Lecture 6 Fertilization PDF

Syngamy: The Start of You Lecture 6  Fertilization – It’s Just Sex???! Getting to fertilization is complex and involves: Environment of female genitalia Cycle-dependent pH Cycle- and tract area-dependent mucopolysaccharides (cervical mucous) Mucopolysaccharides form fern-like...

Syngamy: The Start of You Lecture 6  Fertilization – It’s Just Sex???! Getting to fertilization is complex and involves: Environment of female genitalia Cycle-dependent pH Cycle- and tract area-dependent mucopolysaccharides (cervical mucous) Mucopolysaccharides form fern-like branching structures – packing and density alters viscosity and degree of barrier G-mucous – Sticky, dense block to bacteria and sperm L-mucous – stretchy but opaque mucous – traps ineffective sperm S-mucous – stretchy and clear – oriented and loosely packed – lend directionality to sperm flagellar action Cortes (2014). Revista de Medicina Veterinaria 28(28):103-108 Fertilization – It’s Just Sex (Cont’d) Cervical activity (orgasm – 1 minute before insemination ideal) Pole-ax theory (Desmond Morris “The Naked Ape”) Sperm “up-suck” theory Hormonal priming theory: in pre- industrial times – sex-induce hormonal priming of reproductive cycles in subsistence societies Ciliary action in uterus, fallopian tubes Storage in ampullae? (Bovines accept sperm and save them in a sort of suspended animation – shed slowly Fertilization For Sperm important factors include: Time and vaginal/uterine environment for sperm Sperm count and quality Maturation, Capacitation, Hyperactivation Length of time in reproductive tract before exposure to oocyte Ciliary tractoring up fallopian tubes Pub-Worthy Reproductive Stories Cambridge/Oxford (Oxbridge for short) Study Large number of commuting couples – one lives in the university town, the other work in/stays in London for few day or work week, or commutes daily Committed couples asked to keep private intimate diaries Collect samples after sex – tested sperm count Men Sperm count varies proportional to prospect of conceiving, especially if cost-free Ejaculate from masturbation had low sperm count Ejaculate during protected sex had low sperm count Perform foreplay – higher sperm count Unprotected sex had higher sperm count Illicit unprotected sex had high sperm count Pub-Worthy Reproductive Stories(Cont’d) Women Post-sex specimen sperm count high if from regular partner Post-sex specimen sperm count low if from illicit lover Remember ampulla storage? Or perhaps has to do with reproductive tract mucous, cilia, dynamics/condition/orgasm? Selection of gametes from most attractive mate? 1. Both genders are regulating sperm count 2. In males, this depends upon how much is committed to ejaculate 3. In females, sperm retention is regulated - this is called sperm selection or sperm gating 4. Physiology and “head space” more important than commonly acknowledged Steps to Syngamy Syngamy – the moment that the sperm meets and fuses with the oocyte Sperm nears the cumulus cells surrounding egg Undergoes hypercapacitation – directed and vigorous flagellar activity Enzymes on sperm head (hyaluronidase) help sperm to penetrate between cumulus cells Sperm hits Zona Pellucida – contact with these glycoproteins stimulates acrosome reaction Acrosome reaction acrosomal membrane fuses with sperm plasma membrane acrosomal contents are released into immediate environment Acrosomal enzymes digest a tunnel through the zona pellucida Receptors inside the acrosome are now exposed and can bind to oocyte membrane Actin polymerization under the acrosome begins to polymerize to form a Sea Urchin Acrosome Reaction Sinauer 2000 Receptor stimulus causes cross-membrane influx of Na+ Acrosomal membrane fuses with plasma membrane to release acrosomal contents Actin remodels to form filamentous process Receptors on inner face of what was acrosomal membrane Specificity of Bindin in Urchin Sea Urchin Sperm Penetration Human Fertilization Spermatozoa attach side-on Actin plays less prominent role https://biotexcom.com/wp-content/uploads/2017/03/32-1024x730.jpg Syngamy Induces Cortical Reaction Initiation of Cortical Reaction Lionel Jaffe’s Work p. 417 Fig 9.18 1.Na+ influx at syngamy initiates release of free Ca2+ in cortex 2.Ca2+ wave spreads across cortex of oocyte (zygote now) 3.Ca2+ initiates remodeling of actin in cortex 4.Wave of actin remodeling moves cortical granules to Cortical Reaction (Continued) p. 416 Fig 9.17 Cortical Granules Release: Water swelling agents (glycosaminoglycans (GAGs), hyaluronate, proteoglycans) Enzymes Cleave sperm receptors to prevent additional sperm binding Cross link external vitelline coat - becomes impermeable Perivitelline Space Swells Lifts vitelline coat Increases perivitelline thickness Cortical Actin Remodels Syngamy Completes Totality of sperm absorbed Oocyte/embryo rendered resistant to further fertilization Maternal and paternal pronuclei begin to migrate to fuse How Is Polyspermy Prevented? The problem: there are millions of sperm. How is it that only one enters the egg??? Statistical analysis indicated that cortical reaction (5- 10 minutes duration) too slow to prevent multiple sperm from entering Is there another mechanism also at work? How is polyspermy prevented???