Chapter 2: Human Reproduction

Chapter 2: Human Reproduction

INTRODUCTION TO HUMAN REPRODUCTION

• Humans are sexually reproducing and viviparous organisms.
• The major reproductive events in humans include:
  • Gametogenesis: Formation of gametes (sperms in males, ovum in females).
  • Insemination: Transfer of sperms into the female genital tract.
  • Fertilization: Fusion of male and female gametes to form a diploid zygote.
  • Implantation: Attachment of the blastocyst to the uterine wall.
  • Gestation: Embryonic development.
  • Parturition: Delivery of the baby.
• Key Difference: In males, sperm formation continues even in old men, whereas in females, the formation of the ovum ceases around the age of fifty years.

2.1 THE MALE REPRODUCTIVE SYSTEM

The male reproductive system is located in the pelvis region and comprises a pair of testes, accessory ducts, accessory glands, and the external genitalia.

1. Testes & Scrotum

• Scrotum: The testes are situated outside the abdominal cavity within a pouch called the scrotum.
• Function of Scrotum: It helps maintain the low temperature of the testes (2–2.5°C lower than the normal internal body temperature), which is essential for spermatogenesis.
• Structure: Each adult testis is oval in shape, with a length of about 4 to 5 cm and a width of about 2 to 3 cm, and is covered by a dense covering.
• Testicular Lobules: Each testis contains about 250 compartments known as testicular lobules.

2. Seminiferous Tubules

• Each testicular lobule contains 1 to 3 highly coiled seminiferous tubules where sperms are produced.
• The inner lining of each seminiferous tubule contains two types of cells:
  • Male Germ Cells (Spermatogonia): Undergo meiotic divisions to eventually form sperms.
  • Sertoli Cells: Provide nutrition to the developing male germ cells.

3. Interstitial Spaces (Leydig Cells)

• The regions outside the seminiferous tubules are called interstitial spaces, which contain small blood vessels and interstitial cells or Leydig cells.
• Function: Leydig cells synthesize and secrete testicular hormones called androgens. Other immunologically competent cells are also present here.

4. Male Sex Accessory Ducts

• The accessory ducts transport and store sperms from the testis to the outside. They include the rete testis, vasa efferentia, epididymis, and vas deferens.
• Pathway: Seminiferous tubules open into vasa efferentia through the rete testis. The vasa efferentia leave the testis and open into the epididymis (located along the posterior surface of each testis).
• The epididymis leads to the vas deferens, which ascends to the abdomen and loops over the urinary bladder.
• It receives a duct from the seminal vesicle to form the ejaculatory duct, which opens into the urethra.
• The urethra originates from the urinary bladder, extends through the penis, and ends at the external opening called the urethral meatus.

5. Male Accessory Glands

• The accessory glands include paired seminal vesicles, a single prostate gland, and paired bulbourethral glands.
• Seminal Plasma: The secretions of these glands form the seminal plasma, which is rich in fructose, calcium, and certain enzymes.
• The secretions of the bulbourethral glands specifically help in the lubrication of the penis.

6. External Genitalia (Penis)

• The penis is made up of special tissue that helps in its erection to facilitate insemination.
• The enlarged, sensitive end of the penis is called the glans penis, which is covered by a loose fold of skin known as the foreskin.

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Read Chapter 1 : Sexual Reproduction in Flowering Plants – CLICK HERE

2.2 THE FEMALE REPRODUCTIVE SYSTEM

The female reproductive system is located in the pelvic region and is structurally and functionally integrated to support ovulation, fertilization, pregnancy, birth, and child care. It consists of:

• A pair of ovaries
• A pair of oviducts (fallopian tubes)
• Uterus
• Cervix and Vagina
• External genitalia
• A pair of mammary glands

1. Ovaries (Primary Female Sex Organs)

• Function: Produce the female gamete (ovum) and several steroid hormones (ovarian hormones).
• Location & Size: Located one on each side of the lower abdomen, measuring about 2 to 4 cm in length. They are connected to the pelvic wall and uterus by ligaments.
• Structure: Each ovary is covered by a thin epithelium that encloses the ovarian stroma. The stroma is divided into two distinct zones:
  • Peripheral Cortex: The outer region.
  • Inner Medulla: The inner region.

2. Female Accessory Ducts The oviducts, uterus, and vagina together constitute the female accessory ducts.

• Oviducts (Fallopian Tubes): Each tube is about 10-12 cm long and extends from the periphery of the ovary to the uterus. It consists of three parts:
  • Infundibulum: A funnel-shaped part closer to the ovary. Its edges possess finger-like projections called fimbriae, which help in collecting the ovum after ovulation.
  • Ampulla: A wider part of the oviduct that follows the infundibulum.
  • Isthmus: The last part of the oviduct with a narrow lumen that joins the uterus.
• Uterus (Womb): It is a single, inverted pear-shaped organ supported by ligaments attached to the pelvic wall.
  • Cervix: The uterus opens into the vagina through a narrow cervix. The cavity of the cervix is called the cervical canal, which along with the vagina forms the birth canal.
  • Uterine Wall Layers:
    1. Perimetrium: The external thin membranous layer.
    2. Myometrium: The middle thick layer of smooth muscle. It exhibits strong contractions during the delivery of the baby.
    3. Endometrium: The inner glandular layer that lines the uterine cavity. It undergoes cyclical changes during the menstrual cycle.

3. Female External Genitalia The external genitalia collectively includes:

• Mons pubis: A cushion of fatty tissue covered by skin and pubic hair.
• Labia majora: Fleshy folds of tissue that extend down from the mons pubis and surround the vaginal opening.
• Labia minora: Paired folds of tissue situated just under the labia majora.
• Hymen: A membrane that often partially covers the opening of the vagina. It can be torn during the first coitus, sudden falls, or active sports participation.
• Clitoris: A tiny finger-like structure lying at the upper junction of the two labia minora, just above the urethral opening.

4. Mammary Glands

• Structure: They are paired structures (breasts) containing glandular tissue and a variable amount of fat.
• Glandular Organization: The glandular tissue in each breast is divided into 15-20 mammary lobes.
• Alveoli & Milk Pathway: These lobes contain clusters of cells called alveoli, which secrete milk. The milk is stored in the cavities (lumens) of the alveoli.
• Duct System: Alveoli open into mammary tubules. Tubules of each lobe join to form a mammary duct. Several mammary ducts join to form a wider mammary ampulla, which is connected to the lactiferous duct through which milk is sucked out.

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2.3 GAMETOGENESIS

The primary sex organs (testes in males and ovaries in females) produce gametes (sperms and ovum) through a process called gametogenesis.

1. Spermatogenesis

• Initiation: Begins at puberty.
• Process: Immature male germ cells (spermatogonia) present on the inside wall of seminiferous tubules multiply by mitotic division to increase in number. Each is diploid (46 chromosomes).
• Primary Spermatocytes: Some spermatogonia periodically undergo meiosis to become primary spermatocytes.
• Meiotic Divisions: A primary spermatocyte undergoes the first meiotic (reduction) division to form two equal, haploid cells called secondary spermatocytes (23 chromosomes each). These then undergo the second meiotic division to produce four equal, haploid spermatids.
• Spermiogenesis: The process of transformation of spermatids into spermatozoa (sperms).
• Spermiation: After spermiogenesis, sperm heads become embedded in the Sertoli cells and are finally released from the seminiferous tubules by this process.
• Hormonal Control: Increased secretion of Gonadotropin Releasing Hormone (GnRH) from the hypothalamus acts on the anterior pituitary to stimulate the release of LH and FSH. LH stimulates Leydig cells to secrete androgens (which stimulate spermatogenesis), and FSH acts on Sertoli cells to secrete factors helping in spermiogenesis.

2. Structure of a Sperm

• Body Parts: It is a microscopic structure composed of a head, neck, a middle piece, and a tail, completely enveloped by a plasma membrane.
• Head: Contains an elongated haploid nucleus. The anterior portion is covered by a cap-like structure called the acrosome, which is filled with enzymes that help in the fertilization of the ovum.
• Middle Piece: Contains numerous mitochondria that produce energy for the movement of the tail, facilitating sperm motility.
• Viability Stats: A human male ejaculates about 200 to 300 million sperms during a coitus. For normal fertility, at least 60% of sperms must have normal shape and size, and at least 40% must show vigorous motility.

3. Oogenesis

• Initiation: The process of forming a mature female gamete is initiated during the embryonic development stage, markedly different from spermatogenesis.
• Oogonia Formation: A couple of million gamete mother cells (oogonia) are formed within each fetal ovary. No more oogonia are added after birth.
• Primary Oocytes & Follicles: Oogonia enter prophase-I of meiosis and are temporarily arrested at this stage, called primary oocytes. Each primary oocyte surrounded by granulosa cells forms a primary follicle. Many follicles degenerate from birth to puberty; only 60,000-80,000 remain in each ovary at puberty.
• Tertiary Follicle & Antrum: Primary follicles get surrounded by more granulosa cells and a new theca to become secondary follicles, which soon transform into tertiary follicles (characterized by a fluid-filled cavity called the antrum).
• Completion of Meiosis I: Within the tertiary follicle, the primary oocyte grows and completes its first meiotic division. This is an unequal division resulting in a large haploid secondary oocyte (which retains the nutrient-rich cytoplasm) and a tiny first polar body.
• Graafian Follicle & Ovulation: The tertiary follicle changes into a mature Graafian follicle. The secondary oocyte forms a new membrane called the zona pellucida around itself. The Graafian follicle ruptures to release the secondary oocyte (ovum) from the ovary in a process called ovulation.

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2.4 MENSTRUAL CYCLE

• The reproductive cycle in female primates, such as monkeys, apes, and human beings, is known as the menstrual cycle.
• Menarche: The first menstruation that begins at puberty is called menarche.
• Duration: In human females, menstruation is repeated at an average interval of about 28/29 days.
• Ovulation: One ovum is released during the middle of each menstrual cycle.

The cycle of events starting from one menstruation till the next one is divided into four major phases:

1. Menstrual Phase

• The cycle begins with the menstrual phase, during which menstrual flow occurs, lasting for 3-5 days.
• The menstrual flow results from the breakdown of the endometrial lining of the uterus and its blood vessels, forming a liquid that exits through the vagina.
• Menstruation occurs only if the released ovum is not fertilized.
• A lack of menstruation may be an indicator of pregnancy, but it can also be caused by stress, poor health, or other underlying causes.

2. Follicular Phase (Proliferative Phase)

• During this phase, the primary follicles in the ovary grow to become a fully mature Graafian follicle.
• Simultaneously, the endometrium of the uterus regenerates through a process of proliferation.
• These changes are induced by changing levels of pituitary and ovarian hormones.
• The secretion of gonadotropins (LH and FSH) increases gradually, stimulating follicular development and the secretion of estrogens by the growing follicles.

3. Ovulatory Phase

• Both LH and FSH attain a peak level in the middle of the cycle, which is about the 14th day.
• LH Surge: The rapid secretion of LH leading to its maximum level during the mid-cycle is called the LH surge.
• This surge induces the rupture of the Graafian follicle, thereby releasing the ovum (ovulation).

4. Luteal Phase (Secretory Phase)

• The ovulatory phase is followed by the luteal phase, during which the remaining parts of the Graafian follicle transform into the corpus luteum.
• Function of Corpus Luteum: It secretes large amounts of progesterone, which is essential for the maintenance of the endometrium.
• The maintained endometrium is necessary for the implantation of the fertilized ovum and other events of pregnancy.
• During pregnancy, all events of the menstrual cycle stop, and there is no menstruation.
• Absence of Fertilization: If fertilization does not occur, the corpus luteum degenerates. This causes the disintegration of the endometrium, leading to menstruation and marking a new cycle.

Menopause

• In human beings, the menstrual cycle ceases around 50 years of age, a stage termed menopause.
• Cyclic menstruation is an indicator of the normal reproductive phase, extending between menarche and menopause.

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2.5 FERTILISATION AND IMPLANTATION

1. Insemination and Transport of Gametes

• During copulation (coitus), semen is released by the penis into the vagina, a process known as insemination.
• The motile sperms swim rapidly, pass through the cervix, enter the uterus, and finally reach the ampullary region of the fallopian tube.
• The ovum released by the ovary is simultaneously transported to the ampullary region where fertilisation takes place.
• Key Concept: Fertilisation can only occur if the ovum and sperms are transported simultaneously to the ampullary region. This explains why not all copulations lead to fertilisation and pregnancy.

2. The Process of Fertilisation

• The process of fusion of a sperm with an ovum is called fertilisation.
• During fertilisation, a sperm comes in contact with the zona pellucida layer of the ovum. This contact induces changes in the membrane that block the entry of additional sperms, thus preventing polyspermy and ensuring that only one sperm can fertilise an ovum.
• The secretions of the acrosome help the sperm enter the cytoplasm of the ovum through the zona pellucida and the plasma membrane.

3. Completion of Meiosis II and Zygote Formation

• The entry of the sperm induces the completion of the meiotic division (Meiosis II) of the secondary oocyte.
• This second meiotic division is also unequal, resulting in the formation of a second polar body and a large haploid ovum (ootid).
• Soon after, the haploid nucleus of the sperm and that of the ovum fuse together to form a diploid zygote containing 46 chromosomes.

4. Sex Determination

• The chromosome pattern in a human female is XX, and in a male, it is XY.
• All haploid ova produced by the female contain the X chromosome. Sperms produced by males will contain either the X or the Y chromosome (50% carry X, 50% carry Y).
• If a sperm carrying an X chromosome fertilises the ovum, the zygote will be XX (female baby).
• If a sperm carrying a Y chromosome fertilises the ovum, the zygote will be XY (male baby).
• Therefore, the sex of the baby is determined by the father’s gamete, not the mother’s.

5. Cleavage and Embryonic Development

• As the zygote moves through the isthmus of the oviduct towards the uterus, it undergoes rapid mitotic divisions called cleavage.
• Cleavage forms 2, 4, 8, and 16 daughter cells called blastomeres.
• The embryo with 8 to 16 blastomeres is called a morula.

6. Blastocyst Formation and Implantation

• The morula continues to divide and transforms into a blastocyst as it moves further into the uterus.
• The blastomeres in the blastocyst arrange into two parts:
  • Trophoblast: The outer layer of cells.
  • Inner Cell Mass: An inner group of cells attached to the trophoblast.
• The trophoblast layer gets attached to the endometrium of the uterus, and the inner cell mass differentiates into the embryo.
• After attachment, the uterine cells divide rapidly and cover the blastocyst.
• As a result, the blastocyst becomes embedded in the endometrium of the uterus. This is called implantation, and it leads to pregnancy.

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2.6 PREGNANCY AND EMBRYONIC DEVELOPMENT

1. Formation of Placenta

• Chorionic Villi: After implantation, finger-like projections called chorionic villi appear on the trophoblast. These are surrounded by uterine tissue and maternal blood.
• Placenta: The chorionic villi and uterine tissue become interdigitated (interlocked) with each other to jointly form a structural and functional unit between the developing embryo (foetus) and the maternal body. This is called the placenta.
• Umbilical Cord: The placenta connects to the embryo through an umbilical cord, which helps in the transport of substances to and from the embryo.

2. Functions of Placenta

• Nutritional & Respiratory: It facilitates the supply of oxygen and nutrients to the developing embryo.
• Excretory: It aids in the removal of carbon dioxide and other excretory/waste materials produced by the embryo.
• Endocrine Function: The placenta acts as an endocrine tissue and produces several vital hormones such as:
  • Human chorionic gonadotropin (hCG).
  • Human placental lactogen (hPL).
  • Estrogens and Progestogens.

3. Hormonal Changes During Pregnancy

• Pregnancy-Specific Hormones: Hormones like hCG, hPL, and relaxin (secreted by the ovary in the later phases of pregnancy) are produced in women only during pregnancy.
• Elevated Hormones: The levels of other hormones like estrogens, progestogens, cortisol, prolactin, and thyroxine are increased several-fold in the maternal blood.
• Significance: Increased production of these hormones is essential for supporting foetal growth, bringing about metabolic changes in the mother, and maintaining the pregnancy.

4. Formation of Germ Layers

• Immediately after implantation, the inner cell mass (embryo) differentiates into distinct layers:
  • Ectoderm: The outer layer.
  • Endoderm: The inner layer.
  • Mesoderm: A middle layer that soon appears between the ectoderm and endoderm.
• These three primary germ layers give rise to all the tissues and organs in adults.
• Stem Cells: The inner cell mass contains specialized cells called stem cells, which have the extraordinary potency to give rise to all the tissues and organs.

5. Embryonic Development Timeline (Gestation)

• Human pregnancy lasts for about 9 months.
• First Month: The embryo’s heart is formed. The first sign of a growing foetus is often listening to the heart sound through a stethoscope.
• Second Month: By the end of the second month, the foetus develops limbs and digits.
• First Trimester (12 Weeks): Most of the major organ systems are fully formed (e.g., limbs and external genital organs are well-developed).
• Fifth Month: The first movements of the foetus and the appearance of hair on its head are usually observed.
• Second Trimester (24 Weeks): The body becomes covered with fine hair, eye-lids separate, and eyelashes are formed.
• Third Trimester (9 Months): The foetus is fully developed and is completely ready for delivery.

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2.7 PARTURITION AND LACTATION

1. Gestation and Parturition

• Gestation Period: The average duration of human pregnancy is about 9 months.
• Parturition: The process of delivery of the foetus (childbirth) is called parturition. It involves vigorous contraction of the uterus at the end of pregnancy, which causes the expulsion or delivery of the foetus.

2. Mechanism of Parturition (Neuroendocrine Mechanism)

• Parturition is induced by a complex neuroendocrine mechanism.
• Foetal Ejection Reflex: The initial signals for parturition originate from the fully developed foetus and the placenta. These signals induce mild uterine contractions collectively called the foetal ejection reflex.
• Role of Oxytocin: The foetal ejection reflex triggers the release of the hormone oxytocin from the maternal pituitary gland.
• Positive Feedback Loop: Oxytocin acts on the uterine muscle and causes stronger uterine contractions. These stronger contractions, in turn, stimulate further secretion of oxytocin.
• Delivery: This stimulatory reflex between the uterine contraction and oxytocin secretion continues, resulting in stronger and stronger contractions. This ultimately leads to the expulsion of the baby out of the uterus through the birth canal.
• Placenta Expulsion: Soon after the infant is delivered, the placenta is also expelled out of the uterus.

3. Lactation and Colostrum

• Lactation: The mammary glands of the female undergo differentiation during pregnancy and start producing milk towards the end of pregnancy. This process is known as lactation, and it helps the mother in feeding the new-born.
• Colostrum: The highly nutritious milk produced during the initial few days of lactation is called colostrum.
• Significance of Colostrum: Colostrum contains several antibodies that are absolutely essential to develop resistance and immunity for the new-born babies.
• Breast-feeding during the initial period of infant growth is highly recommended by doctors for bringing up a healthy baby.

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SUMMARY

1. Overview of Human Reproduction

• Reproductive Type: Humans are sexually reproducing and strictly viviparous (giving birth to live young).
• Major Reproductive Events: The sequence includes gametogenesis (formation of gametes), insemination (transfer of sperms into the female genital tract), fertilization (fusion of gametes), implantation (attachment of the blastocyst to the uterine wall), gestation (embryonic development), and parturition (delivery).

2. Male Reproductive System

• Anatomy: It comprises a pair of testes, accessory ducts, accessory glands, and external genitalia (penis).
• Testes Compartments: Each testis contains about 250 testicular lobules, and each lobule contains 1 to 3 highly coiled seminiferous tubules.
• Cell Types: The seminiferous tubules are internally lined by spermatogonia (germ cells) and Sertoli cells (providing nutrition). The interstitial spaces outside the tubules contain Leydig cells, which synthesize and secrete androgens.

3. Female Reproductive System

• Anatomy: It consists of a pair of ovaries, a pair of oviducts (fallopian tubes), a uterus, a vagina, external genitalia, and a pair of mammary glands.
• Ovaries: They produce the female gamete (ovum) and steroid hormones. Ovarian follicles in various developmental stages are embedded in the stroma.
• Uterus Layers: The uterine wall consists of the perimetrium (outer), myometrium (middle), and endometrium (inner).

4. Gametogenesis & Menstrual Cycle

• Spermatogenesis: Results in the formation of sperms from spermatogonia. A mature sperm has a head, neck, middle piece, and tail.
• Oogenesis: The formation of a mature female gamete from oogonia.
• Menstrual Cycle: The reproductive cycle in female primates that starts at puberty (menarche). Cyclic changes in the ovary and uterus are induced by pituitary and ovarian hormones. Normally, only one ovum is released per cycle during ovulation.

5. Fertilization, Pregnancy & Parturition

• Fertilization: After coitus, sperms travel to the ampulla of the fallopian tube where one sperm fertilizes the ovum, forming a diploid zygote.
• Sex Determination: The sex of the embryo is genetically determined by the presence of an X or Y chromosome in the fertilizing sperm.
• Implantation & Gestation: The zygote undergoes mitotic divisions to form a blastocyst, which implants in the uterus. Pregnancy lasts for nine months. The foetus is connected to the maternal body via the placenta.
• Parturition & Lactation: Childbirth is induced by a complex neuroendocrine mechanism involving oxytocin, estrogen, and cortisol. Mammary glands differentiate during pregnancy and secrete milk (lactation) to feed the newborn.

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Reference/Source: NCERT