Fertilization & Embryo Development - Lecture.pdf

MD1020 Reproduction – Week 6 Assoc. Prof. Damien Paris ([email protected])  Development & embryology:  timing & significance  Timeline of embryo development:  sperm capacitation & fertilization  cleavage & blastocyst formation  implantation & early placentation  extra-embryonic membr...

MD1020 Reproduction – Week 6 Assoc. Prof. Damien Paris ([email protected])  Development & embryology:  timing & significance  Timeline of embryo development:  sperm capacitation & fertilization  cleavage & blastocyst formation  implantation & early placentation  extra-embryonic membranes  gastrulation & organogenesis (brief overview) Placental maturation, structure & function covered in Week 7  Summarize the process & timing of fertilization & pre- implantation embryo development  Describe the formation of the embryonic disc & the process of gastrulation  Name the general tissues formed from the 3 germ layers (ectoderm, mesoderm & endoderm)  Explain the process of implantation  Recognize the general function & development of extraembryonic membranes (amnion, yolk sac, allantois)  Describe the general timeline of prenatal development including: (i) duration of pregnancy (incl. embryonic & foetal periods) (ii) timing of major events during embryonic period  Development usually divided into 2 phases:  prenatal (before birth) & postnatal (after birth)  Embryology is the study of embryos & foetuses:  prenatal development:  embryonic period – fertilization until end of 8th week  foetal period – 9th week until birth  embryonic period (fertilization to week 8):  placenta develops  all major adult organs formed (gastrulation & organogenesis)  foetal period (week 9 to birth):  growth & development Marieb & Hoehn 2010  Duration of pregnancy:  38 weeks (266 days) from zygote to birth  spread over three trimesters  Due date for birth:  40 weeks (280 days or 9 months + 7 days) often calculated from LNMP (first day of last normal menstrual period)  clinical significance:  pre-implantation developmental abnormalities (failure to implant within uterus → failed or ectopic pregnancies)  severe embryonic period developmental abnormalities (teratogen exposure & infection during weeks 1-8 → neural tube & other defects)  foetal period developmental abnormalities (malformation or mechanical abnormalities → talipes) ? Wikipedia 7-4-11  fertilization  cleavage & blastocyst formation  blastocyst begins to implant into endometrium  Ejaculated sperm cannot immediately fertilize oocyte  Need to spend several hours in female reproductive tract to attain capacity to fertilize  Oestrogen & vaginal mucus destabilize the sperm plasma membrane & trigger hyperactive motility  Process involves:  removal of protein coating acquired in epididymis  reorganization of plasma membrane to expose binding sites Senger 2005 www.mysbfiles.stonybrook.edu  Swelling of acrosome & fusion of outer acrosomal membrane with overlying plasma membrane  Calcium-dependent event: ↑ intracellular Ca2+ & cAMP  Stimulated by progesterone & a protein (ZP3) on the zona pellucida that surrounds the oocyte  Release of digestive enzymes from acrosome:  hyaluronidase → penetration of corona radiata  acrosin → zona pellucida digestion Acrosome-reacted sperm have a very short lifespan! hyperactive motility hyaluronidase acrosin Sperm Marieb & Hoehn 2010 Corona radiata  capacitation: oestrogen/mucus destabilize sperm PM & hyperactivate motility  acrosome reaction: progesterone & binding to ZP3 causes ↑Ca2+ & release of digestive enzymes from sperm  fertilization: sperm digests zona, binds to sperm-binding receptor & fuses with oocyte PM  polyspermy block: fusion causes ↑Ca2+ & electrical block of oocyte PM (fast block), then cortical granule release which hardens zona & removes spermbinding receptors (slow block) Zona pellucida ZP3 molecules Oocyte plasma membrane Oocyte sperm-binding receptors Cortical granules Acrosomal process Cortical reaction Sperm nucleus Marieb & Hoehn 2010 ?  Cleavage stage:  2 to 8-cells  ↑ cell number without ↑ cell size  totipotent blastomeres 2-cell 8-cell (generate complete individual incl. placenta) morula  Morula stage:  16 to 32 cells  inner & outer cell populations formed  Blastocyst stage:  64+ cells  first cell differentiation event early blastocyst hatching blastocyst (trophectoderm & inner cell mass formation)  pluripotent inner cell mass (generate complete individual excl. placenta)  Morula:     outer & inner cell populations gap junctions b/w inner cells tight junctions b/w outer cells Na+ pumped into morula → influx of water  Blastocyst:  fluid accumulation → blastocoel cavity  ↑ CDX2 in outer cells = trophectoderm differentiation (foetal placenta - chorion)  ↑ OCT4 in inner cells = inner cell mass differentiation (embryo proper)  rapid growth & enzymes → hatching from zona pellucida Senger 2005  implantation completed  placenta & extraembryonic membranes begin development  development of inner cell mass proceeds Marieb & Hoehn 2010  protein-mediated binding of trophoblast to endometrium  digestive enzymes from trophoblast erode endometrial layer Marieb & Hoehn 2010  trophoblast grows to form cytotrophoblast & syncytiotrophoblast  syncytiotrophoblast cells digest & invade endometrium Maternal blood vessels hCG Proliferating syncytiotrophoblast Endometrium Amnion ? Cytotrophoblast Amniotic cavity Yolk sac Bilayered embryonic disc Extraembryonic mesoderm • Epiblast • Hypoblast Endometrial epithelium Chorion being formed Lumen of uterus Marieb & Hoehn 2010  blastocyst enveloped by endometrium  syncytiotrophoblast secretes hCG (from Day 7) → directly stimulates CL to produce progesterone = maintain pregnancy for 8-12 weeks Day 7.5 Day 12 Day 16 Amniotic cavity Chorionic villus Chorion Amnion Forming body stalk Amniotic cavity Yolk sac Bilaminar embryonic disc Allantois Extraembryonic mesoderm • Epiblast • Hypoblast Chorion being formed Marieb & Hoehn 2010  inner cell mass differentiates into epiblast & hypoblast  fluid-filled cavities form in epiblast (amnion) & hypoblast (yolk sac) → bilaminar embryonic disc  out-pocket of hypoblast + extra-embryonic mesoderm form allantois  extra-embryonic mesoderm + cytotrophoblast + syncytiotrophoblast form chorion & chorionic villi → beginning of foetal placenta (see next week) Week 4.5 Decidua basalis Maternal blood Chorionic villus Umbilical blood vessels in umbilical cord Amnion Yolk sac Lumen of uterus Chorion Marieb & Hoehn 2010  amnion: prevents physical trauma, maintains temperature, avoids embryonic structures fusing, permits movement  yolk sac: brief nutrient support, part of future gut, source of early blood cells & vessels  allantois: forms base of umbilical cord  chorion & chorionic villi: with maternal decidua forms true placenta  appearance of primitive streak  gastrulation – formation of 3 primary germ layers Head end Cut edge of amnion Amnion Bilayered embryonic disc Yolk sac (cut edge) Head end of bilayered embryonic disc Right Left Yolk sac Primitive streak Marieb & Hoehn 2010 Tail end Bilaminar embryonic disc, superior view  midline groove (primitive streak) forms along caudal end of bilaminar embryonic disc  primitive streak establishes longitudinal body axis (head & tail) of embryo Head Left Right Tail Primitive streak Marieb & Hoehn 2010  epiblast cells migrate through the primitive streak & form: (i) endoderm – displace cells of hypoblast (ii) mesoderm – fill middle-layer of embryo  remaining epiblast forms (iii) ectoderm  differentiation of ectoderm, mesoderm & endoderm  organogenesis – formation of organ systems All tissues of adult derived from ectoderm, mesoderm & endoderm  each layer has pre-established ‘fate’:  ectoderm – nervous system, skin epidermis  mesoderm – most blood vessels, connective tissues, bone & muscle  endoderm – epithelial linings & associated glands of digestive, respiratory & urogenital systems  epithelium: skin, hair, nails, sensory epithelium, pituitary gland  connective: bones & blood vessels of the head  nerve: brain, spinal cord, peripheral nervous system  epithelium: serous membranes, kidney, gonads, ducts  connective: bone, cartilage, blood, connective tissue proper  muscle: cardiac, skeletal, smooth 1  epithelium: lungs, gastro-intestinal lining, liver, gall bladder, pancreas, urinary tract lining Important! Placentation: nutritive, respiratory, excretory & endocrine functions established Implantation Timeline (weeks) 0 Fertilization 1 2 Gastrulation 3 4 5 6 Organogenesis: all organ systems formed 7 8 12 foetal period (week 9 to birth)  rapid growth & weight gain: from 22 mm (2 g) to 550 mm (3.2 kg)  further differentiation & refinement of body structure  5th month - movements felt by mother  survival if born prematurely after 27-28 weeks Marieb & Hoehn 2010