Embryonic Period Timeline - Embryology

                        Overview
                    
                        Time
                    
                        Landmark
                    
                        Day 0
                    
                        Embryogenesis begins
                    
                        sperm fertilized ovum in ampulla
                    
                        all mitochondria are maternal in origin
                    
                        Week 1
                    
                        Cleavage and blastocyst formation 
                    
                        zygote undergoes mitotic divisions to form blastomeres → morula → blastocyst
                    
                        blastocyst cavity forms two cell masses
                    
                        inner cell mass = embryoblast which will become embryo
                    
                        outer cell mass = trophoblast which becomes placenta 
                    
                        hCG secretion
                    
                        begins after implantation of blastocyst
                    
                        syncytiotrophoblast invades uterine wall and secretes hCG
                    
                        hCG detected in blood at day 8 and in urine at day 10 (pregnancy test)
                    
                        ↓ hCG: ectopic pregnancy or a sign for spontaneous abortion
                    
                        ↑ hCG: multiple pregnancy, hydatidiform mole, or gestational trophoblastic neoplasia
                    
                        Week 2
                    
                        Bilaminar disk formation 
                    
                        consists of 2 germ layers: epiblast and hypoblast
                    
                        2 cavities are present: amniotic cavity and yolk sac
                    
                        placenta has 2 components: cytotrophoblast and synctiotrophoblast
                    
                        notochord forms
                    
                        Week 3
                    
                        Gastrulation 
                    
                        establishment of three embryonic germ layers (ectoderm, mesoderm, and endoderm)
                    
                        initiated by formation of primitive streak
                    
                        Formation of notochord and neural plate
                    
                        notochord induces its overlying ectoderm to become neuroectoderm. 
                    
                        neuroectoderm will form neural plate, fully closes by week 4
                    
                        neural plate forms neural tube and neural crest cells 
                    
                        notochord will become adult nucleus pulposus in the vertebral column
                    
                        Week 3-8
                    
                        Embryonic period
                    
                        major organs systems have begun development 
                    
                        period in embryo is most susceptible to teratogens 
                    
                        hematopoiesis begins around yolk sac at week 3 and is taken over by liver, thymus, spleen, and final and lifetime production is done in bone marrow 
                    
                        week 7 embryo begins sexual differentiation
                    
                        Week 4
                    
                        Initiation of heart beat
                    
                        Formation of upper and lower limb buds
                    
                        Closure of the neural plate
                    
                        Week 8
                    
                        Fetal period
                    
                        fetus begins to move and has appearance of baby
                    
                        Week 10-12
                    
                        Sex of fetus becomes recognizable
                    
                        Phenotypic differentiation completed by week 20

Action	Numeric Key	Letter Key	Function Key
Show Bullets		S	Enter (frontside only)
20%	1	N
40%	2	H
60%	3	F	Enter (backside only)
80%	4	E
100%	5	M
Toss	0	T

Time	Landmark
Day 0	Embryogenesis begins sperm fertilized ovum in ampulla all mitochondria are maternal in origin
Week 1	Cleavage and blastocyst formation zygote undergoes mitotic divisions to form blastomeres → morula → blastocyst blastocyst cavity forms two cell masses inner cell mass = embryoblast which will become embryo outer cell mass = trophoblast which becomes placenta hCG secretion begins after implantation of blastocyst syncytiotrophoblast invades uterine wall and secretes hCG hCG detected in blood at day 8 and in urine at day 10 (pregnancy test) ↓ hCG: ectopic pregnancy or a sign for spontaneous abortion ↑ hCG: multiple pregnancy, hydatidiform mole, or gestational trophoblastic neoplasia
Week 2	Bilaminar disk formation consists of 2 germ layers: epiblast and hypoblast 2 cavities are present: amniotic cavity and yolk sac placenta has 2 components: cytotrophoblast and synctiotrophoblast notochord forms
Week 3	Gastrulation establishment of three embryonic germ layers (ectoderm, mesoderm, and endoderm) initiated by formation of primitive streak Formation of notochord and neural plate notochord induces its overlying ectoderm to become neuroectoderm. neuroectoderm will form neural plate, fully closes by week 4 neural plate forms neural tube and neural crest cells notochord will become adult nucleus pulposus in the vertebral column
Week 3-8	Embryonic period major organs systems have begun development period in embryo is most susceptible to teratogens hematopoiesis begins around yolk sac at week 3 and is taken over by liver, thymus, spleen, and final and lifetime production is done in bone marrow week 7 embryo begins sexual differentiation
Week 4	Initiation of heart beat Formation of upper and lower limb buds Closure of the neural plate
Week 8	Fetal period fetus begins to move and has appearance of baby
Week 10-12	Sex of fetus becomes recognizable Phenotypic differentiation completed by week 20