Compendium Unit 4 Part I

Reproductive System

I.   Human Life Cycle

II.  Male Reproductive System

III. Female Reproductive System

IV. Female Hormone Levels

V.  Control of Reproduction

VI. Sexually Transmitted Diseases

Fetal Development 

I.   Fertilization

II.  Pre-Embryonic and Embryonic Development

III. Fetal Developement

IV. Pregnancy and Birth

V. Development After Birth

Reproductive System

I.   Human Life Cycle

The reproductive system in males and females is fully functional once puberty is complete. This occurs between 14-16 in boys and 11-13 in girls. At this time the individual is able to produce procreate.

In males the reproductive organs or genitals produce sperm in the testes and in females eggs are produced in the ovaries. The sperm travels in ducts which exit the penis. In females the eggs in the uterine tube are transported to the uterus. The penis delivers the sperm to the female vagina during intercourse. In the uterus the fertilized egg develops within the females body.

 The sex hormones produced by the testes and the ovaries affect the features of masculinization and feminization, and other function such as the continuation of pregnancy.

Cell division occurs in the human life cycle as mitosis and meiosis. Mitosis is duplication division meaning the cells carry the same amount of 46 chromosomes when duplicated. Meiosis is reduction division. The cells divide and have half the chromosomes which means 23. Meiosis takes place in the testes to produce sperm and  in ovaries to produce eggs. As the sperm unites with the egg the 23 pairs of chromosomes from the sperm and egg form a zygote that has 46 chromosomes in total.    

II.  Male Reproductive System

 The male reproductive system consists of a pair of testes which are suspended within the scrotum and a network of excretory ducts (epididymis, ductus deferens (vas deferens), and ejaculatory ducts), seminal vesicles, the prostate, the bulbourethral glands, and the penis. Sperm cells pass through the ducts as they leave the testes and move through the urethra.

The epididymis is a tightly coiled tube where the sperm matures and the ductus deferens or vas deferens is where the sperm is stored. The vas deferens curves and empties into the ejaculatory ducts leading to the urethra. The urethra then extends from the urinary bladder to the urethral orifice at the tip of the penis. From this passageway is where sperm and fluids for reproduction and urine from the bladder moves out of the body.

Upon ejaculation sperm leaves the penis in a fluid called semen. Secretions are added from the seminal vesciles, the prostate gland, and the bolbourethral glands.  The seminal fluid contains the sugar fructose that is an energy source for sperm.

The male sexual organ the penis is  the organ of sexual intercourse. The penis includes the shaft, the glans and the foreskin. In a circumcision the foreskin is removed. The male deposits semen into the vagina during intercourse. The penis contains erectile tissue that engorges with blood during sexual arousal resulting in an erection. As sexual stimulation intensifies rhythmic muscle contractions cause the penis to ejaculate. The expulsion of seminal fluid are part of male orgasm. The sensation is centered in the brain but the physical reactions involve the genitals and entire body. 

There can be over 400 million sperm cells in semen that is ejaculated.

The testes which produce sperm and sex hormones lie outside the body within the scrotum. The scrotum regulates the temperature.

Spermatogenesis takes place in the seminiferous tubules. During production spermatogonia divide to produce primary spermatocytes that undergo meiosis I to produce secondary spermatocytes carrying 23 chromosomes. They then undergo meiosis II to produce four spermatids each  with 23 chromosomes. The spermatids then become sperm.

Each sperm cell has three parts: a head, middle piece, and tail. An acrosome at the head produces enzymes which helps penetrate the female ovum. Mitochondria in the middle provides energy for movement of the tail. Ejaculated semen contain several hundred million sperm of which only one needs to enter the egg of a female.  Andogens or male sex hormones are called interstitial cells because they are secreted by cells between the seminiferous tubules.

III. Female Reproductive System

The organs of the female reproductive system produce and sustain the female sex cells called ova, transport these cells to a site where they may be fertilized by sperm, provide a favorable environment for the developing fetus, move the fetus to the outside at the end of the development period, and produce the female sex hormones. The female reproductive system includes the ovaries, fallopian tubes, uterus, vagina, and external genitals. Ovaries are a pair of sex organs that reside in the pelvis on each side of the uterus. Ovaries produce ova (egg cells) and the sex hormones estrogen and progesterone. The genital tract includes the oviducts, uterus, and fallopian tubes. The oviduct also called fallopian tubes include two tubes which extend from the uterus to the ovaries. At the end of the ovaries is a funnel shaped finger like projection called fimbriae. When an egg bursts from an ovary during ovulation, it  moves into an oviduct by the action of the fimbriae and the beating of cilia that lines the oviducts. Once in the oviducts, the egg is moved along by ciliary movement and the peristaltic action of the muscle in the tubes toward the uterus.  An egg lives from 6-24 hours. Thus fertilization or zygote formation takes place in the tubes. A developing embryo arrives at the uterus after several days. Implantation occurs when the embryo embeds in the uterine lining.

The uterus is a muscular organ about the size and shape of an inverted pear. It receives the fertilized egg and provides an appropriate environment for the developing fetus. After childbirth, the uterus is usually larger, then gets smaller after menopause. Development of the embryo or fetus normally takes place in the uterus, The womb accomodates for the growing fetus and is able to stretch to the width of 5cm. The lining of the uterus is called the endometrium which is involved in the formation of the placenta. The opening in the cervix leads to the vagina a fibromuscular tube that extends to the outside. The vagina serves as the passageway for menstrual flow, intercourse, and the birth canal during childbirth.

The external genitals are referred to as the vulva. It includes the labia majora, mons pubis, labia minora, and clitoris. The labia minora are two folds that lye inside the labia majora and extend the the vaginal opening to form the clitoris. The clitoris is the organ of sexual arousal in females.

In females, orgasm culminates in uterine and oviduct contractions, however a woman can become pregnant without having an orgasm.

IV. Female Hormone Levels

The hormones estrogen, and progesterone have important roles in the function of the reproductive system. Hormone levels cycle in the female on a monthly basis.

The non pregnant ovarian cycle includes six stages. As the follicle matures, layers of follicle cells surround a secondary oocyte. The mature follicle then ruptures in a process known as ovulation and a secondary oocyte is released. The follicle then becomes the corpus luteum, and eventually disintergrates. During oogenesis, the chromosome number is reduced to 23. The ovarian cycle is controlled by homones. During the first half of the cycle, FSH from the pituitary causes maturation of a follicle that secretes estrogen and progesterone, and after ovulation LH  converts the follicle into corpus luteum.

The non pregnant uterine cycle takes place simultaneously with the ovarian cycle.  This cycle begins with the follicular phase when FSH is released promoting the maturation of a follicle in the ovary. The ovarian follicle produces increasing levels of estrogen causing the endometrium to rebuild. Ovulation occurs on the 14th day of a 28 day cycle. After ovulation LH promotes the development of the corpus luteum. As a result progesterone is produced by the corpus luteum causing the endometrium to thicken and become secretory and a low level of hormones causes the endometrium to break down as mentruation begins. If pregnany were to occur the corpus luteum does not regress but is maintained and secretes increasing amounts of progesterone, thus menstruation does not occur and the uterine lining is maintained.

      

V.  Control of Reproduction

There are various birth control methods and devices which include birth control pills, diaphragm, and condom. They vary in effectiveness, so the most reliable form is abstinence. Surgical methods include vasectomy in males and tubal litigation in females.

Infertility is the inability of couples to achieve pregnancy. Assisted reproductive technologies include artificial insemination, in vitro fertilization, gamete intrafallopian transfer, surrogate mothers, and intracytoplasmic sperm injection.

VI. Sexually Transmitted Diseases

Sexually transmitted diseases are caused by viruses, bacteria, protists, fungi, and animals. As an example HIV is caused by the human immunodeficiency virus and genital warts by the human papillomavirus, genital herpes by the herpes simplex virus type 2, and hepatitis by the hepatitis virus. Bacteria causes include chlamydia, gonnorrhea, and syphilis. In addition two other bacterial infections include bacterial vaginosis caused by gardnerella vaginalis and trichomoniasis caused by candida albicans.  

Fetal Development

I.   Fertilization

The first cell of an individual forms upon the union of a sperm and a egg to form a zygote, this is known as fertilization. The zygote contains all the genetic info necessary, including half from the mother and half from the father. As the sperm swims toward and implants in the egg of the female, the head of the sperm containing a nucleus fuses with the egg nucleus. As a result the zygote is formed by the cytoplasm and organelles of only the mother. During fertilization a single sperm first drawn into the egg by microvilli of its plasma membrane. The enzymes from the acrosome help the sperm makes it's way through the zona pellucida. The zona pellucida is surrounded by layers of follicular cells called corona radiata. These cells thus nourish the egg when it is in the follicle of the ovary. 

During fertilization, several sperm penetrate the corona radiata,, and several attempt to penetrate the zona apellucida, but only one can enter the egg. After the head of the sperm fuses to the zona pellucida, it releases an enzyme that creates a pathway for the sperm to enter the egg. Polyspermy is the entrance of more than one sperm. This is prevented by the depolarization of the egg's plasma membrane. A s the sperm touches an egg's plasma membrane, it depolarizes from 265 mV to 10mV preventing the binding of other sperm cells. further, the release of enzymes by vescicles called cortical granules cause the zona pellucida to become ad inpenetrable membrane.

II.  Pre-Embryonic and Embryonic Development

The processes involved in embryonic development include cleavage, growth, morphogenesis, and differentiation. Cleavage happens immediately after fertilization, as the zygote begins to divide. Increase in size does not occur. Cell division is mitotic as each cell duplicates with a full set of chromomsomes and genes. Growth takes place as the daughter cells grow in size, and morphogenesis occurs as the embryo takes various shapes. As the cells take on specific structures and functions this is known as differentiation. 

The development of the embryo in the uterus is accomplished by the functions of the extraembryonic membranes. These include the chorion, allantois, yolk sac, and amnion. The function of the chorion is to provide oxygen, nutrients, and take away wastes. It develps into half of the placenta and the blood vessels within the chorionic villi are contonuous with the umbilical blood vessels. The allantois extends away from the embryo. It accumulates the urine produced by the fetus and the blood vessels become the umbilical blood vessels. The umbilical arteries carry O2 poor blood to the placenta and O2 rich blood to the from the placenta. In humans the yolk sac is the first site of blood cell formation and contains plentiful blood vessels. As in shelled animals with a yolk, the placenta takes on the functions of a yolk. And last, the amnion cushions the embryo as it enlarges and the fluid within insulates it from cold and heat.

The stages of embryonic development include the events that occur from fertilization to birth. The gestation period calculated by adding 280 days to the start of the last menstruation. The period known as pre-embyonic development include the events of the first week. In the first week  the zygote divides as it passes from the oviduct to the uterus. This compact ball of embryonic cells called a morula. As the morula further divides the formation of a small cavity appears between cells so there is a inner cell mass surrounded by an outer layer of cells. At this point it is called a blastocyst, however the size has not increased. The inner mass becomes the embryo and the outer becomes the chorion. If the cells of the morula separate, or the inner mass splits and two embryos are present, identical twins with exactly the same chromosomes result. subsequently two different eggs create fraternal twins.

Embryonic development starts from the second week and lasts till the end of the second month. When implantation is successful pregnancy begins. The chorion secretes a hormone that is detected in pregnancy tests called HCG and helps in maintaining the corpus luteum. With progression the inner cell mass becomes the embryonic disk.

Two more extraembryonic membranes form and the amniotic cavity surrounds the embryo as it develops. This event is called gastrulation an example of morphogenesis, as in this case to become tissue layers called the primary germ layers. This process is completed when the disk becomes an embryo with three primary layers; the ectoderm, mesoderm, and endoderm. In the third week the organ systems start to develop and the nervous system is the first to be visible. In the third week the heart begins to form. By the fourth week a body stalk, the future umbilical cord, connects the embryo to the chorion and the fourth extraembryonic membrane, the allantois lies within the body stalk in which it's blood vessels become the umbilical blood vessels and eventually the umbilical cord. In the fifth week limb buds form, the head enlarges, and the sense organs are more visible. The embryo takes on recognizable human features during the six through eighth weeks. With brain development, the head becomes proportional to the the body and a neck develops. Also reflexes along with the nervous system develop. However this all occurs in an embryo the approximate size of an aspirin.            

III. Fetal Development

The hormones at this time function to prevent new follicles from developing, and help to maintain the endometrium and the cessation of menstruation. The blood of the fetus and mother do not mix and the carbon dioxide and wastes move from the fetal side to the maternal side, as the nutrients and oxygen move from the maternal side to the fetus by diffusion. At this time, harmful chemicals can cross the placenta. Of particular importance, there are sensitive periods during which an organ or part is susceptible to substances that can alter it's normal function. 

The umbilical arteries and vein connect the fetus to the mother. The umbilical vein carrying blood rich nutrients enters the liver, merges and with the inferior vena cava 

of the mother. The blood moves to the left atrium, left ventricle, then to the aorta. Oxygen poor blood enters the right atrium and is pumped into the pulmonary trunk, and through the arterial duct it enters the right atrium.

The development of the fetus includes the third through the ninth months of development. In the third month head growth slows and the body begins to increase in size, and external refinements such as fingernails, eyelashes, nipples, eyebrows, and hair appear. Bone is replaced by cartilage in a process called ossification. This process  completes about 18 to 20 years of age. 

Around the third month the presence of the SRY gene forms either male or female genitals. In the fourth month the heartbeat can be heard through a stethoscope. By the fifth through the seventh month the mother can feel movement. The skin is covered by a down called lanugo and a greasy white substance known as vernix caseosa. The eyelids can now open.

By nine months the fetus is near term. Term babies have the have a better chance of survival compared to premature babies. And the fetus usually rotates it's head toward the cervix in preparation for birth. If the baby does not turn, a breech birth occurs in which a cesarean section is needed for delivery.        

IV. Pregnancy and Birth

Hormonal changes can cause fluctuations in energy levels, and the mother may experience fatigue, nausea, vomiting, or loss of appetite. Weight gain occurs due to the mother's own internal changes and the addition of the fetus. Major changes due to placental hormones affect physiological changes. Progesterone relaxes the arteries lowering blood pressure and aldosterone promotes sodium and water retention. Blood volume also increases as cardiac output increases. This  increases blood flow to the kidneys, placenta, skin, and breasts. 

Changes also occur in pulmonary values, such as a 405 increase in vital capacity and tidal volume. Improvements appear in respiratory functions. Levels of oxygen change slightly but carbon dioxide decrease as much as 20%.

Other affects include compression from the enlarged uterus resulting in bladder incontinence and edema or varicose veins due to decreases in venous return. Also, peptide hormones which induce insulin resistance can induce pregnancy related diabetes.

Childbirth normally occurs at the end of the pregnancy period. Toward the end of this period contractions become more stronger and frequent. Labor is evident when contractions happen every 15-20 minutes and last longer than 40 seconds. the stretching of the cevix can release oxytocin. The contractions in the uterus pushs the fetus downward as the crevix continues to stretch. This process repeats until birth.

There are three stages to birth. In the first stage of contractions the baby's head helps to dilate the cervix and if the amniotic mambrane ruptures the fluid that leaks is known as "breaking water". This stage ends when the cervix is fully dilated. In the second stage the head appears. After the rest of the body emerges the umbilical cord is cut. In the third stage known as afterbirth, the placenta is expelled.

V. Development After Birth

In humans, development continues throughout life. The stages of life include infancy, childhood, adolescence, and adulthood. The latter half of adulthood is characterized by aging. This process of progressive changes can increase the risk of illnesses, disease, and death. The study of gerontology, or aging seeks to increase the health of individuals in their latter years.  

References:

http://training.seer.cancer.gov/module_anatomy/unit12_3_repdt_female.html

http://training.seer.cancer.gov/module_anatomy/unit12_2_repdt_male.html