In my last post on What is sex for?, I explained the purpose of sex in human and also the need to reproduce either sexually or asexually. I also discussed about the human reproductive system. Today, I’ll continue from where I stopped, starting by shedding more light on sexual arousal in males, viagra and the reason men fail to have erections, and the female reproductive system.Pixabay
SEXUAL AROUSAL IN MALES
Men become sexually aroused by thinking about sex, as a result of physical stimulation or a combination of both. Nerve impulses from the brain pass down parasympathetic nerves and cause arterioles leading to the penis to dilate. The penis receives more blood than can drain away, spongy erectile tissue in the shaft becomes filled with blood, and an erection results.
Flaccid (non-erect) penises vary greatly in size, largely depending on how much blood is retained in the spongy tissue. When erect, about 90 per cent are between 14 and 16 cm long. The end of the penis, the glans, is particularly sensitive, and continued stimulation from rhythmic thrusting eventually leads to a series of reflexes known as ejaculation. Stored spermatozoa are propelled along the vas deferens by powerful peristaltic waves. As they pass various accessory glands, different secretions are added to the sperm and the final ejaculate is a milky fluid called semen.
Human males ejaculate, on average, about 5 cm3 of semen, which contains between 50 and 200 million sperm. Most sperm never get anywhere near the egg, even after unprotected sex. For most of the monthly cycle, the cervix is blocked by a plug of mucus that sperm cannot penetrate. Only at around ovulation time does the mucus consistency change, allowing sperm to pass through easily.
Viagra: A drug discovered by accident
Viagra is a very popular drug that treats impotence in men. It stimulates an erection in men when they want to have sex. Men fail to have erections because they don’t produce enough of a molecule called cyclic GMP (cGMP). Normally, sexual stimulation causes large amounts of cGMP to be released into the blood. This relaxes the smooth muscle in the arterioles at the base of the penis, allowing blood to flow in to cause an erection.
Viagra mimics the shape of the cGMP molecule, which is the natural substrate of the enzyme phosphodiesterase. When viagra binds to phosphodiesterase, the enzyme is unable to break down cGMP, allowing it to accumulate in the blood. This has the desired effect and an erection is achieved.
Viagra is a drug that has an interesting background. In the mid-1980s, researchers were looking for a drug to treat high blood pressure and angina. They tried to find a molecule that would inhibit the enzyme phosphodiesterase, which would lead to an increase in the amount of available cGMP, resulting in vasodilation and so relieving the symptoms of angina.
Many potential small molecules were screened and the most promising was put into a Phase I clinical trial. No serious side-effects were found in healthy volunteers and a Phase II trial in patients with severe angina was started. Unfortunately, the drug failed to show much effect. However, at the same time further Phase I trials were carried out using higher doses of Viagra. This time there were side-effects the male volunteers in the trial began having frequent erections.
The drug was then trialled for its potential to treat male impotence. In the first trial, in 1994, ten out of the 12 patients reported an improvement in erectile function. Further trials were carried out on a larger sample, and again about 90 per cent of the patients reported an improvement. The drug was finally licensed in 1998. Viagra, sidenafil citrate, is now one of the fastest selling drugs in the world.
THE FEMALE REPRODUCTIVE SYSTEM
The structure of the female reproductive system includes a pair of ovaries, which are the primary female sex organs. These produce egg cells, or ova, and also secrete the hormones oestrogen and progesterone. Each ovary is about 3 to 4 cm across and is attached to the inside of the pelvis by a ligament (a tough band of connective tissue). The oviducts, or Fallopian tubes, connect the ovaries to the uterus (womb). Each tube ends in finger-like fimbriae, which move close to the ovary at the time of ovulation.
The uterus is a compact muscular organ that nourishes, protects and ultimately expels the fetus. The human uterus is able to expand from about the size of a small orange with a capacity of about 10 cm3 to accommodate a full-term baby. This is a 500-fold increase in the capacity of the uterus. The bulk of the uterine wall is made from smooth muscle and is known as the myometrium. The lining of the uterus, the endometrium, consists of two layers. The underlying layer is a permanent basement membrane which produces the surface layer, the decidua. This layer is built up every month and shed during menstruation.
The cervix, or neck of the uterus, is a narrow muscular channel that is usually blocked by a plug of mucus. During sexual arousal the muscles of the vagina (a muscular tube that leads to the outside of the body) relax and glands in the vagina secrete lubricating mucus. This allows the male’s penis to enter without discomfort. During childbirth, the cervix dilates to around 10 cm in diameter to allow the baby to pass through.
Throughout this section I refer to the female gamete as an egg cell. The egg cell is surrounded by several layers of cells and the complete unit is called a follicle. The production of egg cells, oogenesis, takes place within the ovaries of the developing female fetus. At birth, a girl already has about 2 million primary oocytes. Most of these degenerate during childhood and by puberty there are only about 200,000 left. Of these, only about 450 ever mature fully – one per month throughout the female’s reproductive life. As in spermatogenesis, the process of oogenesis is divided into three phases:
- Multiplication: As the female embryo grows, primordial germ cells in the epithelium (outer layer) of the ovary go through a series of mitotic divisions to produce a population of larger cells called oogonia.
- Growth: Oogonia move towards the middle of the ovary where they grow and go through further mitotic divisions to become primary oocytes. Each oocyte is surrounded by a layer of follicle cells. Together they form a primary follicle.
- Maturation: From puberty onwards, a few primary follicles mature each month. Usually, only one completes its development, the rest degenerate. The remaining primary follicle grows larger, becoming an ovarian follicle. Its cells secrete follicular fluid, producing droplets which join together to form a fluid that fills the space known as the antrum. The mature follicle, the Graafian follicle, is almost 1 cm in diameter. It protrudes from the wall of the ovary just before ovulation.
Oogenesis in Eukaryotic Cells.(A) oogonium where the mitotic division occurs (B) differentiation and meiosis I begins (C) primary oocyte (D) meiosis I is completed and meiosis II begins (E) secondary oocyte (F) first polar body (G) ovulation must occur and the presence of the sperm penetration (fertilization) induces meiosis II to completion (H) ovum (I) second polar body. Leiladavids - Own work, CC BY-SA 4.0
Inside the developing follicle, the oocyte begins its first meiotic division. There is no need for more than one egg cell, so the second set of chromosomes formed at meiosis I is discarded, passing into a small cell (with very little cytoplasm) known as the first polar body. This appears to have no function, but it often completes the meiotic division, producing two similar cells; both later break down. After meiosis I, the egg cell is known as a secondary oocyte. It then begins the second meiotic division but gets no further than metaphase. The division is completed only if the egg cell is penetrated by a sperm.
When penetration of the egg cell occurs, meiosis II is completed and the egg cell becomes the mature ovum. This produces another ‘spare’ set of chromosomes, the second polar body, a cell that also degenerates.
On around day 14 of the human female’s menstrual cycle, an egg cell (a secondary oocyte) is released from the ovary in a process called ovulation. Pressure in the antrum builds up and ruptures the Graafian follicle, forcing the egg cell out. The egg cell is released into the body cavity, but very few get lost since the fimbriae of the Fallopian tubes hover close to the ovaries, and the ciliated lining of the tubes creates a current that gently sucks in the egg cells.
Remarkably, a woman can become pregnant even if she has lost an ovary on one side and a Fallopian tube on the other. This suggests that the functioning fimbriae actively seek out the productive ovary by moving right across to the other side of the woman’s body.
The twentieth century saw a revolution in ‘family planning’. People can regulate the number and spacing of their children by using one or more of the wide range of effective methods of contraception now available such as the pill, IUD (intrauterine device), morning after pill, condom, diaphragm, spermicidal gel, coitus interruptus, the rhythm method, vaginal douche, male sterilization (vasectomy), and female sterilization (tubal ligation).
The pill – or, to use its full name, the combined oral contraceptive pill – has been used widely since the 1960s. It contains a mixture of artificially produced oestrogen and progesterone. The pill is effective because it raises the blood hormone levels to those encountered during the secretion phase of the menstrual cycle, or those of early pregnancy. The high level of progesterone inhibits the secretion of FSH, preventing the development of new follicles and therefore egg cells. The high progesterone levels also inhibit menstruation, so the pill is usually taken for 21 days and then not taken for 7 days. This allows menstruation to take place but does not leave enough time for any follicles to develop. Strictly speaking, this is not normal menstruation but a ‘withdrawal bleed’ caused by the drop in progesterone.
This variation of the pill contains low doses of hormones, not enough to prevent ovulation, but enough to change the consistency of the mucus at the cervix. Instead of becoming permeable to sperm at ovulation, the mini-pill causes continued secretion of the thick, non-ovulatory mucus, which prevents sperm getting through.
The morning after pill
An emergency contraceptive that can be taken within 72 hours of unprotected sex to minimize the risk of pregnancy. It works best when taken within 24 hours. The pill contains the female hormone called levonorgestrel – which is an ingredient of ordinary contraceptive pills. This prevents the ovaries from releasing an egg and it also alters the lining of the womb, so a fertilized egg cannot implant. This is a drastic method of contraception and it causes bad nausea and vomiting – it is a last resort rather than a regular method.
THE HUMAN FEMALE MENSTRUAL CYCLE
The average length of the human menstrual cycle is 28 days, but variations from 24 to 35 days are normal, and greater variations are not uncommon. The cycle is divided into four phases:
- The proliferative phase; the endometrium regenerates.
- The ovulation phase; the ovum (egg cell) is released.
- The secretory phase; the endometrium secretes nutrients in preparation for implantation.
- The menstrual phase; the endometrium is shed.
By convention, the first day of the period (the most obvious event) is called day 1 of the menstrual cycle. In the timing of the phases, it is important to relate the events of the menstrual cycle to the hormonal changes.
THE PROLIFERATIVE PHASE
On day 2 of the cycle, the pituitary gland releases follicle stimulating hormone (FSH). This stimulates the development of several ovarian follicles. At around day 6, one of the follicles dominates and begins to secrete oestrogen. The others degenerate. The remaining follicle develops into a Graafian follicle and continues to secrete oestrogen until day 14 of the cycle. Consequently, blood oestrogen levels rise. Oestrogen causes proliferation (growth) of the endometrium to replace the layer lost during the previous menstruation. After 14 days, the repair is complete.
THE OVULATION PHASE
On day 12 to 13, the blood oestrogen level reaches a threshold level, which triggers the release of luteinising hormone (LH), from the anterior pituitary gland. The rapid increase in LH levels triggers ovulation around day 14. The ovum lives for only 24 to 36 hours. During this time, it moves only a few centimetres from the ovary.
THE SECRETORY PHASE
Luteinising hormone has a seond effect: it causes the Graafian follicle to develop into a corpus luteum (‘yellow body’). The name comes from the yellow appearance of the secretory cells that develop inside the ‘remains’ of the Graafian follicle. The corpus luteum secretes oestrogen and progesterone. Progesterone causes spiral-shaped blood vessels to grow into the endometrium. This thickened lining begins to secrete nutrients and mucus to prepare for an embryo to be implanted. During this phase, the high levels of progesterone inhibit the production of FSH. As long as progesterone levels are high, the endometrium is maintained and no new follicles are stimulated. The ‘contraceptive pill’ takes advantage of this inhibition. If the egg cell is not fertilised, the corpus luteum lasts for about 10 to 12 days and then degenerates, ceasing to secrete progesterone.This is a key event because the inhibition of FSH is lifted. The endometrium is no longer protected and the cycle can start again.
THE MENSTRUAL PHASE
The drop in progesterone and oestrogen levels causes the uterine capillaries to rupture, and the endometrium is lost from the body through the cervix, together with some blood. Renewed secretion of FSH begins around day 2, and the cycle begins again.
I will be pausing here for now. In the next post, I will discuss on the events of pregnancy, the placenta, menopause and the hormone replacement therapy.
Thanks for coming.