SEXUAL REPRODUCTION From the time we are teenagers through mid-life or longer, we are capable of sexually reproducing. Sex plays a major role in much of our culture -- we see it in our fashion, literature, music, television and movies. From a biological standpoint, the goal of sex is to merge two sets of genetic information, one from the father and one from the mother, to make a baby that is genetically different from either parent. A fertilized egg is shown below

DEVELOPMENT OF SEX ORGANS When we first develop, we have two sets of organs: one that can develop into the female sex organs (Mullerian duct) and one that can develop into the male sex organs (Wolffian ducts). Which sex organs develop depends on the presence of the male hormone testosterone (in humans, the default sex is female): If the embryo is a male (XY chromosomes), then testosterone will stimulate the Wolffian duct to develop male sex organs, and the Mullerian duct will degrade.

If the embryo is female (XX), then no testosterone is made If the embryo is female (XX), then no testosterone is made. The Wolffian duct will degrade, and the Mullerian duct will develop into female sex organs. The female clitoris is the remnants of the Wolffian duct. If the embryo is a male (XY), but there is a defect such that no testosterone is made, then the Wolffian duct will degrade, and the Mullerian duct will develop into non-functional female sex organs. Sex-organ development is determined by the third month of development.

Plumbing The primary goal of sex is to merge the sperm and egg (fertilization) to make a baby. In many organisms, sex occurs outside of the body. For example, in most fish or amphibians, females lay eggs somewhere (usually on the sea/river bed), the male comes along and sprays the eggs with sperm and fertilization takes place.

In reptiles and mammals (including humans), fertilization takes place inside the body of the female (internal fertilization). This technique increases the chances of successful sexual reproduction. Because we use internal fertilization, our sexual organs are specialized for this purpose. Let's take a closer look at the sexual organs in males and females.

ANATOMY OF MALE REPRODUCTIVE SYSTEM

PHYSIOLOGY (FUNCTION) OF THE MALE REPRODUCTIVE SYSTEM TESTES/TESTIS(SING.) – PRODUCE SPERM AND TESTOSTERONE (MALE SEX HORMONE)

TESTOSTERONE STARTS BEING PRODUCED AT PUBERTY TESTOSTERONE STARTS BEING PRODUCED AT PUBERTY. SOME OF THE EFFECTS ON THE INDIVIDUAL ARE: HAIR GROWTH ON BODY VOICE DEEPENS BODY GROWS: BROADENS/LENGTHENS REPRODUCTIVE SYSTEM BEGINS TO WORK STIMULATES INTEREST IN OPPOSITE SEX

SCROTUM – HOLDS TESTES AND REGULATES TEMPERATURE OF TESTES PENIS – PLACES SEMEN IN VAGINA SEMEN – FLUID CONTAINING SPERM AND SECRETIONS FROM: THE SEMINAL VESICLES PROSTATE GLAND COWPERS GLAND (MOSTLY LUBRICATION)

VAS DEFERENS (SPERM DUCT) - CARRIES SPERM FROM THE EPIDIDYMIS TO THE URETHRA EPIDIDYMIS – STORES SPERM FROM TESTES http://science.howstuffworks.com/human-reproduction3.htm

ANATOMY OF FEMALE REPRODUCTIVE SYSTEM

PHYSIOLOGY ( FUNCTION) OF FEMALE REPRODUCTIVE SYSTEM OVARY – PRODUCES OVA (EGGS) AND ESTROGEN AND PROGESTERONE ESTROGEN & PROGESTERONE – FEMALE SEX HORMONES; START BEING PRODUCED AT MENARCHE. SOME OF THE EFFECTS ON THE INDIVIDUAL ARE: HAIR GROWTH ON BODY BODY SHAPE CHANGES VOICES CHANGES REPRODUCTIVE SYSTEM: MENSTRUAL CYCLE BEGINS STIMULATES INTEREST IN OPPOSITE SEX

FALLOPIAN TUBES – CARRY THE OVA TO THE UTERUS FALLOPIAN TUBES – CARRY THE OVA TO THE UTERUS. EGGS MUST BE FERTILIZED IN TUBES TO DEVELOP UTERUS – PLACE FOR FERTILIZED OVA TO DEVELOP INTO AN EMBRYO, FETUS, AND BECOME A BABY CERVIX – CIRCULAR MUSCLE OPENING TO THE UTERUS THROUGH WHICH SPERM MUST ENTER; DILATES SO THAT BABY CAN PASS THROUGH DURING BIRTH VAGINA – STRUCTURE THAT RECEIVES THE PENIS DURING SEXUAL INTERCOURSE. ALSO CALLED THE BIRTH CANAL SINCE THE BABY PASSES THROUGH THE VAGINA DURING BIRTH URETHRA – CARRIES URINE OUT OF THE BODY http://science.howstuffworks.com/human-reproduction4.htm

MENSTRUAL CYCLE Cycles: Timing is Everything Remember that, biologically, the main goal of sexual reproduction is to have the sperm combine with the egg to make a baby. With respect to the man, age is not critical. Men are capable of producing sperm that can fertilize an egg at any time from the onset of puberty until they die (there have been many cases in which men in their 70s and 80s have conceived children with younger women). In contrast, women typically can release fertilizable eggs from the time of puberty until their late 40s or early 50s. After that time, their ovaries stop releasing eggs and they undergo various biochemical and physiological changes, the sum of which is called menopause. Although women are capable of conceiving a child from puberty to menopause, the timing of sexual intercourse is critical to the success of sexual reproduction. Women have an ovulatory cycle, or menstrual cycle, made up of complex physiological and biochemical changes that are linked to the timing of reproduction. http://science.howstuffworks.com/human-reproduction8.htm

Fertilization The long journey to fertilization must be completed within 12 to 48 hours, before the sperm die. They must first cross the barrier of the cervix, which will be thin and watery if the woman has just ovulated (for our purposes, we will assume that sexual intercourse has occurred within a couple of hours after ovulation). Once the sperm have traversed the cervical mucus, they travel up the moist lining of the uterus into the Fallopian tubes (only one of the Fallopian tubes contains an egg, so many sperm travel in the wrong direction). Fewer than 1,000 sperm out of the millions in the semen actually reach the Fallopian tubes.

Many sperm surround the egg in the Fallopian tube Many sperm surround the egg in the Fallopian tube. The head of each sperm (acrosome) releases enzymes that begin to break down the outer, jelly-like layer of the egg's membrane, trying to penetrate the egg. Once a single sperm has penetrated, the cell membrane of the egg changes its electrical characteristics (depolarizes). This electrical signal causes small sacs just beneath the membrane (cortical granules) to dump their contents into the space surrounding the egg. The contents swell, pushing the other sperm far away from the egg (cortical reaction). The other sperm die within 48 hours. The cortical reaction ensures that only one sperm fertilizes the egg.

The fertilized egg is now called a zygote The fertilized egg is now called a zygote. The depolarization caused by sperm penetration results in one last round of division in the egg's nucleus, forming a pronucleus containing only one set of genetic information. The pronucleus from the egg merges with the nucleus from the sperm. Once the two pronuclei merge, cell division begins immediately. The dividing zygote gets pushed along the Fallopian tube. By approximately four days after fertilization, the zygote has about 100 cells and is called a blastocyst. When the blastocyst reaches the uterine lining, it floats for about two days, finally implanting in the uterine wall by the sixth day after fertilization. Once implanted, the blastocyst secretes hCG, which rescues the corpus luteum and signals that a successful pregnancy has begun.

Fertilized egg, showing two pronuclei beginning to divide (left) and reaching an eight-celled stage within 72 hours (right) (Photo courtesy Georgia Reproductive Specialists) http://science.howstuffworks.com/human-reproduction10.htm