Human Sexual Reproduction: Stages, Fertilization, and Development

Posted on Mar 26, 2025 in Biology

Human Sexual Reproduction

In sexual reproduction, two cells unite (sperm and egg), each from one individual of the opposite sex, to form a zygote. The zygote develops and produces an offspring that is genetically different from the parents.

Puberty and Adolescence

Puberty: The acquisition of sexual maturity during adolescence.

  • Primary Sexual Characteristics: Reproductive organs.
  • Secondary Sexual Characteristics: Hair, voice changes, etc.

Adolescence: The stage from approximately 9 to 18 years of age, during which significant physical and hormonal changes occur.

Menarche: The first menstruation.

Gametogenesis

Spermatogenesis

Spermatogenesis: The process of sperm formation.

  1. Spermatocyte Formation: Germ cells undergo various divisions and growth processes, becoming spermatocytes.
  2. Spermatid Formation: Spermatocytes undergo meiosis (a special cell division) where the chromosome number is halved (from 46 to 23), resulting in spermatids.
  3. Sperm Formation: Spermatids mature into spermatozoa (sperm), which have a head, midpiece, and tail. These are released into the seminiferous tubules.

Oogenesis

Oogenesis: The process of egg formation.

  1. Embryonic Stage: Germ cells undergo division and growth to become oocytes. These are surrounded by follicular cells, forming a follicle. Follicular cells produce estrogen (female sex hormones).
  2. From Puberty: Each month, one follicle and the oocyte within it complete development. The egg lives only a few hours, but if fertilization occurs, it completes processing, reducing the chromosome number from 46 to 23, becoming a mature egg.

Fertilization and Embryo Formation

Fertilization

  1. Fertilization: During intercourse, between 100 and 350 million sperm are deposited in the vagina. However, only one sperm may fertilize the egg.
  2. Zygote Formation: A single cell is formed with two nuclei (pronuclei), one from the father and one from the mother. When they fuse, the result is the zygote, which has 46 chromosomes (23 from each parent).
  3. Zygote Division: The zygote moves into the uterus and forms a group of cells (morula). Upon reaching the uterus, it transforms into a blastocyst.
  4. Implantation: About 7 days after the blastocyst reaches the uterus, it attaches to the endometrium and embeds itself (implantation), allowing the embryo to attach to the uterine blood vessels to form the placenta.

Embryonic Development

Trimesters

  1. 1st Trimester: In the first eight weeks, the embryo becomes a fetus. It lies inside a sac, the amnion, and is immersed in amniotic fluid, which protects and moisturizes it. The placenta connects the fetus with the umbilical cord.
  2. 2nd Trimester: The skeleton ossifies, and the fetus shows some reflexes, such as wrinkling the forehead. The mother’s belly becomes visibly larger.
  3. 3rd Trimester: The fetus gains weight and size, and the circulatory and respiratory systems prepare for birth. The fetus is approximately 50 cm long and weighs about 2.5 to 3 kilos.

Birth

  1. Dilation: Contractions cause the opening of the cervix. Contractions increase in intensity over time. The amniotic sac ruptures, releasing amniotic fluid.
  2. Expulsion: Contractions push the fetus through the cervix.
  3. Placental Stage: The umbilical cord is clamped, and the newborn takes its first breath. The placenta detaches and is expelled through new contractions (afterbirth).