker
Blue stain marks transcriptional activity
P4
Synctitial cleavage in Drosophila is followed by cellularization
Pole plasm forms during cellularization
- mitochondria
- fibrils
- polar granules
no transcription
no translation
germline stabilization
anterior
posterior
Localization of germ cell-less (gcl) gene products
Human males with mutant homolog are often sterile
Germ plasm at the vegetal pole of frog embryos
Marker
for frog
homolog
of fly/worm
translation
blocker,
Nanos
• The frog cells that take up these granules
will become PGCs and migrate to the
gonads as the kidney forms
– Again, no transcription or translation
– Therefore, no differentiation
Conditional Specification of mammalian PGCs
• Posterior of epiblast at the junction of the
primitive streak and extraembryonic
ectoderm
– Cells are no different from other epiblast
– Specified in gastrulation before 3 layers form
– Wnts from endoderm make them competent
– BMPs from extraembryo ectoderm finish it
Picture the blastocyst full of yolk.....
Poor old Henson discovered this node as well but didn’t get the naming rights
Conditional Specification of mammalian PGCs
• Same deal as the others:
– Repress differentiation by repressing gene
expression
• Specified outside embryo forming cells
– Once expression is shut down they can go
back into embryo and not respond to signals
Germ Cell Migration
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•
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Drosophila
Zebrafish
Frogs
Mice
Birds and Reptiles
Germ Cell Migration: Drosophila
As the endoderm
invaginates, the
ectoderm and
mesoderm extend
and converge to
wrap around the
dorsal side to form
the “germ band”
Germ Cell Migration: Drosophila
- mitochondria
- fibrils
- polar granules
no transcription
no translation
germline stabilization
anterior
posterior
Germ Cell Migration: Drosophila
Germ cells
passively ride
endoderm
Germ Cell Migration: Drosophila
Endoderm expresses
repellent molecules
Germ band
is retracting
PGCs and gonad
progenitors in 2
migration streams
Germ Cell Migration: Drosophila
Combination of chemoattraction
and repulsion drive them to gonad
E-cadherin MET
forms epithelium
around PGCs
Germ Cell Migration: Drosophila
• Both mesoderm and PGCs divide through the
larval stage, differentiate at metamorphosis
• At larval-pupal transition anterior PGCs in gonad
become germ line stem cells
• In ovaries, the cells attach top stromal cap
In testes, the cells attach to hub cells
Remember: Zebrafish development occurs very rapidly
24-hours from 1 cell to vertebrate embryo!
Germ Cell Migration: Zebrafish
specification: germ plasm
determination: PGCs by 32-cells
four clusters
join into two
migration of bilateral clusters into developing
gonad follows signal Sdf-1 using receptor CXCR4
Remember: Germ plasm at vegetal pole in frogs
Marker
for frog
homolog
of fly/worm
translation
blocker,
Nanos
Germ Cell Migration: Frogs
During cleavage the germ plasm rises up until it
ends up in the endoderm at top and back near lip
Germ Cell Migration: Frogs
The endoderm below
mesdoderm are PGCs
Germ Cell Migration: Frogs
Migration anterior to gonads at
endoderm-mesoderm boundary
~30 PGCs reach gonads
by fibronectin and Sdf-1
• Remember Sdf-1
• Soluble signal whose receptor is CXCR4
• Common signal for vertebrate germ cells
• Also used by humans to call HSC to bone
marrow, guide lymphocytes, MSC?
Germ Cell Migration: Mice
PGCs formed in extraembryonic epiblast
10-100 cells
@ Day 6.5
in mice
Germ Cell Migration: Mice
Once formed, they migrate directly into the hindgut endoderm
and migrate anteriorly through Day 9 dividing the entire time
They leave
the gut by
the dorsal
mesentary
and enter
the genital
ridges by
Day 12 as
2500-5000
PGCs.
Germ Cell Migration: Mice
• The travelling stem cell niche
– Support cells travel with PGCs to maintain the
undifferentiated stem cell phenotype
– They secrete stem cell factor (SCF)
– The cells follow fibronectin trail
– Sdf-1 also required
Germ Cell Migration: Birds and Reptiles
Germ line cells determined in the
area pellucida, migrate to hypoblast
Migrate to gonads via blood stream
when extraembryonic vessels form
Germ Cell Migration: Birds and Reptiles
Sdf-1 from intermediate mesoderm
draws them out of vessels and through
the mesodermal tissues to the gonad
Bio 127 - Section III
Late Development
Post-Embryonic Development
Gilbert 9e – Chapter 15
Section 4 Encompasses :
•
•
•
•
Development of the Tetrapod Limb
Sex Determination
The Saga of the Germ Line
Post-Embryonic Development
Student Learning Objectives
1. You should understand that development never stops during the life of
the organism and that three major processes occur in the postembryonic animal: metamorphosis, regeneration and aging.
2. You should understand the Direct Development involves young
organisms with the same body plan as the adult; whereas Indirect
Development involves major changes to form the adult body plan.
3. Indirect Development, or metamorphosis, is hormonal reactivation of
4. You should understand that regeneration is the reactivation of
developmental process to restore missing tissues.
5. You should understand that aging and physiological senescence are
an interplay of genetic and environmental influences.
Metamorphosis
• Development of a larval stage and an adult
stage specialized for different functions
– Larvae often specialized for growth, dispersal, etc.
– Adults usually specialized for reproduction
– Example Cecropia moths:
• Larvae are wingless eating machines
• Adults have one day to mate – don’t even have mouth parts!
• Two major types of larvae
– Primary: little to no similarity to adult (sea urchins)
– Secondary: add and subtract parts from similar form
• (insects, amphibians)
Metamorphosis: Sea Urchins
Pluteus
Larvae
Metamorphosis: Sea Urchins
Primary
Larvae:
No trace
of adult
morphology
Metamorphosis: Amphibians
• Hormone(s): T3 and T4
• Four Major Morphological Processes
– Growth of new structures
– Cell death in existing structures
– Remodeling of existing structures
– Biochemical respecification
• Shift in the genes expressed and the physiological
functions they control
Metamorphosis: Amphibians
Tadpole eyes are
on the sides of the
head, frog eyes are
on the front and top
Binocular Vision
New neurons
differentiate
and form new
ipsilateral tracts
Metamorphosis: Amphibians
• Cell death in existing structures
– Apoptosis
• T3 induces apoptosis in tail and gills
• Apoptosis occurs in gut epithelium due to ECM loss
– Phagocytosis
• Macrophages finish off the cells of the tail
• Also destroy larval RBCs to make fresh ones with
the adult hemoglobin protein
Metamorphosis: Amphibians
Remodeling:
- Eyes
- Skull
- Skeleton
- Gut
- Sensory
Metamorphosis: Amphibians
Biochemical Respecification
NH3 = ammonia, amino group
NH4+ = ammonium ion
T3 causes a shift in transcription factor
expression that upregulates these genes.
Metamorphosis: Amphibians
2NH3 + CO2
+ H2O
(urea)
Actions of thyroxine (T4) and tri-iodothyronine (T3)
Thyroid receptors are
found in the nucleus
where they are bound
to inactive promoters.
When thyroid hormone
enters such a cell, it will
bind to receptor and the
combination is an active
transcription factor for
that specific gene.
Cells that make high
levels of deiodinase II
are more responsive to
thyroid stimulation.
Cells that make high
levels of deiodinase III
are less responsive
Actions of thyroxine (T4) and tri-iodothyronine (T3)
• What genes you have your thyroid receptors on is very
important to your function
– Limb muscle cells grow in response to thyroid hormones
– Tail muscle apoptoses in response to thyroid hormones
• How much deiodinase II you make is very important
– Limb buds make a lot and respond to early low levels of T4
– Tails make very little and wait for later very high levels of T4
– This is good!
• How a tissue is organized before T4 is very important
– Thyroid hormones make skin apoptose
– Head and body have basal stem cells, tail does not
• Your tail degenerates during week 4 of
gestation in much the same fashion as the
frogs!
Metamorphosis: Amphibians
• Some amphibian species have evolved
alternatives to metamorphosis: Heterochrony
– Neoteny: Normal gonadal maturity, retention of
juvenile form
– Progenesis: Accelerated gonadal maturity, retention
of juvenile form
– Direct Development: No larval form
Metamorphosis: Amphibians
Neoteny in the Mexican axolotl (salamander)
Normal adult with juvenile form.
Adult form not seen in nature,
resulting from T4 in the pond.
Metamorphosis: Amphibians
Direct development in a common Puerto Rican frog
Two views of the developing
limb buds within the egg
Frogs, not tadpoles,
hatch from the eggs.
Very large, nutrient-rich eggs allow
skipping larval food gathering stage.
Metamorphosis: Insects
Adult = imago
Metamorphosis: Insects
• Larva are eating machines to provide energy for
non-feeding pupal development
• They have both doomed larval cells and
rudiments of imaginal cells for adult
– The larval cells will apoptose in the pupa
– Imaginal disc cells will form wings, legs, antennae,
eye, head, thorax and genitalia
– Histioblasts will form adult abdomen
– Imaginal cell clusters in each organ will form adult
organ as the larval organ degenerates
Metamorphosis: Insects
Locations and developmental
fates of imaginal discs and
imaginal tissues in the third
instar larva of Drosophila
Metamorphosis: Insects
Leg Imaginal Disc
before
pupa
during
pupa
Epidermis cells
form a hollow
tube that coils
Telescopes
out in pupa
Metamorphosis: Insects
Metamorphosis: Insects
A view into the minds of fly-guys....
The cells at center of the disc
secrete Wingless (Wnt) and
Decapentaplegic (TGF-B)
This causes different
expression levels of
transcription factors
Dachsund (green) and
Distal-less (red).
Metamorphosis: Insects
• Like amphibians, control is hormonal in insects
– Presence of juvenile-hormone makes a larval molt
– Shift to steroid 20-hydroxyecdysone gives pupal molt
– Differential timing in the development of pupal
structures is due to 20E receptor expression timing
Regeneration
• Restoration of missing tissues
• Post-embryonic reactivation of development
• Occurs in some form in all species
– Stem cell-mediated regeneration
– Epimorphosis: adult cell de- and re-differentiation
– Morphallaxis: adult cell repatterning
– Compensatory regeneration: adult cell division
Regeneration: Epimorphosis
flatworms,
salamanders
The surviving
cells lose their
specification
and form an
undifferentiated
tissue bulge
Regeneration : Epimorphosis
Vertebrate limb development from apical ectodermal cap (ridge)
Regeneration: Morphallaxis
The hydra is constantly regenerating cells that are sloughed off.
Only the head and
foot aren’t moving
Regeneration: Morphallaxis
• Any cells along the length can become
head, body or foot
• If you cut a hydra up all pieces will form all
structures
• Each piece has a dual gradient already
established and both ends are specified
Regeneration: Morphallaxis
The bud always forms roughly the same distance
anterior-posterior due to the combined gradient
Regeneration: Compensatory Regeneration
• No dedifferentiation occurs in liver regeneration
• All five cell types produce more of themselves
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hepatocytes
duct cells
fat-storing (Ito) cells
endothelial cells
Kupffer macrophages
• Progenitor cell back-up plan: Oval cells
Aging
• Time-related deterioration of physiological
functions necessary for survival and fertilization
– Life span vs. senescence
• Combination of:
– Mutations
– Environmental factors
– Random epigenetic changes
Aging
We’ve learned
not to die young
as often more
than we’ve
extended life
Aging
Hutchinson-Gilford progeria
Mutation repair deficiency
Aging
Low caloric intake
is associated with
long-life in all species.
Figure 15.36 Differential DNA methylation patterns in aging twins (Part 1)
Epigenetic
alterations
Figure 15.37 Methylation of the estrogen receptor gene occurs as a function of normal aging
Can effect
reproductive
capacity
directly