23. Capacitation of mammalian sperms allows them to be activated within the uterus and facilitate fertilization. The following statements were made regarding events occurring during capacitation:
A. removal of cholesterol from sperm head.
B. Removal of non-covalently bound glycoproteins.
C. Increased expression of fibronectin.
D. Decreased permeability of calcium Ions.

Capacitation is a vital physiological process that mammalian sperm undergo to acquire the ability to fertilize an egg. It involves a series of biochemical and structural changes that occur primarily after ejaculation when sperm transit through the female reproductive tract.

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Correct Events During Capacitation:

• A. Removal of cholesterol from sperm head: Cholesterol removal from the sperm plasma membrane by albumin in the female reproductive tract is a key event in capacitation. This increases membrane fluidity and permeability, allowing further biochemical changes necessary for fertilization.

• B. Removal of non-covalently bound glycoproteins: The shedding of surface glycoproteins that are non-covalently attached to the sperm membrane is also crucial during capacitation. This removal exposes receptor sites important for sperm-egg interaction.

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Incorrect or Less Relevant Statements:

• C. Increased expression of fibronectin: There is no evidence that capacitation involves increased expression of fibronectin on sperm; this is not a known hallmark of sperm capacitation.

• D. Decreased permeability of calcium ions: In fact, capacitation involves an increase in calcium ion permeability. Calcium influx is essential for hyperactivation and the acrosome reaction.

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Therefore, the correct events occurring during capacitation are Options A and B.

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Introduction

Capacitation is the complex maturation process mammalian sperm must undergo to fertilize an egg successfully. It involves remodeling the sperm membrane and intracellular signaling cascades, primarily occurring as sperm navigate the female reproductive tract. Understanding capacitation’s molecular basis reveals how fertilization competency is achieved.

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Cholesterol Removal and Membrane Fluidity

The female reproductive tract environment promotes removal of cholesterol from the sperm plasma membrane. Cholesterol stabilizes the sperm membrane, and its removal during capacitation increases membrane fluidity, allowing enhanced permeability and rearrangement of proteins essential for fertilization functions such as the acrosome reaction.

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Shedding of Non-covalently Bound Glycoproteins

Capacitation includes the removal of seminal plasma glycoproteins loosely attached to the sperm surface. This exposes critical molecules required for binding to the zona pellucida of the egg, facilitating successful sperm-egg recognition and fusion.

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The Role of Calcium Ions

Contrary to decreased permeability, capacitation induces increased sperm plasma membrane permeability to calcium ions. Calcium influx regulates several processes including hyperactivated motility necessary for traversing the female tract and preparing for the acrosome reaction.

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Non-Involvement of Fibronectin in Capacitation

Current evidence does not support increased fibronectin expression during capacitation. Sperm surface remodeling during capacitation focuses mainly on cholesterol and glycoprotein removal rather than de novo expression of adhesion molecules like fibronectin.

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Conclusion

Mammalian sperm capacitation involves crucial biochemical changes, notably the removal of cholesterol and loosely bound glycoproteins from the sperm membrane, leading to increased membrane fluidity and exposure of essential receptors. These changes prime sperm for the acrosome reaction and fertilization. Increased calcium permeability is also a hallmark of capacitation, whereas fibronectin expression does not change significantly during this process.