Ovarian and Menstrual Cycles: Puberty, Fertilization, and Fetal Development
The Ovarian Cycle
Ovarian follicles are large cells that form inside cavities in the ovaries, each containing an immature egg. Approximately every 28 days, an egg matures. Future eggs are already present in the fetus. At birth, a girl has approximately 400,000 future eggs, of which only about 400 will mature from puberty (menarche) onwards. After about 50 years, the maturation of eggs completes with menopause. The ovaries also produce the female hormones estrogen and progesterone.
Sexual reproduction, as in humans, means that each gamete carries half of the chromosomes (23). The resulting cell fusion of two gametes (egg and sperm), the zygote, has the full set (46). All human cells are diploid (two sets of chromosomes) except the reproductive cells, which are haploid (one set of chromosomes).
The Ovarian and Menstrual Cycles
Upon reaching puberty, the female begins the process of egg maturation, menarche, approximately once a month. If the egg is not fertilized, it begins a process of destruction and expulsion with a bloody ending. The set of all these processes is called the Menstrual Cycle, including all events that occur between one bleeding (menstruation or period) and the next. This cycle is usually 28 days, although it can be shorter or longer.
Menstrual Cycle Phases
- The pituitary hormones (FSH and LH) signal the ovaries to start maturing an egg. In each cycle, a single ovum develops.
- When the egg matures, the ovaries produce hormones (estrogen and progesterone) that travel to the uterus and induce the development of the endometrium, which becomes thicker and rich in blood vessels.
- At mid-cycle, one egg leaves one of the ovaries (ovulation) and enters the Fallopian tube.
Puberty, Fertilization, and Gestational Development
Fertilization: Once gametes are formed, to produce a new being, the egg and sperm must come together and merge. This process is called fertilization. In humans, fertilization is internal, occurring within the woman’s body, particularly in the Fallopian tubes.
The zygote begins a journey to implant in the uterus. During this journey, it begins to divide and develop as an embryo. From the 16-cell morula stage, it begins to resemble a blackberry. Then, some cells continue to divide and move to a state called blastula. In this state, it reaches the uterus, and implantation or nidation occurs.
Embryo Development
First Trimester: Implantation in the uterus and early development. In the second month, all organs are developed, and some begin to function. It grows rapidly but unevenly, with the head being more distinguished than the body. From the third month, it is called a fetus, is about 3 inches long, and weighs about 10 grams.
Second Trimester: The woman’s belly grows with the increasing size of the uterus. By the fifth month of development, it reaches the navel. The breasts increase in size, and the woman can feel the baby’s movements. All organs are fully developed, and the fetus grows. At the end of this trimester, it is about 30 centimeters long and weighs 1 kilo.
Third Trimester: The uterus reaches maximum development. Organs mature, especially the lungs and adipose tissue under the skin. The fetus changes position and is placed face down. From the seventh month, the fetus would be viable and could survive if born at that time. At the end of pregnancy, the baby will be between 45 and 50 centimeters long and weigh between 2.5 and 3 kilos.