The hypothalamus works like a computer with a program in place which from before birth causes a steady release of hormones which tell the body to grow. There are two ways in which the hypothalamus gets re-programmed. One is that the brain changes the program based on the input it receives through the senses from outside the body. The other way the hypothalamus is re-programmed is by feedback from other parts of the body: packages of hormones (messages) unloaded at the hypothalamus from the train tell the hypothalamus that there is enough of a specific hormone at a specific organ.
The hypothalamus begins functioning before birth and continues until death. The program changes as the body grows. Until about age 7 to 14, the hypothalamus steadily releases hormones which cause gradual growth and also cause a slow and gradual development of sexual organs and glands. Sometime between 7 and 14, the sexual glands have absorbed enough hormones to begin producing hormones. This is the onset of puberty, a time when sexual and reproductive development are speeded up.
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Puberty continues until the child’s body grows into an adult body capable of making babies.
Beginning with puberty, the production of sexual/reproductive hormones is increased. At the end of puberty, there is a peak in the quantity of hormones being produced and a very gradual decrease in production of these hormones begins, eventually causing the body to lose reproductive capacity (menopause in women, senescence in males). However, these hormones continue to be produced throughout life.
Let’s look now at the way this all works. The hypothalamus loads the train with packages of FnRH (Gonad stimulating hormone). The pituitary receives these packages and in response releases FSH & LH (follicle stimulating hormone-FSH and luteinizing hormone-LH).
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In the girl what’s in the package now attached to the ovary is telling it to let a follicle grow so that the egg cell inside the follicle will mature. While the egg is ripening, the follicle loads onto the train a new hormone it has been making called estrogen.
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Some estrogen causes the development of secondary sex characteristics. These include breasts, pubic hair, widening of the hips, etc.
Some estrogen is unloaded at the uterus telling it to start building a new lining—put down a new, thick carpet.
More and more estrogen goes to the hypothalamus as the egg ripens. When the egg is mature, the hypothalamus tells the Pituitary to send a big shipment of LH (luteinizing hormone) to the ovary.
Estrogen is unloaded at several stations:
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This surge of LH will cause the egg to burst out of the follicle (ovulation) and will tell the empty follicle to change its job and start making yet another hormone, progesterone.
Some of the progesterone is unloaded at the uterus, telling it to start making food for a possible fertilization (Company’s coming, stock up the larder). As long as the hypothalamus reads the high level of progesterone, it does not order more GnRH.
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The corpus luteum, however, only survives 14 days. If no pregnancy happens, it begins to deteriorate and the deteriorating corpus luteum puts out less and less progesterone, which is read by the hypothalamus as a message to start another round (i.e. send out GnRH again) and is read by the uterus as a message that there will be na company this month so throw out the provisions (menstruation).
If no pregnancy happens:
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If pregnancy happens, it produces its own hormone (HCG) which acts like LH—keeping the corpus luteum alive to produce progesterone until the developing placenta takes over.
In the girl, the two stimulating hormones act sequentially (that is one after the other)at the same station on the same follicle. The boy’s testicle has two stations, each of which unloads a different hormone and performs a different function in response to it: one station (the Sertoli cells) will make sperm, the other (the Leydig cells) will make testosterone hormone. When the stimulating hormones reach the testicles, FSH causes an increase in the number and size of seminiferous tubules which house the Sertoli cells—the sperm-making compartments. As the Sertoli cells grow, they make sperm. In the neighboring compartment, LH is directing the Leydig cells to make testosterone.
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Testosterone is used by the Sertoli cells in sperm production. Testosterone causes the body to also change visibly (secondary sex characteristics—body growth, penis growth, facial hair, etc.). The hypothalamus is signaled that the sperm-making factory is operating correctly. The hypothalamus reads the testosterone and, in response, maintains the right amount of stimulating hormone output to keep everything in balance.
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The hormones in boys are relatively constant, no stopping and starting every month. There is, however, a cycle of hormone levels which lasts a couple of months in males as well as a daily cycle—more hormones are made during the night than during the day. Boys are always fertile. They have no cycle in which they are more or less fertile.