Feb 12 Lecture Outline

Developmental Cell Biology 220

Bio 220 Homepage

Lecture Schedule

Laboratory Schedule

Assignments

Lecture Outline for February 12, 1999 Rearrangement of Egg Cytoplasm & Cleavage Rearrangement of egg cytoplasm

Morphogenetic determinants are separated into different areas of the newl fertilized egg

After fertilization yellow lipid-containing cytoplasm migrates ventrally.
A yellow crescent is formed in the area which will later become larval muscle cells.
These movements depend on Ca⁺² released after fertilization and construction of microtubules.

After fertlization the surface layers of the animal hemisphere rotate 30^o toward the point of sperm entry.
The newly exposed surface area will later invaginate during gastrulation.
Polymerization of microtubules is involved.

Cleavage

The site of the first cleavage furrow is determined by the point of sperm entry and the subsequent cytoplasmic movements of the newly fertilized egg.
In general, the embryonic volume does not change from that of the newly fertilized egg until after the first12 cell divisions (producing ~ 4000 cells).
Patterns of cleavage (see Table 5.1)

Are determined by the amount and distribution of yolk protein
Cytoplasmic factors influence the angle and timing of the formation of the mitotic spindle.
Based on whether the daughter cells are completely separated or not.

blastocoel formation involves accumulation of proteins within the blastocoel, followed by accumulation of water (by osmosis)
blastocoel formation also involves adhesive connections to the hyaline layer.

Most amphibian eggs (mesolecithal) are examples of radial, holoblastic cleavage.

Because of the concentration of yolk in the vegetal hemisphere the formation of the cleavage furrow in the yolk-containing ventral portion is slower and lags behind that in the animal hemisphere.
Macromeres (large yolk-containing cells) and micromeres (animal pole cells) form.
E-cadherin is important in adhesion between blastula cells.
Experiment: Injecting antisense oligonucleotides complementary to E-cadherin mRNA prevents Xenopus embryos from forming a blastocoel.

Eggs of the snail (Lymnaea peregra) are examples of spiral, holoblastic cleavage.

Cleavage occurs at oblique angles with daughter cells either moving to the right or to the left.
In snails with a shell coil that opened to the right, after the second cell division (Figure 5.14) the micromeres moved to the right; in snails whose coil opened to the left, after the second cell division the micromeres moved to the left.

The direction of daughter cell movement is controlled by the D gene of the mother (cytoplasmic determinants).
DD or Dd mother, all offspring had coils which opened on the right.
dd mother, all offspring had coils which opened on the left.

Eggs of the tunicate (Styela partita) have bilateral holoblastic cleavage; Fig 5.17

During cleavage, cytoplasm that will give rise to different parts of the embryo is segregated into different blastomeres.
At the 8 cell stage: