Quiz 21: Cell Birth, Lineage, and Death



The mouse sperm must penetrate a layer of cumulus cells before it encounters the amorphous zona pellucida (ZP)layer surrounding the oocyte.It is here where the sperm surface protein GaIT encounters and binds ZP3 glycoprotein in the ZP,which triggers the acrosomal reaction.The acrosomal vesicle,containing hydrolytic and proteolytic enzymes and present within the tip of the sperm head,is exocytosed as a result of the acrosomal reaction.These enzymes serve to degrade the ZP,facilitating sperm-egg plasma membrane interactions.Sperm penetration triggers a Ca²⁺ release from within the egg,triggering the exocytosis of egg cortical granules.Enzymes within these granules alter the properties of the ZP to prevent the binding of additional sperm (polyspermy).

Although a relatively inefficient method of cloning,as evidenced from the data that emerged following cloning Dolly the sheep,the technique relies on removing the nucleus from a fully differentiated cell and transplanting it into an enucleated egg of a donor mouse.In one example,the nucleus from an olfactory sensory neuron of a mouse that expresses green fluorescent protein (GFP)was transplanted into an enucleated egg and then differentiated in culture until the inner cell mass stage.GFP-expressing ES cells were isolated and injected into a tetraploid blastocyst host.When this chimeric blastocyst was transplanted into the uterus of a pseudopregnant female,the tetraploid cells formed the placenta and the GFP-expressing cells the embryo proper.If all cells express GFP when the pups were born,this would indicate that their nuclei had been derived from that originally isolated from the GFP-expressing olfactory neuron.