Reproductive System

Introduction.

Adam and Eva, by Durer

Reproduction is the process of generation of new living beings of an species, with the consequent transmission of a part of the genetical information of the parent.

In human beings, reproductor system have a clear sexual dimorphism, what is logical in a species with separated sex.

We can find two parts in the reproductive system. One of the parts is responsible of producing reproductive cells, called gametes. This part is called gonad. The gonads of males are called testes (or testicles) and the cells produced are called spermatozoa. The gonads of females are called ovaries and the cells produced are called ovules. In the gonads are also produced the most important sexual hormones.

The other part of the reproductive system is the group of structures and accessory cells that allow, in some way, the reproductive process.

Male Reproductive System.

Anatomy of the Male Reproductive System.

The male reproductive organ have two parts. The gonads are responsible to produce reproductive cells. And the accessory structures can be divided into glands, conduction systems (tubes and ducts) and organs of support.

Testis are two oval glands, around 5 centimetres long and 2,5 centimetres diameter. They are located in an evagination of the abdomen, formed by a skin sac called scrotum. Testis are covered by a membrane called tunica vaginalis. In the interior, they have other covering, a fibrous capsule called tunica albuginea. This inner tunica forms septa that divide the testicle into various compartments called lobules. 

Each testicle have between 200 and 300 lobules. Each one of these lobules has between two and three seminiferous tubules. In the interior of these ducts spermatozoids are produced. All the seminiferous tubules join in an structure called Rete Testis.

Spermatozoids flow from the seminiferous tubules, through a group of tubules called Efferent Ductules. These ducts join in a coma-shaped structure called Epididymus, located in the posterior part of the testicle.

From the Epididymus the spermatozoids are transported to the Deferent (or Seminal) Ductus. These ducts start in the Epididymus, ascend through the abdomen and cross the prostate. Just before reaching the prostate, they join with a duct from the seminal gland and their name change into ejaculatory ducts. Ejaculatory ducts ends up in the urethra.

The urethra is a 20 centimetres long duct that has three different parts. The first one is the prostatic urethra, a 2 or 3 centimetres long duct that crosses the prostate. The second one is the membranous urethra, a 1 centimetre long duct that crosses the urogenital diaphragm. The third one is the spongy urethra, a 15 centimetres long duct that crosses the penis and exits to the exterior through the external urethral orifice.

We have already seen that there are some gland that release their secretion to the tubes. They are called accessory glands.

The first one is the seminal vesicle. Its a 5 centimetres long sac that release a liquid rich in fructose. Its main function is feeding the spermatozoids. It is around the 60% of all the sperm secreted.

The second gland is the prostate. It is ring-shaped and has the size of a chestnut. Its function is producing and releasing to the sperm a liquid rich in citric acid and phosphatase acid.

Finally, we find the Cowper glands. They are very small and produce a liquid that neutralises the acid pH of the urethra.

It is released between 2,5 and 5 millilitres of sperm per ejaculation. Human sperm has between 50 and 150 millions of spermatozoids per millilitre.

Penis is the structure used to put the spermatozoids beyond the vagina. It is cylindrical-shaped and has three different parts. The root of the penis is the closest part to the abdomen. The body of the penis is the central part. And the glans is the final part of the penis.

Male Reproductive System

The body of the penis has three sac-shaped structures inside. Two of them are called corpora cavernosa. The third one is the corpus spongiosum, and is crossed by the urethra. 

These three sacs are highly vascularised. When the organism suppose that any type of sexual contact is going to take place, the arteries increase their diameter allowing the blood to flood the sacs. At the same time, the veins are closed, so the inner blood pressure of the sacs rises, leading an increase of the volume. This process is called erection.

Male Sexual Hormones.

Hypothalamus produce and releas GnRH. This hormone promotes the release of FSH and LH by the pituitary gland. FSH promotes the production of spermatozoids (spermatogenesis). LH promotes the production of testosterone by the testicle. When the spermatogenesis reaches high levels, testis produce a hormone called Inhibin. This hormone reduce the secretion of FSH and LH by the pituitary gland and GnRH by the hypothalamus.

High levels of testosterone also lead to a reduction of the GnRH production. 

Testosterone is a hormone with low activity. When the testosterone reaches the target tissues is transformed into DHT (Dihidrotestosterone) due to the action of an enzyme called 5α-Reductase.

Testosterone and DHT promote all the male corporal developmental patterns, such us morphology of the genitals during the embryonic development. During puberty, they promote the typical male characteristics of adults: muscular development, distribution of hair, etc. These hormones stimulate the synthesis of proteins and consumption of fats. They are also related to our sexual and social behavior.

Female Reproductive System.

Anatomy of the Female Reproductive System.

Female gonads are called ovaries. They produce female gametes, called ovules. These ovules develop from secondary oocytes, that are not transformed into ovules until the fecundation. Ovary produces, besides, some hormones. The most important hormones are progesterone, estrogens, relaxin and inhibin.

The accessory structures of the female reproductive system are the Fallopian tubes, the Uterus (also called womb), the vagina and the external genitals, also called vulva.

Ovaries are two oval glands, with a size and shape similar to an almond, located in the pelvic cavity, side by side with the uterus. The ovary is covered by a tunic called tunica albuginea. Under this layer, we find the germinal epithelium. Tunica albuginea and germinal epithelium surround the stroma. In the estroma are produced the ovarian follicles, that evolve and release the oocytes (that will be transformed into ovules).

When the ovules are released by the ovary, they are transported to a duct called Fallopian tube. It is a 10 centimetres long tube that is connected to the uterus. The part of the tube that is in contact with the ovary is called infundibulum. The infundibulum is conic-shaped crowned with a group of finger-shaped extensions called fimbriae.

In the middle of the tube there is a dilated portion called ampulla. This is the place where, in theory, the ovule must be fecundated.

The final part of the Fallopian Tube is called isthmus. Is connected to the uterus through an orifice called ostium.

The uterus or womb is the sac where the embryonic development takes place. Before the first pregnancy, it is 7,5 centimetres long and 5 centimetres wide. The uterus has three different parts. The upper part is wider and is called fundus. The central part is called body. The lower part, that is narrower and connects to the vagina is called cervix (or neck of uterus).

Female Reproductive System

The uterus wall have three layers. The outer one is a membranous layer called perimetrium. Under the perimetrium there is a muscular layer, responsible of the uterus contraction during the delivery, called myometrium. Finally, the inner layer is called endometrium.

Endometrium have two parts. The outer layer of the endometrium,adjacent to the myometrium, is called basal layer. The inner part is called functional layer. The functional layer is responsible of protecting the ovule after fecundation and during the initial stages of the pregnancy. This layer is renewed every 28 days. The destruction and elimination of this layer is called menstruation. Menstruation is the final part of the menstrual cycle. The menstrual cycle must ensure that the uterus is in optimum conditions to support the fecundated ovule in the fertile days. 

The vagina is a duct that connects the uterus to the exterior. Through this duct the blood and the remains of endometrium are discharged during the menstruation. And the babies exit to the exterior during the delivery. Besides, the penis of the male must penetrate through the vagina to release the spermatozoa as close to the uterus as possible, generally near the cervix.

It is a membranous tube located between the urinary bladder and the rectum. It is open to the exterior by the vaginal orifice. This orifice can be partly covered by a membrane called hymen.

Female Reproductive System.

The opening of the vagina to the exterior is surrounded by the external genitals, called vulva. One of the components of the external genitals is the Mons Pubis or mons Venus, an elevation of fat tissue covered by skin and hair, which main function is protect the pubis arch against damages during sexual relations. Other components, under the Venus Mons, are the labia majora, two cutaneous folds covered by hair. Covered by these majora labia there are two little flaps of skin without hair that protect the opening of the vagina. The skin located between the minor labia is called vestibule. In the upper junction of the minor flap there is a little cylindrical erected mass, called clitoris. This structure is rich in sensitive nerves.

Female Reproductive Cycle.

During the fertile years, the female reproductive system carries out cyclical changes affecting mainly to the uterus and ovaries. These cyclical changes are called reproductive cycle and can be divided into menstrual cycle and ovaries cycle. All the process is controlled by the sexual hormones.

The cycle starts after menstruation. During menstruations, the ovule produced in the cycle is released and eliminated with the functional endometrium layer. After the elimination of the ovule, new follicles in the ovary start their development. Firstly about twenty follicles start to develop, however only one of them will finish the process. The development accelerates above all after the sixth day.

The development of the follicles is a response to the high levels of FSH and LH. These hormones promote, at the same time, the production of estrol and  β-stradiol by the follicles. Low concentration of strogens inhibits the production of FSH and LH by the pituitary gland. The concentration of FSH drops. The amount of FSH is not high enough to promote the development of all the follicles, so only one of the can complete the developmental process. This follicle matures and becomes into a Graaf follicles.

The concentration of LH continues raising slowly. Graaf follicle keeps on producing strogens. Strogens promotes the growth of the endometrium in the uterus.

High levels of strogens promote secretion of GnRH by the hypothalamus. As a result, LH and FSH levels rise too. When LH level reaches a peak point, the ovule is released from the Graaf follicle. The ovule left the ovary and is transported to the Fallopian tube.

At the moment, the strogen level drops abruptly, because the follicle without ovule reduce the production of those hormones. The Graaf follicle is transformed into the corpus luteum, that produce strogens and progesterone. The release of the ovule takes place in the fourteenth day of the cycle.

The progesterone produced by the corpus luteum stimulates the growth of the new endometrium. The uterus will be ready to protect the embryo if the fecundation occurs.

Now, if no fecundation takes place the corpus luteum keeps on producing strogens and progesterone. Both hormones, secreted at the same time, inhibit the secretion of LH and GnRH. The yellow body is transformed into the corpus albicans. The corpus albicans stops producing hormones. So levels of progesterone and strogens drop abruptly. This drop stops the inhibition of the production of LH and FSH. So the level of these two hormones raises again. The new elevation of LH and FSH levels leads the menstruation. The cycle ends up, and we are again in the first stage. 

However, if fecundation takes place the fecundated ovule is implanted into the endometrium. This fecundated ovule is called chorion. The chorion produces a hormone called hCG. This hormone inhibit the transformation of the corpus luteum into the corpus albicans. The corpus luteum keep on producing progesterone and strogens, so the menstruation is not activated.

After being implanted in the endometrium, the fecundated ovule starts the production of the placenta. The placenta produces strogens and progesterone. At the moment, the corpus luteum degenerates, because it is not necessary to make hormones.

Female Reproductive System

Fecundation.

During fecundation, the genetic information of the ovule and the spermatozoa join. Both cells are haploids, they have only a half of the chromosomes (n), because they have been generated by meiosis from diploid cells.

When the genetic information of ovule and spermatozoa join, the new cell is diploid again. This diploid cell divides and grow, forming after nine months the new human being. 

During an ordinary sexual relation, the male release in the vagina, near the cervix, between 300 and 500 million spermatozoa in average. Only 1% arrives at the ovule. And only one complete the fecundation.

After the fusion, the cell carries out successive divisions. This process is called segmentation. During the first days, it divides without changing its size, only increasing the number of cells. After 96 hours we can find a structure called morula. After 5 days, the morula has been transformed into a blastula. The blastula adheres to the endometrium. The process is called implantation.

After this, the real embryonic development stars. This process will last nine months. The placenta will be formed in three months. That structure carries out the nutrition of the zygote, providing nutrients and oxygen. After the three first months the zygote is called foetus.

The foetus will be attached to the placenta until delivery. Foetus and placenta are chained by a structure called umbilical cord, that transports nutrients from the circulatory system of the mother to the foetus and waste products from the foetus to the circulatory system of the mother (this waste products will be filtrate do and eliminated by the kidneys of the mother).