Proyectos de Investigación: Crecimiento de Plantas.

Proyectos de investigación en segundo de ESO.

Diferencias de crecimiento en plantas regadas con agua destilada y con agua con iones.
En los vídeos se explican los resultados. Están en inglés, por pertenecer al programa bilingüe.
Para la grabación de los vídeos se usaron cámaras de fotos de teléfonos móviles (viejos y desechados), en los que se instaló el programa tina-timelapse para toma programada de fotos.
El montaje se ha realizado con iMovie y el audio con GarageBand.

Human Parasites: Lice

Parasitism can be defined as a relationship that takes place between two living beings of different species, where one of them feeds on the another without killing it, but causing damage. In other words, one of the species obtains benefits, whereas the other one suffers.

Louse

Human beings have many different parasites. Some of them are nematodes, such as intestinal roundworms, others are Platyhelminthes, such as tapeworms, although the most of them belong to the phylum Arthropoda, something logical because three quarters of the whole animals are arthropods.

Some parasite arthropods live outside our body and only contact with us in order to get their substance. Mosquitos or ticks are two examples. Others, however, live on our body, so that they can easily access their food. The main examples of these kinds of bugs are fleas and lice. Curiously, all of them feed on the same organic product: our blood.

The parasites that live on their hosts are usually extremely adapted to them. In general, they have only one or few favourite preys. The fleas that live on dogs are different to the fleas that live on human beings. Dog fleas can live on our skin, but not for long. In fact, they will move to their original prey as soon as possible. During the fifteenth century rat fleas transmitted an extremely severe disease, called black death, because they attacked people when their host (the rat) died due to the pest and they needed a temporally living being to survive. Sadly, at that moment rats were very abundant in cities, because of the lacking of basic hygienic conditions.

Lice (Pediculus humanus) are, doubtless, the most frequent parasite capable of living on our skin. They rarely transmit diseases. Nevertheless, they are a recurrent and uncomfortable infestation, above all in children. As we said, human lice are exclusive for us, and different to lice of other mammals. What is more, there are even slightly different lice adapted to dwell on different races. And there are three different types of lice according to the zone of our body they prefer to invade.

The first type is the louse that is adapted to live on our head (Pediculus humanus sp capitis), attached to our hair. It is, by far, the most frequent one. This louse always lives there, and the females lay eggs that mature attached to the base of the hair. These eggs are called nits. The mature animal feeds on our blood, biting our skin and causing the typical stinging.

The second one is the bodily louse (Pediculus humanus sp corporis). This animal is adapted to feed on the skin that is not completely covered by hair. It can not live on our skin, because the hairs are too thin and not abundant enough, so that it usually lives in our clothes, mainly in the folds of shirts, jackets or sweaters. It leaves the clothes and moves to our skin just to obtain blood, coming back to the clothes after feeding. These lice are not so frequent, and sometimes they can penetrate in our epidemic tissue to protect themselves.

Crab louse

The last type of louse is called crab louse (or simply crab) (Pthirius pubis). This louse is adapted to live, feed and reproduce on our genital zone, attached to pubic hair. It can not live on our head, merely because the section of the hair is very different (the hairs on our head have a circular or slightly ellipsoidal section, whereas pubic hairs has a triangular section). If the population of crab lice rises too much, some of them can migrate to other places with similar hairs, above all our armpits and our eyebrows.

The Manna for the Acari.

Biologists define commensalism as an inter-specific relationship, that takes place between two different species where one of them obtains some profit without affecting the other one.

Dust Mites, by Jacopo Werther

One typical example of commensalism is the relationship between human beings and those myriads of acari, also called dust mite, that live in our pillows, sheets or mattresses. Some of these small arthropoda are adapted to live close to humans and feed on our dead epithelial cells that are permanently being removed from our skin. The animal obtains its food without affecting us, unless we are allergic to them. But, even in that case, we must talk about commensalism because the dust mites are not responsible for the damage, it is our defensive system which is working incorrectly. Summing up, the bug causes indirect damage and after all, it is not to blame.

Dust mites are one of the smallest known animals, they measure around 250 microns so they are hardly visible to the naked eye. They belong to the phylum arthropoda, and the class arachnida, like spiders or scorpions and just like them, they have eight legs and their body has two main parts, the cephalothorax and the abdomen. As they feed on dead organic matter, we can say that they are scavengers.

Evolution has transformed some of these animals into our ordinary neighbours. They dwell in places where dust and organic matter are especially abundant. And we lose lots of dead epithelial cells when we are sleeping in our beds. Due to this, they are glad to combine our rest and their meal.

The fact of being surrounded by animals capable to live eating dead parts of our body is, in my opinion, slightly overwhelming. Superficial cells of our skin are shed constantly. Indeed, a fraction of the dust that it is built up in our house is made up of these cells we have released. This is the reason why we talk about dust allergy; we should talk about allergy to dust mites, because the animals that live in the dust are the real allergen.

Skin Anatomy

Our skin is a complex organ with three parallel layers. The lower one, in contact with other inner tissues, above all bones and muscles, is called hypodermis. This is a connective tissue, rich in adipocytes (also called fat cells) and fibrillar proteins. Just above this layer we can find the dermis, a thick layer made also of connective tissue, although this one is richer in water, elastic fibers and fibroblasts, the cells that produce and support the fibrilar system.

Human skin

Finally, the upper layer is called epidermis and it is made of epithelial tissue. The lowest level of this layer is called basal layer and it is made of epithelial cells that are dividing constantly. They produce new epithelial cells that move to upper levels. New cells push older cells, so that the longer the time a cell has been produced, the higher the level it can be found.

The cells move through the epidermis towards the surface and at the same time, they carry out a maturative process, they build up a fibrilar protein called keratin that make them harder. They also lose all their inner organelles and, step by step, they are transformed into a block of keratin. After finishing the process, after arriving in the surface, they are dead. To sum up, all our exterior is a sort of sheet, a tissue of keratin and dead cells (called squamous cells). The older cells are removed and other cells take their place. This is the way our body keeps our exterior in optimum condition.

From the dust mites point of view, these lost cells are a sort of manna released by those enormous animals that God created in order to provide them their food.

Nervous System.

Introduction.

Cutting the Stone (Hieronymus Bosch)

The nervous system and the endocrine system are the main systems to control and regulate the body.

The nervous system is responsible for receiving stimuli from the environment or interior parts of the body (detected by the sense organs and propioceptors), processing this information and deciding the appropriate response. The information must be conducted, first from the sense organs and propioceptors to the integration centres (Central nervous system), and then from the integration centres to the effectors, which are always muscles or glands.

Due to this, the nervous system has sensitive, integrative and motor functions.

Although the endocrine system is the other one which is responsible for controlling the body, its functions are different. The responses coordinated by the nervous system are quicker (they take place in milliseconds), but are not so lasting.

The endocrine system, however, coordinates slower responses (they take place in seconds), but are lasting (growth, for instance, is coordinated by the endocrine system and lasts for years).

Anatomy of the Nervous System.

Parts of the Nervous System.

The nervous system can be divided according to two different criteria.

First, we can divide the nervous system according to anatomical criteria. And we have two different anatomical parts: the central nervous system and the peripheral nervous system. The central nervous system can be also divided into spinal cord and brain, where the integration centres can be found. The peripheral nervous system transmits information from the receptors to the central nervous system and from the central nervous system to the effectors.

Second, we can divide the nervous system according to functional criteria, so we can find the somatic nervous system and the autonomous nervous system. The somatic nervous system carries out conscious or voluntary actions (all the movements of skeletal muscles). The autonomous nervous system carries out non conscious actions (such as digestion, arterial pressure, etc.).

Third, we can find  two divisions into the autonomous nervous system, called sympathetic and parasympathetic nervous system. The autonomous organs receive (generally) innervation from both divisions, and one of them activates the organ whereas the other one inactivates the organ. In other words, sympathetic and parasympathetic divisions have usually opposite functions (there are some exceptions, there are organs that have only sympathetic or parasympathetic innervation, and there are organs where both divisions have the same function). The sympathetic division is mainly related to actions that promote energy consumption, whereas parasympathetic is mainly related to actions that promote energy saving. For instance, sympathetic nerves increase the heart beat rate and parasympathetic nerves reduce the heart beat rate.

In this lesson we are going to study the nervous system according to anatomical parameters, first analysing the central nervous system and then the peripheral nervous system.

All the central nervous system is covered by three protective layers called meninges. We talk about cranial meninges to the protective layers of the brain and spinal meninges to the protective layers of the spinal cord.

The outer layer is called dura mater and is attached to the bone surface (cranial bones and vertebrae). The second layer, lying under the dura mater, is vaguely similar to a spider web and is called arachnoid. The third one, attached to the nervous tissue, is called pia mater. Between the arachnoid and the pia mater there is a space filled by a liquid called cerebrospinal fluid. This liquid not only provides protection to the nervous system, but also nutrients. 

Meninges

Central Nervous System: Spina Cord.

The spinal cord is a thin bundle made up of nervous tissue, covered and protected by the vertebral column. The spinal cord transports information from the afferent nerves to the brain and from the brain to the efferent nerves. So the spinal cord is the place where the afferent nerves arrive at, and the place where efferent nerves leave from. It is also the place where spinal reflexes take place.

The spinal cord is cylindrical, with a slightly ellipsoidal cross-section. In the interior, there is an H-shaped dark coloured zone (similar also to a butterfly), made up of grey matter (mainly non myelinated neurones). This is the part where integrative actions are carried out. The peripheral part of the cord is made up of white matter, what it means mainly myelinated neurones. This white zone is responsible for transmitting  the information upwards (to the brain) and downwards (to the efferent nerves). The four prolongations of the central grey zone are called dorsal horns (there are two anterior and two posterior dorsal horns).

These four dorsal horns divide the peripheral white zone into four funiculi (called anterior, posterior and left or right lateral funiculi). As we said, these funiculi are responsible for transmitting the information.

The spinal cord is crossed by a central duct called central canal. There are, besides, two fissures, one of them in the anterior part, the other one in the posterior part. The anterior fissure is wider and more important than the anterior one, and is called anterior medial sulcus (or fissure). The posterior fissure is narrower and is called posterior medial sulcus.

Spinal Cord: Section.

The spinal cord is not only related to the transmission of information, but also to the spinal reflexes. Spinal reflexes are simple, but very fast responses to external stimuli. These responses are carried out extremely quickly because the information is not transmitted to the brain, they are coordinated in the grey zone of the cord. They allow immediate responses to serious injuries, accidents or problems that must be solved quickly, such us burns, hits or abrupt muscle contractions.

Central Nervous System: The Brain.

The brain is the main organ where the information is analysed. After the analysis, this organ decides the responses and send orders to the effector organs. Furthermore, this is the place where are generated our sensations, feelings or emotions and where our memories are stored.

The brain has three parts:

• Brain Stem:
• Medulla oblongata.
• Pons.
• Midbrain.
• Cerebrum.
• Cerebellum.

Let’s analyse these parts one by one:

• Brainstem:
• Medulla oblongata: it is the lower part of the brainstem, located just at the end of the spinal cord. All the nervous funiculi form the spinal cord crosses the medulla to the brain. Part of the medulla tract crosses from the left to right and from the right to the left in the decussation of the pyramids. In the medulla oblongata are also located the the cardiovascular centre, that control the heart beat rate, strength of the heart contraction and the arterial pressure, and the respiratory centre, that control the breathing rate.
• Pons: the pons is located between the medulla oblongata and the midbrain. It is only two centimetres long, and connects the medulla oblongata with to the rest of the brain, and som parts of the brain in a parallel way. In the pons are located some important nuclei that control sleep and warning, and the pneumotaxic centre that controls inhalation and exhalation during breathing.
• Midbrain: This is the upper part of the stem-brain. In the midbrain there are some important nuclei related to coordination and movement of the eyes.
• Cerebrum: The cerebrum is a large neural mass, divided into two hemispheres linked by the corpus callosum. The surface of the cerebrum is not smooth, it has many folds that increase its  surface area. We are going to analyse the most important parts of the cerebrum: 
• Lobes: Each hemisphere of the cerebrum is divided into four lobes, called frontal, parietal, temporal and occipital lobe.
• Cerebral cortex: The cerebral cortex is the superficial part of the cerebrum. It is made up of grey matter, what it means that the neurones are not myelinated. The cerebral cortex can be divided into different areas according to their function. The motor area is the part of the cortex in charge of the control of skeletal muscles. The sensory area is the part of the cortex in charge of analysing the information from sense organs and receptors. Finally, the association areas are responsible for the coordination between the sensory and the motor areas.
• Basal Ganglia: located in basal zones of the hemispheres, they control some automatic movements of skeletal muscles, such as the movement of the arms when we walk or run (in order to keep balance).
• Thalamus: Located between the midbrain and the upper part of the cerebrum, the thalamus is control the transmission of information from lower to upper parts of the brain. It is also related to some specific sensations, such as the pain.
• Hypothalamus: Located under the thalamus, it is the main controller of homeostasis and chemical balance. It is the main control centre of the endocrine system (secretion of hormones).
• Lambic System: It is surrounding the thalamus. It is the main emotional centre of the brain. It is related to primary reactions, such us hate, love, aggression or attack-defence reactions. It is also related to the memory (this is why emotional facts are lasting remembered and some of them are absolutely unforgettable).
• Cerebellum: It is in the lower posterior part of the cranium. It is divided into two hemispheres, and has also an anterior and a posterior lobe in each hemisphere. The cerebellum compares the movements that our body is doing with the movements that it must be doing according to the orders of the motor area of the cortex. The cerebellum detects mistakes in the movements and sends information to the motor cortex in order to correct them. So, it does not control the muscles directly, but send the information to the part of the cerebrum that controls the muscles. Furthermore, it is related to some automatic movements to control our balance (the movement of our arms that helps us not to keep our balance, for instance). Finally, it also controls some repetitive movements that we can carry out in a semi-automatic way (writing, for example).

Anatomy of the Central Nervous System

Peripheral Nervous System.

The nerves transmit information from the sense organs to the central nervous system and from the central nervous system to the muscles and glands. The nerves part of the receptors in the sense organs to the spinal cord or directly to the brain, or of the spinal cord or the brain to the effector organs. The nerves connected to the spinal cord are called spinal nerves. The nerves connected to the brain are called cranial nerves.

There are twelve pairs of cranial nerves and thirty one pairs of spinal nerves. The spinal nerves exits of the spinal cord through the gap between the vertebrae. Each pair of nerves control a corporal band surrounding their vertebral zone. There are eight pairs of cranial nerves, twelve pairs of dorsal nerves, five pairs of lumbar nerves, five pairs of sacral nerves and one pair of coccygeal nerves.

The neurones that made up the nerves are myelinated to improve and accelerate the transmission of information. 

Dermatomes.

Autonomous Nervous System.

As we analysed before, the autonomous nervous system carries out autonomous or involuntary actions. There are two divisions in the autonomous nervous system: sympathetic and parasympathetic.

The sympathetic division is made of nerves that part of the dorsal and lumbar zones of the spinal cord. Just after leaving the cord, the sympathetic nerves form ganglia called sympathetic ganglia. All these ganglia make a ganglia chain located adjacent to the vertebral column. The information is transmitted from these ganglia to the effector organs.

The parasympathetic nerves part of the brainstem or of sacral zones of the spinal cord. The ganglia associated to these nerves are not located near the vertebral column, but near or inside the effector organ innervated.

As we studied before, when both sections of the autonomous nervous system control an organ, they usually carry out opposite actions. So, if one of the sections activates an organ, generally the other one deactivates it. The parasympathetic section is usually related to actions that preserve energy, whereas the sympathetic section is related to actions that consume energy. For instance, the sympathetic system increases the heartbeat and breathing rates, whereas parasympathetic system lowers heartbeat and breathing rates.

Physiology of the Nervous System.

The functioning of the nervous system is mainly based on the ability of the neurones to send signals one another when they are in contact by synapsis. The synapsis is the communication established between adjacent neurones. It is chemical communication and the chemical messenger transmitted from one neurone to another is called neurotransmitter.

The chemical messenger received by a neurone by the synapsis is quickly transmitted to the whole cell by an electrical process. It is related to the ionic differences between the inner and the outer part of the neurone. The inner part of the neurone is richer in anions, whereas the outer part of the cell is richer in cations. Due to this, there is a polarisation of the membrane, negative in the interior and positive in the exterior, that leads to an electrical potential of -60mV. After receiving the chemical signal from another neurone, the cell open ionic channels that allow the free transport of some anions and cations. The membrane depolarises, transmitting the impulse form the synapsis to the whole neurone. The signal will be also transmitted by synapsis to all the neurones connected with the activated neurone.

When a receptor detects one stimulus, it transmits the signal to an associated neurone. This neurone transmits the information to the nerves, connected to the spinal cord. The spinal cord transmits the information to the brain (the spinal reflexes are an exception), where it is analysed to find out the required response. The first place where the information is analysed is the cerebral cortex, but different parts of the brain will work to decide what to do. After that, the motor cortex will send the order to the muscle that must be moved and the cerebellum will analyse if the order is being carried out properly.

Or maybe the brain decides that the response must be executed by the autonomous nervous system, that will send the information to inner organs and glands. For instance, if the stimulus is related to an increase of our body temperature, the autonomous nervous system will order the sweat gland to release sweat to the skin.

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).