Human Reproduction Questions Class 12 Biology

ICSE Class 12 Biology

Topic-1
Male and Female Reproduction System

Very Short Answer Type Questions

Question. Name the pouch in which the human testes are present.
Answer :
 Scrotum. 

Question. Name the cells that nourish the germ cells in the testes. Where are these located in the testes ?
Answer : 
Name of cells—Sertoli cells.
Location—They are found lining-up the inner surface of seminiferous tubules. 

Question. Write the location and function of Leydig cells in human
Answer :
 Leydig cells are present at interstitial region between seminiferous tubules. They synthesize and secrete androgens.

Question. Write the location and function of the Sertoli cells in human
Answer : 
In the seminiferous tubules of testis.
Function : Nourishes the sperms or germ cells.

Question. List different parts of human oviduct through which the ovum travels till it meets the sperm for fertilization.
Answer : 
Fimbriae, infundibulum, ampulla and ampullaryisthmic junction.

Question. Give reasons for the following :
The human testes are located outside the abdominal cavity.
Answer : 
For spermatogenesis, it is essential to maintain the temperature 2-2.5°C lower than normal internal body temperature.

Question. Write the function of Seminal vesicle.
Answer :
 Seminal vesicle produces an alkaline secretion. It neutralizes the acidic environment of female reproductive tract and male urethra during coitus.

Short Answer Type Questions – l

Question. Write the location and functions of myometrium and endometrium.
Answer : Myometrium : Middle layer of uterus, contractions of the uterus during delivery/child birth/parturition,
Endometrium : Inner layer of uterus, cyclic changes during menstruation/implantation of embryo.

Question. Why are the human testes located outside the abdominal cavity ? Name the pouch in which they are present.
Answer : Testes are located outside the abdominal cavity within a sac called scrotal sac or scrotum. Scrotum keep the testes temperature at 2oC lower than the body temperature. The lower temperature is required for proper functioning of testes and for spermatogenesis.

Question. Write the location and functions of the following in human testis :
(i) Sertoli cells
(ii) Leydig cells
Answer : (i) Location : Lined inside the seminiferous tubules. 
Function : Provide nutrition to the germ cells / sperms
(ii) Location : Outside seminiferous tubules / Interstitial space.
Function : Synthesize or secrete male hormones / Testicular hormones / Androgens. 

Short Answer Type Questions – ll

Question.

The above diagram shows human male reproductive system
(i) Identify ‘X’ and write its location in the body.
(ii) Name the accessory gland ‘Y’ and its secretion.
(iii) Name and state the function of Z.
Answer : (i) X — Testicular lobules, location : Testis, outside the abdominal cavity scrotum.
(ii) Y — Seminal vesicle, seminal plasma.
(iii) Z — Epididymis, function : Storage of sperms.

Question. Name the male accessory glands in humans and write their functions.
Answer : (Paired) seminal vesicles, prostate, bulbourethral glands (paired)
Functions : Secretions constitute the seminal plasma, which is rich in fructose / calcium and certain enzymes, lubrication of penis

Question. Differentiate between vas deferens and vasa efferentia. U [Outside Delhi Set-III, 2014]
Answer :

Question. Name and explain the role of inner and middle walls of the human uterus.
Answer : Inner : Endometrium.
Supports foetal growth, helps in placenta formation after implantation.
Middle : Myometrium. ½
Exhibits strong contraction during delivery of baby

Question.

The above diagram shows a part of the human female reproductive system.
(i) Name the gamete cells that would be present in ‘X’ if taken from a newborn baby.
(ii) Name Y and write its function.
(iii) Name Z and write the events that take place here.
Answer :
(i) X = Primary oocytes
(ii) Y = Fimbriae, collection of ovum
(iii) Z = ampullary – isthmic junction/fallopian tube, the ovum encounters the sperms/ fertilization takes place.

TOPIC-2
Gametogenesis and Menstrual Cycle

Very Short Answer Type Questions

Question. Differentiate between spermatogenesis and spermiogenesis.
Answer :

Question. When do the oogenesis and the spermatogenesis initiate in human females and males, respectively ?
Answer : Oogenesis starts in female in their foetal stage while spermotogenesis in males starts at puberty.

Question. Mention the difference between spermiogenesis and spermiation.
Answer : Spermiogenesis is the process of development of spermatozoan from spermatids, while spermiation is the maturation and releasing of sperms.
Spermiogenesis occurs over sertoli cells while during spermiation, the sperms detach from sertoli cells.

Question. How does the sperm penetrate through the zona pellucida in the human ovum ?
Answer : Acrosome present at the tip of the sperm helps to penetrate through the zona pellucida in human ovum.

Question. Where is acrosome present in humans ? Write its function.
Answer : It is present at the tip or head of the sperm. It helps to penetrate through the zona pellucida in human ovum.

Question. Where is acrosome present in humans ? Write its functions. 
Answer : The acrosome is a cap like structure, present in the head region of human sperm. It contains enzymes which help in dissolving the envelop of ovum to facilitate the entry of sperm into the ovum through zona pellucida and plasma membrane.

Question. Write the function of each of the following :
(i) Luteinizing hormone in human males
(ii) Middle piece of human sperm.
Answer : (i) Luteinizing hormone (LH) stimulate leydig cell to secrete androgens which in turn stimulate spermatogenesis.
(ii) There are numerous mitochondria in the middle piece of sperms which produce energy which is required for the movement of sperms.

Question. Write the effect of the high concentration of L.H. on a mature graafian follicle.
Answer : Rupture of graafian follicle, release of ovum / oocyte / ovulation.

Question. Write the physiological reason, why a woman generally cannot conceive a child after 50 years of age ?
Answer : In human beings, menstrual cycle ceases around at 50 years of age and there will be no production of egg. It is the phase in a woman’s life when menopause occurs, ovulation and menstruation stop. Hence, a woman cannot conceive a child after 50 years of age.

Short Answer Type Questions – l

Question. How is ‘oogenesis’ markedly different from ‘spermatogenesis’ with respect to the growth till puberty in the humans ?
Answer : Oogenesis is initiated at the embryonic stage.
Spermatogenesis begins only at puberty. 

Question. Name the stage of human embryo at which it gets implanted. Explain the process of implantation.
Answer : The embryo gets implanted at blastocyst stage.
The trophoblast gets attached to the endometrium uterine wall of mother by a process called implantation and the inner mass of cells gets differentiated as embryo. After attachment the uterine cells divide rapidly and cover the blastocyst. 
As a result the blastocyst gets embedded in the endometrium of the uterus.

Question. Enumerate the events in the ovary of a human female during :
(i) Follicular phase,
(ii) Luteal phase of menstrual cycle.
Answer : (i) Follicular phase : Follicular phase is also called the proliferative phase. During the phase, the FSH released by the pituitary stimulates the growth of the primary ovarian follicles and also causes maturation of the primary oocyte in this follicle. The follicular cells of the Graafian follicle secrete estrogen. Due to an increased level of estrogen the uterine endometrium becomes thick, more vascular and more glandular. This phase lasts for about 10 to 14 days or until ovulation occurs. 
(ii) Luteal phase : Development of corpus luteum,
secretion of progesterone.

Question. (i) Arrange the following hormones in sequence of their secretion in a pregnant woman.
(ii) Mention their source and the function they perform : hCG; LH; FSH; Relaxin.
Answer : (i) FSH, LH, hCG, relaxin (all four hormones in correct sequence = (1 if less than four correct = 
(ii) FSH : Anterior pituitary, stimulates follicular development
LH : Anterior pituitary,

Question. Mention the relationships between pituitary and ovarian hormones during a menstrual cycle.
Answer : FSH stimulate follicular development and secretion of estrogen.
LH induces ovulation and development of corpus luteum which secretes progesterone.

Question. Mention the relationship between concentration of luteinising hormone and maintenance of endometrium in the human uterus.
Answer : (Mid cycle) LH surge 􀁯 formation of corpus luteum 􀁯 progesterone, maintain the growth of endometrium. 

Short Answer Type Questions – l

Question. Explain the events in a normal woman during her menstrual cycle on the following days :
(i) Ovarian event from 13-15 days
(ii) Ovarian hormones level from 16 to 23 days
(ii) Uterine events from 24 to 29 days.
Answer : (i) Rupture of Graafian follicle leads to ovulation/ release of ovum.
(ii) Estrogen level is low.
(iii) Disintegration of endometrium and menstrual cycle begins again.

Question. Explain the hormonal control of spermatogenesis in humans. 
Answer : Spermatogenesis is under the control of endocrine hormones. In human male reproductive system, spermatogenesis starts at the age of puberty due to significant increase in the secretion of gonadotropin-releasing hormone by hypothalmus.
The increased level of GnRH then acts on the anterior pituitary gland and stimulates secretion of the two gonadotropins : Luteinising hormone (LH) and follicle stimulating hormone (FSH). LH acts on leydig cells and stimulates synthesis and secretion of androgens like testosterone which stimulates
spermatogenesis. FSH acts on the sertoli cells and stimulates secretion of some factors which help in the process of spermiogenesis. Excess of testosterone inhibits the secretion of LH and GnRH. A glycoprotein called inhibin suppresses FSH synthesis. Thus normal release of testosterone is under negative feed back control.

Question.. Explain the events in a normal woman during her menstrual cycle on the following days :
(i) Pituitary hormone levels from 8 to 12 days
(ii) Uterine events from 13 to 15 days
(iii) Ovarian events from 16 to 23 days.
Answer : (i) FSH and LH levels – low
(ii) Endometrium is highly vascularised, proliferative phase of uterine lining.
(iii) Formation of corpus luteum, secretion of progesterone.

Question. Identify A, B, C and D with reference to gametogenesis in humans in the flow chart given below :

Answer : A – Pituitary
B – FSH
C – Spermatogenesis
D – Spermiogenesis

Question. When do the oogenesis and spermatogenesis initiate in human females and males respectively ?
Answer : Oogenesis in females is initiated in the foetal or embryonic stage whereas spermatogenesis in males start at puberty. Sperm formation (spermatogenesis) in men continues even upto old age but formation of ovum in women ceases at about the age of 50 years with menopause.

Long Answer Type Questions

Question. Differentiate between spermatogenesis and oogenesis.
Answer : Oogenesis is different from that of spermatogenesis in the following ways: –
(a) In spermatogenesis, continuous production of sperms takes place from puberty to old age. Whereas, in oogenesis, the oocytes are generated before birth/ in the foetus.
(b) A large number of these oocytes degenerate during the phase from birth to puberty. The oocytes continuously decrease in number and it gets completely exhausted at menopause.
(c) A primary spermatocyte completes the first meiotic division leading to formation of two equal sized secondary spermatocytes whereas, Primary oocyte divides unequally by the first meiotic division resulting in the formation of a large haploid secondary oocyte and a tiny first polar body. 1
(d) Meiotic division of secondary oocyte gets
temporarily arrested at Prophase-I stage. It is completed only when a sperm comes in contact with the zona pellucida layer of the ovum. There is no such event in spermatogenesis.
(e) Spermatogonium produces 4 functional spermatozoa whereas an oogonium produces one functional ovum and 3 non-functional polar bodies.

Question. Explain the process of development of a mature human female gamete from the time it is initiated till the formation of ootid.
Answer :

Question. Explain the process of spermatogenesis in humans.
Answer : Spermatogenesis is the formation of sperm in seminiferous tubules of testis. It has two stages :
(i) Formation of Spermatids
(ii) Spermiogenesis.
In the formation of spermatids, the sperm mother cells or Spermatogonia produce spermatids and in spermiogenesis, the spermatids are transformed into sperms or spermatozoan. At the time of sexual maturity, the primary germ cells divide mitotically to form a large number of spermatogonia which later on grow in size and form primary spermatocytes. The primary spermatocyte undergo meiosis-I to produce small haploid secondary spermatocytes which divide by meiosis II to form spermatids. The spermatids under the influence FSH is metamorphosed into spermatozoa. The schematic representation of spermatogenesis is as follows

Question. The graph given below shows the variation in the levels of ovarian hormones during various phases of menstrual cycle : E & A
(i) Identify ‘A’ and ‘B’.
(ii) Specify the source of the hormone marked in the diagram.
(iii) Reason out : why A peaks before B.
(iv) Compare the role of A and B.
(v) Under which condition will the level of B continue to remain high on the 28th day?
Answer : (i) A–Estrogen B–Progesterone
(ii) A–Maturing ovarian follicle/Graafian follicle B–Corpus luteum
(iii) Formation of Graafian follicle (releases estrogen) is followed by the formation of corpus luteum (releases progesterone)
(iv) Role of A (Estrogen)—Leads to changes in the ovary and uterus/regeneration of endometrium through proliferation Role of B (Progesterone)—Maintenance of endometrium for implantation of the fertilized ovum / maintenance of other events of pregnancy
(v) In case of pregnancy

Question. Given below is the diagram of a human ovum surrounded by a few sperms. Observe the diagram and answer the following questions :

(i) Compare the fate of sperms shown in the diagram.
(ii) What is the role of zona pellucida in this process ?
(iii) Analyze the changes occurring in the ovum during the process.
(iv) How is the entry of sperm into the ovum facilitated ?
(v) Specify the region of female reproductive system where the event represented in the diagram takes place.
Answer :
(i) A is able to penetrate / fertilize the ovum, whereas B and C are unable to penetrate / fertilize // B and C will degenerate.
(ii) Zona pellucida ensures the entry of only one sperm into the ovum
(iii) Induces completion of meiotic division of the secondary oocyte, formation of second polar body and a haploid ovum
(iv) Enzymes of acrosome help ‘acrosome‘ is written)
(v) Ampullary – isthmic junction of the fallopian tube

Question. (i) Briefly explain the events of fertilization and implantation in an adult human female.
(ii) Comment on the role of placenta as an endocrine gland. 
Answer : (i) (a) On the 14th day of menstrual cycle, ovulation takes place which releases secondary oocyte.
(b) This secondary oocyte is caught by fimbriae and it starts moving up the fallopian tube.
(c) In the meantime, sperms which have been deposited in vagina start moving up and reach the fallopian tube.
(d) The two meet at ampullary isthmus junction and fuse together.
(e) After entry of sperm secondary oocyte completes its meiosis II, changes the ovum and fuse with sperm’s pronuclei leading to zygote formation.
(f) This zygote after sometime starts dividing. It changes to morula (8-16 celled), which continues to divide to form the blastocyst. The morula moves further into the uterus.
(g) The cells in the blastocyst are arranged into an outer trophoblast and an inner cell mass.
(h) The trophoblast gets attached to the uterine endometrium and the process is called implantation. This leads to pregnancy. The inner cell mass gets differentiated to form the embryo. 3
(ii) Placenta as endocrine gland produces :
(a) hCG (Human Chorionic Gonadotropin)
(b) hPL (Human Placental Lactogen)
(c) Oestrogen
(d) Progesterone
(e) In the later stages of pregnancy, it also releases relaxin. These hormones support foetal growth and help in maintaining pregnancy.

Question. When and where are primary oocytes formed in a human female ? Trace the development of these oocytes till ovulation (in menstrual cycle). How do gonadotropins influence this developmental process ?
Answer : (i) The primary oocytes are formed in the foetal ovary during the development of the foetus. The ovum is formed by the process of oogenesis.
(ii) During embryonic growth, millions of gamete mother cells (oogonia) are formed in the foetal ovary. These cells undergo meiosis, but get temporarily arrested at prophase and are called primary oocytes. Before reaching puberty, a large number of primary oocytes degenerate and the remaining ones get surrounded by layers of granulosa cells and a new theca. These are called secondary follicles. The secondary follicles are then converted into tertiary follicles that have characteristic fluid-filled cavity called antrum. At this stage, the primary oocyte present within the tertiary follicle completes meiosis, which results in the formation of haploid secondary oocyte and a tiny polar body. This tertiary follicle further changes into the Graafian follicle. The secondary oocyte in the Graafian follicle is surrounded by the zona pellucida. Then, the Graafian follicle ruptures to release the ovum by ovulation.
(iii) Role of gonadotropins : The gonadotropins (Follicle Stimulating Hormone and the Luteinising Hormone) are released by the anterior portion of the pituitary gland. The maturation of the primary follicles into the Graafian follicles occurs during the follicular phase. The secretion of gonadotropins increases during this phase, cause follicular growth and the growing follicles produce oestrogen. The LH and FSH are at their peak in the middle of the cycle (14th day) and cause the rupture of the Graafian follicles to release ovum. This phase is called the ovulatory phase.

Question. Explain the ovarian and uterine events that occur during a menstrual cycle in a human female, under the influence of pituitary and ovarian hormones respectively.
Answer :
Menstrual cycle is the reproductive cycle in all primates and begins at puberty (menarche). In human females, menstruation occurs once in 28 to 29 days. The cycle of events starting from one menstruation till the next one is called the menstrual cycle. During the middle of the menstrual cycle one ovum is released (ovulation). The cycle starts with the menstrual flow (3 to 5 days), caused due to the breakdown of the endometrium of the uterus. Blood vessels ruptures, causing bleeding. But this occurs only when the ovum is not fertilised. It is followed by the follicular phase where the primary follicles mature into Graafian follicles. This causes the regeneration of the endometrium.
These changes are brought about by ovarian and pituitary hormones. In this phase, the release of gonadotropins (LH and FSH) increases. This causes follicular growth and the growing follicles produce oestrogen. The LH and FSH are at their peak in the middle of the cycle (14th day) and cause the rupture
of the Graafian follicles to release ovum. This phase is called the ovulatory phase. The remains of the Graafian follicles get converted into the corpus luteum, which secretes progesterone for the maintenance of the endometrium. In the absence of fertilisation, the corpus luteum degenerates, thereby causing the disintegration of the endometrium and the start of a new cycle.

Question. (a) Explain menstrual cycle in human females.
(b) How can the scientific understanding of the menstrual cycle of human females help us a contraceptive measure ?
Answer :
(a) (i) Menstrual Phase : Menstrual flow occurs / due to breakdown of endometrial lining of uterus, when fertilization does not occur.
(ii) Follicular Phase : Primary follicles grow into mature graafian follicles and endomentrium regenerates through proliferation, changes induced by pituitary and ovarian hormones.
(iii) Ovulatory Phase : LH surge, induces rupture of graafian follicle and release of secondary oocyte /ovum during middle of cycle (i.e. 14th day).
(iv) Luteal phase : Ruptured graafian follicle transforms into corpus luteum which secrete large amount of progesterone, essential for maintaining endometrium. 
(b) Because ovulation occurs during mid cycle chances of fertilization are very high so, couples should abstain from coitus between day 10 – 17.

TOPIC-3
Fertilization and Post-Fertilization Events

Very Short Answer Type Questions

Question. Name the embryonic stage that gets implanted in the uterine wall of human female.
Answer : Blastocyst.

Question. Explain the function of myometrium and endometrium in human females.
Answer : (i) The myometrium exhibits strong contraction during parturition.
(ii) Endometrium prepares uterus for pregnancy by undergoing cyclic changes.

Question. How is the entry of only one sperm and not many ensured into an ovum during fertilization in humans?
Answer : During fertilization, a sperm comes in contact with the zona pellucida layer of the ovum and induces changes in the membrane that blocks the entry of additional sperms.

Question. During fertilization hundreds of sperms are in the vicinity of an egg cell; but only one sperm enters the ovum. How is this achieved.
Answer : During fertilization when a sperm comes in contact with the egg it induces changes in zona pellucida of ovum and this blocks the entry of any other sperm.
This ensures the entry of only one sperm.

Question. What stimulates pituitary to release the hormone responsible for parturition ? Name the hormone.
Answer : Foetal ejection reflex that originates from the fully developed foetus and placenta is responsible for parturition. The hormone is oxytocin.

Question. Explain the function of umbilical cord.
Answer : Umbilical cord connects placenta with the foetus. It helps in the transport of nutrients, respiratory gases and metabolic wastes to and fro between mother and foetus.

Question. Mention the function of trophoblast in human embryo.
Answer : It helps in implantation and leads to pregnancy.

Short Answer Type Questions – l

Question. Where does fertilization occur in humans ? Explain the events that occur during this process.
Answer :
Fertilization in humans occurs at ampullaryisthmic junction of the fallopian tube. Secretions of acrosome of sperms help it to enter the ovum through zona pellucida and plasma membrane. It induces meiosis II to form haploid ovum (oocyte/ ootid). The fusion of sperm and ovum to form zygote is called fertilization.

Question. (i) Where do the signals for parturition originate from in humans ?
(ii) Why is it important to feed the newborn babies on colostrum ?
Answer : (i) From the fully developed foetus / placenta / foetal ejection reflex.
(ii) It contains antibodies (IgA), provides immunity to the child.

Question. Differentiate between morula and blastocyst.
Answer :
Morula is a mulberry like solid mass of 8-16 cells called blastomeres, formed by cleavage of zygote, while the blastocyst is a hollow sphere of 64 cells formed by the rearrangement of blastomeres of morula. It has a cavity called blastocoel and an inner cell mass within. It also has an outer envelop of cells called the trophoblast.

Question. Explain the events that follow upto fertilization when the sperms come in contact with the ovum in the fallopian tube of a human female.
Answer : The secretion of the acrosome help the sperm to enter into the cytoplasm of ovum through zona pellucida and the plasma membrane, this induces
the completion of second meiotic division of the secondary oocyte, forming second polar body and a haploid ovum, soon the haploid nucleus of the sperm and the ovum fuse together to form a diploid zygote.

Question. What happens to corpus luteum in human female if the ovum is (i) fertilized, (ii) not fertilized?
Answer :
(i) Corpus luteum continues to secrete progesterone to maintain pregnancy, it persists and produces progesterone.
(ii) It disintegrates and changes into corpus albicans.

Question. When and where do chorionic villi appear in humans ? State their function.
Answer : Chorionic villi are finger like projections that arise from the trophoblast layer that develops in the blastocyst after it has undergone implantation in
the uterus. The chorionic villi and uterine tissue get interdigitated with each other to form a structural and functional unit between embryo (foetus) of maternal body, called as placenta. Placenta is connected with the embryo by umbilical cord. It helps to transport substances to and from the embryo. 

Question. State the role of Oxytocin in parturition. What triggers its release from the pituitary ?
Answer : Oxytocin acts on uterine muscle, and cause stronger uterine contraction, leading to expulsion of the foetus/baby out of uterus. The signals from fully formed foetus and the placenta which induce mild uterine contraction/ foetal ejection reflex triggers release of oxytocin.

Question. Write the function of each one of the following :
(i) (Oviducal) Fimbriae
(ii) Oxytocin
Answer :
(i) Collection of ovum released by ovary.
(ii) Causes uterine contraction for parturition / promotes milk ejection.

Short Answer Type Questions – ll

Question. (i) How is placenta formed in the human female ?
(ii) Name any two hormones which are secreted by it and are also present in a non–pregnant woman.
Ans. (i) After implantation, the chorionic villi that appear on trophoblast, interdigitate with the uterine tissue, jointly form placenta. 
(ii) Estrogen, progesterone.

Question. Describe the process of parturition in humans.
Answer :
(i) Signals originate from the fully developed foetus and placenta.
(ii) Induce mild uterine contractions (foetal ejection reflex).
(iii) Triggers release of oxytocin (from maternal pituitary).
(iv) Oxytocin acts on uterine muscles and causes stronger uterine contractions.
(v) Stimulatory reflex between the uterine contraction and oxytocin secretion continues resulting in stronger and stronger contraction.
(vi) Expel the baby from the uterus

Question. State any three functions of placenta in human female.
Answer :
(i) Supply oxygen to the embryo/nutrients to the embryo.
(ii) Removes carbon-dioxide/removes waste material produced by the embryo.
(iii) Produces hormones like hCG—human
Chorionic Gonadotropin/hPL—human
Placental Lactogen/ estrogen/progesterone.

Question. Describe the embryonic development of a zygote upto its implantation in humans.
Answer : Zygote moves through isthmus and undergoes cleavage (forming morula), morula continues to divide and transform into blastocyst (as it moves further into uterus), Blastomeres in the blastocyst are arranged into an outer layer trophoblast, and inner cell mass, the trophoblast layer gets aattached to endometrium, uterine cells divide and cover the blastocyst.

Question. After implantation, interdigitation of maternal and foetal tissues takes place. Identify the tissues involved and justify their role.
Answer : After implantation, interdigitation of maternal and foetal tissues results in the formation of structural and functional unit between embryo and maternal body called placenta. It facilitates supply of oxygen and nutrients to the embryo, removal of carbon dioxide and excretory material. Also acts as endocrine tissue and produces hormones like hCG, hPL, estrogen, progesterone, relaxin. 

Long Answer Type Questions

Question. Arrange the terms given below in their order of occurrence describing their structure and function in the early development of the human embryo: Implantation; cleavage; inner cells mass; trophoblast; blastomeres; endometrium; morula; blastocyst.
Answer : Correct order = cleavage, blastomeres, morula, blastocyst, trophoblast, inner cell mass, endometrium, implantation.
Cleavage : Mitotic division starts as the zygote moves through the oviduct towards the uterus and form (2/4/8/16) daughter cells called blastomeres.
Embryo with 8 to 16 blastomeres is called a morula. Morula continues to divide and transforms into a blastocyst. Blastomeres in blastocyst are arranged into an outer layer called trophoblast and an inner group of cells attached to trophoblast called the inner cell mass. The trophoblast layer then gets attached to the endometrium and inner cell mass differentiates as the embryo. Ultimately blastocyst becomes embedded in the endometrium of uterus. This is called Implantation.

Question. (i) Where in the fallopian tube does fertilization occur in humans ? Describe the development of a fertilized ovum upto implantation.
(ii) How is polyspermy prevented in humans ?
Answer : (i) Ampullary region / ampullary isthmic junction.
(a) Cleavage occurs in zygote to form 2 – 4 – 8 – 16 daughter cells / upto 16 daughter cells called blastomeres.
(b) 8 – 16 / 16 blastomeres stage called morula.
(c) Morula continues to divide and transform blastocycst (as it moves further into uterus).
(d) Blastomeres in the blastocyst are arranged into an outer layer called trophoblast which gets attached to endometrium.
(e) Inner group of cells called inner cell mass get differentiated into embryo.
(f) Uterine cells divide rapidly and covers blastocyst / implantation. × 6
(ii) When a sperm comes in contact with a zona pellucida layer of ovum, it induces changes in membrane to block entry of additional sperms.

Question. Mention the site of fertilisation of a human ovum.
List the events that follow in sequence until the implantation of the blastocyst.
Answer : The site of fertilisation is the ampullary isthmic junction (fallopian tube).
Events :
Fertilization (Zygote formation), → Cleavage, → 8-16 Blastomeres stage – Morula → Continuously divides and transforms into Blastocyst (as it moves into uterus) → Blastomeres in the Blastocystarrange to form outer layer trophoblast and inner cell mass → Trophoblast attaches to endometrium and inner cell mass differentiates as embryo → After attachment of blastocyst the uterine cells cover it → Embed blastocyst in the endometrium – implantation.

Question. (i) Name the hormones secreted and write their functions :
(a) By corpus luteum and placenta (any two).
(b) During Follicular phase and parturition.
(ii) Name the stages in a human female where :
(a) Corpus luteum and placenta co-exist.
(b) Corpus luteum temporarily ceases to exist.
Answer : (i) (a) Corpus luteum : progesterone, essential for maintenance of the endometrium. Placenta : hCG / human chorionic gonadotropin, produce during pregnancy / stimulates and maintains the corpus luteum / to secrete progesterone / growth of mammary glands. HPL / human placental lactogen, produced during pregnancy. Estrogen, maintenance of pregnancy / supporting foetal growth / metabolic changes in mother (Any two)
(b) Follicular phase : LH / FSH, stimulates follicular development / secretion of estrogen by growing follicles. Parturition : oxytocin causes stronger uterine contraction / relaxin, secreted during (later stage) the pregnancy / softens symphysis pubis.
(ii) (a) Pregnancy / gestation.
(b) Menstruation / proliferative phase / ovulatory phase / follicular phase.

Question. During the reproductive cycle of a human female, when, where and how does a placenta develop?
What is the function of placenta during pregnancy and embryo development ?
Answer : After implantation uterus, chorionic villi and uterine tissue become interdigitated (physically fused).
Placenta facilitates supply of oxygen / nutrients to the embryo.
Removal of carbon dioxide / waste material / excretory material produced by the embryo. Production of hCG / hPL / estrogens / progesterone.

Question. (i) Briefly explain the events of fertilization and implantation in an adult human female.
(ii) Comment on the role of placenta as an endocrine gland.
Answer : (i) Fertilization :
(a) Sperm comes in contact and enters the secondary oocyte.
(b) Activates/induces secondary oocyte to complete meiosis II, leads to formation of ovum/ootid.
(c) The haploid nucleus of sperm and ovum fuse to form a diploid zygote completing the process of fertilization.
Implantation :
(a) Trophoblast layer of blastocyst attaches to the endometrium (of the uterus).
(b) The uterine cells divide rapidly and cover the blastocyst.
(c) The blastocyst becomes embedded in the endometrium and the implantation is completed.
(ii) (a) hCG (human chorionic gonadotropin)
(b) hPL (human placental lactogen)
(c) estrogen
(d) progesterone

Question. Where does fertilization occur in humans ? Explain the events that occur during this process.
Answer : (i) Ampullary Isthmic junction in fallopian tube / fallopian tube.
(ii) The sperms come in contact with zona pellucida.
(iii) Induces change in the membrane.
(iv) Blocks entry of other sperms / ensures only one sperm fertilizes the ovum / prevents polyspermy.
(v) The secretion of acrosome helps the sperm to enter the cytoplasm.
(vi) Entry of sperm induces completion of second meiotic division forming ovum and 2nd polar body.
(vii) The haploid nucleus of sperm and ovum fuse.
(viii) Formation of diploid zygote, fertilisation completed.

Question. ‘Parturition is induced by a complex Neuro endocrine mechanism’. Justify
Answer : Parturition is a process where vigorous contraction of the uterus at the end of pregnancy causes expulsion/delivery of the foetus.
The signals for parturition originate from the fully developed fetus and the placenta which induce mild uterine contractions called foetal ejection reflex.
This triggers release of oxytocin from the maternal pituitary. Oxytocin acts on the uterine muscles and causes stronger uterine contractions, which
in turn stimulates further secretion of oxytocin. The stimulatory reflex between the uterine contraction and oxytocin secretion continues resulting in stronger and stronger contractions. This leads to expulsion of the baby out of the uterus through the birth canal – parturition, after the infant is delivered; the placenta is also expelled out of the uterus.

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