.......... REPRODUCTION II. THE PROCESS OF THE HUMAN REPRODUCTION

Introduction. Interestingly we invest a lot of energy to give rise to cells that are nor for our use but to generate other individuals, they are called the sexual or reproductive cells. Moreover, many of our life actions are related to the reproductive process, such as finding the person we want to have children with, living with this person, taking care of children, etc.

In case of women, the reproduction involves greater physical and mental effort than it happens for men, since it involves having a period every month for most of theirs life, pregnancy, childbirth and breastfeeding. In conclusion, the woman is the main character of this story. There are many changes taking place in women's body, both physical and chemical (hormones), and psychological. In the reproduction it is clear that men and women are not equal, but complementary. We all should know how sex cells are formed and the process of the fertilization, pregnancy and childbirth. Well, it all is expained in this chapter.

Activities to do. Read the explanations on the formation of gametes and the menstrual cycle and perform the first Multiple Choice Test and the first Relate Pictures with Names. Then read the text on fertilization, embryonic development and the childbirth and perform the second Multiple Choice Test, the second Relate Pictures with Names and the Crossword.

1 . The formation of sex cells. The sex cells or gametes are special cells which have half the chromosomes that have the cells of the rest of the body, which are called the somatic cells. As a result, when two different types of sex cells unite in order to form the first somatic cell of the new being, the number of chromosomes suitable for those cells is recuperated. Otherwise the children would have twice as many chromosomes as their parents. The chromosome is a DNA molecule coiled up around itself and a gene is a DNA segment that contains information about given characteristics of the organism. In human, somatic cells have 46 chromosomes and gametes have 23 chromosomes.

The pass from somatic cells to sex cells is called meiosis and consists of two successive cell divisions. There are two types of meiosis: the spermatogenesis or generation of male sexual cells, which are the spermatozoa and the oogenesis or generation of female sexual cells, which are the egg cells. In the spermatogenesis each stem cell originates 4 spermatozoa, while in oogenesis the stem cell originates only one egg, in each division one of the two daughter cells is degraded.

There are two different types of chromosomes that determine the sex of the individual, first one are called sex chromosomes, which are the X chromosome and the Y chromosome, and the remaining 44 chromosomes are called non-sex chromosomes that are common equaly for women and men. If inside the cells are two X chromosomes the individual is female and if there is an X and an Y the individual is a man. In the following drawing the gametogenesis in the rat is showed. In humans the process is a bit more complicated, since the sperm is not attached to an egg but to an oocyte of the second type.

2. The menstrual cycle. The detachment of the endometrium is called a menstruation or a period. The endometrium is the layer that cover the inside of the uterus. This process is accompanied by a loss of blood and discomfort that may be serious, it lasts 3 to 5 days and repeats every 28-32 days, for this it is called the menstrual cycle. The menstruation is controlled by two pituitary hormones, FSH and LH.

1) The FSH stimulates ovarian follicle to mature and the ovaries to produce the hormone called estrogen, which stimulates the uterine endometrium to get thicker.

2) The LH forces the already ripe follicle to release its egg (ovulation) and to transform into the corpus luteum which produces the progesterone hormone that stimulates the continuation of the thickening of the endometrium. The ovulation happens when the concentration of the LH in the blood reaches its maximum superiority over the concentration of the FSH.

3) When the progesterone hormone starts to decrease it produces the detachment of the endometrium or menstruation, which lasts from 3 to 5 days. The egg is released about two weeks after the beggining of the menstruation and lives around 24 hours, during which it can be fertilized.

3. The fertilization. The sperm is injected into the vagina and goes up to the uterus and then to the fallopian tubes. If it does not find any egg it dies after about 3 or 4 days. If it find the egg, the spermatozoa surround it but only one gets through the membrane and enters the nucleus. This, containing 23 chromosomes, joins the ovum nucleus, which also contain 23 chromosomes and a cell is formed with a core containing 46 chromosomes, called the zygote, which is the first cell of the new individual. Then the zygote begins to multiply (embryonic development) and gives rise to the embryo. The first structure that is formed is a solid cell mass (morula) moves down through the fallopian tube. A mass with a hollow interior (blastula) is formed from it. It is implanted on the uterine's mucous membrane (nesting). This happens about six days after fertilization.

4. The embryonic growth. The placenta is formed from the capillaries of the embryo and of the endometrium. This organ allows providing oxygen (O2) and nutrients from the maternal blood to the embryo and transfer carbon dioxide (CO2) and excretory products (mainly urea) produced by the embryo to maternal blood. After nine weeks, the embryo has a length of 3 to 4 cm, weighs about 2 to 4g, has almost all organs and a human form, so it is called a fetus. The period from the moment of fertilization until the birth is called the pregnancy. It usually endures about nine months.

5. The labor. It is the process of the expulsion of the fetus. It can distinguish three phases:

a) The dilation phase. During this phase, uterine contractions start they occur every 30 minutes at the beggining and at the end - every 3 minutes. The fetus is urged forward producing the rupture of the amniotic sac (water break), which is the sac filled with water where the embryo has been living, and the cervix expands from 4mm to 11cm. This phase lasts around 8 hours.

b) The expulsion phase. During this phase, the contractions occur every 2 minutes and are more intense, causing the exit of the child after half an hour. Then the child is put upside down and given a pat on the back to empty the water from the lungs and begin to breath (first cry), and the the umbilical cord is cut and tied. A few days later it dries and falls off leaving a scar called navel.

c) The phase of birth. During this phase, about 20 minutes after birth, the contractions restart and the placenta is ejected, which was previously detached, it is a process that is accompanied with small bleeding.

6. Standards of prevention of sexually transmitted diseases. The next topic discusses sexually transmitted diseases such as AIDS and hepatitis B and C. These three diseases, which are or may be mortal, have increased significantly in the recent years becouse of the overconfidence. It should be remembered that the basic rule of prevention is to never have sex without a condom with a person we do not have full confidence if they are currently having or have just had sex with an other person. In a doubt is always better to abstain or use a condom.

Crossword (Spanish activity. Activity in English is under construction)

Graphs and questions about the embryonic growth