Answer of Question of Reproduction & Development

Answer of Question of Reproduction & Development

Q.11. Describe in detail the production and transport of sperm in human male.

Ans. The male gonads or sex organs, are the testes — oval glandular structures that form in the dorsal portion of the abdominal cavity from the same embryonic tissue that gives rise to the ovaries in females.

Scrotum

In the human male the testes descend about the time of birth from their points of origin into the scrotal sac (scrotum), a pouch whose cavity is initially continuous with the abdominal cavity via a passageway called the inguinal canal. After the testes have descended through the inguinal canal into scrotum, the canal is slowly plugged by growth of connective tissue, so that the scrotal and abdominal cavities are no longer continuous. The temperature in a scrotum is about 2°C below that in the abdominal cavity. The lower temperature is necessary for active sperm production and survival.

Testes

Each testis contains over eight hundred tightly coiled seminiferous tubules, which produce thousands of sperms each second in healthy young men. The walls of the seminiferous tubules are lined with two types of cells.

1. Spermatogenic cells, which give rise to sperms, and

2. Sustentacular cells, which nourish the sperms as they form, also secrete a

fluid (as well as the hormone inhibin) into the tubules to provide a liquid medium for the sperms. Between the seminiferous tubules are clusters of endocrine cells, called interstitial cells (Leydig cells), that produce testosterone. Inhibin and testosterone, both are androgens, the male sex hormones.

Duct system

A system of tubes carries the sperm, that the testes produce, to the penis. The seminiferous tubules merge into a network of tiny tubules called the rete testis (L.rete, net), which merges into a coiled tube called the epididymis. The epididymis has three main functions:

1. It stores sperm until they are mature and ready to be ejaculated.
2. It contains smooth muscle that helps propel the sperm toward the penis by peristaltic contractions, and
3. It serves as a duct system for sperm to pass from the testis to the ductus deferens.

The ductus deferens (formerly called the vas deferens or sperm duct) is the dilated continuation of the epididymis. Continuing upward after leaving the scrotum, the ductus deferens passes through the lower part of the abdominal wall via the inguinal canal. The ductus deferens then passes around the urinary bladder and enlarges to form the ampulla. The ampulla stores some sperm until they are ejaculated. Distal to the ampulla, the ductus deferens becomes the ejaculatory duct. The urethra is the final ein-tion of the reproductive duct system. Fig. 7.7.

Accessory Glands

Three sets of accessory glands add secretions to the semen, the fluid that is ejaculated:

1. A pair of seminal vesicles contributes about 60% of the total volume of the semen. The fluid from the seminal vesicles is thick and clear, containing mucus, amino acids, vitamin C, prostaglandins, and large amount of fructose (sugar), which provides energy for the sperm and helps to neutralize the natural protective acidity of the vagina. (The pH of vagina is about 3 to 4, bui sperm motility is enhanced when it increases to about 6).
2. The prostate gland is the largest of the semen secreting glands. It secretes its products directly into the urethra through several small ducts. Prostatic fluid is thin and milky, contains several enzymes, cholesterol, buffering salts and phospholipids.
3. The bulbourethral glands are a pair of small glands along the urethra below the prostate. Before ejaculation they secrete a clear, alkaline and viscous fluid that lubricates the urethra to facilitate the ejaculation of semen. Bulbourethral fluid also carries some sperm released before ejaculation, which is one reason for the high failure rate of the withdrawal method of birth control.

Penis

The penis has two functions,

1. It carries urine through the urethra to the outside during urination, and
2. It transports semen through the urethra during ejaculation.

The human penis is composed of three cylindrical strands of spongy erectile tissue derived from modified veins and capillaries: two corpora cavernosa and the corpus spongiosum. The corpus spongisum extends beyond the corpora cavernosa and becomes the expanded tip of the penis called the glans penis. The human glans is covered by a fold of skin called the foreskin, or prepuce, which may be removed by circumcision. Fig. 7.7

Sperm

A mature human sperm consists of a head, midpiece, and tail. The head contains the haploid nucleus, which is mostly DNA. The acrosome, a cap over most of the head, contains an enzyme called acrosin that assists the sperm in penetrating the outer layer surrounding a secondary oocyte. The sperm tall contains an array of microtubules that bend to • produce whip like movements. The spiral mitochondria in the midpiece supply the ATP necessary for these movements.

Q.12. What are the major constituents of semen? What are the functions of seminal fluids?

Ans. Semen is a mixture of seminal fluid and sperm cells. The seminal fluid is secreted by three sets of glands: the seminal vesicles secrete water, fructose, prostaglandins, and vitamin C; the prostate, secrete water enzymes, cholesterol, buffering salts, and phosphohpids; and the bulbouretheral glands secrete a clear, alkaline fluid. The average human ejaculation produces 3 to 4m1 of semen and contains 300 to 400 million sperms.

The seminal fluid has a variety of functions:

1. It serves as a vehicle for transport of sperm.
2. It lubricates the passage through which the sperm must travel.
3. As an effective buffered fluid, it helps protect the sperm from the harmful effects of the acids in the female genital tract.
4. It contains much sugar (mostly fructose) which the active sperm can use as a source of energy.

Q. 13. What is inguinal hernia? How does it occur?

Ans. In the human male the testes descend about the time of birth from their point of origin into the scrotum via a passage way called the inguinal canal. After the testes have descended through the inguinal canal into the scrotum, the canal is slowly plugged by growth of connective tissue, so that the scrotal and abdominal cavities are no longer continuous. Sometimes the inguinal canal fails to close properly, and even when it does, it remains a point of weakness and is easily broken open again when subjected to excessive strain, as when a man lifts a heavy object. The opening resulting from insufficient closure or from later rupture is known as an inguinal hernia, or rupture; it is the most common type of hernia in human males. If the hernia is large, it must be repaired surgically to prevent a loop of intestine from slipping through the opening into the scrotal sac, where the intestine may become caught so tightly that its blood supply is cut off and gangrene results. Inguinal hemia is largely a human hazard, attributable to our two —legged stance, which places much strain on the lower abdomen; such hernias are very infrequent in mammals that walk on four legs.

0.14.  Explain how hormones control the human male reproductive function?

Ans.  Hormonal control of male reproductive functions:

Before a male can mature and function sexually, special regulatory hormones must come into play.

1. Male sex hormones are collectively called androgens (Gr.andros, man + gennan, to produce).
2. The hormones that travel from the brain and pituitary gland to testes (and ovaries in the female) are called gonadotropins.
3. The interstitial cells produce the male sex hormone testosterone.
4. When the level of testosterone in the blood decreases, the hypothalamus is stimulated to secrete GnRH (gonadotropin — releasing hormone).
5. GnRH stimulates the secretion of FSH (follicle — stimulating hormone) and LH (luteinizing hormone), also called ICSH (interstitial cell stimulating hormone), into the blood stream. FSH causes the spermatogenic cells in the seminiferous tubules to initiate spermatogenesis,’ and LH stimulates the interstitial cells to secrete testosterone.
6. The cycle is completed when testosterone inhibits the secretion of LH, and another hormone,. inhibin, is secreted. Inhibin inhibits the secretion of FSH from the anterior pituitary.. This cycle maintains a constant rate (homeostasis) of spermatogenesis. See table below.

Q.15.    How do LH and FSH function in males?

Ans. The LH induces the interstitial cells of the testes to produce more testosterone. The testosterone, together with FSH, induces maturation of the seminiferous tubules and stimulates them to begin sperm production (spermatogenesis). Maintenance of spermatogenesis over long periods requires the continued presence of testosterone (and the LH that induces it) and FSH.

Q.16. How do the two kinds of mammalian reproductive cycles — estrous and menstrual, differ from each other?

Ans. The cyclic reproductive patterns of mammals are of two types: estrous cycle. characteristic of most mammals, and menstrual cycle, characteristic only of the anthropoid primates (monkeys, apes, and humans). These two cycles differ in two important ways:

1. In estrous cycle, females are receptive to males only during brief periods of estrus, or “heat”, whereas in the menstrual cycle receptivity may occur throughout the cycle.
2. A menstrual cycle, but not an estrous cycle, ends with collapse and discharge of the inner portion of the endometrium (uterine lining). In an estrous animal, each cycle ends with the uterine lining simply reverting to its original state, without the discharge characteristic of the menstrual cycle.

Q.17. Explain how the female reproductive system in human is more complex than males?

Ans. The reproductive role of human females is more complex than that of males. Not only do females producexametes (eggs or ova), but after fertilization, they also nourish, carry, and protect the developing embryo. After the offspring is born, the mother may nurse it for a time. Another difference between the sexes is the monthly rhythmicity of the female reproductive system.

18. What does the human female reproductive system consists of?

Ans. The female reproductive system consists of a number of structures with specialized functions:

1. Two ovaries produce eggs and the female sex hormones estrogen and progesterone.
2. Two uterine (fallopian) tubes, one from each ovary, carry eggs from the ovary to the uterus. Fertilization usually occurs in the upper third of the uterine tube.
3. If fertilization occurs, the uterus receives the blastocyst and houses the developing embryo.
4. The vagina receives semen from the penis during sexual intercourse. It is the exit point for menstrual flow and is the canal through which the baby passes from the uterus during child birth.
5. The external genital organs have protective functions and play a role in sexual arousal.
6. The mammary glands, contained in the paired breasts, produce milk for the newborn baby.

Q.19. What are the external genital organs in human female?

Ans. The female external genitalia are the clitoris two sets of labia surrounding the clitoris 0 e labia majora and labia minoray mons pubis, vestibular glands and vaginal opening. As a group, these organs are called the vulva.

Q.20. How does the production and tansport of the egg takes place in the human female?

Ans. The female gonads, the ovaries produce both ova and female sex hormones. Ovaries: The ovaries are located in the lower part of the abdominal cavity, one on each side of the uterus. At the time of birth, a girl’s ovaries already contain a huge number of primordial egg cells, or oocytes, (about 100,000). Of these, only several hundred will release egg cells during the woman’s reproductive years. A cross section of an ovary reveals rounded vessicles called follicles, which are the actual centers of egg production (oogenesis). The cells of the follicle also produce the primary female sex hormones, the estrogens. The egg cell is expelled from the follicle in the process of ovulation. The remaining follicular tissue then grows within the ovary to form a solid mass called the corpus luteum (“yellow body”). The corpus luteum secretes additional estrogen and progesterone, the hormone that maintains the uterine lining during pregnancy. If the egg cell is not fertilized, the corpus luteum disintegrates, and a new follicle matures during the next cycle.

Uterine tubes:

The female reproductive system is not completely closed, and the egg cell is released into the abdominal cavity near the opening of the uterine tubes or fillopian tubes. The uterine tube has a funnel – like opening, and cilia on the inner epithelium lining the tubes help collect the egg cell by drawing fluid from the body cavity into the tube. The cilia also convey the egg cell down the duct to the uterus, also known as the womb. Fertilization usually Occurs in the uppermost third of the uterine tube. A fertilized oocyte (zygote) continues its journey toward the uterus, where it will implant. The journey takes four to seven days. If fertilization does not occur, the ova degenerates in the uterine tbue.

Uterus:

Each uterine tube empties directly into the upper end of the uterus (womb). This organ, which is about the size of a fist, lies in the lower portion of the abdominal cavity just behind the bladder. It has very thick muscular walls and a mucous lining containing many blood vessels. The neck of the uterus is the cervix, which opens into the vagina. The uterus has three layers of tissues:

1. The outer layer (perimeirium) extends beyond the uterus to form the two broad ligaments that stretch from the uterus to the lateral walls of the pelvis.
2. The middle muscular layer (myometrium [Grmyo, muscle + metra, womb]) makes up most of the uterine wall.
3. The endometrium is the specialized mucous membrane that contains an abundance of blood vessels and simple glands.

The cervix leads to vagina, a muscular tube 8 to 10cm long. The wall of the vagina is composed mainly of smooth muscle and elastic tissue.

External Genitalia

The external female genitalia are collectively termed the vulva. The vulva includes the mans pubis, labia majora, labia minora, vestibular glands, clitoris, and vaginal opening.

Hymen

The opening of the vagina in young human females is partly closed by a thin membrane called the hymen. Traditionally the hymen has been regarded as the symbol of virginity, to be destroyed the first time sexual intercourse takes place.

Mammary glands

The mammary glands (L.,mammae, breasts) are modified sweat glands that produce and secrete milk, which drains into a series of ducts, opening at the nipple. Fatty (adipose) tissue forms the main mass of the mammary gland of a nonlactating female. The absence of estrogen in males prevents the development of both the secretory apparatus and the adipose deposits, so male breasts remain small and the nipples are not connected to the ducts. For details, see chapter 1 question 14.

Q.21. What events take place in the menstrual cycle and in ovarian cycle?

Ans. The term menstrual cycle refres specifically to the changes that occur in the

uterus. By convention, the first day of a woman’s menstrual period, the first day of menstruation, is designated day 1 of the cycle. The menstrual flow phase of the cycle, during which menstrual bleeding (loss of most of the endometrium) occurs, usually persists for a few days. Then the thin remaining endometrium begins to regenerate and thicken for a week or two, which is called the proliferative phase of the menstrual cycle. During the next phase, the secetory phase, usually about two weeks in duration, the endometrium continues to thicken, becomes more vascularized, and develops glands that secrete a fluid rich in glycogen. If an embryo has not implanted in the uterine lining by the end of the secretary phase, a new menstrual flow commences, marking day 1 of the next cycle. Fig. 7.9.

Figure 7.9

Major Events in the Female Ovarian and Menstrual Cycles. The two charts correlate the gonadotropins. ovarian hormones, follicle development ovulation and changes in uterine anatomy during the cycles.

Paralleling the menstrual cycle is an ovarain cycle. It begins with the follicular phase, during which several follicles in the ovary begins to grow, of which only one usually continue to enlarge and mature. The maturing follicle develops an internal fluid – filled cavity and grows very large, forming a bulge near the surface of the ovary. The follicular phase ends with ovulation, and the ovulatory phase, when the follicle and adjacent wall of the ovary rupture, releasing the egg cell. The follicular tissue that remains in the ovary after ovulation is transformed into the corpus luteum, that secretes female hormones during the luteal phase of the ovarian cycle. The next cycle, begins with a new growth of follicles. On average, it takes 28 days to complete one menstrual cycle, although the range may be from 22 to 45 days. During this time, the following hormonal events take place:

1. The controlling center for ovulation and menstruation is the hypothalamus. It releases, on a regular cycle,,GnRH, which stimulates the anterior pituitary to secrete FSH and LH.

FSH promotes the development of the oocyte in one of the immature ovarian follicles.

1. The follicles produce estrogen„ causing a buildup and proliferation of the endometrium, as well as the inhibition of FSH production.
2. the elevated estrogen level about midway in the cycle triggers the anterior pituitory to secrete LH, which causes the mature follicle to enlarge rapidly and release the secondary oocte (ovulation). LH also causes the collapsed follicle to become another endocrine tissue, the corpus luteum.
3. The corpus luteum secretes estrogen and progesterone, which act to complete the development of the endometrium and maintain it for 10 to 14 days.
4. If the oocyte is not fertilized, the corpus luteum disintegrats into a corpus albicans, and estrogen and progesterone secretion cease.
5. Without estrogen and progesterone, the endometrium breaks down, and menstruation occurs. The menstruation flow is composed mainly of sloughed off endometrial cells, mucus, and blood.
6. As progesterone and estrogen levels decrease further, the pituitary renews active secretion of FSH, which stimulates the development of another follicle, and the monthly cycle begins again.

e monthly menstrual cycle ceases in old age (45-50 years), a condition called as meno ause. A summary of events of menstrual cycle is given below:

Q.22. Name the major human female reproductive hormones, their source and functions.

Ans. Major Human Female Reproductive Hormones

 

Estrogens (estrin, esterone, and estradiol), progesterone, human chorionic gonadotropin, human chorionic somatomammotropin (hCS) hormones are released from placenta during pregnancy which maintain pregnancy.

Q.23. How do hormonal regulation in the pregnant female is brought about? Apis. Hormonal regulation in the pregnant female:

Ans. Pregnancy sets into motion a new series of physiological events. The ovaries are directly affected because, as the embryo develops, the cells of the embryo and placenta release the hormone human chorionic gonadotropin (hCG), which prevents the corpus luteum from disintegrating. The progesterone that it secretes is necessary to maintain the uterine lining. After a time, the placenta takes over progesterone production, and the corpus luteum degenerates. By the end of two weeks following implantation, the concentration of hCG is so high in the female’s blood, and in her urine as well, that an hCG immunological test can check for pregnancy. As the embryo develops, other hormones are secreted. For example, prolactin and oxytocin induce the mammary glands to secrete and eject milk after childbirth. Oxytocin and prostaglandins also stimulate the uterine contractions that expel the baby from the uterus during childbirth Fig. 7.10.

Q.24. Describe the events of prenatal development, from zygote to newborn.

Ans. Events of prenatal development: from zygote to newborn: The development of a human being can be divided into prenatal (before birth) and postnatal (after birth) periods. During the prenatal period, the developing individual begins life as a zygote, then becomes a ball of cells called a morula, and eventually becomes a blastocyst that implants in the endometrium. From two weeks after fertilization until the eighth week of its existence, the individual if called an embryo. From nine weeks until birth, it is-a fetus. During or after bit^, it is called a newborn, or baby.

Pregnancy is arbitrarily divided into three — month periods called trimesters. The first trimester begins at fertilization, and during this time most of the organs areformed. The next two trimesters are mainly periods of growth for the fetus.

The First Trimester:

The first trimester is the time of most radical change for both the mother and the baby. Fertilization occurs in the oviduct. About 24 hours later, the resulting zygotet begins dividing, a process called cleavage. By about 1 week after fertilization, cleavage has produced a hollow ball of cells, an embryonic stage called the blastocyst. The next stage of development occurs when the blastocyst adheres to the uterine wall and implants. During implantation, the outer cells of the blastocyst, called the trophoblast, invade the endometrium. Implantation is usually completed 11 to 12 days after fertilization, from then on, the female is considered to be pregnant. The embryo obtains nutrients directly from the endometrium during the first 2 to 4 weeks of development. Meanwhile, tissues grow out from the developing embryo and mingle with the endometrium to form the placenta. One of the unique feature of mammalian development is that most of the cells of the early embryo make no contribution to the embryo’s body, giving rise instead to supportive and protective membranes. Only the inner cell mass gives rise to the embryonic body. Eventually, these cells arrange in a flat sheet that undergoes a gastrulation similar to that of reptiles and birds. Once gastrulation is complete, the rest of the first trimester is devoted to organogenesis and growth. The heart begins beating by the fourth week and can be detected with a stethoscope by the end of the first trimester. By the end of the eighth week, all the major structures of the adult are present in the rudimentary

form. At this point, the embryo is called a fetus. Athough well differentiated, the fetus is only 5cm long by the end of the first trimester. Because of its rapid organogenesis, the embryo is more sensitive during first trimester to such threats as radiation and drugs can cause birth defects.

The second trimester:

During the second trimester, the fetus grows rapidly to about 30cm and is very active. The mother may feel movements during the early part of the second trimester, and fetal activity may be visible through the abdominal wall by the middle of this time period. During the, six month the upper and lower eyelids of fetus separate, and the eyelashes form. During the seventh month, the eyes open. During this period, the bones begin to ossify. During the second trimester, the uterus will grow enough for the pregnancy to become obvious.

The third Trimester:

The third and final trimester is one of rapid growth of the fetus to about 3— 3.5 kg in weight and 50cm in length. During the last month, fetal weight doubles. During third trimester, fetal activity may decrease as the fetus fills the available space within the embryonic membranes. As the fetus grows and the uterus expands around it, the mother’s abdominal organs become compressed and displaced, leading to frequent urination, digestive blockages, and strains in the back muscles.

Q.25. What is the function of the placenta in human female?

Ans. After implantation of the embryo in the uterine lining, the embryonic membranes form the umbilical cord, through which blood vessels, contributed by the allantois run to a large structure, the placenta, formed from the embryonic membranes (primarily the chorion) and from the adjacent uterine tisue.

The placenta is the organ that sustains the embryo and fetus throughout the pregnancy and through which gases, nutrients, the maternal and fetal systems. The tiny, fingerlike projections that were sent out from the blastocyst during implantation develop into numerous chorionic which contain embryonic blood vessels. These blood vessels do not merge with those of the mother; the two bloodstreams remain separate throughout the

pregnancy. The umbilical cord connects the placenta to the abdomen of the fetus. Two fetal umbilical arteries and one umbilical vein spiral about each other in the umbilical cord. Fig. 7.12.

Besides serving as a medium for the transfer of material between maternal and fetal bloodstreams, the placenta also serves as an endocrine gland.

1. It continues to secrete hCG and also produce estrogen and progesterone.
2. Preparation of the mammary glands for secretion of milk requires two additional hormones, prolactin (PRL) and human placental lactogen (hPL). PRL is produced by the anterior pituitary.
3. The placenta also synthesize a peptide hormone called relaxin that facilitates labour and delivery.
4. Prostaglandin that also facilitate labour.

Q.26. What events take place during parturition?

Ans. About 266 days after fertilization, or 280 days from the beginning of the last menstrual period, the human infant is born. Birth, or parturition (L.Parturire, to be in labor), begins with a series of strong. rhythmic contractions of the uterine musculature, called labor. The hormone relaxln, produced by the ovaries and placenta, causes the mother’s pelvic bones to slightly separate so that the baby can pass

through the birth canal. Changing hormone. levels initiate parturition. Pituitary gland secretes adrenocorticotropie hormone (ACTH), which stimulates the adrenal glands to secrete steroids. The steroids stimulate the placenta to produce. prostaglandin that, alongwith the hormone oxytocin; cause the uterus to begin powerful muscular contractions. The contractions increase in frequency from 2 to 18 hours. During that time, the cervix becomes fully dilated, and the amniotic sac ruptures. Fig. 7.13.

Childbirth occurs in three stages:

1. In the first stage the neck (cervix), or opening of the uterus into the vagina, is enlarged by pressure from the baby in its bag of amniotic fluid, which may be ruptured at this time.
2. In the second stage, the baby is forced out of the uterus and through the vagina to the outside.
3. In the third stage, the placenta, or after birth, is expelled from the mother’s body, usually within 10 minutes after the baby is born.

Q.27. How does lactation occur in human female?

Ans. Lactation (Llactare, to suckle) includes both milk secretions (production) by the mammary glands and milk release from the breasts. Mammary glands, a unique characteristic of mammals evolved from sweat glands in the skin. During pregnancy, the breasts enlarge in response to increasing levels of the hormone prolactin. Before birth, placental secretions of estrogen and progesterone inhibit milk secretion from the breasts. After the placenta has been expelled from the uterus, the concentrations of estrogen and progesterone drop, and the breasts begin to produce copious amount of milk. Fig. 7.14.

The mother’s breasts do not actually release milk until one to three days after the baby is born. During these first days, the suckling baby receives colostrum, a

high — protein fluid present in the breast at birth. The colostrum functions as:

1. It contains an abundance of maternal antibodies and thus helps strengthen the baby’s immune system
2. It also functions as a laxative, removing fetal wastes, called meconium, retained in the intestines.

After about three days, the prolactin secreted from the pituitary stimulates milk production. The newborn’s suckling stimulates the pituitary to release oxytocin as well as prolactin. Oxytocin triggers milk release from the mammary glands and prolactin stimulates continued production of milk by the mammary glands.

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