Chapter 2: Human Reproduction

Class 12 Biology | NCERT-Based Content | RBSE CBSE Board 2025-26

1. Introduction

Human beings are sexually reproducing and viviparous organisms. The reproductive events in humans include the formation of gametes (gametogenesis), transfer of sperms into the female genital tract (insemination), fusion of male and female gametes (fertilisation), formation and development of blastocyst (cleavage), attachment of the blastocyst to the uterine wall (implantation), formation of embryo (embryogenesis), development of fetus (organogenesis), and delivery of the baby (parturition).

Reproduction is the process by which living organisms produce new individuals of their own kind. It is essential for the continuation of the species. In human beings, puberty marks the beginning of reproductive ability. Puberty is the period of life when sexual maturity is attained with significant changes in psychological, physiological and anatomical features.

In human males, puberty begins at about the age of 13-14 years. The major changes include increased height, development of facial hair, deepening of voice, enlargement of reproductive organs and initiation of sperm formation. In females, puberty begins earlier, at about 10-12 years. The major changes include breast development, widening of hips, beginning of menstrual cycle and maturation of reproductive organs.

2. The Male Reproductive System

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

2.1 Testes

The testes are the primary male sex organs which produce sperms and hormones. In adult males, each testis is oval in shape with a length of about 4 to 5 cm and width of about 2 to 3 cm. The testes are located outside the abdominal cavity within a pouch called the scrotum. The scrotum helps in maintaining the low temperature of the testes (2-2.5°C lower than the normal internal body temperature) which is necessary for spermatogenesis.

Each testis is covered by a dense covering called tunica albuginea. The testis is divided into about 250 testicular lobules by septa. Each lobule contains one to three highly coiled seminiferous tubules in which sperms are produced. The regions outside the seminiferous tubules contain small blood vessels and interstitial cells or Leydig cells. The Leydig cells synthesize and secrete androgens, the male sex hormones. The seminiferous tubules are lined by two types of cells: spermatogonia (male germ cells) and Sertoli cells. The Sertoli cells provide nourishment and support to the developing germ cells.

2.2 Male Accessory Ducts and Glands

The male sex accessory ducts include rete testis, vasa efferentia, epididymis and vas deferens.

Rete testis and Vasa efferentia: The seminiferous tubules of each testis open into rete testis through a network of tubules. The rete testis leads to vasa efferentia which connect the testis to the epididymis.

Epididymis: The epididymis is a long coiled tubule located along the posterior surface of each testis. It stores sperms and also helps in their maturation.

Vas deferens: The vas deferens (or ductus deferens or sperm duct) is a muscular tube that carries sperms from the epididymis to the urethra. It ascends to the abdomen and loops over the urinary bladder. The vas deferens receives a duct from seminal vesicle and opens into urethra as the ejaculatory duct.

Urethra: The urethra is the terminal portion which opens to the exterior through the penis. It is a common passage for both urine and sperms.

The male accessory glands include paired seminal vesicles, a prostate gland and paired bulbourethral glands (Cowper's glands). The secretions of these glands constitute the seminal plasma which is rich in fructose, calcium and certain enzymes. The secretions of bulbourethral glands also help in the lubrication of the penis.

2.3 The Penis

The penis is the male external genitalia. It is made up of special tissue that helps in erection of the penis to facilitate insemination. The enlarged end of the penis called the glans penis is covered by a loose fold of skin called foreskin.

           Urinary Bladder
                 │
    ┌────────────┼────────────┐
    │     Vas    │            │
    │   deferens │   Seminal  │
    │     ↓      │   Vesicle  │
    │  ┌─────────┴────┐       │
    │  │  Prostate    │       │
    │  │   Gland      │       │
    │  └─────────┬────┘       │
    │            │            │
    │      Bulbourethral      │
    │         Gland           │
    │            │            │
    │        Urethra          │
    │            │            │
    │         Penis           │
    │      (Erectile)         │
    │            │            │
    │      Glans Penis        │
    └────────────┴────────────┘

  External to Abdominal Cavity:
    ┌───────────────┐
    │   Scrotum     │
    │               │
    │   ┌───────┐   │
    │   │Testis │   │ ← Seminiferous tubules
    │   │(Lobules)  │   + Leydig cells
    │   └───┬───┘   │
    │       │       │
    │  Rete testis  │ ← Connects tubules
    │       ↓       │
    │ Vasa efferentia│ ← Multiple ducts
    │       ↓       │
    │  Epididymis   │ ← Sperm storage
    └───────────────┘

3. The Female Reproductive System

The female reproductive system consists of a pair of ovaries along with a pair of oviducts, uterus, vagina and external genitalia located in the pelvic region. These parts along with a pair of mammary glands are integrated structurally and functionally to support the processes of ovulation, fertilisation, pregnancy, birth and child care.

3.1 Ovaries

The ovaries are the primary female sex organs which produce the female gamete (ovum) and several steroid hormones (ovarian hormones). The ovaries are located one on each side of the lower abdomen. Each ovary is about 2 to 4 cm in length and is connected to the pelvic wall and uterus by ligaments. Each ovary is covered by a thin epithelium which encloses the ovarian stroma. The stroma is divided into two zones: a peripheral cortex and an inner medulla.

3.2 Fallopian Tubes (Oviducts)

Each fallopian tube is about 10-12 cm long and extends from the periphery of each ovary to the uterus. The part closer to the ovary is the funnel-shaped infundibulum. The edges of the infundibulum have finger-like projections called fimbriae, which help in collection of the ovum after ovulation. The infundibulum leads to a wider part of the oviduct called ampulla. The last part of the oviduct, isthmus, has a narrow lumen and joins the uterus.

3.3 Uterus

The uterus is a single pear-shaped organ also called the womb. It is divided into three parts: a fundus (upper rounded region), a body (main central region) and a cervix (narrow lower part opening into vagina). The wall of the uterus has three layers of tissue. The external thin membranous layer is called perimetrium, the middle thick layer of smooth muscle is called myometrium and the inner glandular layer is called endometrium. The endometrium undergoes cyclical changes during the menstrual cycle. The myometrium exhibits strong contractions during delivery of the baby.

3.4 Vagina

The vagina is a large muscular tube extending from the cervix to the exterior. It receives the penis during coitus and forms the birth canal during parturition.

3.5 External Genitalia

The female external genitalia includes mons pubis, labia majora, labia minora, hymen and clitoris. The mons pubis is a cushion of fatty tissue covered by skin and pubic hair. The labia majora are fleshy folds of tissue which extend from the mons pubis and surround the vaginal opening. The labia minora are paired folds of tissue under the labia majora. The opening of the vagina is often covered partially by a membrane called the hymen. The clitoris is a tiny erectile structure located at the upper junction of the labia minora.

3.6 Mammary Glands

Mammary glands are one of the characteristic features of all mammals. A functional mammary gland is present in each breast. The mammary glands are modified sweat glands. They contain 15-20 mammary lobes. Each mammary lobe is made up of clusters of cells called alveoli. The alveoli secrete milk which is stored in the lumen of alveoli. The alveoli open into mammary tubules which join to form a mammary duct. Several mammary ducts join to form a wider mammary ampulla which is connected to the lactiferous duct. Each lactiferous duct opens separately through the nipple.

       Infundibulum (Fimbriae)
              ↓
       ┌──────────────┐
       │    Ovary     │ ← Follicles
       └──────┬───────┘   (cortex + medulla)
              │
       ┌──────┴──────┐
       │   AMPULLA   │ ← Fertilisation site
       └──────┬──────┘
              │
       ┌──────┴──────┐
       │   ISTHMUS   │ ← Narrow part
       └──────┬──────┘
              ↓
       ┌────────────┐
       │  Uterus    │
       │            │
       │  Fundus    │
       │            │
       │  Body      │
       │  (Layers:) │
       │ -Perimetrium
       │ -Myometrium │
       │ -Endometrium│
       │            │
       │  Cervix    │
       │     │      │
       │ Cervical   │ ← Opening to vagina
       │  canal     │
       └─────┬──────┘
             │
        ┌────┴────┐
        │ Vagina  │
        │(Birth   │
        │ canal)  │
        └─────────┘

External Genitalia:
- Mons pubis
- Labia majora
- Labia minora
- Hymen
- Clitoris

4. Gametogenesis

Gametogenesis is the process of formation of gametes, i.e., male gametes in males and female gametes in females. The formation of male gametes is called spermatogenesis and the formation of female gametes is called oogenesis.

4.1 Spermatogenesis

Spermatogenesis is the process of formation of haploid spermatozoa (sperms) from diploid spermatogonial stem cells in the testes. It begins at puberty and continues throughout life. The process occurs in the seminiferous tubules of the testes and takes about 74 days to complete.

Spermatogenesis involves three phases: multiplication phase, growth phase and maturation phase.

Multiplication Phase: The immature male germ cells called spermatogonia (sing. spermatogonium) undergo repeated mitotic divisions to increase their number. Some of these cells periodically undergo meiosis while others continue to divide by mitosis to maintain the spermatogonial population.

Growth Phase: Each spermatogonium grows in size by accumulating food reserves and DNA to become a primary spermatocyte. The primary spermatocyte is diploid (2n).

Maturation Phase: The primary spermatocyte undergoes the first meiotic division (meiosis I) to produce two haploid (n) cells called secondary spermatocytes. Each secondary spermatocyte then undergoes the second meiotic division (meiosis II) to produce two haploid (n) spermatids. Thus, each primary spermatocyte gives rise to four haploid spermatids.

Spermiogenesis: The transformation of spermatids into mature spermatozoa (sperms) is called spermiogenesis. After spermiogenesis, sperms are released from the seminiferous tubules by the process called spermiation.

MULTIPLICATION PHASE (Mitosis)
        ┌────────────────┐
        │ Spermatogonia  │ (2n)
        │   (Diploid)    │
        └────────┬───────┘
                 │ Repeated
                 │ Mitosis
        ┌────────┴───────┐
        │  Spermatogonia │ (2n)
        │  (Many cells)  │
        └────────┬───────┘

GROWTH PHASE
                 │ Growth
                 ↓
        ┌────────────────┐
        │    PRIMARY     │ (2n)
        │ SPERMATOCYTE   │ (Diploid)
        └────────┬───────┘

MATURATION PHASE
                 │ MEIOSIS I
                 ↓
        ┌────────┴────────┐
        │                 │
   ┌────┴────┐      ┌────┴────┐
   │SECONDARY│      │SECONDARY│ (n)
   │SPERMATO-│      │SPERMATO-│ (Haploid)
   │ CYTE    │      │ CYTE    │
   └────┬────┘      └────┬────┘
        │ MEIOSIS II      │ MEIOSIS II
   ┌────┴────┐      ┌────┴────┐
   │    │    │      │    │    │
  ┌┴┐  ┌┴┐  ┌┴┐    ┌┴┐
  │S│  │S│  │S│    │S│  (n - Haploid)
  │P│  │P│  │P│    │P│  Spermatids
  │E│  │E│  │E│    │E│
  │R│  │R│  │R│    │R│
  │M│  │M│  │M│    │M│
  │A│  │A│  │A│    │A│
  │T│  │T│  │T│    │T│
  │I│  │I│  │I│    │I│
  │D│  │D│  │D│    │D│
  └┬┘  └┬┘  └┬┘    └┬┘

SPERMIOGENESIS
   │    │    │      │
   ↓    ↓    ↓      ↓
  [Sperm cells with
   head, tail formed]

SPERMIATION (Release)

RESULT: 1 Primary Spermatocyte → 4 Sperms
Duration: About 74 days

Structure of Sperm

A human sperm is a microscopic structure composed of a head, neck, middle piece and a tail. The whole body of sperm is enveloped by plasma membrane. The head contains an elongated haploid nucleus, the anterior portion of which is covered by a cap-like structure called acrosome. The acrosome is formed from Golgi complex and contains enzymes that help in fertilisation of the ovum. The middle piece possesses numerous mitochondria which produce energy for the movement of the tail that facilitates sperm motility essential for fertilisation. The human male ejaculates about 200-300 million sperms during a coitus of which 60 percent sperms must have normal shape and size and at least 40 percent of them must show vigorous motility for normal fertility.

        ┌─────────────┐
        │  ACROSOME   │ ← Cap-like structure
        │  (Enzymes)  │   (from Golgi)
        ├─────────────┤
        │             │
        │   NUCLEUS   │ ← Haploid (n)
        │    (DNA)    │   Genetic material
        │             │
        └──────┬──────┘
               │
        ┌──────┴──────┐
        │    NECK     │ ← Connecting piece
        └──────┬──────┘
               │
        ╔══════╧══════╗
        ║   MIDDLE    ║ ← Mitochondria
        ║   PIECE     ║   (Energy for
        ║ [Mitochondria]  movement)
        ╚══════╤══════╝
               │
               │
        ═══════╪═══════
        ═══════╪═══════ ← TAIL (Flagellum)
        ═══════╪═══════   for motility
        ═══════╪═══════
        ═══════╪═══════
               │
               ▼
           (Movement)

HEAD: Acrosome + Nucleus
MIDDLE PIECE: Mitochondria
TAIL: Flagellum for swimming

4.2 Oogenesis

Oogenesis is the process of formation of a mature female gamete or ovum. It occurs in the ovaries. It is initiated during the embryonic development stage when millions of gamete mother cells (oogonia) are formed within each fetal ovary. These oogonia undergo mitotic division and start dividing. Some of these cells enter prophase I of meiotic division I and get temporarily arrested at this stage. These are called primary oocytes. Each primary oocyte then gets surrounded by a layer of granulosa cells and is called a primary follicle. A large number of these follicles degenerate during the period from birth to puberty. Therefore, at puberty only about 60,000-80,000 primary follicles are left in each ovary.

The primary follicles get surrounded by more layers of granulosa cells and a new theca layer to form secondary follicles. The secondary follicle soon transforms into a tertiary follicle which is characterized by a fluid-filled cavity called antrum. At this stage the primary oocyte within the tertiary follicle grows in size and completes its first meiotic division. This is an unequal division resulting in the formation of a large haploid (n) secondary oocyte and a tiny first polar body. The secondary oocyte retains the bulk of the nutrient-rich cytoplasm.

The tertiary follicle further changes into a mature follicle or Graafian follicle. The Graafian follicle ruptures to release the secondary oocyte (ovum) from the ovary by the process called ovulation. It is important to note that the second meiotic division of the secondary oocyte is completed only after fertilisation.

5. Menstrual Cycle

The reproductive cycle in the female primates (monkeys, apes and humans) is called the menstrual cycle. The first menstruation begins at puberty and is called menarche. In human females, menstruation is repeated at an average interval of about 28 days. The cycle of events starting from one menstruation till the next one is called the menstrual cycle. One ovum is released during the middle of each menstrual cycle.

The menstrual cycle consists of four phases:

5.1 Menstrual Phase (Day 1-5)

Menstruation marks the beginning of the menstrual cycle (day 1). The menstrual flow results due to the breakdown of the endometrial lining of the uterus and its blood vessels which forms liquid that comes out through the vagina. Menstruation occurs only if the released ovum is not fertilised. The menstrual flow lasts for 3-5 days.

5.2 Follicular Phase (Day 6-13)

The follicular phase overlaps with the menstrual 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 proliferation. These changes in the ovary and the uterus are induced by changes in the levels of pituitary and ovarian hormones. The secretion of gonadotropins (LH and FSH) increases gradually during the follicular phase. These hormones stimulate follicular development and secretion of estrogens by the growing follicles. Estrogens stimulate the proliferation of endometrium.

5.3 Ovulatory Phase (Day 14)

Both LH and FSH attain a peak level in the middle of the cycle (about the 14th day). Rapid secretion of LH leading to its maximum level during the mid-cycle is called LH surge. LH surge induces rupture of the Graafian follicle and thereby the release of the ovum (secondary oocyte). This process is called ovulation.

5.4 Luteal Phase (Day 15-28)

During the luteal phase, the remaining parts of the Graafian follicle transform into the corpus luteum. The corpus luteum secretes large amounts of progesterone which is essential for the maintenance of the endometrium. The endometrium becomes thick and richly vascularized. This is necessary for implantation of the fertilised ovum and other events of pregnancy. If fertilisation does not occur, the corpus luteum degenerates and this causes disintegration of the endometrium leading to menstruation, marking a new cycle.

In human beings, menstrual cycles cease around 50 years of age. This is called menopause.

6. Fertilisation and Implantation

6.1 Fertilisation

Fertilisation is the fusion of a sperm with an ovum. During copulation (coitus), semen is released by the penis into the vagina (insemination). The motile sperms swim rapidly through the cervix, enter the uterus and finally reach the junction of the fallopian tube and the uterus (ampullary-isthmic junction). This is the site of fertilisation.

The ovum released from the ovary is also transported to the ampullary-isthmic junction where fertilisation takes place. Fertilisation can occur only if the ovum and sperms are transported simultaneously to the ampullary-isthmic junction. This is facilitated by the fimbriae of the fallopian tube.

The process of fertilisation involves several sequential steps. The sperm comes in contact with the zona pellucida layer of the ovum and induces changes in the membrane that block the entry of additional sperms. Thus, it ensures that only one sperm can fertilise an ovum. The secretions of the acrosome help the sperm to penetrate the zona pellucida and the plasma membrane of the ovum. This induces the completion of the meiotic division of the secondary oocyte. The second meiotic division is also unequal and results in the formation of a second polar body and a haploid ovum. Soon the haploid nucleus of the sperm and that of the ovum fuse together to form a diploid zygote.

6.2 Implantation

The mitotic division of the zygote starts as it moves through the fallopian tube towards the uterus. These mitotic divisions are called cleavage. The embryo with 8 to 16 blastomeres is called a morula. The morula continues to divide and transforms into a blastocyst as it moves further into the uterus. The blastomeres in the blastocyst are arranged into an outer layer called trophoblast and an inner group of cells called the inner cell mass.

The trophoblast layer attaches to the endometrium and the inner cell mass gets differentiated 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. Implantation leads to pregnancy.

7. Pregnancy and Embryonic Development

After implantation, the inner cell mass (embryo) differentiates into an outer layer called ectoderm and an inner layer called endoderm. A middle layer called mesoderm appears between the ectoderm and the endoderm. These three layers give rise to all tissues and organs in adults. It is therefore called the germinal layers.

The human pregnancy lasts for about 9 months, which is called the gestation period. During pregnancy, the embryo develops in the uterus. The outer layer of the blastocyst called the trophoblast layer differentiates into two layers: an outer syncytiotrophoblast and an inner cytotrophoblast. These layers along with the uterine tissue form the placenta.

7.1 Placenta

The placenta is a disc-shaped structure attached to the uterine wall through villi on one side and to the embryo through the umbilical cord on the other side. The placenta facilitates the supply of oxygen and nutrients to the embryo and removal of carbon dioxide and excretory wastes from the embryo. The placenta also acts as an endocrine gland and produces several hormones like human chorionic gonadotropin (hCG), human placental lactogen (hPL), estrogens and progesterone. The detection of hCG in urine forms the basis of pregnancy tests.

In the placenta, the villi of the trophoblast are surrounded by the uterine tissue and maternal blood. However, the maternal blood and the fetal blood do not come in direct contact. Nutrients and oxygen diffuse from the maternal blood into the fetal blood.

7.2 Embryonic Development

After one month of pregnancy, the embryo's heart is formed. The first signs of limbs and digits are developed by the end of the second month. By the end of 12 weeks (first trimester), the major organ systems are formed. The limbs and external genital organs are well developed. The first movements of the fetus and appearance of hair on the head are usually observed during the fifth month. By the end of 24 weeks (end of second trimester), the body is covered with fine hair, eyelids separate and eyelashes are formed. By the end of nine months of pregnancy, the fetus is fully developed and is ready for delivery.

8. Parturition and Lactation

8.1 Parturition

The average duration of human pregnancy is about 9 months which is called the gestation period. Vigorous contraction of the uterus at the end of pregnancy causes expulsion/delivery of the fetus. This process of delivery of the fetus is called parturition. Parturition is induced by a complex neuroendocrine mechanism.

The signals for parturition originate from the fully developed fetus and the placenta. These induce mild uterine contractions called fetal ejection reflex. This triggers the release of oxytocin from the maternal pituitary. Oxytocin acts on the uterine muscle and causes stronger uterine contractions, which in turn stimulate further secretion of oxytocin. This is a positive feedback mechanism. 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, i.e., parturition. Soon after the infant is delivered, the placenta is also expelled out of the uterus.

8.2 Lactation

Lactation is the production of milk by mammary glands. The mammary glands show differentiation during pregnancy and start producing milk towards the end of pregnancy by a process called lactogenesis. This helps the mother in feeding the newborn. The milk produced during the initial few days of lactation is called colostrum which contains several antibodies absolutely essential to develop resistance in the newborn baby. Breast-feeding during the initial period of infant growth is recommended by doctors for bringing up a healthy baby.

The process of milk production and its ejection from the mammary glands is hormonally regulated. Prolactin from the anterior pituitary stimulates the mammary glands to secrete milk. Oxytocin causes milk ejection from the mammary glands.

9. Practice Questions

Multiple Choice Questions

Very Short Answer Questions (1 mark)

Short Answer Questions (2-3 marks)

Long Answer Questions (5 marks)