17. Reproduction
Contents of the chapter
17.1 Reproduction
Reproduction is a condition for the survival of living organisms. Animals demonstrate breeding behaviors that are specific to their species. Mammals often have specific breeding seasons.
Females of different mammal species have different estrous cycles. In some species, females go into heat only once a year, whereas in other species they can go into heat several times a year. Humans have menstrual cycles instead of estrous cycles.
The success of reproduction depends on a number of factors. The most important of these are the living conditions and the choice of partner, the latter of which is influenced by biological, mental and social factors.
17.2 Sex
The biology of reproduction is a story that is 4,5 billion years long. The origin of sexual reproduction can be traced to early prokaryotes, which first developed around two billion years ago. Animals that reproduce sexually have two biological genders, called sexes: female and male.
In general, an individual's biological sex can be distinguished based on its genitals, shape, size, appearance or behavior. In some animal species, gender is determined by the influence of the environment. For example, flea eggs develop into females in good conditions, whereas some of the eggs develop into males due to malnutrition or cold temperatures.
In many reptile species (e.g. turtles, lizards, alligators, and crocodiles), sex is determined by the temperature of the eggs during hatching. For these species, the temperature of the environment has the same function as the sex hormones in the XY system, where the gender of an individual is determined by its sex chromosomes.
Oysters are hermaphrodites, but their male and female genitals do not develop simultaneously. Usually, young oyster individuals are males. In old age, they become females.
In mammals and birds, sex is an inherited characteristic. This means that an individual's sex is determined by the chromosomes it receives from their parents.
17.3 The formation of gametes
Germ cells are cells that create reproductive cells called gametes. Germ cells are located only in the gonads. They are called oogonia in females and spermatogonia in males. In females, germ cells are found in the ovaries, whereas male germ cells are found in the testes. During oogenesis, germ cells divide to produce ova, or eggs, in females.
Germ cells are the only cells that can undergo meiosis as well as mitosis.
When germ cells divide by mitosis, they produce genetically identical cells. The daughter cells created during this process have a full set of chromosomes (diploid). Diploid cells have 23 pairs of chromosomes.
When germ cells divide by meiosis, the daughter cells have half a set of chromosomes (haploid gametes). The cells formed in meiosis are all genetically different. This is because the chromosomes - and genes - separate and are reshuffled during the process.
In males, a single diploid cell yields four haploid sperm cells through meiosis. In females, diploid cells divide unevenly. Most of the cytoplasm is segregated into one daughter cell, which becomes the egg cell or ovum, while the smaller polar bodies only get a small amount of cytoplasm.
The figure on the right displays the formation of gametes.
2n is the chromosome number of the fertilized ovum and normal cells (46 in humans). The letter n describes one set of chromosomes, which in humans is 23 chromosomes.
The parent cell divides into two cells in the ovary or the testicle. At the same time, the number of chromosomes is halved (2n → n, a cell with 46 chromosomes is divided into two cells with 23 chromosomes).
The cells are further divided into two cells. The result is four gametes.
Ordinary human cells have 46 chromosomes, two of which are sex chromosomes. Chromosomal sex is determined at the time of fertilization; a chromosome from the sperm cell, either X or Y, fuses with the X chromosome from the egg cell.
There are two types of sperm. Some sperm have the sex chromosome X, whereas others have the sex chromosome Y. If a sperm cell with the sex chromosome X fertilizes the egg, the result will be a female. If a sperm cell with the sex chromosome Y fertilizes the egg, the result will be a male. Therefore, the chromosomes of a female are 44 + XX, whereas the chromosomes of a male are 44 + XY.
17.4 The male reproductive system and gametes
The male genitals are the testicles (testes), the duct system (made up of the epididymis and the deferent duct), the accessory glands (seminal vesicles and prostate gland) and the penis.
The deferent duct or vas deferens is continuous with the epididymis, which connects it to the prostatic urethra. All boys are born with a foreskin, a fold of skin at the end of the penis that covers the glans (head).
The penis, which usually hangs limp, becomes hard when a male is sexually excited. When this happens, the tissues inside the penis fill with blood, and the penis becomes stiff and erect. This is called an erection. When the erect penis is stimulated, muscles around the reproductive organs contract and force the semen through the duct system and urethra. Semen is pushed out of the male's body through his urethra — this process is called ejaculation. Each ejaculation can contain 2-6 ml of sperm. 1 ml of sperm contains 100 million male gametes.
Sperm cells are produced in the testes (singular testis) and stored in the epididymis. Compared to the abdominal cavity, the temperature required for sperm to form is a few degrees lower than the individual's body temperature.
Sperm cells are 0.06 millimeters long. In the main part of the cell, there is a nucleus (with a so-called haploid or simple (n) genome). The mitochondria of the middle part release energy for movement. The tail of the sperm cell helps it to move.
17.5 Female reproductive system and gametes
The female genitals comprise of the vulva, labia, vagina, clitoris, uterus, fallopian tubes and ovaries. In addition to the vaginal opening, the labia also protect the urethra. The clitoris is a small sensory organ, located toward the front of the vulva above the vagina.
The two ovaries are located in the lower abdomen. The ovaries produce hormones. During the menstrual cycle, the ovaries also produce ova or egg cells.
The fallopian tubes begin near the ovaries, and thin tubes lead to the uterus. The pear-shaped uterus has a thick muscular wall. The elastic muscles of the vagina can stretch during intercourse and childbirth.
At birth, a female has millions of oocytes in their ovaries. At the middle of each menstrual cycle, follicular cells leave the ovary together with a single ovum. This is called ovulation. The ovum, or egg cell, is about 0.1 millimeters in diameter. It is one of the largest cells in the human body, and it can just about be distinguished with the naked eye.
17.6 The menstrual cycle
A female human has all the oocytes they will ever have at birth. During the 30-35-year maturation period, about 400 of these oocytes will have time to mature. Normally, each ovary releases an ovum in alternating months. Oocytes do not mature or get released during pregnancy. Diseases, the use of birth control pills together with major mental stress can affect ovulation.
The first menstrual periods come at an average age of 11 to 13. A period happens because of the changes in the body's hormonal balance. At this point, the girl has grown enough mass for the period to start, averaging 47 kilograms. Generally, menstruation is quite irregular in its early stages.
The menstrual cycle lasts about 28 days and is calculated to begin on the first day of the menstrual period.
An average menstrual cycle progresses as follows:
- Days 1 to 4: Menstrual bleeding begins. This happens when the lining of the uterus, the endometrium, is shed.
- Day 5: A new ovum begins to mature in the ovarian follicle. This is affected by the hormone secreted by the pituitary gland.
- Days 5 to 14: The follicle grows and produces estrogen, which causes important mucosal regeneration.
- Day 14: Ovulation occurs. An ovum detaches from the erupted follicle and travels to the fallopian tube. The onset of the follicle is caused by another hormone (LH) secreted by the pituitary gland.
- Days 15 to 20: A corpus luteum is formed at the site of a follicle, or sac, that has matured and released its ovum. It is made up of lutein cells, which secrete progesterone. This thickens the uterus.
- Days 21 to 28: If fertilization has not occurred, the corpus luteum becomes inactive. Decreased progesterone production triggers mucosal degeneration.
A female human stops menstruating between the ages of 45 and 55. At first, the menstrual cycle becomes irregular, and gradually the menstruation stops completely. The name menopause comes from a period of change that lasts for a couple of years.
