Gametogenesis and Fertilisation PDF

Gametogenesis and fertilisation A quote "Nothing in Biology Makes Sense Except in the Light of Evolution" Theodosius Dobzhansky Learning objectives • Understand the significance and importance of meiosis in gametogenesis. • Consider similarities and differences in gametogenesis between males and fe...

Gametogenesis and fertilisation A quote "Nothing in Biology Makes Sense Except in the Light of Evolution" Theodosius Dobzhansky Learning objectives • Understand the significance and importance of meiosis in gametogenesis. • Consider similarities and differences in gametogenesis between males and females. • Understand the mechanisms of fertilisation. • Describe the components of the zygote. • Consider some of the chromosome abnormalities that may occur during fertilisation. • Understand the correlation between the amount of yolk and evolution. Gametes provide the continuity of life between generations of a species, by passing on chromosomal DNA which contains developmental information for the species. Gametogenesis is a process by which diploid (2n) precursor cells undergo cell division and differentiation to produce mature haploid (1n) gametes. At fertilisation, the fusion of haploid gametes produces a diploid zygote, the beginning of a new individual of the species. Gametogenesis Gametogenesis describes the origin, development and maturation of gametes. Female gametes are also known as eggs or oocytes; the term ova (ovum in singular) is more theoretical than real. Male gametes are also known as sperm or spermatozoa (spermatozoon in singular). Gametes are cells specialised in sexual reproduction. They contain half of the normal number of chromosomes of the species (haploid: 1n). Meiosis is the type of cell division that allows sexual reproduction, since it reduces the number of chromosomes of the species to one half (from diploid cells with 2n chromosomes to haploid cells with 1n chromosomes), making it possible to combine two gametes to form a new individual. The cells that form gametes are germ cells, as opposed to somatic cells. The ploidy (number of chromosomes) of germ cells is the same as somatic cells (diploid cells: 2n). Only during the formation of gametes does meiosis occur, reducing the number of chromosomes to half (1n). Gonads are the organs where the gametes are produced. They contain germ cells that undergo reductional division (meiosis) and generate gametes. In males, the gonads are the testicles. In females, the gonads are the ovaries. Spermatogenesis It is the process by which spermatozoa (several hundred million per ejaculation) are formed. Spermatogenesis is a continuous process that begins in puberty and goes on until old age, and sometimes during the whole life of the male. Spermatogenesis duration offers a wide range of variation: 34 days in mice; 36 days in stallions; 74 days in humans. Before birth, male germ cells called spermatogonia (spermatogonium in singular) migrate to the testes where they divide by mitosis until they reach the seminiferous tubules where they stay quiet until puberty that is when spermatogenesis begins. It comprises two phases: spermatocytogenesis and spermiogenesis. • • Spermatocytogenesis. - Spermatogonia (2n) proliferate by mitosis in order to keep their population producing themselves and also producing primary spermatocytes (2n) which start meiosis. - Primary spermatocyte (2n) produces two secondary spermatocytes (1n) via meiosis I. - Secondary spermatocytes (1n) divide into spermatids (1n) via meiosis II. Spermiogenesis is the transformation of spermatids into sperm cells (spermatozoa). In this differentiation process, spermatids undergo changes in cellular size and shape such as the formation of the acrosome, the appearance of the flagellum, the reduction of the cytoplasm and the increase in the number of mitochondria. The acrosome is a structure that contains a large number of digestive enzymes. It is located at the anterior end of the sperm cell and is formed through the union of Golgi complex vesicles. The function of the acrosome is to release its enzymes when the sperm cell meets the egg cell to break the external covering of the female gamete, thus making fertilisation possible. The flagellum of the sperm cell is formed from the centrioles that migrate to the region posterior to the nucleus. Its function is to promote locomotion towards the egg cell. The reduced cytoplasm of sperm cells decreases the cell size and weight and provides a more hydrodynamic shape (elongated nucleus) for its locomotion in fluids. The high concentration of mitochondria at the base of the flagellum of the sperm cell is necessary for supplying energy to the flagellum for it to vibrate and move the sperm cell. https://sway.office.com/ZkdUwxb0nwCmK7MA#content=jTs3J1psZ8om5P 1 - Spermatogenesis. Sequential stages in the development of the male gamete. Oogenesis Oogenesis in mammals Oogenesis is a complex process that starts before birth in the embryo and is only completed when the sexual cycles begin at puberty. After puberty, the ovaries release cyclically mature female gametes able to be fertilised until the age when fertility is lost (menopause). • Before birth, oogonia (oogonium in singular) or female germ cells (2n) migrate to the ovaries where they divide by mitosis until they all give rise to the primary oocytes (2n) by mitosis; primary oocytes initiate meiosis I but they stop at the prophase stage and become surrounded by a layer of follicular cells. Each primary oocyte surrounded by a single layer of follicular cells is referred to as a primordial follicle. • At birth, the normal ovary contains a species-specific number of primary follicles which stay dormant until puberty. Therefore, the total number of follicles is determined early in life, and follicle depletion leads to reproductive senescence. • After puberty, sexual cycles begin, and selective follicles mature at each cycle in response to circulating follicle stimulating hormone (FSH) from the pituitary. The developing follicles grow from the stage of the primary follicle to secondary and finally tertiary follicle. By this time (the last stage of follicular maturation) the primary oocytes resume the division process. After finishing the first meiotic division (meiosis I), the oocyte I forms two cells: the oocyte II (n) and the first polar body (n). The oocyte II is bigger because it receives almost all the cytoplasm and the cytoplasmic structures of the oocyte I as a strategy for metabolite and nutrient storage. The first polar body is very small and has almost no cytoplasm; it either disintegrates or stays attached to the oocyte II. Ovulation occurs at this stage once the first meiotic division has been completed and a secondary oocyte and the first polar body have been formed within the dominant follicle. At ovulation, the secondary oocyte is released from the ovary enveloped by its membranes (pellucid zone and corona radiata) and enter the oviduct. Fertilisation occurs when an oocyte II and a sperm combine and fuse their pronuclei. • After fertilisation, the oocyte II completes meiosis II producing a fertilised ovum or egg and a second polar body. The second polar body is a very small cell that has almost no cytoplasm and stays attached to the egg which receives the entire cytoplasmic content of the oocyte II. Therefore, oocytes II only complete the second meiotic division if fertilisation occurs. https://sway.office.com/ZkdUwxb0nwCmK7MA#content=CpCswF9HnsRsJC 2 - Oogenesis. Sequential stages in the development of the female gametes. Under normal circumstances, fertilisation takes place inside of the fallopian tube, then the fertilised eggs develop into the morula and blastocyst stages while they migrate through the fallopian tube toward the uterus. If secondary oocytes are not fertilised, they degenerate without completion of the meiosis II and are reabsorbed by the female reproductive tract. https://sway.office.com/ZkdUwxb0nwCmK7MA#content=30qRtlFm5LUuBU 3 - Places where ovulation, fertilisation and cleavage occur in mammals. Oogenesis in birds At the embryonic stage, the female bird´s reproductive system starts to develop as paired structures, but after hatching, the right ovary and oviduct degenerate, and therefore, only the left ovary and left oviduct remain and become functional in adult birds. In birds, like mammals, the ovary holds a number of primary follicles, each containing a primary oocyte, which has not yet begun to grow, encircled by a granulosa layer of small follicular cells. 4 - Analogies and differences of the genital system between mammals and birds. In contrast with mammals, follicular development in birds mainly entails a substantial increase in the size of the oocyte as a result of the cumulation of yolk, without forming a fluid-filled follicle. During the breeding season, small primary follicles start to develop by producing oestrogen, which stimulates the liver to produce yolk material. The yolk precursors are subsequently blood-borne and finally taken up by growing follicles. In oocytes which have begun to accumulate yolk, the yolk will appear as a viscous fluid with suspended granules and globules of various sizes and appearances. As the primary oocyte increases in size, it moves nearer to the surface of the ovary. 5 - Oogenesis in birds. Sequential stages of oogenesis in birds. 6 - Follicles of birds in different stages of development. During the later stages of growth, the oocyte increases greatly in size and distends the surface of the ovary until it is suspended by a stalk. The largest follicle produces high levels of progesterone that induce the luteinising hormone surge that leads to ovulation. A bird ovary in the breeding season resembles a bunch of grapes with the developing follicles arranged in a hierarchy with the largest destined to ovulate first, the second largest ovulating the following day and so on. Outside of the breeding season, however, the oviduct is reduced in size and the ovary resembles a collection of millet seeds. https://sway.office.com/ZkdUwxb0nwCmK7MA#content=SwSbe4OIkLBbnq 7 - Places where ovulation, fertilisation and cleavage take place in birds. When a mature follicle is released from the ovary and enters the oviduct, it consists of only the "yolk". Whether or not the oocyte is fertilised, the additional components of the egg are added along the oviduct. The "white" (protein to nourish the embryo in later stages of development) is added in the oviduct as the egg passes along it. The shell is the last layer added, just before laying. Fertilisation Fertilisation is the union of a haploid oocyte and a haploid spermatozoon, producing a diploid cell, called zygote, capable of developing into a new individual. Fertilisation begins with gamete fusion (zygote formation) and ends with the initiation of zygote cell division (the start of cleavage). The fusion of a spermatozoon with an oocyte takes place in the uterine tube, near the ovary. The spermatozoa are propelled from the vagina to the uterine tube by the active movement of their tails and contraction of the female genital tract. In overview, fertilisation comprises the following steps: Sperm-zona pellucida binding. For fertilization to take place, a spermatozoon must bind to a specific glycoprotein (ZP) on the zona pellucida surrounding the oocyte; this species recognition process prevents union with foreign sperm. Acrosomal reaction and penetration of sperm. After binding to the zona pellucida, the spermatozoon releases degradative enzymes. The enzymes break down the zona pellucida, allowing the sperm cell to penetrate the zona pellucida barrier. Once a sperm penetrates the zona pellucida, it binds to and fuses with the plasma membrane of the oocyte and the sperm head is incorporated into the oocyte cytoplasm. Activation of the oocyte. The oocyte is activated when the spermatozoon and oocyte plasma membranes fuse. The oocyte immediately cancels its membrane potential (via Ca++ influx) and enzymes are released by exocytosis from oocyte cytoplasmic granules (cortical reaction). This causes permanent changes in the composition of the zona pellucida and prevents fusion by additional sperm (polyspermy prevention). At the same time, the secondary oocyte completes meiosis giving rise to an ovum and a second polar body. Amphimixis. It is referred to the fusion of the male and female haploid nuclei (also called pronuclei). This restores the original diploid condition in the zygote nucleus and allows the first mitotic division to start. In birds, fertilisation also occurs at the beginning of the oviduct (infundibulum) but unlike mammals, multiple sperms typically bind and fuse with the oocyte (physiological polyspermy); however, only one reaches the female pronucleus and fuses with it (amphimixis). https://sway.office.com/ZkdUwxb0nwCmK7MA#content=Uin9d9eiiqAHzI 8 - Sequential stages of fertilisation in mammals. Zygote The zygote is the fertilised egg that results from the union of a female gamete (egg, or ovum) with a male gamete (sperm). In animal development, the zygote stage is brief and is followed by cleavage, when the single cell becomes subdivided into smaller cells (the two-cell stage occurs on average between 22 and 26 hours after fertilisation). The zygote contains all the essential factors for development, but they exist solely as an encoded set of instructions localized in the genes of chromosomes. Both the sperm and the egg contribute DNA, or genetic material, to the newly formed zygote, and this genetic material is packaged as chromosomes. Nearly all the cytoplasm of the zygote comes from the female gamete (only one centriole is contributed by the sperm). For that purpose, the egg cytoplasm is well stock with nutrients (yolk) and all the organelles necessary for zygotes’ survival. These include the mitochondria, endoplasmic reticulum, and a variety of molecules, including mRNAs. Additionally, the egg contributes the membranes (coverings) that protect the embryo before implantation. 9 - The cytoplasm of the zygote. https://sway.office.com/ZkdUwxb0nwCmK7MA#content=190YiDQcOOGKyz Egg membranes The zygote is well protected by the same membranes that surround the female gamete. These coverings are produced either by the egg itself or by follicle cells in the ovary or by glands in the oviduct. 10 - The covering membranes of the zygote https://sway.office.com/ZkdUwxb0nwCmK7MA#content=LNGX94Y6Nu2g0E The primary membrane is equivalent to the cytoplasmic membrane that it is found in all the cells. 11 - Cytoplasmic membrane of the zygote in mammals 12 - Cytoplasmic membrane of the zygote in birds The secondary membranes in mammals consist of: • The pellucid zone (Latin, zona pellucida = transparent zone) is a specialized glycoprotein layer surrounding the developing oocyte within each follicle within the ovary. This thick membrane is thought to be formed by secretions from the oocyte and the follicle granulosa cells. It consists of four types of zona pellucida glycoproteins referred to as ZP1, ZP2, ZP3 and ZP4 which have different roles in fertilization. 13 - Pellucid zone • Corona radiata. Adhering to the outer surface of the zona pellucida are several layers of cells, derived from those of the follicle and collectively constituting the corona radiata. 14 - Corona radiata The secondary membrane in birds is equivalent to the vitelline membrane, which is the layer of proteins and receptors that surround the outer surface of the plasma membrane of an egg at ovulation. The species-specificity between these receptors plays an important role at fertilisation and prevents breeding between different species. 15 - Vitelline membrane The tertiary membranes are specific of egg-laying animals (reptiles and birds). They are produced and added by the oviduct as the egg goes down toward the cloaca. They consist of: • White Albumen: it is found outside the vitelline membrane. It is formed of inner denser albumen and outer less dense albumen. The albumen is formed of water and proteins. The egg yolk, which contains the egg nucleus and associated nutritive materials, is suspended in the albumen by one or two spiral bands of tissue called chalazae (singular: chalaza). 16 - White albumen 17 - Chalazae • Shell Membrane: the shell membrane is formed around the albumen. It is a double membrane. The two membranes adhere closely and are separated by an air space at the blunt end of the egg. This membrane is formed of keratin. 18 - Shel membrane • Shell: the shell is the outer covering of eggs in land animals. It is formed of calcium carbonate. The shell is porous (there are about 7,000 pores in a chicken eggshell). This allows the transfer of gases through the shell. Carbon dioxide and moisture are given off through the pores and are replaced by atmospheric gases, including oxygen. 19 - Shell Nutritive cytoplasm: yolk or vitellus The yolk, also known as vitellus, is the nutritive material that accumulates in the cytoplasm of the female gamete during its development and maturation in the ovary and has the function of nourishing the embryo. 20 - The yolk or vitellus https://sway.office.com/ZkdUwxb0nwCmK7MA#content=03BxxlhYbVfMu5 Depending on the amount of yolk in them, vertebrate eggs are classified as: • Oligolecithal or microlecithal (little yolk; oligo = little; lecithal = yolk): In oligolecithal eggs, the amount of yolk is much less than the amount of cytoplasm. These eggs are very small in size. The eggs of many invertebrates, marsupials and eutherian mammals are of this type. • Mesolecithal or mediolecithal (middle yolk; meso = middle; lecithal = yolk): Here yolk is moderate in amount. The eggs of may fishes and amphibians are of this type. • Polylecithal or macrolecithal (a lot of yolk; poli = a lot; lecithal = yolk): Enormous amount of yolk is present in macrolecithal eggs and here yolk is several times greater than the cytoplasm. The eggs of reptiles, birds and monotremes (egg-laying mammals) are included in this type. Depending on the distribution of yolk in them, vertebrate eggs are classified as: • Isolecithal (iso = equal): The yolk in these eggs is uniformly distributed through the cytoplasm. Examples are many invertebrates and mammals including man. • Centrolecithal: Yolk is concentrated in the interior of the egg and the cytoplasm is distributed as a thin layer on the outside of the yolk, as in insects and many other arthropods. • Telolecithal (telo or tele = distant): Yolk becomes more abundant and tends to concentrate on one hemisphere of the egg. Because of the uneven distribution of yolk, such an egg is said to have a vegetal pole, where the concentration of yolk is the greatest and an animal pole, where the concentration of yolk is the smallest. In fact, in macrolecithal eggs, the amount of yolk is so massive that it occupies almost all the vegetal pole, and the active cytoplasm and the nucleus remain confined to a small cap at the animal pole. Examples are of fishes, amphibians, and reptiles, birds and monotremes eggs. https://sway.office.com/ZkdUwxb0nwCmK7MA#content=RyTha6mt2OqHtV 21 - Yolk content of the eggs and evolution Yolk and evolution What impact has yolk had on animal evolution? The answer is clearly straight: Embryonic development depends entirely on nutritional reserves that are predominantly derived from vitellogenin proteins and stored in egg yolk. Therefore, the increased complexity observed in animal evolution only was possible producing eggs with increased yolk content. Because egg size is largely set by yolk platelet content, there is a crucial trade-off between the number of eggs and the size of those eggs. Fishes typically produce large numbers of eggs at one time with little yolk. The arrival of the amphibians was likely associated with massive increases in egg size and yolk content. This increased yolk content might have driven developmental innovations. The segmentation of the yolk-laden egg had to overcome the drawback of dividing a giant cell. In the gastrula stage, the increased yolk content also interfered with the cellular movement which had to diverge from the ancestral pattern toward new mechanisms of gastrulation and thereby news branches in evolution. In reptiles and birds, the increased complexity of the embryo demands a larger amount of nutrients stored in a shelled egg. In birds, the proportion of yolk differs between altricial and precocial birds. The former, which hatch so undeveloped that they require significant parental care and thus need less stored energy, generally have eggs that contain about 25 per cent yolk. Precocial birds, which can walk and feed themselves shortly after hatching, have eggs with about 40 per cent yolk. As might be expected, the eggs of precocial birds tend to be heavier in proportion to body weight than those of altricial birds. During mammalian evolution, the emergence of lactation (mammalian ancestors already had milk before they stopped laying eggs) set the stage for mammals' progressive loss of egg yolk. Lactation reduced dependency on the egg as a source of nutrition for the offspring. On the other hand, the development of the placenta in marsupials and eutherian mammals involved, among others complex transformation, the retention of the developing egg within the female reproductive tract, and consequently, the embryo development ceased to rely on the yolk content of the egg. This meant the gradual degeneration and loss of the yolk genes and yolk content in the mammalian egg. Chromosomes and normal development Cells contain chromosomes, string-like structures that comprise all of the animal's genetic material, called genes. The number of chromosomes does not correlate with the apparent complexity of an animal or a plant: in humans, for example, the diploid number is 2n = 46 (that is, 23 pairs), compared with 2n = 78 in dogs, 2n = 62 in horses, or 2n=60 in cattle. Haploid gametes (sperm and eggs) only contain one single chromosome from each pair. During fertilisation, the embryo receives one chromosome of each pair from each parent resulting in a new normal diploid individual. 22 - Nucleus of the zygote https://sway.office.com/ZkdUwxb0nwCmK7MA#content=QYhOZxFVs2oRv6 Chromosomes and sex determination Mammals have an XY sex-determination system: Since the female is XX, each of her eggs has a single X chromosome. The male, being XY, can generate two types of sperm: half bear the X chromosome, half the Y. If the egg receives another X chromosome from the sperm, the resulting individual is XX, forms ovaries, and is female; if the egg receives a Y chromosome from the sperm, the individual is XY, forms testes, and is male. Therefore, primary sex is determined in mammals just at fertilisation. Birds have a ZW sex-determination system: The sex chromosomes in birds are designated Z and W. The female is the heteromorphic (ZW) sex and the male homomorphic (ZZ). Therefore, the male gametes always have one Z chromosome and the female gametes can contain Z or W chromosome. That way, the sex is predetermined in the female gamete before fertilization. Chromosome Abnormalities A genetic disorder is a genetic problem caused by one or more abnormalities in the genome, especially a condition that is present from birth (congenital). This may happen as a result of various circumstances: variations in the number of chromosomes (aneuploid/polyploid); chromosome pieces attached to the wrong chromosome (translocations); loss of a piece of a chromosome (deletions); part of a chromosome is upside down (inversions); changes in the gene's DNA sequence (mutations). Chromosomally abnormal embryos have a low implantation rate. If they do implant, the pregnancy often results in miscarriage or the birth of a baby with physical problems or developmental delay. In human, chromosome disorders occur in well over half of all first-trimester pregnancy losses. Nevertheless, some chromosomal abnormalities give rise to viable embryos that are born with a series of abnormalities known as genetic syndromes. Regarding abnormalities in the number of chromosomes, polyploid organisms are those containing more than a paired set of all chromosomes. They can be triploid, tetraploid, pentaploid, etc. In contrast with polyploid, if the abnormality happens in a particular chromosome is called aneuploidy. Aneuploids organisms have a number of chromosomes either greater or smaller than that of the wild type in a particular chromosome. Some examples of aneuploidy are: Monosomic syndromes (meaning “one chromosome”). There is only one copy of some specific chromosome, instead of the usual two copies found in its diploid progenitor. The Turner syndrome (X0) is a well-known sex-chromosome monosomic aneuploidy. Affected individuals have a characteristic, easily recognisable female’s phenotype. Trisomic syndromes (meaning “three chromosomes”). There are several examples of viable trisomic syndromes. The combination XXY (1 in 1000 male births) results in Klinefelter syndrome. Another combination, XYY, also occurs in about 1 in 1000 male births and it is known as supermale syndrome. In humans, the Down syndrome is a trisomy of chromosome 21.