Human Reproduction | Class-12 Notes | CBSE Biology

⮚              Introduction

✔    Gametogenesis: Process of formation of gametes.

✔    Insemination: Process of transfer of the male gametes (sperm) into the female genital tract.

✔    Implantation: Process of attachment of blastocyst to the uterine walls.

✔    Gestation: The event of embryonic development.

✔    Parturition: The process of delivery of the baby.

⮚              The male reproductive system

✔    Location: Pelvis region

✔    Consists of :

o   Pair of Testes: (Sing. Testis), also called testicles

o   Accessory ducts: (Rete Testis, Vasa Efferentia, Epididymis, Vas Deferens)

o   Glands: (Paired Seminal Vesicle, Prostate gland, Bulbourethral gland)

o   External Genitalia: (Penis)

The Testis (Primary Sex Organ)

✔    Location: Inside a pouch like structure called scrotum, Present outside the abdominal cavity.

✔    Shape : Oval

✔    Size: Length 4-5 cm; Width 2-3 cm.

✔    Role of scrotum / Why are testes located outside the abdominal cavity?

o   The production of the sperm requires 2-2.5⁰ C less than the normal body cavity. To maintain this lower temperature the testes are located outside the abdominal cavity.

Structural Organisation

✔     Each of the testis has about 250 compartments called testicular lobules.

✔    Each of the testicular lobules has 1-3 highly coiled structure called seminiferous tubules.

o   Sperm in developed in the seminiferous tubules.

o   The seminiferous tubules are lined by two types of cells on the inside

▪    Spermatogonia – Male germ cells

●    Produces sperm by meiotic cell division.

▪    Sertoli Cells

●    Provides nutrition to the developing sperm cells.

▪    Immunologically competent cells

✔    Interstitial spaces: The region outside the seminiferous tubules, within the testicular lobules.

o   It consists of :

▪    Small blood vessels

▪    Leydig cells or interstitial cells.

●    Releases Androgen (Testicular Hormone)

Male Accessory ducts: (Secondary Sex Organ)

✔    The seminiferous tubule opens into the vasa efferentia through the rete testis.

✔    The vasa efferentia opens into the epididymis (located on the posterior side of the testis).

✔    The epididymis then opens into the vas deferens.

✔    The vas deferens then receives the duct from the seminal vesicle and opens into urethra as the ejaculatory duct.

o   Role of ejaculatory duct: to store and transport the sperms from the testis to the outside through urethra.

✔    Urethra starts from the Urinary Bladder and extends through the penis.

✔    The external opening of the penis is called the urethral meatus.

The Male Accessory Glands: (Secondary Sex Organ)

✔    Consists of seminal vesicle (paired), Bulbourethral gland (paired) and a prostate gland.

✔    Together they secrete Seminal Plasma

✔    Prostate gland: Milky secretion. It nourishes and activates the spermatozoa to swim.

✔    Seminal Vesicle: Secrete mucus and alkaline fluid containing fructose.

o   Only place where fructose is secreted in the body.

o   It acts as the substrate for the energy production by sperm.

✔    Bulbourethral gland/ Cowper’s gland: Release mucus fluid that helps in the lubrication of the penis.

✔    Sperm + Seminal Plasma = Semen

The Penis: (External Genitalia)

✔    Consists of specialized tissue – erectile tissue.

✔    This helps in erection, which is necessary for the process of insemination.

✔    The Glans Penis (enlarged end of penis) is covered by the Foreskin (loose fold of skin)

➢  The Female reproductive system

✔    Location: Pelvis region

✔    Consists of

o   Pair of Ovaries :(Sing. Ova)

o   Accessory ducts   🙁Paired Fallopian Tube, Uterus, Cervix, Vagina)

o   External Genitalia   🙁Vulva)

o   Glands   🙁Paired Mammary gland)

Ovaries (Primary Sex Organs)

✔    Location: One on each side of lower abdomen.

✔    Size: 2 to 4 cm in length

✔    Role of Ovaries

o   Production of Ovum/Egg (Female gamete)

o   Secretion of steroid hormones

▪    Estrogen, Progesterone, Relaxin

✔    Each ovary is covered by epithelium, and contains the ovarian stroma.

✔    The ovarian stroma is made up of:

o   Peripheral cortex

o   Inner medulla

Fallopian Tube / Oviduct (Secondary Sex Organ)

✔    Tube like structure that connects the uterus to the periphery of the ovary.

✔    Length : About 10-12 cm long

✔    Oviduct is divided into 3 parts:

o   Infundibulum : Funnel shaped structure closest to the ovary

▪    The edges contain finger like structure called Fimbriae that help in collection of the egg after ovulation and pass it on to the oviduct.

o   Ampulla : Middle wider structure of the oviduct.

o   Isthmus : Closest to the uterus and joins with it.

Uterus / Womb (Secondary Sex Organ)

✔    Shape: Inverted Pear.

✔    The uterus opens into the vagina via a narrow opening called the cervix.

✔    The cavity of the cervix is called cervical canal.

✔    Cervical Canal + Vagina = Birth Canal

✔    Uterine wall has 3 tissue layer:

o   Perimetrium: Outer most layer; thin membrane.

o   Myometrium: Middle Layer; Thick muscular (Smooth muscle)

▪    Exhibits strong contraction during the process of child birth.

o   Endometrium: Innermost layer; Glandular

▪    Undergoes cyclic change and play important role in the menstruation.

External Genitalia

✔    Consists of :

o   Mons Pubis

▪    A cushion of fatty tissue covered by skin and pubic hair

o   Labia Majora

▪    Fleshy folds of tissue, which extend down from the mons pubis and surround the vaginal opening.

o   Labia Minora

▪    Paired folds of tissue under the labia majora.

o   Hymen

▪    Membrane covering the opening of the vagina.

▪    It is often torn during the first coitus (intercourse).

▪    It can also be broken by a sudden fall or jolt, insertion of a vaginal tampon, active participation in some sports like horseback riding, cycling, etc.

▪    In some women the hymen persists even after coitus.

●    Therefore, the presence or absence of hymen is not a reliable indicator of virginity or sexual experience.

o   Clitoris

▪    Tiny finger like structure

▪    Present at the upper junction of the two labia minora above the urethral opening.

Mammary Gland (Secondary Sex Organ)

✔    Paired structure consists of glandular tissue and variable amount of fat.

✔    The glandular tissue is divided into 15-20 mammary lobes.

✔    The mammary lobes contain cluster of cells called alveoli.

o   The cells of alveoli secrets Milk.

o   It is stored in the lumens (cavities) of alveoli.

✔    The alveoli open into mammary tubules.

✔    The mammary tubules join to form a mammary duct.

✔    Several mammary ducts join to form a wider mammary ampulla

✔    The ampulla is connected to lactiferous duct through which milk is sucked out.

➢  Gametogenesis

✔    It refers to the process of development of sperm and ovum in the testis and ovaries respectively.

✔    Spermatogenesis is the process of gametogenesis in males while Oogenesis is the process of gametogenesis in females.

Spermatogenesis

✔    Process begins at the age of puberty.

✔    The process of Spermatogonium (Pl.– Spermatogonia) developing into the spermatozoa.

✔    Location of spermatogonia: inside wall of seminiferous tubules.

✔    Step #1 : Mitotic division of spermatogonia to increase in number.

o   Ploidy level : Diploid (46 chromosomes)

✔    Step #2 : Some of the spermatogonia undergoes meiosis. These are called the primary spermatocytes.

o   Ploidy level : Diploid (46 chromosomes)

✔    Step #3 : At the end of Meiosis-1, two cells are formed called the secondary spermatocytes.

o   Ploidy level : Haploid (23 chromosomes)

✔    Step #4 : The secondary spermatocytes undergoes meiosis-2 and produces four cells, called the spermatids.

o   Ploidy level : Haploid (23 chromosomes)

✔    Step #5 : Spermatids are transformed into the spermatozoa (sperms) by the process of spermiogenesis.

✔    Step #6 : The sperms cells heads then gets embedded into the Sertoli cells to complete the development process.

✔    Spermiation : The process of release of fully developed sperms from the seminiferous tubules.

Hormonal Regulation:

✔    The process is regulated by gonadotropin releasing hormone (GnRH) secreted from the hypothalamus.

✔    The GnRH stimulates the anterior pituitary and secrets two gonadotropins: luteinising hormone (LH) and follicle stimulating hormone (FSH).

Structure of Sperm

✔    Single celled structure.

✔    Consists of a head, neck, a middle piece and a tail.

✔    A single plasma membrane covers the entire sperm.

✔    Sperm Head : Contains nucleus with haploid set of chromosomes

o   Acrosome– cap like structure on the anterior portion of head.

▪    Filled with enzymes that is essential for the process of fertilization/

✔    Middle Piece : Contains numerous mitochondria.

o   It is essential for the energy production which is needed for the sperm movement (motility).

✔    Tail : Helps in the sperm motility.

✔    A single human male ejaculation consists of about 200 to 300 million sperms.

o   at least 60 per cent sperms must have normal shape and size and at least 40 per cent of them must show vigorous motility.

✔    The testicular hormones or androgens maintain the functions of male sex accessory ducts and glands.

Oogenesis

✔    The process of formation of the female gamete that starts during the embryonic development stage.

✔    Step #1: The ovary of the foetus develops a couple of million gamete mother cells (oogonia).

o   No more oogonia are added later.

✔    Step #2: The oogonia undergoes meiosis, but the processes temporarily arrested at the Prophase-I stage. These are called primary oocytes.

o   Step #3: The primary oocyte gets surrounded by a layer of granulosa cells and is called the primary follicle.

▪    At puberty only 60,000-80,000 primary follicles are left in each ovary, while the remaining degenerate.

o   Step #4: The primary follicle then turns into the secondary follicle as it gets surrounded by more layers of granulosa cells and a new theca.

o   Step #5: The secondary follicle develops into the tertiary follicle and it develops a fluid filled cavity called the antrum.

▪    The primary oocyte within the tertiary follicle grows in size and completes its first meiotic division at this stage.

✔    Step #6: The primary oocyte after the first meiotic division forms two unequal cells.

o   Large Cell – Haploid Secondary Oocyte

▪    Contains most of the nutrient rich cytoplasm of the primary oocyte.

o   Smaller Cell – First Polar Body

✔    Step #7: The tertiary follicle then develops into the mature follicle or graafian follicle.

o   Step #8: The secondary oocyte gets a new membrane called the Zona Pellucida.

✔    Step #9 : Ovulation– The graafian follicle ruptures to release the secondary oocyte (ovum) under the influence of LH.

➢  Menstrual Cycle

✔    It refers to the reproductive cycle (Cyclic ovarian and uterine changes) in the female primates.

✔    Menarche – The first menstruation during puberty.

✔    Menopause – The stoppage of the menstruation at around 50 years of age.

✔    Menstrual Cycle – Events between two menstruation that repeats about every 28/29 days.

✔    Consists of 4 phases: Menstruation, Follicular, Ovulatory, Luteal phase.

✔    During pregnancy there is no menstruation or menstrual cycle.

Menstrual Phase

✔    First phase lasts for 3-5 days

✔    The endometrium lining of the uterus breaks down.

✔    The blood vessels and other tissues come out via the vagina.

✔    It occurs only if there is no fertilization.

o   The absence is an indication of pregnancy.

✔    Menstruation does not occur in some of the following situations also

o   Stress

o   Poor Health condition.

Follicular Phase (Proliferative Phase)

✔    Follicular development occurs

✔    Hormones: The level of LH and FSH (gonadotropin) gradually increases.

o   Stimulates follicular development.

o   Stimulates the secretion of estrogen by developing follicle.

✔    Ovarian Change: The primary follicle develops into the mature follicle or graafian follicle.

✔    Uterine Change: The broken endometrium regenerated via cell proliferation.

Ovulatory Phase (Ovulation)

✔    The level of the LH and FSH is highest during the middle of the cycle (14^th day).

✔    The LH surge induces the rupture of the graafian follicle and releases the ovum.

Luteal Phase (Secretory Phase)

✔    The graafian follicle changes into corpus luteum.

o   It secretes the hormone progesterone.

o   Progesterone is required to maintain the thick endometrium.

o   It is essential in case of fertilization for the implantation to occur.

✔    In the absence of fertilisation, the corpus luteum degenerates.

o   It forms corpus albicans.

o   Disintegration of the endometrium occurs that leads to the menstruation.

o   A new cycle begins

➢  Fertilization

✔    The process of fusion of the male and the female gamete is known as fertilization.

✔    Sexual intercourse / coitus/ copulation leads to the release of semen into the vagina of female (process is referred to as insemination).

✔    The sperm swims through the cervix, uterus and finally reaches to the ampullary region of the fallopian tube.

✔    If ovulation has occurred and the egg is present in the ampullary region then the fertilization occurs.

✔    This is why all the copulation need not lead to pregnancy.

✔    Process of fertilization

o   The ovum is surrounded by the plasma membrane and the zona pellucida

o   The acrosome present at the tip of the sperm helps the sperm to enter into the cytoplasm of the ovum.

o   At this time the Meiosis-II division of the secondary oocyte is completed.

▪    The division is unequal

●    Larger cell: Haploid Ovum (ootid)

●    Smaller cell: Second Polar Body

o   The haploid nucleus of the sperm and the ovum fuses together to form the diploid cell called the Zygote.

✔    What ensures that only one sperm can fertilise an ovum?

o   When the first sperm comes in contact with the zona pellucida, there is a change in the membranes. This change prevents additional sperm from fertilizing the ovum.

➢  Sex-determination

✔    Genetically determined at the zygote stage.

✔    Human male has a chromosomal arrangement of 44 Autosome + XY Sex Chromosome.

✔    Human female has a chromosomal arrangement of 44 Autosome + XX Sex Chromosome.

✔    The haploid gametes in females consists of only X type of sex chromosome.

✔    The haploid gametes in males can contain either the X type or the Y type of sex chromosome.

✔    There is a 50% chance of the male gamete with X sex chromosome to fuse with the female gamete with X chromosome.

o   This leads to the formation of the zygote with chromosomal arrangement of 44 autosomes + XX sex chromosomes.

o   The child born will be a female child.

✔    There is also a 50% chance of the male gamete with Y sex chromosome to fuse with the female gamete with X chromosome.

o   This leads to the formation of the zygote with chromosomal arrangement of 44 autosomes + XY sex chromosomes.

o   The child born will be a male child.

➢  Events leading to Implantation

✔    Post fertilization the zygote moves through the isthmus to reach the uterus.

✔    Mitotic division leads to the formation of 2, 4, 8, 16 cell stage embryos.

o   Each of the cells are called the blastomere.

✔    Embryo with 8 to 16 blastomeres is called Morula.

✔    The morula transforms into a Blastocyst due to rapid cleavage.

o   It consists of around 70-100 cells.

o   The blastocysts are well organized.

▪    Outer later – Trophoblast

●    Gets attached to the endometrium

▪    Inner group of cells attached to trophoblast- Inner Cell Mass (ICM)

●    Differentiates as embryo

▪    Fluid filled cavity – Blastocoel (Blastocyst cavity)

✔    Post the attachment of the trophoblast to the endometrium, the uterine cells divide rapidly and covers the blastocyst.

o   The blastocyst gets embedded in the endometrium of the uterus.

o   This process is called Implantation.

➢  Pregnancy And Embryonic Development

Placenta

✔    It is a structural and functional unit between developing embryo (foetus) and maternal body.

Development of Placenta

o   Post implantation, chorionic villi are formed that are surrounded by uterine tissue and maternal blood.

▪    Chorionic villi are finger like projections on trophoblast.

o   Interdigitation of the chorionic villi and the uterine tissue takes place and together they form the placenta.

✔    Role of Placenta

o   Supply of oxygen to the developing embryo.

o   Supply of necessary nutrients.

o   Removal of CO₂ and other excretory/waste materials.

o   Acts as endocrine tissue and releases hormones like

▪    human chorionic gonadotropin (hCG)

▪    human placental lactogen (hPL)

▪    estrogens

▪    progestogen

✔    Placenta is connected to the embryo through the umbilical cord.

Hormones during Pregnancy

✔    hCG, hPL and relaxin are released only during pregnancy, hence are the pregnancy hormone.

✔    Relaxin is released by the ovary.

✔    The levels of estrogen, progesterone, cortisol, prolactin, thyroxine, etc., increases many folds during pregnancy.

o   It is essential for supporting the fetal growth, metabolic changes in the mother and maintenance of pregnancy.

Embryonic Development

✔    The ICM contains the stem cells.

o   They have the ability to develop into all the tissues and organs.

✔    After implantation the ICM differentiates into ectoderm (outer germ layer) and endoderm (inner germ layer).

✔    Mesoderm (middle germ layer) appears in between the ectoderm and endoderm.

✔    The ectoderm, mesoderm and the endoderm together give rise to all the tissues and organs.

Events during the gestation period (development period of 9 months)

✔    End of One Month

o   the embryo’s heart is formed.

✔    End of Second Month

o   the foetus develops limbs and digits (fingers/toes)

✔    End of Third month / 12 weeks (First Trimester)

o   most of the major organ systems are formed

o   the limbs and external genital organs are well developed.

✔    End of Fifth Month

o   first movements of the foetus.

o   appearance of hair on the head.

✔    End of Six Month / 24 weeks (Second Trimester)

o   body is covered with fine hair

o   eye-lids separate

o   eyelashes are formed

✔    End of Nine Month / 36 weeks (Third Trimester)

o   the foetus is fully developed and is ready for delivery.

➢  Parturition

✔    It refers to the process of child birth.

o   A complex neuro endocrine event.

✔    Signal for parturition originates from the fully developed foetus and the placenta.

o   It induces mild contraction of the uterine muscles.

o   It is referred to as the foetal ejection reflex.

✔    The foetal ejection reflex induced the release of the pituitary hormone- Oxytocin.

✔    Oxytocin acts on the uterine muscle and induces stronger uterine contraction.

o   It further stimulates the secretion of more oxytocin.

✔    The stimulatory reflex between the uterine contraction and oxytocin secretion continues.

✔    This results in stronger and stronger contractions.

✔    This leads to expulsion of the baby out of the uterus through the birth canal.

o   The placenta is also expelled out of the uterus.

➢  Lactation

✔    It refers to the process of secretion of milk by the mammary glands.

✔    The mammary glands of the female undergo differentiation during pregnancy and start producing milk towards the end of pregnancy.

✔    The milk produced during the initial few days of lactation is called colostrum.

o   It contains various antibodies (IgA) absolutely essential to develop resistance.

✔    Breast-feeding during the initial period of infant growth is recommended by doctors for bringing up a healthy baby.

***********************************