Revision: Reproduction >> Human Reproduction Biology (Theory) ISC (Science) ISC Class 12 CISCE

Puberty is the process of physical and hormonal changes by which a child’s body matures into an adult body capable of sexual reproduction to enable fertilization.

Reproduction is a process that results in the formation of new individuals of the same kind, though usually with slight genetic, structural and physiological variations.

1. 1. Umbilical cord
   2. Placenta
   3.  Amniotic fluid
2. Functions of placenta: 
   • It allows the diffusion of oxygen, nutrients and immune products from the mother to the foetus.
   • It allows the diffusion of waste material generated by the foetus to the mother for excretion.
3. Functions of amniotic fluid:
   • The amniotic fluid prevents the foetus from injury and shocks.
   • It allows the foetus some amount of movement.
   • It prevents the amnion from sticking to the foetus.
   • It maintains an even pressure all around the foetus.
4. Gestation:
   • The time period required for the development of the foetus inside the uterus is called gestation. 
   • The normal gestation period in humans is 280 days (9 months).
5. Male sex chromosomes:
   • One ‘X’ chromosome and one ‘Y’  chromosome (XY).
   • Female sex chromosomes: Two ‘X’ chromosomes (XX).

The two sexes, male and female, often show outwardly differentiating features called secondary sexual characters.

or

The characters by which we can differentiate between male and female sexes externally are called secondary sexual characters.

The sex organs which produce gametes and sex hormones are called primary sex organs.

The sex organs that do not produce gametes but assist in reproduction by supporting, transporting, or nourishing gametes are called secondary sex organs.

The secretion of glands with spermatozoa is a whitish, viscous fluid known as seminal fluid or semen.

The process of expulsion of semen from the urethra is called ejaculation.

Hernia is an abnormal condition that is caused when the intestine, due to the pressure in the abdomen, bulges into the scrotum through the inguinal canal.

Spermatogenesis is the process of formation and development of sperm cells (spermatozoa) in the testes from spermatogonial stem cells through mitosis and meiosis.

A pair of oval male reproductive glands that produce sperms and secrete testosterone are called testes.

The fold of peritoneum that supports and attaches the testis is called mesorchium.

The condition in which one or both testes fail to descend into the scrotum is called cryptorchidism.

The fibrous cord extending from the caudal end of the testis to the scrotal wall that helps in testicular descent is called gubernaculum.

The two testes (popularly called testicles) are oval organs which are contained in a thin-walled sac of skin called the scrotum (or scrotal sac).

or

The testes of a male remain suspended in a pouch, called the scrotum.

Seminiferous tubules (semini: sperm, ferous: bearing) where the sperms are produced. The process is called spermatogenesis.

The opening of the vagina in young females is partially closed by a thin membrane called the hymen (or virgin knot).

The external female genitalia is called the vulva.

When the follicle enlarges and fills with fluid as the egg grows, it is called a Graafian follicle.

The remnant of the follicle persists for some time to convert into a yellow mass called corpus luteum (corpus: body, luteum: yellow).

A maturing egg contained in a cellular sac is called the follicle.

A structure in the ovary consisting of an ovum surrounded by layers of granulosa cells that supports the growth and maturation of the ovum is called an ovarian follicle.

The process by which a mature Graafian follicle ruptures and releases the secondary oocyte from the ovary into the fallopian tube is called ovulation.

The white fibrous tissue formed by degeneration of the corpus luteum when fertilization does not occur is called the corpus albicans.

The series of changes in the ovary involving follicle development, ovulation, and formation and degeneration of the corpus luteum is called the ovarian cycle.

The funnel-shaped opening of the oviduct that picks up the released egg with the help of cilia is called the oviducal funnel.

The lower part of the uterus is very narrow and is called cervix.

The thin membrane partially covering the vaginal opening in young females is called the hymen.

The skin over the centre of the elevation has a darkly pigmented circular area called areola.

The glandular tissue is divided into 15–20 mammary lobes that radiate outward from the nipple and contain clusters of milk-producing glands, called alveoli.

A projection overlies the central part of the areola that is known as nipple.

Spermiogenesis is the process of transforming spermatids into mature, flagellated spermatozoa (sperms).

The process of transformation of a circular spermatid to a spermatozoon is called spermiogenesis.

Spermiation is the process of releasing mature spermatozoa. In this, spermatozoa are shed into the lumen of the seminiferous tubule for transport.

After spermiogenesis, sperm heads become embeded in sertoli cells from which they obtain their nutrition and finally get released into the lumen of seminiferous tubules. This process of release of mature sperms from sertoli cells is called spermiation.

Gametogenesis is the process by which male and female primary sex organs produce gametes (sperms and ova).

or

Gametogenesis is the process by which male and female gametes are formed from germinal cells in the gonads through a series of developmental stages.

Gametogenesis is the process of formation of gametes i.e., sperms and ovary from the primary sex organs in all sexually reproducing organisms. Meiosis plays the most significant role in the process of gametogenesis.

Formation of haploid sperms (male gametes) from diploid spermatogonia (sperm mother cells) is called spermatogenesis.

The process of release of mature sperms from Sertoli cells into the lumen of seminiferous tubules is called spermiation.

or

Spermiogenesis (also called spermateleosis) is the process of transformation (metamorphosis) of spermatids into mature spermatozoa (sperms).

The formation of ova in the ovary from primordial germinal cells is said to be oogenesis.

Puberty is the period during which the immature reproductive systems of boys and girls mature and become capable of reproducing.

or

The period during adolescence when the reproductive tissues mature and the body undergoes changes to attain sexual maturity is called puberty.

Adolescence is the time period between the beginning of puberty and adulthood. During this period, the body undergoes several changes alongside reproductive maturity. It begins around the age of 11 and lasts till 18 or 19 years of age. The period of adolescence may vary from person to person.

The release of the ovum from the ovary is called ovulation.

It is the stage when a girl menstruates for the first time.

The rhythmic series of changes in the sex organs throughout the reproductive life of a female primate (e.g., monkeys, apes, and human beings) from puberty to menopause is called the menstrual cycle (L. mensis = month, lunar month).

Menarche is the onset of menstruation in a young female at about the age of 13 years (arche: beginning).

or

The appearance of the first menstrual flow in the life of a girl is called menarche.

Menopause is the permanent stoppage of menstruation in females at about the age of 45 years (pause: stop).

or

The permanent disappearance of menstrual cycles in the life of a woman is called menopause.

A series of rapid mitotic divisions of the zygote without an increase in size is called cleavage.

An extra-embryonic sac involved in waste storage and placental development is called allantois.

A hollow embryonic stage formed after morula in humans is called blastocyst.

The process of formation of blastula from morula is called blastulation.

A solid, mulberry-like ball of cells formed after cleavage is called morula.

Cleavage is the process of early mitotic division of the zygote to generate a multicellular morula stage.

A study of the development of an organism from fertilization to the formation of young one is known as embryogeny.

The formation of a zygote by the union of male and female gametes is known as fertilization.

The union of a male gamete with the egg cell to form a zygote is called fertilization.

The fusion of the male gamete (sperm) and female gamete (ovum) to form a zygote is called fertilization.

or

The formation of a zygote by the union of a sperm and an ovum is called fertilization.

A disc-like structure attached to the uterine wall that supplies food and oxygen to the foetus and removes waste is called placenta.

or

The intimate connection established between the foetal membrane and the uterine wall is known as placenta.

After nine months of pregnancy, the fully developed foetus is ready for delivery. The process of childbirth is called parturition.

or

Parturition is the act of expelling the full-term foetus from the mother's uterus at the end of gestation.

The term 'lactation' refers to the secretion and ejection of milk by the mammary glands.

• Humans are sexually reproducing and viviparous (giving birth to young ones, not eggs).
• 6 stages of reproduction in order: Gametogenesis → Insemination → Fertilisation → Implantation → Gestation → Parturition.
• Gametogenesis (gamete formation), Insemination (sperm transfer), Fertilisation (fusion → zygote), Implantation (blastocyst attaches to uterine wall), Gestation (embryo development), Parturition (delivery of baby).
• Sperm formation continues throughout a man's life; ovum formation ceases at around age fifty (menopause) in women.
• Testes (males) produce sperm; Ovaries (females) produce eggs; these are the primary reproductive organs (gonads).
• Features like beards (males) and breasts (females) are secondary sexual characteristics; they do not directly participate in reproduction.

1. Primary sex organs (testes and ovaries) produce gametes and hormones, whereas secondary sex organs facilitate the reproductive process without producing gametes.
2. Human development is divided into the prenatal (embryonic) period inside the mother's womb and the postnatal period extending from birth to death.
3. The reproductive process begins with gametogenesis (formation of sperm and ova) followed by insemination (transfer of sperm into the female tract).
4. Fertilization unites the gametes to form a zygote, which then undergoes rapid mitotic divisions called cleavage to form a multicellular blastocyst.
5. The blastocyst implants into the uterine wall, establishes a placenta, and undergoes gastrulation and organogenesis to form specific tissues and organ systems.
6. The gestational period ends with parturition (childbirth), and it is notable that males can produce gametes lifelong, while females cease production around age fifty.

• The male reproductive system is located in the pelvic region and includes testes, accessory ducts, accessory glands, and external genitalia (penis and scrotum).
• Testes are the primary sex organs present in the scrotum; they contain seminiferous tubules where sperm are produced, while Leydig cells secrete testosterone, and Sertoli cells provide nourishment.
• Accessory ducts - rete testis → vasa efferentia → epididymis → vas deferens → ejaculatory duct → urethra—help in storage, maturation, and transport of sperm.
• Accessory glands - seminal vesicles, prostate gland, and bulbourethral glands - add secretions like fructose, enzymes, and mucus, which nourish sperms and aid in their movement.
• The penis acts as the external genital organ, containing the urethra, and helps in the transfer of sperm during ejaculation.

1. Testes are a pair of oval-shaped glands responsible for producing sperm, with an adult testis measuring roughly 4–5 cm in length, 2–3 cm in width, and weighing about 25 g.
2. During early foetal life, they develop in the abdominal cavity and descend into the scrotum through the inguinal canal during the 7th month of development.
3. The testes are structurally supported by a peritoneal fold called the mesorchium and anchored to the scrotal wall by a fibrous cord known as the gubernaculum.
4. Clinical conditions related to this descent include cryptorchidism (the failure of the testes to descend into the scrotum) and inguinal hernia (when an intestinal loop enters the scrotum through an unclosed inguinal canal).

1. The testes are located outside the abdomen in the scrotum, which is divided internally by the septum scroti.
2. The scrotum maintains a temperature 2-2.5°C below normal body temperature – a critical requirement for spermatogenesis – through a lack of fatty insulation and involuntary muscle adjustments.
3. Testicular location varies among mammals; they descend only during breeding seasons in some (e.g., bats, llamas) and remain permanently internal in others (e.g., elephants, whales).

• Each testis is enclosed by three protective layers – tunica vaginalis, tunica albuginea, and tunica vasculosa.
• Internally, each testis is divided into about 250 testicular lobules.
• Each lobule contains highly coiled seminiferous tubules, which are the site of spermatogenesis.
• The wall of seminiferous tubules is lined by germinal epithelium consisting of spermatogonia, Sertoli cells, and interstitial cells.
• Spermatogonia are diploid male germ cells that undergo meiosis to form haploid sperms.
• Sertoli cells act as nurse cells, providing nourishment and support to developing sperms, while Leydig cells secrete testosterone.
• Sperms move from seminiferous tubules into efferent ducts, then to the epididymis for maturation and temporary storage before entering the vas deferens.

• Semen is a viscous, milky, and alkaline fluid (pH 7.2 to 7.7) consisting of sperm suspended in accessory gland secretions.
• These secretions nourish the sperm (via fructose), neutralise vaginal acidity (via calcium and bicarbonates), and stimulate motility (via prostaglandins).
• An average ejaculation yields 2.5 to 5 ml of fluid, with sperm cells making up only about 10% of the total semen volume.
• Upon ejaculation, semen temporarily coagulates due to the protein fibrinogen, then liquefies after approximately 20 minutes due to the enzyme plasmin.
• Male sterility can be indicated by a sperm count below 20 × 10⁶ per ml, high numbers of non-motile or abnormal sperm, oligospermia (abnormally low sperm count), or azoospermia (complete absence of sperm).

• Includes - Ovaries, Fallopian tubes, Uterus, Cervix, Vagina, External genitalia, Bartholin's glands, Mammary glands - all in the pelvic region.
• Ovaries - outer cortex (follicles) + inner medulla; produce ova and ovarian hormones; release one ovum monthly after puberty.
• Fallopian tube - 3 parts: Infundibulum (fimbriae collect ovum) → Ampulla (site of fertilisation) → Isthmus (connects to uterus); cilia push egg towards uterus.
• Uterus - 3 layers: Perimetrium (outer), Myometrium (muscular), Endometrium (inner, menstrual changes); opens into the vagina via the cervix (birth canal).
• Zygote implants in the endometrium; the placenta connects the embryo to the mother for nutrient and waste exchange till birth.
• External genitalia - Mons pubis, Labia majora, Labia minora, Hymen, Clitoris. Bartholin's glands provide lubrication.
• Mammary glands produce milk for newborns. Puberty begins at 10–14 years in females.

• Ovaries are the primary female sex organs that produce ova and secrete the hormones estrogen and progesterone.
• They are almond-shaped, solid structures located in the lower abdominal cavity and attached by the mesovarium.
• Each ovary is covered by germinal epithelium and internally divided into cortex and medulla.
• The cortex contains ovarian follicles with immature oocytes, supported by granulosa cells.

• Mammary glands are paired structures containing 15 to 20 lobes with milk-producing alveoli, ducts, a pigmented areola, and a nipple.
• The hormone prolactin stimulates the alveoli to produce milk, while oxytocin triggers its ejection following childbirth.
• The initial milk secreted, known as colostrum, is highly concentrated with essential nutrients and antibodies for the newborn.
• Following the period of lactation or the onset of menopause, the glandular tissue naturally shrinks and undergoes atrophy.
• Weaning is the gradual developmental process of replacing a mother's milk with solid foods in an infant's diet.
• Clinically, breast cancer often presents as a painless lump, is diagnosed through mammograms or biopsies, and is treated with surgery, radiation, or chemotherapy.

• Gametogenesis is the process of formation of male and female gametes in the reproductive organs.
• It occurs in the testes of males and ovaries of females.
• Spermatogenesis produces sperms, while oogenesis produces ova (eggs).
• In this process, diploid germ cells form haploid gametes for sexual reproduction.

• Spermiogenesis is the process by which spermatids are transformed into mature spermatozoa (sperms).
• During spermiogenesis, the nucleus becomes condensed, the acrosome develops, and the tail is formed.
• Mitochondria arrange around the middle piece to supply energy for sperm movement.
• The mature sperm becomes specialised for motility and fertilisation of the ovum.
• One primary spermatocyte finally produces four sperms after meiosis and spermiogenesis.
• The release of mature sperms from Sertoli cells into the lumen of seminiferous tubules is called spermiation.

• The release of gonadotropin-releasing hormone (GnRH) stimulates the anterior pituitary gland to secrete luteinizing hormone (LH) and follicle-stimulating hormone (FSH).
• LH triggers Leydig cells to produce testosterone, while FSH prompts Sertoli cells to release factors that support sperm development.
• Spermatogenesis requires an environment cooler than the core body temperature, which is naturally maintained by the testes residing in the scrotal cavity.
• Sperm achieve maturation and full motility as they travel through the epididymis, aided by stimulating secretions from the seminal vesicles and prostate.

• Puberty marks the transition to reproductive maturity, starting around age 10 for girls (thelarche) and age 11 for boys (gonadarche).
• Girls experience breast development, hip widening, and the onset of menstruation (menarche) driven by rising estrogen, FSH, and LH.
• Boys develop increased muscle mass, broader shoulders, facial hair, and a deepening voice due to elevated testosterone.
• Both sexes undergo a rapid growth spurt and develop pubic and axillary hair influenced by sex and growth hormones.
• Hormonal shifts increase skin and sweat gland activity, commonly leading to acne and heightened emotional and psychological changes.

• The maturation of the connection between the hypothalamus and anterior pituitary allows GnRH to trigger the secretion of LH and FSH.
• These gonadotropins stimulate the gonads to produce sex steroids, primarily testosterone in boys and estradiol in girls.
• Consequently, the initially low childhood levels of LH, FSH, prolactin, and sex steroids rise steadily to reach adult concentrations.
• Prior to these gonadal changes, adrenal androgens increase starting around age eight to promote pubic and axillary hair growth in both sexes.

• The menstrual cycle is a series of monthly changes in females of reproductive age.
• The beginning of the cycle is called menarche, and its permanent stop is called menopause (around 45–50 years).
• It occurs mainly in primates, including humans, and involves changes in the ovaries and uterus.
• The cycle is controlled by hormones (gonadotropins and ovarian hormones) and repeats every ~28 days.
• In the middle of the cycle, ovulation occurs, where an egg is released from one of the ovaries.

• Both LH and FSH reach their peak levels in the middle of the cycle, around the 14th day.
• Hormones such as hCG, hPL, and relaxin are produced in women only during pregnancy.
• In pregnancy tests, the first morning urine contains the highest concentration of hCG.
• The activation of sperm for fertilising the egg is termed capacitation.
• The zona pellucida functions to prevent an increase in the size of the embryo.
• Nervous and emotional influences, such as emotional disturbances, can delay or prevent menstruation.

• Following fertilization, the single-celled zygote undergoes rapid mitotic divisions known as cleavage to form a solid, multicellular ball of cells termed the morula.
• The morula continues to divide and transforms into a hollow structure called the blastocyst, which consists of an outer trophoblast layer and an inner cell mass.
• By approximately the seventh day post-fertilization, the blastocyst embeds itself into the endometrial lining of the mother's uterus in a critical process called implantation.
• The inner cell mass subsequently undergoes gastrulation, a phase where it differentiates into three primary embryonic germ layers: the ectoderm, mesoderm, and endoderm.
• During organogenesis, these three germ layers systematically give rise to all specific tissues, internal organs, and organ systems, typically beginning with the formation of the heart.
• A temporary structural and functional organ known as the placenta develops from trophoblastic and maternal tissues to facilitate the essential exchange of nutrients, gases, and wastes.
• The complete sequence of embryonic and foetal development is sustained throughout a gestation period of approximately nine months, ultimately culminating in childbirth.

• Fertilization in humans is the fusion of male and female gametes (sperm and ovum) and is an internal process occurring in the oviduct, leading to the formation of a diploid zygote.
• During copulation, millions of sperms are released into the vagina, but only one sperm reaches the ovum due to selection and protective barriers around the egg.
• Before fertilization, sperm undergo capacitation and move towards the ovum; interaction between sperm and egg involves recognition molecules like fertilizin (ovum) and antifertilizin (sperm).
• The sperm’s acrosome releases enzymes that help digest the egg coverings (corona radiata and zona pellucida), allowing sperm entry through the membrane.
• After entry, the sperm nucleus and the ovum nucleus fuse (syngamy), forming a zygote (2n); this process restores the diploid chromosome number (46 in humans).
• Fertilization triggers completion of meiosis II in the oocyte, formation of the second polar body, and formation of the female pronucleus and male pronucleus.
• In humans, sperm carry 22+X or 22+Y chromosomes, while ova carry 22+X only, determining the genetic sex of the offspring.

• Meaning – The placenta is a temporary structural and functional connection between foetal and maternal circulation, formed by chorionic villi (finger-like projections of trophoblast) interdigitating with uterine tissue and maternal blood.
• Attachment - The placenta is attached to the wall of the uterus on one side and connected to the baby via the umbilical cord on the other side.
• Umbilical cord – Contains two small arteries (carry blood towards the placenta) and one large vein (returns blood to the foetus).
• Functions - Supplies oxygen and nutrients to the foetus; removes CO₂ and excretory wastes from foetal blood to maternal blood.
• Hormones produced – The placenta produces hCG (human Chorionic Gonadotropin), hPL (human placental lactogen), oestrogen, and progesterone. By the end of the 1st trimester, progesterone production shifts to the placenta.

• Parturition is the process of childbirth, i.e., the delivery of the foetus at the end of the gestation period.
• It is controlled by a neuroendocrine mechanism involving signals from the fully developed foetus and placenta.
• Foetal signals trigger uterine contractions known as the foetal-ejection reflex (labour pain).
• Hormones like ACTH and corticosteroids from the foetus stimulate the release of oxytocin from the mother’s pituitary gland.
• Parturition occurs in three stages: dilation, expulsion of the baby, and afterbirth (placenta removal).

• Lactation involves the mammary glands secreting and ejecting milk to nourish and protect the newborn, initiated after childbirth.
• Prolactin, released by the anterior pituitary, stimulates alveolar cells to produce milk, a process called lactogenesis.
• Oxytocin, from the posterior pituitary, triggers the let-down reflex by causing myoepithelial cells to contract and eject milk when the baby suckles.
• Colostrum, the initial yellowish milk produced for a few days post-birth, is low in fat but highly concentrated in proteins, lactose, and essential antibodies.
• The abundant IgA antibodies present in colostrum transfer crucial natural passive immunity to the infant, protecting them while their own immune system develops.