Menstruation is a unique biological process among mammals, with only a few species, including humans, experiencing it. For a long time, scientists have been trying to understand why humans menstruate, with various theories proposed over the years. Initially, people believed that menstruation was a way to "clean" the uterus of toxins, but this theory has been debunked. Another theory suggested that menstruation evolved to flush out pathogens, but this idea has also been discredited.
Recent research suggests that menstruation is a byproduct of the evolutionary struggle between the fetus and its host. The most invasive type of placenta, found in humans, may have led to the development of spontaneous decidualization, a process where the uterine lining prepares for implantation before fertilization occurs. This process may help the host maintain control over resources and prevent the fetus from becoming too attached.
The "choosy uterus" hypothesis proposes that spontaneous decidualization helps the uterus detect and terminate pregnancies with lethal genetic abnormalities. Research has shown that embryos with aneuploidies are more metabolically active and may be sensed by decidualized cells, leading to early miscarriage.
While the exact mechanisms behind menstruation are still not fully understood, scientists believe that spontaneous decidualization is the key to understanding why humans menstruate. Further research is needed to understand how this process evolved and how it can be used to help people with reproductive issues.
Here are the key facts extracted from the text:
1. Few mammals experience menstruation.
2. Only a few non-human primates, some species of bats, elephant shrews, and an African mouse menstruate.
3. Menstruation is a month-long cycle regulated by hormones in humans and these other animals.
4. The lining of the uterus, or the endometrium, is shed and expelled during menstruation.
5. A mixture of blood, endometrial cells, mucus, and other tissue flows out of the vagina for three to seven days during menstruation.
6. Hormones trigger the ovaries to produce around five to twenty tiny sacs called follicles, each containing an immature egg cell.
7. One or two dominant follicles grow in diameter and become ready to release a mature egg cell.
8. The dominant follicle releases its mature egg, which begins its journey down the fallopian tube.
9. The spent follicle turns into a tiny hormonal powerhouse called the corpus luteum, which releases hormones that act on the lining of the uterus.
10. The corpus luteum shrinks and dissolves if the egg doesn't get fertilized and implant, causing the uterus to shed the extra tissue and kick off another menstruation.
11. Scholars in ancient Rome believed that contact with menstrual blood could harm crops, kill bees, and affect dogs.
12. The concept of "menotoxin" was proposed in 1920 to describe a supposedly toxic substance in menstruating people's sweat, but it was later debunked.
13. Research suggests that bacteria from the genital tract can cling to the tails of sperm cells, but this doesn't explain why humans menstruate.
14. The current hypothesis is that menstruation is a byproduct of the evolutionary struggle between the fetus and its host.
15. Spontaneous decidualization may have evolved to mediate this relationship and help the host keep control over their resources.
16. The choosy uterus hypothesis suggests that spontaneous decidualization helps the uterus detect genetic red flags in embryos and terminate pregnancies early.
17. Research suggests that embryos with lethal aneuploidies are more metabolically active than healthy ones and may be detected by the decidualized cells in the outer layers of the endometrium.
18. Around 4,000 mammals, including humans, deliver nourishment to their young before birth through a placenta.
19. Placentas are temporary organs that grow at the beginning of pregnancy and are expelled immediately following birth.
20. Humans have a hemochorial placenta, which is the most invasive kind of placenta, where the fetal tissue burrows through and erodes some of the maternal endometrial tissue.