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- When tracking cholesterol throughout the membrane, we find that it is distributed in all parts of the sperm membrane prior to capacitation. During capacitation, phospholipid scramblase is activated, which allows for cholesterol lateral redistribution. The sperm membrane then undergoes a cholesterol efflux, losing these molecules to albumin, functioning as an effective cholesterol “sink.” Lipid rafts, defined as “cholesterol-enriched domains in the membrane,” then become suspects that may be involved in capacitation (Gestel et al. 2005).
- Lipid rafts can be detected by the presence of caveolin-1 and flotillin-1. When testing boar sperm cells, Gestel et al. found that both of these molecules were present. Caveolin-1 was initially present in both the apical head site and the tail, while flotillin-1 was present in the apical head site only. Sperm cells having undergone capacitation showed a redistributed concentration of both of these molecules at the apical ridge head area, compared to uncapacitated sperm cells (not exposed to bicarbonate).
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- Gestel and colleagues assert that this redistribution “represents a capacitation-dependent reorganization of lipid rafts,” despite a previous assertion by Cross that lipid rafts may be disrupted or separated during capacitation. Although Cross reported a moving apart of raft markers, Gestel et al. show a lack of change of detergent-resistant membranes (DRMs, evidence of lipid rafts) during capacitation. Additionally, the work of Gestel et al., Flesch et al., and Shadan et al. combine to support the proposal of microdomains of lipid rafts combining to form a larger domain during capacitation. Larger domains are proposed to aid zona-pellucida binding, the interaction of sperm and egg, not discussed in this review.
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| This image is used with permission from senior author of: van Gestel RA, Brewis IA, Ashton PR, Helms JB, Brouwers JF, Gadella BM. August 2005. Capacitation-dependent concentration of lipid rafts in the apical ridge head area of porcine sperm cells. Molecular Human Reproduction 11(8):583-90. |
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This webpage was made as a part of student projects in Dr. Dorcas' Animal Physiology class at Davidson College.
