Internal fertilization in mammals is the process where the male deposits sperm directly into the female's reproductive tract, and fertilization of the egg by the sperm occurs inside the female's body. This contrasts with external fertilization, where the eggs and sperm are released into the environment.
Here's a breakdown of the process:
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Insemination: The male introduces sperm into the female's reproductive tract, typically the vagina. This occurs through sexual intercourse.
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Sperm Transport: Millions of sperm are released, but only a fraction make it to the egg. The sperm must travel through the cervix, uterus, and finally into the fallopian tubes (oviducts).
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Egg Release (Ovulation): An egg is released from the ovary and enters the fallopian tube.
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Fertilization: Fertilization typically occurs in the ampulla, the widest part of the fallopian tube. A single sperm penetrates the outer layers of the egg (zona pellucida and corona radiata). Upon penetration, the egg undergoes changes to prevent other sperm from entering. The sperm and egg nuclei (containing the genetic material) fuse, forming a zygote.
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Zygote Development and Transport: The zygote begins to divide through mitosis, undergoing cleavage. This results in the formation of a morula (a solid ball of cells) and then a blastocyst (a hollow ball of cells with an inner cell mass). While dividing, the developing embryo travels down the fallopian tube towards the uterus.
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Implantation: The blastocyst reaches the uterus and implants itself into the uterine lining (endometrium). This marks the beginning of pregnancy. The implanted blastocyst secretes hormones that maintain the uterine lining and prevent menstruation.
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Gestation: Following implantation, the embryo (and later fetus) develops within the uterus for a specific gestation period characteristic of the mammalian species.
In summary, internal fertilization in mammals involves sperm being deposited into the female reproductive tract, the sperm traveling to meet the egg in the fallopian tube where fertilization occurs, and the resulting zygote developing into an embryo that implants in the uterus.
