22. In an experiments, sperm removed from epididymis of a male mouse was added in a dish containing appropriate media and oocyte. No fertilization was seen. However, when sperm from epididymis were directly placed in uterus of an ovulated female, she became pregnant. These observation suggest that
    (1) The sperm need to travel some distance to attain fertilizing ability.
    (2) The oocyte secrets some biochemicals or factors which help sperm to fertilize.
    (3) The hormones in body help sperm to attain fertilizing ability.
    (4) The contents of female reproductive tract interact with sperm and activate it for fertilization

 

 

The observations in the experiment suggest that the sperm require interaction with the female reproductive tract environment to gain fertilizing ability. Specifically, although sperm removed from the epididymis and directly placed with an oocyte in vitro did not fertilize, when the same sperm were introduced directly into the uterus of an ovulated female, fertilization and pregnancy occurred. This indicates that the female reproductive tract plays an active role in activating sperm for fertilization, a process known as capacitation.

Therefore, the correct explanation is:

(4) The contents of female reproductive tract interact with sperm and activate it for fertilization.

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Introduction

Fertilization competence in mammalian sperm is not innate immediately upon ejaculation. Mouse sperm stored in the epididymis are initially functionally immature and require further activation, known as capacitation, which occurs within the female reproductive tract. Capacitation involves physiological and biochemical changes that prepare sperm for interaction with and fertilization of the egg.

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The Necessity of the Female Reproductive Environment

• The failed fertilization in vitro with epididymal sperm and success when sperm are placed directly in the female uterus highlights the importance of female tract factors.

• These factors include changes in pH, ionic composition, cholesterol removal from the sperm membrane, hyperpolarization of the sperm membrane potential, and activation of signaling pathways.

• Capacitation enhances sperm motility (hyperactivation) and primes the sperm for the acrosome reaction necessary to penetrate the egg’s protective layers.

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Supporting Molecular and Cellular Evidence

• In vivo studies demonstrate increased protein tyrosine phosphorylation in sperm during capacitation within the female tract.

• Sperm calcium ion channels, especially CatSper, become activated during capacitation, regulating motility and acrosome reaction capability.

• Without capacitation, sperm remain unable to bind and penetrate the egg zona pellucida, making fertilization impossible.

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Why Other Options Are Less Complete

• While sperm may need to travel some distance (option 1), the critical factor is the biochemical changes induced by the female tract, not simply distance.

• The oocyte secretes signaling molecules (option 2) that attract sperm, but these do not replace the capacitation process essential for fertilizing ability.

• Hormonal effects (option 3) generally regulate female tract environment and oocyte maturation; direct sperm activation is mediated largely by female tract secretions and physicochemical conditions rather than systemic hormones acting directly on sperm.

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Conclusion

The female reproductive tract environment is indispensable in activating sperm through capacitation, transforming them from functionally inert cells into competent fertilizing agents. This activation enables sperm to undergo the acrosome reaction and sperm-egg fusion, underpinning successful fertilization and pregnancy.