Human Reproduction: From Fertilization to Childbirth and Puberty
Human Reproduction: From Fertilization to Childbirth
Human reproduction occurs when the egg produced by a woman combines with the sperm produced by a man. Reproduction is internal, meaning it occurs within the reproductive tract of the woman. The process can be broken down into the following stages:
- Production of gametes in specialized reproductive organs.
- Fertilization: The union of two gametes within the female reproductive system, forming a single cell called the zygote.
- Development of the zygote within the female reproductive tract.
- Childbirth, or the birth of a fully formed baby.
- Development of the child: Maturation of the sexual organs and the transformation from child to adult.
The Female Reproductive System and Female Gametes
Female Gametes: Eggs
The egg is the female reproductive cell, or gamete. It is a very large cell, about 1 mm in diameter, that matures within the ovary in a capsule called the Graafian follicle. This follicle opens and releases the egg into the fallopian tube. The egg is surrounded by a mass of much smaller cells that form a protective sheath. Once expelled, the egg remains fertile for about 24 hours. If it is not fertilized during this time, it dies.
Male Gametes
Sperm
Sperm are produced continuously in the seminiferous tubules. From there, they migrate. They are much smaller and more numerous than eggs. Every day, several hundred million sperm mature in the testicles. Their release occurs during ejaculation, in which semen is ejected. Semen comprises sperm, a lubricating liquid, seminal fluid, and prostatic fluid. Sperm are deposited in the vagina and remain alive for only three to five days.
The Female Reproductive Cycle
Female sex hormones are estrogen and progesterone. Estrogens are produced in the ovaries and are responsible for the maintenance of secondary sex characteristics. They also induce the formation of the Graafian follicle. After releasing the egg, the Graafian follicle produces progesterone, which induces the growth and maturation of the endometrium. If fertilization does not occur, the egg dies, the follicle degenerates and stops producing progesterone, and the endometrium is shed, causing menstruation.
Fertilization and Early Pregnancy
The sex act, or coitus, involves the insertion 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 process of fertilization. The resulting cell is called the zygote, or egg cell.
Early Pregnancy
Three important structures develop during early pregnancy:
- The placenta: A structure that develops in the endometrium and serves as a connection between the mother and the embryo.
- The umbilical cord: Connects the embryo to the placenta and consists of a tube through which three blood vessels pass.
- The amniotic sac: A bag filled with amniotic fluid, inside which the embryo remains.
The Development of Pregnancy
Pregnancy is the process that begins with fertilization and ends with delivery. The changes are very significant. The zygote becomes an embryo, then a fetus, and finally a fully formed human being.
Delivery
The fully formed baby exits through the vagina. Delivery consists of three phases:
- Dilation: The cervix begins to dilate, increasing in diameter. The amniotic sac breaks, releasing the amniotic fluid, which is between two and five liters of clear fluid. This is commonly known as “breaking waters.”
- Expulsion: The fetus is pushed outward by uterine contractions and the force exerted by the mother pushing with her diaphragm.
- Delivery: Between five and fifteen minutes after the expulsion of the fetus, the placenta is delivered, along with the remains of the endometrium.
Changes at Sexual Maturity
Children
Babies are born with primary sexual characteristics: the penis and scrotum in boys and the vulva in girls.
Puberty
During puberty, secondary sexual characteristics develop. Girls experience their first menstruation, and boys begin producing sperm. In girls, breasts grow, while in boys, bone thickness and muscle mass increase.