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Embryo Development Timeline Fertilization is day 0. All other events are based on this day Week 1 0 hour Fertilization occurs in ampulla 0-24 hours 1 – cell zygote isthmus 24 hours 2 – cell embryo isthmus 36 hours 4 – cell embryo isthmus Day 3 8 – cell embryo isthmus Day 4 Embryo compaction occurs in oviduct to uterus ~ Day 4-6 Blastocyst embryo enters uterus on Days 6-8 Blastocyst Implantation uterus o Note: this is day 20-22 after onset of menstruation Week 2 Day 6 – 8 blastocyst attachment and implantation Day 8 Bilaminar embryo (epiblast and hypoblast) completely formed o Soon after this formation of primitive yolk sac ~ Day 9 Approximately 24 hours after first lacunae appears w/in extraembryonic mesoderm, extraembryonic coelom (chorionic cavity) forms o Simultaneously, 2nd wave of proliferating and differentiating hypoblast cells (extraembryonic endoderm) forms the secondary/definitive yolk sac Day 11-13 --> Start of formation of primitive placenta primary villi cellular columns of cytotrophoblast Week 3 Week 4 Beginning of week 3 hemangiogenesis begins in walls of yolk sac o Sometime within week 3 (specifically 2 days after end of primitive hemangiogenesis), definitive hemangiogenesis occurs Day 15 Gastrulation begins at caudal end of embryonic disk w/ appearance of primitive streak o Soon after notochord devs and grows toward rostral end of embryo Day 16 -17 Secondary villi primary villi plus mesenchymal core Day 21 tertiary villi secondary villi plus blood capillaries which differentiate into blood vessels that connect up with embryonic vessels later in dev o Primary placenta established by end of third week w/ formation of tertiary villi Paraxial mesoderm (somite or somitomere) migrates into pharyngeal arch during week 3 Development of auditory and vestibular periphery starts in 3rd week End of third week cells at lateral edge of neural plate proliferate and form elevated neural folds flanking neural groove leads to developing CNS Note: gastrulation, notochord development, and tertiary villi all complete by end of week 3 Limb buds appear upper limb growth precedes lower limb growth by about 2 days Tongue develops Development of eye begins in 4th week Neurulation and embryo folding o Neural tube starts fusing neural crest cells lose contact w/ neuroectoderm undergo epithelia-mesenchymal transformation and migrates dorsally and ventrally w/in embryo o Prior to neural tube closure, small aggregates of ectoderm incorporated w/in surface ectoderm adjacent to neural tube form ectoderm placodes Play important role in dev of sensory organs, hypophysis, teeth, and cranial ganglia End of fourth week, fusion of neural tube complete o Neural tube broader at cephalic end At end of week 4, face region is composed of 5 prominences frontonasasl, 2 maxillary, 2 mandibular Week 5 Neurulation and embryo folding Growth period of embryo decidua capsularis pushed toward decidua parietalis (all the way up to week 22 where they fuse) Nasal placodes invaginate during 5th week lateral and medial nasal prominences appear Further development of chorionic villi, anchoring villi, and invasion of extravillous cytotrophoblast Week 6 Cartilage forms in mesenchyme around vertebrae beginning in 6th week Neurulation and embryo folding Palate shelves initially grow downward alongside the tongue during 6th week Further development of chorionic villi, anchoring villi, and invasion of extravillous cytotrophoblast Week 7 Neurulation and embryo folding Palate shelves reorient their growth horizontally (now above tongue) and fuse w/ primary palate during 7th week Further development of chorionic villi, anchoring villi, and invasion of extravillous cytotrophoblast Fusion of secondary palate (~7th to 8th week) End of 7th week primary lens fibers Ossification of vertebra begins around 7-8 weeks (?) continues to 25 years or so Week 8 Neurulation and embryo folding Villi associated with decidua capsularis are stretched by growth and compressed until they lose their blood supply formation of amniochorionic membrane Week 10 Placenta becomes major source or progesterone (although CL secretes progesterone until end of 4th month)