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The Human Reproductive System

Structures of the male reproductive system

Bulb of penis

The bulb of the penis is the expanded posterior part of the corpus spongiosum of the penis. It lies in the interval between the crura of the penis. It contains a slightly dilated and angulated portion of the urethra.

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Bulbourethral (Cowper’s) gland

The bulbourethral gland is pea-sized structures located on the sides of the urethra below the prostate gland. It produces a clear, slippery fluid that empties directly into the urethra. These fluids lubricate the urethra and neutralises any acidity that may be present due to residual drops of urine in the urethra.

Corona

The corona is a rounded projecting border that forms the circumference of the base of the glans. It is located behind the penis and overhangs a deep retroglancular sulcus

Corpora cavernosum penis

The corpus cavernosum penis is one of the pair of sponge-like regions of erectile tissue in the penis. It contains irregular spaces which are filled with blood during erection. These spaces are lined by endothelium and separated by connective tissue septa.

Corpus spongiosum penis

The corpus spongiosum penis is the median column of erectile tissue located between and ventral to the two corpora cavernosa penis. Posteriorly It expands into the bulb of penis and terminates as enlarged glans penis anteriorly.

Ductus (vas) deferens

The ductus deferens is a long muscular tube that travels from the epididymis into the pelvic cavity to just behind the bladder. It transports mature sperm to the urethra in preparation for ejaculation.

Ejaculatory duct

Ejaculatory duct is formed by the fusion of vas deferens and the seminal vesicles. They open into the urethra and mix sperm stored in the ampulla with fluids secreted by the seminal vesicles and transport these to the prostate.

Epididymis

The epididymis is a long, coiled tube that rests on the backside of each testicle. It transports and store sperm cells that are produced in the testes. It also brings the sperm into maturity, since the sperm that emerge from the testes are immature and incapable of fertilisation.

Glans penis

The glans penis, commonly referred to as the head of the penis, is a sensitive bulbous structure at the end of the penis. It is the conical expansion of the corpus spongiosum. It is anatomically homologous to the glans clitoris in females.

Membranous Urethra

The membranous urethra is a short portion of the urethra which is situated between the prostatic urethra and spongy urethra. It connects the parts of the urethra passing through the prostate gland and the penis.

Penis

The penis is the male organ for sexual intercourse. It is cylindrical in shape and consists of three internal chambers which are made of special sponge-like erectile tissue.

Peritoneum

The peritoneum is a serous membrane that forms the lining of the abdominal cavity and folds inward to enclose the viscera. It covers most of the intra-abdominal organs including the urinary bladder and is composed of a layer of mesothelium supported by a thin layer of connective tissue. It both supports and serves as a conduit for blood and lymph vessels and nerves for the abdominal organs.

Prepuce (foreskin)

The Prepuce is a loose double-layered fold of skin and mucos membrane that covers the glans penis. It protects the urinary meatus when the penis is not erect.

Prostate

The prostate or prostate gland is walnut-sized and is located below the urinary bladder in front of the rectum. It contributes additional fluid to ejaculate. These fluids help nourish the sperm.

Prostatic urethra

The prostatic urethra is the prostatic part of the male urethra. It is about 2.5 cm in length and traverses the prostate. It includes seminal colliculus and the ejaculatory and prostatic ducts open into the int.

Pubic symphysis

The pubic symphysis is a cartilaginous joint that joins the superior rami of the left and right pubic bones at the front of the pelvis. It is located at the back of the urinary bladder and above the external genitalia. It is attached to the suspensory ligament of the penis.

Scrotum

The scrotum is a loose pouch-like sac of skin that hangs behind the penis. It contains the testicles as well as many nerves and blood vessels. It protects and acts as a climate control system for the testes.

Seminal vesicle

The seminal vesicles are sac-like pouches that attach to the vas deferens near the base of the bladder. They produce fructose that provides sperm with a source of energy and helps with the sperms’ motility. The fluid of the seminal vesicles makes up most of the volume of a male’s ejaculatory fluid.

Suspensory ligament of penis

The suspensory ligament of penis is a fibrous band of tissues at the deep fascial layer that extends from the pubic symphysis to the deep fascia of the penis. It is attached to the pubic symphysis and anchors the roof of the penis.

Testes

The testes are oval-shaped male sex glands. They are located in the scrotum and secured at either end by spermatic cord. They produce male sex hormone testosterone. Within the testes, there are coiled masses of tubular structures called seminiferous tubules where sperm cells are produced through spermatogenesis.

Urethra

The urethra is a tube like structure that carries urine from the urinary bladder to outside of the body. It also expels semen when males reach orgasm. When the penis is erect, during sex, the flow of urine is blocked from the urethra, allowing only semen to be ejaculated at orgasm.

Urinary bladder

The urinary bladder is a muscular sac located in the pelvis that stores urine and contracts to expel it from the body. Urine enters the bladder from the kidneys through the ureters and is excreted from the body through the urethra. An adult human bladder can hold 0.6 litres of urine.

Structures of the female reproductive system

Ampulla of uterine tube

The ampulla of uterine tube is the second portion of the uterine tube. It is an intermediate dilated portion that curves over the ovary and has a complexly folded mucosa with a columnar epithelium of mostly ciliated cells between which are secretory cells.

Bartholin’s glands

Bartholin’s glands are located next to the vaginal opening. They are homologous to bulbourethral glands in males and secrete mucus for vaginal lubrication. They secrete relatively minute droplets of fluid just before female orgasms moistening the labial opening of the vagina.

Cervix

The cervix is the lower end of the uterus. It is located at the top of the vagina and is about one inch long. The cervical canal passes through the cervix. It allows the menstrual period and fetus to pass from the uterus into the vagina and sperm to pass from the vagina into the uterus.

Clitoris

The clitoris is a small sensitive protrusion in females compared to the penis in males located where the two labia minora meet. It is covered by a fold of skin, called the prepuce. Similar to the penis, the clitoris is very sensitive to stimulation and can become erect.

Endometrium

The endometrium is the membrane lining the uterus. It prevents friction between the opposed walls of the myometrium, thereby maintaining the patency of the uterine cavity. During menstrual cycle or estrous cycle, it grows to a thick, blood vessel rich, glandular tissue layer which provides an optimal environment for the implantation of a blastocyst upon arrival in the uterus.

Fallopian tubes

The fallopian tubes, also known as oviductus or uterine tubes, are two very fine tubes lined with ciliated epithelia that lead from the ovaries into the uterus. It serves as a conduit for the ovulated oocyte to enter the uterine cavity following fertilisation.

Fimbrae of uterine tube

The fimbrae of uterine tube is the irregularly branched or fringed processes surrounding the ampulla at the abdominal opening of the uterine tube. Most of its lining epithelial cells have cilia that beat toward the uterus.

Fundus of uterus

The fundus of uterus is the upper rounded extremity of the uterus above the openings of the fallopian tubes opposite the cervix. The height of the fundus, measured from the top of the pubic bone, is measured routinely during pregnancy to determine the growth rate of the fetus.

Infundibulum of uterine tube

The infundibulum of uterine tube is the third part of the uterine tube. It terminates with the ostium of fallopian tube and is surrounded by fimbrae, one of which, the ovarian fimbrae is attached to the ovary.

Isthmus of uterine tube

The isthmus of uterine tube is a narrow portion of the uterine tube adjoining the uterus. It is the first part of the uterine tube and is constricted.

Labia majora

The labia majora enclose and protect the other external reproductive organs. It is large, fleshy and contains sweat and oil-secreting glands. It is covered with hair after puberty.

Labia minora

The labia minora are two longitudinal cutaneous folds on the human vulva situated between the labia majora. The labia minora does not have any pubic hair and is a horizontal piece of skin that covers the vestibule.

Lateral fornix

The lateral fornix is the portion of the fornix of the vagina lateral to the uterine cervix. The fornix extends into the recesses created by the extension of the cervix into the vaginal space.

Myometrium

The myometrium is the middle layer of the uterine wall. It consists of a muscular tissue made up of smooth muscle cells and supports stromal and vascular tissue.

Ovaries

The ovaries are small oval shaped glands located on either side of the uterus. They produce secondary oocytes and hormones including progesterone, oestrogen and inhibin, a peptide that is an inhibitor of follicle stimulating hormone synthesis and secretion.

Suspensory ligament

The suspensory ligament is a fold of peritoneum. It extends out from the ovary to the wall of the pelvis and conveys the ovarian artery, lymphatic vessels, vein and nerve plexuses.

Uterus

The uterus is a hollow, pear-shaped organ within which the fetus develops during gestation. It is located in the pelvis between the bladder and the rectum. The cervix part of the uterus opens into the vagina and the other is connected on both sides to the fallopian tubes.

Vagina

the vagina is a muscular tube lined with mucus membranes. It connects the uterus and cervix to the outside of the body and is located between the urethra and the anus. It allows the flow of menstrual blood and for sexual intercourse. It is also the passageway through which a baby is born.

Human reproduction is a form of sexual reproduction. It involves sexual intercourse between a male and a female where the male and the female reproductive system interact resulting in pregnancy followed by childbirth. The entire process starts with the production of male and female germ cells. The development and production of male and female germ cells called gametes required to form a new offspring through meiotic cell division is known as gametogenesis. The gamete referred to as sperm in males are produced by the testes through a process called spermatogenesis. In females the gamete referred to as egg or ova is produced by the ovaries through a process called oogenesis.

Spermatogenesis takes place in the walls of the seminiferous tubules in the testes. Through meiosis spermatogonia divide to form spermatocytes. Spermatocytes undergo meiosis to form haploid spermatids. The transformation of spermatids to sperm or spermatozoa takes place in the wall adjacent to the lumen of the tubule. The heads of the developing sperms are embedded in sortil cells present in the testes and their tails project into the fluid-filled lumen. Eventually the sperms mature and are detached from the sertoli cells.

Oogenesis takes place in the ovary of the females. Primordial germ cells in the ovary multiply to form oogonia, which start their first meiotic division to become oocytes in the fetus. This division is not completed until each oocyte is ovulated. The second division is only completed on fertilisation. Each meiotic division is unequal, so that one large ovum is produced with a much smaller polar body.

The testes are oval shaped glands in males about the size of olives located in the scrotum. Normally there are two testes present which are secured at either end by spermatic cord. Each testis contains about 250 lobules and each lobule contains 1 to 3 tightly coiled seminiferous tubules. These tubules produce sperm through spermatogenisis and consist of two types of cells: sertoli and spermatogenic cells that assist in spermatogenisis. Sertoli cells support, protect and nourish the sperm. They also secrete androgen binding protein like testosterone which is a male sex hormone. The testes are surrounded by a thick capsule called tunica albuginea from which a conical mass of connective tissue, the mediastinum testis, projects into the testes.

The ovaries are small oval shaped glands in females. They are located on either side of the uterus and each ovary is attached to the fimbrae of the fallopian tube. They produce eggs through oogenesis and the female sex hormones: progesterone, oestrogen and also inhibin, which is a peptide inhibitor of follicle stimulating hormone (FSH) synthesis and secretion. The ovaries are covered with germinal epithelium underneath which is a connective tissue called tunica albuginea. The outer region of the ovary is called the cortex and the inner region is called the medulla. The cortex consists of ovarian follicles and stroma cells in between.

Sexual intercourse also known as coitus is the insertion of the male’s penis into the female’s vagina for the purpose of reproduction. To engage in coitus, the erect penis is inserted into the vagina and the male and female stimulate themselves through back and forth motion until they orgasm which accompanies male ejaculation. During ejaculation, a series of muscular contractions delivers semen containing male gametes or sperm cells from the penis into the vagina. The sperm cells then travel from the vagina through the cervix and into the uterus and finally into the fallopian tubes by flagellation. Out of the millions of sperm cells present in ejaculation only one succeeds in fertilization with a fertile ovum or egg present in the fallopian tube and results in the formation of a new embryo.

Fertilistion is the union of sperm and egg that occurs in the ampulla of the fallopian tube. It is the initiation of prenatal development. Fertilisation involves the penetration of the egg or oocyte by the sperm, fusion of the sperm and the oocyte to for a zygote followed by the fusion of their genetic material. Within hours of conception the zygote undergoes cell division and within the first week the fertilised egg travels towards the uterus, where the continued growth of the zygote will occur in the form of an embryo. In the uterus, the zygote fuses itself to the uterine wall and the cells of the zygote differentiate into embryoblast cells and trophoblast cells. (See further detail in Task 2)

Females of reproductive age, between 11-16 years, experience cycles of hormonal activity that repeat at about 1-month intervals, this is called as the menstrual cycle. A female body prepares for a potential pregnancy, regardless of their choice, with every cycle which involves the shedding of the uterine lining. The average cycle takes about 28 days and occurs in 3 phases: the follicular phase, the ovulatory phase and the luteal phase. Fertility varies over the 28 days and day 11-14 is the most fertile. There are four major hormones that stimulate or regulate the activity of cells or organs involved in the menstrual cycle: follicle-stimulating hormone (FSH), Luteinising hormone (LH), estrogen and progesterone.

The follicular phase starts on day 1 of the period and the following events occur:

* FSH and LH are released from the brain and transported to the ovaries through blood.

* The released hormones stimulate the growth of about 15-20 eggs in the follicle of the ovaries.

* The hormones also trigger in the production of estrogen.

* The increased level of estrogen blocks the production of FSH creating a balance of hormones that allows the body to limit the number of follicles that complete maturation or growth.

* As the phase progresses, one follicle in one ovary becomes dominant and continues to mature and produce oestrogen. The rest of the follicles stop growing and die.

The ovulatory phase starts in about 14 days after the start of follicular phase. It is the midpoint of the menstrual cycle and the following events occur:

* The rise oestrogen from the dominant follicle triggers the production of LH from the brain.

* This causes the dominant follicle to release its egg from the ovary.

* After the release of the egg or ovulation, the egg is captured by finger-like projections present at the end of the fallopian tubes called fimbrae. The fimbrae sweep the egg into the tube.

* During this phase, the amount and thickness of mucus produced by the cervix is increased. If coitus occurs during this time, the thick mucus capture’s the male’s sperm, nourishes it, and helps it move towards the egg for fertilisation.

The lutleal phase starts immediately after ovulation and the following events occur:

* After releasing the egg, the empty follicle develops into a new structure called the corpus luteum.

* The corpus luteum secretes oestrogen and progesterone. Progesterone prepares the uterus for the implantation of the fertilized egg.

* If coitus has taken place and conception occurs, the fertilised egg will travel through the fallopian tube to implant in the uterus. This event results in the female to be pregnant.

* The egg passes through the uterus if not fertilised. This results in the lining of the uterus to break down and shed as it is not needed to support pregnancy. The next menstrual period then begins.

Follicle stimulating hormone (FSH) is produced and secreted by gonadotropes in the anterior lobe of the pituitary glands. It regulates the development, growth, pubertal maturation and reproductive processes in males and females. It acts alongside with LH in reproduction. In females, FSH stimulates the growth and maturation of immature Graafian follicles. As the follicles grow, it releases inhibin which blocks the production of FSH. FSH levels fall in late follicular phase which is critical in selecting only the most advanced follicle to proceed to ovulation. It then slightly rises at the end of the luteal phase which is necessary to start the next ovulatory cycle. In males, FSH increases the production of androgen binding protein by the sertoli cell present in the testes which is necessary for spermatogenesis.

Oestrogen is a family of complex hormones with similar characteristics. FSH stimulates the ovarian follicles to secrete oestrogen. LH stimulates the Graafian follicle to secret corpus luteum, which is an empty graafian follicle after ovulation transformed into a yellow body filled with cells containing yellow substance that in turn secretes oestrogen. Day 0-14 in the menstrual cycle is referred to as oestrogen phase. Oestrogen is also produced in smaller amounts by other organs such as liver, adrenal glands and breasts. These secondary sources are especially important in postmenopausal females. Oestrogen maintains the endometrium of the uterine wall during menstrual cycle. It triggers the maturation of reproductive organs, helps in the development of sexual characteristics, assists in lactation, regulates bone density in a foetus, promotes blood flow within the uterus, protects the foetus form the effects of androngens in the female system and also maintains regulates and triggers the production of other hormones.

Luteinising hormone LH is produced and secreted by gonadotropes in the anterior lobe of the pituitary gland. In females, LH helps regulate menstrual cycle and ovulation. The rise in oestrogen level triggers the release of LH. This rise of LH also known as LH surge occurs in about day 14 midway through the cycle which in turn triggers ovulation. LH maintains luteal function for the first two weeks during the menstrual cycle. LH causes the graaafian follicle to rupture and release the egg. It supports thecal cells in the ovary that provide androgens and hormonal precursors for estradiol, a type of oestrogen, production. In males, LH acts upon Leydig cells present in the testes. It is responsible for the production of testosterone, which stimulates endocrine activity and intratesticular activity such as spermatogenesis. The LH level rise and fall during the monthly menstrual cycle.

Progesterone is a female sex hormone that regulates menstrual cycle and ovulation. During ovulation the temperature of the body in females rises by 0.5�C. LH stimulates the graafian follicle to secret corpus luteum, which is an empty graafian follicle after ovulation transformed into a yellow body filled with cells containing yellow substance that in turn secretes progesterone. Day 15-28 in the menstrual cycle is referred to as progesterone phase. In females progesterone levels are low during preovulatory phase of the menstrual cycle, rise after ovulation and higher during the luteal phase. In the menstrual cycle day Progesterone converts the endometrium to its secretory stage to prepare the uterus for implantation and at the same time it affects the vaginal epithelium and cervical mucus, making the mucus thick and impermeable to sperm cells. Levels of progesterone decrease if pregnancy does not occur, which results in menstruation. The fertility varies over the 28 day period during the menstrual cycle and the most fertile day is between day 11-14. Progesterone decreases the contractility of the uterine smooth muscle and also inhibits lactation during pregnancy. In males the levels are similar to those of the females during the follicular phase.

Oestrogen and testosterone is present in both males and females. In males, the oestrogen is produced by the adrenal cortex. Its levels are significantly higher in women of reproductive age. Oestrogen promotes female secondary sexual characteristics, such as breast development and widening of the pelvis. In males, oestrogen regulates certain functions of the reproductive system necessary for the maturation of the sperm. It is also necessary for a healthy libido. Other functions of oestrogen are as follows: decelerate height growth, accelerate metabolism, reduces muscle mass, stimulates endometrial growth, increases uterine growth and vaginal lubrication, thicken the vaginal wall and maintenance of vessel and skin. Excess oestrogen in males causes development of breasts, obesity, long-term liver conditions like cirrhosis and some genetic conditions.

In males, testosterone is synthesised from cholesterol in the testes by Leydig cells. It maintains the function of the testes throughout the reproductive life in males. Females also produce testosterone relatively in lower levels than males by the thecal cells of the ovaries, by the placenta and by zona reticularis of the adrenal cortex. Testosterone promotes male secondary sexual characteristics, such as deepening of voice, growth of Adam’s apple and broad shoulders. Other fuctions of testosterone are as follows: enlargement of sebaceous glands, increased libido and frequency of erection or clitoral engorgement, facial hair, increased muscle strength and mass, growth of spermatogenic tissue in testes and phallic enlagement or clitoromegaly. Excess testosterone in females cause male pattern hair growth, especially on their faces and chests; some women experience virilisation, which is increased muscle mass; enlargement of the clitoris; deepening of the voice, male pattern baldness; acne and increased perspiration.

Bibliography

Boyle, M and Senior, K . (2002). Biology. London: Harper Collins Publishers Limited.

EHD. (2009). Ovary. Available: http://www.ehd.org/images/prenatal_article/ovary-adv.jpg. Last Accessed 04 Feb 2009.

E-Rham. (2009). Testes. Available: http://www.e-rham.com/img/atcl4_testes.gif. Last Accessed 03 Feb 2009.

Kent, M . (2000). Advanced Biology. Oxford: Oxford University Press.

Tortora, G.J. and Derrickson, B. (2007). Principles of Anatomy and Physiology. United States of America: John Wiley & Sons, Inc.

Task 2

Journey taken by the gametes to achieve fertilisation

The sperm cells or spermatozoa are produced in the seminiferous tubules of the testes. The journey of the sperm starts from these tubules. The newly produced sperm cells move towards the epididymis where it is stored. The epididymis also transports the sperm to vas deferens from where the sperm cells continue its journey to the urethra. Sperm cells continue its journey via the urethra and it is secreted into the vaginal cavity through ejaculation by the penis during coitus. The sperm cells then travel from the vagina through the mucus in the cervix and into the uterus and finally into the fallopian tubes by flagellation. The movement of the sperm in the genital tract is also assisted by the contractions of the vagina, cervix, uterus and oviducts.

The eggs or ova are produced in the graafian follicle of the ovaries. The journey of an egg starts form this follicle. During ovulation the graafian follicle breaks open and releases the egg. The released egg is captured by the fimbrae, which consist of tiny hair-like cilia that beat in waves, and the egg enters the uterine cavity. Other cells in the tube’s endothelium nourish the egg and lubricate its path during its stay inside the fallopian tube. Once inside the fallopian tube, the egg and the sperm meet and the egg is fertilised. The fertilised egg will travel through the fallopian tube to implant in the uterus. If the egg does not become fertilised within 24 to 36 hours after ovulation, it will deteriorate and be removed by the body’s immune system, like any other dead cell in the body.

Fertilisation and implantation

Fertilisation involves the fusion of sperm and ovum or oocyte to form a zygote, a fertilised egg. The secondary oocyte is covered by: the corona radiata and the zona pellucida. The sperm penetrates these layers through capacitation, which involves the stimulation of acrosome reaction. The acrosome present in the head of the sperm secretes digestive enzymes that break down these layers and the sperm penetrates into the oocyte. When the sperm penetrates the zona pellucid a cortical reaction occurs, where cortical granules inside the oocyte fuses with the plasma membrane of the cell forming a protective fertilisation membrane. This protective layer forms along the entire membrane of the oocyte and prevents other sperm to fuse with the oocyte allowing only a single sperm to enter the diploid secondary oocyte. The sperm fuses with the oocyte, enabling fusion of their cell membranes and preparing for the fusion of their genetic material. Within 12 hours, the first meiotic cell division takes place, causing a rapid cell division which results in the formation of morulla cells. Further cell division forms blastocyst. The blastocyst is a structure in early embryogenesis before implantation. The blastocyst move towards the oviduct through the action of tiny hair-like cilia and it takes about 6 to 7 days to reach the oviduct.

Implantation is the process of attachment of the embryo to the endometrial lining of the uterine wall. It occurs within 1 to 2 days after arrival of the blastocyst in the uterus, a week after fertilisation. The outer membrane of the blastocyst called the trophoblast causes an inflammatory-type response causing the development of the endometrium. The trophoblast is what attaches the egg to the inside of the uterus and penetrates its way into the uterus. At this point of innteraction between the trophoblast and the endometrium, the placenta starts to develop. Implantation can sometimes cause bleeding which occurs when the egg implants into the uterus. Once implantation happens the blastocyst becomes the embryo. The embryo then begins to secrete a hormone called human chorionic gonadotrophin (hCG), which stimulates the corpus luteum to secrete progesterone. The secreted progesterone maintains the lining of the endometrium and inhibits the secretion of FSH.

The developing embryo

The embryonic period lasts between week 1 to 8 and after the 8th week foetal period starts and the embryo is then called the foetus. The following events occur during week 1 to 12:

Week 1 to 4

* The embryo receives nutrients for the rich blood supply to the uterus and it consists of two layers of cells.

* The embryo’s growth centres around an axis, which will become the spine and spinal cord.

* The brain, spinal cord, heart and gastrointestinal tract begin to form.

* Formation of blood vessels and blood forms in yolk sac, chorion and allantois.

* The body system begins to form; the eyes, ears and the primary brain vesicles begin to develop.

* The embryo is about 4 to 5 millimetres long.

Week 5 to 8

* The heart becomes four chambered, primary brain vesicles develop into secondary brain vesicles and ossification or bone formation begins.

* The brain develops three recognizable divisions and appears to be a prominent bulge in the head region.

* The facial features begin to develop and appear to be more human like and the nose develops, which is flat.

* The eyes and hair begin to form along with all essential organs.

* The first muscle movements take place.

* The embryo sit in a C shape with its head bowed and legs flexed upward.

* It weighs about 1/3 of an ounce and measures about 2 inches in length.

Week 9 to 12

* The embryo is now called the foetus

* The nose develops a bridge and the heartbeat can be detected.

* The brain continues to enlarge, the ossification continues and the body system continues to develop.

* The foetus begins to move unobserved by the mother.

* Early fingernails start to develop.

* The external genitalia are still not distinguishable as male of female.

The role of placenta in reproduction

The placenta is a membranous vascular organ that develops in females during pregnancy after fertilisation. It is formed partly from membranes surrounding the developing foetus and partly from the lining of the uterus. The placenta functions as an exchange surface allowing the passage of nutrients such as food and oxygen between the blood of the mother and of the foetus. Waste products such as urea and carbon dioxide are removed from the blood from the foetus to the mother and the mother nourishes the foetus with oxygen, glucose, vitamins, antibodies and amino acids etc. via the placenta. The exchange of nutrients and wastes is possible due to its thin membrane, which allows for easy diffusion and active transport of materials. The exchange surface of the placenta contains many mitochondria with folded microvilli that provide a large surface area for easy diffusion and active transport of materials. Pathogen fighting antibodies also pass through the placenta. The flow of blood in the mother is opposite to that of the foetus. This forms a counter-current system. This mechanism is vital because, if the mother and foetus’ blood flow in the same direction, equilibrium would rapidly reach and the blood would take up no more oxygen. It also makes diffusion easier.

The placenta also has metabolic and endocrine functions. It secretes placental lactogen or somatomammotropin, which increases the amount of glucose and lipids in the mother’s blood; hormones that maintain pregnancy such as progesterone that maintains the endometrium, oestrogen that causes proliferation of the endometrium; and human chorionic gonadotrophin (hCG), which inhibits FSH secretion and stimulates the corpus luteum to secrete progesterone. This results in increased blood sugar levels during pregnancy. Placenta also produces insulin-like growth factors (IGFs).

The role of hormones during birth and lactation

Childbirth also known as parturition or labour is the process of bringing forth a child, a fully developed foetus from the uterus to the vagina and finally to the outside world. Childbirth is categorized in three stages of labour: the shortening and dilation of the cervix, descent and delivery of the infant, and delivery of the placenta. In some cases, childbirth is achieved through caesarean section, which is the removal of infant through the abdomen, rather than through the vagina. Two hormones namely oxytocin and prostaglandins take part in the muscular contraction of the uterus during labour. Oxytocin is secreted by the anterior pituitary gland and prostaglandins by the placenta. These hormones are released when the fetus stimulates the cervix and vagina. The hormones enhance the contraction of the uterine smooth muscle to facilitate birth. This contractions causes tension and pressure on the cervix, which in turn act as stimuli that brings about further secretion of oxytocin, resulting in more powerful contractions. The intense contractions push the foetus out of the uterine wall and the cervix. The cervix dilates and allows the passage of the baby out it. The placenta and the umbilical cord are expelled after childbirth.

Lactation is the secretion of milk by the mammary glands in females. The first milk that a breastfed baby receives is called colostrum. Colostrum contains no fat but contains higher amounts of white blood cells and antibodies than mature milk. Closostrum provides the baby immunity until it is able to develop its own. It is especially high in immunoglobulin A (IgA), which coats the lining of the baby’s immature intestines and helps to prevent germs from invading the baby’s system. Normal milk is secreted at about 3-4 days. Prolactin or lactogenic hormone secreted by the anterior lobe of the pituitary gland stimulates the production of milk by the mammary glands. The sucking of nipples by the baby acts as a stimulus and causes the pituitary glands to secrete oxytocin. Oxytocin travels in the blood and causes contraction of the muscular myoepithelial cells surrounding the milk glands. The contraction ejects the milk into the milk ducts and cisterns and into the infant’s mouth. During the period of lactation, the pituitary continues to secrete Prolactin, which also maintains the milk ducts.

Bibliography

Boyle, M and Senior, K . (2002). Biology. London: Harper Collins Publishers Limited.

Choice To Live With. (2009). Fetal Development. Available: http://www.choicetolivewith.com/FetalDevelopment/Week3/implantation.jpg. Last Accessed 20 Jan 2009.

Davidson. (2009). Fertilization. Available:http://www.bio.davidson.edu/Courses/Molbio/MolStudents/spring2005/Champaloux/fertilization.jpg. Last Accessed 15 Jan 2009.

Kent, M . (2000). Advanced Biology. Oxford: Oxford University Press.

Natracare. (2009). Placenta. Available: http://www.natracare.com/images/help_for_schools/illustrations/placenta.gif. Last Accessed 2 Feb 2009.

Tortora, G.J. and Derrickson, B. (2007). Principles of Anatomy and Physiology. United States of America: John Wiley & Sons, Inc.

Amendment 1

(a) What do you mean the scrotum acts as a climate control for the testes? Refer to temperature in the answer.

The scrotum acts as a temperature regulator and keeps the testis, where the sperm is produced, at a slightly lower temperature one or two degree below the body temperature. The lower temperature is ideal for sperm production and higher temperatures may be damaging to sperm count. In warm environments, the skin of the scrotum thins and the scrotum lengthens to move the testicles away from the body to keep the temperature of the testis cooler. In cold environments, the skin thickens and the scrotum is drawn close to the body to warm the testis.

(b) Answer each question below in one sentence only.

(i) What do you mean by a dominant follicle?

Graafian follicle.

(ii) What are gonadotropes?

Gonadotropes are cells in the anterior pituitary gland, which produce the gonadotropins follicle-stimulating hormone (FSH) and luteinizing hormone (LH).

(iii) Where does FSH come from and what is its main effect on the ovary?

FSH is synthesised and secreted by gonadotropes in the anterior pituitary gland and it stimulates the growth of immature Graafian follicles to maturation in the ovary.

(iv) Where does LH come from and what is its main effect in the middle of a cycle?

LH is synthesised and secreted by gonadotropes in the anterior pituitary gland and it triggers ovulation in the middle of a cycle.

(v) Where does oestrogen come from and what does it do to the lining of the uterus in the menstrual cycle?

The ovarian follicles in the ovary secrete oestrogen and it stimulates the growth of the lining of the uterus.

(vi) Where in the ovary does progesterone come from and what does it do to the lining of the uterus?

Progesterone is secreted by the corpus luteum in the ovary and it converts the lining of the uterus to its secretory stage to prepare the uterus for implantation.

(vii) What happens when progesterone secretion stops at the end of a menstrual cycle?

When progesterone secretion stops at the end of a menstrual cycle the lining of the uterus breaks down and menstrual bleeding occurs.

(viii) State not less than 4 secondary sexual characteristics in the female caused by oestrogen.

* Enlargement of breasts

* Growth of body hair, including underarm and pubic hair

* Vaginal and uterine growth

* Widening of hips

(ix) State not less than 4 secondary sexual characteristics in the male caused by testosterone.

* Growth of facial hair

* Enlargement of larynx and deepening of voice

* Heavier skull and bone structure

* Increased muscle mass and strength

(x) What are “androgens”?

Androgens are steroid hormones such as testosterone or androsterone that controls the development and maintenance of masculine characteristics.

(xi) Rewrite References.

Boyle, M and Senior, K . (2002). Biology. London: Harper Collins Publishers Limited. P 446-462.

Kent, M . (2000). Advanced Biology. Oxford: Oxford University Press. P 246-261.

Tortora, G.J. and Derrickson, B. (2007). Principles of Anatomy and Physiology. United States of America: John Wiley & Sons, Inc. P 1057-1089.

Amendment 2

Journey of the egg

The eggs or ova are produced in the Graafian follicle of the ovaries where its journey for fertilization starts. During ovulation, the Graafian follicle breaks open and releases the egg. The released egg is captured by the fimbriae consisting of tiny hair-like cilia that beat in waves and the egg then enters the tissue fluid of the pelvic cavity. The egg is then “wafted” by the finger-like projections in the funnel of fallopian tube and move towards the fallopian tube. Once inside the fallopian tube, the egg and the sperm meet, and the egg is fertilised. The fertilized egg then starts to move towards the uterus for implantation where it is swept along by peristaltic contractions of the oviduct and movement of the cilia lining the oviduct.

Amendment 3

What is capacitation? Coment on hyaluronidase enzyme.

Capacitation is the change in the sperm head membrane that prepares it for fusion with the egg membrane. Hyaluronidase enzyme is released by the acrosome of the sperm cell after it has reached the oocyte. It digests proteins in the zona pellucida that surrounds the oocyte and enables the sperm nucleus to enter the oocyte for fusion with the egg nucleus.

Amendment 4

(a) Why is the foetus and maternal blood flow not joined by blood vessels?

The foetus and maternal blood flow is not joined by blood vessels but separated by the placenta. If this was not the case then the mother’s immune system would reject the foetus like any other foreign material and also the blood pressure of the mother is too high compared to that of the foetus that it would damage and burst open the foetal blood vessels and kill the foetus.

(b) Name one chemical stored in the placenta.

Calcium

(c) How does the mother get rid of (i) CO2 (ii) Urea, entering to blood from the foetal blood (iii) Which substances give “passive immunity” to foetus?

The CO2 and urea from the foetal blood pass through the placenta and enter the blood of the mother. The mother’s blood then carry the CO2 to the lungs through which it is exhaled and urea to the kidneys through which it is excreted in urine. Maternal antibodies (MatAb) such as Immunoglobulin G provide “passive immunity” to the foetus.

Amendment 5

State correctly where oxytocin comes from.

Oxytocin is secreted by the posterior pituitary gland and prostaglandins in the placenta.

Amendment 6

Bibliography

Boyle, M and Senior, K . (2002). Biology. London: Harper Collins Publishers Limited. P 446-462.

Kent, M . (2000). Advanced Biology. Oxford: Oxford University Press. P 246-261.

Tortora, G.J. and Derrickson, B. (2007). Principles of Anatomy and Physiology. United States of America: John Wiley & Sons, Inc. P 1057-1089.

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