The menstrual cycle is a periodic cycle observed in primate mammals responsible for preparing the female body for pregnancy and delivery of the baby. 

Person holding the red bag on the body

Menstruation begins during the menstrual cycle, which is bleeding from the uterus. This menstruation period lasts for around 4-8 days. An overall menstrual cycle lasts for 24-38 days. However, it is sometimes delayed for some women. In this period, hormones such as luteinizing and follicle-stimulating hormones are released to regular estrogen and progesterone levels. Estrogen and progesterone are responsible for ensuring the female body for pregnancy. The menstrual cycle involves four phases: menstrual, follicular, ovulation, and luteal phases. During these phases, the egg, when released from the ovule to become an unfertilised egg, will remove the egg and the endometrial lining from the body as blood. However, if the egg becomes fertilised with male sperm, three events will occur to fertilisation. These events are; pre-fertilisation, fertilisation, and post-fertilisation events. The events include the formation of sex cells to a series of events after fertilisation. 

Menstrual cycle and three fertilisation events

In primate females, the menstrual cycle begins after reaching reproductive maturity (puberty) and lasts until menopause. During this period, a female’s body undergoes multiple changes to prepare the body for pregnancy until the delivery of the offspring. This cycle lasts for around 24-38 days which differs for each woman. During this time, hormones involved are Luteinizing Hormone (LH) and Follicle-Stimulating Hormone (FSH), induced by the pituitary gland. These two hormones regulate the estrogen and progesterone levels in a woman’s body. Estrogen and progesterone are steroidal hormones whose essential function is to control and prepare women for reproduction. 

A female’s body undergoes four phases of the menstrual cycle; menstrual, follicular, ovulation, and luteal phases. In menstrual phase is the beginning stage of blood flow that occurs when the egg is not fertilised. However, the initial stage of menstruation occurs from the follicular phase in the body. The follicular phase begins during the first day of menstruation. Therefore, the period of the follicular phase and menstrual phase overlaps until ovulation. In this phase, ovaries contain follicles, each having an immature egg. The eggs grow in the follicle, during which endometrial lining (a cushioning) is developed in the uterus that contains capillaries for blood circulation. This immature egg finally matures to break off from the follicle, is released into the fallopian tube, and travels towards the uterus for the sperm to fertilise the egg. In the luteal phase, the follicle transforms into Corpus Luteum (CL) after the release of the egg, producing estrogen and progesterone hormones responsible for the endometrial lining in the uterus. If the female does not become pregnant (egg unfertilised), the egg dies and reduces CL that gets reabsorbed. This leads to a decrease in the estrogen and progesterone levels, initiating the period. The endometrial lining is shed as blood and mucus, thus beginning the menstrual cycle. 

Pre-fertilisation event

Three events occur during sexual reproduction: pre-fertilisation, fertilisation, and post-fertilisation events. 

 Multiple white wiggly things around a multicoloured circle

In males and females, before fertilisation, gametes (sex cells) are formed, the process called gametogenesis. Similar morphological structures of male and female gametes are called homogametic or isogamete. In contrast, if the morphological structure between male and female gametes is dissimilar, they are called heterogametes. The gametes are haploid cells from each diploid parent; in males, they are called sperms, while in females, they are called the ovum. Specialised cells called meiocytes are diploid cells that play an essential role in gametogenesis. These meiocytes undergo meiosis (reduction division) twice to produce four haploid cells called gametes; mature sperm and egg cell. The formation of sperm from spermatocytes is known as spermatogenesis in males. Whereas in females, the ovum (egg cell) formation from an oocyte is called oogenesis

The male sperms are mobile. In humans, the sperms have tails that aid in their mobility, while in plants, the pollen grains (containing male gametes) are dispersed from the anther. These pollen grains are carried by pollinators such as wind, water, animals, and insects, to the stigma (female reproductive part). In comparison, the female gametes are stationary. In humans, the female gamete (egg) is present in the fallopian tube, waiting for the sperm to fertilise the egg. During conception, when the male releases millions of sperm, they swim towards the egg, and only one sperm can fuse with the egg. This fusion is called syngamy. Whereas in plants, the female egg cell is present in the ovary of the pistil (female reproductive organ). Pollen grains that stick to the stigma forms a pollen tube to transfer male gametes through the tube into the ovary and fertilises the egg cell. This process is gamete transfer.

Fertilization event

When male and female gametes fuse with each other is called fertilisation to form a diploid zygote. Different types of fertilization occur for various species, such as external and internal fertilisation. In most aquatic animals, like fish and some amphibians, the male and female gametes are released into the outside environment in the water, where they fertilise. Sometimes, the male gametes may not reach the female gamete; therefore, the males produce millions of sperms. 

The majority of plants and animals reproduce via internal fertilisation. Internal fertilisation occurs within the female body. The male sperm swim towards the egg in the female body to find and fuse with the egg, leading to fertilisation. 

Post-fertilization event 

Post-fertilisation is the event after which the haploid male and female gamete fuse together to produce a diploid zygote. The development of zygotes is different for various species, additionally depending on the environmental conditions and the life cycle of an organism. For instance, a zygote in fungi undergoes a resting phase by forming a thick covering to protect it from any damage before its germination. The diploid zygote undergoes meiosis to produce haploid spores to develop into the haploid individual; for organisms having a haploid life cycle. 

The zygote is a single diploid cell that multiples through mitosis cell division to form into an embryo; the process is called embryogenesis. Depending on where the zygote develops, they are classified into oviparous and viviparous. Oviparous are organisms where the zygote is developed inside eggs outside the female body, after which they hatch, such as birds. In contrast, the zygote is developed inside the body of female organisms are called viviparous organisms, like humans. The embryo develops into a seed in plants, while the ovary develops into a fruit. 

A brown substance inside a transparent covering

The development of the zygote is as follows. A zygote multiplies from a single cell to sixteen cell stage; this cell mass is called a morula. As the morula keeps multiplying, it transforms into a blastula, a stage during which a hollow mass containing fluid is formed. The next stage is blastocyst, when the cells inside the hollow mass start to form differentiating cells, thus beginning the embryo’s development. The blastocyst implants itself onto the endometrial (thick wall) lining in the uterus. This implantation will help the embryo receive nutrition, excreting wastes, and exchange gases from the mother. The embryo grows and develops into a fetus delivered out from the mother’s body. 

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