WEEK 3: Gametogenesis Notes

Gametogenesis

Oogenesis and Folliculogenesis

• Occurs in the ovaries.

Spermatogenesis

• Generation of spermatozoa and occurs in the seminiferous tubules.

Gametogenesis Defined

• Generation and maturation of sex cells.

Primordial Germ Cells (PGCs)

• Generative sex cells (germplasm).

Germplasm

• No obvious germplasm in mammals.

• Easily distinguished germplasm in animals other than mammals.

Primary Spermatocyte and Oocyte

• Formed during growth and differentiation.

Secondary Spermatocyte and Oocyte

• Formed during meiosis 1.

Spermatids and Oocyte

• Formed during meiosis 2.

Phases of Gametogenesis

1. Generation of germ cells and migration to the gonads.

2. Multiplication of germ cells in the gonads (mitosis).

3. Reduction in chromosome number by half (meiosis).

4. Maturation and differentiation.

Generation of Germ Cells and Migration to the Gonads

• Arise outside the gonads.

• Recognizable at an early stage of development.

Amniotes

• The birds, reptiles, and mammals.

• PGCs identified in the yolk sac endoderm.

• PGCs are large-sized, with a high content of alkaline phosphatase.

• In humans, PGCs are recognizable at 24 days post-fertilization in the yolk sac endoderm.

• Hannel & Eddy: PGCs originally reside in the epiblast of the gastrula.

• Ginsburg: Localized the region in the extra-embryonic mesoderm posterior to the primitive streak in the 7.25th day embryo.

Inner Cell Membrane

• Will split into the epiblast.

PGCs in Humans

• By the 3rd week, PGCs wander in an amoeboid manner.

• Primary ectoderm → yolk sac wall → near the exit of the allantois.

Genital Ridges

• Forming gonads of the embryo.

Rodents (as a Mammalian Model)

• Development of PGCs depends on signals.

• Radical expression of pluripotency markers.

Bone Morphogenetic Protein

• It is a signal factor.

Proposed Migration Pathway

• Primary ectoderm → yolk sac endoderm → allantois → dorsal mesentery → left and right genital ridges.

• Yolk sac endoderm: PGCs become extraembryonal.

• Allantois: PGCs become specified at the base.

• Left and Right genital ridges: In the embryo proper.

Teratoma

• A type of tumor.

Proliferation of Germ Cells

• Mitotic patterns in the gonads differ widely between males and females.

• For males, the process of mitosis is continuous.

• For females, the process of mitosis stops early around the 5th month of oogenesis.

Reduction in Chromosome Number by Half

• In males, meiosis starts at puberty.

• In females, meiosis starts earlier; happens every month.

Meiosis I

• Reducing the chromosome number by half (reduction); long prophase.

Leptotene Stage

• 46 chromosomes, 23 maternal chromosomes and 23 paternal chromosomes.

• The chromosome material is thin and only on one side of the nucleus.

Zygotene Stage

• Pairing of homologous chromosomes.

• Referring to the synapting stage.

Pachytene Stage

• Chromosomes become thicker and shorter, and pairing is done.

Diplotene

• The process of chiasmata happens (exchange of genetic material).

Metaphase I

• Homologous chromosomes meet at the equatorial plane.

Anaphase I

• Homologous chromosomes separate and move toward their respective poles.

Meiosis II

• Ordinary mitotic division (equational division).

• Sister chromatids are the ones that separate.

• Diplotene and Metaphase II: During oogenesis, the stage where meiosis I and II are arrested.

Distinct Features of Oogenesis

1. Oocytes undergo arrested stages.

2. Unequal cytoplasmic division.

3. Only one functional oocyte produced per cycle; polar bodies are produced.

Mammalian Oogenesis

• PGCs → oogonia → primary oocytes → meiosis (leptotene, zygotene, Pachtene) → diplotene (1st arrest cellular molecular changes) → Diakinesis → Metaphase I → Anaphase I → Telophase I → secondary oocyte and first polar body.

• Secondary oocyte (haploid) → Meiosis II → Metaphase II (2nd arrest meitotic arrest released during fertilization) → anaphase II → telophase II → mature oocyte and second polar oocyte.

• Mature oocyte: Functional oocyte = zygote oocyte.

• First polar body → polar body.

Oogenesis Stages

Hormonal Control of Oocyte Maturation

• Follicle Stimulating Hormone (FSH): Stimulates the increase of granulosa cells.

• Luteinizing Hormone (LH): Will target the theca externa and interna.

Hormone Interactions

FSH to Granulosa
ewline to Estrogen to Female
ewline to LH to Theca
ewline to Testosterone to Membrana Granulosa

• FSH → Granulosa cells (secrete and synthesize) → estrogen (stimulates) → female reproductive activity → LH surge → Theca cells (produce) → Testosterone (crosses) → Membrana granulosa (FSH & cAMP general system) → testosterone (acromatase and cyclic AMP generating system) convert to → estradiol/estrogen (acts) → follicle cells → Formation of more LH receptors (in response to large LH surge before ovulation) → Secondary follicle → graafian follicle.

• FSH → Granulosa cells (secrete and synthesize) → aromatase.

Sequence of Follicle Maturation in the Ovary

• Ovulation happens in the middle of the month.

Oogenesis in Avian

Ovarian Follicle

• The yellow yolk.

Cytoplasm/Blastodisc

• Lying on top of the yellow yolk.

Nucleus of Pander

• Surrounding the blastodisc.

Chalazae

Blastoderm

• Fertilized blastodisc.

Oogenesis in Amphibians

• Mitosis is continuous.

• Egg maturation requires 3 years.

Previtellogenic Phase

• Before the accumulation of the yolk.

Vitellogenic Phase

• Accumulation of the yolk; formation of cortical granules.

Balbiani Body

• Intense RNA synthesis and formation of lampbrush chromosomes and nucleoli.

• In most animals, growing oocytes are actively transcribing genes.

Spermatogenesis

• PGCs → spermatozoa → mitotic multiplication → meiosis → spermiogenesis.

• Mitosis in males occurs throughout life.

• PGCs at the genital ridge → sex cords → seminiferous tubules → tubular epithelium → sertoli cells.

Stem Cells

• Synchronized cell divisions.

Type A Spermatogonia (A_4)

• Can undergo cell division again, undergo apoptosis, or become primary spermatocytes.

Type B Spermatogonia

• Last to undergo mitosis and become primary spermatocytes.

Secondary Spermatocytes

• Are produced under meiosis; they came from being primary spermatocytes.

Spermatids

• Undergo spermiogenesis and come from secondary spermatocytes.

Cytoplasmic Bridges

• Interconnect the sperm with each other.

Categories of Changes During Spermiogenesis

A. Reorganization of the Cytoplasm

• Golgi apparatus

• Centrioles

• Mitochondria

B. Progressive Reduction in Nuclear Size/Compaction of the Nucleus

• Nuclear elongation

• Loss of water

• Centrioles

• Elimination of RNA; leaving only DNA

• Not all histones are replaced.

• In humans, 15-20% remain in nucleosomal configuration.

Sperm Anatomy

Head

• Acrosome

• Nucleus

Neck

Midpiece

• Mitochondria

Tail (flagellum)

• Normal Sperm

• Abnormal Sperm includes: Giant head, Micro head, Double body, Double head, Long middle piece, and Rough middle piece.