Human Reproduction: From Fertilization to Birth
Fertilization and Reproduction
Fertilization is internal, meaning the egg and sperm link inside the female reproductive system. Reproduction occurs through the following processes:
- The production of gametes: Sperm and ova are created.
- Fertilization: This is the union of two gametes within the female apparatus to form the zygote.
- The development of the zygote: Inside the female reproductive tract, the zygote becomes an embryo, and this, in turn, a fetus.
- The birth of a fully formed baby.
- Infant development: The maturation of sexual organs and the transformation of the baby into an adult capable of producing gametes and reproducing.
Changes Leading to Sexual Maturity
Childhood
When a baby is born, their reproductive tract is already formed: the penis and scrotum in boys, and the vulva in girls. These are the primary sexual characteristics.
Puberty
Between ten and fourteen, girls have their first menstruation, and boys begin producing spermatozoa. Girls’ breasts grow, and their hips widen. In boys, bone thickness and muscle mass increase, their voices deepen, and beard and pubic hair appear.
Adolescence
This stage involves sexual maturation and the development of secondary sexual characteristics. Individuals feel the need for independence, explore their feelings and their bodies, and sexual desire appears.
Adulthood
The end of development occurs once adolescence has passed, around twenty-five years of age.
Gametes
Female Gametes: Ova
The reproductive cell or female gamete is an almost 1mm in diameter cell. It matures inside the ovary in a structure called a Graafian follicle.
Male Gametes: Sperm
Sperm are produced continuously in the seminiferous tubules, from where they migrate over several days to the epididymis. They develop a long flagellum and exit to the outside through ejaculation in the form of semen.
The Female Reproductive Cycle
The female sex hormones are estrogen and progesterone. The minimum level of estrogen activates the release of pituitary hormones, prompting the formation of a Graafian follicle in the ovary. When the estrogen level is at its maximum, ovulation occurs: the follicle breaks, the egg leaves the ovary, and it is picked up by the fallopian tube. After releasing the egg, the Graafian follicle becomes the corpus luteum, which produces progesterone, inducing the growth and maturation of the endometrium. If there is no fertilization, the egg dies, the corpus luteum degenerates, and estrogen and progesterone production stops. The endometrium then breaks down, causing menstruation. The set of events that take place between two successive menstruations is called the menstrual cycle (lasting around 28 days).
Fertilization and Early Pregnancy
Sexual intercourse is the introduction of the penis into the vagina. During ejaculation, sperm are released inside the vagina. The genetic material of the sperm and the egg fuse, completing the fertilization process. The resulting cell is called the zygote.
The onset of pregnancy: The egg attaches to the endometrium (implantation or nesting). It is then called the embryo.
Placenta: An organ that serves as a connection between the mother and the embryo.
The umbilical cord: Connects the embryo with the placenta.
The amniotic sac: A sac filled with amniotic fluid.
Gestation or Pregnancy
This is a process that begins with fertilization and ends with childbirth.
Childbirth
Dilation
The cervix begins to dilate, and the vaginal plug is expelled.
Expulsion
The fetus is pushed to the outside by the contractions of the uterus. The umbilical cord is cut.
Delivery
Between five and fifteen minutes after the expulsion, the placenta is expelled.
Complications of Childbirth
- A bad fetal position: If the fetus does not have its head towards the mother’s pelvis.
- Insufficient dilation of the cervix and vagina.
- Umbilical cord clamping: If the cord is compressed, blood flow is cut off.