Step one: Contact between the sperm and the eggshell

Step two: Contact between the sperm and the egg’s plasma membrane

Fertilization: a multistep process (ECM)

Sperm-eggshell contact triggers the acrosome reaction Example 1-- the sea urchin enzymes Species specificity (equivalent to zona pellucida)

attracts/activates sperm Figure 7.8

Sperm-eggshell contact triggers the acrosome reaction Example 2: Mammals Sperm have ZP3 receptor

Figure 7.8

BLOCKING POLYSPERMY 1.Fast/transient: prevent sperm fusion 2.Slow/permanent: removal of other bound sperm

Fast block 1-3 seconds lasts 1 min Slow block seconds permanent

How do we know this? We can measure these events directly! Figure 7.17Membrane potential of sea urchin eggs before and After fertilization. resting -70 (sperm can fuse with egg) (sperm can’t fuse with egg) +20 mV

Cortical granules: poised for release 15,000 cortical granules in a sea urchin egg contain enzymes that clip the egg’s bindin receptor and any attached sperm alter the vitelline envelope (zona pellucida in mammals) Blocking polyspermy: SLOW BLOCK Figure 7.6

Sperm-eggshell contact triggers the acrosome reaction Example 2: Mammals Sperm have ZP3 receptor

A wave of increased calcium can be visualized moving across the egg from the point of sperm entry Figure 7.20

Fast block 1-3 seconds lasts 1 min Slow block seconds permanent

Fig Fig Cleavage and Gastrulation Gastrulation

Many embryos fail to successfully complete early cleavage divisions or gastrulation CLEAVAGE GASTRULATION

EARLY CLEAVAGE  rapid increase in the number of cells after fertilization  egg contents distributed to cells, often asymmetrically blastomeres

Early BlastomeresNormal Cells The cell cycle is different during cleavage Much shorter cell cycles during early development-- no growth phases and cells rely on stored material Figure 5.1 mitosis DNA synthesis

Mitosis-promoting factor (MPF) Mitosis is dependent on presence of Cyclin B Figure 5.1 Activation of MPF controls start of cleavage

actintubulin mitotic spindle contractile ring Fig. 5.2 CLEAVAGE and the cytoskeleton

II. MEROBLASTIC (INCOMPLETE CLEAVAGE) A. Telolecithal (Dense yolk throughout most of cell) Fig. 5.3 Different organisms have distinct cleavage patterns

What controls the pattern of cleavage? 1.Amount and distribution of yolk in the egg 2.Factors in the egg that influence the angle and timing of mitotic spindle formation

Sea urchin egg yolk-rich Fig. 5.6

II. MEROBLASTIC (INCOMPLETE CLEAVAGE) A. Telolecithal (Dense yolk throughout most of cell) Fig. 5.3 Different organisms have distinct cleavage patterns

Discoidal cleavage in fish and birds  creating a cellular region above the dense yolk Figure 7.40 zebrafish

What controls the pattern of cleavage? 1.Amount and distribution of yolk in the egg 2.Factors in the egg that influence the angle and timing of mitotic spindle formation

Radial cleavage in sea urchins: simple planes of cleavage a) Placement of mitotic spindles

Synchronous vs. asynchronous cleavage 1  2  4  8  even cell stages odd numbered cell stages are frequent b) Timing of mitotic spindle formation

c) Symmetric vs. asymmetric cell divisions Fig. 5.6

Mammalian cleavage is unique

Rotational cleavage in mammals Figure 8.16 meridional equatorial (radial cleavage)

Synchronous vs. asynchronous cleavage 1  2  4  8  even cell stages odd numbered cell stages are frequent

Compaction during mammalian development: A sudden change in cell adhesion 8-cell embryos: uncompacted compacted

Compaction requires the cell adhesion protein E-cadherin Control embryos Embryos treated with antibodies blocking E-cadherin

Development of a Human Embryo From Fertilization to Implantation Fig cell stage compaction

Early mammalian development follows (in function) the pattern seen in other animals morula

hatch from zona pellucida Fig Fig Development of a Human Embryo From Fertilization to Implantation

End of cleavage = mid-blastula transition controlled by nuclear/cytoplasmic (N/C) ratio

The Mid-Blastula Transition Growth phases added to Cell Cycle Embryo starts transcribing its own RNA Cells start moving around in preparation for gastrulation

Mid-blastula transition and the N/C ratio Lets say you have an organism which normally undergoes 4 cell divisions before the MBT pre-cleavage: N/C ratio = 1/16 = (close to 0) after 4 cell divisions (1 > 2 > 4 > 8 > 16): 16/16 = 1