2. Cell Specialization Flashcards
Phases of clevage
synthesis
NO G1 and G2 phase
there’s only S and M
- mitosis promoting factor has 2 components CDC (cycline dependant kinase) and cyclin B
- cyclin B protein is degraded allowing cell to pass to S phase
What happens in cytokineses
- cleavage furrow starts/pinches
- formed by microfilliments (made of actin)
- microtubules help seperation (made of å-tubulin)
- plan of cleavage is perpendicular to spindle fibers
- as cycles continue maternal stores get depleted and sygotic genes turn on
- “mid blastula transition”
Differentiation
Generation of specialized type of cells is
Differentiation.
Journey of differentiated cells
Uncommitted
Stages of Commitment
Specification
Determination
DifferentiationStages of Commitment
Specification
Determination
Specification
The first stage of commitment of cell or tissue fate during which the cell or tissue is capable of differentiating autonomously (i.e by itself) when placed in an environment that is neutral with respect to the developmental pathway. At this stage of specification, cell commitment is still capable of being reversed.
Fate of a cell is capable of determining autonomously on its own.
Capable of differentiating by itself in a neutral environment (ex: if placed in a tissue culture dish). Here the cell commitment is reversible. No influencing signals.
• Test for specification relies only on culturing cells in neutral environment without inductive signals.
• If placed in a non-neutral environment the cell fate can
change for example if placed in a different region of an
embryo the cell can follow the fate of other cells in the new location rather than their original fate
Determination
The second and irreversible state of cell or tissue commitment in which the cell or tissue capable of differentiating autonomously even when placed into another region of an embryo in a non-neutral environment
The commitment is irreversible.
Made up its mind and cannot be influenced
Two major ways of commitment:
Conditional Specification
Autonomous Specification
A slight variation occurs in the case of insects
that exhibit syncitial specification
Conditional Specification
“the ability of the cells to achieve their respective fates by interactions with other cells. What a cell becomes is in large measure specified by paracrine factors secreted by its neighbors”
•The cells interact with other cells to achieve their
respective cell fates.
• No macromolecular determinants of egg involved.
• Paracrine factors.
• If a blastomere is removed other neighbouring cells
compensate for the missing cells.
• Usually preceeded by massive cell rearrangements
(epiboly, migration or ingression).
• Predominates in vertebrates seen in few invertebrates.
• The cleavage pattern is variable no invariant fate
assigned to cells.
Autonomous Specification
“a mode of cell commitment in which the blastomere inherits a determinate, usually a set of transcription factors from the egg cytoplasm, and these transcription factors regulate gene expression to direct the cell into a particular path of development”
“of it’s own
what contents is, is inherrent in cell
The cells inherits morphogenetic determinants from the
egg cytoplasm.
• RNA
• transcription factors
• Proteins
• If a blastomere is removed other cells do not
compensate for the missing cell.
• If a blastomere is transplanted in another embryo its fate
does not change.
• Predominates in invertebrates.
Asymmetric distribution - 3 modes
• Molecules are bound to the egg cytoskeleton and are
inherited passively.
• Molecules can be transported along the cytoskeleton.
• Molecules can be associated with centrosomes and can
be transferred to one of the daughter cells
tail muscles from sperm entrance
sperm enter though veg pole
yellow part will form tail muscle ‘yellow crescent’
-tail is formed due to determinates there (RNA molecules = macho are floating around these induce cells to take on mesodermal fate)
-As the sperm pronucleus moves towards the egg pronucleus the yellow cytoplasm moves laterally and marks the location where the future tail muscles would develop.
In Stiu Hybridization
- label cDNA which binds to mRNA
- make holes so cells can take up cDNA
- treat cells with alkaline phosphatase.
- binds with digioxin (tag attatched to antibody which binds to digioxin)
- wash excess, treat with substrate, converts to puruple coloured product
- colored product that accumulates where the mRNA is present.
-if cytoplasm with macho is forced in other cells
it develops into tail muscles
Summary: Muscle in tunicates
- A yellow myoplasm is segregated in the egg.
- The myoplasm is segregated into a small number of
blastomeres at cleavage. - Progeny of these blastomeres and the cells adjacent to
them become muscle. - The determinants in the yellow crescent may be
maternal mRNAs which are segregated to vegetal
cytoplasm. These mRNA encode a transcription factorspolypeptides
which regulate gene expression (macho-1
mRNA provide evidence for this localization - If the myoplasm from an egg is injected into other
blastomeres, they will form muscle. - If the cytoplasm from B4.1 blastomere is injected into
other blastomeres (eg a4.2), then muscle proteins are
expressed in these cells. - If nuclei from other blastomeres (or even larvae) are
injected into anucleate egg fragments, the nuclei express
genes characteristic of the cytoplasm into which they are
placed-only the ones in “yellow” myoplasm express
muscle genes
[BUT NOTE: descendants of B4.1 blastomere (at 128 cell
stage) also give rise to endoderm-so lineage is not completely
given over to muscle “fate”!!]
