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UNIT-VI : REPRODUCTION
CHAPTER-
SEXUAL REPRODUCTION IN FLOWERING PLANTS
Topic-
Sexual Reproduction
in Flowering Plants
Concepts Covered Structure of a flower, male and female reproductive structures,
development of male and female gametophytes.
Revision Notes
Flower Flowers are the site of sexual reproduction in flowering plants. Parts of a typical angiospermic flower are: sepals, petals, stamens and pistils. The four whorls of the flower are attached on a central axis called thalamus. A flower can be bisexual (contains both male and female reproductive parts) or unisexual (only one of the reproductive parts is present). Male Reproductive Structures Androecium (Whorl of Stamens) Androecium consists of a whorl of stamens. The number and length of the stamens are variable in flowers of different species. A stamen has three parts namely, anther, filament and connective. (a) Anther It is the terminal and bilobed part of stamens attached with filament. A bilobed anther is called dithecous. Each lobe has two pollen sacs or microsporangia. Therefore, the anther is tetrasporangiate. A longitudinal groove runs lengthwise separating the theca. (b) Filament It is the long and slender stalk part of the stamen. Its proximal end is attached to the thalamus or petals of the flower. (c) Connective The structure which connects the anther lobes is known as connective. Transverse section of an anther The anther is tetragonal in a structure consisting of four microsporangia or pollen sacs located at the corners, two in each lobe. The microsporangia develop to become pollen sacs. They extend longitudinally throughout the length of an anther. These are packed with pollen grains. Structure of microsporangium or pollen sac It is circular and is generally surrounded by wall layers namely, (a) Epidermis (b) Endothecium (c) Middle layers (d) Tapetum The first two layers perform the function of protection and help in dehiscence of anther to release the pollens. The middle layers and the innermost layer, (tapetum) nourishes the developing pollen grains. The cells of the tapetum possess dense cytoplasm and more than one nuclei. When the anther is young, a group of compactly arranged homogenous cells called sporogenous tissues occupies the centre of each microsporangium.
Key Words
Homogenous: Common origin or environ-
ment.
Dehiscence: Splitting or bursting
Viability: Ability to survive.
Microsporogenesis When the anther develops, each cell of sporogenous tissue undergoes meiotic division to form microspore tetrads. Each cell of sporogenous tissue is a microspore mother cell (MMC) or pollen mother cell (PMC). The process of formation of microspores from a pollen mother cell (PMC) through meiosis is called microsporogenesis. Dehiscence of anther The microspores get arranged in a group of four cells and each group is called microspore tetrad. As the anthers mature and dehydrate, the microspores dissociate from each other and develop into pollen grains. From each microsporangium, thousands of pollen grains are formed and released due to the dehiscence of anther.
Pollen grain (Male gametophyte) Pollen grain germinate and give rise to male gametophyte. These are spherical, measuring about 25- micrometers in diameter. Pollen grains are well preserved as fossils due to the presence of sporopollenin, a tough, resistant and stable material. A pollen grain has a two-layered wall namely, exine and intine. (a) Exine Exine is the hard outer layer which is made up of sporopollenin. The sporopollenin is one of the most resistant organic materials. It can withstand high temperature and strong acids and alkali. It cannot be degraded by enzymes. The exine has apertures called germ pores where sporopollenin is absent. (b) Intine It is the inner, thin and continuous layer that is made up of cellulose and pectin. A mature pollen grain contains two cells namely, vegetative cell and generative cell. (i) Vegetative cell It is the bigger cell having abundant food reserve and a large irregularly shaped nucleus. (ii) Generative cell It is the smaller cell that floats in the cytoplasm of the vegetative cell. It is spindle shaped with dense cytoplasm and a nucleus. The pollen grains are generally shed at the 2-celled stage in flowering plants. In other plants, the generative cell divides mitotically to give rise to the two male gametes before pollen grains are shed in a 3-celled stage. Once they are shed, pollen grains have to land on the stigma before they lose viability. The period of pollen grains remaining viable varies and depends on the prevailing temperature and humidity. The viability of pollen grains of some cereals such as rice, wheat, etc. is 30 minutes while some members of Leguminosae, Rosaceae & Solanaceae have viability for months. Pollen grains of some plants like Parthenium are allergic for some people leading to chronic respiratory disorders such as asthma, bronchitis , etc. Pollen grains are rich in nutrients. Pollen tablets are used as food supplements. Pollen consumption in the form of tablets and syrups increases the performance of athletes and race horses. It is possible to store pollen grains for years in liquid nitrogen (–196°C). The pollens stored in the pollen banks for crop breeding programmes which deals with the selection of superior phenotypes for the development of improved and new varities. Female Reproductive Structures Gynoecium (Pistil) It represents the female reproductive part of the flower. If it consists of a single pistil or carpel then, it is known as monocarpellary or if it has more than one pistil or carpel then, it is called multicarpellary. When there is more than one carpel, they may be fused then the pistil is known as syncarpous or may be free then, it is known as apocarpous. Each carpel has three parts namely stigma, style and ovary. (a) Stigma It is a landing platform for pollen grains. Key Words
Placenta: The surface of the carpel to
which the ovules are attached.
Integuments: Outer hard protective
layer in plants.
Degenerate: To loose structural or
physical ability. (b) Style It is an elongated slender part beneath the stigma. (c) Ovary It is the basal swollen part of the carpel. Inside the ovary is the ovarian cavity called the locule where the placenta is located. Placenta contains the ovules or megasporangia. The number of ovules in an ovary may be one as seen in wheat, paddy, mango, etc., or many as seen in papaya, watermelon, orchids, etc. Mnemonics
1. Concept: Male Reproductive Structures
Mnemonic: A sk F or C onnectivity
Interpretation: Anther, Filament,
Connective
2. Concept: Structures of microsporangium
or pollen sac
Mnemonic: E ating T omato
Interpretation: Endothecium, Tapetum
3. Concept: Female Reproductive Structures
Mnemonic: S mall S oft O rnament
Interpretation: Stigma, Style, Ovary
Megasporogenesis The formation of haploid megaspores from the diploid megaspore mother cell (MMC) as a results of meiosis is called megasporogenesis. A single megaspore mother cell is differentiated in the micropylar region of the nucellus. The megaspore mother cell is a large cell containing dense cytoplasm and a prominent nucleus. The megaspore mother cell undergoes meiotic division resulting in the production of four haploid megaspores. Female gametophyte (Embryo sac) In most of the flowering plants, only one megaspore is functional while the other three degenerate. The functional megaspore develops into the female gametophyte or embryo sac. This method of embryo sac formation from a single megaspore is termed as monosporic development. Development of Female gametophyte The nucleus of the functional megaspore divides mitotically to form two nuclei which move towards the opposite poles, forming a two- nucleated embryo sac. Two more sequential mitotic nuclear divisions result in the formation of the four-nucleated and later the eight-nucleated stages of the embryo sac are formed These divisions are strictly free nuclear, i.e., nuclear divisions are not followed immediately by cell wall formation. After eight-nucleate stage, the organisation of the typical female gametophyte or embryo sac takes place. Generally six of the eight nuclei are surrounded by cell walls and organised into cells. The remaining two nuclei called the polar nuclei are found below the egg apparatus in the large central cell. Distribution of the cells within the embryo sac The three cells consisting of two synergids and one egg cell which are grouped at the micropylar end constitute the egg apparatus. The synergids have special cellular thickenings at the micropylar tip called filiform apparatus. The filiform apparatus helps to guide the pollen tubes into the synergid. Three cells at the chalazal end organise as the antipodals. Thus, a typical mature angiosperm embryo sac at maturity is eight-nucleate and seven-celled. Fig 1.6: A diagrammatic view of a typical anatropous ovule Fig 1.7: A diagrammatic view of the mature embryo Topic- Pollination and Fertilisation
Concepts Covered Modes of Pollination, Pollen-Pistil Interaction, Artificial
Hybridisation, Double Fertilisation Revision Notes Modes of Pollination The process of transfer of pollen grains from the anther to the stigma of a pistil is known as pollination. There are few external agents which help the plants for pollination to take place. Pollination is of three types based on the source of pollens namely, (a) Autogamy (b) Geitonogamy (c) Xenogamy Autogamy When the pollen grains are transferred from the anther to the stigma of the same flower, it is
known as autogamy. In flowers with exposed anthers and stigma, a complete autogamy is rare and hence the anthers and stigma should lie close to each other to enable self-pollination. Along with this there should be synchrony in pollen release and stigma receptivity. Plants like Viola (common pansy), Oxalis and Commelina produce two types of flowers namely Chasmogamous flowers and Cleistogamous flowers. (a) Chasmogamous flowers Flower are similar to flowers of other species with exposed anthers and stigma. (b) Cleistogamous flowers They do not open at all. Geitonogamy When the pollen grains are transferred from the anther to the stigma of another flower of the same plant, it is known as geitonogamy. It is structurally cross-pollination but genetically self-pollination. It is genetically similar to auto gamy because the pollen grains come from the same plant. Xenogamy When the pollen grains are transferred from anther to the stigma of a different plant, it is known as xenogamy. It brings about genetically different types of pollen grains to the stigma. Agents of pollination: There are two types of agents of pollination namely: (a) Biotic agents (b) Abiotic agents Abiotic Agents There are two abiotic agents namely, wind and water which help pollination to takes place. Pollination by Wind The pollination taking place by the wind is called anemophily. Wind and water pollinated flowers are not very colourful and do not produce nectar. Wind pollinated flowers often have a single ovule in each ovary. Numerous flowers remain packed into an inflorescence. Example – In corn cob, the tassels are the stigma and style wave in the wind to trap pollen grains. Wind pollination is commonly seen in grasses. Key Words
Synchrony: Fluctuation of multiple pop-
ulations of different places in the same way.
Stigma receptivity: Ability of stigma to
support viable anther for germination. Characteristics of Anemophilous flowers The flowers produce an enormous amount of pollen. The pollen grains are light and non-sticky so that they can be transported through wind currents. They often possess well-exposed stamens for easy dispersal of pollens into wind currents. They have large, feathery and sticky stigma to trap air-borne pollen grains. Pollination by Water The pollination taking place by water is called hydrophily. It is limited to about 30 genera, mostly monocotyledons. In Vallisneria , t he female flowers reach the surface of the water by the long stalk and the male flowers or pollen grains are released on to the surface of the water. These male flowers or pollen grains are carried by water currents and reach the female flowers. In sea grasses, the female flowers remain submerged in water and the long, ribbon-like pollen grains are carried inside the water and reach the stigma. The pollen grains of most of the water-pollinated species have a mucilaginous covering to protect from wetting. Not all aquatic plants use hydrophily. For example, in aquatic plants like water hyacinth, water lily, etc., the flowers emerge above the level of water for entomophily or anemophily i.e., for pollination to takes place by insects or wind. It is seen in Vallisneria & Hydrilla (freshwater), Zostera (marine sea-grasses), etc. Biotic Agents Some flowering plants use animals as pollinating agents like Bees, butterflies, flies, beetles, wasps, ants, moths, birds (sunbirds and hummingbirds) bats, some primates (lemurs), arboreal (tree- dwelling) rodents, reptiles (gecko lizard & garden lizard) etc. When the pollination takes place by insects, it is known as entomophily. Often flowers of animal pollinated plants are specifically adapted for a particular species of animal. When the animal comes in contact with the anthers and the stigma, pollen grains may get stuck to the body of the animals, which results in pollination. S ome plants provide safe places as a floral reward to lay eggs as seen in Amorphophallus , the tallest flower. There is a very close obligatory symbiotic relationship between the species of moth ( Pronuba ) and the plant Yucca. They cannot complete their life cycles without each other. The moth deposits its eggs in the locule of the ovary and the flower gets pollinated by the moth. The larvae of the moth come out of the eggs as the seeds start developing. Characteristics of Entomophilous Flowers Flowers are large, colourful, fragrant and rich in nectar.
Since two types of fusions viz. syngamy and triple fusion take place in an embryo sac, it is called double fertilisation. The central cell after triple fusion becomes the primary endosperm cell (PEC) and develops into the endosperm while the zygote develops into an embryo. It is an event unique to flowering plants. Topic- Post-fertilisation Changes and Special Modes of Reproduction
Concepts Covered Embryo and its Development Structure and types of Seed Fruit
and its types Apomixis and Polyembryony Revision Notes Embryo and its Development Post-fertilisation Events The development of endosperm and embryo, the maturation of ovule(s) into seed(s) and ovary into fruit are post-fertilisation events. Endosperm Development The primary endosperm cell divides repeatedly by mitosis to form a triploid endosperm tissue. Endosperm cells are filled with reserve food materials that are used for the nutrition of the developing embryo. During the endosperm development, the primary endosperm nucleus undergoes successive mitotic nuclear divisions to give rise to free nuclei. This stage is called free-nuclear endosperm. Then the endosperm becomes cellular due to the cell wall formation. For example, the tender coconut water is a free- nuclear endosperm that is made up of thousands of nuclei and the surrounding white kernel is the cellular endosperm. Embryo Development The embryo develops at the micropylar end of the embryo sac where the zygote is situated. The zygotes divides only after the formation of a certain amount of endosperm to provide nutrition to the developing embryo. The development of embryo is similar in monocotyledons and dicotyledons up to the octant stage. The zygote gives rise to the pro-embryo and subsequently to the globular, heart-shaped and mature embryo. Dicotyledonous Embryo It has a central embryonal axis and two lateral cotyledons. The portion of the embryonal axis above the level of cotyledons is the epicotyl, which terminates into the plumule (stem tip). The cylindrical portion below the level of cotyledon is hypocotyl that terminates into the radicle (root tip). The root tip is covered with a root cap. Fig 1.8: Fertilised embryo sac Fig. 1.9: Stages in embryo development in a dicot show- ing zygote and primary endosperm nucleus Fig 1.10: A typical dicot embryo
Monocotyledonous Embryo They possess only one cotyledon. In the grass family, the cotyledon is called the scutellum which is situated lateral to the embryonal axis. At its lower end, the embryonal axis has the radicle and root cap enclosed in an undifferentiated sheath called coleorhiza. The portion of the embryonal axis above the level of attachment of the scutellum is the epicotyl. It has a shoot apex and a few leaf primordia enclosed in a hollow foliar structure called coleoptile. Seed Seed is the final product of sexual reproduction. It is the fertilised ovule formed inside fruits. It consists of the seed coat(s), cotyledon(s) and an embryonal axis. The cotyledons are simple, thick and swollen due to the storage of food as seen in most of the dicots. Mature seeds may be non-albuminous or albuminous. Fig 1.11: L.S of an embryo of grass Fig 1.12: L.S (grain of maize) Fig 1.13: L.S. (monocot seed of onion) Non-albuminous or Non-endospermic Seeds These seeds have no residual endosperm as it is completely consumed during embryo development. Examples - Pea, groundnut, beans. Key Word
Pericarp: Part of fruit formed from the
wall of the ripened ovary. Albuminous or Endospermic Seeds These seeds retain a part of the endosperm as it is not completely used up during embryo development. Examples: wheat, maize, barley, castor, coconut, sunflower. In some seeds like black pepper, beet, etc., the remnants of nucellus also persistent. It is called the perisperm. I nteguments of ovules harden as tough protective seed coats. It has a small pore (micropyle) through which oxygen and water enter into the seed during germination. As the seed matures, its water content gets reduced and the seeds become dry (10-15 % moisture by mass). The general metabolic activity of the embryo slows down. The embryo may enter a state of inactivity (dormancy). If favourable conditions are available such as adequate moisture, oxygen and suitable temperature, they germinate. Fruit The ovary develops into a fruit after pollination and fertilisation. The transformation of ovules into seeds and ovary into fruit proceeds simultaneously. The wall of the ovary develops into a pericarp. The fruits may be fleshy as seen in guava, orange, mango, etc., or may be dry as seen in groundnut, mustard, etc.,
CHAPTER-
HUMAN REPRODUCTION
Topic-
Human Reproductive System
Concepts Covered Structure of Male Reproductive
System, Structure of Female Reproductive System
Revision Notes
Reproductive System Male Reproductive System It consists of : (a) A pair of testes (b) Accessory ducts (c) Accessory glands (d) External genitalia Testes Testes are the primary sex organs that produce sperms and testosterone hormone. Testes are located in the scrotum present in between upper thighs. The low temperature (2 – 2.5°C less than the normal internal body temperature) in the scrotum helps for the proper functioning of testes and spermatogenesis. Each testis is oval in shape and has about 250 (
- compartments called testicular lobules. Each lobule is filled with connective tissue and contains 1-3 coiled yellow seminiferous tubules in which sperm are produced. Seminiferous tubule is lined internally with spermatogenic cells called spermatogonia or primary male germ cells and sertoli cells or supporting cells. Spermatogonia undergo meiotic divisions and leads to sperm formation. Sertoli cells give shape and nourishment to developing spermatogenic cells and therefore also called as nurse cells. The regions outside the seminiferous tubules are the interstitial spaces which contain small blood vessels and interstitial cells or Leydig cells. The Leydig cells are endocrine in nature and secrete testicular hormones called androgens. Immunologically competent cells are also present. Accessory Ducts The duct system includes rete testis, vasa efferentia, epididymis and vas deferens. The seminiferous tubules open into the vasa efferentia through rete testis. The vasa efferentia open into the epididymis. The epididymis leads to vas deferens that ascends into the abdomen and loops over the urinary bladder. It receives a duct from the seminal vesicle and opens into the urethra as the ejaculatory duct. These ducts store and transport the sperms from the testis to the outside through urethra. The urethra originates from the urinary bladder and extends through the penis to its external opening called the urethral meatus. Accessory Male Genital Glands It includes paired seminal vesicles, prostate 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. Seminal vesicles produce seminal fluid and form 60 – 70% of semen. The secretion of bulbourethral glands is alkaline and rich in mucus. It helps in the lubrication of the penis, supplies nutrient to sperms and provides an alkaline medium to counteract the acidity of the uterus. External Genitalia The penis is the male external genitalia. It is made up of special tissue that helps in the erection of the penis to facilitate insemination. The enlarged end of the penis is called the glans. The penis is covered by a loose fold of skin called foreskin. IMPORTANT DIAGRAMS Fig 2.1 : Human Male Reproductive System
Fig 2.2 : Sectional view of human seminiferous tubule The Female Reproductive System It includes a pair of ovaries, accessory ducts and external genitalia. Ovaries They are the primary female sex organs that produce ova or the female gametes. It secretes many steroid ovarian hormones such as estrogen and progesterone. Ovaries are located on both sides of the lower abdomen. Each ovary is about 2-4 cm in length. The ovaries are connected to the pelvic wall and uterus by ligaments. Each ovary is covered by a thin epithelium which encloses the ovarian stroma. The stroma has outer cortex and an inner medulla. The ovary contains groups of cells known as Ovarian or Graafian follicles. Each follicle carries a centrally placed ovum. Accessory Ducts It includes two oviducts or fallopian tubes, cervix, a uterus and vagina. Each oviduct is 10-12 cm long and has four parts namely, infundibulum, ampulla, isthmus and uterine part. Uterus It is single and also called the womb. The shape of the uterus is like an inverted pear. It is supported by ligaments attached to the pelvic wall. The uterus opens into the vagina through a narrow path called cervix. The cavity of the cervix is called the cervical canal which along with the vagina forms the birth canal. The wall of the uterus is thick and muscular and is differentiated into three layers of tissue namely, (a) The external thin membranous perimetrium. (b) The middle thick layer of smooth muscle, myometrium. (c) The inner glandular layer called the endometrium. The endometrium undergoes cyclic changes during the menstrual cycle while the myometrium exhibits strong contraction during delivery of the baby. The vagina opens to the exterior between the urethra and anus. The lumen of the vagina is lined by a glycogen- rich mucous membrane consisting of sensitive papillae and Bartholin’s glands. The secretions of Bartholin’s glands lubricate the penis during sexual activity. External Genitalia It includes the mons pubis, labia majora, labia minora, hymen and clitoris. The external genitalia are collectively called the vulva. Mons pubis is a cushion of fatty tissue covered by skin and pubic hair. The labia majora are a pair of large thicker fleshy folds of tissue, which surround the vaginal opening. The labia minora are a pair of narrow fleshy folds of tissue found below labia majora. The opening of the vagina is often covered partially by a membrane called the hymen. Mammary Glands A pair of mammary glands containing glandular tissue and fat is present in the chest region. The glandular tissue of each breast has 15- mammary lobes containing clusters of cells called alveoli. The cells of alveoli secrete milk which is stored in the cavities or lumen of alveoli. The alveoli open into mammary tubules. The 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. Mnemonics
1. Concept: Accessory Male Genital
Glands
Mnemonics: S upreme P ower in B ack
or S even P ieces of B anana
Interpretations: Seminal vesicles,
Prostate, Bulbo-urethral glands
2. Concept: Structure of Oviducts.
Mnemonics: I A m I ntelligent than U
Interpretations: Infundibulum,
Ampulla, Isthmus, Uterine part.
3. Concept: Female External genitalia
Mnemonics: M obile's L ight L ed H im
C razy.
Interpretations: Mons pubis, Labia
majora, Labia minora, Hymen Clitoris.
which contains lytic enzymes, that help in fertilisation of the ovum. Key Word
Spermatids: Formed after the second
meiotic division from spermatocyte and develop into spermatozoa. (b) Neck Behind the head is a neck containing proximal and distal centrioles. The distal centriole of the neck is connected to the axial filament. (c) Middle Piece It is composed of axial filament surrounded by numerous mitochondria and cytoplasm. Mitochondria produce energy for the sperm motility. (d) Tail It consists of a central axial filament. The sperm moves in fluid medium and female genital tract by the undulating movement of the tail. Sperms are transported through the accessory ducts. The secretions of the epididymis, vas deferens, seminal vesicle and prostate are essential for maturation and motility of sperms. The seminal plasma and sperms together constitute the semen. The human male ejaculates about 200-300 million sperms during a coitus ejaculation. For normal fertility at least 60% of sperms must have a normal shape and size and 40% of them, must show vigorous motility. Key Diagram : Fig 2.4 : Structure of a Sperm Oogenesis It is the process of formation and maturation of the ovum. It takes place in Graafian follicles. It is initiated in embryonic stage when millions of egg mother cells (oogonia) are formed within each ovary. No oogonia are formed and added after birth. Oogonia multiply to form primary oocytes which enter into prophase-I of the meiosis and get temporarily arrested at that stage. Each primary oocyte gets surrounded by a layer of granulosa cells to form a primary follicle. A large number of primary follicles degenerate during the phase from birth to puberty. Therefore at puberty, only 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 to form secondary follicles. The secondary follicles get transformed into a tertiary follicle. It has a fluid-filled cavity (antrum). The theca layer forms an inner theca interna and an outer theca externa. The primary oocyte within the tertiary follicle grows in size and undergoes first unequal meiotic division to form a large haploid secondary oocyte and a tiny first polar body. The secondary oocyte retains the nutrient-rich cytoplasm of the primary oocyte. It is unknown, whether the first polar body divides further or degenerates. The tertiary follicle further changes into the mature follicle (Graafian follicle). The secondary oocyte forms a new membrane (zona pellucida). The Graafian follicle now ruptures to release the secondary oocyte (ovum) from the ovary. This is called ovulation.
Key Diagram : Fig 2.5 : Schematic representation of (a) Spermatogenesis (b) Oogenesis Structure of Ovum It is a spherical or oval and non-motile female gamete. It is about 0.2 mm in diameter. The human ovum is non cleidoic (without shell) and alecithal (without yolk). Ovum has four membranes namely, (a) Plasma membrane (Oolemma) : Innermost layer. (b) Vitelline membrane : Attached to the plasma membrane. (c) Zona pellucida : Transparent non-cellular, thick, glycoprotein rich layer found outer to the vitelline membrane. (d) Corona radiata : Outer layer is formed of follicle cells. These cells are held together by a mucopolysaccharide called hyaluronic acid. Fig 2.6 : Structure of human ova Fig 2.7 : Graafian follicle Fig 2.8 : Sectional view of ovary Menstrual Cycle The reproductive cycle in the human female and related primates is called the menstrual cycle.
Mnemonics
1. Concept: Structure of Sperm
Mnemonics: H igh N ode M agnification T ime
Interpretations: Head, Neck, Middle piece, Tail
2. Concept: Structure of Ovum
Mnemonics: P lease cross V ia Z ebra C rossing
Interpretations: Plasma membrane Vitelline membrane, Zona pellucida, C orona radiata
Topic- Fertilisation and Post-Fertilisation Events
Concepts Covered Fertilisation Pregnancy Placenta Formation and Functions
Lactation Parturition Revision Notes Fertilisation The process of fusion of male gamete (sperm) with the female gamete (ovum) is called fertilisation. During copulation, semen is released through the penis into the vagina (insemination). After insemination, the sperms swim through the cervix and enter into the uterus and reach the ampullary-isthmic junction of the oviduct where fertilisation takes place. The process of fertilisation takes place as follows : Sperms → vagina → cervical canal → uterus → isthmus ↓ Fertilisation ← Ampullary-isthmic Junction ampulla ↑ Ovum (from ovary) → fimbriae → infundibulum → Fertilisation (sperm + ovum → zygote) occurs only if ovum and sperms are transported simultaneously. So all copulations do not lead to fertilisation and pregnancy. As soon as sperm contacts with zona pellucida, it induces changes in the membrane that block entry of additional sperms. With the help of enzymes of the acrosome, which dissolve the zona pellucida and plasma membrane of the ovum, the sperm enters into the cytoplasm of the ovum. This induces second meiotic division of the secondary oocyte to form a second polar body and a haploid ovum (ootid). The haploid nuclei of the sperm and ovum fuse together to form a diploid zygote. Implantation The mitotic division (cleavage) starts as the zygote moves through the isthmus of the oviduct towards the uterus and forms 2, 4, 8, 16 daughter cells called blastomeres. The embryo with 8-16 blastomeres is called a morula. Morula continues to divide and transforms into a large mass of cells called the blastocyst, which moves further towards the uterus. Key Word
Chorionic villi: Tiny projections of
placental tissue that look like fingers and contain the same genetic material as the foetus. The blastomeres in the blastocyst are arranged into an outer layer (trophoblast) and an inner group of cells (inner cell mass) attached to the trophoblast. The trophoblast layer then gets attached to the endometrium and the inner cell mass gets differentiated into three germ layers namely, outer ectoderm, middle mesoderm and inner endoderm forming 3-layered structure (gastrula) leading to the formation of the embryo. After attachment, 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. Fig 2.10: Diagram of a Blastocyst Pregnancy and Embryonic Development After implantation, the finger-like projections called chorionic villi appear on the trophoblast which is surrounded by the uterine tissue and maternal blood. The chorionic villi and uterine tissue become interdigitated with each other and form a structural and functional unit between the developing embryo and the maternal body called the placenta.
The placenta is a structural and functional unit between the embryo (foetus) and the maternal body. The placenta is connected to the embryo by an umbilical cord. The umbilical cord helps to transport substances to and from the embryo. Functions of Placenta It acts as a barrier between the foetus and mother. Soluble inorganic and organic materials, nutrients, hormones, antibodies, etc. can pass through the placenta from the mother to the foetus. It helps in the gas exchange between mother and foetus. It helps to eliminate nitrogenous wastes of foetus. It acts as an endocrine gland by secreting several hormones like human Chorionic Gonadotropin (hCG), human Placental Lactogen (hPL), oestrogens, progesterone and relaxin. Pregnancy During pregnancy, levels of estrogen, progestogen, cortisol, prolactin, thyroxine, etc. are also increased in maternal blood. They support the foetal growth, metabolic changes in the mother and maintain pregnancy. Three germ layers (ectoderm, endoderm, mesoderm) give rise to all tissues (organs) in adults. The stem cells in inner cell mass have the potency to give rise to all the tissues and organs. Human pregnancy (gestation period) lasts 9 months (for cats : 2 months, dogs : 2 months, elephants : 21 months). Changes in Embryo during Pregnancy After one month of pregnancy : The heart is formed. End of second month : Limbs and digits are developed. End of 12 weeks (first trimester) : The major organs such as limbs, external genital organs etc., are well developed. During 5th^ month : The first movement of foetus and appearance of hair on the head. End of 24 weeks (second trimester) : Body is covered with fine hair, eyelids separate and eye lashes are formed. End of 9 months : Ready for delivery. Parturition (Labour) and Lactation The process of giving birth to young ones after the gestation period of nine months is known as parturition. Parturition is induced by a neuroendocrine mechanism. The signals originating from the foetus and placenta induce mild uterine contractions (foetal ejection reflex). This causes the release of oxytocin from the maternal pituitary. Oxytocin causes stronger uterine muscle contractions which in turn stimulate further secretion of oxytocin. This process is continued leading to the expulsion of the baby out of the uterus through the birth canal. After parturition, the umbilical cord is cut off. The placenta and remnants of the umbilical cord are expelled from the maternal body after parturition. This is called “after birth”. Lactation The mammary glands produce milk towards the end of pregnancy by the process called lactation. The yellowish milk produced during the initial few days of lactation is called colostrum. The colostrum contains several antibodies essential to develop resistance for newborn babies.
CHAPTER-
REPRODUCTION HEALTH
Revision Notes
Reproductive Health
- The term ‘reproductive health’ simply refers to healthy reproductive organs with normal functions. According to WHO (World Health Organisation), the word ‘reproductive health’ means a total well-being in all aspects of reproduction i.e., physical, emotional, behavioural and social. Problems Associated with Reproductive Health : (a) Rapid increase in the human population is called population explosion. (b) Lack of awareness and sex education in people. (c) Spread of myths and misconceptions about sex- related aspects. (d) Common occurrence of sexually transmitted diseases due to lack of knowledge of hygiene of reproductive organs. (e) Illegal abortions and destruction of a foetus [foeticides] which is mainly done for a female foetus. (f) Sex abuse and sex-related crime. Strategies of Reproductive Health Programmes: To ensure total reproductive health, several pro- grammes like reproductive health programmes and family planning were started in 1951.
- Improved programmes covering wider reproduc- tion related areas are currently in operation under
2. Artificial Methods : This involves mechanical or barrier methods. (a) Condoms : These are rubber or latex sheaths that are put on the penis before coitus (copulation).These check pregnancy by preventing the deposition of sperms in the vagina. These also prevent the spread of the (STDs) including AIDS, syphilis, etc. Female condoms are also available called femidoms. (b) Diaphragms and cervical caps : These are reuseable rubber barriers and fitted in the vagina of a female to check the entry of sperms in the uterus. (c) Intra Uterine Devices (IUDs) : These are inserted by doctors or expert nurses in the uterus through the vagina. These include : - Non-medicated IUDs (e.g., Lippes loop) - Copper releasing IUDs (e.g., Copper T) - Hormone releasing IUDs (e.g., Progestogen) : Make the uterus unsuitable for implantation and the cervix hostile to the sperms. - IUDs increase the phagocytosis of sperms. The Cu ions suppress the motility and fertilising capacity of sperms. - IUDs are ideal contraceptives for females who want to delay pregnancy or spacing in children. 3. Chemical Methods : These are of the following types : (i) Spermicidal tablets, jellies, paste and creams introduced in the vagina before coital activity. These kill sperms. Common spermicidal chemicals used are lactic acid, citric acid, potassium permanganate, zinc sulphate etc. (ii) Physiological (Oral) Devices : These are the hormonal preparation in the form of pills for females. - The pills are usually small doses of progestogens or progestogen–estrogen combinations in the form of tablets (pills). - Pills are taken daily for 21 days starting within the first five days of the menstrual cycle. After a gap of 7 days (during which menstruation occurs) it has to be repeated in the same pattern as long as the female desires to prevent conception. - They inhibit ovulation and implantation as well as alter the quality of cervical mucus to prevent the entry of sperms. - Pills are very effective with lesser side effects. - Saheli : It is a new oral contraceptive for females. It was developed by the Central Drug Research Institute (CDRI) Lucknow. It contains a non-steroidal preparation. It is a 'once a week' pill with very few side effects and high contraceptive value.
- Drawbacks of Oral Contraceptives : Nausea, abdominal pain, breakthrough bleeding, irregular menstrual bleeding, breast cancer etc. (iii) Injectables/Implants
- Progestogens alone or in combination with an oestrogen is used by females as injections or implants under the skin.
- Their mode of action is similar to that of pills and their effective periods are much longer. These are also effective within 72 hours of coitus. Thus it has been found to very effective as emergency contraceptives. Key Word
Implantation : The attachment of ferti-
lised egg to the wall of uterus at the begin- ning of pregnancy.
4. Sterilisation or Surgical Methods : These methods block gamete transport and so prevent conception. These include the following measures : (a) Male sterilisation : It is a permanent method of birth control in which either testes are surgically removed, called castration, or cutting of the vas deferens, called Vasectomy. The vas deferens is exposed and cut through a small incision on the scrotum to prevent the passage of sperms. (b) Female sterilisation : Methods of female sterilisation include : (i) Ovariectomy involves surgical removal of ovaries. (ii) Tubectomy involves cutting or tying up of fallopian tubes through a small incision in the abdomen or through vagina. (iii) Tubal ligation involves blocking of fallopian tubes by an instrument called a laparoscope. Medical Termination of Pregnancy (MTP) - Intentional or voluntary termination of pregnancy before full term is called MTP or induced abortion.
- 45 to 50 million MTPs are performed in a year all over the world (i.e., 1/5th^ of the total number of conceived pregnancies). - MTP helps to decrease the population. - Because of emotional, ethical, religious and social issues many countries have not legalised MTP. - Government of India legalised MTP in 1971 with some strict conditions to check indiscriminate and illegal female foeticides which are reported to be high in India. Importance of MTP
- To avoid unwanted pregnancies due to casual intercourse or failure of the contraceptive used during coitus or rapes.
- Essential in cases where continuation of the pregnancy could be harmful to the mother or to the foetus or both.
- MTPs are safe during the first trimester, (up to 12 weeks of pregnancy). 2nd-trimester abortions are very risky. Problems Related to MTPs
- Majority of the MTPs are performed illegally.
- Misuse of amniocentesis (a foetal sex determination test based on the chromosomal pattern in the amniotic fluid).
- MTP for a female child causes sex imbalance in society. Amniocentesis
- It is a prenatal diagnostic method to determine the sex of the developing baby. This method has both positive and negative application. This method is legally banned in India. (a) Positive application
- It helps to detect any genetically controlled congenital disease or any metabolic disorders in the foetus. (b) Negative application
- People use this method for female foeticide, which causes sex imbalance in society. Sexually Transmitted Diseases (STDs)
- Diseases transmitted through sexual intercourse are called Sexually transmitted diseases (STDs)/ Venereal diseases (VD) or Reproductive tract infections (RTI). e.g., Gonorrhoea, syphilis, genital herpes, chlamydiosis , genital warts, trichomoniasis, hepatitis-B and HIV leading to AIDS.
- Hepatitis-B and HIV are also transmitted: (a) By sharing of injection needles, surgical instruments, etc. (b) By transfusion of blood. (c) From infected mother to foetus.
- Except Hepatitis B, genital herpes, HIV and other diseases are completely curable if detected early and treated properly.
- Early symptoms : Itching, fluid discharge, slight pain, swellings, etc., in the genital region.
- Absence or less significant early symptoms and the social stigma deter the infected persons to consult a doctor. This leads to pelvic inflammatory diseases (PID), abortions, stillbirths, ectopic pregnancies, infertility, cancer of the reproductive tract, etc.
- All persons are vulnerable to STDs. These are very high among persons in the age group of 15- years.
- Prevention : (a) Avoid sex with unknown partners/multiple partners. (b) Always use condoms during coitus. (c) In case of doubt, go to a qualified doctor for early detection and get complete treatment. Infertility
- It is the inability of male or female to produce children.
- The reasons for this may be physical, congenital, diseases, drugs, immunological or even psychological. Assisted Reproductive Technologies (ART) (1) In vitro fertilisation (IVF– test tube baby programme) : In this method, ova from the wife/ donor and sperms from the husband/donor are collected and are induced to form zygote under simulated conditions in the laboratory. This is followed by Embryo transfer (ET). It is of 2 types : (a) Zygote Intra Fallopian Transfer (ZIFT) : Transfer of zygote or early embryos (with up to 8 blastomeres) into the fallopian tube. (b) Intra Uterine Transfer (IUT) : Transfer of embryos with more than 8 blastomeres into the uterus. The embryo formed by in vivo fertilisation (fertilisation within the female) is also used for such transfer to assist those females who cannot conceive. Key Words
Chlamydiosis : A sexually transmitted
disease caused by the bacteria chlamydia trachomatis. The disease infects both men and women.
Blastomeres: Cells formed by the cleav-
age of zygote or fertilised ovum which later produce morula. (2) Gamete Intra Fallopian Transfer (GIFT) : Transfer of an ovum from a donor into the fallopian tube of another female who cannot produce ovum, but can provide a suitable environment for fertilisation and development. (3) Intra Cytoplasmic Sperm Injection (ICSI) : A laboratory procedure in which a single sperm (from a male partner) is injected directly into an egg (from a female partner). Then the fertilised egg is implanted into the woman’s uterus. (4) Artificial Insemination (AI) technique:
- The semen collected from the husband or a healthy donor is artificially introduced into the vagina or the uterus (IUI– intra-uterine insemination) of the female.
- This technique is useful for the male partner having an inability to inseminate female or low sperm counts, etc. (5) Surrogacy
- Here, a woman (surrogate mother) bears a child for a couple unable to produce children, because the wife is infertile or unable to carry.
- The surrogate is impregnated either through artificial insemination or through the implantation of an embryo produced by in vitro fertilisation.
