The unfertilized mouse egg has a round and highly villated main body and a "nipple" that is unvillated and buds off on fertilization to form the second polar body. Fluorescent markers stain the body more intensely than the nipple, which has been assumed to result from surface amplification due to microvilli. Using fluorescence recovery after photobleaching and microfluorescence photometry, we have measured the membrane protein diffusion and concentration on the main body and nipple region of unfertilized and on fertilized CD-1 mouse eggs. Two general membrane protein labels were used: rhodamine-labeled succinylated concanavalin A and trinitrobenzene sulfonate visualized with a rhodamine Fab fragment of a sheep anti-trinitrophenyl. We found that while the diffusion coefficient was the same on the nipple and main body, considerably higher recovery was observed on the nipple for both probes. The ratio of intensity of fluorescence on the nipple to main body was significantly lower for the concanavalin A stain than for the trinitrophenyl stain, indicating that true concentration gradients exist beyond those that result from surface amplification. The effect of fertilization was not general. No effect was observed for the concanavalin A stain for either diffusion coefficient or percent recovery. For the trinitrophenyl stain, percent recovery decreased approximately twofold while diffusion coefficient increased approximately threefold.

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