An image from an electron microscope showing sperm fertilizing an egg, taken from www.sciencemuseum.org.uk/.../lifecycle/40.aspI. Introduction
II. Human Life Cycle
III. Male Reproductive System
IV. Female Reproductive system
- Ovarian Cycle
V. Birth Control
VI. STDs
VII. Stages of Development
- Pre-Embryonic
- Embryonic
- Fetal
VIII. Birth
IX. Aging
X. Conclusion
Introduction
I enjoyed reading these units because they deal with subjects I have been immersed in over the last 13 years. I spent my early days as a health educator in a women’s health clinic and now serve as the project coordinator of an aging program. I suppose I’ve come full circle. I guess the biggest lesson I have learned from working in a women’s clinic and through these chapters is the importance of understanding your body and how it works. So many women are completely unaware of how their reproductive systems work and are therefore woefully unequipped when making important decisions on pregnancy, contraception, and STD protection. The same is true of men. My husband knew something was “different” when he discovered his cancer but it was not until we looked it up that he knew he might have a serious problem. Over the next few paragraphs I will describe the male and female reproductive organs and their functions, discuss contraception and protection from STDs, and provide an overview of development, pregnancy, and birth.
Human Life Cycle
During puberty, between ages 11 – 13 for girls and 14 – 16 for boys, children reach sexual maturity. Sexual maturity means an individual is now capable of producing children. The reproductive systems of males and females differ significantly but have some similar functions.
Both produce gametes, in males, sperm are produced in the testes, in females the eggs are formed in the ovaries
Eggs reach the uterus via the fallopian tubes, and sperm are carried through ducts through the penis and are ejaculated. The penis delivers sperm to the vagina during sexual intercourse.
Testes and ovaries produce hormones that result in male/ female features and in females facilitate pregnancy. The uterus is the female organ that allows a fertilized egg to grow and develop.
In the human life cycle meiosis occurs in the ovaries (females) and testes (males) resulting in eggs and sperm. Sperm and eggs each contain only 23 chromosomes as compared to other cells that contain 46. Ordinary cells undergo division called mitosis resulting in another cell containing 46 chromosomes. Meiosis requires two cell divisions resulting in the haploid 23 chromosomes. During fertilization the sperm and egg meet to form the zygote which has 46 chromosomes.
Graphic of the human life cycle taken from Human biology page 320, Sylvia S. Mader
Male Reproductive System
The primary sexual organs in males are the testes which are suspended within the scrotum. Sperm are produced in the testes but mature within the epididymis. After maturation, sperm are stored in the vas deferens which consists of two long tubes leading to the ejaculatory duct which leads to the urethra. Ejaculate and urine both exit the penis through the urethra. The seminal vesicles, the prostrate gland, and bulbourethral glands all add secretions to semen which is the fluid that sperm is ejaculated in. Sperm require a particular environment to be more viable and components added by the seminal vesicles and the glands help create this environment. At birth all males have a foreskin over the penis which is usually removed by circumcision.
The penis is the male sexual organ. To facilitate erection, throughout the penis there is spongy tissue with expandable blood spaces. During sexual arousal these spaces fill with blood, causing erection. The sphincter closes off the the bladder so no urine enters the urethra during erection. Sperm enter the urethra via the vas deferens and muscle contractions trigger ejaculation. There are over 400 million sperm expelled during ejaculation.
Graphic of the male reproductive system taken from http://www.riversideonline.com/health_reference/Mens-Health/MC00023.cfm
As stated before, sperm and male sex hormones are produced in the testes. Testosterone is the main male sex hormone and is important for normal development and functioning of the male sexual organs. Testosterone also contributes to male physical characteristics like larger muscles and a deeper voice. The hypothalamus controls the functioning of the testes by secreting a hormone called gonadotropin-releasing hormone, GnRH.
Inside the testes are compartments called lobules which contain seminiferous tubules. This is where the production of sperm takes place. The process is known as spermatogenesis. Spermatogonia divide to produce spermatocytes which undergo meiosis I to become secondary spermatocytes which contain 23 chromosomes. These secondary spermatocytes will then go through meiosis II yielding 4 spermatids which become sperm. Throughout this process, sertoli cells provide support and nourishment. Spermatogenesis is a 74 day process.
A graphic from the text showing testis, sperm, and spermatogenesis. Taken from Human Biology page 323, Sylvia S. MaderFemale Reproductive System
Eggs and the sex hormones, estrogen and progesterone are produced in the ovaries, the female gonads. Oocytes within ovarian follicles mature during the ovarian cycle. Females are born with more then two million follicles but only a few, (400) mature. Usually women only produce one egg per month while they are reproductive.
Meiosis in females is known as oogenesis. An egg bursts from the ovaries during ovulation and is swept into the fallopian tubes, AKA oviducts, by finger-like projections called fimbriae. The egg then travels down the fallopian tubes to the uterus. Fertilization of the egg by a sperm usually takes place in the fallopian tubes. After fertilization, the egg implants on the endometrium of the uterus. If an egg implants in the fallopian tubes it is know as an ectopic pregnancy. The fallopian tubes can not support an embryo and do not expand like the uterus. The uterus is the organ where development of the fetus occurs. The cervix, the opening of the uterus is found at the back of the vagina. During intercourse sperm enter the uterus via the cervix and travel into the fallopian tubes in search of an egg. cervical cancer used to be the leading cause of cancer deaths in women but its incidence has been greatly reduced by the pap smear, a test for changes in cervical cells.
A graphic of the female reproductive system taken from www.northstar.k12.ak.us/.../markey/webpage.html
Ovarian Cycle
The female sex hormones estrogen and progesterone are responsible for female characteristics like breast development and body hair. These hormones also play an important role in the ovarian cycle. The average ovarian cycle lasts about 28 days. The cycle begins when lower levels of estrogen and progesterone cause the endometriun to disintegrate and pass out of the vagina, this is called menstruation. Menstruation is usually a sign that a women is not pregnant. Day one of an ovarian cycle is the first day of menstruation. Around 6-13 days later estrogen is increased by a new follicle and the endometrium begins to thicken in preparation for implantation in the event of a pregnancy. This is known as the proliferative phase. Ovulation usually occurs around day 14. During days 15-28 of the cycle, the corpus luteum increases production of progesterone causing the endometrium to thicken even further. This is the secretory phase and the body is now ready to receive a developing embryo.
A chart showing female hormone levels from Human Biology page 329, Sylvia S. Mader.During pregnancy the embryo implants on the uterine wall and menstruation does not occur. The corpus luteum will increase the amounts of progesterone because of a hormone secreted by the placenta, human chorionic gonadotropin (HCG). HCG is the hormone detected in a pregnancy test. Later on the placenta will produce its own progesterone and some estrogen.
Hormones during pregnancy, Human Biology page 330, Sylvia S. MaderBirth Control
There are many different methods available to women wanting to prevent pregnancy. The only sure way is abstinence, abstaining from sexual intercourse. However, contraceptives like birth conrol pills or Depo-provera can be about 98%-99% effective when used correctly. This methods use synthetic hormones (estrogen and/or progesterone) to inhibit ovulation. There are some side effects or risks like menstrual changes, weight gain, headaches, and blood clots that may occur with these methods. Hormonal methods do not provide protection from STDs.
The intrauterine device (IUD) is a piece of plastic, in some cases with copper or hormones, that is placed inside the uterus. It changes the environment of the uterus so that the egg does not implant on the uterine wall. Some women experience heavy cramping and bleeding with the IUD. Infections can also occur and it is recommended to be in a monogamous relationship while using the IUD.
Tubal Ligation and Vasectomy are surgical methods that bring about sterilization. In males the vas deferens is cut. Ejaculation still occurs but the semen is without sperm. In a tubal ligation the oviducts are cut, preventing the sperm and egg from reaching each other.
Barrier methods include the diaphragm and the male and female condoms. The diaphragm is used with a spermicide and is fitted by a clinician. Barrier methods, particularly the male and female condom, offer some protection against STDs but must be used correctly the whole time there is sexual contact. Even when used correctly, barrier methods are not 100% protection against STDs.
Photographs depicting different birth control methods, form Human Biology, page 331, Sylvia S. Mader
STDs
There are many different sexually transmitted diseases that people should be aware of. Some are bacterial, some are viral. Chlamydia and Gonorrhea are examples of bacteria and can be treated by antibiotics. Years ago, when I first started in women's health, Chlamydia was the most common STD. Things have either changed or we just know more. Human Papillomaviruses, AKA HPVs are the most common STDs today. HPVs may cause warts, but not always. What is really scary about HPVs is that some strains are linked to cervical cancer. There is no known cure for HPV but a new vaccine can protect women from some dangerous strains. Unless a woman has regular pap smears she may never know she has the virus. So get your pap smears ladies! By the way, condoms do not necessarily protect from HPV because they may not cover the entire area affected.
Other viruses include HIV and Hepatitis B and C. Hep A is not sexually transmitted. There is a vaccine for Hep B. Hepatitis affects the liver and is actually a more hearty virus than HIV, it just doesn't get the press, go figure.
These images show human embryos at various stages of development. Taken from hesc.stanford.edu/research/programs/human.html
Pre-Embryonic Development
The pre-embryonic stage consists of the first week of development. After the zygote has begun to divide it becomes a morula, a complex Ball of embryonic cells. The morula becomes a blastocyst. The cells of the blastocyst will arrange themselves into am inner cell mass with an outer cell layer.
A graphic of pre-embryonic development taken from Human Biology page 356, Sylvia S. Mader
Embryonic Development
The embryonic development stage begins the second week and continues through the second month. Implantation occurs in the second week. HCG is secreted by the chorion which helps maintain the corpus luteum. The inner cell mass becomes the embryonic disk. The amniotic cavity surrounds the embryo and the yolk sac is a site for blood formation.
Gastrulation is morphogenesis in which cell move to form tissue layers called primary germ layers. At the end of gastrulation the embryonic disk becomes the embryo with the ectoderm, mesoderm, and endoderm. Each of these layers is associated with different parts of the body as illustrated below.
Illustration showing the three germ layers and their significance. Taken from Human Biology page 358, Sylvia S. Mader
Development of the heart and nervous system begins in the third week. In the fourth and fifth weeks a body stalk appears, this is the future umbilical cord. Little buds appear that will be future arms and legs. During weeks six through eight the embryo begins to look human. This is an important stage, drugs and alcohol can affect a developing embryo before the mother even knows she is pregnant.
Fetal Development
By the third month it is possible to determine the gender by ultrasound. The placenta is functioning and provides nourishment for the fetus. The placenta also secretes progesterone and estrogen. Cartilage begins to be replaced by bone during the third and fourth months. The skull has six areas where the bone is not fully formed. These are called fontanels. The fontanels provide the skull flexibility during birth.
The mother will usually feel movement around the fifth month. If the child is born at this time it is possible for it to survive, albiet there can be many problems for a premature child. During weeks eight and nine the fetus grows bigger. It positions itself in a head down position in preparation for birth. If a child does not come out head first it is called a breech birth. Because of the dangers in delivering a child in this position many women will have a cesarean section.
Birth
There are three stages to parturition, the process of giving birth. During Stage one uterine contractions cause the cervical canal to thin, a process called effacement. The amniotic membrane often ruptures during this stage, an event that is often referred to as one's "water breaking." Contractions are regular, unlike Braxton Hicks contractions, but they may be relatively far between for some time. Once the cervix is fully dilated, stage one is over.
During stage two, contractions are closer together, about every 1-2 minutes. The woman will feel a desire to push at this point. The baby is delivered during stage two. Sometimes a doctor may have to make a small incision called an episiotomy to enlarge the opening of the vaginal orifice.
Stage three is the delivery of the afterbirth, the placenta and membranes. This can occur as many as fifteen minutes after the baby is born.
The different stages of birth, taken from Human Biology page 370, Sylviia S. Mader.
Aging
There are different stages of life, infancy, childhood, adolescence, and adulthood. development does not end at birth, chidren continue to grow and age. Gerontology is the study of aging. This field, is increasing in interest because it is estimated that by the year 2030 over 20% of our population will be over the age of 65. You should know that Yavapai County is of particular interest because currently 25% of our population is over age 60! You can find this information through the census website, link below.
Aging has an effect on many of our body systems. Bone density decreases and in some cases can lead to osteoporosis. Skin becomes thinner and less elastic causing wrinkles to form.
Women reach menopause somewhere between 45 and 55. Men go through andropause but some sperm continue to be produced until death. It is a myth that older adults are not sexually active. In fact, incidence in STDs in adults over 45 is increasing. This is a particular problem in assisted living communities. There is lots of infomation on this at the CDC website and the Center for Healthy Aging, link below.
I think it is important to point out that there is much wisdom and life experience to be found in older adults. It grieves me that the US is an ageist society that almost worships youth. Many other cultures revere their oldest members. Aging does not mean that we have to become weak and sick. People can stay healthy longer by staying active and eating right. The adults I work with here in town are mostly very active and some are in their nineties. I am a richer person for knowing them.
I got this image from MSN office clipart. I like it because these ladies look like they are having a good time. Good for them!
Conclusion
Reproduction is a fascinating subject. It is even more interesting given the fact that I have had two children and have experienced it firsthand. It is easy for me to get on my soapbox about the aging issue as I live and breathe it every day.
Works Cited
Human Biology, Sylvia S. Mader
Professor Frolich's Power Point
hesc.stanford.edu/research/programs/human.html (this will not go live but you can find it under the photo)
http://www.healthyagingprograms.org/
http://quickfacts.census.gov/qfd/states/04/04025.html
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