Human Reproduction: From Gametes to Fetus Development

Sperm

• Midpiece: This is located between the head and the tail. It contains mitochondria at the base of the flagellum, which provide energy for movement. It also contains centrioles that control the sperm's movement.
• Tail: The flagellum is made of proteins and is the part that moves.

Egg

The egg contains the zona pellucida, vitelline membrane, deutoplasm, nucleus, and corona radiata. It carries 23 chromosomes. It is elongated, visible to the human eye, immobile, and has a reserve of nutrients.

Ovarian Cycle

After puberty, oogenesis takes place in women approximately every 28 days. One egg matures and is released. The ovarian cycle has three phases:

• Follicular Phase

  This phase lasts about 14 days and is caused by the follicle-stimulating hormone produced by the pituitary gland. Only one egg reaches full maturity.

• Ovulation

  This is the release of an egg as a result of the production of luteinizing hormone. The body temperature increases by 0.5ºC.

• Luteal Phase

  The corresponding area in the ovary transforms into a mass of cells that produce and secrete progesterone. After 10-12 days, it decays, starting a new cycle.

Menstrual Cycle

This is a series of periodical changes that occur in women's sex organs. It prepares the uterus for the implantation of the zygote. It only happens if the egg has not been fertilized. The menstrual cycle has three phases:

• Proliferative Phase

  The endometrium is built up. This phase is started by estrogens and lasts about 11 days.

• Luteal Phase

  The endometrium reaches its maximum thickness and prepares to receive and provide nutrients to an embryo. This phase is started by progesterone and lasts about 12 days.

• Menstruation

  This phase only occurs when there is no embryo. The endometrium comes off and is expelled from the body with some blood. It lasts 4-5 days. Emotional circumstances influence the menstrual cycle.

Fertilization

Fertilization occurs in the fallopian tubes. It can happen when sperm and an egg meet inside a fallopian tube. Sperm can live up to 5 days after ejaculation, and an egg can live up to 48 hours after ovulation. They have to meet during this period. When the sperm surrounds the ovum, the acrosome secretes a substance that breaks the outer membrane. When the two recognize each other, the sperm binds to the egg's zona pellucida.

When the sperm's nucleus enters the egg, the two gametes join. This fertilizes the egg, and a zygote is formed. This causes physical and chemical changes, like the formation of a barrier around the zygote, preventing more sperm from entering. The fusion of the nuclei results in a zygote with 46 chromosomes.

Gestation

Gestation is the period between fertilization and birth, lasting approximately 9 months. The embryo gradually begins to form special tissues and organs, but it still does not look like a baby. A few days after fertilization, it becomes an embryo and attaches to the uterus through implantation. There are three phases of embryonic development:

• Cleavage

  The zygote divides several times to form a mass of cells (morula) that becomes a hollow sphere of cells (blastula).

• Gastrulation

  Three cell layers form: the mesoderm and an outer layer (ectoderm). These then become tissues and organs.

• Organogenesis

  This is the longest phase, during which bodily structures form.

About 60 days after fertilization, it is a fetus and looks like a baby.