Oogenesis: Prenatal and Postnatal Stages

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Questions and Answers

What is the primary role of ovarian follicles in the process of oogenesis?

  • Providing support to the developing oocyte. (correct)
  • Facilitating meiosis II in the secondary oocyte.
  • Producing the zona pellucida around the oocyte.
  • Secreting estrogen to prepare the uterine lining.

How does the number of primary oocytes change from before birth to after puberty in females?

  • Decreases from approximately 7 million to about 4000. (correct)
  • Remains constant throughout the individual's life.
  • Increases steadily until menopause.
  • Increases significantly due to mitotic divisions.

At what stage of oogenesis does the primary oocyte arrest before puberty?

  • Prophase I (correct)
  • Metaphase II
  • Telophase II
  • Anaphase I

What triggers the release of GnRH (Gonadotropin-Releasing Hormone) after puberty, initiating the menstrual cycle?

<p>Signals from the hypothalamus. (A)</p> Signup and view all the answers

Which hormones are directly stimulated by GnRH (Gonadotropin-Releasing Hormone) to regulate the menstrual cycle?

<p>Follicle-Stimulating Hormone (FSH) and Luteinizing Hormone (LH) (B)</p> Signup and view all the answers

Approximately how long is the typical human menstrual cycle, which controls oogenesis?

<p>28 days (D)</p> Signup and view all the answers

What event marks the beginning of the menstrual cycles in females?

<p>Puberty (A)</p> Signup and view all the answers

In what specific stage of meiosis is the secondary oocyte arrested until fertilization occurs?

<p>Metaphase II (D)</p> Signup and view all the answers

What is the main distinction between primary and secondary oocytes regarding meiotic division?

<p>Primary oocytes are diploid (2n), while secondary oocytes are haploid (1n). (C)</p> Signup and view all the answers

What is folliculogenesis?

<p>The development of follicles within the ovary. (D)</p> Signup and view all the answers

What is the direct role of the zona pellucida in oogenesis and fertilization?

<p>Facilitating sperm binding and inducing the acrosomal reaction. (D)</p> Signup and view all the answers

How do follicular cells support the growing oocyte in unilaminar primary follicles?

<p>Forming a simple cuboidal epithelium around the oocyte. (D)</p> Signup and view all the answers

What structural feature characterizes multilaminar primary follicles?

<p>A stratified epithelium of granulosa cells. (C)</p> Signup and view all the answers

What is the significance of gap junctions between granulosa cells in a developing follicle?

<p>They facilitate communication and coordination among the granulosa cells. (A)</p> Signup and view all the answers

What initiates the transition from primordial follicles to primary follicles?

<p>The release of follicle-stimulating hormone (FSH). (C)</p> Signup and view all the answers

What is the role of liquor folliculi in secondary follicles?

<p>Accumulating in intercellular spaces between granulosa cells. (B)</p> Signup and view all the answers

Which structural innovation defines the formation of a secondary follicle during oogenesis?

<p>The appearance of the antrum. (C)</p> Signup and view all the answers

What is the main function of the theca interna cells in secondary follicles?

<p>Secreting androgens like androstenedione. (C)</p> Signup and view all the answers

How does the structure of the theca externa contribute to the function of secondary follicles?

<p>Providing structural support and merging with surrounding stroma. (B)</p> Signup and view all the answers

Which process is completed in secondary (antral) follicles, leading to the formation of a secondary oocyte and the first polar body?

<p>The first meiotic division (A)</p> Signup and view all the answers

What is the result of the chromosomes dividing equally between two daughter cells during secondary oocyte formation?

<p>One cell retains almost all of the cytoplasm to nourish the oocyte. (C)</p> Signup and view all the answers

What is the chromosomal content of the secondary oocyte compared to the primary oocyte?

<p>The secondary oocyte has half the chromosome number of the primary oocyte. (C)</p> Signup and view all the answers

What characterizes the structure known as the cumulus oophorus?

<p>A small hillock of granulosa cells surrounding the oocyte within the antrum. (D)</p> Signup and view all the answers

What is the composition of the fluid found within the antrum of a Graafian follicle?

<p>Follicular fluid containing hyaluronic acid, growth factors, and steroids. (B)</p> Signup and view all the answers

What is the term of the granulosa cells directly surrounding the zona pellucida in a Graafian follicle?

<p>Corona radiata (A)</p> Signup and view all the answers

At what specific stage of development is the oocyte within the Graafian follicle arrested prior to ovulation?

<p>Metaphase II (A)</p> Signup and view all the answers

Over what approximate time period does a primordial follicle typically develop into a mature follicle?

<p>90 days (D)</p> Signup and view all the answers

What is the 'stigma' in the context of ovulation?

<p>An area of tissue compaction with blocked blood flow on the tunica albuginea. (D)</p> Signup and view all the answers

Which hormone directly triggers ovulation?

<p>Luteinizing hormone (LH) (D)</p> Signup and view all the answers

Following ovulation, where does the secondary oocyte and its corona radiata typically enter?

<p>The fimbriated end of the oviduct (C)</p> Signup and view all the answers

During each menstrual cycle, what usually happens to the follicles that do not become the dominant follicle?

<p>They undergo atresia. (A)</p> Signup and view all the answers

What crucial role does estrogen play when produced by growing follicles, even if their oocytes are not used?

<p>Preparing the reproductive tract for embryo implantation. (A)</p> Signup and view all the answers

What is the term for the degenerative process by which most ovarian follicles are eliminated?

<p>Atresia (D)</p> Signup and view all the answers

Which cellular event is a key component of follicular atresia?

<p>Apoptosis and detachment of granulosa cells (A)</p> Signup and view all the answers

What distinguishes atretic follicles from healthy follicles within the ovary?

<p>Atretic follicles are undergoing degeneration. (B)</p> Signup and view all the answers

What is the approximate size range of a mature oocyte?

<p>120-130μm (C)</p> Signup and view all the answers

What cellular structures surround and support the oocyte?

<p>Zona pellucida and cumulus ooforus cells (C)</p> Signup and view all the answers

How is meiosis II completed during fertilization?

<p>It is induced to continue when a sperm penetrates the oocyte. (A)</p> Signup and view all the answers

What is the outcome of meiosis II after fertilization occurs?

<p>Two cells are produced again: the fertilized oocyte and the second polar body. (B)</p> Signup and view all the answers

How does the process of oogenesis ensure that the resulting oocyte receives adequate cytoplasmic material?

<p>By ensuring that one daughter cell retains almost all of the cytoplasm during meiotic divisions, forming a polar body. (C)</p> Signup and view all the answers

What is the significance of the fibrous theca externa in the secondary follicle?

<p>It provides structural support and merges with surrounding stroma. (D)</p> Signup and view all the answers

Which of the following represents the correct order of events in folliculogenesis, leading to ovulation?

<p>Primordial follicle → primary follicle → secondary follicle → Graafian follicle (C)</p> Signup and view all the answers

In the context of oogenesis, how do granulosa cells support the developing oocyte in a multilaminar primary follicle?

<p>By forming a stratified epithelium and communicating through gap junctions. (D)</p> Signup and view all the answers

How does the production of estrogen by growing follicles, even those whose oocytes are not destined for ovulation, contribute to the reproductive process?

<p>It prepares the reproductive tract to potentially support an embryo. (B)</p> Signup and view all the answers

Flashcards

Oogenesis

The process of producing a mature oocyte from immature germ cells, occurring in the ovaries and continuing in the oviducts.

Prenatal Oogenesis

The stage where the majority of oogenesis steps occur.

Oogonium

A diploid germ cell that divides by mitosis during fetal life.

Prophase I Arrest

The stage where primary oocytes stop dividing until puberty.

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Postnatal Oogenesis

Occurs after puberty where GnRH is released creating a menstrual cycle.

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Menstrual Cycle

The monthly cycle controlled by hormones that regulates oogenesis.

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Folliculogenesis

The sac-like development of follicles in the ovaries that support the oocyte development.

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Ovarian Follicles

The primary role of these structures is to provide support for the developing oocyte within the ovary.

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Primordial Follicles

Formed during fetal life; consist of a primary oocyte arrested in prophase I, surrounded by follicular cells.

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Unilaminar Primary Follicle

A follicle with follicular cells undergoing mitosis forming a simple cuboidal epithelium.

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Multilaminar Primary Follicle

A follicle with follicular cells that continue to proliferate forming a stratified structure called the granulosa.

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Zona Pellucida

Secreted by oocytes; important for sperm receptors, binding sperm and inducing the acrosomal reaction.

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Liquor Folliculi

The accumulation of fluid in intercellular spaces, leading to the formation of a cavity.

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Antrum

The cavity formed by fluid-filled spaces coalescing, crucial for follicle maturation and selection.

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Secondary (Antral) Follicles

Follicles outside primary follicles that differentiate stromal cells to form a second distinct tissue.

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Theca Interna

The inner well-vascularized endocrine tissue layer with steroid cells that is part of the secondary follicles.

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Theca Externa

The outer more fibrous layer containing fibroblasts and smooth muscles that is part of the secondary follicles.

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Secondary Oocytes

The cell resulting from the first meiotic division.

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Follicular Fluid

Contains hyaluronic acid, growth factors, and hormones that help with binding proteins.

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Cumulus Oophorus

Specialized collection of granulosa cells around the oocyte.

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Graafian Follicle

A mature follicle ready for ovulation, containing the corona radiata and large antrum.

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Corona Radiata

The granulosa cells surrounding the zona pellucida.

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Ovulation

The rupture of the follicle with the release of the oocyte from the ovary, typically around the 14th day of a cycle.

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Follicular Atresia

The degenerative process in which follicles degenerate and die.

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Atretic Follicles

The state of follicles in maturation that later undergo degeneration.

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Oocyte

A large cell with the main role of feeding zygotes during early mitotic divisions.

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Fertilization

The fusion process between sperm and oocyte that later creates a new zygote.

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Meiosis II

The process completed if fertilization occurs where the sperm penetrates the oocyte.

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Second Polar Body

The relatively small cell that results from the asymmetrical division of the secondary oocyte during meiosis II.

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Study Notes

  • Oogenesis is the process of producing a mature oocyte from immature germ cells.
  • It occurs in the ovaries and continues in the oviducts.
  • The process starts in early fetal life and continues during puberty.

Prenatal Oogenesis

  • Majority of the steps occur prenatally.
  • Oogonium divides many times in fetal life through mitosis.
  • Oogonia grow before birth to produce primary oocytes.
  • Most primary oocytes undergo atresia before birth.
  • Females are born with all the primary oocytes they will ever have.
  • Primary oocytes do not divide further.
  • Primary oocyte starts the first meiosis before birth but stops (arrests) in prophase I until puberty.

Number of Primary Oocytes During Life

  • Before birth, there are ~7 million.
  • At birth, there are ~2 million.
  • Before puberty, there are ~400,000.
  • After puberty (during the reproductive phase), there are ~4,000.

Postnatal Oogenesis

  • After puberty, GnRH is released from the hypothalamus.
  • GnRH causes FSH and LH release
  • These hormones control the menstrual cycle, which takes almost 28 days.
  • In each cycle, 5-15 primordial follicles start to grow, but only one oocyte gets ready for fertilization.
  • Oogenesis is controlled by the menstrual cycle (~28 days).
  • After puberty, menstrual cycles begin.
  • Oocyte begins meiosis and is arrested in prophase I during fetal life.
  • As the menstrual cycle starts, the oocyte completes the first meiotic division.
  • It enters the second meiosis and arrests in metaphase II in the menstrual cycle again.
  • Meiosis continues if fertilization occurs.

Arrest Points in Oogenesis

  • Primary oocyte is arrested in diplotene of prophase I.
  • It completes meiosis I and enters into meiosis II.
  • Secondary oocyte is arrested in metaphase II until activation or fertilization, which results in a mature egg.

Folliculogenesis

  • Oocyte development occurs within follicles.
  • Folliculogenesis is the development of follicles.
  • Follicles are sac-like structures where oocytes grow.
  • Oogenesis and folliculogenesis occur in conjunction with each other.

Ovarian Follicles

  • Ovarian follicles' primary role is oocyte support.
  • From birth, the ovaries contain numerous immature, primordial follicles.
  • These follicles each contain an immature primary oocyte.
  • After the first menstruation, a clutch of follicles begins folliculogenesis.
  • An ovarian follicle consists of an oocyte surrounded by one or more layers of epithelial cells.
  • The follicles formed during fetal life are primordial follicles, consisting of a primary oocyte (2N) arrested in prophase I enveloped by a single layer of flattened follicular cells.
  • These follicles are in the ovarian cortex.
  • The oocyte in the primordial follicle is ~25 µm in diameter and contains chromosomes in the first meiotic prophase.
  • The basal lamina surrounds the follicular cells.
  • It forms a clear boundary between the follicle and the vascularized stroma.
  • With the release of follicle-stimulating hormone (FSH) from the pituitary, a small group of primordial follicles begins folliculogenesis each month.
  • This involves the growth of the oocyte (meiosis continues), proliferation, and changes in the follicular cells and stromal fibroblasts around each follicle.
  • An oocyte can grow to a diameter of about 120 µm.
  • Follicular cells undergo mitosis and form a simple cuboidal epithelium around the growing oocyte.
  • The follicle is called a unilaminar primary follicle.
  • Follicular cells continue proliferating, forming a stratified follicular epithelium called the granulosa which communicates through gap junctions.
  • Follicular cells are now termed granulosa cells.
  • The follicle becomes a multilaminar primary follicle surrounded by a basement membrane.

Zona Pellucida

  • Between the oocyte and the first layer of granulosa cells, extracellular material accumulates, called the zona pellucida.
  • The zona pellucida contains four glycoproteins secreted by the oocyte.
  • It is essential for sperm receptors, binding specific proteins on the sperm surface and inducing acrosomal activation.
  • A fluid (liquor folliculi) begins to accumulate in the intercellular spaces between granulosa cells.
  • Fluid-filled spaces coalesce to form a single large cavity called an antrum.
  • As a result of this, secondary follicles are developed.
  • Stromal cells outside each growing primary follicle differentiate to form the follicular theca.
  • This differentiates further into two distinct tissues around the follicle.
    • A well- vascularized endocrine tissue, the theca interna, with steroid-producing cells secreting androstenedione.
    • A more fibrous theca externa with fibroblasts and smooth muscle merges gradually with the surrounding stroma.
  • First meiotic division is completed

Secondary Oocytes

  • The chromosomes are equally divided between the two daughter cells, but one retains almost all of the cytoplasm.
  • The cell is now the secondary oocyte (1N), and the other becomes the first polar body (PB).
  • Secondary oocyte contains half the chromosome number (N) of the parent cell.
  • The oocyte begins the second meiotic division but arrests again at metaphase until fertilization.
  • Follicular fluid contains hyaluronic acid, growth factors, plasminogen, fibrinogen, the anticoagulant heparan sulfate proteoglycan, and high concentrations of steroids (progesterone, androstenedione, and estrogens) with binding proteins.
  • As the antrum develops, the granulosa cells around the oocyte form a small hillock, the cumulus oophorus, which protrudes into the antrum

Graafian Follicle

  • Granulosa cells surrounding the zona pellucida make up the corona radiata.
  • They accompany the oocyte when it leaves the ovary at ovulation.
  • The large antrum of a mature or (preovulatory follicle / graafian follicle) accumulates follicular fluid and expands to a diameter of ~2 cm.
  • It contains a secondary oocyte (1N) arrested at metaphase II.
  • A preovulatory follicle forms a bulge at the ovary surface visible with ultrasound imaging.
  • A mature follicle normally develops from a primordial follicle over a period of about 90 days.

Ovulation

  • It's the release of oocyte from the ovary.
  • Occurs around the 14th day of a cycle.
  • Hours before ovulation, the mature dominant follicle bulging against the tunica albuginea develops an ischemic area, the stigma, in which tissue compaction has blocked blood flow.
  • Usually only one oocyte is ovulated during each cycle, but sometimes either no oocyte or two or more oocytes may be expelled.
  • Ovulation occurs in response to the LH surge.
  • The secondary oocyte and its corona radiata cells leave the ruptured follicle at the ovarian surface to enter the fimbriated end of the oviduct.
  • During a menstrual cycle, one follicle becomes dominant and develops farther than the others.
  • The dominant follicle undergoes ovulation, while the other follicles undergo atresia.
  • Large growing follicles produce estrogen before becoming atretic follicles.
  • This estrogen stimulates the preparation of the reproductive tract to transport and sustain the embryo if the oocyte from the dominant follicle is fertilized.

Follicular Atresia

  • Most ovarian follicles undergo the degenerative process called atresia. Follicular cells and oocytes die and are disposed of by phagocytic cells..
  • Follicles at any stage of development including nearly mature follicles may become atretic.
  • Atresia involves apoptosis and detachment of the granulosa cells, autolysis of the oocyte, and collapse of the zona pellucida.
  • Atretic follicles are follicles (in various stages of maturation) undergoing degeneration.
  • After a graafian follicle ovulates, the remaining secondary follicles degenerate.

Oocyte

  • A large cell having the potential to feed the zygote during early mitotic divisions.
  • The biggest cell of the body at ~120-130μm.
  • It is surrounded by zona pellucida and cumulus oophorus cells.
  • Feeds and protects the oocyte.

Fertilization

  • The process where sperm and oocyte come together to form a new zygote.
  • If fertilization occurs, meiosis II is induced to continue when a sperm penetrates the oocyte.
  • Meiosis II finishes, and two cells are produced again.
  • The chromatids are divided between the two daughter cells, but again, one of these cells is big (oocyte) ,and smaller become the second polar body.
  • The second polar body (PB) is extruded.

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