LESSON 4: MECHANISMS OF DIFFERENTATION Flashcards
changes in cellular biochemistry and function are preceded by a process resulting in the __________of the cell to a certain fate
commitment
- cell is capable of differentiating autonomously when placed in neutral environment.
- fate of the cell is specified early on but
Specification
Stages of Commitment
1.Specification
2.Determination
3. Differentiation
Type of Cell: Keratinocyte (Epidermal cell)
Differentiated cell product: ?
Specialized function: ?
Differentiated cell product: Keratin
Specialized function: Protection against abrasion, desiccation
Type of Cell: Erythrocyte (red blood cell)
Differentiated cell product: ?
Specialized function: ?
Differentiated cell product: hemoglobin
Specialized function: transport of oxygen
- 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
Type of Cell: Leydig Cell
Differentiated cell product: ?
Specialized function: ?
Differentiated cell product: Testosterone
Specialized function: Male Sexual Characteristics
Type of Cell: Lens Cell
Differentiated cell product: ?
Specialized function: ?
Differentiated cell product: Crystallins
Specialized function: Transmission of light
Type of Cell: T Lymphocyte
Differentiated cell product: ?
Specialized function: ?
Differentiated cell product: Cytokines
Specialized function: Destruction of foreign cells, regulation of immune response
Type of Cell: B Lymphocyte
Differentiated cell product: ?
Specialized function: ?
Differentiated cell product: Immunoglobulins
Specialized function: Synthesis of antibodies
Type of Cell: Melanocyte
Differentiated cell product: ?
Specialized function: ?
Differentiated cell product: Melanin
Specialized function: Pigment production
Type of Cell: Pancreatic Islet
Differentiated cell product: ?
Specialized function: ?
Differentiated cell product: Insulin
Specialized function: Regulation of carbohydrate metabolism
Type of Cell: Chondrocyte (Cartlilage cell)
Differentiated cell product: ?
Specialized function: ?
Differentiated cell product: Chondroitin sulfate; type II colagen
Specialized function: Tendons and ligaments
Type of Cell: Myocyte (Muscle)
Differentiated cell product: ?
Specialized function: ?
Differentiated cell product: Actin and Myosin
Specialized function: muscle contraction
Type of Cell: Osteoblasts (bone- forming cell)
Differentiated cell product: ?
Specialized function: ?
Differentiated cell product: Bone Matrix
Specialized function: skeletal support
Type of Cell: Hepatocyte (Liver cell)
Differentiated cell product: ?
Specialized function: ?
Differentiated cell product: serum albumin; numerous enzyme
Specialized function: production of serum proteins and numerous enzymatic functions
Type of Cell: Tubule Cell of hen oviduct
Differentiated cell product: ?
Specialized function: ?
Differentiated cell product: Ovalbumin
Specialized function: Egg white proteins for nutrition and protection of the embryo
Strategies of Specification (Type)
1.Autonomous Specification
2.Conditional Specification
3. Syncytium Specification
Type of Cell: Follicle cell of insect ovary
Differentiated cell product: ?
Specialized function: ?
Differentiated cell product: Chorion proteins
Specialized function: Eggshell proteins for protection of embryo
Type of Cell: Neurons
Differentiated cell product: ?
Specialized function: ?
Differentiated cell product: Neurotransmitters (Acetylecholine, serotonin, etc.)
Specialized function: Transmission of communication signals in the nervous system
-blastomere inherits a set of transcription factors from the egg cytoplasm
-regulate gene expression, directing the cell into a particular path of development
-different regions of the egg contain different morphogenetic determinants (transcription factors or their mRNAs— that will influence the cell’s development)
-the cell “knows” what it is to become very early and without interacting with other cells
Autonomous Specification
What influence the cells development according to autonomous specification?
Morphogenetic Determinants
contain a yellow-pigmented cytoplasm that has within it the mRNA for a muscle-specific transcription factor
(Macho - blastomeres that acquire this region will give rise to muscle cells)
B4.1 blastomeres
Autonomous specification in Tunicate (Macho factor)
-if the macho-containing yellow cytoplasm is placed into the cells - will form tail muscles
-if the cells normally containing this cytoplasm are removed - will not form tail muscles.
- tail muscles of tunicates are formed autonomously by acquiring the mRNA for a transcription factor from the egg cytoplasm. (Whittaker 1973; Nishida and Sawada 2001).
Conditional Specification: When a cell becomes dependent on its position in the embryo. Its fate is determined by interactions with_____________
neighboring cells.
Conditional Specification: If cells are removed from the embryo, the remaining cells can_________________ for the missing part
regulate and compensate
-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
Germ Plasm Theory first testable model of cell specification is proposed by
August Weismann in 1888
- in which each cell of the embryo would develop autonomously
-proposed that the sperm and egg provided equal chromosomal contributions, both quantitatively and qualitatively, to the new organism
Germ Plasm Theory
Who experimented on each blastomeres from a 2-cell embryo. Wherein each isolated blastomere regulated its development to produce a complete organisms
Hand Dreisch (Isolation Experiment)
Based on William Roux experiment: Poking a hot needle and killing one cell in a 2-cell frog embryo thus killing one cell but the other cell continuous to develop thus proving what?
specification was autonomous, and that all the instructions for normal development were present inside each cell (but this is rejected afterwards)
- specification of presumptive cells within such a syncytium
-predominates in most insect classes
-specification of body regions by interactions between cytoplasmic regions prior to cellularization of the blastoderm
-variable cleavage produces no rigid cell fates for particular nuclei
-after cellularization, both autonomous and conditional specification are seen
syncytial specification
cytoplasm that contains many nuclei
syncytium
- morphogen gradients of transcription factors within a cell
-intracellular proteins are made in specific sites in the embryo, diffuse over long distances, and form gradients in which the highest concentration is at the point of synthesis and gets lower as the morphogen diffuses away from its source and degrades over time
-concentration of specific morphogens at a particular site tells the nuclei where they are in relation to the source of the morphogens.
syncytial specification
produced by the anterior most portion; concentration that is highest in the anterior and declines toward the posterior.
Bicoid
Syncytial specification in Drosophila melanogaster: long axis of the Drosophila egg is spanned by ____________ by bicoid and caudal
opposing gradients
posterior most portion of the egg forms a posterior-to-anterior gradient of this transcription factor.
Caudal
Syncytial specification in Drosophila melanogaster: Bicoid is from _______ and caudal from the _______
Bicoid: Anterior
Caudal: Posterior
Specification of the germ layers: Vegetal Hemisphere cell
Cells of the gut and associated organs (endoderm)
Specification of the germ layers: animal hemisphere balstomeres
Ectoderm (skin and nerves)
Specification of the germ layers: Mesodermal cells
From the internal cytoplasm around the equator
Specification of the germ layers: General fate map
Imposed on the embryo by the vegetal cells
Two major functions of vegetal cells
1.differentiate into endoderm
2. 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
destroyed by antisense oligonucleotides, the entire embryo becomes epidermis, with no mesodermal or endodermal components.
VegT transcripts
Soxl7 is transcription factor that activates genes that specify cells to be ________
endoderm
VegT activates a set of genes that encodes the ____ transcription factor (activating the genes that specify cells to be endoderm)
Sox17
VegT codes/ activates ____, which instruct the cell layers above them to be mesoderm
Nodal Paracrine Factors
Nodal Paracrine Factors: Secreted from the vegetal cells in the nascent endoderm and signal the cells above them to express __________ (activate the eomesodermin and Brachyury (Xbra) genes - specified as mesoderm)
phosphorylated Smad2
phosphorylated Smad2 activate the__________ and ______________ - specified as mesoderm)
eomesodermin and Brachyury (Xbra) genes
Eomesodermin and Smad2 proteins - activate the zygotic genes for the_______________, thus creating a positive feedforward loop that is critical in sustaining the mesoderm.
VegT proteins
The absence of the induction of Eomesoderin and Smad2 protein may lead to what?
Cell becoming an ectoderm
The fundamental germ layers are becoming specified in what stage in the development?
Late Blastula Stage
Categories under determination of embryonic axes
1.Dorso-Ventral Axis
2.Antero-Posterior Axis
3.Left-Right Axis
Categories under Dorso-Ventral Axis
1.Spermann’s Organizer
2.Nieuwkoop Center
3. Regional Specificity
mass of ventral cells, (the belly piece)
Bauchstiick
anterior-posterior axis formation is linked to the formation of the dorsal-ventral axis
Xenopus (other amphibians)
-University of Freiburg, Germany
-Nobel Prize for Spemann in 1935
-early newt blastomeres have identical nuclei, each capable of producing an entire larva
Hans Spemann and Hilde Mangold
Spemann did an experiment, wherein he created a ligature to an embryo of Triturus taeniatus restricting one half of the embryo. He concluded what?
early amphibian nuclei were genetically identical and that each cell was capable of giving rise to an entire organism
-process, whereby CNS forms through interactions with the underlying mesoderm
-one of the principal ways that the vertebrate body becomes organized
-first mesodermal cells to migrate over the dorsal blastopore lip - induce the ectoderm above them to produce anterior structures (forebrain)
-mesoderm that involutes later will signal the ectoderm to form more posterior structures (hindbrain and spinal cord)
-dorsal blastopore lip and its descendants - “the organizer“; region is different from all the other parts of the embryo
Primary Embryonic Induction
When the egg was divided perpendicular to the first cleavage plane, some ________ substance was not equally distributed into the two halves
cytoplasmic substance
“Something in the region (grey crescent) of the gray crescent was essential for proper embryonic development.”
Hans Spemann’s Experiment
_________, which showed that the gray crescent region gives rise to those cells that form the dorsal lip of the blastopore
fate maps
dorsal lip cells are committed to invaginate into the ________, initiating _________ and the formation of the _________________.
-blastula
-gastrulation
- head endomesoderm and notochord
found that the cells of the early gastrula were uncommitted, but that the fates of late gastrula cells were determined
1918
-derived from the gray crescent cytoplasm
- its fate autonomously determined.
-transplanted into the presumptive belly skin region of another gastrula
-continued to be dorsal
blastopore lip
dorsal lip of the blastopore
future amphibian development depends on the interaction of cells that are rearranged during __________
gastrulation
Dorsal Lip of the blastophore initiate what?
gastrulation and embryogenesis in the surrounding tissue
dorsal lip of an early T. taeniatus gastrula was removed and implanted into the region of an early T. cristatus gastrula fated to become ____________
ventral epidermis (belly skin)
becoming organs that normally they never would have formed
Host cells
-induced the host’s ventral tissues to change their fates to form a neural tube and dorsal mesodermal tissue ( somites)
-organized host and donor tissues into a secondary embryo with clear anterior-posterior and dorsal-ventral axes.
Dorsal lip cells and their derivatives (notochord and head endomesoderm) as the organizer
with the host tissues to form a complete neural plate from host ectoderm
Dorsal Lip Cells
During normal development, Dorsal lip cells and its derivatives “organize” the dorsal ectoderm into a _______ and transform the flanking mesoderm into the __________________ (Spemann 1938).
-neural tube
-anterior-posterior body axis
key induction —in which the progeny of dorsal lip cells induce the dorsal axis and the neural tube
Primary embryonic induction
__________________showed that the dorsal lip of the blastopore, along with the dorsal mesoderm and pharyngeal endoderm that form from it, constituted an “organizer” able to instruct the formation of embryonic axes.
Spemann and Mangold
These cells are in the right place at the right time, at a point where two signals converge.
Organizer
tells the cells that they are dorsal
first signal
These signals interact to create a polarity within the mesoderm that is the basis for specifying the organizer and for creating dorsal-ventral polar
Organizer: First Signal and Second Signal
these cells are mesoderm.
second signal
A)Isolated animal cap cells become a mass of ciliated ectoderm, isolated equatorial (marginal zone) cells become mesoderm, and isolated vegetal cells generate gut-like tissue.
B) If animal cap cells are combined with vegetal cap cells, many of the animal cells generate mesodermal tissue
mesodermal induction by vegetal endoderm (Nieuwkoop and Nakamura and Takasaki)
Nakamura and Takasaki (1970) showed that the mesoderm arises from the ____________ at the border between the animal and vegetal poles
marginal (equatorial) cells
Dorsal most vegetal cells of the blastula, which are capable of inducing the organizer (Gerhart et al.1989).
Nieuwkoop center
REGIONAL SPECIFICITY OF INDUCTION: induced balancer & oral apparatus
Archenteron Roof
-Self- differentiating tissue
-Induces adjacent tissues (VE) to:
>change their fates & form neural tube and
somites
>organize the host & donor tissues into 20
embryo with clear antero-posterior and dorso-
ventral axes.
DLBlastopore - The Primary Organizer
Nieuwkoop Center lies on the _____& will give rise to the ____________.
-center
-left & right sides
Cortical rotation toward the sperm entry point defines the_____________-.
midline of the embryo
-promotes the formation of cement gland, eyes, & nasal placode.
-Inhibits Nodal, Wnt & BMP4
CERBERUS
necessary but insufficient for induction of trunk and posterior region
Fibroblast growth factor (FGF)
ventral mesoderm inducers; antagonistic to cerberus
BMP4 and Wnt
FACTORS ASSOCIATED WITH AXIAL DEVELOPMENT
1.Maternal Location
2.Nieuwkoop Center
3.Spemann Organizer
4.Fibroblast growth factor
5.Goosecoid
6. BMP
epidermal and ventral development
BMP
-defined by the dense area of cells (Posterior Marginal Zone)
-established the other body axes of the embryo.
-where the Primitive Streak will develop
Antero- Posterior Axis
____________ –> bilateral
Symmetric embryo via gravity.
Radial blastoderm
Transplantation of posterior marginal zone cell from one chick blastoderm to another sometimes results in additional ____________
Primitive Streak
Chick embryo: ____________ of the blastoderm becomes the site of the future primitive streak – the posterior end of the embryo.
Uppermost side
cell fate is determined after 32-cell stage depending on the position
Blastocyst
embryo proper
Inner Cell Mass
extraembryonic membrane and placenta
Trophectoderm (Mouse)
Axes Formation in Mouse: 6.5 days after fertilization – _____ begin to form the primitive streak at the posterior pole.
epiblast
Mouse: ________________ = Dorso-ventral axis
Embryonic- Abembryonic axis
The gut rotates _________ during development resulting in a right sided stomach
asymmetrically
The axes formation in mouse requires __________, which is the most anterior region including the brain
Anterior Visceral endoderm
Axes Formation In Mouse: primitive streak elongates and form the ___________________ of other vertebrates
node analogous to the organizer
Organ handedness is specified by _____
iv gene
- a condition in which there is a reversal in the position of the internal organs.
- established by complex epigenetic and genetic cascades.
SITUS INVERSUS
condition where organs of normal and inverted asymmetry are resent in the same animal mutant mice)
heterotaxy
Responsible in the formation of the dorsal lip of the blastophore
Beta- Catenin
Activate Beta-catenin
Goosecoid
Direct the movement of the cell to the blastophore
Goosecoid
