Lecture 5 - Mechanism of Differentiation Flashcards
different types of specification of cell fate
- autonomous specification
- conditional specification
- syncytial specification
process of cell differentiation from zygote
- zygote
- multicellular organism (differentiated cells)
- vertebrate body plan: antero-posterior, dorso-ventral
changes in cellular biochemistr and function are preceded by a process resulting in the __ of the cell to a certain fate
commitment
Stages of Commitment
- specification
- determination
- differentiation
- cell is capable of differentiating autonomously when placed in neutral environment
- fate of the cell is specified early on but the cell fate is still reversible
specification
- cell differentiates autonomously even if placed in another region of the embryo
- cell fate is irreversible or fixed
determination
generation of specialized cell types
differentiation
- the cell “knows” what it is to become very early and without interacting with other cells
- blastomere inherits a set of transcription factors from the egg cytoplasm
- regulate gene expression, directing the cell into a particular path of development
autonomous specification
where does the blastomere inherit a set of transcription factors in autonomous specification
egg cytoplasm
transcription factors or their mRNAs that will influence the cell’s development
morphogenic determinants
blastomeres that acquire this region will give rise to muscle cells
macho
example of autonomous specification
tail muscles of tunicates form autonomously by acquiring the mRNA for a transcription factor from the egg cytoplasm
- ability of cells to achieve their respective fates by interacting with other cells
- what a cell becomes is in large measure specified by paracrine factors secreted by its neighbors
conditional specification
what specifies the fate of a cell in conditional specification
paracrine factors secreted by neighboring cells
- first testable model of cell specification proposed by August Weismann in 1888
- each cell of the embryo would develop autonomously
- proposed that the sperm an egg provided equal chromosomal contributions, both quantitatively and qualitatively, to the new organism
Germ Plasm Theory
proposed the Germ Plasm Theory
August Weismann 1888
what is the Germ Plasm Theory about
- only germ cells contained all the inherited determinants
- somatic cells were each thought to contain a subset of the determinants - type of determinant will determine its differentiated type
used a hot needle to kill one of the cells i a 2-cell frog embro (only the right of left half of a larva developed)
Wilhelm Roux
what did Roux do
- destroy one cell of a 2-cell from embryo
- resulted in development of only one half of the embryo
- did isolateion experiments
- each of the blastomeres from a 2-cell embryo - complete larva
- each isolated blastomere regulated its development to produce a complete organism
Hans Dreisch
what did Hans Dreisch do
- isolation experiments
- each isolated blastomere regulated its development to produce a complete organism
another experiment of Dreisch
pressure-plate experiment alter distribution of nuclei
- diffusable biochemical molecule that can determine the fate of a cell by its concentration
- cells exposed to high levels activate different genes than those cells exposed to lower levels
morphogen
cytoplasm that contains many nuclei
syncytium
- specification of presumptive cells within a syncytium
- predominates in most insect classes
- specification of body regions by interaction between cytoplasmic regions prior to cellularization of the blastoderm
syncytial specification
specification of body regions in syncytial specification is due to the interactions between what
cytoplasmic regions prior to cellularization of blastoderm
produce no rigid cell fates for particular nuclei
variable cleavage
- produced by the anteriormost portion
- concentration that is highest in the anterior and declines toward the posterior
bicoid
posterior-most portion of the egg forms a posterior-to-anterior gradient of this transcription factor
caudal
specification of germ layers:
animal hemisphere blastomeres
ectoderm (skin and nerves)
specification of germ layers:
vegetal hemisphere cells
cells of the gut and associated organs (endoderm)
specification of germ layers:
mesodermal cells
from the internal cytoplasm around the equator
specification of germ layers:
general fate map
imposed on the embryo by the vegetal cells
imposed on the embryo by the vegetal cells
general fate map
Two major functions of vegetal cells
- differentiate into endoderm
- induce the cells immediately above them to become mesoderm
- transcription factor tethered to the vegetal cortex
- involved in the “bottom-up” specification of the frog embryo
- critical in generating both the endodermal and mesodermal lineages
VegT
VegT is critical in generating what
endodermal and mesodermal lineages
when destroyed by antisense oligonucleotides, the entire embryo becomes epirdermis
VegT transcripts
what does VegT activate
set of genes that encode Sox17 transcription
- transcription factor
- activates genes that specify celld to be endoderm
Sox17
instruct the cell layers above them to become mesoderm
nodal paracrine factors
what is the signal that the nodal paracrine factors signal the cells above them to express
Smad2
what does the Smad2 activate
eomesodermin
activate the zygotic genes for the VegT proteins, thus creating a positive feedforward loop that is critical in sustaining the mesoderm
- eomesodermin and
- Smad2 proteins
specified as endoderm by Sox17
vegetal cells
specified as mesoderm (eomesodermin)
equitorial cells
what does the vegetal regio nof the oocyte accumulate
mRNA for transcription factor
- VegT (future dorsal region) and
- Nodal pracrine factor Vg1
what happens at late blastula stage
- Vg1 mRNA is translated
- Vg1 induces future dorsal mesoderm to transcribe the genes for several Wnt antagonists
activates nuclear genes encoding VegT
- eomesoderm with the help of activated
- Smad2 from Nodal proteins
In this way, VegT expressio has gone from _________ to _________
- maternal mRNAs in presumptive endoderm
- nuclear expression in the presumptive mesoderm
what are the different embryonic axes
- dorso-ventral axis
- antero-posterior axis
- left-right axis
under the dorso-ventral axis
- Spemann’s Organizer
- Nieuwkoop Center
- Regional specificity
anterior-posterior axis formation is linked to the formation of the dorsal-ventral axis
Xenopus (other amphibians)
specify that region of the egg to be the dorsal region of the embryo
transcription factor β-catenin in the region of the egg opposite the point of sperm entry
what does the expression of certain genes, that were induced by the β-catenin, initiate
movement of involuting mesoderm
what is established in the movement of mesoderm
anterior-posterior axis of the embryo
- process whereby CNS forms through interactions with the underlying mesoderm
- one of the principal ways that the vertebrate body becomes organized
primary embryonic induction
what does the first mesodermal cell to migrate over the dorsal blastopore lip induce
ectoderm above them to produce anterior structures (forebrain)
what will the mesoderm that involutes later signal to the ectoderm
form more posterior structures (hindbrain and spinal cord)
dorsal blastopore lip and its descendants
“the organizer”
concluded that early amphibian nuclei were genetically identical and that each cell was capable of giving rise to an entire organism
Hans Spemann and Hilde Mangold
what did Spemann concluded from his experiment
early amphibian nuclei were genetically identical and that each cell was capable of giving rise to an entire organism
what stage did a single nucleus enter the as-yet undivided half
16-cell stage
result when constricting longitudanlly with ventral and dorsal side separated
only the future dorsal side gave rise to a normal larva
- a “germinal localization” and it is a marker of the dorsal side of the fertilized egg, the future embryo, and the adult
- cortical region of the newly fertilised egg of frogs and some salamanders that forms just after fertilization on the side opposite sperm penetration
gray crescent
what did the belly piece contain
- epidermal cells (ectoderm)
- mesenchyme (mesoderm)
- gut cells (endoderm)
the belly piece
Bauchstiick
gives rise to those cells that form the dorsal lip of the blastopore
gray crescent
showed that the gray crescent region gives rise to those cells that from the dorsal lip of the blastopore
fate maps
dorsal lip cells are committed to invaginate into the what
blastula
what do the dorsal lip cells initiate
- gastrulation and
- formation of head endomesoderm and notochord
where does the importance of the gray crescent material lie
ability to initiate gastrulation
what happens during gastrulation
crucial changes in cell potency
when did they found that the cells of the early gastrula were uncommited, but that the fates of late gastrla cells were determined
1918
when were cells uncomitted yet
early gastrula
when were cells determined
late gastrula
what did the cells of the early new gastrula exhibit
conditional (i.e. regulative, induction-dependent) development
what did the cells of the late new gastrula exhibit
autonomous development
- derived from the gray crescent cytoplasm
- its fate autonomously determined
dorsal lip of the blastopore
why are the dorsal lip cells and their derivatives organizer
- induced host’s ventral tissues to change their fate to form neural tube and dorsal mesodermal tissues (somites)
- organized host and donor tissues into a secondary embryo with clear anterior-posterior and dorsal-ventral axes
first signal
tells cells that they are dorsal
second signal
these cells are mesoderm
isolated animal cap cells become a mass of what
ciliated ectoderm
isolated equitorial (marginal zone) cells become what
mesoderm
isolated vegetal cells generate what
gutlike tissue
did the experiment of showing mesodermal induction by vegetal endoderm
- Nieuwkoop
- Nakamura and Takasaki
animal cap cells combined with vegetal cap cells generate what
mesodermal tissue
why were the animal cap cells converted to mesoderm
factors released from vegetal cells
group of cells in a blastula that signals the development of the Spemann-Mangold organizer
Nieuwkoop center
showed that the mesoderm arises from the marginal (equitorial) cells at the border between the animal and vegetal poles
- Nakamura
- Takasaki
recombined single vegetal blastomeres from a 32-ell Xenopus embryo with the uppermost animal pole tier
Dale and Slack
what did the experiment of Dale and Slack show
dorsal most vegetal cells induced the animal pole cells to become dorsal mesoderm
- responsible for specifying the micromeres of the sea urchin embryo
- key player in the formation of the dorsal amphibian tissues
- experimental depletion of this molecule results in the lack of dorsal structures
p-Catenin
the primary organizer
DLBlastopore (??)
defines the midline of the embryo
cortical rotation toward the sperm entry point
model for the induction of the organizer
- B-catenin + Tcf-3
- siamois protein + TGF-β
- goosecoid
play an important role in Spemann’s organizer phenomenon
goosecoid
B-catenin degraded
ventral
B-catenin stabilized
dorsal
promotes the formation of cement glands, eyes, and nasal placode
cerberus
- ventral mesoderm inducers
- antagonist to cerberus
BMP4 and Wnt
necessary but insufficient for induction of trunk and posterior region
FGF
= Nieuwkoop center
maternal location - VegT, B-catenin
Spermann Organizer
- Noggin,
- Chordin,
- Xlim antagonize ventralizing BMPS
FGF
favor post mesoderm
goosecoid
dorsalizes mesoderm
BMP
epidermal and ventral development
- defined by the dense area of cells
- establish the other body axes of the embryo
- where the primitive streak will develop
Antero-Posteror Axis
radial blastoderm ->
bilateral symmetric embryo via gravity
occurs as it passes thru the shell gland with the pointed end first
rotation
becomes the site of the future primitive streak - the posterior end of the embryo
uppermost side
what happens to transplantation of posterior marginal zone cells from one chick blastoderm to another
sometimes result in an additional primitive streak
embryo proper
inner cell mass
extraembryonic membrane and placenta
trophectoderm
= dorso-ventral axis
embryonic-abembryonic axis
6.5 days after fertilization
epiblast begin to form the primitive streak at posterior pole
what does the primitive streak form
node analogous to the organizer of other vertebrates
most anterior region including the brain
anterior visceral endoderm
what occurs in the cells on the left side of a region called the node
surge of calcium ions
where does heart point
left
lobe of left lung
1
lobe of right lung
4
how does the gut rotate and what is its result
- asymmetrically
- right sided stomach
specifies organ handedness
iv gene
- large, double fold of tissue that hangs down from the stomach and covers the upper abdominal organs
- attaches the stomach to the transverse colon
greater omentum
attaches the stomach and the duodenum to the liver
lesser omentum
prevent ATPase which lead to L-R patterning defects
heart treated w/ lansoprazole
- a condition in which there is a reversal in the position of the internal organs
- established by complex epigenetic and genetic cascades
situs inversus
gene control of left-right asymmetry:
activin
activin -> ActR -> Shh (right of node)
gene control of left-right asymmetry:
lefty
lefty -> Shh (left) -> nodal -> pitx2
left is expressed in the _____ and acts as what
- midline of Node
- acts as barrier to these signals
- respiratory problem
- immotile cilia due to lack of dynein arms on cilia
Kartagener’s syndrome
homozygous mutatio of inv gene leads to what
complete randomization
rare condition where many organs in the chest and abdomen are formed abnormally, in the wrong position, or are even missing.
Heterotaxy syndrome
causal agent for X-linked heterotaxy
ZIC3
folded in L-R disturbance of the internal organs
ZF1-2
