Fertilisation and Implantation

Fertilisation

Fertilisation is the fusion of a haploid male gamete (sperm) and haploid female gamete (ovum) to form a diploid zygote. In human internal fertilisation take place. Fertilisation is a process that: (i) Restore diploid number of chromosomes (ii) Determine sex of the new organism (iii) Activation of zygote to start a series of mitotic divisions called cleavage.

During copulation (coitus) semen is released by the penis into the vagina, it is known as insemination. The motile sperms swim rapidly, pass through the cervix, enter into the uterus and finally reach the ampullary region of the fallopian tube. So fertilisation take place in ampulla of fallopian tube. The ovum released by the ovary is also transported to the ampullary region where fertilisation take place. Fertilisation can only occur if the ovum and sperm are transported simultaneously to the ampullary region. This is the reason why not all copulations lead to fertilisation and pregnancy.

Fertilisation and Implantation

Events of Fertilisation

The process of fusion of a sperm with an ovum is called fertilisation. During fertilisation, a sperm comes in contact with the zona pellucida layer of the ovum and induces changes in the membrane that block the entry of additional sperms. Thus it enusres that only one sperm can fertilise an ovum. The secretions of the acrosome help the sperm enter into the cytoplasm of the ovum through the zona pellucida and the plasma membrane. This induces the completion of the meiotic division of the secondary oocyte. The second meiotic division is also unequal and results in the formation of a second polar body and a haploid ovum (ootid). Soon the haploid nucleus of the sperms and that of the ovum fuse together to form a diploid zygote. All this process can be grouped into following reproductive events:
(1) Acrosomal Reaction
(2) Cortical Reaction
(3) Sperm Entry
(4) Karyogamy
(5) Activation of Egg

Significance of Fertilisation

  1. Fertilisation provides stimulus to the egg to complete its maturation.
  2. Fertilisation activates the ovum to develop into a new individual by repeated mitotic divisions.
  3. Fertilisation restores the diploid number of chromosomes, characteristic of the species (46 in humans), in the zygote.
  4. Fertilisation combines the characters of two parents. This introduces variations, which make the offsprings better equipped for the struggle for existence and contribute to evolution of the race.
  5. Fertilisation determines the sex of the young one to be developed from the zygote in humans.
  6. Fertilisation membrane developed after the entry of the sperm prevents the entry of additional sperms into the ovum.
  7. Fertilisation introduces centrioles which are missing in the ovum.

Cleavage

Cleavage is the rapid mitotic division in the fertilised egg. Cleavage begins almost immediately after fertilisation and continues during its passage down the fallopian tube to the uterus. During cleavage the embryo does not increase in size. The cells produced do not grow after division. The embryo with 8-16 solid ball of cells is called morula. Cells of morula is called blastomeres.

Morula

The morula continues to divide and transforms into blastocyst. The blastomeres in the blastocyst are arranged into an outer layer called trophoblast and an inner group of cells attached to trophoblast called the inner cell mass. The inner cell mass is the source of embryonic stem cells from which all body structures are formed. The inner cell mass looks like a knob at one pole. It is called embryonal knob. The cells of trophoblast which are in contact with the inner cell mass (embryonal knob) are known as cells of Rauber. Blastocyst contain a fluid filled cavity, known as Blastocoel.

Role of zona pellucida

After the formation of blastocyst, zona pellucida becomes thinner and finally disappears. The function of the zona pellucida is to prevent the implantation of the blastocyst at an abnormal site. It does not expose the sticky and phagocytic trophoblast cells till the blastocyst reaches the proper implantation site.

Implantation

Implantation is the process by which the developing attaches itself to the wall of the mother's uterus and stimulates the development of the placenta. At the time of implantation embryo is present in form blastocyst. So Implantation is the attachment of blastocyst to the uterine wall (in endometrium). It take place about 7 days after fertilisation. With implantation, pregnancy is established. During implantation trophoblast layer gets attached to the endometrium and the inner cell mass gets differentiated as the embryo. After attachment, the uterine cells divide rapidly and covers the blastocyst. As a result, the blastocyst becomes embedded in the endometrium of the uterus. This is completion of implantation.Implantation

Menstrual Cycle (Reproductive Cycle)

Menstrual cycle is the cyclic change in the reproductive tract of primate females, including human, apes and monkeys. The cyclical changes in the ovary and the uterus during menstrual cycle are induced by changes in the levels of pituitary and ovarian hormones. Menstrual cycle in human female starts at the age of 12-15 years and continues upto the age of 45 years. Menstrual cycle is absent during pregnancy, may be suppressed during lactation and permanently stops at menopause. First menstruation is called menarche and last menstruation is called menopause. In human duration of menstrual cycle is 28 days.

Phases of Menstrual Cycle

The menstrual cycle consists of 3 phases:
1. Follicular phase (Proliferative phase/ preovulatory phase)
2. Luteal phase (Secretory phase/ post ovulatory phase)
3. Menstrual phase (Bleeding phase)

Menstrual Cycle

Menstrual phase

The cycle starts with bleeding phase in its first 3 to 5 days. During this bleeding the part of the layer of endometrium gets shed off. The menstrual flow is a liquid that contain endometrial lining of the uterus, blood. It comes out through vagina. Menstruation only occurs if the released ovum is not fertilised. Lack of menstruation may be indicative of pregnancy. However, it may also be caused due to some other causes like stress, poor health etc. Menstrual fluid contains fibrinolysin, so it doesn't clot.

Proliferative phase

During this phase, the primary follicles in the ovary grow to become a fully mature Graafian follicle and simultaneously the endometrium of uterus regenerates through proliferation. The secretion of gonadotropins (LH and FSH) increases gradually during the follicular phase, and stimulates follicular development as well as secretion of estrogens by the growing follicles. Both LH and FSH attain a peak level in the middle of cycle (about 14th day). Maximum level of LH during mid cycle is called LH surge. LH surge induces rupture of Graafian follicle and thereby the release of ovum (Ovulation).

Luteal phase

During this remaining parts of the Graafian follicle transform as the corpus luteum. The corpus luteum secretes large amounts of progesterone which is essential for maintenance of the endometrium. Such an endometrium is necessary for implantation of the fertilised ovum and other events of pregnancy. During pregnancy all events of the menstrual cycle stop and there is no menstruation. In absence of fertilisation, the corpus luteum degenerates. This causes disintegration of the endometrium leading to menstruation, marking a new cycle.

DRGP Special Points

  • Progesterone is known as pregnancy hormone, it is important to maintain the pregnancy.
  • The time period between ovulation and next menstrual bleeding is always constant.
  • Ovulation occurs at 14th day (mid cycle). In this stage both LH and FSH attain a peak.
  • Menstrual fluid contains fibrinolysin, so it doesn't clot.

Seed and Fruit

Seed and fruit are post fertilisation structures in flowering plants. Seed is developed from ovule and fruit is developed from ovary. Following transformation take place after fertilisation:

Seed and Fruit

Seed

In angiosperms, the seed is the final product of sexual reproduction. It is often described as a fertilised ovule. Seeds are formed inside fruits. A seed typically consists of seed coat(s), cotyledon(s) and an embryo axis. The cotyledons of the embryo are simple structures, generally thick and swollen due to storage of food reserves (as in legumes).

Integuments of ovules harden as tough protective seed coats. The micropyle remains as a small pore in the seed coat. The facilitates entry of oxygen and water into the seed during germination. As the seed matures, its water content is reduced and seeds become relatively dry (10-15 per cent moisture by mass). The general metabolic activity of the embryo slows down. The embryo may enter a state of inactivity called dormancy, or if favourable conditions are available (adequate moisture, oxygen and suitable temperature), they germinate.

Question: What is perisperm?
Answer:
In some seeds such as black pepper and beet, remnants of nucellus are also persistent. This residual, persistent nucellus is the perisperm.

Type of Seeds

Mature seeds may be non-albuminous or albuminous. Non-albuminous seeds have no residual endosperm as it is completely consumed during embryo development (e.g., pea, groundnut, sunflower). Albuminous seeds retain a part of endosperm as it is not completely used up during embryo development (e.g., wheat, maize, barley, castor).

Importance of Seed

  • Seeds can remain dormant for many years and germinate on return of favourable conditions.
  • Seeds have better adaptive strategies for dispersal to new habitats and help plant species to be colonized in different areas.
  • Seeds have sufficient food reserves to initiate embryo development and seedling development till the photosynthesis process is initiated.
  • Hard seed coat insure protection to the young embryo from environment stress.
  • Can be easily stored for future usage Long term viability of most of the seeds.
  • Seeds are the product of sexual reproduction it promotes diversity.

Fruit

As ovules mature into seeds, the ovary develops into a fruit, i.e., the transformation of ovules into seeds and ovary into fruit proceeds simultaneously. The wall of the ovary develops into the wall of fruit called pericarp. The fruits may be fleshy as in guava, orange, mango, etc., or may be dry, as in groundnut, and mustard, etc. Many fruits have evolved mechanisms for dispersal of seeds.

Types of Fruit

True fruits

Fruits develop only from the ovary and are called true fruits. Example: Cucumber, tomato, coconut etc.

False fruits

When floral parts other than ovary, particularly thalamus contributes to fruit formation, such fruits are called false fruits. Example: apple, strawberry, cashew, etc., the thalamus also contributes to fruit formation. Such fruits are called false fruits.

False Fruit

Parthenocarpic fruits

Fruits develop without fertilisation. Such fruits are called parthenocarpic fruits and this process is known as parthenocarpy. It can be induced through the application of growth hormones and such fruits are seedless. Example: Orange, banana, water melon, grapes etc.

Apomixis : Seeds without ferilisation

A phenomenon which produce seeds without fertilisation. Apomixis is a form of asexual reproduction that mimics sexual reproduction. 

Methods:
1. In some species, the diploid egg cell is formed without reduction division and develops into the embryo without fertilisation.
2. In some case the nucellar cells surrounding the embryo sac start dividing and develop into the embryos (sporophytic budding).
3. In some case vegetative reproduction take place. Example: Asteraceae and grasses.

Importance of Apomixis

Apomicts have several advantages in horticulture and agriculture.
1. Clonal reproduction through seeds. We can produce large number of same varieties of seed without fertilisation.
2. New hybrids produced in lesser time.
3. Disease free plants can be produced.
4. Cost effective.

Polyembryony

Formation of many embryos inside a single seed is called polyembryony. It was discovered by Leeuwenhoek in citrus seeds (Orange). Polyembryony is commonly found in Gymnosperms (conifers) but it is also found in some of Angiospermic plants such as Orange, Lemon, onion, groundnut, mango, Opuntia etc. When many embryos are formed from separate-separate embryosac inside the ovule is called false or pesudo-polyembryony. When many embryos are formed inside the single embryo sac of the seed is called true polyembryony.

Polyembryony  may occurs due to a number of reasons like:

  • Cleavage of zygote.
  • Embryo develops by the fertilization of synergids
  • Embryo develops by fertilization of antipodal cells.
  • Number of embryos develop from the tissues of nucellus and integuments (Sporophytic budding).

Angiosperm Embryo

Embryo develops at the micropylar end of the embryo sac where the zygote is situated. Most zygotes divide only after certain amount of endosperm is formed. This is an adaptation to provide assured nutrition to the developing embryo. The zygote forms and embryo by mitotic divisions. The early stages of the embryo development, called embryogeny, are similar in both monocotyledons and dicotyledons. The zygote gives rise to the proembryo and subsequently to the globular, heart shaped and mature embryo.

Development of Angiosperm Embryo

  • The zygote divides transversely to form two cells.
  • The cell towards the micropyle is the suspensor cell and the one farthest away is the embryo cell.
  • The embryo cell divides mitotically and gives rise to a structure called proembryo.
  • The proembryo undergoes a number of divisions in various planes. It goes through various stages like globular, heart-shaped, etc. and then developed into a mature embryo.
  • The remaining chain of cells formed by division of suspensor cell is called the suspensor with a basal cell below. As the suspensor elongates it pushes the embryonic cells deeper into the endosperm to get nutrition.
  • A few cells of embryo nearest to suspensor develop into hypocotyl and radicle. Other cells give rise to epicotyl, plummule and cotyledons.
  • Portion of embryonal axis above cotyledon is epicotyl while below it is hypocotyl.
  • Epicotyl terminates in plumule while hypocotyl terminates in radicle. Plumule give rise to the shoot while radicle gives rise to root tip.
  • Dicot plants have two cotyledons while monocot plants have only one cotyledon.
  • As the embryo develops the integument cells become lignified and form a covering called testa or the seed coat.

Stage of development angiosperm embryo

Difference between dicot and monocot embryo

A typical dicot embryo consists of an embryonal axis and two cotyledons. The portion of embryonal axis above the level of cotyledons is the epicotyl, which terminates with the plumule or stem tip. The cylindrical portion below the level of cotyledons is hypocotyl that terminates at its lower end in the radicle or root tip. The root tip is covered with a root cap. Dicot and monocot embryo

Embryos of monocotyledons possess only one cotyledon. In the grass family the cotyledon is called scutellum that is situated towards one side of the embryonal axis. At its lower end, the embryonal axis has the radical and root cap enclosed in an undifferentiated sheath called coleorrhiza. The portion of the embryonal axis above the level of attachment of scutellum is the epicotyl. Epicotyl has a shoot apex and a few leaf primordia enclosed in a hollow foliar structure, the coleoptile.

Difference between mocot and dicot embryo

 

Endosperm And Types Of Endosperm

The primary endosperm cell divides repeatedly and forms a triploid endosperm tissue. The cells of this tissue are filled with reserve food materials and are used for the nutrition of the developing embryo. It also protects the embryo from mechanical injury. In gymnosperms, the endosperm is haploid (n) and formed before fertilisation. On the other hand, in angiosperms it is triploid (3n) and formed after fertilisation. Endosperm may be completely consumed by developing embryo before it matures or it may persist in the mature seed and used up during seed germination.

The endosperm is developed from primary endosperm nucleus (PEN) which is formed as a result of triple fusion sperm nucleus which is formed as a result of triple fusion. It is a nutritive tissue that supplies food material to the growing embryo and also the seedling. The primary endosperm nucleus is generally triploid (as it is formed by the fusion of one of the male gametes with two polar nuclei). It divides to form a large triploid endosperm. Endosperm is absent in members of families Orchidaceae, Podostarnonaceae and Trapaceae.

Types of Endosperm

There are three types of Endosperm:
1. Nuclear Endosperm
2. Cellular Endosperm
3. Helobial Endosperm

Types of endosperm

Nuclear Endosperm

This type of endosperm is the most common in Angiosperm. In this type of endosperm, the primary endosperm nucleus divides to form many free nuclei. The division of the nucleus is not accompained by wall formation. These free nuclei formed in this way start arranging towards the periphery in the cytoplasm and after sometime wall formation start form periphery towards the centre of the sac. This type of endosperm mostly found in Dicotyledon (polypetalae). Nuclear endosperm is also present in Capsella. The liquid syncytium of coconut is in fact watery endosperm of nuclear type.

Cellular endosperm

In this type division of the primary endosperm nucleus is immediately followed by the wall formation. The first division results in the formation of two equal sized chambers: chalazal and micropyler chambers. The subsequent divisions are followed by regular cell wall formation. So that endosperm is remains cellular from the biginning. Cellular endosperm is characteristic of gamopetalae. Example: Petunia, Datura.

Helobial endosperm

This type of endosperm is intermediate between cellular and nuclear types. The first division of PEN results in the formation of large micropylar cell and a small chalazal cell. The nucleus of the micropylar cell divides freely. But that of the chalazal cell may remain undivided or divide only a few times. The endosperm so formed is called helobial. The chalazal endosperm cell is sometimes described as basal apparatus. Such a type of endosperm is characteristic of Helobieae (an order of monocotyledons).

All types of endosperm become cellular at a very late stage of development. It is generally believed that nuclear endosperm is primitive, helobial intermediate and cellular advanced.

Double Fertilisation

The fusion of male gamete with female gamete (egg) is called fertilisation. In Angiosperm a specific form of fertilisation takes place which is known as double fertilisation. In this two sets of fertilisation take place or we can say female gametophyte (embryo sac) fuse with two male gametes. One male gamete fertilised with egg cell and other with polar nuclei. The process was discovered by S.G. Nawaschin and Guignard. Fertilisation occurs in four steps:

  • Germination of pollen on stigma
  • Growth of pollen tube and entry into ovule
  • Pollen tube entry into embryo sac
  • Fusion of gametes.

Germination of pollen on stigma

Pollen germination

  • In gymnosperms, the pollen grains usually land directly on the nucellus, while in angiosperms, they fall on the stigma.
  • After being deposited on the stigma, the pollen grain absorbs liquid from the moist surface of the stigma, expands in size.
  • The stigma plays an important role in the germination of pollen grain.
  • The stigma, scretes fluid containing liquids, gums, sugar and resins. The chief function of the stigmatic secretion is to protect the pollen as well as the stigma from desiccation.
  • The intine form a tube like structure through a germ pore. This tube like structure are known as pollen tube.
  • It continues to elongate.

Growth of pollen tube and entry into ovule

  • Each pollen tube grows through the stigma and style to the ovary.
  • Each pollen tube has two nuclei, a vegetative or tube nucleus and a generative nucleus.
  • The tube nucleus is at the growing tip of the pollen tube and disintegrates.
  • The generative nucleus divides to give rise to two male gametes or nuclei.
  • After reaching ovary, the pollen tube enters the ovule. Pollen tube may enter the ovule by any one of the following routes:

Entrance of pollen tube

  1. Progamy: Entrance of the pollen tube through the micropyle is known as progamy. It is the most common type.
  2. Chalazogamy: Entrance of the pollen tube through chalazal region is known as chalazogamy.
  3. Mesogamy: Entrance of the pollen tube through the middle part or integument or through funicle is known as Mesogamy.

Pollen tube entry into embryo sac

  • The pollen tube enters the embryo sac only from the micropylar end irrespective of its mode of entry into the ovule.
  • The pollen tube either passes between a synergid and the egg cell or enters into one of the synergids through filiform apparatus.
  • This filiform apparatus of synergids direct the grwoth of pollen tube by secreting some chemical substances (chemotropic secretion).
  • After entrance of pollen tube in embryosac, synergid starts degenerating.
  • The tip of pollen tube enlarge and ruptures to release both male gametes.

Fusion of gametes (Double Fertilisation)

The fusion of male gamete with female gamete is called fertilisation. After pollination, pollen germination takes place. Male gametes enter into embryo sac by pollen tube. At this stage there are two male gametes.

Double fertilisation

  1. First Fertilisation: One of the male gametes moves towards the egg cell and fuses with its nucleus and completing syngamy. This results in the formation of a diploid cell, zygote. This fertilisation is known as generative fertilisation.
  2. Second Fertilisation: The other male gamete moves towards the two polar nuclei and fuses with them to produce a triploid primary endosperm nucleus (PEN). This involves the fusion of three haploid nuclei hence it is termed as triple fusion. It is also knwon as vegetative fertilisation.

Since two types of fusion, syngamy take place in an embryo sac the phenomenon is termed double fertilisation. PEN develops into endosperm and the zygote develops into an embryo. Total number of nuclei involved in double fertilisation is five, 2 in syngamy + 3 in triple fusion.

Significance of Double Fertilisation

  • Female gametophyte stops its growth at eight nucleate stage, further growth continues after double fertilisation.
  • Syngamy leads to diploid zygote, which later on changes into embryo.
  • Triple fusion makes endopserm, which provide nutrition to the developing embryo. So it is necessary for the formation of viable seeds.
  • Secondary nucleus stops its division before fertilisation. So triple fusion induces this dormant nucleus to regain its division power.

Question: What is siphonogamy?

Answer: The fertilisation involving formation of pollen tube is called siphonogamy.

Oogenesis

The formation of egg (female gamete) in ovary (gonads) is knwon as Oogenesis. Just like testis ovary also contain germinal epithelium. The germinal epithelial cells are small diploid cells. These cells undergo a number of stages to produce egg.

Important stages of oogenesis

There are three main stages of oogenesis

1. Multiplication phase: The primordial germinal cells divide repeatedly to form the oogonia. This initial steps occur prior to birth. By the time the foetus is 25 weeks old, all the oogonia that she will ever produce, are already formed by mitosis. No more oogonia are formed and added after birth.

2. Growth phase: Only some of the oogonia increase in size and undergo a growth phase to form the primary oocytes. Oogonia start division and enter into prophase-I of the meiotic division and get temporarily arrested at that stage, called primary oocytes. Each primary oocytes then gets surrounded by a layer of granulosa cells and is called the primary follicle. A large number of these follicles degenerate during the phase from birth to puberty. Therefore, at puberty only 60,000-80,000 primary follicles are left in each ovary. Primary oocytes are diploid in nature. The growth phase of the oogenesis is comparatively longer than the growth phase of spermatogenesis. In the growth phase, the size of the primary oocytes increase enormously.

Oogenesis

3. Maturation phase: The primary oocytes undergoes the first meiotic division to produce two haploid cells. Since there is an unequal division of the cytoplasm, one of the cells is large and receives most of the cytoplasm (secondary oocytes) and other is small (Secondary oocytes). Ovulation occurs in secondary oocyte stage. The second part of the meiosis take place only after the contact of sperm. The nucleus of secondary oocytes divides again to form another polar body and Ootid. As there is no metamorphosis in Ootid, it may be called as Ovum.

Structure of Ovum

Ovum is the immotile female reproductive cell. It is produced in the ovary. A single ovum is released from the ovary at regular intervals. Mammalian eggs have very less amount of yolk, so the eggs are oligolecithal and isolecithal or microlecithal. The egg has 2 egg membranes, Zona pellucida and Corona radiata.

  1. Zona pellucida: This is a transparent membrane like covering and is a primary membrane secreted by the ovum/oocyte itself.
  2. Corona radiata: This is a layer of follicular cells. These cells are attached to the surface of egg through "hyaluronic acid". This is a secondary membrane, which is secreted by the ovary. These eggs don't have tertiary membrane.

Ovum

Spermatogenesis

Spermatogenesis

The formation of sperms (male gamete) in testis (gonads) is known as Spermatogenesis. Spermatogenesis occurs in the seminiferous tubules of testes. Each tubule contains two types of cells: Germinal epithelial cells and Sertoli cells. The germinal epithelial cells or primary germ cells are small diploid cells. These cells undergo a number of stages to produce spermatids. The Sertoli cells are large elongated supporting cells present in between primary germ cells. They provide nourishment to the developing sperms. The spermatids get embedded in the Sertoli cells and transformed into sperms.

seminiferous tubules

Important stages of spermatogenesis

There are four important stages of spermatogenesis:

  1. Maturation phase:In this phase cells of germinal epithelium multiply by mitotic division and increase in nummber. This cells are knwon as spermatogonia (sing. spermatogonium). Each spermatogonium is diploid and contains 46 chromosomes.
  2. Growth phase:In this phase some of the spermatogonia increase in size by accumulation of nourishing materials. They become double in volume and called primary spermatocytes. Each primary spermatocyte is diploid and contain 46 chromosomes.
  3. Maturation phase:Each primary spermatocytes undergoes first meiotic division and form two haploid secondary spermatocytes. These undergo second meriotic division to produce a total of four equal haploid spermatids.
  4. Spermiogenesis:It is transformation of an unspecialised spermatid into a sperm. The spermatids gets embedded in sertoli cell. Following transformations occurs:

    • Losing cytoplasm.
    • Condensation of nucleus.
    • Formation of tail.
    • Formation of acrosome from golgi body.
    • Formation of nebenkarn sheath from mitochondria.

 

Spermatogenesis

 

Hormonal control of spermatogenesis

 

Hormonal Control of spermatogenesis

 

Structure of sperm

 

Male gamete