This is a summary of the course 10 theory lessons on developmental biology. Personally, after learning this summary, I got an 8.1 for theme test (without rematch)!
This is a summary of the course 10 theory lessons on developmental biology. After learning this summary, I myself obtained an 8.1 for ...
Development biology COURSE 10BM
Lesson 1:
Preformism: with the spreading of the Preformism theory, sperm cells were seen as cells that could
carry the preformed offspring! People thought little humans were in sperm cells.
Preformism VS epigenesis
Organisms develop from miniature versions of themselves, homunculi
Believed that the form of living things exist prior to their development.
Offspring develop from a tiny fully-formed embryo contained within the head of
a sperm cell.
Who was was the first “thinker” to elaborate an idea** about embryo development and
transmitted it to the European culture?
Aristotle: the idea of epigenesis: his writings shaped Western philosophy and natural science for
greater than two thousand years. Aristotle established methods for investigation and reasoning He
provided a preliminary theory on how embryos generate and develop: an organism develops
gradually from undifferentiated material. Later called Epigenesis.
Epigenesis: William Harvey (1578-1657, UK)
One of his life’s major work, published in 1651 “On Animal Generation”
He was the first to show that life came from eggs (i.e. embryos from yolk)
Recapitulation: Johann Friedrich Meckel (1790s, Germany) and Etienne Serres (1824, France)
Also known as “Embryological parallelism”: Development of the embryo goes through stages
that represent evolution of ancestors
Theory of recapitulation by Ernst Haeckel: “Ontogeny recapitulates Phylogeny”
Ontogeny = Embryonic Development represents
Phylogeny = History of Evolution
In our development there are also part of evolution. If you look at embryos from different species
they look alike. We develop in a similar way among organisms. This is conserved during evolution.
Recapitulation: Each species seems to recapitulate ancestral morphologies during its own
development. Organisms have common genetic developmental plans.
Organisms arise by a relatively slow process of progressive change = development
Development begins with a single cell—the fertilized egg, or zygote (fully undifferentiated
cell), which divides mitotically to produce all the cells of the body (fully differentiated
cells). Development is differentiation and multiplication of cells. So cells will both multiply
and differentiate once you develop
Development does not stop at birth, or even at adulthood. Every day we replace >1 gram
of skin cells. Bone marrow generates millions of new red blood cells every minute.
Regeneration of severed parts (starfish). Metamorphosis (frogs,
butterflies)
Developmental biology is the discipline that studies embryonic development and
other processes related to organisms’ development.
So what is the most undifferentiated cell? zygote
, From undifferentiated differentiated. What are the processes that can differentiate cells?:
When you have a zygote (single cell) = undifferentiated. At this point it starts the differentiation
process which is driven by multiple molecular mechanisms like cellular polarization, small set of
proteins etc.
1. Cellular polarization (this can take place in multiple tissues, not only zygote):
Extrinsic regulation Intrinsic regulation
• cell is in close contact with an • Regulators of self-renewal within the cell are
external niche localized asymmetrically
• only one daughter cell can • They are inherited by only one of the two daughter
maintain contact with the niche cells
Extrinsic: something from outside
What is the initial process what can break symmetry? The first cell is totally symmetric. The enter of
the sperm can break this. This starts to chain reaction establishment of polarity polarity
maintenance mitosis (anaphase)
Actin (part of scaffold of cell) and myosin will go to more one side of the cell and not to the other
Symmetry breaking in the early C. elegans embryo
(1) PAR Proteins: An internal cellular signal for Polarisation
Par proteins = Partitioning defective genes
Homologs throughout the animal kingdom
Organizing cell asymmetry, co-ordinating polarisation of cytoskeleton
Key features of PAR polarity factors
PAR polarity is well conserved in different cell types of many organisms
They create protein domains which have distinct compositions for different functions
PAR proteins interface in the lumen
PAR proteins localize in a mutually exclusive manner
Par proteins drive their own asymmetric localisation
Anterior PAR are required to prevent posterior PAR
proteins from localising at the anterior and vice versa
PAR proteins become asymmetrically localised after
fertilisation and form the basis of cell polarity. PAR proteins
act in a mutually exclusive manner! PAR proteins work the
same in C. elegans as in humans. Migrating cells: PAR proteins
PAR-2 is on one side and PAR-3
is on other side. What happens
when you have a mutant that
doesn’t have PAR-2. If you do a
staining of PAR-2 you don’t get a signal. If you stain PAR-6 you get
strange distribution on other side. So PAR-2 inhibits PAR-6 and once
PAR-2 is gone the PAR-6 can go to the other side.
, so cellular polarization asymmetric cell division
The first cell (zygote) division is asymmetric
in wildtype situation anterior is bigger than posterior . when there is loss of PAR-
2 or PAR-3 you can see that there is no difference in the cells. So PAR is involved
in asymmetry
par mutants show defects in cell polarity
Cells can be polarised by spindle localisation
The mitotic spindle:
Spindle poles give an orientation to cell division
(e.g. Anterior-posterior, Dorsal-Dentral)
The Spindle is subjected to mechanical forces and can rotate
Microtubular scaffold that allows the cells to
divide. 2 poles: spindle poles that in
metaphase (when chromosomes are aligned) the poles pulls the
chromosomes to the two sides of the cells.
The spindle apparatus aligns with respect to polarity axis
Orientation of cell division depends on the position of the mitotic spindle relative to the axis
of polarity
Many scientific works have shown that improper spindle orientation and disease and tumour
formation are linked
Spindle orientation is an essential determinant of cell fate specification
Spindle positioning regulates orientated cell divisions (C.
elegans)
In later divisions, the axis rotates 90 o
Pulling forces acting on astral microtubules are critical for
spindle positioning in one-cell C. elegans embryos.
There is also evidence indicating that cortical forces
contribute to centration/rotation. First division: Anterior
Posterior axis
The way how a cell division goes can actually decide
whether differentiation takes place
par-proteins take place Second division: Dorsal-
in asymmetry lateral axis
Par genes are necessary for correct
spindle positioning and cell divisions in C. elegans zygote. Their
functional domains interact with microtubules in order to influence the
spindle position
Cell polarity creates asymmetry in the forces that pull on astral
microtubules
This is dependent on the presence
of PAR proteins (PAR-3
(blue), PAR-2 (red))
that regulate force
generators, likely
dynein patches
localized at the cortex
(green).
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