GAMETOGENESIS | SPERMATOGENESIS | OOGENESIS | ZOOLOGYTALKS

Gametogenesis is a biological process by which diploid or haploid precursor cells undergo cell division and differentiation to form mature haploid gametes. Depending on the biological life cycle of the organism, gametogenesis occurs by meiotic division of diploid gametocytes into various gametes, or by mitotic division of haploid gametogenous cells. For example, plants produce gametes through mitosis in gametophytes. The gametophytes grow from haploid spores after sporic meiosis. The existence of a multicellular, haploid phase in the life cycle between meiosis and gametogenesis is also referred to as alternation of generations.

GAMETOGENESIS | SPERMATOGENESIS | OOGENESIS

BASIC CONCEPT OF GAMETOGENESIS

Gametogenesis is the process by which male and female sex cells or gametes, i.e., sperms and ova are formed respectively in the male and female gonads (testes and ovaries). The gametes differ from all other cells (= somatic cells) of the body in that their nuclei contain only half the number of chromosomes found in the nuclei of somatic cells. Meiosis forms the most significant part of process of gametogenesis.Gametogenesis for the formation of sperms is termed spermatogenesis, while that of ova is called oogenesis. Both spermatogenesis and oogenesis comprise similar phases of sequential changes.

TYPES OF EGGS

Based on the quantity of yolk, the eggs are of the following types:
- Alecithal eggs: The ova or eggs with no yolk are called alecithal. But this is idealistic feature because even in man and other eutherian mammals where egg size is smallest, the egg cytoplasm is not completely free from yolk.
- Microlecithal eggs: They contain very small amount of yolk, e.g., eggs of sea urchin, tunicates, and amphioxus. In marsupials (kangaroo) and eutherian mammals (man) eggs contain very little amount of yolk and hence these eggs are called alecithal (almost free of yolk).
- Mesolecithal eggs: These eggs contain moderate amount of yolk, e.g., eggs of Petromyzon (lamprey), lung fish, frogs and toads.
- Macrolecithal eggs: They contain large amount of yolk, e.g., eggs of insects, sharks, bony fishes, reptiles, birds and prototherian mammals.
Based on the distribution of yolk in the cytoplasm eggs are of the following types:
- Homolecithal eggs: The yolk is uniformly distributed all over the ooplasm (cytoplasm of the egg) e.g., eggs of echinoderms and potochordates.
- Telolecithal eggs: The yolk is concentrated in the vegetal half e.g., eggs of amphibians
- Meiolecithal eggs: The yolk is very large which occupies nearly the entire ooplasm, leaving free only a small disc-like area of cytoplasm for the nucleus e.g., eggs of reptiles, birds and egg laying mammals.
- Centrolecithal eggs: The yolk is localized at the centre e.g., eggs of insects.

SPERMATOGENESIS

Spermatogenesis process in which spermatozoa are produced from spermatogonial stem cells by way of mitosis and meiosis division. The initial cells in this pathway are called spermatogonia, which yield primary spermatocytes by mitosis. The primary spermatocyte divides meiotically (Meiosis I) into two secondary spermatocytes; each secondary spermatocyte divides into two spermatids by Meiosis II. These develop into mature spermatozoa, also known as sperm cells. Thus, the primary spermatocyte gives rise to two cells, the secondary spermatocytes, and the two secondary spermatocytes by their subdivision produce four spermatozoa.

Spermatozoa are the mature male gametes in many sexually reproducing organisms. Thus, spermatogenesis is the male version of gametogenesis, of which the female equivalent is oogenesis. In mammals it occurs in the seminiferous tubules of the male testes in a stepwise fashion. Spermatogenesis is highly dependent upon optimal conditions for the process to occur correctly, and is essential for sexual reproduction. DNA methylation and histone modification have been implicated in the regulation of this process. It starts at puberty and usually continues uninterrupted until death, although a slight decrease can be discerned in the quantity of produced sperm with increase in age (see Male infertility).

Purpose of Spermatogenesis

Spermatogenesis produces mature male gametes, commonly called sperm but specifically known as spermatozoa, which are able to fertilize the counterpart female gamete, the oocyte, during conception to produce a single-celled individual known as a zygote. This is the cornerstone of sexual reproduction and involves the two gametes both contributing half the normal set of chromosomes (haploid) to result in a chromosomally normal (diploid) zygote. To preserve the number of chromosomes in the offspring – which differs between species – each gamete must have half the usual number of chromosomes present in other body cells. Otherwise, the offspring will have twice the normal number of chromosomes, and serious abnormalities may result.

In humans, chromosomal abnormalities arising from incorrect spermatogenesis results in congenital defects and abnormal birth defects (Down Syndrome, Klinefelter’s Syndrome) and in most cases, spontaneous abortion of the developing fetus.

Location for Spermatogenesis

Spermatogenesis takes place within several structures of the male reproductive system. The initial stages occur within the testes and progress to the epididymis where the developing gametes mature and are stored until ejaculation. The seminiferous tubules of the testes are the starting point for the process, wheres permatogonial stem cells adjacent to the inner tubule wall divide in a centripetal direction—beginning at the walls and proceeding into the innermost part, or lumen—to produce immature sperm. Maturation occurs in the epididymis.

The location [Testes/Scrotum] is specifically important as the process of spermatogenesis requires a lower temperature to produce viable sperm, specifically 1°-8 °C lower than normal body temperature of 37 °C (98.6 °F). Clinically, small fluctuations in temperature such as from an athletic support strap, causes no impairment in sperm viability or count.

Duration for Spermatogenesis

For humans, the entire process of spermatogenesis is variously estimated as taking 74 days (according to tritium-labelled biopsies) and approximately 120 days (according to DNA clock measurements). Including the transport on ductal system, it takes 3 months. Testes produce 200 to 300 million spermatozoa daily. However, only about half or 100 million of these become viable sperm.

Stages for Spermatogenesis

The entire process of spermatogenesis can be broken up into several distinct stages, each corresponding to a particular type of cell in human. In the following table, ploidy, copy number and chromosome/chromatid counts are for one cell, generally prior to DNA synthesis and division (in G1 if applicable). The primary spermatocyte is arrested after DNA synthesis and prior to division.

Spermatocytogenesis

The process of spermatogenesis as the cells progress from primary spermatocytes, to secondary spermatocytes, to spermatids, to SpermSchematic diagram of Spermatocytogenesis Spermatocytogenesis is the male form of gametocytogenesis and results in the formation of spermatocytes possessing half the normal complement of genetic material.

In spermatocytogenesis, a diploid spermatogonium, which resides in the basal compartment of the seminiferous tubules, divides mitotically, producing two diploid intermediate cells called primary spermatocytes. Each primary spermatocyte then moves into the adluminal compartment of the seminiferous tubules and duplicates its DNA and subsequently under goes meiosis I to produce two haploid secondary spermatocytes, which will later divide once more into haploid spermatids. This division implicates sources of genetic variation, such as random inclusion of either parental chromosomes or chromosomal crossover, to increase the genetic variability of the gamete.

Each cell division from a spermatogonium to a spermatid is incomplete; the cells remain connected to one another by bridges of cytoplasm to allow synchronous development. It should also be noted that not all spermatogonia divide to produce spermatocytes; otherwise, the supply of spermatogonia would run out. Instead, spermatogonial stem cells divide mitotically to produce copies of them, ensuring a constant supply of spermatogonia to fuel spermatogenesis.

Structure of sperm

During spermiogenesis, the spermatids begin to form a tail by growing microtubules on one of the centrioles, which turns into basal body. These microtubules form an axoneme. The anterior part of the tail (called midpiece) thickens because mitochondria are arranged around the axoneme to ensure energy supply. Spermatid DNA also undergoes packaging, becoming highly condensed. The DNA is packaged firstly with specific nuclear basic proteins, which are subsequently replaced with protamines during spermatid elongation. The resultant tightly packed chromatin is transcriptionally inactive.

The Golgi apparatus surrounds the now condensed nucleus, becoming the acrosome. Maturation then takes place under the influence of testosterone, which removes the remaining unnecessary cytoplasmand organelles. The excess cytoplasm, known as residual bodies, is phagocytosed by surrounding Sertoli cells in the testes. The resulting spermatozoa are now mature but lack motility, rendering them sterile. The mature spermatozoa are released from the protective Sertoli cells into the lumen of the seminiferous tubule in a process called spermiation.

The non-motile spermatozoa are transported to the epididymis in testicular fluid secreted by the Sertoli cells with the aid of peristaltic contraction. While in the epididymis the spermatozoa gain motility and become capable of fertilization. However, transport of the mature spermatozoa through the remainder of the male reproductive system is achieved via muscle contraction rather than the spermatozoon’s recently acquired motility.

Role of Sertoli cells

At all stages of differentiation, the spermatogenic cells are in close contact with Sertoli cells which are thought to provide structural and metabolic support to the developing sperm cells. A single Sertoli cell extends from the basement membrane to the lumen of the seminiferous tubule, although the cytoplasmic processes are difficult to distinguish at the light microscopic level.

Sertoli cells serve a number of functions during spermatogenesis; they support the developing gametes in the following ways:

Maintain the environment necessary for development and maturation, via the blood-testis barrier
Secrete substances initiating meiosis
Secrete supporting testicular fluid
Secrete androgen-binding protein (ABP), which concentrates testosterone in close proximity to the developing gametes.
Testosterone is needed in very high quantities for maintenance of the reproductive tract, and ABP allows a much higher level of fertility
Secrete hormones affecting pituitary gland control of spermatogenesis, particularly the polypeptide hormone, inhibin.
Phagocytose residual cytoplasm left over from spermiogenesis.
Secretion of anti-Müllerian hormone causes deterioration of the Müllerian duct.
Protect spermatids from the immune system of the male, via the blood-testis barrier.
Contribute to the spermatogonial stem cell niche.

The intercellular adhesion molecules ICAM-1 and soluble ICAM-1 have antagonistic effects on the tight junctions forming the blood-testis barrier. ICAM-2molecules regulate spermatid adhesion on the apical side of the barrier (towards the lumen).

Influencing factors

The process of spermatogenesis is highly sensitive to fluctuations in the environment, particularly hormones and temperature. Testosterone is required in large local concentrations to maintain the process, which is achieved via the binding of testosterone by androgen binding protein present in the seminiferous tubules. Testosterone is produced by interstitial cells, also known as Leydig cells, which reside adjacent to the seminiferous tubules. Seminiferous epithelium is sensitive to elevated temperature in humans and some other species, and will be adversely affected by temperatures as high as normal body temperature. Consequently, the testes are located outside the body in a sack of skin called the scrotum.

The optimal temperature is maintained at 2 °C (man)–8 °C (mouse) below body temperature. This is achieved by regulation of blood flow and positioning towards and away from the heat of the body by the cremasteric muscle and the dartos smooth muscle in the scrotum. Dietary deficiencies (such as vitamins B, E and A), anabolic steroids, metals (cadmium and lead), x-ray exposure, dioxin, alcohol, and infectious diseases will also adversely affect the rate of spermatogenesis. In addition, the male germ line is susceptible to DNA damage caused by oxidative stress, and this damage likely has a significant impact on fertilization and pregnancy. Exposure to pesticides also affects spermatogenesis.

Hormonal control

Hormonal control of spermatogenesis varies among species. In humans the mechanism is not completely understood; however it is known that initiation of spermatogenesis occurs at puberty due to the interaction of the hypothalamus, pituitary gland and Leydig cells. If the pituitary gland is removed, spermatogenesis can still be initiated by follicle stimulating hormone (FSH) and testosterone. In contrast to FSH, LH appears to have little role in spermatogenesis outside of inducing gonadal testosterone production.

FSH stimulates both the production of androgen binding protein (ABP) by Sertoli cells, and the formation of the blood-testis barrier. ABP is essential to concentrating testosterone in levels high enough to initiate and maintain spermatogenesis. Intratesticular testosterone levels are 20–100 or 50–200 times higher than the concentration found in blood, although there is variation over a 5- to 10-fold range amongst healthy men. FSH may initiate the sequestering of testosterone in the testes, but once developed only testosterone is required to maintain spermatogenesis.

However, increasing the levels of FSH will increase the production of spermatozoa by preventing the apoptosis of type A spermatogonia. The hormone inhibin acts to decrease the levels of FSH. Studies from rodent models suggest that gonadotropins (both LH and FSH) support the process of spermatogenesis by suppressing the proapoptotic signals and therefore promote spermatogenic cell survival.

The Sertoli cells themselves mediate parts of spermatogenesis through hormone production. They are capable of producing the hormones estradiol and inhibin. The Leydig cells are also capable of producing estradiol in addition to their main product testosterone. Estrogen has been found to be essential for spermatogenesis in animals. However, a man with estrogen insensitivity syndrome (a defective ERα) was found produce sperm with a normal sperm count, albeit abnormally low sperm viability; whether he was sterile or not is unclear. Levels of estrogen that are too high can be detrimental to spermatogenesis due to suppression of gonadotropin secretion and by extension intratesticular testosterone production. Prolactin also appears to be important for spermatogenesis.

OOGENESIS

Oogenesis: Ovogenesis or oögenesis is the creation of an ovum (egg cell). It is the female form of gametogenesis; the male equivalent is spermatogenesis. It involves the development of the various stages of the immature ovum.
Oogenesis in mammals – In mammals, the first part of oogenesis starts in the germinal epithelium, which gives rise to the development of ovarian follicles, the functional unit of the ovary.

Oogenesis consists of several sub-processes: oocytogenesis, ootidogenesis, and finally maturation to form an ovum (oogenesis proper). Folliculogenesis is a separate subprocess that accompanies and supports all three oogenetic sub-processes.

Oogonium —(Oocytogenesis)—> Primary Oocyte —(Meiosis I)—> First Polar Body (Discarded afterward) + Secondary oocyte —(Meiosis II)—> Second Polar Body (Discarded afterward) + Ovum It should be noted that oocyte meiosis, important to all animal life cycles yet unlike all other instances of animal cell division, occurs completely without the aid of spindlecoordinating centrosomes.

The creation of oogonia

The creation of oogonia traditionally doesn’t belong to oogenesis proper, but, instead, to the common process of gametogenesis, which, in the female human, begins with the processes of folliculogenesis, oocytogenesis, and ootidogenesis.

Maturation of the oocyte in amphibians

The egg is responsible for initiating and directing development, and in some species (as seen above), fertilization is not even necessary. The accumulated material in the oocyte cytoplasm includes energy sources and energy-producing organelles (the yolk and mitochondria); the enzymes and precursors for DNA, RNA, and protein syntheses; stored messenger RNAs; structural proteins; and morphogenetic regulatory factors that control early embryogenesis. A partial catalogue of the materials stored in the oocyte cytoplasm, while a partial list of stored mRNAs .Most of this accumulation takes place during meiotic prophase I, and this stage is often subdivided into two phases, previtellogenesis (Greek, “before yolk formation”) and vitellogenesis. Cellular components stored in the mature oocyte of Xenopus laevis.

The eggs of fishes and amphibians are derived from an oogonial stem cell population that can generate a new cohort of oocytes each year. In the frog Rana pipiens, oogenesis takes 3 years. During the first 2 years, the oocyte increases its size very gradually. During the third year, however, the rapid accumulation of yolk in the oocyte causes the egg to swell to its characteristically large size). Eggs mature in yearly batches, with the first cohort maturing shortly after metamorphosis; the next group matures a year later.

Growth of oocytes in the frog. During the first 3 years of life, three cohorts of oocytes are produced. The drawings follow the growth of the first-generation oocytes. (After Grant 1953.)

Vitellogenesis occurs when the oocyte reaches the diplotene stage of meiotic prophase. Yolk is not a single substance, but a mixture of materials used for embryonic nutrition. The major yolk component in frog eggs is a 470-kDa protein called vitellogenin is synthesized in the liver and carried by the bloodstream to the ovary (Flickinger and Rounds 1956). This large protein passes between the follicle cells of the ovary, and is incorporated into the oocyte by micropinocytosis, the pinching off of membrane-bounded vesicles at the bases of microvilli (Dumont 1978).

In the mature oocyte, vitellogenin is split into two smaller proteins: the heavily phosphorylated phosvitin and the lipoprotein lipovitellin. These two proteins are packaged together into membrane-bounded yolk platelets). Glycogen granules and lipochondrial inclusions store the carbohydrate and lipid components of the yolk, respectively.

CHEMICAL AND METABOLIC EVENTS DURING GAMETE FORMATION

Metabolism is the set of life-sustaining chemical transformations within the cells of living organisms. The three main purposes of metabolism are the conversion of food/fuel to energy to run cellular processes, the conversion of food/fuel to building blocks for proteins, lipids, nucleic acids, and some carbohydrates, and the elimination of nitrogenous wastes. These enzyme-catalyzed reactions allow organisms to grow and reproduce, maintain their structures, and respond to their environments. The word metabolism can also refer to the sum of all chemical reactions that occur in living organisms, including digestion and the transport of substances into and between different cells, in which case the set of reactions within the cells is called intermediary metabolism or intermediate metabolism.
Metabolism is usually divided into two categories: catabolism, the breaking down of organic matter, for example, by cellular respiration, and anabolism, the building up of components of cells such as proteins and nucleic acids. Usually, breaking down releases energy and building up consumes energy.
The chemical reactions of metabolism are organized into metabolic pathways, in which one chemical is transformed through a series of steps into another chemical, by a sequence of enzymes. Enzymes are crucial to metabolism because they allow organisms to drive desirable reactions that require energy that will not occur by them, by coupling them to spontaneous reactions that release energy. Enzymes act as catalysts that allow the reactions to proceed more rapidly. Enzymes also allow the regulation of metabolic pathways in response to changes in the cell’s environment or to signals from other cells.
The metabolic system of a particular organism determines which substances it will find nutritious and which poisonous. For example, some prokaryotes use hydrogen sulfide as a nutrient, yet this gas is poisonous to animals. The speed of metabolism, the metabolic rate, influences how much food an organism will require, and also affects how it is able to obtain that food.
A striking feature of metabolism is the similarity of the basic metabolic pathways and components between even vastly different species For example, the set of carboxylic acids that are best known as the intermediates in the citric acid cycle are present in all known organisms, being found in species as diverse as the unicellular bacterium Escherichia coli and huge multicellular organisms like elephants These striking similarities in metabolic pathways are likely due to their early appearance in evolutionary history, and their retention because of their efficacy

CONCLUSION

Gametogenesis, by definition, is the development of mature haploid gametes from either haploid or diploid precursor cells. The precursor cells undergo cell division in order to become gametes. This may sound like a very technical definition, but by the end of this lesson you’ll understand it.
Organisms can be either diploid or haploid. Those that are diploid, like you and me, have two copies of their DNA per cell. Those that are haploid have one copy of their DNA per cell. As mentioned in the definition, gametes are all haploid. So, if you are already a haploid cell, you undergo regular cell division (mitosis). However, if you are diploid you have to make haploid gametes. That is, you have to create cells with only one copy of DNA each. This is also done by a special type of cell division called meiosis. During the process of mitotic cell division a cell makes a complete copy of its DNA. Then, when the cell divides, the DNA is split between the two daughter cells. Thus, each daughter cell gets a complete, exact copy of the parent cell’s genetic information (DNA). This type of cell division is a one-step process.
Meiotic cell division is a two-step process. Meiosis begins with a diploid cell that has two copies of DNA. One comes from the father and one from the mother. The cell divides twice producing four haploid cells. The first division is called Meiosis I. It involves replication of chromosomes but allows ‘gene shuffling’ between the maternal and paternal chromosomes. The second division is called Meiosis II. It results in haploid cells.
What is gene shuffling? Imagine you have a red blanket from your dad and a light blue one from your mom. Mitotic cell division keeps the blankets (or chromosomes) separate. So, all cells have red blankets and light blue blankets. Meiosis allows the cells to make a patchwork quilt out of mom and dad’s blankets. This creates genetic diversity which is an advantage to sexual reproduction