Molecular principles of development summary 7-11

Molecular principles of development
Book = Wolpert et al (5th edition)

Lecture 7: Germ cells, fertilization, and sex I
THE DEVELOPMENT OF THE GERM CELLS
Today: Focus on the adult mouse to a fertilized egg which start developing again.
Pay attention, don’t confuse!
Germ layers = 3 mayor tissues which develop early during embryonic development.
This lecture and the next is about germ cells = oocyte & sperm that will result in a fertilized
oocyte/zygote. So a completely different event during embryonic development.
During early embryonic development you can distinguish between the somatic cells or germ cells
Somatic cells = differentiating cells or stem cells.
Germ cells = sustain a species, develop into a sperm and egg cell.

Function of germ cells:
- preservation of genetic integrity (protection from mutations or other events)
- generation of genetic diversity
- transmission of genetic information to the next generation
GERM CELL CYCLE

This lecture;
1) Germ cell specification
2) Germ cell migration
3) Meiosis
4) Genomic imprinting
Next lecture;

5) Fertilization
6) Sex determination
7) X chromosome inactivation




This lecture we discuss the germ cell cycle:
The oocyte has to be fertilized by the sperm cell. Already very early on you get specialization of
primordial germ cell precursors which will further develop into primordial germ cells.
During embryonic development these cells which migrate to the genital ridge. Here they can further
develop into a sperm cell/ oocyte.  so a lot of movement is happening. They are not made at the
site where they end up. They are made in a completely different space and during development they
start moving.
This lecture: how are these primordial germ cells specified? Germ cell migration.

1) SPECIFICATION AND MIGRATION


1

,This lecture is focused on 3 organisms: drosophila, zebrafish and mouse
by what mechanisms are germ cells specified? How do germ cells migrate to the correct site?

EARLY GERM CELL DEVELOPMENT IN DROSOPHILA
Drosophila: Once the oocyte is fertilized how do germ cells develop?
Orange (P) = fertilized germ cell.
The oocyte contains a specialized cytoplasm > pole plasm or germ
plasm (present at the posterior end of the oocyte) there is gradient of
certain molecules in the oocyte:
polar granules (on the posterior site of fertilized oocyte)
Fig.10.1 in Wolpert: 3 different drosophila oocytes with different
genotypes (P.Y.G) pole plasm removed from posterior site to anterior
side. So germ cells develop on the anterior side.
Conclusion: there are instructive signals in the cytoplasm which make
the germ cells develop. The germ cells are derived from the Y oocyte, so
only the cytoplasm specifies the germ cells! If you transplant those
germ cells to another oocyte (G) in the anterior side. You see the germ
cells from G and Y further develop. These germ cells are specified very
early on before stem cells and other somatic cells derive.
Maternal genes are involved in pole plasm formation.
A gradient which are set up for the germ cell specification:
Oskar is expressed at posterior pole of the oocyte (by the mother)
oskar organizes the germ plasm and directs localization of the posterior determinant nanos.
Bicoid is on the anterior side. Mechanism involved there are a bit
similar.
= gradient set up in the oocyte by the mother!
Bicoid and oskar mRNA are delivered into oocyte by nurse cells.
Once provided they still need to go anterior or posterior: microtubules +
motor proteins
dynein transports bicoid mRNA towards minus end of microtubules 
anterior end of the oocyte.
kinesin transports oskar mRNA towards the plus end of microtubules  posterior end of the
oocyte.


Experiment)
untranslated region put into oskar gene. Turns out that untranslated
region is important for localization of bicoid and oskar. Oskar gene +
untranslated gene of bicoid  oskar gene wil end up at the anterior
side (were normally the bicoid would be).
ZEBRAFISH
Zebrafish works a bit similar:
germ plasm is present at the cleavage furrows, consists mainly of mRNA.
There is germ plasm, zebrafish specify the germ cells at the 32-cell stage were the germ plasm is
incorporated into the cells.
32-cell stage the germ plasm is found in distinct primordial germ cells



2

, Experiment) Germ plasm can be successfully be removed from 4 cell stage embryos:
removal result in absence of germ cell lineage
germ plasm contains components that are essential for germ cell development
2) GERM CELL MIGRATION ZEBRAFISH
Zebrafish = 4 different location were the germ cells are (sphere stage) but they have to end up at 2
locations at the general ridge> you need migration. The germ cells push aside other cells to get to
their end point.
What are the signals to provide migration?
1 very important gradient important: SDF1 gradient guides migration of primordial germ cell.
Based on this gradient the cells can sense where they are in the embryo and they can migrate.
They also use CXCR4 receptor for SDF1.
CXCR4 on the surface of germ cells
SDF1 in the embryo
Both are needed for the zebrafish to produce!
GERM CELL MIGRATION IN MOUSE
Slightly different in mouse:
No evidence for germ plasm in mouse so germ cells need cell-cell interactions in the embryo to
induce germ cell specification.
Cultured embryonic stem cells can give rise to germ cells and somatic cells.

Primordial germ cells in proximal epiblast (prospective extra-embryonic mesoderm), express
blimp1.
Primordial germ cells have the potential to form all germ layers
proximal epiblast: germ cells location
blimp 1 > represses genes associated with somatic development and activated genes characteristic of
germ cells. Germ cells have to remain germ cells, so repression of somatic development.




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