Hormones released by the ovaries and testes cause sexual development during puberty, a period of rapid growth and sexual maturation that usually starts between the ages of 9 and 15. At the end of puberty, the male and female reproductive organs are fully developed and become fully functional.

Reproduction in humans is when the male gamete (sperm) fuses together with the female gamete (ovum/egg).

At first, it is just one single cell, which duplicates over and over until after 9 months……

TA-Dahh! - A baby is born!

The main role of the male reproductive system is to make and deliver sperm.

The testes are the main organs of the male system. Two testes are held in an external sac called the scrotum. The testes make sperm in tiny tubes called seminiferous tubules. The sperm mature and are stored in an epididymis. A tube called a vas deferens carries sperm from each testis to the urethra within the penis.

Along the way, secretions of several glands form a nutrient-rich fluid called seminal fluid. The combination of sperm and seminal fluids is called semen. Semen leaves the body through the urethra. Contractions eject semen from the penis in a process called ejaculation.

A mature sperm cell consists of a head that contains the nucleus, a midpiece that is packed with mitochondria, and a flagellum that propels the sperm.

The main roles of the female reproductive system are to make eggs and prepare the female body to nourish an embryo.

The ovary is the main organ of the female system. Each ovary has thousands of follicles, which are clusters of cells that surround an egg. A mature egg moves through the Fallopian tube to the uterus, which is connected to the outside of the body by the vagina.

Beginning in puberty, the female body goes through a menstrual cycle, a series of events that prepares the body to care for a fertilized egg. The menstrual cycle has four phases:

· Follicular phase: An egg matures in its follicle.

· Ovulation: The mature egg is released from the ovary.

· Luteal phase: The follicle develops into a structure called the corpus luteum.

· Menstruation: The lining of the uterus falls away and leaves the body through the vagina if the egg is not fertilized.

Fertilization and Early Development Fertilization is the joining of a sperm and an egg. Following fertilization, a series of events called development begins.

A fertilized egg is called a zygote. The zygote divides and undergoes repeated rounds of mitosis and develops into a hollow ball of cells called a blastocyst.

About a week after fertilization, the blastocyst attaches to the wall of the uterus in the process of implantation. At the same time, cells of the blastocyst start to specialize through differentiation. Some cells migrate to form two cell layers—the ectoderm and the endoderm.

A third layer of cells is produced by a process called gastrulation, in which cells from the ectoderm migrate to form the mesoderm. The three layers eventually develop into the different organs of the embryo. During neurulation, the notochord and the neural tube form. The neural tube eventually develops into the brain and spinal cord.

As the embryo develops.......

membranes for protection and nourishment also form. Part of one membrane combines with the uterine lining to form the placenta. Mother and embryo/fetus exchange gases, food, and waste products across the placenta. The umbilical cord connects the embryo/fetus to the placenta.

After eight weeks of development, the embryo is called a fetus. By the end of three months, most organs are fully formed.

Later Development Another six months of development occurs before birth.

During months 4–6, the fetus’s tissues become specialized and organs such as the heart begin to function.

During months 7–9, the fetus’s organ systems mature as the fetus grows in size and mass. The lungs and the central nervous system complete their development.

Childbirth occurs about nine months after fertilization, when hormones cause contractions in the mother’s uterus. The contractions first push the baby out through the vagina. Then, more contractions expel the placenta and amniotic sac from the uterus. Shortly after birth, the mother’s breast tissue begins to produce milk that contains everything the baby needs for the first months of life.

The placenta is a barrier to many harmful agents, but some are able to pass through it, such as viruses that cause AIDS and German measles. Alcohol, drugs, and smoking also have negative effects on embryos and fetuses. Prenatal care and advancements in medical technology have lowered the infant mortality rate.

Reproduction REVIEW