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The reproductive system has multiple functions in animals. The most important one is the production of gametes, haploid cells specialized in the transmission of the genetic information. Male gametes are the spermatozoids and female gametes are the oocytes. The fusion of one spermatozoid and one oocyte results in a diploid cell known as zygote. One zygote divides and gives rise many cell lineages that after differentiation form a complete new organism.

The reproductive system is morphologically different in males compared to females. The differences arise very early during the embryo development. Building male or female reproductive structures will later determine the morphological features of the adult animal, mainly the sexual secondary features like muscle development, bone structures, body hair, mammilar structures, fat distribution, behavior, etcetera. This is the so-called sexual dimorphism. Being a male or a female is mostly determined by sexual chromosomes. In humans, XX sexual chromosomes give female morphology and XY chromosomes give male morphology. However, in other vertebrates, like reptiles and fish, environment variables, like temperature, may determine the sex of the animal. Some fish species may behave as hermaphrodites, so that they can produce male and female gametes.

Reproductive system consists of several parts: gonads, ducts, vesicles and reproductive organs.

Female reproductive  system
Drawing of the female reproductive system.

Sistema reproductor femenino

Ovaries are the female gonads. They are a couple of structures that produce oocytes, which are the female gametes. Oocytes mature inside the ovarian follicles, which are made up of somatic cells plus one oocyte. The oocyte maturation takes place inside the follicles. Ovaries also synthesize and release female hormones like progesterone and estrogen.

Oviducts (Fallopian tubes or uterine tubes) are the ducts used by the oocyte to travel from the ovary to the uterus. If fertilization has occurred in the oviduct, the embryo attaches (implantation) to the wall of the uterus, which is the specialized part of reproductive female ducts where development of the embryo occurs. The walls of the uterus undergoes periodic changes influenced by hormones (estrogens) released by the ovary every time an oocyte is released from its follicle. The vagina is the last part of the female reproductive ducts, connects the uterus with the exterior, and is the receptacle of the male reproductive organ.

The vulva is the external reproductive female organ. It is formed by skin folds known as labia. There are the outer labia major and the inner labia minor. The clitoris, the main structure for sexual stimulation, is located in the most rostral part of the bulba.

Male reproductive system

Male reproductive system in humans
Drawing of the male reproductive system in humans.

Testicles are the male gonads. They produce spermatozoids, the male gametes, and release testosterone, a male hormone. Unlike ovaries, testicles are included in a bag of tissue known as scrotum, located outside the abdominal cavity. The seminiferous tubules are the testicular structures in charge of producing spermatozoids. These ducts are made up of somatic cells that form an outer layer of tissue enclosing the germinal cells, which produce gametes by meiosis.

Spermatozoids are produced in the seminiferous tubules, and are conducted by the epididymis (there are two epididymis ducts, one per testicle), which are connected with the deferent ducts. Ejaculatory ducts continue after the deferent ducts, and they converge into the urethra, which has an open end allowing the gametes to leave body.

Seminal vesicles and prostate, glands associated to the ejaculatory ducts and urethra, respectively, release the liquid solution that together with spermatozoids forms the semen. Cowper glands, or bulbourethral glands, release a fluid for lubrication during sexual arousal.

She male reproductive organ is known as penis. It is composed of spongy and cavernous bodies and by the glande. Urethra runs through the interior of the pennis and conducts the semen to the exterior of the body. Urethra is also used by the excretory system for releasing the urine.

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