Information

In the process of oogenesis, when the egg completes meiosis 2, what actually happens?


oogenesis is the process of egg formation in females. The process of formation of the ovum is as follows

  1. oogonium
  2. primary oocyte
  3. secondary oocyte
  4. ovum

Though the original question is terribly unclear; seemingly the question wanted to mean "what should be the correct time-order of the following structures in the process of oogenesis in human?".


Fig-Human oogenesis. Source:*

Oogonium(in zoology), is a diploid cell (female gametogonium), that gradually gives rise to primary oocyte

In case of human oogenesis, this primary oocyte (diploid) get divided into 2 cells through meiosis-1… a (1) secondary oocyte and a (2) first-polar-body.

(In human) After that, meiosis-2 take place; where the secondary oocyte give rise to 2 haploid cell: an (1) Ovum and a (2) secondary polar body. In other hand; the first polar body also due to meiosis-2 division, gives 2 haploid cells (both are secondary polar body).

So the order would be

Oogonium(1)-->Primary oocyte (2)--> secondary oocyte (3)--> ovum (4) if they meant the temporal order of these structures.

Reference:

1:*Advanced BIOLOGY: Principles and Application; by C.J. Clegg and D.G. MacKean; John Murray publishing; copyright 1994.


At the end of meiosis, four haploid cells have been produced, but the cells are not yet gametes. The cells need to develop before they become mature gametes capable offertilization. The development of haploid cells into gametes is called gametogenesis.

How much DNA is in a gamete? The sperm cell forms by meiosis and spermatogenesis. Because it forms by meiosis, the sperm cell has only half as much DNA as a body cell. Notice the three distinct segments: a head piece, a flagella tail and a midpiece of mostly mitochondria. What is the role of each section?

Gametogenesis may differ between males and females. Male gametes are called sperm. Female gametes are called eggs. In human males, for example, the process that produces mature sperm cells is called spermatogenesis. During this process, sperm cells grow a tail and gain the ability to &ldquoswim,&rdquo like the human sperm cell shown in Figure below. In human females, the process that produces mature eggs is called oogenesis. Just one egg is produced from the four haploid cells that result from meiosis. The single egg is a very large cell, as you can see from the human egg in Figure below.

A human sperm is a tiny cell with a tail. A human egg is much larger. Both cells are mature haploid gametes that are capable of fertilization. What process is shown in this photograph? Notice the sperm with the head piece containing the genetic material, a flagella tail that propels the sperm, and a midpiece of mostly mitochondria, supplying ATP.

Spermatogenesis and Oogenesis

During spermatogenesis, primary spermatocytes go through the first cell division of meiosis to produce secondary spermatocytes. These are haploid cells. Secondary spermatocytes then quickly complete the meiotic division to become spermatids, which are also haploid cells. The four haploid cells produced from meiosis develop a flagellum tail and compact head piece to become mature sperm cells, capable of swimming and fertilizing an egg. The compact head, which has lost most of its cytoplasm, is key in the formation of a streamlined shape. The middle piece of the sperm, connecting the head to the tail, contains many mitochondria, providing energy to the cell. The sperm cell essentially contributes only DNA to the zygote.

On the other hand, the egg provides the other half of the DNA, but also organelles, building blocks for compounds such as proteins and nucleic acids, and other necessary materials. The egg, being much larger than a sperm cell, contains almost all of the cytoplasm a developing embryo will have during its first few days of life. Therefore, oogenesis is a much more complicated process than spermatogenesis.

Oogenesis begins before birth and is not completed until after fertilization. Oogenesis begins when oogonia (singular, oogonium), which are the immature eggs that form in the ovaries before birth and have the diploid number of chromosomes, undergo mitosis to form primary oocytes, also with the diploid number. Oogenesis proceeds as a primary oocyte undergoes the first cell division of meiosis to form secondary oocytes with the haploid number of chromosomes. A secondary oocyte only undergoes the second meiotic cell division to form a haploid ovum if it is fertilized by a sperm. The one egg cell that results from meiosis contains most of the cytoplasm, nutrients, and organelles. This unequal distribution of materials produces one large cell, and one cell with little more than DNA. This other cell, known as a polar body, eventually breaks down. The larger cell undergoes meiosis II, once again producing a large cell and a polar body. The large cell develops into the mature gamete, called an ovum (Figure below). The unequal distribution of the cytoplasm during oogenesis is necessary as the zygote that results from fertilization receives all of its cytoplasm from the egg. So the egg needs to have as much cytoplasm as possible.

Maturation of the ovum. Notice only one mature ovum, or egg, forms during meiosis from the primary oocyte. Three polar bodies may form during oogenesis. These polar bodies will not form mature gametes. Conversely, four haploid spermatids form during meiosis from the primary spermatocyte.


Developmental Biology. 6th edition.

Oogenesis—the differentiation of the ovum𠅍iffers from spermatogenesis in several ways. Whereas the gamete formed by spermatogenesis is essentially a motile nucleus, the gamete formed by oogenesis contains all the materials needed to initiate and maintain metabolism and development. Therefore, in addition to forming a haploid nucleus, oogenesis also builds up a store of cytoplasmic enzymes, mRNAs, organelles, and metabolic substrates. While the sperm becomes differentiated for motility, the egg develops a remarkably complex cytoplasm.

The mechanisms of oogenesis vary among species more than those of spermatogenesis. This difference should not be surprising, since patterns of reproduction vary so greatly among species. 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 (Figure 19.19). 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 (Pinkerton et al. 1961). 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. As Figure 19.19 indicates, primary oocytes continue to die even after birth. Of the millions of primary oocytes present at birth, only about 400 mature during a woman's lifetime.

Figure 19.19

Changes in the number of germ cells in the human ovary over the life span. (After Baker 1970.)

Oogenic meiosis also differs from spermatogenic meiosis in its placement of the metaphase plate. 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 (Figure 19.20). 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.

Figure 19.20

Polar body formation in the oocyte of the whitefish Coregonus. (A) Anaphase of first meiotic division, showing the first polar body pinching off with its chromosomes. (B) Metaphase (within the oocyte, arrow) of the second meiotic division, with the first (more. )

In a few species of animals, meiosis is severely modified such that the resulting gamete is diploid and need not be fertilized to develop. Such animals are said to be parthenogenetic (Greek, “virgin birth”). In the fly Drosophila mangabeirai, one of the polar bodies acts as a sperm and �rtilizes” the oocyte after the second meiotic division. In other insects (such as Moraba virgo) and in the lizard Cnemidophorus uniparens, the oogonia double their chromosome number before meiosis, so that the halving of the chromosomes restores the diploid number. The germ cells of the grasshopper Pycnoscelus surinamensis dispense with meiosis altogether, forming diploid ova by two mitotic divisions (Swanson et al. 1981). All of these species consist entirely of females. In other species, haploid parthenogenesis is widely used not only as a means of reproduction, but also as a mechanism of sex determination. In the Hymenoptera (bees, wasps, and ants), unfertilized eggs develop into males, whereas fertilized eggs, being diploid, develop into females. The haploid males are able to produce sperm by abandoning the first meiotic division, thereby forming two sperm cells through second meiosis.


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What is Oogenesis? (with pictures)

Oogenesis is the production of an ovum or egg cell, the female gamete or sex cell. It is one type of gametogenesis, or sex cell production, the other being the male process of spermatogenesis. Oogenesis happens in all sexually reproductive species, and it includes all of the immature stages of the ovum. As it matures, the ovum passes through five stages in mammals: the oogonium, the primary oocyte, the secondary oocyte, the ootid, and the ovum.

In most sexually reproductive species, the ovum contains half of the genetic material of a mature individual. Reproduction takes place when the egg cell is fertilized by the male gamete, or sperm. The sperm also contains half the genetic material of a mature individual, so the embryo formed by fertilization will contain a full set of genetic material, half from the ovum and half from the sperm.

The first stage of the immature ovum is the oogonium, formed by mitosis in the very early life of the organism. In mitosis, a cell replicates its DNA — its genetic material — before dividing into two identical daughter cells. Mitosis is also a method of asexual reproduction. In animals, sex cells or gametes, including egg cells, are only formed by meiosis, in which a cell divides without replication, resulting in daughter cells with only half the number of chromosomes of the parent cell. All other body cells are formed by mitosis.

In the first stage of oogenesis, the oogonium undergoes oocytogenesis, creating the primary oocyte through mitosis. Like the oogonium, the primary oocyte is a diploid cell, containing two complete sets of chromosomes. Sex cells are haploid cells, containing only half the amount of chromosomes in a diploid cell. Haploid cells are formed from diploid cells by meiosis.

Through ootidogenesis, a form of meiosis, the primary oocyte produces the haploid secondary oocyte. The process of ootidogenesis is halted halfway through, which is called dictyate, until ovulation, when it is completed to produce the released egg or ootid. In the final stage, the ootid develops into the ovum, the mature egg cell. In humans and other mammals, the secondary oocyte does not become an ootid until is is ready to be released during the menstrual cycle.

In protists, such as algae, and gymnosperms, the non-flowering seed-bearing land plants, oogenesis begins not in the oogonium, but in a specialized structure called the archigonium. In flowering plants, it takes place within the megagametophyte, or embryo sac, contained in the ovule in the flower's ovary. When the egg cell is mature, the ovule will become the seed, which protects and nourishes the egg cell. In some organisms, notably the parasitic roundworm ascaris, the meiosis period only begins if the sperm comes into contact with the primary oocyte.

In addition to her role as a InfoBloom editor, Niki enjoys educating herself about interesting and unusual topics in order to get ideas for her own articles. She is a graduate of UCLA, where she majored in Linguistics and Anthropology.

In addition to her role as a InfoBloom editor, Niki enjoys educating herself about interesting and unusual topics in order to get ideas for her own articles. She is a graduate of UCLA, where she majored in Linguistics and Anthropology.


Hormonal control of oogenesis

Oogenesis is controlled by FSH, LH, estrogen, and progesterone.

  • FSH stimulates development of egg cells that develop in structures called follicles, which are located within the ovaries.
  • LH also promotes development and maturation of eggs and induction of ovulation.
  • Estrogen is the reproductive hormone in females that assists in ovulation and regrowing the lining of the uterus it is also responsible for the secondary sexual characteristics of females such as breast development.
  • Progesterone assists in endometrial re-growth and inhibition of FSH and LH release.

These hormones together regulate the ovarian and menstrual cycles. The ovarian cycle governs the preparation of endocrine tissues and release of eggs, while the menstrual cycle governs the preparation and maintenance of the uterine lining. These cycles occur concurrently and are coordinated over a 22–32 day cycle, with an average length of 28 days:

  • The first half of the ovarian cycle is the follicular phase. Slowly rising levels of FSH and LH cause the growth of follicles on the surface of the ovary. This process prepares the egg for ovulation. As the follicles grow, they begin releasing estrogens. Estrogen levels increase over the course of the follicular phase as the follicles continue to develop. In the menstrual cycle, menstrual flow occurs at the beginning of the follicular phase when estrogen levels are low (when the follicles are only just beginning to develop) rising levels of estrogen then cause the endometrium to proliferate (grow), replacing the blood vessels and glands that deteriorated during the end of the last cycle.
  • Ovulation occurs just prior to the middle of the cycle (approximately day 14), when the high level of estrogen produced by the developing follicles causes FSH and especially LH to rise rapidly, then fall. The spike in LH causes ovulation: the follicle which is most mature ruptures and releases its egg. The follicles that did not rupture degenerate and their eggs are lost. The level of estrogen decreases when the extra follicles degenerate.
  • Following ovulation, the ovarian cycle enters its luteal phase, and the menstrual cycle enters its secretory phase, both of which run from about day 15 to 28. The cells in the follicle undergo physical changes and produce a structure called a corpus luteum, which produces estrogen and progesterone. The progesterone facilitates the regrowth of the uterine lining and inhibits the release of further FSH and LH. The uterus becomes prepared to accept a fertilized egg, should fertilization occur. The inhibition of FSH and LH by progesterone prevents any further eggs and follicles from developing. The level of estrogen produced by the corpus luteum increases to a steady level for the next few days estrogen enhances the effects of progesterone.
  • It takes about seven days for an egg to travel through the Fallopian tube from the ovary to the uterus, and it must be fertilized while in the Fallopian tube:
    • If no fertilized egg is implanted into the uterus, the corpus luteum degenerates and the levels of estrogen and progesterone decrease. The endometrium begins to degenerate as the progesterone levels drop, initiating the next menstrual cycle. The decrease in progesterone also allows the hypothalamus to send GnRH to the anterior pituitary, releasing FSH and LH and starting the cycles again. The figure below visually compares the ovarian and uterine cycles as well as the hormone levels controlling these cycles.
    • If a fertilized egg implants in the endometrial lining of the uterine wall, the embryo produces a hormone called human chorionic gonadotropin (hCG) that maintains the corpus luteum. The ovary continues to produce progesterone at high levels, and the menstrual cycle is arrested for the duration of the pregnancy. Because hCG is unique to pregnancy, it is the hormone detected by pregnancy tests.

    The figure below visually compares the ovarian and uterine cycles as well as the hormone levels controlling these cycles.

    Rising and falling hormone levels result in progression of the ovarian and menstrual cycles. Image credit: modification of work from OpenStax Biology and OpenStax Anatomy and Physiology modification of work by Mikael Häggström)

    This video provides a great overview of the human female reproductive system, emphasizing many of the points described above:


    The sister chromatids are pulled apart by the kinetochore microtubules and move toward opposite poles. Non-kinetochore microtubules elongate the cell.

    Figure 1. The process of chromosome alignment differs between meiosis I and meiosis II. In prometaphase I, microtubules attach to the fused kinetochores of homologous chromosomes, and the homologous chromosomes are arranged at the midpoint of the cell in metaphase I. In anaphase I, the homologous chromosomes are separated. In prometaphase II, microtubules attach to the kinetochores of sister chromatids, and the sister chromatids are arranged at the midpoint of the cells in metaphase II. In anaphase II, the sister chromatids are separated.


    How many eggs are produced from one primary oocyte?

    Considering this, how many ovum are produced from one primary oocyte?

    what is a primary oocyte? Primary oocyte. From Biology-Online Dictionary | Biology-Online Dictionary. Definition. noun, plural: primary oocytes. The oocyte that arises from the oogonium via the process of oocytogenesis, and gives rise to secondary oocyte and polar body after first meiotic division (meiosis I).

    Subsequently, one may also ask, how many eggs are produced by Oogenesis?

    In human females, the process that produces mature eggs is called oogenesis. Just one egg is produced from the four haploid cells that result from meiosis.

    What is primary oocyte and secondary oocyte?

    primary oocyte into the ovum is called the oogenesis. The primary oocyte undergoes meiosis 1 to produce a secondary. oocyte and a polar body. The secondary oocyte undergoes meiosis 2 to produce the ootid and another polar body.


    Sexual Reproduction in Humans - The First Stages

    Like in plants it is the male gamete that needs to be transferred to the female gamete. The female gamete is fertilised and develops inside the mother's body so the reproductive systems of both males and females are highly adapted for this.

    Male reproductive system and sperm production

    Production of sperm is called spermatogenesis.

    It takes place in the gonads of the male - the testes. Over 100 million can be made in one day!

    Each testis is composed of numerous tiny tubes called seminiferous tubules. It is in the walls of these tubules that sperm production actually takes place.

    Development begins in the outer side of the wall in a layer of cells called the germinal epithelium. As the immature sperm cells become more mature they move to the inner side and break way into the lumen of the tubule to be carried away to the epididymis for storage. The process of this production is shown in the next two diagrams.

    The Sertoli cells are present to provide nourishment to the developing gametes and to aid with the development of the specialised shape of the sperm cells.

    At the end of this production line, mature sperm look like this:

    In between the tubules, inside the testes, are interstitial cells called Leydig cells. These secrete the hormone testosterone.

    There are also blood vessels in close proximity, delivering nutrients and carrying away some testosterone to other target cells for the development and maintenance of secondary sexual characteristics, e.g. facial and pubic hair, deepening of the voice. The testosterone also stimulates the cells inside the testis involved in spermatogenesis.

    Hormonal control of spermatogenesis

    The control centres are the pituitary gland and the hypothalamus in the brain.

    The hypothalamus secretes GnRH (gonadotrophin releasing hormone). This is released into the blood and stimulates the anterior lobe of the pituitary gland.

    The anterior lobe of the pituitary gland secretes ICSH (interstitial cell stimulating hormone).

    ICSH: this stimulates the leydig cells that produce testosterone.

    FSH: this stimulates the seminiferous tubules, including the Sertoli cells. They produce sperm in response.

    Note: Testosterone also acts on the seminiferous tubules and stimulates sperm production.

    The testosterone feeds back to the hypothalamus and pituitary gland to switch off GnRH and ICSH release.

    The Sertoli cells produce a hormone called inhibin that feeds back to the pituitary gland to switch off FSH release.

    Since the action of the interstitial cells and Sertoli cells are inhibited, less testosterone and inhibin are released as a result. The inhibition of the hypothalamus and pituitary is lifted and the process can start again. Due to the levels of the hormones and their effects, the process is not noticeably cyclical - there aren't noticeable peaks and troughs in the levels of the hormones.

    Female reproductive system and egg production

    The production of eggs is called oogenesis. It takes place in the ovaries and begins before birth.

    The outer layer of the ovary (the germinal epithelium) produces primary oocytes. It also produces follicle cells that congregate around the oocytes, forming a structure called the primary follicle.

    By the time a baby girl is born, the primary oocytes in the primary follicles have started the first meiotic division but the process halts at the first stage (prophase I).

    After puberty one of these develops each month. It completes meiosis I to form a secondary oocyte and first polar body (the latter of which will eventually disintegrate). The follicle cells around it proliferate to form a wall many cells thick called the theca. Fluid collects inside the structure to form a fluid-filled cavity.

    The whole structure is called a Graafian follicle.

    At a time controlled by hormones the secondary oocyte is released from the Graafian follicle and it leaves the ovary - a process called ovulation. The secondary oocyte with some surrounding follicle cells leaves the ovary and enters one of the oviducts. What is left behind on the surface of the ovary turns into a structure called the corpus luteum.

    Hormonal control of oogenesis

    As with males, the process of egg production is controlled by 2 centres in the brain - the hypothalamus and the anterior lobe of the pituitary gland. Unlike males, however, the process is cyclical with rises and falls in hormone levels. The cycle, called the menstrual cycle - takes about 28 days, with ovulation occurring in the middle at about day 14.


    Useful Notes on Gametogenesis (Spermatogenesis and Oogenesis)

    The origin and development of gametes is called gametogenesis (Fig. 3(B).1).

    This may be divided into spermatogenesis and oogenesis. Spermatogenesis deals with the development of male sex-cells called sperms in the male gonad or testis.

    Oogenesis is the development of female sex-cells called ova or eggs in the female gonad or ovary.

    1. Spermatogenesis:

    The entire process of spermatogenesis can be divided into following two phases:

    (A) Formation of Spermatid:

    The male gonad known as testis is the site of spermatogenesis. In each vertebrate a pair of testes remains attached to dorsal body wall by a connective tissue called mesorchium. Each testis is formed of thousands of minute elongated and coiled tubules called seminiferous tubules. The inner lining of seminiferous tubules is called as germinal epithelium and is made of primordial germ cells (Primary germ cells) as well as some supporting nutritive cells. The primordial germ cells give rise to spermatids through the following steps (Fig. 3(B).2).

    1. Multiplication Phase:

    The primary germ cells multiply by repeated mitotic division. The cells produced after the final mitotic divisions are known as spermatogonia or sperm mother cells.

    The spermatogonia do not divide for sometime but increase in size by accumulating nutritive materials from the supporting cells. In mammals such supporting cells are called cells of Sertoli. The enlarged spermatogonia are now called primary spermatocytes.

    During the phase of maturation, the primary spermatocytes divide by meiosis consisting of two successive divisions. The first division is reductional or disjunctional reducing the chromosome number from 𔃲n’ to ‘n’. These cells are celled secondary spermatocytes. Second division is equational resulting in formation of four daughter cells called spermatids.

    (B) Spermiogenesis (Spermatoleosis):

    This is the second phase of spermatogenesis during which the spermatids produced at the end of first phase are metamorphosed into sperm cells. The spermatid is a typical cell containing a nucleus and cytoplasmic organelles such as mitochondria, golgi bodies, centriole etc, but the nucleus only contains haploid number of chromosomes.

    During spermiogenesis or spermatoleosis the following transformations occur in the spermatids:

    1. The large spherical nucleus becomes smaller by losing water and usually changes its shape into elongated structure.

    2. The Golgi bodies condense into a cap called acrosome in front of the nucleus.

    3. Nucleus and the acrosome combinedly form the head of the developing sperm while the cytoplasm with mitochondria and centrioles move downwards and form the cylindrical middle piece behind the head (Fig. 3(B).3).

    4. The two centrioles of middle piece develop axial filaments which are bunched into a single thread and extend behind in the form of a long vibratile tail. Thus, spermatid is transformed into a motile sperm divisible into head, middle piece and tail.

    2. Oogenesis:

    It occurs in the ovary of female animals. It is comparable to spermatogenesis so far as nuclear changes are concerned. But the cytoplasmic specialization in oogenesis is different from spermatogenesis.

    It is divisible into following three phases:

    1. Multiplication Phase:

    The primary germinal cells of the ovary with diploid number of chromosomes (2n) divide several times mitotically so as to form a large number of daughter cells known as oogonia (Fig. 3(B) .4).

    2. Growth Phase:

    The oogonium does not divide but increases in size enormously to form a primary oocyte. The growth is associated with both nuclear and cytoplasmic growth. The nuclear growth is due to accumulation of large amount of nuclear sap and is termed as germinal vesicle. The cytoplasmic growth is associated with increase in number of mitochondria, endoplasmic reticulum and Golgi complex and accumulation of reserve food material called yolk or vitellin.

    3. Maturation phase:

    The primary oocyte undergoes two successive divisions by meiosis. The first division is meiosis-I and two unequal daughter cells are produced. The large cell is called secondary oocyte containing haploid (n) set of chromosomes (due to reductional or disjunctional division) and entire amount of cytoplasm. The smaller cell is called first polar body or polocyte containing ‘n’ number of chromosomes and practically no cytoplasm.

    The secondary oocyte and first polar body then undergo second maturation division by meiosis-II which is an equational division. As a result of this division one large ovum is formed containing entire amount of cytoplasm and ‘n’ number of chromosomes and a second polar body like the first polar body.

    Simultaneously, the first polar body may divide into two polar bodies or may not divide at all. Thus only one functional ovum is formed and the two or three polar bodies soon degenerate. In vertebrates the first polar body is formed after the primary oocyte is released from ovary and has entered into the oviduct. The second polar body is formed only when the sperm enters into ovum during fertilization.

    (C) Ripening of Egg:

    Oogenesis is followed by the formation of protective coverings called egg membranes. Primary membrane is formed surrounding the plasma membrane of ovum and is secreted by the ovum itself. It is called vitelline membrane in frog and zona pellucida in rabbit. The secondary membrane called chorion is formed from ovarian follicle cells. The tertiary membranes are secreted in oviduct when the ovum passes from ovary to outside. The egg white (albumin), calcareous shell etc. come under this category (Fig. 3(B).5).

    The ripe ovum is spherical or oval and non-motile. Depending upon the amount of yolk, it may be as small as 0.15 mm as in mammals (microlecithal) it may be 2 mm as in frog (mesolecithal) or it may be as large as 30 mm as in hen (megalecithal).

    In a ripe ovum, the polarity is fixed. The top-most point is animal pole and the bottom point is vegetal pole. The density of yolky cytoplasm increases from the animal pole towards the vegetal pole. In frog, the animal hemisphere is highly pigmented and appears black while the vegetal hemisphere is highly pigmented and appears white.

    1. The process leads to formation of germ cells or gametes.

    2. The normal body cells known as somatic cells are diploid (2n) where as the germ cells are haploid (n).

    3. During fertilization one halpoid sperm unites with one haploid ovum to form a normal diploid somatic cell thus keeping the chromosome number constant generation after generation.

    4. During first maturation division, the reshuffling of paternal and maternal genes take place resulting in variation.


    Watch the video: Oogenesis (January 2022).