Human Reproduction: Characteristics, Process, and Systems

Characteristics of Sexual Reproduction

Human reproduction is sexual. This means that it's necessary for two individuals of different sexes to take part.

1. Each sex produces a different type of sex cell or gamete.
2. Fertilization is internal; it takes place inside the female reproductive system.
3. We are viviparous; the fetus grows in the mother's womb.
4. We are sexually dimorphic. This means that males and females have physical differences. This happens in many species.

Process of Reproduction

1. Gametogenesis: formation of the gametes.
2. Fertilization: joining of the male and female gametes.
3. Embryo Development: cell division and differentiation.
4. Childbirth: birth of the baby.
5. Development: cells increase and mature.

Changes in Adolescence

Adolescence begins with puberty, a stage characterized by anatomical and physiological changes in girls' and boys' bodies. It is until 14 and 15 years old.

• Physical changes: mammary glands develop, menstruation, sebaceous glands develop (acne), body changes shape and grows taller, underarm and pubic hair grows, sweat glands develop (body odor), sex organs mature, facial and body hair grows, voice deepens.
• Psychological and social changes: sudden mood changes, greater need for independence, more impulsive behavior, sexual desire begins, a feeling of invulnerability, a search for relations with peers and support from friends, a need to belong to a social group.

Gametes

Specialized reproductive cells, or sex cells, that transfer genetic information from each parent to their descendants.

Spermatozoa

Male gametes, small, mobile cells. In their mature state, they are made up of:

• Head: most of this is taken up by the nucleus, which contains the paternal genetic material. The tip, called the acrosome, is a vesicle with enzymes that dissolve the outer membrane of the ovum. This allows the spermatozoon to penetrate the ovum.
• Midpiece: It contains many mitochondria which supply the spermatozoon with the energy it needs to move.
• Tail: long flagellum that gives the spermatozoon cell movement. Spermatozoa are formed through a process called spermatogenesis. Begins in puberty. After their formation in the testicles, the spermatozoa are stored in the epididymis and in the vas deferens. From there, they exit the body through the urethra during ejaculation in the form of semen.

Semen

White viscous fluid that contains the spermatozoa and various other substances produced by the glands of the male reproductive system that feed and activate the spermatozoa.

Ova

Female gametes are called ova. Are large, spherical cells that do not move. Made up of:

• Nucleus: contains the maternal genetic material.
• Cytoplasm: has nutritive substances that feed the embryo in its first stages of development.
• Protective covering: protect the ovum. Innermost layer is called the plasma membrane, which surrounds the cytoplasm. Next is the pellucid membrane, and then the corona radiata, which is made up of follicle cells.

Formation of Ova

This process is called oogenesis and takes place in the ovaries.

• Before birth: ova development begins in the ovaries in the embryonic stage, around the fourth month of pregnancy. Stem cells divide many times and grow, becoming oocytes. Each oocyte stops dividing. It then surrounds itself with cells that protect and feed it, building a follicle.
• During childhood: when girls are born, their ovaries contain about 400,000 follicles. However, only about 450 mature.
• During puberty and fertile life: ovulation takes place approximately every 28 days. A follicle grows and matures, releasing the oocyte into the Fallopian tube. If the oocyte is fertilized, it continues to mature and becomes an ovum. For convenience, the oocyte that leaves the Fallopian tube is also called an ovum.

Fertilization

Joining of the ovum with a spermatozoon. After intercourse, or copulation, when the penis enters the vagina, the spermatozoa travel up through the female internal genitalia.

1. Some spermatozoa manage to reach the oocyte and surround it. Then they release enzymes from their acrosomes to weaken the oocyte's protective covering until one spermatozoon enters. The oocyte becomes an ovum and surrounds itself with a fertilizing membrane.
2. Nuclei of both gametes fuse and form the first cell of the new individual: the egg cell, or zygote.
3. On its way towards the uterus, the zygote begins to divide, becoming a mass of complex cells called an embryo.
4. Once in the uterus, the embryo enters and implants itself in the endometrium. This happens six or seven days after fertilization.

Pregnancy or Gestation

Begins with the implantation of the embryo in the endometrium. At the same time, new structures form that allow the embryo to grow and survive.

• Amniotic sac: thin membrane containing amniotic fluid that protects the embryo.
• Umbilical cord: connects the embryo to the placenta.

Childbirth of Three Stages

Takes place at the end of the pregnancy and consists:

1. Cervix dilates, amniotic sac breaks.
2. Strong contractions widen cervix, lasting 8-12 hours.
3. Baby born headfirst, cord cut, first breath taken.
4. Placenta expelled within 5-20 minutes.

Reproductive System