Developmental Biology
Lecture
Cleavage
Gilbert reading: chapters with "Cleavage" – in eds. 6, 7, or 8: chap 8 OR ed.9, chap 5 OR ed.10, chap5+:
"Early development in selected invertebrates”
PLUS: Cleavage in fly, frog, z-fish, bird, mammal - - In editions 6-10 -pages to read: ed 6: 263-6, 305-7, 345-9, 354-5, 363-8
OR ed 7: 221-228, 239-246, 263-6, 303-6, 339-343, 345-8, 354-5, 363-8
OR ed. 8:211-5, 217-223, 229-235, 237-241, 243-5, 253-5, 291-4, 325-8, 336-7, 348-350.
OR ed. 9: 159-163, 165-172, 178-184, 187-9, 192-6, 242-3, 273-5, 287-8, 300-1.
OR ed. 10: 153-19, 161-6, 179-182, 217-224, 232-3, 241-4, 271-4, 285-7, 298-302.
I. Cleavage - The phase of very rapid cell divisions during which the large volume of the fertilized egg is
divided up to create many smaller nucleated cells (blastomeres) of the embryo (blastula)
A. Cleavage commences following fertilization
B. Cell division is carried out by two coordinated processes carried out by the cytoskeleton
1. cytokinesis- cell division- by the contractile ring of actin microfilaments
2. karyokinesis-nuclear division- by tubulin microtubules of mitotic spindle
C. The end of cleavage is marked by:
1. a significant decrease in cell division rate
2. the beginning of dramatic morphogenetic movements known as Gastrulation
3. operationally - a new ratio between volume of nucleus and cytoplasm (see I.D3)
D. For most species, there is no net increase of volume during cleavage
1. cells cycle from M phase-S phase-M phase, skipping G1 and G2 (Gap & growth)
2. subsequent cleavages produce smaller and smaller cells
3. the ratio of cytoplasmic to nuclear volume decreases as cleavage continues
a. this change allows for titrating out factors on the newly made chromatin
-this signals transitions (via proteins like cyclin) to: end cleavage,
start transcription, activate specific genes, start gastrulation
THAT IS CLEAVAGE. THE REST IS DETAILS FOR EACH ORGANISM.
Cleavage is coordinated and programmed differently for different groups of organisms,
largely according to the parameters of:
***The amount and distribution of yolk in the cytoplasm
***Factors influencing the timing and placement (angle) of the mitotic spindle
II. RADIAL HOLOBLASTIC CLEAVAGE- little yolk- simplest to visualize and understand
A. all cleavages are (with respect to the animal-vegetable axis):
parallel to a-v axis
(meridional) m
or perpendicular to a-v axis
(equatorial)e
B. in Synapta, divisions are m, m, e, then alternating m, e until a 256 cell blastula is formed
1. The resulting blastula is a single cell thick ball of cells surrounding a central
cavity called a blastocoel
- this blastula is radially symmetrical around any meridian
C. Sea urchins undergo radial holoblastic cleavage with some modifications
1. At 16 cells, the embryo becomes a collection of 8 animal mesomeres,
4 vegetal macromeres and 4 vegetal micromeres
2. Micromeres form at the proper time, even if early cleavages are disrupted
(by hypotonic sea water exposure or shaking)
3. the blastula changes shape at the end of cleavage by forming tight junctions
between cells, and by the swelling of the blastocoel (by osmotic pressure)
III. UNEQUAL RADIAL HOLOBLASTIC CLEAVAGE- AMPHIBIANS
A. high concentration of vegetal yolk slows the cleavage furrow of amphibians vegetally
B. equatorial cleavage furrows are displaced toward the animal pole
1. cleavage-faster on the animal pole side, more divisions & smaller cells are formed
2. the blastocoel forms at early cleavages, and later serves to separate a and v tissues
IV. SPIRAL HOLOBLASTIC CLEAVAGE
A. cleavage is "spiral", cells touch each other more extensively, and gastrulation starts early
B. the spiraling proceeds in a genetically predetermined direction for a given species
1. genetic mutants show (in the snail Limnaea peregra) that the coiling direction of
the snail is a direct consequence of the direction of spiraling of cell divisions
2. the mutants coiling is dependent on the genotype of the snail's mother
this is a maternal effect due to the way morphogens are deposited in the egg
V. ROTATIONAL HOLOBLASTIC CLEAVAGE - MAMMALS
A. divisions are slowest in animal kingdom
B. the cleavage planes of the second cleavage are rotated 90° from each other-"rotational"
C. cleavage is asynchronous- can see odd numbers of cells
D. embryos undergo compaction at the 8 cell stage
1. a number of cell surface protein mediate compaction (e.g. uvomorulin)
2. Compaction is stabilized by tight junctions, inner cells - connected by gap junctions
E. the 16 cell morula consists of 1-2 internal cells surrounded by a group of external cells
1. the outer cells become the trophoblast (trophectoderm) cells (then the chorion)
will implant and nourish the embryo
2. the embryo is derived from the inner cell mass - later called the blastocyst
a. from the 1-2 cells, plus an occasional trophoblast cell at the 32 cell stage
b. at 64 cells, there is no cross contribution
3. expts with allophenic mice (fusion of 2 + blastulas)-embryo stems from 1-3 cells
4. experiments with allophenic mice, experiments to inactivate 1 of 2 cells of a rabbit
embryo, and the phenomena of fraternal twins point out that early mammalian
development is a clear case of Regulatory development
VI. MEROBLASTIC CLEAVAGE TYPES
DISCOIDAL CLEAVAGE- Fish, Bird, Reptiles
Due to very heavy yolk deposits at the vegetal pole, cleavage takes place only in the
blastodisc, at the animal pole- the blastoderm is first formed as a monolayer
Later the subgerminal space, area pellicula, and area opaca appear
SUPERFICIAL CLEAVAGE- Insects and other arthropods
Heavy yolk concentration is a the center of the egg. Nuclei proliferate in a common
cytoplasm (syncytial blastoderm) then migrate to the periphery and cellularize
VII. Many genes and proteins who have the fundamental role of controlling entry to mitosis in the cell
cycle are conserved (and are even interchangeable) from yeast to humans.
A central protein is a tyrosine kinase known as pp34cdc2 (in S. pombe), MPF in Xenopus,
CDC28 in S. cerevisiae
regulating proteins include cyclins (cdc13), CSF (c-mos), string (cdc25), & other oncogenes