Download Timeline Part I

Embryo Development Timeline
Fertilization is day 0. All other events are based on this day
Week 1
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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
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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
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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
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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
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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
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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
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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
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Neurulation and embryo folding
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Villi associated with decidua capsularis are stretched by growth and compressed until they lose
their blood supply  formation of amniochorionic membrane
Week 10
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Placenta becomes major source or progesterone (although CL secretes progesterone until end of
4th month)