Lecture 2A
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Questions and Answers

What is the result of the first meiotic division in primary sex cells?

  • Differentiation of primary spermatocyte
  • Formation of spermatids
  • Production of secondary sex cells (correct)
  • Formation of polar bodies

    • What is the function of germplasm in gametogenesis?

  • To undergo meiosis
  • To produce mRNA-binding proteins
  • To create a cell-dense association of mRNAs and proteins (correct)
  • To differentiate primary sex cells

    • What is the difference between oogenesis and spermatogenesis in terms of the number of sex cells produced?

  • Oogenesis produces one functional sex cell, while spermatogenesis produces four sex cells (correct)
  • Oogenesis produces four sex cells, while spermatogenesis produces one functional sex cell
  • Both oogenesis and spermatogenesis produce one functional sex cell
  • Both oogenesis and spermatogenesis produce four sex cells

    • What is the term for the cells that undergo meiosis to produce sex cells?

    <p>Primary sex cells</p> Signup and view all the answers

    What is the result of the second phase of meiosis in secondary sex cells?

    <p>Production of spermatids and mature oocyte</p> Signup and view all the answers

    What is the process called that transforms primordial germ cells into specialized sex cells?

    <p>Gametogenesis</p> Signup and view all the answers

    Where does oogenesis take place?

    <p>Ovary</p> Signup and view all the answers

    What is the term for the stage where primary sex cells undergo one more round of growth and differentiation?

    <p>Gonial stage</p> Signup and view all the answers

    In which stage of development can the germ cells be recognized in anuran amphibians?

    <p>Unfertilized egg stage</p> Signup and view all the answers

    What is the marker for primordial germ cells (PGCs) in amniotes?

    <p>Alkaline phosphatase</p> Signup and view all the answers

    In humans, when can primordial germ cells (PGCs) be recognized in the yolk sac endoderm?

    <p>3 weeks of embryonic development</p> Signup and view all the answers

    What is the extraembryonic membrane most intimate with the developing embryo?

    <p>Amnion</p> Signup and view all the answers

    How do primordial germ cells (PGCs) migrate from the yolk sac to the allantois?

    <p>By amoeboid motion</p> Signup and view all the answers

    What is the signal factor that is necessary for the development of primordial germ cells (PGCs)?

    <p>Bone morphogenetic protein (BMP)</p> Signup and view all the answers

    What is the region in the embryo where the primordial germ cells (PGCs) are located before migrating to the gonads?

    <p>Extraembryonic mesoderm</p> Signup and view all the answers

    What is the structure that connects the hindgut to the genital ridges?

    <p>Dorsal mesentery</p> Signup and view all the answers

    What are the pluripotency gene markers of the primordial germ cells (PGCs)?

    <p>Oct-4+, NANOG, and SOX2</p> Signup and view all the answers

    What is the proposed migration pathway of the primordial germ cells (PGCs)?

    <p>From the yolk sac endoderm to the allantois to the hindgut to the genital ridges</p> Signup and view all the answers

    Where do the primordial germ cells (PGCs) become specified?

    <p>In the allantois</p> Signup and view all the answers

    What is the fate of the PGCs that do not reach their destinations during migration?

    <p>They develop into teratomas</p> Signup and view all the answers

    During which phase do the PGCs undergo mitotic division?

    <p>The proliferation phase</p> Signup and view all the answers

    What is the difference between mitotic patterns in males and females?

    <p>Mitotic patterns differ in timing and duration between males and females</p> Signup and view all the answers

    When do meiosis begin in males?

    <p>At puberty</p> Signup and view all the answers

    What happens to the oogonia in females by the seventh month of embryonic development?

    <p>They enter the first meiotic division</p> Signup and view all the answers

    How many oocytes are retained at puberty in females?

    <p>400,000</p> Signup and view all the answers

    What is the fate of the 400,000 oocytes retained at puberty?

    <p>Only 400-500 can complete maturation</p> Signup and view all the answers

    What is the destination of the PGCs in males?

    <p>The testes</p> Signup and view all the answers

    What is the term for the process of formation of a new germ layer during gastrulation?

    <p>Gastrulation</p> Signup and view all the answers

    At what stage do PCGs migrate through the forming hindgut?

    <p>After E7.25</p> Signup and view all the answers

    What is the fate of migrating PCGs that stray from their final destination?

    <p>They form teratomas</p> Signup and view all the answers

    What is the proposed mechanism by which migrating PCGs find their final destination?

    <p>Chemoattractants</p> Signup and view all the answers

    What is the primary difference in mitotic patterns between males and females?

    <p>Mitosis is continuous in males, but stops early in females</p> Signup and view all the answers

    At what stage do oogonia enter the primary oocyte stage?

    <p>7th month of embryonic development</p> Signup and view all the answers

    What is the number of germ cells left in females at puberty?

    <p>400,000</p> Signup and view all the answers

    What is the duration of reproductive years in females?

    <p>35 years</p> Signup and view all the answers

    What is the purpose of the final proliferation phase of germ cells?

    <p>To increase the population of germ cells</p> Signup and view all the answers

    At what stage do the primary oocytes in females enter meiosis?

    <p>During embryonic development, around the 5th month</p> Signup and view all the answers

    What is the end result of meiosis I in terms of chromosome number?

    <p>Halved chromosome number</p> Signup and view all the answers

    What is the purpose of synapsis in meiosis I?

    <p>To align homologous chromosomes</p> Signup and view all the answers

    What is the term for the exchange of genetic material between homologous chromosomes during meiosis I?

    <p>Crossing over</p> Signup and view all the answers

    What is the result of meiosis II?

    <p>Four haploid daughter cells with unique combinations of chromosomes</p> Signup and view all the answers

    When do males start meiosis?

    <p>At the onset of puberty and continuously throughout life</p> Signup and view all the answers

    What is the significance of the bouquet stage in meiosis I?

    <p>It's the stage where chromatin material is very thin and thread-like</p> Signup and view all the answers

    What is the term for the point of exchange in crossing over during meiosis I?

    <p>Chiasmata</p> Signup and view all the answers

    How many primary oocytes are matured in females per month?

    <p>1</p> Signup and view all the answers

    What is the significance of the diplotene stage in meiosis I?

    <p>It's the stage where distinct evidence of crossing over is seen</p> Signup and view all the answers

    What is the primary difference between spermatogenesis and oogenesis in terms of meiosis?

    <p>Oogenesis has two arrested phases, while spermatogenesis does not</p> Signup and view all the answers

    What is the outcome when homologous chromosomes fail to separate during meiosis?

    <p>Nondisjunction resulting in chromosomal abnormalities</p> Signup and view all the answers

    What is the result of the second meiotic division in spermatogenesis?

    <p>Four haploid daughter cells</p> Signup and view all the answers

    What is the significance of chiasma formation in prophase I?

    <p>It allows for genetic material exchange through crossing over</p> Signup and view all the answers

    What is the characteristic of chromosomes in diakinesis?

    <p>They are separating or pulling apart</p> Signup and view all the answers

    What is the consequence of nondisjunction of chromosome 21 during meiosis?

    <p>Down syndrome</p> Signup and view all the answers

    What is the effect of nondisjunction in the first meiotic division?

    <p>One cell receives 24 chromosomes, and the other receives 22 chromosomes</p> Signup and view all the answers

    What is the characteristic of the secondary spermatocyte in spermatogenesis?

    <p>It has a haploid number of chromosomes</p> Signup and view all the answers

    What is the significance of the sex chromosomes in spermatogenesis?

    <p>They determine the sex of the offspring</p> Signup and view all the answers

    What is the characteristic of Klinefelter syndrome?

    <p>It is characterized by an extra X chromosome, affecting only males</p> Signup and view all the answers

    What is the characteristic of the follicular histology during the fetal period?

    <p>Absence of follicle cells around the oogonium</p> Signup and view all the answers

    What is the stage of meiosis during which the primary oocyte is arrested in the fetal period?

    <p>Diplotene stage of meiosis I</p> Signup and view all the answers

    What is the characteristic of the secondary follicle?

    <p>Appearance of a small cavity within the granulosa layer</p> Signup and view all the answers

    What is the term for the translucent layer surrounding the oocyte?

    <p>Zona pellucida</p> Signup and view all the answers

    What is the stage of meiosis during which the secondary oocyte is released from the ovary?

    <p>Metaphase stage of meiosis II</p> Signup and view all the answers

    What happens to the granulosa cells after the release of the oocyte from the ovary?

    <p>They form the corpus luteum</p> Signup and view all the answers

    What is the term for the liquid-filled cavity within the granulosa layer of the tertiary follicle?

    <p>Antrum</p> Signup and view all the answers

    What is the characteristic of the primary oocyte stage after birth?

    <p>Increased size of the oocyte</p> Signup and view all the answers

    What is the term for the structure that forms after the release of the oocyte from the ovary?

    <p>Corpus luteum</p> Signup and view all the answers

    What is the stage of meiosis during which the chromosome number is reduced by one-half?

    <p>Metaphase stage of meiosis II</p> Signup and view all the answers

    At what stage is the first meiotic arrest lifted off in females?

    <p>At puberty, with the release of FSH and LH</p> Signup and view all the answers

    What is the significance of diplotene stage in oogenesis?

    <p>It is the stage where chromosomes are prone to damage</p> Signup and view all the answers

    What is the outcome of the completion of meiosis I in oogenesis?

    <p>Production of a secondary oocyte and a 1st polar body</p> Signup and view all the answers

    What is the reason for the higher probability of birth defects in older women?

    <p>The oocyte is exposed to chromosomal damage for a longer period</p> Signup and view all the answers

    What is the characteristic of cytoplasmic division in oogenesis?

    <p>It is equal, with each cell receiving the same amount of cytoplasm</p> Signup and view all the answers

    What is the stage at which the second meiotic arrest is lifted off in females?

    <p>At fertilization, with the onset of sperm penetration</p> Signup and view all the answers

    What is the fate of the 1st polar body in oogenesis?

    <p>It degenerates and is absorbed by the body</p> Signup and view all the answers

    What is the significance of the diplotene stage in relation to birth defects?

    <p>It is the stage where chromosomes are prone to damage, leading to birth defects</p> Signup and view all the answers

    What is the characteristic of oogenesis in terms of the number of functional oocytes produced?

    <p>One functional oocyte is produced per cycle</p> Signup and view all the answers

    What is the stage at which the oocyte becomes a mature oocyte in oogenesis?

    <p>At the completion of meiosis II</p> Signup and view all the answers

    Study Notes

    Gametogenesis

    • Definition: The development of gametes (sex cells) from primordial germ cells, consisting of a series of changes that transform them into specialized sex cells.

    Oogenesis

    • Oogenesis occurs in the ovaries.
    • The process includes the formation of investments around the oocyte.
    • It involves a series of changes that transform the primordial germ cells into specialized sex cells.

    Spermatogenesis

    • Spermatogenesis occurs in the seminiferous tubules of the testis.
    • It involves a series of changes that transform the primordial germ cells into specialized sex cells.

    General Overview of Gametogenesis

    Gonial Stage

    • Oogonium and spermatogonium undergo one more round of growth and differentiation to become primary sex cells: primary spermatocyte and primary oocyte.
    • Primary sex cells embark on the first meiotic division (meiosis I).

    Primary Sex Cell Stages

    • Primary sex cells complete the first meiotic division, begetting the secondary sex cell stage: secondary spermatocyte and secondary oocyte.
    • Secondary sex cells enter the second phase of meiosis (meiosis II).

    Phases of Gametogenesis

    1. Generation of Germ Cells and Migration to the Gonads

    • Germ cells arise outside the gonads.
    • Recognizable at an early stage of development in some vertebrates, such as anuran amphibians (frogs).

    2. Multiplication of Germ Cells in the Gonads (Mitosis)

    • Germ cells undergo mitotic division to increase their population.

    3. Reduction in Chromosome Number by Half (Meiosis)

    • Meiosis I: Primary spermatocyte and primary oocyte embark on the first meiotic division, reducing the chromosome number by half.
    • Meiosis II: Sister chromatids separate, each acting as a chromosome.

    4. Maturation and Differentiation

    • Maturation of the gametes into specialized sex cells.

    Migration of Primordial Germ Cells (PGCs)

    • PGCs arise in the yolk sac endoderm and migrate to the genital ridges.
    • Migration is guided by chemoattractants secreted by the genital ridges.

    Mitosis in Gametogenesis

    In Males

    • Mitosis is continuous from embryonic development to adulthood.
    • Each batch of mitosis is completed within a few weeks.

    In Females

    • Mitosis stops early, around the 5th month of embryonic development.
    • The oogonia enter the primary oocyte stage and meiosis I is arrested until puberty.

    Meiosis

    Meiosis I

    • The first phase of meiosis, where the chromosome number is reduced by half.
    • Primary spermatocyte and primary oocyte embark on the first meiotic division.

    Meiosis II

    • The second phase of meiosis, where sister chromatids separate, each acting as a chromosome.
    • Proceeds by ordinary mitotic division.

    Abnormalities in Meiosis

    Nondisjunction

    • The failure of chromosomes to separate, which can occur in the first and second meiotic divisions.
    • Can result in abnormalities, such as Down syndrome (trisomy 21) and neurological disorders (trisomy 18).

    Lifecycles of Germ Cells

    Murine Germ Cells

    • After fertilization: zygote → cleaving forming a 2-cell embryo → giving rise to the blastocyst.
    • Primordial germ cells (PGCs) are specified at E7.25 (black arrow) at the base of the mesodermal allantois.
    • PGCs migrate through the forming hindgut, dorsal mesenterium, and genital ridges.

    Human Lifecycles

    • In males, meiosis starts at puberty and proceeds in waves.
    • In females, meiosis starts early, around the 5th month of embryonic development, and is arrested until puberty.

    Teratomas

    • A bizarre form of tumor, resulting from the failure of PGCs to reach their final destination.
    • Can be fatal.### Gametogenesis
    • Normal gametogenesis involves the production of gametes (sperm and egg cells) with a haploid number of chromosomes (23) through the process of meiosis.
    • Spermatogenesis:
      • Spermatogonium (diploid) undergoes growth and becomes a primary spermatocyte (diploid).
      • Primary spermatocyte undergoes first meiotic division (reductional division) to produce two secondary spermatocytes (haploid).
      • Secondary spermatocytes undergo second meiotic division (equational division) to produce four spermatids (haploid).
    • Oogenesis:
      • Oogonium (diploid) undergoes mitotic division to increase in number.
      • Primary oocyte (diploid) undergoes first meiotic division (reductional division) to produce a secondary oocyte (haploid) and a first polar body.
      • Secondary oocyte undergoes second meiotic division (equational division) to produce a mature oocyte (haploid) and a second polar body.

    Abnormal Gametogenesis

    • Nondisjunction can occur during meiosis, resulting in abnormal gametes with an incorrect number of chromosomes.
    • Klinefelter syndrome:
      • Results from a sperm with an extra X chromosome (XXY) fertilizing an egg cell.
      • Causes neurological disorders and affects only males.
      • Characterized by an extra X chromosome (47, XXY).

    Timing and Duration of Meiosis

    • Spermatogenesis:
      • Meiosis is continuous throughout life.
      • Mitosis is continuous from the embryonic stage.
    • Oogenesis:
      • Meiosis is arrested at the diplotene stage of meiosis I.
      • Meiosis resumes at puberty with the onset of hormonal changes.
      • Meiosis is arrested again at metaphase II of meiosis II until fertilization.

    Distinct Features of Oogenesis

    • Oocytes undergo arrested stages of meiosis.
    • There is an equal cytoplasmic division.
    • There are polar bodies produced.
    • The first meiotic arrest is lifted off at puberty with the onset of hormonal changes.
    • The second meiotic arrest is lifted off at fertilization.

    Diplotene Stage

    • The diplotene stage is the longest stage of meiosis I.
    • It is the stage where the chromosomes are most prone to damage.
    • The longer the diplotene stage, the greater the chance of chromosomal damage.
    • Older women have a higher probability of giving birth to children with birth defects due to the longer duration of the diplotene stage.

    Fetal Period

    • Oogonium undergoes mitotic division to increase in number.
    • Primary oocyte is formed before or at birth.
    • Primary oocyte undergoes growth and increases in size.
    • Follicle cells (granulosa cells) surround the primary oocyte.
    • The primary oocyte is arrested in the diplotene stage of meiosis I.

    Follicular Development

    • Primordial follicle: primary oocyte with a incomplete layer of follicle cells.
    • Primary follicle: primary oocyte with a complete layer of follicle cells.
    • Secondary follicle: primary oocyte with multiple layers of follicle cells and a small cavity (antrum).
    • Tertiary follicle (Graafian follicle): primary oocyte with a large antrum and a distinct layer of follicle cells (theca externa and theca interna).

    Ovulation and Corpus Luteum Formation

    • The secondary oocyte is released from the ovary during ovulation.
    • The granulosa cells (theca externa and theca interna) and the antrum liquid are released and form the corpus luteum.
    • The corpus luteum eventually degenerates and becomes part of the ovary again.

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