Gametogenesis 2009

Gametes – reproductive cells Ovum Spermatozoon Gametogenesis – differentiation of highly specialized sex cells capable of uniting at fertilization 1. Origin of the germ cell 2. Multiplication in the gonads by mitosis 3. Reduction of chromosomes – meiosis 4. Final stages of maturation and differentiation

Origin of primordial cells Germ cell can be recognized very early – vegetal pole cytoplasm in the zygote Epiblast – temporary residence in extraembryonic tissues – recognizable at 24 ED in the endoderm of yolk sac Migration within mesenchyme of posterior wall of yolk sac (near the allantois), gut, and dorsal mesentery (4 -6 week) to the gonads Extracellular matrix and chemotactic influence from gonad – resident germ cells induce formation of gonads Number of cells increases during migration

Proliferation Oogonia and spermatogonia Proliferative phase of development – from thousands to about 7 million (in female) – mitosis Oogonia – division during 2.-5. months By the seventh month oogonia entere the prophase of first meiotic division and end proliferative phase Spermatogonia enter meiosis after puberty, mitotic capability continues as long as the male is capable of reproduction

Meiosis Reduction of normal number of chromosomes From diploid to haploid Two maturation divisions without new DNA synthesis Reductional division Equational meiotic division Recombination of genetic information Random distribution of maternal and paternal chromosomes Exchanging of portions of homologous chromosomes by crossing over

First Meiotic Division Prophase I Leptotene Zygotene Pachytene Diplotene Diakinesis Metaphase I Anaphase I Telophase I and Interphase Second Meiotic Division

In Mammals - initiation of germ line development- maintain pluripotency within germ cells Activation of differentiation – inductive signal from trophoblast Proliferation and survival – trophic factors Extracellular matrix – direct the migration Final differentiation

Spermatogenesis - 64 days Mitotic multiplication – spermatogonia (Type A – stem cell population, Type B – leave mitotic cycle - preleptotene spermatocytes) Meiosis - Primary spermatocytes Secondary spermatocytes Spermiogenesis – Spermatides – transformation into extremely specialized cells – spermatozoa (concentration of chromatin, decrease of size, formation of acrosome, flagellum)

Male germ cells Sertoli cells – isolation of germ cells, support and nutrition Degradation of residual bodies Synthesis of signal molecules (Anti-Müllerian factor) Synchronization of development- waves

Spermiogenesis Nucleus – concentration of chromatin – head Golgi complex- proacrosomal granules - acrosome Centrioles – achorage of flagellum Axoneme – microtubules (9+2) and dynein Mitochondria – spiral investment around proximal part of flagellum – mitochondrial helix Residual body

Spermatozoon Head (nucleus and acrosome) Neck (proximal centriole) Middle piece (flagellum, centriole, mitochondrial helix) Tail - flagellum

Sperm maturation Newly formed spermatozoa are not capable of fertilization. Maturation in genital tract – activation – increase of motility Capacitation – final step of sperm maturation- changes in acrosome, preparing the enzyme release (in female genital tract), changes in sperm membrane Sperm attraction and hyperactivation Acrosome reaction – fusion of the acrosome with plasma membrane, extension of the acrosomal process

Oogenesis Oogonium gives arise to only one ovum – first and second polar body (DNA and only little cytoplasma) First meiotic division is not completed untill puberty Meoitic arrest occurs during prophase I (diplotene) – egg builds up its stores of yolk Second arrest during metaphase II – mitosis is finished after fertilization

Lampbrush chromosomes Active transcription during meiosis Synthesis of RNA – genes loop out

Oogenesis At birth – 1 milion oocytes Surrounded by a layer of follicular cells (granulosa cells) – follicle Only 400 (one per menstrual cycle) reach maturity Atresia (degeneration) Folliculogenesis Primordial Primary Secondary Graafian follicle - Ovulation

Egg Egg accumulates yolk as reservoir of food (energy) for embryo Proteins (Amino acids, Energy) Ribosomes and tRNA- proteosynthesis after fertilization mRNA – early development - morphogenic factors

Coverings of eggs Zona pellucida – Glycoproteins, GAG, Hyaluronic acid, Sialic adid. It is produced by oocyte ZP-3 Sperm receptor and induction of acrosome reaction Corona radiata – follicular cells

Fertilization It is an interaction between sperm and oocyte Spermatozoon binds to specific sperm receptor in the zona pellucida (ZP3). It induces release of enzymes from acrosome Penetration the zona pellucida Sperm and oocyte fuse Cortical reaction – cortical granules release to perivitelline space (between oocyte and zona pellucida) – alteration of receptors for sperms – prevent polyspermy

Prevention of polyspermy Fast block of polyspermy – change the electrical potential Slow block of polyspermy - cortical granules -enzymes – proteases – clip off binding receptor Fertilization envelope – space between zona pellucida and egg - GAG, peroxidase, and hyalin – zona reaction

Fertilization Fusion with sperm induces oocyte to resume meiosis – second polar body and definitive oocyte Fertilized oocyte = zygote Female and male pronuclei Membrane disapears Replication First mitotic division 24 hours

Imprinting Egg-derived genome is functionally different from sperm-derived Imprinting is inactivation of gene depending on gender - prevent parthenogenesis Maternal genes are important for embryo development (receptor for IGFII) Paternal genes are important for placenta development (IGFII – Beckwith-Wiederman sy)

Cleavage Mitotic division without cell growth Daughter cells (Blastomeres) get smaller - embryo does not change in size Mitotic division is equal and total 4 cells – 40 hours 3ED – 6-12 cells 4ED – 16 -32 cells – morula (mulberry)

Segregation of blastomeres into embryoblast and trophoblast Starting at 8 cell stage – changes in intercellular juctions – compaction – polarization of cells Tight junction and gap junctions – outer cell mass Cells in centre – inner cell mass (embryoblast) and outer cell mass – (trophoblast). Fluid is collected – blastocyst cavity Blastocyst – Embryonic pole Abembryonic (vegetative) pole

Genetic regulation of germ cell formation, proliferattion, migration, and development Regulatory gene cascade – sequential activation of genes that direct the initial induction and development, proliferation, survival, migration and differentiation of the germ cells Maternal effect genes – germ plasm in zygote

Twins and embryonic stem cells Monozygotic twins - before hatching – at 5.ED – dichorionic Later monochorionic,diamniotic Monochorionic monoamniotic Conjoined twins (after ED9) Inner cell mass – embryonic stem cells