Fertilisation and Pregnancy Recognition PDF

Fertilisation and pregnancy recognition Associate Professor Roslyn Bathgate Images from Senger, P. L. (2003) Pathways to Pregnancy and Parturition, 2nd edn. Current Conceptions Inc unless otherwise stated Learning objectives Chromosomal allocation in oogenesis and spermatogenesis Th...

Fertilisation and pregnancy recognition Associate Professor Roslyn Bathgate Images from Senger, P. L. (2003) Pathways to Pregnancy and Parturition, 2nd edn. Current Conceptions Inc unless otherwise stated Learning objectives Chromosomal allocation in oogenesis and spermatogenesis The events that occur prior to and during fertilisation Pre-implantation embryo stages What is maternal recognition of pregnancy What causes it When does it occur  Spermatozoa are 1N Oogenesis and Primary oocytes are 2N spermatogenesis Secondary oocytes are 1N Fertilisation Deposition of sperm Adequate numbers of live and motile Formation of sperm reservoirs Capacitation of sperm Ovulation of viable ovum(a) Transport of ovum(a) Meeting of viable sperm and viable ovum(a) Acrosome reaction Polyspermy block Decondensation Pronuclear formation Syngamy Cleavage Bathgate 2004  Species Site Volume of Duration of Semen Deposition Cow Vagina ejaculate (ml) 4-8 ejaculation 1-3 sec Ewe Vagina 1 1-2 sec Sow Uterus 150-500 5-20 min The vagina is a hostile environment (for sperm) with Mare Uterus 50-120 10-60 sec acidic pH and leucocytes Bitch Uterus 5-10 5-30 min Sperm reservoirs are created in the female tract includes ‘tie’ Vaginal depositors have small volume, high Queen Vagina 0.1 10-30 sec concentration, rapid deposition ejaculate (sheep, cattle, Woman Vagina 0.3-3.0 1-3 sec cat) Uterine depositors have large volume, low concentration, slower deposition ejaculate (pig, dog) Sperm in the female tract Fertilisation occurs in the oviduct Sperm are usually in place before fertilisation Sperm must traverse vast physical distance and overcome attack from leucocytes During transport, sperm must undergo preparation for fertilisation (capacitation and hyperactivation) McDonald L.E (1989) Veterinary Endocrinology and Reproduction Lea & Febiger Sperm transport The first sperm to reach ampulla couldn’t have got there on their own. Swimming keeps spermatozoa in suspension and the rapid transport is helped by contraction of the uterus – is this assisted by female orgasm? Beating of cilia in fallopian tubes may help Oestrogen from follicles (and seminal plasma) stimulates an increase in uterine contractility increases cervical mucus amount and fluidity Oxytocin from the posterior pituitary (and seminal plasma) Stimulates smooth muscle contractions BUT Progestagens/prostaglandins cause deterioration of cervical mucus and inhibit sperm atransport Sperm preparation for fertilisation Capacitation Binding to oviductal epithelium Acrosome reaction Sperm motility – conversion to hyperactiviation Ovulation and ovum transport Ovulation occurs randomly on the ovarian surface (except in the horse) Oocytes must be picked by the fimbria and guided into the oviduct, aided by the cumulus oophorus and corona radiata Beating of the cilia moves the oocyte along the ampulla Approximate time take to traverse the ampulla varies Sheep 72h Cattle 90h Pig 50h Dog 168h Cat 148h Human 48-72h Fertile lifespan Species Capacitation Sperm Ova of gametes in (hr) (hr) (hr) vivo Cow 4 30-48 8-12 Ewe 1.5 24 10-25 Sow 3-6 24-48 8-10 Mare Unknown 70-140 6-8 Bitch 7 150-240 96 Queen 2-24 24-48 26 Woman 2-6 30-45 6-24 Fertilisation and cleavage Zygote = a fertilized ovum Decondensation = breakdown of chromatin to release sperm DNA into an active form Syngamy = the fusion of the male and female pronuclei Cleavage = splitting of cells Nelson 2005 An Introduction to Behavioral Endocrinology, 3rd ed Sinauer Associates, Inc - Resources  First cleavage occurs at ~24hrs Synchrony of division is soon lost Oocytes and Zygote 2 cell embryos 8 cell Morulae Blastocysts Blastocysts of some species elongate before implantation  Early pregnancy Embryos pass down the oviduct to Plasma progesterone levels rise as the CL the uterus as they develop forms, preparing the uterus for acceptance of the embryo Preimplantation Losses If the embryo is lost prior to the antiluteolytic signal then subsequent oestrus will occur at normal time (as if conception never occurred) If female is late to return to oestrus, it is can be assumed there was conception followed by embryonic loss Litter bearing animals tend to ovulate more embryos than the uterus can carry so there must be embryo loss Polytocous species have minimum number of embryos (~4) for establishment of pregnancy Normally, failure to conceive at one oestrus is not strongly correlated with failure to conceive at subsequent oestrus(es) eg cow 1st oestrus = 60% conception, 2nd oestrus = 60% of remaining 40% (ie 24%), 3rd oestrus = 60% remaining 16% (ie 10%), so 94% of original animals are pregnant after 10-12 weeks Causes of pre-implantation loss Fertilisation failure Non-functional spermatozoa or a fault with the oocyte Genetic A fault with zygote/embryo during replication Physiological Maternal environment fails to support embryo due to a faulty in the endocrine system or other pathology Preimplantation embryo growth Species 2 cell 4 cell 8 cell M BL hBL Implant Bitch D3-7 - - - - D13-15 D18-20 Cow 24h D1.5 D3 D4-7 D7-12 D9-11 D30-35 Ewe 24h D1.3 D2.5 D3-4 D4-10 D7-8 D15-16 Mare 24h D1.5 D3 D4-5 D6-8 D7-8 D16 fixed D36-38 Queen - - - D5 D8 D10-12 D13-14 Sow 14-16h D1 D2 D3.5 D4-5 D6 D11 Woman 24h D2 D3 D4 D5 D5-6 D6-12 Clear = in the oviduct Shaded = in the uterus Maternal recognition of pregnancy When pregnancy occurs – an embryonic signal stops luteolysis Species dependent The embryo must be in place at the right time to prevent luteolysis Prevention of luteolysis 4 Ovary F X CL 3 6 1 5 Uterus 2 The CL produces P4 which primes the uterus to produce PGF2α PGF2α stimulates formation of Ot receptors (OtR) on the uterine endometrium OtR respond to pulses of ovarian Ot, causing release of PGF2α PGF2α destroys the CL The presence of an embryo stops this pathway Embryonic signalling in ruminants IFN-Ƭ produced by BL IFN-Ƭ stops OTr synthesis IFN-Ƭ causes uterine protein production to promote the conceptus (Ƭ = trophoblastic origin) Embryonic signalling in pigs The conceptuses (minimum 2/horn) secrete E2 PGF2α is diverted to the uterine lumen (minimal access to circulation) CL maintained E2 also causes myometrial contractions to distribute embryos within uterus Embryonic signaling in horses and eCG Conceptus secretes proteins that appear to suppress uterine PGF2α production (possible role of equine chorionic gonadotropin; eCG) eCG produced by the chorion maintains the production of P4 via formation of accessory CL Embryo migration probably replaces the elongation stage embryo in other species Human chorionic gonadotropin (hCG) Produced by the trophoblastic cells on the blastocyst (chorion) Aids pregnancy maintenance by facilitating production of P4 from the ovary Preventing fetal (embryonic) rejection The fetus should be identified as a foreign body – so why isn’t it rejected? Trophoblast cells produce factors (eg P4 and IFN-Ƭ) which act as immunosuppressants Probably why embryo can implant at ‘foreign’ sites Trophoblast cells are coated with sialomucin (blocks binding) and have fewer histocompatibility antigens The trophectoderm is continuous and may act as a dialysis membrane to keep fetal and maternal proteins separated The number of uterine leukocytes decreases in pregnancy But there is still enough to reject other foreign tissue! The uterus secretes factors that act as immunosuppressants (eg PGE2 and transforming growth factor; TGF-ß) Wait – what? Use this padlet link to ask anonymous questions about this material (or endorse ones already there, if you are also confused about a it) https://padlet.com/Roslyn_Bathgate/what- in-the-what-now-fv63ej26xyxcgr4u