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?

Primary sex cells Signup and view all the answers

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

Production of spermatids and mature oocyte Signup and view all the answers

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

Gametogenesis Signup and view all the answers

Where does oogenesis take place?

Ovary 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?

Gonial stage Signup and view all the answers

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

Unfertilized egg stage Signup and view all the answers

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

Alkaline phosphatase Signup and view all the answers

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

3 weeks of embryonic development Signup and view all the answers

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

Amnion Signup and view all the answers

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

By amoeboid motion Signup and view all the answers

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

Bone morphogenetic protein (BMP) 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?

Extraembryonic mesoderm Signup and view all the answers

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

Dorsal mesentery Signup and view all the answers

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

Oct-4+, NANOG, and SOX2 Signup and view all the answers

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

From the yolk sac endoderm to the allantois to the hindgut to the genital ridges Signup and view all the answers

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

In the allantois Signup and view all the answers

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

They develop into teratomas Signup and view all the answers

During which phase do the PGCs undergo mitotic division?

The proliferation phase Signup and view all the answers

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

Mitotic patterns differ in timing and duration between males and females Signup and view all the answers

When do meiosis begin in males?

At puberty Signup and view all the answers

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

They enter the first meiotic division Signup and view all the answers

How many oocytes are retained at puberty in females?

400,000 Signup and view all the answers

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

Only 400-500 can complete maturation Signup and view all the answers

What is the destination of the PGCs in males?

The testes Signup and view all the answers

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

Gastrulation Signup and view all the answers

At what stage do PCGs migrate through the forming hindgut?

After E7.25 Signup and view all the answers

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

They form teratomas Signup and view all the answers

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

Chemoattractants Signup and view all the answers

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

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

At what stage do oogonia enter the primary oocyte stage?

7th month of embryonic development Signup and view all the answers

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

400,000 Signup and view all the answers

What is the duration of reproductive years in females?

35 years Signup and view all the answers

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

To increase the population of germ cells Signup and view all the answers

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

During embryonic development, around the 5th month Signup and view all the answers

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

Halved chromosome number Signup and view all the answers

What is the purpose of synapsis in meiosis I?

To align homologous chromosomes Signup and view all the answers

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

Crossing over Signup and view all the answers

What is the result of meiosis II?

Four haploid daughter cells with unique combinations of chromosomes Signup and view all the answers

When do males start meiosis?

At the onset of puberty and continuously throughout life Signup and view all the answers

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

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

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

Chiasmata Signup and view all the answers

How many primary oocytes are matured in females per month?

1 Signup and view all the answers

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

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

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

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

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

Nondisjunction resulting in chromosomal abnormalities Signup and view all the answers

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

Four haploid daughter cells Signup and view all the answers

What is the significance of chiasma formation in prophase I?

It allows for genetic material exchange through crossing over Signup and view all the answers

What is the characteristic of chromosomes in diakinesis?

They are separating or pulling apart Signup and view all the answers

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

Down syndrome Signup and view all the answers

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

One cell receives 24 chromosomes, and the other receives 22 chromosomes Signup and view all the answers

What is the characteristic of the secondary spermatocyte in spermatogenesis?

It has a haploid number of chromosomes Signup and view all the answers

What is the significance of the sex chromosomes in spermatogenesis?

They determine the sex of the offspring Signup and view all the answers

What is the characteristic of Klinefelter syndrome?

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

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

Absence of follicle cells around the oogonium Signup and view all the answers

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

Diplotene stage of meiosis I Signup and view all the answers

What is the characteristic of the secondary follicle?

Appearance of a small cavity within the granulosa layer Signup and view all the answers

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

Zona pellucida Signup and view all the answers

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

Metaphase stage of meiosis II Signup and view all the answers

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

They form the corpus luteum Signup and view all the answers

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

Antrum Signup and view all the answers

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

Increased size of the oocyte Signup and view all the answers

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

Corpus luteum Signup and view all the answers

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

Metaphase stage of meiosis II Signup and view all the answers

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

At puberty, with the release of FSH and LH Signup and view all the answers

What is the significance of diplotene stage in oogenesis?

It is the stage where chromosomes are prone to damage Signup and view all the answers

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

Production of a secondary oocyte and a 1st polar body Signup and view all the answers

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

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

What is the characteristic of cytoplasmic division in oogenesis?

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

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

At fertilization, with the onset of sperm penetration Signup and view all the answers

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

It degenerates and is absorbed by the body Signup and view all the answers

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

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

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

One functional oocyte is produced per cycle Signup and view all the answers

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

At the completion of meiosis II 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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