Spermatogenesis and Oogenesis PDF

Spermatogenesis -PGC differentiate into spermatogonia at puberty -Spermatogonia serve as stem cells for spermatogenesis -Golgi orient to tip of head to form acrosome -globular actin between nucleus and Golgi -mitochondria and centrosome with tubulin segregate towards tail -flagella grows -cyt...

Spermatogenesis -PGC differentiate into spermatogonia at puberty -Spermatogonia serve as stem cells for spermatogenesis -Golgi orient to tip of head to form acrosome -globular actin between nucleus and Golgi -mitochondria and centrosome with tubulin segregate towards tail -flagella grows -cytoplasm sloughed off Spermatogonia (diploid) undergo mitosis to form primary spermatocytes (diploid) Primary spermatocytes (diploid) undergo meiosis I to form 2 Secondary Spermatocytes (haploid) Secondary spermatocytes (haploid) undergo meiosis II to form 4 spermatids (haploid) Spermatids differentiate into spermatozoa Sperm Maturation -hormone mediated -sperm endocytose epididymosomes (epigenetic information in form of miRNA) 1. Capacitation- destabilizes acrosome- ready to fertilize on contact 2. Hyperactivation- induced by near oocyte environment. ZP3 (zona pellucida ligand), progesterone (cumulus cells) -sperm flagella amplitude and frequency increases Oogenesis Prenatal -Oogonia undergo mitotic division -Oogonia differentiate giving rise to primary oocyte (diploid) -Primary oocyte arrest in prophase of meiosis I Postnatal -At or just before ovulation, primary oocyte completes first meiotic division giving rise to secondary oocyte (haploid) and first polar body -Secondary oocyte enters second meiotic division but held in metaphase 2 -Fertilization does not occur, secondary oocyte degenerates -Fertilization occurs, secondary oocyte finishes meiosis II giving rise to an ovum and a second polar body Mucopolysaccharides -fern like branching G-mucopolysaccharides- dense, sticky- does not let sperm through L-mucopolysaccharides- block ineffective sperm S-mucopolysaccharides- stretchy and clear- lead directionality to sperm flagellar action Syngamy Sperm and egg fusion 1. Sperm nears the cumulus cells - Undergoes hypercapacitation- directed and vigorous flagellar activity - Hyaluronidase helps sperm penetrate between cumulus cells - Sperm hits zona pellucida- stimulates acrosome reaction 2. Acrosome reaction - Acrosomal membrane fuses with sperm plasma membrane - Acrosomal contents are released into the immediate environment - Acrosomal enzymes digest a tunnel through the zona pellucida - Receptors in the acrosome are now exposed and can bind to the oocyte membrane - Actin polymerization under the acrosome begins to polymerize and form a filament - Filament extends to oocyte surface- inner acrosomal receptors can contact oocyte Sea Urchin Acrosome Reaction - Na+ influx - Acrosomal membrane fuses with plasma membrane to release acrosomal contents - Actin remodels to form a filamentous process - Receptors on inner face of what was acrosomal membrane bind egg Sea Urchin Sperm Penetration 1. Sperm contacts jelly layer 2. Acrosomal reaction 3. Digestion of jelly layer 4. Sperm binding to vitelline envelope 5. Fusion of acrosomal process membrane and egg membrane Syngamy Induces Cortical Reaction 1. Contact - Sperm cell contacts eggs jelly coat - Triggering exocytosis from sperm's acrosome 2. Acrosome Reaction - Hydrolytic enzymes released from acrosome make hole in jelly coat - Growing actin filaments form acrosomal process - Structure protrudes from sperm's head and penetrates jelly coat - Bind to receptors in the egg cell membrane 3. Contact and fusion of sperm and egg - Hole is made in vitelline layer allowing contact and fusion of the gamete plasma membranes - The membrane becomes depolarized resulting in the fast block to polyspermy 4. Entry of sperm into nucleus 5. Cortical Reaction - Release of Ca2+ in the egg's cytosol - Cortical granules fuse with plasma membrane and release their contents - Swelling of perivitelline space, hardening of the vitelline layer, and clipping of sperm-binding receptors Initiation of Cortical Reaction 1. Na+ influx triggers release of Ca2+ in cortex 2. Ca2+ spreads across cortex of oocyte 3. Ca2+ initiates remodelling of actin in cortex 4. Wave of actin remodelling moves cortical granules to the surface 5. Cortical grandules fuse with the plasma membrane 6. Cortical granules contents released into perivitelline space -release water swelling agents (hyaluronate) -enzymes cleave receptors -enzymes cross link vitelline coat Fast Block to Polyspermy -high voltage prevents polyspermy -low voltage facilitates polyspermy Nicotine Inhibits Fast Block -nicotine inhibits amplitude of electrical block- polyspermy ensues -fertilization potential amplitude is reduced If voltage clamped high- no fertilization Release voltage clamp- fertilization Fertilization occurs= depolarization Fast block to polyspermy commences Cleavage -no G1 or G2 because Cdk1-cyclinB -kinase promotes mitosis -cyclin-Cdk active at M phase -cyclin is degraded after M phase and resynthesized before next M phase -cleavage embryo filled with Cdk1-cyclinB -after mitosis only cyclin B associated with nucleus is depredated -cleavage divisions are very rapid -no cell growth= cells get smaller each division -cells are transcriptionally silent- zygotic genome inactive- embryo relies on oocyte stores -cell divisions synchronous Telolecithal- in fish, chicks, reptiles. Yolk takes up most of the egg, yolk is dense Mesolecithal- amphibians. Yolk concentrated to one hemisphere of the egg. Isolecithal- Mammals. Yolk is concentrated evenly across the egg Centrolecithal- Fruit Fly. Yolk is concentrated in the middle of the egg. Cytoplasm is peripheral Cleavage Divisions in Frogs Mesolecithal- yolk concentrated in lower half of the egg -holoblastic cleavage- complete cleavage -first 2 divisions are vertical 3^rd^ is horizontal and closer to top -top cells divide faster= less yolk -after 4 cell stage cleavage is asymmetric Cleavage Divisions in Fish, Birds Telolecithal- dense yolk Meroblastic cleavage- incomplete divisions -only upper region of egg is free of yolk -first 5 cell divisions are vertical and incomplete -6^th^ division is horizontal and separates yolk from upper blastomeres Cleavage Divisions in Mammals Isolecithal- yolk evenly distributed Holoblastic cleavage- cleavage divisions go completely through the egg/ blastomeres -each blastomere is of equal size Cleavage Divisions in C.elegans -holoblastic and asymmetric -fates of cells are very lineage restricted Meroblastic Fruitfly Cleavage 1\. DNA replication and nuclear division without cell cleavage 2\. Nuclei migrate to periphery 3\. Pole-plasm containing cells collect posteriorly (presumptive germ cells) 4\. Cells partially partition but remain open at bottom 5\. around division 13 cellular partition completes Weismann\'s Theory of Determinative Development- cleavage partitions different ingredients to different daughter cells -directed future development Roux's Evidence for Mosaic Development- destroy half of a cell with a hot needle. The damaged cell did not divide as normal and did not develop. Undamaged cell has developed into something resembling half a normal embryo -if you tie a loop of baby hair and tighten slowly to separate blastomeres, both develop normally, but ½ regular size Driesch's evidence for regulative development- separation of cells at two cell stage resulted in the death of one cell. The surviving cell developed into a small but otherwise normal larva -sea urchin cleavage is regulative -half of regular size TH Morgan- repeated Driesch's experiment on frogs. Both blastomeres developed into complete embryos. Regulative Development- cell fate is established later, via cell-cell interactions Mosaic Development- cell fate is established/restricted at each cell division. - C.elegans -most organisms develop by a mix of both strategies Mid-Blastula Transition Transition of maternal control of development to zygotic control Early Blastula - Cleavage divisions are rapid, lack gap phases - Absence of transcription - MRNA\'s and protein deposited in egg during oogenesis carry out all cellular processes Late Blastula - Slowing of cell cycle, introduction of gap phases, G1, G2 - Asynchronous divisions - Zygotic transcription - Many maternal mRNAs are actively destroyed - Development of cell-cell adhesions/connections Experiment 1- Removal of cytoplasm only -MBT occurs earlier -fewer cell divisions and cell number Experiment 2- Addition of cytoplasm only -MBT occurs later -more cell divisions, and more cells Experiment 3- Create haploid frog embryo -less DNA -MBT occurs later -more cells Experiment 4- Inject surplus of non-frog DNA -MBT occurs earlier with more cells MBT triggered when nuclear/cytoplasmic ratio reaches critical point -nuclear divisions begin slowing down at cycle 10 MBT at cycle 14 -dramatic slowing of cell cycle -zygotic transcription activated, maternal mRNAs destroyed -cellularization- cleavage furrows form between nuclei -onset of gastrulation -mitotic cyclins are present in the egg before fertilization -each mitosis in embryo, overall mitotic cyclin levels go down due to being destroyed after use -by cycle 11 overall cyclin levels drop enough to get slowing of cell cycle -longer gap between cell divisions-\> some genes able to be transcribed -transcription of genes that promote destruction of Cdc25 -even longer gap in cell division-\> allows time for most genes to be transcribed - Genes include genes involved in maternal mRNA degradation, genes required for gastrulation Compaction in Mammals Transition from blastula-morula -high level expression of E-cadherin- cell-cell adhesion molecule \- all cells begin to associate tightly with each other -cells become polarized \- compaction results in outer and inner cells -inner cells give rise to embryo -outer cells give rise to placenta -Cells at different locations will give rise to different cell types -e,g mammals - Inner cell mass-\> embryo proper + some extra-embryonic - Trophectoderm-\> extra-embryonic Caviation to create fluid filled centre- the blastocoel