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BIOLOGY 624 Fall 2009
Lecture #4:
Reading: Gilbert, Ch8:211-215, 243-249; Ch9:254-258;
Click on this link to access the Powerpoint for Lect. #4
A. Animal development commences after fertilization (in most species)
with a series of fast mitotic divisions that collectively are called the
CLEAVAGE STAGE, because a uniform embryonic volume is divided
into smaller and smaller cells.
1. The outcomes of the cleavage cycles are (Slide 2):
a. rapid increase in cell number
b. distribution of egg contents, often asymmetrically, to the cells
2. Cleavage is accomplished by rapid cycling between the S (DNA synthesis)
and M (mitosis) phases of the cell cycle, with no intervening growth (G1 or G2)
phases. (Slide 3 & 4)
3. Cleavage patterns differ among organisms – parameters that affect
cleavage patterns are (Slide 5):
a. amount and location of yolk in the egg
b. how complete cleavage is
c. relationship of the cleavage planes to each other
d. timing, ie are divisions synchronous (same time) or asynchronous
B. These attributes are mixed and matched in different ways to provide
cleavage programs for each type of embryo. We will briefly discuss two
types of cleavage:
1. Cleavage in the worm C. elegans:
a. each cleavage division in worms has complete cytokinesis
(HOLOBLASTIC) – ie two separate cells are made
b. the divisions are ROTATIONAL, that is after the first division the division
planes are at 90o to each other
c. early cleavage divisions (and subsequent ones) are diagrammed as a
lineage diagram in the worm – the relationships are invariant (Slide 6).
Worm cleavage I (slide 7)1. point of sperm entry is the posterior pole
2. the sperm centriole organizes microtubules to ferry determinants to
anterior or posterior (likely similar to what we learned about localization during
fly oogenesis).
3. cytoplasmic “streams” also move molecules around
4. the first division is ASYMMETRIC in size, forming a large AB daughter and
a small P1 daughter (which contains the P granules).
a. these divisions occur in the context of asymmetric localization of maternal
proteins that affect cell fates
b. proteins called PARs initiate this process, their correct placement ensures
that transcription factors (and other things) get to the right places (slide 8, 9)
c. as might be expected from this localization, the specification of some cell
fates is only dependent on whether they receive a critical determinant: for
example, SKN-1 is a transcription factor required to form pharyngeal tissue
(Slide 10)
d. However, some cells require interactions with neighbors to adopt their
correct cell fate: for example, EMS divides into E (endoderm) and MS
(muscle), but E does not form unless P2 cell is in contact with it. This is
shown by a simple but elegant experiment done by Bob Goldstein (Slide 11)
e. at the level of molecules, this interaction requires Wnt signaling (Slide 12)
2. Cleavage in the mammal (mouse):
a. Mammalian cleavage is very slow compared to most other embryos, one
cleavage each 12-24 hr (see O’Farrell et al review for interesting approach to
this paradox). (Slide 13)
b. it is ASYNCHRONOUS, so you can see 3 or 6 cells in normal embryos
c. cleavage is ROTATIONAL and HOLOBLASTIC as in the worm (Slide 14)
d. However, there is little evidence that determinants are segregated to
individual blastomeres during mammalian cleavage. Instead, it seems that
relative placement in the embryo and subsequent interactions with neighbors
set up the lineages:
1. The 8-cell embryo is called a MORULA. At this time a dramatic event
called COMPACTION occurs – the cells suddenly lose definition and form a
compact ball of cells joined by tight junctions that seal the ball (Slide 15)
2. The next division gives 16 cells and some are on the inside. They form
gap junction interconnections.
3. Very simply, the outside cells become TROPHOBLAST – cells that will give
rise to only extraembryonic tissue including the cells for implantation, while the
inner cells become INNER CELL MASS (ICM) and go on to form the entire
embryo plus some extraemb. lineages. At this point the ICM cells are
equivalent in potential – they each can contribute to all lineages of the
organism. (Slide 16)
C. The mid-blastula transition – this is the controlled end of the
cleavage cycles and transition to more “normal” cell cycling (except in
the mammal which does not have abbreviated cell cycles – see O’Farrell
review for an alternative explanation) (Slide 17)
1. The transition is controlled by the ratio of nuclear material to cytoplasm
(N/C ratio). Early on there is a lot of cytoplasm relative to nuclear material,
thus the ratio is a fraction, but as nuclear material increases and cytoplasm
stays constant the ratio approaches 1.
2. The cell cycle lengthens and incorporates G1 (and G2) for the first time
3. The embryo starts to synthesize RNA (zygotic transcription) and
sometimes maternal RNAs are actively destroyed
4. Cells become motile (prelude to gastrulation).
Cowan CR and Hyman AA. (2004). Asymmetric cell division in C. elegans:
cortical polarity and spindle positioning.