Embryology   Biology 441   Spring 2011   Albert Harris

Lecture notes for Wednesday, Feb 2, 2011 Fertilization Sperm meets egg; sperm fuses with egg; egg refuses any other sperm. Three important fusions of plasma membranes in fertilization: 1) Fusion of acrosomal membrane with sperm plasma membrane 2) Fusion of sperm plasma membrane with oocyte plasma membrane 3) Fusion of oocyte plasma membrane with cortical granule membranes The acrosome contains enzymes that digest holes through jelly coats around the oocyte, so that the sperm can get into contact. It amounts to a giant lysosome; fusion of the sperm plasma membrane with the membrane surrounding the acrosome releases the contents. Even mammal eggs have a vitelline membrane (like a jelly shell). Almost all species have vitelline membranes around their oocytes Urchins, frogs, birds, mammals, including humans. In addition, frogs and salamanders have several additional jelly layers secreted around their egg cells, secreted by oviduct cells. In birds and reptiles, the egg white ("albumin") is secreted around the oocyte, by the upper end of the oviduct, and then the shell is secreted by cells of the lower end of the oviduct. (bird and reptile eggs have already been fertilized, before these layers) If an egg ("oocyte" = egg cell) gets fertilized by 2 sperm, that will make the embryo triploid, and therefore infertile or dead. "Polyspermy" means fertilization by more than one sperm. Many mechanisms have evolved to 'lock the doors' as soon as one sperm had managed to fertilize the egg: "Blocks to polyspermy" The "fast block" to polyspermy uses a propagated decrease in electrical voltage across the plasma membrane of the oocyte. This depolarization is closely equivalent to nerve action potentials! (and presumably evolved before nerves! Protozoa also use it.) Before being fertilized, the voltage is about 70 millivolts more positive outside the cell than inside = 70 mv more negative inside than out! This voltage is caused by leakage of potassium ions (+ charge) from high concentrations inside the cell to low outside. NOTICE THE PARADOX! The positive charge outside is being caused by higher concentrations of (positively-charged) potassium ions inside the cell. Their leakage outward creates this voltage. (This is like the "resting potential" of nerve cells, muscle cells, and 99% of biologists don't know, but also almost all other cell types) When the oocyte is fertilized, then ion channels open in its plasma membrane that let sodium ions leak in (& also calcium ions). The membrane voltage depolarizes, as a result. The wave of depolarization propagates across the egg surface, in only a minute or so. (much is slower than a nerve impulse) The oocyte membrane (somehow!) won't fuse with the sperm membrane after it has depolarized. Just under the plasma membrane of oocytes are thousands of vesicles, each with its own membrane around it. The increased calcium concentration causes these to fuse with the plasma membrane, releasing the content of the ~ 20,000 "Cortical vesicles" The contents of these vesicles causes slow blocks to polyspermy Different kinds of slow blocks to polyspermy: a) Enzymes in the cortical vesicles digest away adhesion molecules on the oocyte surface, that are needed for sperm to stick to eggs. Mammals have only this kind of slow block to polyspermy; it is called "the zona reaction". Phenomena in humans and mammals tend to be called by special names even when the same phenomena also occur in lower animals. For example, the mammal vitelline membrane is called the zona pellucida. b) In sea urchins, and amphibians, enzymes in the cortical granules digest adhesion molecules that stick the vitelline membrane to the plasma membrane of the oocyte. This allows the jelly coat to lift away from the egg cell's surface (tending to move sperm away, too)

This also allows frog eggs to rotate, so the heavier (yolkier) white side rotates downward. In a not-yet-fertilized batch of frog eggs, you can see many of the eggs still have their white sides upward, randomly. And if you flip the mass of eggs over, the cells can't rotate. You can really impress parents, friends and future in-laws with the ability to distinguish fertilized from unfertilized eggs that frogs lay in their swimming pool or bird-bath! c) In sea urchins, a special protein in the cortical granules precipitates onto the inner surface of the vitelline membrane, to form a sperm-proof layer called "the fertilization membrane" Notice differences in the location of fertilization (in different kinds of animals) (external & internal etc.) Sea urchins: simply release sperm and oocytes into sea water Sea squirts: release sperm into sea water; but hold oocyte in body;      (sperm get sucked in during filter-feeding) Frogs, most salamanders, & most teleost fish: male and female come together, female releases eggs into the water, and then the male releases sperm onto the eggs. Some salamanders: Males come to ponds about a week before females, and deposit gooey mucus blobs ("spermatophores"). Then, on the next rainy night, the females come to the pond and pick up the spermatophores with their rear ends and lay eggs past them. Many fish, & all reptiles birds and mammals: Male inserts sperm into the lower end of the female oviduct, and sperm cells meet the oocytes "coming through the rye", as it were. (I was listening to "Thistle & Shamrock" on the radio as I typed this, and they were playing a song by Robert Burns) Cytoplasmic rearrangements inside oocytes (after fertilization) Sea squirts   formation of yellow crescent      (control muscle cell differentiation) Teleost fish   cytoplasm forms a lump at the animal pole Frogs & Salamanders   rotation of cortical cytoplasm relative to deeper cytoplasm, forming gray crescent.      (controls subsequent location of blastopore)

In nematodes, the A-P axis of the embryo is controlled by where the sperm fertilizes the oocyte. And UNC's own Bob Goldstein discovered this!! additional videos of cleavage, gastrulation and neurulation

 

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