Chapter at a Glance
- Human are unisexual and viviparous.
- Sexual reproduction is the only method of reproduction in humans. Reproductive structure in humans are divided into- male reproductive system and female reproductive system.
- The major reproductive events in human beings are:
- Gametogenesis – Formation of gametes.
- Insemination – Transfer of sperms into female genital tract.
- Fertilisation – Fusion of male and female gametes leading to formation of zygote.
- Implantation – Formation and development of blastocyst and its attachment to the uterine wall.
- Gestation – Time from conception to birth and embryonic development.
- Parturition – Delivery of baby (the process of birth).
These reproductive events occur after puberty (= period when reproductive organs starts functioning).
• The characters which distinguish the male from the female externally are called secondary sex characters.
MALE REPRODUCTIVE SYSTEM
• Male reproductive system performs two major functions – spermatogenesis and transfer of sperm.
• It consists of a pair of testis, duct system & associated glands and external genitalia.
• Testis (the primary reproductive structure) are male gonads and produce male gametes (sperms) and male sex hormones (testosterone).
• Each testis has 250 compartments called testicular lobules. Each testicular lobules contain 1 – 3 seminiferous tubules.
• Seminiferous tubule is the site of sperm formation by a process called spermatogenesis. It is a place where germination, maturation, and transportation of the sperm cells take place.
• Seminiferous tubules are made up of columnar Sertoli cells surrounded by spermatogenic cells on the epithelial interior and stem cells exteriorly.
• The region outside the seminiferous tubule is called interstitial space. These spaces contain interstitial cells or Leydig cell.
• Leydig cell secretes male sex hormone (called testosterone) for the growth and maintenance of germinal epithelial cells and the development of secondary sex characteristics.
• Spermatogonia undergo meiosis and form sperms while sertoli cells (also called subtentacular cells) provide nutrition to the germs cells.
• Male sex accessory ducts include rete testis, vasa efferentia, epididymis, a vas deferens, and an ejaculatory duct. These ducts store and transport the sperms from the testis to the outside through urethra.
• Vasa efferentia carries spermatozoa from the rete testis to the epididymis.
• The penis is the male external genitalia. It contains urethra which serves as common canal for urine and semen.
• The male accessory glands include paired seminal vesicles, a prostate and paired bulbourethral glands.
• Seminal vesicles are the glands that contribute major portion of the seminal fluid.
• Secretions of seminal vesicle, prostate gland and bulbourethral gland are called semen.
FEMALE REPRODUCTIVE SYSTEM
• It consists of a pair of ovaries, oviducts, uterus, vagina and associated glands and external genitalia.
• Ovaries are female gonads and responsible for producing female sex hormones (oestrogen) and gametes (ova).
• Each ovary is covered bya thin epithelium which encloses the ovarian cortex.
• Scattered throughout the cortex are many rounded or oval bodies, called ovarian or Graafian follicles at various stages of development.
• Each follicle contains a large ovum surrounded by many layers of follicle cells.
• Graafian follicle is a mature ovarian follicle that contains a liquid-filled cavity (called antrum) and ruptures during ovulation to release an egg.
• Every month the most mature ovum from one ovary is released.
• At birth female ovaries contain 2 million follicles, each with an ovum that has begun meiosis but which is arrested in prophase of the first meiotic division. At this stage the ova are called primary oocyte.
• In the cortex both young and mature follicles along with a mass of yellow cells called corpus luteum is also present.
• Corpus luteum contain a yellow pigment lutein and secrete the hormone progesterone during pregnancy and hormone relaxin toward the end of pregnancy.
• Oviducts (fallopian tubes) is a pair of long duct which extends from the periphery of each ovary to uterus. It is the site offertilisation.
• Fallopian tube is divided into four parts: infundibulum (a funnel-shaped opening near the ovary to which fimbriae are attached), fimbriae (finger-like, ciliated projections, responsible to capture the ovum from the surface of the ovary after ovulation), ampulla (widest section of the uterine tubes where fertilisation usually occurs) and isthmus (a narrow section of the uterine tubes connecting the ampulla to the uterine cavity).
• Uterus (womb) is a hollow muscular organ which receives embryo and provide optimal conditions for its survival and growth.
It consists of three layers: endometrium (inner layer which is highly specialised and essential to menstrual and reproductive function), myometrium (middle layer which makes up most of the uterine volume and composed primarily of smooth muscle cells) and perimetrium (outer layer which is a thin layer of tissue made of epithelial cells that envelop the uterus).
• Vagina is meant to receive male’s penis during copulation and also allow menstrual flow.
• Bartholin’s glands secrete fluid which serves as a lubricant during copulation.
• Mammary gland is a gland located in the breasts offemales that is responsible for lactation, or the production of milk. Mammary glands only produce milk after childbirth.
• The basic components of the mammary gland are the alveoli (hollow cavities, a few millimetres large) lined with milk-secreting cuboidal cells and surrounded by myoepithelial cells. These alveoli join up to form groups known as lobules, and each lobule has a lactiferous duct that drains into openings in the nipple.
• Gametogenesis is a process in which a haploid sex cell (n)- sperm and ova are formed from a diploid primary germ cells (called gametogonia) present in the reproductive organs (called gonads-testes and ovary) through meiosis and cell differentiation.
-Production of mature sperm cell in the testis is known as spermatogenesis.It starts at the age of puberty.
-Spermatogonia (2N) divide mitotically into primary spermatocytes (2N), which divide by meiosis-I into secondary spermatocytes (IN).
-Secondary spermatocytes (lN) transformed into spermatozoa (lN) by spermiogenesis.
-Each spermatozoan or sperm is a long, flagellated, motile cell and consists of a head (comprising nucleus and acrosome), a neck (containing two centrioles), a middle piece (containing mitochondria) and a tail.
–Oogenesis is the process of formation of mature female gamete called ova.
-It is initiated during embryonic development stage when a couple of million gamete mother cell (oogonia) are formed within each foetal ovary.
-When more layers of granulosa cell and a new theca are laid down then it is called secondary follicles.
-The secondary follicle transform into a tertiary follicle which is characterised by a fluid filled cavity (called antrum).
-Tertiary follicle further undergo changes into mature follicle (or Graafian follicle).
-The Graafian follicle now ruptures to release the secondary oocyte from the ovary by a process called ovulation.
-The mature ovum or egg or secondary oocyte is spherical in shape, contains nucleus & the largest cell in the body.
• If the secondary oocyte is fertilised by a sperm, chemical changes will trigger the completion of meiosis II and the formation of another polar body (the first polar body may also undergo a second division to form a third polar body).
• Once meiosis II is complete the mature egg forms an ovum, before fusing its nucleus with the sperm nucleus to form a zygote.
• Ovum is larger and non- motile without the flagella.
• It involves cyclic changes in female’s reproductive tract culminating in menstruation that is flow of cast off uterine and fallopian tube lining along with blood and tissue fluid through the vagina.
• Beginning of menstruation or first menstruation at puberty (called menarche) usually varies. It usually occurs between 12 and 15 years.
• When menstrual cycles permanently cease due to the natural depletion of ovarian oocytes from aging, the condition is called menopause.
• Menopause marks the permanent end of fertility and the average age of menopause is 51 years.
• Two key groups of hormones which control and coordinate the menstrual cycle are pituitary and ovarian hormones.
Pituitary hormones (FSH and LH) are released from the anterior pituitary gland and act on the ovaries to develop follicles. Ovarian hormones (oestrogen and progesterone) are released from the ovaries and act on the uterus to prepare for pregnancy.
• Menstrual cycle consists of menstrual phase, proliferative phase (follicular phase) and secretory phase (luteal phase).
FERTILISATION AND EMBRYONIC DEVELOPMENT
• During copulation, the semen is transferred into the vagina. The motile sperms and the ovum released from the ovary reaches the ampullary-isthmic junction, where fertilisation takes place.
• Fertilisation is a process in which male and female gametes are fused together, initiating the development of a new organism with the combination of genes of both the parents. The product of fertilisation is a cell called zygote.
• After the formation of zygote, it starts moving through fallopian tube towards the uterus.
• The zygote undergoes successive (mitotic) divisions called cleavage (also called segmentation), as it moves through the isthmus of fallopian tube towards the uterus and form blastocyst which embedded in the endometrium; this process is called implantation and its leads to pregnancy.
• After implantation chorionic villi & uterine tissue interdigitated with each other & jointly form a structural & functional unit between developing embryo & maternal body called placenta.
• Placenta supply oxygen & nutrients to the embryo & remove carbon dioxide & excretory/waste materials produced by the embryo.
• Expulsion of the young one at the end of gestation is known as parturition. Parturition is induced by a complex neuro-endocine mechanism. The signals for parturition originate from the fully developed foetus and the placenta which induce mild uterine contraction called foetal-ejection reflex.
• Gestation period is 280 days in humans.
• Lactation is a process of milk production. Human milk is secreted by the mammary glands, which are located within the fatty tissue of the breast.
• The natural production of breast milk (lactation) is triggered by a complex interaction between three hormones -oestrogen, progesterone and human placental lactogen – during the final months of pregnancy.
• The mammary glands undergo differentiation during pregnancy and starts producing milk towards the end of pregnancy.
• The milk produced during the initial few days oflactation is called colostrum.
• Colostrum is especially high in nutrients, fats, and antibodies, to protect the newborn from infection.
- NCERT Revision Notes for Class 12 (All Subjects)
- CBSE Class 12 Biology Notes
- NCERT Solutions for class 12th Biology Chapter 3 Human Reproduction