Medical Embryology Lecture 2: Gametogenesis PDF

1/30/25, 12:59 PM lecture 2: Gametogenesis 🎉 RemNote More! 1.18 - YouTube Annotator, New PDF Reader, Ask AI, Voice Typing, and 🤱 medical embryology...

1/30/25, 12:59 PM lecture 2: Gametogenesis 🎉 RemNote More! 1.18 - YouTube Annotator, New PDF Reader, Ask AI, Voice Typing, and 🤱 medical embryology Share lecture 2: Gametogenesis 12 / 12 Gametogenesis (divided into 4 phases) → The process of producing gametes (sex cells) and the process that occurs before fertilization; process of formation and genetic/phenotypic maturation of gametes Gametogenesis is divided into 4 phases: 1. extraembryonic origin of the primordial germ cells and their migration to the gonads 2. An increase in the number of PGCs by mitosis 3. A reduction in chromosomal material by meiosis 4. Structural and functional maturation of gametes (oogenesis, spermatogenesis) the first phase of gametogenesis is identical in males and females, whereas distinct differences exist between the male and female patterns in the last three phases location of PGCs in the 16-somite human embryo (midsagittal section) PGCs Phase 1 Origin of the PGCs: PGC precursors originate in the epiblast prior to gastrulations; requires inductive signaling moves to the yolk sac (extraembryonic region) where they are determined; seen in the lining (endoderm) of the yolk sac at the angle of the allantois PGCs re-enter the embryo and migrate to the developing gonads https://www.remnote.com/w/678ec44ccab9e1fef78d5af4/lecture-2-Gametogenesis-lMUyKqe19laP3U3A4 1/6 1/30/25, 12:59 PM lecture 2: Gametogenesis The arduous journey in brief: Between 4-6 weeks the PGCs; exit from the yolk sac enter the hindgut epithelium and then migrate through the the dorsal mesentery, until they reach the primordia of the gonads PGC migration can be traced 1. Alkaline phosphatase staining; enzyme marker used for PGCs 2. GFP linked to a germ cell specific protein (i.e. stromal cell derived factor -1; SDF1) How do PGCs migrate and know where to go? 1. Amoeboid movements during initial migration 2. cytoplasmic processes link adjacent PGCs 3. Chemoattractants (chemical substances that cause cells to move toward a specific location) secreted by genital ridges 4. migration by extending pseudopod (integrin- fibronectin interactions) 5. follow extracellular matrix "roadways" lined with fibronectin (feet like things that helps them move across the membrane) https://www.remnote.com/w/678ec44ccab9e1fef78d5af4/lecture-2-Gametogenesis-lMUyKqe19laP3U3A4 2/6 1/30/25, 12:59 PM lecture 2: Gametogenesis Phase 2: increase in the number of germ cells by mitosis during the migration journey and after the PGCs arrive in the gonads they undergo a series of mitotic divisions number of PGCs increases exponentially from hundreds to millions of cells pattern of mitotic proliferation differs markedly between male and female PGCs 1. oogonia: max # reached during gestation 2. spermatogonia: able to divide postnatally https://www.remnote.com/w/678ec44ccab9e1fef78d5af4/lecture-2-Gametogenesis-lMUyKqe19laP3U3A4 3/6 1/30/25, 12:59 PM lecture 2: Gametogenesis Phase 3: Reduction in Chromosomal Number by Meiosis biological significance of meiosis is humans is similar to that in other species Reduction of number of chromosomes from diploid (2N) to haploid (1N) so that species number of chromosomes can be maintained from generation to generation Independent re-assortment of paternal and maternal chromosomes to allow for better mixing of genetic characteristics further redistribution of maternal and paternal genetic information through the process of crossing-over during the first meiotic division Gamete → A mature male or female reproductive cell. Fertilization → The process by which a sperm cell fuses with an egg cell to form a zygote. Mitosis: Gender differences in germ cells mitosis Females 1. Oogonia undergo intense period of mitotic activity in embryonic ovary from 2nd to 5th month of pregnancy (highest number around 70 million PGCs around 20 weeks) 2. population of germ cells increases from only a few thousand to nearly 7 million (highest number ever detected) 3. Atresia of oogonia begins soon after and continues through life Males 1. mitosis begins early embryonic testes 2. continues throughout life ⇒ seminiferous tubules lined with germ cells 3. beginning puberty, subpopulations of spermatogonia undergo periodic waves of mitosis ⇒ progeny enter meiosis in synchronous groups after puberty https://www.remnote.com/w/678ec44ccab9e1fef78d5af4/lecture-2-Gametogenesis-lMUyKqe19laP3U3A4 4/6 1/30/25, 12:59 PM lecture 2: Gametogenesis Meiosis difference between mitosis and meiosis Mitosis: 2 genetically identical daughter cell centromere between sister chromatids splits... one chromatid from each chromosome migrates to each pole of mitotic spindle results: genetically equal daughter cells (diploid) Meiosis: 4 gametes all haploids; not genetically identical to the mother cells due the crossing over preparation: DNA duplication end of meiosis I: genetically different daughter cells (diploid) no duplication between meiosis II: centromeres between sister chromatids divide results: genetically different daughter cells (haploid) Primordial germ cells (PGCs) → earliest recognizable precursors of gametes, arise outside the gonads, migrate into the gonads during early embryonic development First recognizable at 24 days after fertilization; 'set aside' in the endodermal layer of the yolk sac Integrins → Transmembrane receptors that mediate cell-cell and cell- matrix adhesion. Fibronectin → A glycoprotein that mediates cell adhesion SDF-1 → Stromal cell-derived factor-1 is a chemokine that plays a crucial role in cell migration and development. Oogonia → precursor of the oocyte Spermatogonia → precursor of a mature sperm Teratomas → the PGCs migrate to these places instead of the gonads causing the tumor to grow https://www.remnote.com/w/678ec44ccab9e1fef78d5af4/lecture-2-Gametogenesis-lMUyKqe19laP3U3A4 5/6 1/30/25, 12:59 PM lecture 2: Gametogenesis an unusual tumor in the newborn located at the base of the tailbone (coccyx), more common in female than in males babies, usually not malignant (although very large), cured by surgery after birth (occasionally cause trouble before birth) Sacrococcygeal teratoma → in a fetus 44.8%; is usually discovered through a blood test at 16 weeks showing a high alpha fetoprotein (AFP) amount Massive oropharyngeal teratoma → 1-2% of cases; associated with a high neonatal mortality rate due to severe airway obstruction (obstruct airway of the newborn) https://www.remnote.com/w/678ec44ccab9e1fef78d5af4/lecture-2-Gametogenesis-lMUyKqe19laP3U3A4 6/6