Download Early Cleavage Patterns

BIOLOGY 52/SECTION 7 DEVELOPMENT-DR. LILJEGREN
Lecture 3
Early Cleavage Patterns
Fertilization>Cleavage>Gastrulation>Organ Formation. Cleavage and Gastrulation are two
critical stages of embryogenesis.
1.
After fertilization, early development in most animals starts with a series
of rapid cell divisions called CLEAVAGE that serves several purposes:
a.
rapidly increases cell number, dividing the cytoplasm of the egg into
smaller and smaller cells called blastomeres
i. Length of "cell cycle" is regulated in most cell types.
ii. In the early embryo it's much shorter- many divisions very rapidly
iii. How so fast? No growth phases (just mitosis and DNA synthesis) &
relies on stored material- no transcription. So faster, and no volume
increases
iv. Transition from fertilization to cleavage caused by activation of MPF
(mitosis-promoting factor). Active MPF dependent on presence of
cyclin B, which accumulates during S and is then degraded after cells
reach M phase.
v. Cyclin regulators stored in egg cytoplasm, so cell cycle doesn’t require
zygotic transcription for numerous cell divisions.
b.
distributes egg contents, often asymmetrically, to the cells
2.
What controls the process of cleavage?
a.
The cleavage plane forms perpendicular to the mitotic spindles
(microtubules made of tubulin).
b.
The contractile ring (microfilaments made of actin) that forms creates a
cleavage furrow.
c.
Placement of the mitotic spindle is regulated by attachment sites that
form on the inside of the cell membrane. The composition of the sites is
unknown, but in some animals is controlled by cytoplasmic factors
made by mom. This is an example of a MATERNAL EFFECT, in which the
phenotype of the EMBRYOS is controlled by the genotype of the MOM.
3.
Different organisms have different patterns of cleavage.
Cleavage is quite variable among organisms, but like most developmental
processes the underlying mechanisms and goals are similar. The type of
cleavage primarily depends on:
a.
the amount and location of yolk in the egg
Cell divisions occur at a faster rate in yolk-free parts of egg, in general
yolk represses cleavage. Yolk-rich pole = vegetal pole, vs. animal pole
1. Completeness of cleavage
• HOLOBLASTIC is complete cleavage, cleavage furrow extends through
the entire egg. Sea urchins, snails, frogs, mammals.
• MEROBLASTIC is partial cleavage (usually stops when it hits yolk).
Yolk inhibits membrane formation. Flies, fish & birds
2. Pattern of cleavage
• ISOLECITHAL (=equal yolk) eggs have a little yolk that is evenly
distributed. Sea urchins, nematodes (C. elegans), mammals (including
mouse and human). Since little yolk, embryos have to obtain food
elsewhere.
• TELOLECITHAL eggs have a lot of yolk with only one small spot that is
free of yolk. This is where the embryo develops. Fish (zebrafish) and
birds (chick)
• CENTROLECITHAL eggs have yolk in the egg center. Most insects, like
flies.
b.
factors in the egg cytoplasm that influence the angle of the mitotic
spindle and the timing of its formation.
1. The relationship of the cleavage planes to each other and egg axis:
• MERIDIONAL cuts through embryo poles like a geographic meridian
• EQUATORIAL cuts through the area of greatest diameter
2. timing of mitotic spindle formation
• cleavages can be SYNCHRONOUS, occuring at the same time. ie. sea
urchin
• or ASYNCHRONOUS, occuring at different times. ie. mammals
3. symmetric (equal): daughter cells are same size vs. assymetric
(unequal) cell divisions: some daughter cells are a different size
4.
Molluscs and Spiral cleavage:
a.
Unlike radial cleavage, cleavage planes are not parallel or perpendicular to the
animal-vegetal axis of the egg, rather cleavage planes are oblique.
b.
Cells touch one another at more places
c.
Fewer cell divisions before gastrulation so easier to make fate maps
5.
Frogs (A type of HOLOBLASTIC cleavage): displaced radial cleavage due to more yolk
(moderate yolk disposition--between isolecithal and telolecithal). Cell divisions much
slower as can be seen by the cleavage furrows.
6.
Fish and bird cleavage:
a. Fish and bird eggs are telolecithal (lot of yolk with only one small spot free of yolk).
So cell divisions only occur in the small disc of cytoplasm=DISCOIDAL cleavage pattern
7.
Insect (fly) cleavage:
a.
Fly embryos are CENTROLECITHAL (yolk at center of egg). They have a
central yolk-rich ENDOPLASM and a peripheral yolk-free area called the
PERIPLASM. Cells do not divide until after several rounds of nuclear
division.
b.
Laser confocal micrographs showing stained chromatin. Nuclei start to
divide in the endoplasm, but move to the periplasm, where they align
against the outer membrane. Divisions of the cytoplasm only occur in the
cell’s peripheral region=SUPERFICIAL cleavage pattern.
c.
POLE CELL NUCLEI get to posterior end first, they are pinched off to form
POLE CELLS that will be the germ line of the embryo.
d.
rest of nuclei at edge make incomplete cleavage planes after 13
rounds of mitosis. This is the CELLULAR BLASTODERM STAGE. Finally,
nuclei are pinched off into cells.
8.
Mammalian cleavage is unique in several ways:
(most studies have used mice as model organisms)
a.
First, initial cell divisions in mammalian eggs are slow (12-24 hours)
compared to embryos that develop outside the mother (ie. flies 10
minutes), presumably because mammalian embryos are protected.
b.
c.
d.
e.
f.
g.
h.
Second, mammals have a ROTATIONAL cleavage pattern. In mammals,
the first cleavage is MERIDIONAL, but during the second cleavage, one
blastomere divides meridionally and the other blastomere divides
equatorially.
Third, mammalian cleavage is asynchronous from an early timepoint. In
other words, mammlian blastomeres do not all divide at the same time.
So, embryos frequently contain odd numbers of cells, ie. 3 or 6
blastomeres, instead of exponential 2-to 4-to 8-cell stages.
Fourth, unlike most other animals, zygotic transcription occurs during
early cleavage in mammals, and produces proteins necessary for
subsequent cleavages to occur. In mice, this switch from maternal to
zygotic control occurs at the 2-cell stage.
Fifth, at the 8 cell stage the mammalian embryo undergoes
COMPACTION. The outer cells form tight junctions with each other,
while the inner cells form gap junctions, enabling small molecules and ions
to pass between them. Compaction requires the cell adhesion protein
E-cadherin.
The 16 cell stage embryo is called the morula, and it has a small group of
internal cells surrounded by a larger group of outer cells. This
environment of outer vs. inner cells sets the stage for differentiation in
development. Most of the descendents of the outer cells form the
TROPHOBLAST (trophectoderm), while the inner cells become the
INNER CELL MASS. These cells will give rise to all parts of the embryo
proper and some extraembryonic structures, while the trophoblast cells
will participate in placenta formation and implantation. This distinction
between trophoblast and inner cell mass cell fate is the first
differentiation event in mammalian development.
The BLASTOCYST forms at 32-64 cells by the outer (trophoblast) cells
secreting fluid into the morula to form a cavity called the BLASTOCOEL.
Hatching from the zona pellucida is prevented until the blastocyst reaches
the uterus. Premature hatching/implantation in the oviduct is known as
ectopic or tubal pregnancy and can cause a life-threatening
hemorrhage.
9.
After fixed number of divisions, Cleavage ends in a controlled manner
called the MID-BLASTULA TRANSITION (MBT)
a.
if it ended haphazardly there would be huge differences in cell numbers
among embryos of the same species.
b.
controlled by the nuclear/cytoplasmic ratio, which is almost 0 in a
fertilized egg with tons of cytoplasm, but approaches 1 in cells after many
nuclei are made without extra cytoplasm.
c.
during the MBT the cell cycle lengthens and often G1 is used for the first
time.
d.
the embryo starts to synthesize its own RNA (often maternal RNA is
actively destroyed at this time). Remember this is a difference between
mammals and other animals
e.
the cells become motile (prelude to gastrulation).
10.
Maternal effect mutations and cleavage: SNAILS
a. In a certain kind of snail, the shell normally spirals in a right or DEXTRAL
COIL. Mutants make embryos with left or SINISTRAL COILS.
b. The direction of the coils follows the cleavage pattern. Remember that the
cleavage plane forms perpendicular to the mitotic spindles, whose placement
is regulated by the spindle attachment sites.
c. In snails, cytoplasmic factors made by mom determine these attachment
sites.
d. Alfred Sturtevant came up with the idea in 1923 that left-coiling snails are
caused by a recessive maternal effect mutation, ie. the phenotype of the
EMBRYOS is controlled by the genotype of MOM. Since the mutation is
recessive, this means that homozygous D/D mothers and heterozygous D/d
mothers make right-handed embryos, and homozygous d/d mothers make
left-handed embryos.
e. How does this work? (to be continued)