Gametogenesis gah

For students studying embryology

1. Gametogenesis Dr. Gauri Haval A.G. College Pune

2. Gametogenesis is the production of  gametes  from haploid precursor cells. In animals and higher plants, two morphologically distinct types of gametes are produced (male and female) via distinct differentiation programs. Animals produce a tissue that is dedicated to forming gametes, called the  germ line . Individual  germline  cells are called  germ cells . During the process of gametogenesis, a germ cell undergoes meiosis to produce  haploid cells  that directly develop into gametes. Hence, in animals, meiosis is an integral part of gametogenesis.

3. Spermatogenesis Means formation of sperm ; male gametes. Oogenesis Means formation of ovum ; female gametes. Sperm and ovum are highly specialized sex cells. Both types possess three main phases: Period of multiplication : The primordial germ cells multiply by mitotic cell division giving rise to oogonia in case of females and spermatogonia in case of males. 2. Period of growth : During these phase both oogonia (female gamete) and spermatogonia (male gamete) grow into primary oocyte or primary spermatocyte.

4. 3. Period of maturation : In case of female , the primary oocyte undergoes two meiotic cell division, the first gives rise to secondary oocyte and primary polar body. The secondary undergoes second meiotic division giving rise to mature ova and secondary polar body (mature ova leach oogonia). In case of male , each primary spermatocyte divides meiotically into secondary spermatocytes and intern to spermatid (4 spermatid l each primary spermatocyte).

5. Spermiation The mature spermatozoa released from the protective Sertoli cells into the lumen of the seminiferous tubule and a process called spermiation then takes place, which removes the remaining unnecessary cytoplasm and organelles . The resulting spermatozoa are now mature but lack motility, rendering them sterile. The non-motile spermatozoa are transported to the epididymis in testicular fluid secreted by the Sertoli cells with the aid of peristaltic contraction . In the epididymis they acquire 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.

6. Oogenesis In some species, such as sea urchins and frogs, the female routinely produces hundreds or thousands of eggs at a time, whereas in other species, such as humans and most mammals, only a few eggs are produced during the lifetime of an individual. In those species that produce thousands of ova, the  oogonia  are self-renewing stem cells that endure for the lifetime of the organism. In those species that produce fewer eggs, the oogonia divide to form a limited number of egg precursor cells. In the human embryo, the thousand or so oogonia divide rapidly from the second to the seventh month of gestation to form roughly 7 million germ cells  . After the seventh month of embryonic development, however, the number of germ cells drops precipitously. Most oogonia die during this period, while the remaining oogonia enter the first meiotic division 

7. These latter cells, called the  primary oocytes , progress through the first meiotic prophase until the diplotene stage, at which point they are maintained until puberty. With the onset of adolescence, groups of oocytes periodically resume meiosis. Thus, in the human female, the first part of meiosis begins in the embryo, and the signal to resume meiosis is not given until roughly 12 years later. In fact, some oocytes are maintained in meiotic prophase for nearly 50 years. 

8. When the primary oocyte divides, its nucleus, called the  germinal vesicle , breaks down, and the metaphase spindle migrates to the periphery of the cell. At telophase , one of the two daughter cells contains hardly any cytoplasm, whereas the other cell has nearly the entire volume of cellular constituents. The smaller cell is called the  first polar body , and the larger cell is referred to as the  secondary oocyte . During the second division of meiosis, a similar unequal cytokinesis takes place. Most of the cytoplasm is retained by the mature egg (ovum), and a second polar body receives little more than a haploid nucleus. Thus, oogenic meiosis conserves the volume of oocyte cytoplasm in a single cell rather than splitting it equally among four progeny.

9. Polar body formation

10. 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.  Most of this accumulation takes place during meiotic prophase I, and this stage is often subdividedintotwophases ,  previtellogenesis  (Greek, “before yolk formation”) and  vitellogenesis . 

11. 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.

12. 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 . It is not made in the frog oocyte (as are the major yolk proteins of organisms such as annelids and crayfishes), but is synthesized in the liver and carried by the bloodstream to the ovary. 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. 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.

13. As the yolk is being deposited, the organelles also become arranged asymmetrically. The cortical granules begin to form from the Golgi apparatus; they are originally scattered randomly through the oocyte cytoplasm, but later migrate to the periphery of the cell. The mitochondria replicate at this time, dividing to form millions of mitochondria that will be apportioned to the different blastomeres during cleavage. (In  Xenopus ,  new mitochondria will not be formed until after gastrulation is initiated.) As vitellogenesis nears an end, the oocyte cytoplasm becomes stratified. The cortical granules, mitochondria, and pigment granules are found at the periphery of the cell, within the actin-rich oocyte cortex. Within the inner cytoplasm, distinct gradients emerge. While the yolk platelets become more heavily concentrated at the vegetal pole of the oocyte, the glycogen granules, ribosomes, lipid vesicles, and endoplasmic reticulum are found toward the animal pole. Even specific mRNAs stored in the cytoplasm become localized to certain regions of the oocyte.

14. Amphibian oocytes can remain for years in the diplotene stage of meiotic prophase. This state resembles the G 2  phase of the cell division cycle (see Chapter 8). Resumption of meiosis in the amphibian primary oocyte requires progesterone. This hormone is secreted by the follicle cells in response to gonadotropic hormones secreted by the pituitary gland. Within 6 hours of progesterone stimulation,  germinal vesicle breakdown  ( GVBD ) occurs, the microvilli retract, the nucleoli disintegrate, and the chromosomes contract and migrate to the animal pole to begin division. Soon afterward, the first meiotic division occurs, and the mature ovum is released from the ovary by a process called  ovulation . The ovulated egg is in second meiotic metaphase when it is released

15. Entry into the mitotic (M) phase of the cell cycle (in both meiosis and mitosis) is regulated by  mitosis-promoting factor , or  MPF  (originally called “maturation-promoting factor” after its meiotic function). MPF contains two subunits,  cyclin B  and the  p34 cdc2  protein. The p34 protein is a cyclin -dependent-kinase—its activity is dependent upon the presence of cyclin . Since all the components of MPF are present in the amphibian oocyte, it is generally thought that progesterone somehow converts a pre-MPF complex into active MPF . Reference: Gilbert SF. Developmental Biology. 6th edition. Sunderland (MA): Sinauer Associates; 2000. Oogenesis. Available from: https://www.ncbi.nlm.nih.gov/books/NBK10008/

16. All the material necessary for the beginning of growth and development must be stored in the mature egg (the ovum ). The meiotic divisions that form the oocyte conserve its cytoplasm (rather than giving half of it away), and the oocyte either synthesizes or absorbs proteins, such as yolk, that act as food reservoirs for the developing embryo. Thus, birds' eggs are enormous single cells, swollen with their accumulated yolk.

17. E gg also has a remarkable cytoplasmic storehouse that it has accumulated during its maturation . Proteins. It will be a long while before the embryo is able to feed itself or obtain food from its mother . The early embryonic cells need a supply of energy and amino acids. In many species, this is accomplished by accumulating yolk proteins in the egg. Many of the yolk proteins are made in other organs (liver, fat body) and travel through the maternal blood to the egg.

18. Ribosomes and tRNA. The early embryo needs to make many of its own proteins, and in some species , there is a burst of protein synthesis soon after fertilization. Protein synthesis is accomplished by ribosomes and tRNA, which exist in the egg. The developing egg has special mechanisms to synthesize ribosomes, and certain amphibian oocytes produce as many as 1012 ribosomes during their meiotic prophase. Messenger RNA. In most organisms, the instructions for proteins made during early development are already packaged in the oocyte. It is estimated that the eggs of sea urchins contain 25,000 to 50,000 different types of mRNA. This mRNA, however, remains dormant until after fertilization Morphogenetic factors. Molecules that direct the differentiation of cells into certain cell types are present in the egg. They appear to be localized in different regions of the egg and become segregated into different cells during cleavage

19. Protective chemicals. The embryo cannot run away from predators or move to a safer environment , so it must come equipped to deal with threats. Many eggs contain ultraviolet filters and DNA repair enzymes that protect them from sunlight; some eggs contain molecules that potential predators find distasteful; and the yolk of bird eggs even contains antibodies.

20. Egg Envelopes Egg envelope on the basis of origin are of three types. ( 1) Primary egg envelops - These are secreted by egg.e.g . Vitelline membrane-( Zona pellusida & Zona Reticulate) ( 2) Secondary egg envelops - These are secreted by ovary. e.g.- Chorion on insects . (3) Tertiary egg envelops - These are secreted by oviduct. e.g.- Jelly coat of frog, Albumin, shell membrane and shell of Hen

21. Egg Membranes Enclosing the cytoplasm is the egg plasma membrane . This membrane must regulate the flow of certain ions during fertilization and must be capable of fusing with the sperm plasma membrane. Primary Egg membrane : Outside the plasma membrane is the vitelline envelope, which forms a fibrous mat around the egg. This envelope contains at least eight different glycoproteins and is often involved in sperm-egg recognition. The vitelline envelope is essential for the species-specific binding of sperm. The envelope thus formed bears different names in different animals – it is known as the vitaelline envelope or vitelline membrane in insects, molluscs, amphibians, and birds; in tunicates and fishes it is usually called the chorion . In mammals the envelope of an exactly similar nature is called the zona pellucida . The zona pellucida thus takes the place of the zona radiata . The jelly coat surrounding the eggs of sea urchins belongs in the same group. Vitelline membrane. In insects a second, thicker coat is secreted by the follicle cells on top of the vitelline envelope. This second envelope is called the chorion .

22. Secondary Envelope The second group of egg envelopes is those which are secreted by oviducts and other accessory parts of the genital organs while the egg is passing from the ovary to the exterior. The eggs of amphibians are surrounded by a layer of jelly, which protects the egg and sometimes serves to make the eggs adhere to one another and to submerged objects such as water plants. This jelly is secreted as the eggs pass through the oviducts. When the amphibian egg is deposited in water, the jelly absorbs water and swells. In the oviparous sharks and rays the egg is surrounded in the oviducts (in the special parts called the shell glands) by a hard shell of a complicated shape. The shell is drawn out into long twisted horns which serve to entangle the eggs among seaweed.

23. The most complicated egg envelopes, however, are found in the eggs of birds, where no less than five envelopes can be distinguished, the innermost being the vitelline envelope—a very thin envelope covering the surface of the yellow of the egg (which is the true egg cell). This envelope actually has a double origin . An inner layer of the envelope is produced in the ovary, in the space between the oocyte and the follicle cells. This layer is composed of very rough fibers . An outer, more finely fibrous layer is then formed on top of the first layer when the egg enters the upper portion of the fallopian tube. The next envelope is the white of the egg.

24. Over most of the surface of the egg the shell envelopes are in contact with one another, but at the blunt end of the egg they are separated; the inner envelope adheres to the egg white, and the outer envelope adheres to the shell, leaving a space in between filled with air. The outermost envelope is the shell, which consists chiefly of calcium carbonate (CaCO 3 ), about 5 gm. in a hen’s egg. The shell is pierced by many fine pores which are filled by an organic (protein) substance related to collagen . The envelopes of a bird’s egg are secreted one after another as the egg proceeds down the oviduct. The whole process takes slightly longer than 24 hours. After the egg has been released from the ovary, it quickly passes into the oviduct and descends through it for about three hours, during which most of the egg white is secreted and envelops the egg cell. The lowest portion of the oviduct is widened and is termed the uterus. Here the egg remains for 20 to 24 hours, while the remainder of the egg white and eventually the shell envelopes and the shell itself are secreted. Not only do the envelopes of a bird’s egg protect the egg cell, but the egg white serves also as an additional source of nourishment and is gradually used up in the course of the development of the embryo.

25. Secondary membranes: Many types of eggs also have an egg jelly outside the vitelline envelope. This glycoprotein meshwork can have numerous functions, but most commonly is used either to attract or to activate sperm.