Human Reproduction and Development Stages

Posted on Nov 26, 2024 in Biology

Human Reproduction

Females produce female gametes (eggs) and males produce male gametes (sperm). These two cells join to form a zygote.

Internal Fertilization

The egg and sperm unite in the female’s reproductive tract, and the new human being develops there.

Reproduction Steps

  1. Gamete Production: Specialized reproductive organs produce gametes.
  2. Fertilization: The union of both gametes inside the female reproductive tract forms the zygote.
  3. Zygote Development: Within the female reproductive tract, the zygote develops into an embryo and then a fetus.
  4. Birth: A fully formed baby is born.
  5. Child Development: The child develops, matures sexually, and transforms into an adult capable of producing gametes and reproducing.

Primary Sex Characteristics (Birth)

At birth, a baby has a formed reproductive tract: penis and scrotum (male) and vulva (female). The presence of these reproductive organs corresponds to the primary sexual characteristics.

Puberty (10-14 years)

Puberty is the first maturation period of the sexual organs, and sperm production begins. At this stage, the mature reproductive organs begin to produce hormones that cause the appearance of secondary sexual characteristics. In girls, bone thickness increases, hips widen, and pubic and armpit hair appears. In boys, bone thickness and muscle mass increase, the voice deepens, and beard, pubic, and armpit hair appear.

Adolescence (16-19 years)

While developing secondary sexual characteristics (2-5 years), the need for independence, exploration of feelings and body, and sexual desire emerge. Emotional support is sought from friends and peer groups. Sometimes there is conflict with adults. Adolescence is reached, and the body is ready for reproduction.

Young Adulthood (20-25 years)

In developing countries, many women have children by this age. However, in our society, teenagers do not usually have children and spend more years in education, acquiring a situation that allows them to have a family with stability and security.

Female Reproductive System

  • Ovaries: Almond-shaped and sized, attached to the uterus by a ligament. Ovules mature within them.
  • Fallopian Tubes: Ducts that connect each ovary with the uterus. Their interior is lined with cilia that push the egg.
  • Clitoris: Organ with many nerve endings, highly sensitive. Located at the union of the labia minora.
  • Uterus: Hollow organ internally coated with a highly vascular epithelium (endometrium) where the fertilized egg implants.
  • Cervix: A narrowing that connects the uterus to the vagina.
  • Vagina: Muscular-walled passage from the uterus to the exterior. It opens to the outside through the vaginal opening.
  • Labia Majora: Skin folds located between the thighs, covering the rest of the external genitalia.
  • Labia Minora: Thin and sensitive skin folds covered by the labia majora.

The egg is a very large reproductive cell that matures within the ovarian follicle (Graafian follicle). The follicle opens and releases the egg into the fallopian tube, surrounded by a mass of much smaller cells that form a protective envelope. Once ejected, the egg remains fertile for about 24 hours. If not fertilized during that time, it dies.

Male Reproductive System

  • Testicles: Glands in the scrotum that produce sperm.
  • Prostate: Walnut-sized gland that surrounds the urethra. It produces prostatic fluid that protects sperm against the acidity of the urethra and vagina.
  • Deferent Ducts: Located below the epididymis, they converge into the urethra.
  • Seminal Vesicles: Glands that produce seminal fluid in which sperm swim. They empty into the deferent ducts.
  • Urethra: Communicates with the exterior. Its muscular walls contract during ejaculation, pushing sperm.
  • Epididymis: Long, very thin tube coiled on top of each testicle. Sperm mature inside.
  • Penis: Cylindrical organ whose tip (glans) is widened and covered by the foreskin (retractable skin).
  • Scrotum: Pouch formed by skin and mucous membranes that contains the testicles and keeps them outside the abdomen. The skin has many sebaceous glands.

Sperm are continuously produced in the seminiferous tubules. They move from there for several days to the epididymis, where they complete their maturation and develop a long flagellum to swim quickly. Sperm exit through ejaculation, where semen (formed by sperm, lubricating fluid, seminal fluid, and prostatic fluid) is expelled and deposited inside the vagina. Sperm remain alive for 3-5 days, during which fertilization can occur.

Female Sex Hormones

Estrogen and progesterone are the female sex hormones. Estrogen is produced in the ovaries and is responsible for the development and maintenance of secondary sexual characteristics. It also induces the formation of the Graafian follicle in the ovary. Estrogen production follows a cycle of about 28 days. When its blood concentration is at its maximum, ovulation occurs (follicle rupture with the release of the egg into the fallopian tubes). After releasing the egg, the Graafian follicle produces progesterone, which induces the growth and maturation of the endometrial tissue that lines the uterus where the fertilized egg will implant. When the progesterone level is at its maximum, the endometrium is fully developed. If fertilization does not occur, the egg dies, the follicle degenerates, it stops producing progesterone, and the endometrium is shed, causing menstruation.

Pregnancy and Childbirth

When fertilization occurs, the zygote produces a substance that prevents the Graafian follicle from degenerating, so it continues to produce progesterone, which in turn keeps the endometrium alive and functional. As a result, menstruation does not occur. The zygote begins to divide, soon forming two identical cells. During the few days it takes to travel to the uterus, it becomes a few hundred cells that form a hollow sphere of a little over half a centimeter in diameter. When this spherical cell mass reaches the uterus, implantation occurs (it attaches to the endometrium). From that point on, it is called an embryo.

Embryonic and Fetal Development

  • Placenta: An organ that develops in the endometrium and serves as a connection between mother and embryo.
  • Umbilical Cord: Connects the embryo with the placenta and consists of a tube through which three blood vessels run: two arteries that carry nutrients from the mother to the embryo, and one vein that carries waste substances from the embryo to the mother.
  • Amniotic Sac: A sac filled with amniotic fluid within which the embryo remains.

1st Trimester

The head, trunk, and limbs are forming, as well as the internal organs and the circulatory, excretory, and reproductive systems. By the third month, the heart begins to beat. At the end of the trimester, the embryo is called a fetus. It measures 10cm and weighs 50-80 grams. The woman perceives her pregnancy; menstruation does not come, and she may feel physiological changes (nausea, increased sense of smell, changes in appetite, etc.).

2nd Trimester

The fetal nervous system matures and responds to stimuli, and its movements can be felt. The circulatory and excretory systems are completed. From the fourth month, the sex can be determined. At the end of the second trimester, the fetus measures 30cm and weighs more than 1kg. The uterus expands, and the belly perimeter begins to increase. The breasts enlarge, and the woman may perceive some symptoms due to the functioning of the fetus’s internal organs.

3rd Trimester

All organs mature. The fetus increases in size and weight, develops a fat layer under the dermis, and moves a lot. It spends time awake and asleep. Finally, it turns its head down and engages in the mother’s pelvis. At the end, it measures 50cm and weighs between 2.5-4kg. The belly is very bulky due to the large dilation of the uterus to accommodate the fetus. The uterus presses on some internal organs, such as the bladder or bowel, which can cause some discomfort.

Stages of Childbirth

  1. Dilation: The cervix begins to dilate. The vaginal muscles relax, and the uterine muscles dilate rhythmically. The uterus begins to contract, the amniotic sac breaks, and the amniotic fluid is released (water breaking). This phase can last between 8-12 hours.
  2. Expulsion: The fetus is pushed outward by uterine contractions and by the force exerted by the mother pushing with the diaphragm. It comes out through the cervix and vagina, which are dilated to the maximum. This phase lasts between 15-30 minutes. Once outside, the baby takes its first breath, and the umbilical cord is cut, forming the navel scar.
  3. Delivery of the Placenta: Between 5-15 minutes after the expulsion of the baby, the placenta comes out with the remains of the endometrium.

Childbirth Complications

  • Poor fetal position: Forceps (large tweezers) are inserted into the uterus to hold the baby and gently pull it out.
  • Insufficient cervical dilation of the uterus and vagina: The mother is given an IV with oxytocin to produce dilation and uterine contractions.
  • Compression of the umbilical cord: If compressed, blood flow between mother and fetus is cut off, and the fetus suffers from a lack of oxygen that could cause cerebral injuries.

Infertility

Male: Insufficient sperm production, production of immature sperm with low mobility, malformations that prevent sperm from fertilizing an egg, or disorders that prevent the passage of sperm, such as inflammation of the deferent ducts.

Female: Blockages in the fallopian tubes, lack of ovulation due to hormonal imbalances, malnutrition, other health problems, or difficulty in implanting the zygote in the endometrium.

Assisted Reproduction

  • Artificial Insemination: When infertility is due to the inability of sperm to fertilize the egg, semen from a man is introduced into the woman’s uterus when she is about to ovulate.
  • In Vitro Fertilization: When women have problems with ovulation or embryo implantation, women undergo hormone treatment to produce several eggs, which are extracted and fertilized in the laboratory with the man’s sperm. When the zygotes have begun to divide, several are implanted in the maternal uterus. It is common for several to implant, resulting in a multiple pregnancy.

Contraception Methods

Natural: Ogino method, basal temperature, observation of cervical mucus.

Mechanical: Condoms, diaphragm, intrauterine device (IUD).

Chemical: Spermicides, contraceptive pills.

Surgical: Tubal ligation, vasectomy.

Sexuality and Sex

Sexuality: Set of anatomical, physiological, and psychological characteristics that define each sex and represent a series of emotional and behavioral phenomena related to sex, which mark the human being throughout development and their role in society.

Sex: Biological condition that distinguishes between male and female individuals.

Sexual Intercourse: Sexual contact between people. It is driven by attraction or sex drive and features a set of reactions that occur in the body in response to stimuli related to sexuality.

Sexual Health Advice

  • Wash often.
  • Use condoms.
  • Avoid promiscuity.
  • Avoid denigrating attitudes towards women.
  • Seek a trusted adult.
  • See your doctor.
  • Ignore advice from uninformed people.

In girls: The appearance of menstruation.

In boys: The first emissions of sperm.

Adolescence: Period of gradual transition from childhood to adulthood.

Puberty: Set of transformations of the child’s body into an adult.

Boy: Growth spurt, deeper voice, oilier skin.

Girl: Breast development, growth spurt, oilier skin, sometimes juvenile acne.

Sperm: The testicles produce sperm that can move thanks to the beating of their flagella.

Spermatozoon: Male gamete or reproductive cell.

Ovary: Genitalia that produce ovules.

Male Reproductive System: Bladder, vas deferens, seminal vesicles, prostate, urethra, testicle, penis.

Female Reproductive System: Fallopian tube, ovary, fimbriae, uterine walls, cervix, vagina, vulva.