Biology 5 Marks Questions

The process of the formation of a mature ovum in the ovary from the oogonia in females is known as oogenesis.
Germ cell of the female foetus divide to produce oogonia (gamete mother cell). No more oogonia are formed or added after birth. A diploid oogonium or egg mother cell increases in size and gets transformed into a diploid primary oocyte. This diploid primary oocyte undergoes meiosis or reductional division to form two unequal haploid cells. The smaller cell is known as the first polar body, while the larger cell is known as the secondary oocyte. This secondary oocyte undergoes meiosis II or equational division and gives rise to a second polar body and an ovum. Hence, in the process of oogenesis, a diploid oogonium produces a single haploid ovum while two or three polar bodies are produced.

Menstrual cycle is the cyclic change (itf' the reproductive tract of primate female. This period is marked by a characteristic event repeated almost every month (28 days with minor variation) in the form of a menstrual flow (i.e shedding of the endometrium of the uterus with bleeding). It may be temporarily stopped only in pregnancy.
The hormones that regulates menstrual cycles are:

1. FSH (Follicle stimulating hormone).
2. LH (Luteinizing hormone).
3. Oestrogens.
4. Progesterone.

The process of the production of sperms from the immature germ cells in males is called spermatogenesis. The male germ cell of seminiferous tubules multiply mitotically to increase in numbers. Each spermatogonium is diploid and contains 46 chromosomes. Some of the spermatogonia called primary spermatocytes periodically undergo meiosis. A primary spermatocyte completes the first meiotic division (reduction division) leading to formation of two equal, haploid cells called secondary spermatocytes, which have only 23 chromosomes each. The secondary spermatocytes undergo the second meiotic division to produce four equal, haploid spermatids. The spermatids are transformed into spermatozoa (sperms) by the process called spermiogenesis.

1. Corpus luteum:

It acts as an endocrine gland and secretes progesterone hormone which is essential for endometrium of uterus.

2. Endometrium:

It is innermost layer of uterus responsible for nutrition and development of the foetus. It undergoes cyclic changes during menstrual cycle and prepares itself for implantation of blastocyst and placentation.

3. Acrosome:

Acrosome of sperm head contains hydrolytic enzymes that help in dissolving membranes of the ovum for fertilization.

4. Sperm tail:

It helps in the movement of sperm essential for fertilisation.

5. Fimbriae:

It help in collection of the ovum after ovulation.

Vigorous contraction of the uterus at the end of the pregnancy causes expulsion/ delivery of the foetus. This process of delivery of the foetus (childbirth) is called parturition.
Parturition is induced by a complex neuroendocrine mechanism. The signals for parturition originate from the fully developed foetus 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 muscle 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. Soon after the infant is delivered, the placenta is also expelled out of the uterus.

Following figure shows various stages of zygote development till implantation:

1. The zygote divides into two-cell stage called cleavage. This development takes place when the zygote is still in the isthmus.
2. The cleavage undergoes further rounds of mitosis to form 2, 4, 8 and finally 16-celled stage. These daughter cells are called blastomere.
3. The 16-celled stage is called morula.
4. The morula changes into blastocyst; with more rounds of meiosis. At this stage, the blastomeres are arranged in an outer layer of cells; called trophoblast and an inner mass of cells.
5. Implantation to uterine wall happens at the blastocyst stage.

Corpus luteum is illustrated by ‘g’ and the hormone influencing its formation is luteinising hormone (LH).

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.

Primary follicle grows into Graafian follicle under the influence of LH and FSH leading to regeneration of endometrium (under the influence of estrogen).

Oogenesis is initiated at the embryonic stage whereas spermatogenesis begins only at puberty.

Gametogenesis, i.e., spermatogenesis in males starts at puberty and oogenesis in females, start during embryonic development at foetal stage.

Formation of Graaffian follicle (releases estrogen) is followed by the formation of corpus luteum (releases progesterone).

Diagrammatic sectional view of a seminiferous tubule (enlarged). Hormonal control of spermatogenesis.

Schematic representation of spermatogensis.

Vas deferens: It stores and transports the sperms from the testes to the outside through urethra.

Seminiferous tubules → Rete testis → Vasa efferentia → Epididymis → Vas deferens → Ejaculatory duct.
OR



Schematic representation of spermatogensis.

1. Zona pellucida.
2. The acrosome releases enzymes that dissolve the zona pellucida and plasma membrane of the ovum for the sperm to enter the cytoplasm of the ovum.
3. Ampullary-isthmic junction of fallopian tube.
4.  

• The zygote undergoes mitosis in quick succession (cleavage) as it moves through the isthmus towards the uterus and forms 2, 4, 8 and 16 daughter cells, called blastomeres.
• The embryo with 8-16 blastomeres, is called a morula.

Uterus (Womb):

• It is a single, hollow, muscular, pear-shaped structure supported by ligaments and attached to the pelvic wall.
• It is present between the urinary bladder and the rectum.
• The lower part of the uterus is very narrow and is called cervix. The cavity of the cervix is called cervical canal, which forms the birth canal along with vagina.
• The wall of the uterus has three layers of tissue:
• Perimetrium: Outer, thin covering of the uterus wall.

• Myometrium: Middle, thick layer of smooth muscle fibres which contracts strongly during delivery of the baby.
• Endometrium: Inner layer, that contains glands and many blood vessels. It undergoes cyclical changes during the menstrual cycle.

Follicular stimulating hormone (FSH) stimulate follicular development. Luteinising hormone (LH) ruptures Graafian follicle and thereby release ovum.

1.  

A. Placental villi.

B. Umbilical cord with its vessels.

C. Embryo.

2.  

3. Vigorous contraction of the uterus at the end of pregnancy causes delivery of the foetus. by the process known as parturition. It is induced by a complex neuroendocrine mechanism. The signals originate from the fully developed foetus and the placenta which induces uterine contractions known as foetal ejection reflex. This triggers release of oxytocin from the maternal pituitary which acts on the uterine muscle and causes stronger uterine contractions, that in turn stimulates further secretion of oxytocin. The stimulatory reflex continues causing stronger contractions which leads to the expulsion of the baby out of the uterus through the birth canal known as parturition.

The mitotic division starts as the zygote moves through the isthmus of the oviduct called cleavage towards the uterus and forms 2, 4, 8, 16 daughter cells called blastomeres. The embryo with 8 to 16 blastomeres is called a morula. The morula continues to divide and transforms into blastocysts, it moves further into the uterus.

1. The blastomeres in the 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 the inner cell mass gets differentiated as the embryo.
2. After attachment, the uterine cells divide rapidly and covers the blastocyst. As a result the blastocyst becomes embedded in the endometrium of the uterus. This is called implantation and it leads to pregnancy.

a-ovum.

Schematic representation of oogenesis:
Systematic representation of oogenesis.

“d” is the developing tertiary follicle.
“e” is the Graafian follicle.

Pituitary hormones: FSH: Secreted by the anterior lobe of pituitary gland, it stimulates follicular development as well as secretion of estrogens by the growing follicles.LH:

1. It induces rupture of Graafian follicle and thereby release of ovum (ovulation).
2. Its secretion also stimulates follicular development along with FSH.

Ovarian hormones: Estrogen: It stimulates the proliferation of the endometrium of the uterine wall. Progesterone: It is essential for maintenance of the endometrium for implantation of fertilised ovum and other events of pregnancy. It is secreted by corpus luteum.

1.  

• Sertoli cells: Secrete factors which help in the process of spermiogenesis/ provide nutrition to germ cells.
• Spermatogonia: Divide to produce spermatids/ sperms.
• Leydig cells: systhesis or section of androgens/ testosterone.

2. The role of pituitary and sex hormones in the process of spermatogenesis is as follows:

• Pituitary hormones: LH/ luteinizing hormone – acts on leydig cell and arouses synthesis and secretion of androgens, FSH/ follicle stimulating hormone- acts on sertoli cells and arouses secretion of some factor that help in spermiogenesis.
• Sex hormone: (Androgen/ testosterone) arouse process of spermatogenesis.

Fertilisation:

• The process of fusion of a sperm (male gamete) with an ovum (female gamete) is called fertilisation.
• During coitus, semen is released by the penis into the vagina (insemination).
• The motile sperms swim rapidly through the cervix, enter into the uterus and reach the ampullary – isthmic junction of the oviduct.
• A sperm comes in contact with the zona pellucida layer of the ovum and induces changes in the membrane to block the entry of additional sperms.
• The enzymes of the acrosome of sperm help to dissolve zona pellucida and plasma membrane of the ovum and sperm head is allowed to enter into the cytoplasm of the ovum, i.e., secondary oocyte.
• Ultimately diploid zygote is produced by the fusion of a sperm and an ovum.

Implantation:

• Zygote divides rapidly by mitotic division as it moves through isthmus of oviduct towards uterus. This is called cleavage. As a result 2, 4, 8, 16 daughter cells are produced which are termed as blastomeres.
• Embryo with 8–16 blastomeres is called a morula.
• The morula continues to divide and transforms into a large mass of cells called blastocyst, which passes further into the uterus.
• Blastomeres in the blastocyst are arranged into an outer layer called trophoblast and an inner group of cells attached to trophoblast called inner cell mass.

• Transport of ovum, fertilisation and passage of growing embryo through fallopian tube.
• The trophoblast layer gets attached to the cells of the endometrium and the inner cell mass gives rise to the embryo.
• After attachement, the cells of endometrium divide rapidly and cover the blastocyst.
• So, the blastocyst gets embedded in the endometrium of the uterus. This is called implantation, which leads to pregnancy.

Epididymis: Maturation of spermatids to functional spermatozoa takes place here and the sperms are stored temporarily till ejaculation.

S. No.	Hormones Secreted	Their functions
(i) By corpus luteum	1. Progesterone 2. Estradiol	Essential for maintaining endometrium for implantations of fertilised ovum. Also inhibits release of FSH.
By placenta	1. Human Placental lactogen	Ensures supply of energy to foetus. It has anti-insulin property.
	2. hCG (Human chorionic gonadotropin)	Maintains corpus luteum in pregnancy and stimulates release of progesterone.
(ii) During Follicular Phase	1. LH 2. FSH	Stimulates follicular development as well as secretion of estrogen by growing follicles.
During parturition	Oxytocin	Leads to contraction of smooth muscles of myometrium of the uterus during child birth.

Placenta develops after implantation of embryo in human female in the uterus. After implantation, finger-like projections called chorionic villi are surrounded by the uterine tissue and maternal blood. The chorionic villi and uterine tissue become interdigitated with each other and together form a structural and functional unit between developing embryo and maternal body called placenta.

Spermatogenesis:The process of formation of spermatozoa (sperms) from diploid spermatogonia is called spermatogenesis.
It includes the following phases:
Multiplication phase: The male germ cells (spermatogonia) present on the inside wall of seminiferous tubules multiply by mitotic division and increase in numbers.
Growth phase: Spermatogonia grow and increase in size and form primary spermatocytes. Each spermatogonium is diploid and contains 46 chromosomes.
Maturation phase or formation of spermatids: Some of the spermatogonia called primary spermatocytes periodically undergo meiosis. A primary spermatocyte completes the first meiotic division (reduction division) leading to formation of two equal haploid cells called secondary spermatocytes, which have only 23 chromosomes each. The secondary spermatocytes undergo the second meiotic division to produce four equal haploid spermatids.
Differentiation phase: The spermatids are transformed into spermatozoa (sperms) by the process of spermiogenesis. The sperm’s head gets attached to Sertoli cells to draw nourishment and are finally released from the seminiferous tubules by the process called spermiation.

The two accessory glands are:

1. Prostate gland: It surrounds the urethra and produces a milky secretion which forms a considerable part of semen. This secretion contains citric acid, lipids and enzymes. The secretion nourishes and activates the spermatozoa to swim.
2. Seminal vesicles: These secrete mucous and a watery alkaline fluid that contains fructose which provides energy to the sperms.

Spermatogenesis occurs at puberty in testes.

S. No.	Spermatogenesis	Oogenesis
(i)	It is the process of formation of haploid spermatozoa from diploid male germ cells of the testes.	It is the process of formation of haploid ova from the gamete mother cells (oogonia) in the ovary.
(ii)	It occurs in testes.	It occurs in ovaries.
(iii)	A primary spermatocyte divides to form two secondary spermatocytes by meiosis I.	A primary oocyte divides by meiosis I to form one secondary oocyte and a polar body.
(iv)	Spermatogonia changes to primary spermatocyte.	Oogonia changes to primary oocyte.
(v)	A secondary spermatocyte divides to form two spermatids by meiosis II.	A secondary oocyte divides by meiosis II to form ovum and a polar body.
(vi)	No polar body is formed.	Polar bodies are formed.
(vii)	Four spermatozoa are produced from one primary spermatocyte.	One ovum/ egg is produced from one primary oocyte.

Structure of a sperm.

The female reproductive system perform the following functions:

1. It forms the female gamete, i.e., the egg.
2. It receives the male gamete, i.e., sperm, for fertilisation.
3. It provides the site for fertilisation.
4. It provides suitable conditions for the development of foetus.
5. It produces hormones that control the stages of ovulation and maintain pregnancy.

The female reproductive system consists of a pair of ovaries along with a pair of oviducts,uterus, cervix, vagina and the external genitalia located in pelvic region. These parts are integrated structurally and functionally to support the processes ofovulation, fertilization, pregnancy and birth.
Labels:

1. Ovary (production of gamete).
2. Isthmus-ampullary junction(site of fertilization).
3. Uterine endometrium (site of implantation).
4. Cervix and vagina (birth canal).

Oogenesis starts during the embryonic developmental stages in the female foetus.The hormones involved are:

1. Follicle stimulating hormone (FSH)
2. Luteinising hormone (LH).
3. Progesterone.

1. Follicle stimulating hormone (FSH):

• FSH stimulates the growth and development of ovarian follicles into a mature Graafian follicle.
• It also stimulates the secretion of estrogen from the follicle cells.

2. Luteinising hormone (LH):

• LH stimulates the rupture of mature follicle and ovulation.
• It also stimulates the formation of corpus luteum from the ruptured follicle and secretion of progesterone from it.

3. Progesterone:

• Progesterone is necessary to maintain the endometrium and implantation.
• In the absence of fertilisation, the corpus luteum degenerates and consequently the endometrium also degenerates leading to menstruation.

Mammary glands are paired structures which contain glandular tissue and variable amount of fat.

• In each breast, glandular tissue is divided into 15 – 20 mammary lobes. These lobes have clusters of cells called alveoli.
• The cells of alveoli secrete milk, which is stored in the cavities (lumens) of alveoli. The alveoli open in mammary tubules.
• Tubules from each lobe join to form a mammary duct.
• Many mammary ducts join to form a wider mammary ampulla.
• A mammary ampulla is connected to the lactiferous duct. Milk is sucked out through the lactiferous duct.

1.  

2.  

1.  

S. No.	Vas deferens	Vasa efferentia
1.	Vas deferens arises from epididymis and opens into urethra.	Vasa efferentia arise from rete testis and open into epididymis.
2.	This conducts the sperm to the urethra and then to outside.	They conduct the sperms out of the testes.
3.	It receives ducts of the associated glands.	They do not receive ducts of any associated glands.
4.	It ascends into the abdominal cavity.	They remain inside the scrotum (extra abdominal).

2.  

S. No.	Spermatogenesis	Spermiogenesis
1.	It is the process of formation of spermatozoa in the testes.	It is the process of transformation of spermatids into spermatozoa.
2.	It involves cell divisions.	It does not involve any cell division.
3.	Spermatogonia of the seminiferous tubules are involved.	Sertoli cells of the seminiferous tubules are involved.

An umbilical cord connects placenta with the embryo.

S. No	Process of gametogenesis	Males	Females
(i)	Time of initiation of the process.	Puberty.	Embryonic development at foetal stage.
(ii)	Products formed at the end of the process.	Four spermatozoa are produced from one primary spermatocyte.	One ovum and two polar bodies are produced from one primary oocyte.

1. Spermatogenesis is the process of the production of sperms from the immature germ cells in males. It takes place in seminiferous tubules present inside the testes. During spermatogenesis, a diploid spermatogonium (male germ cell) increases its size to form a diploid primary spermatocyte. This diploid primary spermatocyte undergoes first meiotic division (meiosis I), which is a reductional division to form two equal haploid secondary spermatocytes. Each secondary spermatocyte then undergoes second meiotic division (meiosis II) to form two equal haploid spermatids. Hence, a diploid spermatogonium produces four haploid spermatids. These spermatids are transformed into spermatozoa (sperm) by the process called spermiogenesis.

2.  

• Acrosome: It is flat, oval in shape and composed of a large haploid nucleus and a small anterior cap-like structure, called acrosome. The acrosome lies at the tip of the nucleus and is formed from the Golgi complex. It contains hydrolytic enzymes and is used to contact and penetrate the egg (ovum) during fertilisation.
• Middle piece: It is cylindrical in shape and known as powerhouse of the sperm. It possesses many mitochondria to produce energy for the movement of the tail that facilitates sperm motility essential for fertilization.

Release of gonodotropins (FSH and LH) from pituitary, during follicular phase or 5-14 days of menstrual cycle leads to growth of primary follicle to Graafian follicle (GF) in the ovary. Estrogen from growing follicle helps proliferation of uterine endometrium or its repair. High level of LH at middle 14th day of the menstrual cycle leads to rapture of GF causing release of ovum. This is called ovulation. The remaining cells of GF transform into corpus luteum (CL) under the influence of LH. CL secretes progesterone that maintains endometrium in preparation for pregnancy. Level of FSH and LH fall due to rise of progesterone and estrogen (25th day of the cycle), leading to degeneration of CL. Level of progesteron falls, leading to disintegration of uterine endometrium and menstruation starts (0-5 day of the cycle).

Hormone ‘X’: Luteinising hormone.
Hormone ‘Y’: Follicle stimulating hormone.

The primary function of the female reproductive system is to produce the female egg cells which are essential for reproduction These are called as the ova or oocytes. It is important to know that, the entire system is designed for transporting the ova to the exact fertilization site. Further, the fertilization process of an egg after interaction with sperm usually happens in the fallopian tubes.
The later phase of the fertilized egg is associated with getting rooted into the uterus walls. This is counted as the beginning of the early stages of pregnancy. In case fertilization or implantation doesn’t occur, then the system is aimed to menstruate. Moreover, the female reproductive system is responsible for producing female sex hormones which maintain the reproductive cycle.

Parts of Female Reproductive System
The female reproductive system comprises of parts which are both internal and external to the body.

1. External Reproductive Parts:

The external parts of the female reproductive system include:

• Labia majora: It protects and encloses the other outside reproductive organ. The labia majora can be tagged as outsized and fleshy and is analogous to the male scrotum. It contains sweat as well as oil-secreting glands. The labia majora gets covered with hair right after puberty.
• Labia minora: These are translated as ‘small lips’, and can be quite small close to 2 inches wide. Labia minora rest just in the interior of labia majora. It surrounds the primary opening of the vagina and urethra.
• Bartholin’s glands: The location of these glands rest beside the vaginal opening and is responsible to manage a fluid discharge.
• Clitoris: Both the labia minora encounter at the clitoris, which is a small and delicate protrusion that is analogous to the penis in the male reproductive system. This part of the female reproductive system is covered by a skin fold, termed as prepuce. Similar to the male penis, the clitoris is sensitive to stimulus and can turn erect.

2. Internal Reproductive Parts:

The internal parts of the female reproductive system include:

• Vagina: It is a canal which joins the cervix to the external portion of the body. You can even call it as the birth canal.
• Womb or Uterus: It is a pear-shaped, hollow organ which is ‘home’ for a developing fetus. Further, the uterus separates into two parts; namely, the cervix, and the corpus. The corpus easily enlarges for holding a developing baby.
• Ovaries: These are oval-shaped glands which are small and are located on both side of the uterus. Ovaries produce harmones and eggs.
• Fallopian tubes: They are narrow tubes which attach to the upper portion of the uterus. Fallopian tubes act as tunnels for the egg cells. Therefore, they transport the egg cells from the ovaries the to uterus.

c- Developing follicle;
h- Regressing corpus luteum.

• Rapid release of luteinising hormone ruptures Graafian follicle and release ovum (ovulation).
• Corpus luteum secretes large amount of progesterone hormone that is essential for maintenance of the endometrium required for implantation of blastocyst leading to pregnancy.
• Placenta produces several hormones like human chorionic gonadotropin (hCG), human placental lactogen (hPL). Relaxin is also produced during later phase of pregnancy. Level of other hormones like estrogens, progestogens, cortisol, prolactin and thyroxine also increases which is essential for supporting fetal growth, metabolic changes in mother and maintenance of pregnancy.
• Parturition signals originate from the fully developed foetus and the placenta induce mild uterine contractions which triggers release of oxytocin from pituitary. Oxytocin acts on the uterine muscle causing stronger uterine contractions.

1. During pregnancy.
2. During menstruation.

Oogenesis: The process of formation of a mature female gamete is called oogenesis. It occurs in the ovaries. It consists of the following three phases:

1. Multiplication phase: Oogenesis is initiated during the embryonic development stage when a couple of million gamete mother cells (oogonia) are formed within each foetal ovary. No more oogonia are formed and added after birth. These cells start division and enter into prophase-I of the meiotic division. They get temporarily arrested at this stage and are called primary oocytes.
2. Growth phase: Each primary oocyte then gets surrounded by a layer of granulosa cells. This structure is called the primary follicle. A large number of these follicles degenerate during the phase from birth to puberty. At puberty, only 60,000 to 80,000 primary follicles are left in each ovary. The primary follicles get surrounded by more layers of granulosa cells and a new theca to form secondary follicles. Systematic representation of oogenesis.

3. Maturation phase: In the first maturation phase, the secondary follicle soon transforms into a tertiary follicle. The primary oocyte within the tertiary follicle grows in size and completes its first meiotic division to form a large, haploid, secondary oocyte and a tiny first polar body. The tertiary follicle changes into a mature follicle-the Graafian follicle-which ruptures to release the secondary oocyte (ovum) from the ovary by a process called ovulation. The second maturation phase occurs after fertilisation when the meiotic division of the secondary oocyte is complete. This second meiotic division results in the formation of a second polar body and a haploid ovum (ootid).

Diagrammatic sectional view of a seminiferous tubule (enlarged)Hormonal control of spermatogenesis.

Sectional view of ovary.

A- Estrogen.
B- Progesterone.

Fertilisation: The process of fusion of a sperm (male gamete) with an ovum (female gamete) is called fertilisation. During coitus, semen is released by the penis into the vagina (insemination). The motile sperms swim rapidly through the cervix, enter into the uterus and reach the ampullary – isthmic junction of the oviduct. A sperm comes in contact with the zona pellucida layer of the ovum and induces changes in the membrane to block the entry of additional sperms. The enzymes of the acrosome of sperm help to dissolve zona pellucida and plasma membrane of the ovum and sperm head is allowed to enter into the cytoplasm of the ovum, i.e., secondary oocyte.
Ovum surrounded by few sperms Ultimately diploid zygote is produced by the fusion of a sperm and an ovum.

The placenta also acts as an endocrine gland as it secretes the following hormones:

1. Human chorionic gonadotropin (hCG) .
2. Human placental lactogen (hPL) .
3. Progestogens.
4. Estrogens.

The increased production of these hormones during pregnancy is essential for supporting the fetal growth and metabolic changes in the mother.

Trophoblast: It is the outer layer of blastocyst which sends finger-like projections called chorionic villi into uterine stroma. Chorionic villi and uterine tissues jointly form the placenta.

After implantation, uterus, chorionic villi and uterine tissue become interdigitated (physically fused) to form a structural and functional unit between the developing embryo and the maternal body called placenta.
Functions:

1. Placenta facilitates supply of oxygen and nutrients to the embryo.
2. Removes carbon dioxide, waste material and excretory material produced by the embryo.
3. Produces hCG/ hPL, estrogens and progestrogens.

To ensure the fusion of one sperm.

Graafian follicle ruptures to release the ovum (secondary oocyte) and remaining parts of the Graafian follicle transform into corpus luteum.

Seminiferous tubules: Sperms are produced here by the process of spermatogenesis.

Diagrammatic sectional view of a seminiferous tubule (enlarged) . Hormonal control of spermatogenesis.

1. 5th day to 12th day of the cycle: Follicular phase (Proliferative phase).
2. 14th day of the cycle: Ovulatory phase (release of ovum) followed by luteal phase.
3. 16th day to 25th day of the cycle: Luteal phase.
4. FSH is secreted by the anterior pituitary which stimulates the ovarian follicle to secrete estrogen, which in turn stimulates the proliferation of the endometrium of the uterine wall.

Systematic representation of oogenesis.
Hormones involved are:
LH/ Luteinising hormone.
FSH/ Follicle stimulating hormone.
Estrogen.

Hypothalamus secretes gonadotropin releasing hormone (GnRH) which stimulates the anterior lobe of pituitary gland to secrete LH (Luteinising hormone) and FSH (Follicle stimulating hormone).
LH stimulates Leydig cells for the secretion of testosterone from the testes.
FSH stimulates Sertoli cells of the testes to secrete an androgen-binding protein (ABP) that concentrates testosterone in seminiferous tubules. It also causes the secretion of some factors which help in spermiogenesis.

1. Gonadotropin releasing hormone is released by the hypothalamus; it stimulates the pituitary to release follicle stimulating hormone (FSH) and luteinising hormone (LH), both of which act on the ovary.
2. FSH stimulates the development of ovarian follicle into mature Graafian follicle and also secretion of estrogens by the follicle cells.
3. LH stimulates ovulation, formation of corpus luteum from the ruptured follicle and secretion of progesterone by the corpus luteum.
4. Progesterone is necessary to maintain the endometrium for implantation of the embryo and other events of pregnancy.

1. Pituitary hormone:

• FSH stimulates maturation of follicle.
• Rapid secretion of LH (LH surge) induces rupture of Graafian follicle, thereby leading to ovulation (release of ovum).

2. Ovarian hormone:

• Estrogen stimulates follicular development.
• Progesterone produced by corpus luteum helps to maintain endometrium.

In the absence of fertilisation corpus luteum degenerates and the endometrium disintegrates leading to menstruation.

1.  

2. ln human embryo, the primary oocyte grows in size, completes meiosis-I and forms a larger cell, the secondary oocyte and a smaller cell, the first polar body.

In the follicular phase, following events occur:

1. The primary follicles in the ovary grow to form a fully mature Graafian follicle.
2. The endometrium of uterus regenerates through proliferation.
3. The secretion of gonadotropins (LH and FSH) gradually increases.

d – cells of corona radiata.

The process of formation of a mature female gamete is called oogenesis. Oogenesis starts during embryonal stages where millions of gamete mother cells-oogonia are formed in foetal ovary. Oogonia starts division and enters prophase-I and gets temporarily arrested at this stage to form primary oocytes. Each primary oocyte gets surrounded by a layer of granulosa cells forming primary follicle. From birth to puberty, large number of follicles degenerate (follicular atresia) and at puberty 60,000-80,000 follicles are left in each ovary. Primary follicles form secondary follicles, surrounded by more layers of granulosa cells. Secondary follicle transforms into tertiary follicle with a fluid-filled cavity called antrum, having theca externa and theca interna and primary oocyte completes Ist meiotic division (grows in size) forming two very unequal haploid cells, large haploid secondary oocyte and a tiny first polar body. Tertiary follicle transforms into mature Graafian follicle which bursts to release ovum/secondary oocyte (ovulation). The hormones involved are FSH (Follicle Stimulating Hormone) and LH (Luteinising Hormone) secreted by anterior pituitary in response to production of GnRH. (Gonadotropin Releasing Hormone). FSH stimulates follicular growth and maturation of oocyte. Granulosa cells of follicles produce oestrogen. In the presence of both LH and oestrogen ovulation occurs.

The complete neuro-endocrine mechanism.

In human males, the process is completed in the testes (seminiferous tubules) whereas in females, it is completed in the fallopian tube (oviduct).

Menstrual Phase:

• The soft tissue of endometrial lining of the uterus disintegrates causing bleeding.
• The unfertilised egg and soft tissues are discharged.
• It lasts for 3-5 days.

Diagrammatic presentation of various events during a menstrual cycle.

Ovaries: During the first phase of menstrual cycle, growth of follicles occur and primary oocyte matures in the follicle. The mature Graafian follicle bursts to release the ovum. In luteal phase, the remaining follicle forms corpus luteum, an endocrine gland that produces progesterone.
Uterus: The inner walls of the uterus are highly vascularised and muscular in nature and is known as endometrium. This wall takes active part in menstrual cycle because it breaks down due to the lack of progesterone. As a result, bleeding occurs. In the follicular phase, uterus enlarges in size.
Fallopian tube: The wall of the fallopian tubes thickens and its cilia and their movements are increased.

S. No.	Hormone	Source	Function
(i)	FSH	Anterior pituitary lobe.	Stimulates growth of ovarian follicles and maturation of primary oocytes.
(ii)	LH	Anterior pituitary lobe.	Induces ovulation and maintains corpus luteum.
(iii)	hCG	Chorionic cells of placenta.	Maintains the corpus luteum and stimulates it to secrete progesterone.
(iv)	Relaxin	Ovary.	Helps during child birth by relaxing the pelvic muscles as well as muscles of the cervix.

Vagina: It receives the semen from the male during coitus. It serves as the birth canal during parturition.

A is able to penetrate/ fertilize the ovum, whereas B and C are unable to penetrate/ fertilise B and C will degenerate.

Oxytocin acts on uterine muscle for stronger contraction.

Estrogen and Luteinising hormone (LH).

1. Corpus luteum-d.
2. Secondary oocyte-c.
3. Antrum-b.
4. Primary follicle-e.
5. Blood vessels-a.

Structure of a sperm (Spermatozoa):It consists of four parts–head, neck, middle piece and tail-enveloped by a plasma membrane.
Head: It is the enlarged end of a sperm, containing the large haploid nucleus, i.e., condensed chromatin body and is capped by acrosome. The acrosome contains hydrolytic enzymes that help in dissolving membranes of the ovum for fertilisation.
Neck: It contains proximal centriole which is necessary for the first cleavage division of zygote and the distal centriole that is connected to the tail filament.
Middle Piece: It contains a number of mitochondria that provide energy for the movement of the tail that facilitate sperm motility, essential for fertilisation.
Tail: It consists of axial filaments surrounded by the plasma membrane. It helps the sperms to swim in a fluid medium.
A human male ejaculates about 200-300 million sperms during a coitus. Seminal plasma along with the sperms constitute the semen.

Structure of a sperm.

Morula is formed in the upper portion of oviduct, i.e., isthmus. The haploid nucleus of the sperms and that of the ovum fuse together to form a diploid zygote. Zygote divides rapidly as it moves through the isthmus of the oviduct by mitotic division called cleavage and as a result 2, 4, 8, 16 daughter cells are produced, which are termed as blastomeres. The embryo with 8-16 blastomeres is called a morula.

Spermatogenesis occur in seminiferous tubules.
Schematic representation of spermatogensis.

Seminal plasma is a liquid medium which helps the sperm to move and nourishes it.

Month 1: Human development starts when a sperm fuses with an egg to create a zygote. A zygote is a single-cell that contains two copies of chromosomes-one copy from each parent. In the week following fertilization, the zygote undergoes rapid cell division and becomes a mass of cells known as a blastocyst. After more cell division, the blastocyst splits in half.
Month 2: This month, the embryo’s development shifts into high gear. Its tongue, teeth and eyelids start to form. Its limbs grow longer and stronger, and its palate is nearly complete. Also in this time period, the embryo’s gastrointestinal tract separates from its urogenital organs and its heart begins beating-twice as fast as yours, in fact.
Month 3: This is the month of the heartbeat. Using a special tool called a Doppler monitor, doctors can detect the tiny thump-thumps of a 10-week-old fetus.
Month 4: Get out the headphones and tune the radio dial to Mozart-this month, the fetus can hear its mother’s heartbeat, her voice and other outside noises. The fetus is also developing at warp speed; by now, all its major organs are complete.
Month 5: Finally, the baby really starts kicking! “Quickening” is when a mother senses her potential punter in action for the first time, and this milestone moment usually happens during pregnancy’s fifth month.
Month 6: Month 6 marks the end of the second trimester. At this point, the fetus’s essential organs-its kidneys, heart and lungs-are fully formed. The facial features are becoming more recognizable. It also can hiccup and react to loud “outside” noises by blinking, shifting or kicking. By month’s end, the fetus will measure about 12 or 13 inches long and weigh roughly 2 pounds.
Month 7: The 7-month-old fetus can blink, and its eyes may even remain open for short period of time. Hands and feet are becoming even more active. Also in this phase: Taste buds form and protective fat tissue makes its debut. The fetus’s hearing is fully developed and, in boys, its testicles have moved to the groin. By month’s end, the baby-to-be will measure 14 to 16 inches long and weigh anywhere from 2 ½ to 3 ½ pounds.
Month 8: This month, the fetus’s brain develops rapidly, and all of its organs except the lungs are mature. An 8-month-old fetus stretches 16 to 18 inches long and weighs between 4 and 6 pounds. And as the baby-to-be grows larger, space in the womb becomes scarce. Expectant mothers should still count on catching a few elbows every day, but the elaborate somersault sequences should stop until delivery day. Other exciting changes during this period? The fetus’s fingernails now reach beyond its fingertips and its skin is starting to smooth.
Month 9: The finish line is in sight! In this final month of development, the fetus’s fat layers thicken to help keep it warm outside the womb, and the protective layers of vernix caseosa and lanugo largely disappear. By now, the fetus’s lungs are mature, its skin pink and smooth, and its toenails fully grown. The baby-to-be can also execute an array of reflexes, such as head turning, blinking and grasping. At this late stage, it stretches between 20 to 22 inches long, and weighs about 7½ pounds.

Oogenesis:The process of formation of a mature female gamete is called oogenesis. It occurs in the ovaries. It consists of the following three phases:

1. Multiplication phase: Oogenesis is initiated during the embryonic development stage when a couple of million gamete mother cells (oogonia) are formed within each foetal ovary. No more oogonia are formed and added after birth. These cells start division and enter into prophase-I of the meiotic division. They get temporarily arrested at this stage and are called primary oocytes.
2. Growth phase: Each primary oocyte then gets surrounded by a layer of granulosa cells. This structure is called the primary follicle. A large number of these follicles degenerate during the phase from birth to puberty. At puberty, only 60,000 to 80,000 primary follicles are left in each ovary. The primary follicles get surrounded by more layers of granulosa cells and a new theca to form secondary follicles.

Systematic representation of oogenesis.

3. Maturation phase: In the first maturation phase, the secondary follicle soon transforms into a tertiary follicle. The primary oocyte within the tertiary follicle grows in size and completes its first meiotic division to form a large, haploid, secondary oocyte and a tiny first polar body. The tertiary follicle changes into a mature follicle – the Graafian follicle – which ruptures to release the secondary oocyte (ovum) from the ovary by a process called ovulation. The second maturation phase occurs after fertilisation when the meiotic division of the secondary oocyte is complete. This second meiotic division results in the formation of a second polar body and a haploid ovum (ootid).

Sectional view of ovary Oogenesis is completed when the sperm penetrates the ovum.

Endometrium proliferate (glands become cork-screw shaped) highly vascularised, high regeneration anticipating implantation of the fertilised ovum.

Scrotum: Testes are situated in scrotum where the temperature remains 2-2.5°C less than the internal body temperature, which is essential for the formation of sperms.

S. No.	Cells	Location	Function
i.	Leydig cells	Outside seminiferous tubules in interstitial space.	Synthesis and secretion of testicular hormones called androgens.
ii.	Sertoli cells	Inside the seminiferous tubules.	Provide nutrition to the germ cells.
iii.	Primary spermatocyte	Inner wall of seminiferous tubules.	Undergo meiotic divisions to form secondary spermatocyte and then haploid sperms.

Both LH and FSH attain a peak level in the middle of cycle (about 14th day).
Rapid secretion of LH leading to its maximum level during the mid-cycle called LH surge induces rupture of Graafian follicle and thereby the release of ovum (ovulation).
The remaining cells of ovarian follicles are stimulated by the LH to transform into corpus luteum. The corpus luteum secretes large amount of progesterone which is essential for maintenance of the endometrium.

Rapid release of luteinising hormone ruptures Graafian follicle and release ovum (ovulation) .
Corpus luteum secretes large amount of progesterone hormone that is essential for maintenance of the endometrium required for implantation of blastocyst leading to pregnancy.

Changes in the primary oocyte:

• Each primary oocyte gets surrounded by a single layer of granulosa cells and is known as primary follicle.
• Primary follicle get surrounded by more layers of granulosa cells and a new theca and form secondary follicle.
• A secondary follicle transforms into tertiary follicle characterised by a fluid-filled space, the antrum.
• Primary oocyte undergoes first meiotic division and produces two unequal haploid cells-a large haploid secondary oocyte and a tiny first polar body.
• The tertiary follicle further changes into the mature Graafian follicle.
• The secondary oocyte forms a new membrane called zona pellucida around it.

Sectional view of ovary.

f; secondary oocyte.

Menstrual cycle is regulated by hypothalamus through the pituitary gland. At the end of menstrual phase, the pituitary FSH gradually increases resulting in follicular development within the ovaries. As the follicles mature, estrogen secretion increases resulting in a surge in FSH and LH. The surge of LH is responsible for ovulation and formation of corpus luteum that secretes progesterone which helps in maintaining the endometrium for implantation.

Produces the hormone progesterone, causes proliferation of the endometrium which gets highly vascularised. It is essential for the implantation of the fertilised ovum and maintains the same during pregnancy.

Oogenesis: The process of formation of a mature female gamete is called oogenesis. It occurs in the ovaries. It consists of the following three phases:

1. Multiplication phase: Oogenesis is initiated during the embryonic development stage when a couple of million gamete mother cells (oogonia) are formed within each foetal ovary. No more oogonia are formed and added after birth. These cells start division and enter into prophase-I of the meiotic division. They get temporarily arrested at this stage and are called primary oocytes.
2. Growth phase: Each primary oocyte then gets surrounded by a layer of granulosa cells. This structure is called the primary follicle. A large number of these follicles degenerate during the phase from birth to puberty. At puberty, only 60,000 to 80,000 primary follicles are left in each ovary. The primary follicles get surrounded by more layers of granulosa cells and a new theca to form secondary follicles.

Systematic representation of oogenesis.

3. Maturation phase: In the first maturation phase, the secondary follicle soon transforms into a tertiary follicle. The primary oocyte within the tertiary follicle grows in size and completes its first meiotic division to form a large, haploid, secondary oocyte and a tiny first polar body. The tertiary follicle changes into a mature follicle-the Graafian follicle-which ruptures to release the secondary oocyte (ovum) from the ovary by a process called ovulation. The second maturation phase occurs after fertilisation when the meiotic division of the secondary oocyte is complete. This second meiotic division results in the formation of a second polar body and a haploid ovum (ootid).

Acrosome: Acrosome contains hydrolytic enzymes that help in dissolving membranes of the ovum for sperm entry ensuring fertilisation.
Middle Piece: It contains a number of mitochondria that provide energy for the movement of the tail and provides motility to sperm.

Flow chart showing the hormonal control of the human male reproductive system.

Unequal cytoplasmic division of the oocyte is to ensure the retention of bulk of cytoplasm in one cell called secondary oocyte instead of sharing it with two. It has to provide nourishment for the developing embryo during early stages, so it is essential to retain as much cytoplasmic materials it could in a single daughter cell.

A- Maturing ovarian follicle/ Graafian follicle.
B- Corpus luteum.

c- zona pellucida.
It protects ovum and regulates interaction between ovum and sperms during fertilisation.

The sequence of secretion of hormones is:

1. FSH.
2. LH.
3. hCG.
4. Relaxin.

In the luteal phase, following events occur:

1. The parts of Graafian follicle remaining after the rupture, transform into the corpus luteum.
2. The corpus luteum secretes large amounts of progesterone.

Menstrual Cycle:

• The rhythmic series of changes that occur in the reproductive organs of female primates (monkeys, apes and human beings) is called menstrual cycle.
• It is repeated at an average interval of about 28/ 29 days.
• The first appearance of menstruation at puberty is called menarche.
• The menstrual cycle has four phases. These are:

1. Menstrual Phase:

• The soft tissue of endometrial lining of the uterus disintegrates causing bleeding.
• The unfertilised egg and soft tissues are discharged.
• It lasts for 3-5 days.

Diagrammatic presentation of various events during a menstrual cycle.

2. Follicular Phase/ Proliferative Phase:

• The primary follicles in the ovary grow and become a fully mature Graafian follicle.
• The endometrium of the uterus is regenerated due to the secretion of LH and FSH from anterior pituitary and ovarian hormone, estrogen.
• It lasts for about 10 to 14 days.

3. Ovulatory Phase:

• Rapid secretion of LH (LH surge) induces rupture of Graafian follicle, thereby leading to ovulation (release of ovum).
• It lasts for only about 48 hours.

4. Luteal Phase/ Secretory Phase:

• In this phase the ruptured follicle changes into corpus luteum in the ovary and it begins to secrete the hormone progesterone.
• The endometrium thickens further and their glands secrete a fluid into the uterus.
• If ovum is not fertilised, the corpus luteum undergoes degeneration and this causes disintegration of the endometrium leading to menstruation.
• Estrogen and progesterone levels rise during this phase.
• It lasts for only 1 day.
• During pregnancy all events of the menstrual cycle stop and there is no menstruation. The menstrual cycle permanently stops in females at the age of around 50 years. This is called menopause.

FSH and LH are the two pituitary gonadotropins which play important role during follicular and ovulatory phases of menstrual cycle.

• FSH stimulates the release of ovum from the Graafian follicle. It also stimulates the development and subsequent rupture of Graafian follicle. Thus, FSH plays major role during follicular and ovulatory phases.
• The given figure shows varying levels of LH, FSH and steroid hormones during follicular and ovulation phases of menstrual cycle.
• At this stage, level of estradiol (a steroid hormone) increases. This suppresses the level of Luteinizing hormone (LH). Once the level of estradiol reaches a threshold level; its effect is reversed.
• After that, level of estrogen (another steroid hormone) increases. This stimulates a surge in levels of LH. High level of LH stimulates development of corpus luteum and proliferation of endometrium. This marks the beginning of the luteal phases.

Oogenesis is initiated during the embryonic development stage when a couple of million gamete mother cells (oogonia) are formed within each fetal ovary (about third month of foetal ovary), no more oogonia are formed and added after birth.
These cells start division and enter into prophase-I of the meiotic division and get temporarily arrested at that stage, called primary oocytes. Each primary oocyte then gets surrounded by a layer of granulosa cells and then called the primary follicle. 10 follicle contains 10 oocyte. A large number of these follicles degenerate during the phase from birth to puberty.
The primary follicles get surrounded by more layers of granulosa cells and a new theca and called secondary follicles. The theca layer is organised into an inner theca interna and an outer theca externa. Theca internal secrete estrogen hormone. The secondary follocle soon transforms into a tertiary follicle which is characterised by a fluid filled cavity called antrum.
It is important to draw your attention that it is at this stage that the primary ococyte within the tertiary follicle grows in size and completes its first meiotic division. It is an unequal division resulting in the formation of a large haploid secondary oocyte and a tiny first polar body. The tertiary follicle further changes into the mature follicle or Graafian follicle.

In the epididymis sperms undergo physiological maturation, acquiring increased motility and fertilizing capacity.