Section 2 - Understanding Kallmann's SyndromeNext Question

6.What happens during normal puberty ?

The physical features of males and females can be divided into two main groups: the primary sexual characteristics and the already briefly mentioned secondary sexual characteristics. The first group is directly involved in sexual reproduction, whilst the second group is not, indicating that puberty has occurred due to the action of gonadal hormones. The primary and secondary sexual characteristics are listed in Table 1.

Table 1 - the primary and secondary sexual characteristics

Puberty is not something which happens overnight; it often takes three or four years to complete. It is the transition period during which boys acquire the physical characteristics of adult men and girls those of adult women. Research has shown that generally, girls reach puberty by the age of 15, a year or two earlier than boys.

Take a look at Figure 5. Normally, special cells in the hypothalamus secrete a hormone called GnRH, the "Gonadotrophin Releasing Hormone". GnRH is then delivered to the pituitary gland via capillaries in the pituitary stalk. As its name suggests, GnRH releases other hormones (the gonadotrophins) from the pituitary gland. The suffix -trophin means "the nourishment of" and so a literal translation of "gonadotrophin" is "the nourishment of the gonads". There are two main gonadotrophins: LH and FSH, the "Luteinizing Hormone" and "Follicle Stimulating Hormone", respectively. In some countries, GnRH is also known as LHRH, the "Luteinizing Hormone Releasing Hormone", although this can be a little misleading because it is not only LH, but also FSH which is released by GnRH / LHRH.

Function of the gonadotrophins in men during and after puberty

LH and FSH target different cells with different functions in the testes. In simple terms, LH encourages masculinity and FSH promotes fertility.

In the normal male, LH targets special cells called Leydig or interstitial cells and it is here that almost 95% of the main male sex hormone testosterone is made. The remaining 5% of testosterone is manufactured by the adrenal glands above the kidneys (answer to Question 3). Testosterone is responsible for the development of the male body's secondary sexual characteristics during puberty (Table 1). FSH, with a little help from some testosterone, stimulates the production of sperm cells, a process known as spermatogenesis. Sperm cell production can take up to 76 days to be completed from beginning to end.

Function of the gonadotrophins in women during and after puberty

A healthy woman also has levels of the gonadotrophins LH and FSH in her blood but her body responds rather differently to these hormones than a man's. Whereas a man has relatively constant levels of LH and FSH, a woman has a regular monthly cycle during which her gonadotrophin levels vary from day-to-day. This is called the menstrual cycle and this is illustrated in Figure 6.

In a woman, LH and FSH are both released into the bloodstream in response to the GnRH delivered by the hypothalamus to the pituitary gland. Together with LH and FSH, two other hormones regulate the menstrual cycle. These hormones are oestrogen and progesterone. In addition, oestrogen encourages the development of the female secondary sexual characteristics (Table 1) and most of it is made by the ovaries, but a little oestrogen is also made by the adrenal glands. Progesterone is responsible for thickening the active layer of the uterus or womb, preparing it for possible pregnancy. The main sources of progesterone are the ovaries, but during pregnancy, the production of progesterone is taken care of by the placenta, an organ attached to the uterus which holds and nourishes the growing embryo. LH and FSH are indirectly responsible for the presence of oestrogen and progesterone in the blood.

For the first few days of the menstrual cycle, a certain amount of bleeding occurs. This is known as menstruation. During menstruation (1), progesterone levels are low. Since progesterone helps to maintain the active layer (endometrium) of the uterus, low levels result in the shedding of much of the endometrium, taking tiny blood vessels which have grown over the previous few weeks with it. At this point, the blood contains medium levels of LH and FSH and only a little oestrogen.

Over the next week or so, oestrogen, LH and FSH begin to increase until about halfway through the menstrual cycle, when ovulation (2) occurs. It is during ovulation that an egg is released from the ovary and slowly travels to the uterus. Only one sperm cell is needed to fertilise the released egg cell.

Just before ovulation, LH, FSH and oestrogen levels are high but progesterone is still low. Soon after ovulation, hormone levels decrease rapidly except for progesterone, which gradually reaches its peak level on around the 21st day of the menstrual cycle (3) and the endometrium is therefore at its thickest.

If the released egg is not fertilised during this time, the blood begins to "lose" progesterone. When progesterone is at its lowest level (4), the thick endometrium can no longer be supported. It begins to degenerate, menstruation occurs and the menstrual cycle starts again (1).