Main Objectives!1) Identify the periods of prenatal development and phases of human embryogenesis!2) Describe the formation of the bilaminar disc (-> what forms the embryo)!3) Describe the formation of the amniotic and chorionic cavities!4) Describe the formation of uteroplacental circulation!5) Understand how congenital abnormalities develop!!1) Periods of prenatal development and human embryogenesis!2nd week: implantation completed. Amnion, chorion, and yolk sac form, and the placenta begins to •develop.!2 periods of prenatal development= embryo + fetus!•Embryo: fertilization-> end of 8th week!◦Fetus: beginning of 9th week to birth!◦Periods of human embryogenesis (only focusing on first 3 today)!•Gametogenesis- formation of haploid gametes (egg and sperm, sex cells)!◦Fertilization- gametes join to form a zygote!◦Takes place in the ampulla of oviduct (fallopian tube)!‣Cleavage- series of rapid cell divisions that form the morula and then the blastocyst!◦Gastrulation!◦Formation of the tube within a tube body plan!◦Organogenesis!◦! First Week of Development!1. Fertilization!2. Formation of the zygote! (w/diploid nucleus)!3. Cleavage- series of mitotic divisions, rapid increase in number of cells (blastomeres)!4. Formation of the morula (12-32 blastomeres)!- Reaches uterus between 3-4 days of development!5. Formation of the blastocyst- when morula reorganizes cells into inner cell mass (embryoblast) andan outer cell mass (trophoblast)!- Morula begins to absorb fluid -> blastocyst cavity!- Embryoblast cells form a compact mass on one side of the cavity!- Trophoblast organizaes into a thin, single-layered epithelium!- Day 5, blastocyst hatches from the zona pellucida (part from the egg) and is now free to interact w/the endometrium 6. Implantation begins!- Day 6, blastocysts attaches to endometrial epithelium!- Trophoblast differentiates into 2 layers!- cytotrophoblast, the inner layer of cells!- syncytiotroohoblast, the outer layer. It extends thru the endometrial epithelium and invades the endometrial connective tissue!- By end of 1st week, Blastocyst is superficially implanted in the compact layer of the endometrium and deriving its nourishment from the eroded maternal tissues.!!Clinical correlations!!- Twinning (#5 in first week of development)!- Fraternal (dizygotic): product of the fertilization of 2 ovulated eggs, dev. as 2 individs!- Identical (monozygotic twins): product of a single fertilized egg, subdivision and splitting of the inner cell mass of a blastocyst and formation of 2 embryos enclosed in a common trophoblast!- Splitting cell mass can not completely separate-> 2 embryos in a single amnion-> conjoined twins - Ectopic pregnancy (#6 in first week of development)!- Implantation anywhere other than the uterine cavity!- Epithelium at abnormal responds with increased vascularity-> vessels can rupture-> life threatening!!Second Week of Development!1. Development of bilaminar embryonic disc!2. Implantation is completed!3. Development of the amniotic and chorionic cavities and primary and secondary yolk sac!!!Day 7: Embryoblast reorg's into 2 layers: epiblast (external, upper layer of columnar cells adj. to the amniotic cavity) and the hypoblast (internal lower layer of cuboidal cells adj. to the blastocyst cavity)!2 layers= the bilaminar embryonic disc! !!!!!!!!Day 8: Formation of Amniotic Cavity!- Fluid collects between epiblast and overlying trophoblast (this area will be the amniotic cavity)!!!!!!!!!Day 9!- Layer of epiblast cells diff. into a thin membrane= amnion which separates the new cavity from the trophoblast...!Amniotic cavity expands and by 8th week, it encloses the entire embryo!!!!!!!Day 9 cont. formation of Yolk Sac - Hypoblast cells prolif. and migrates to form !the yolk sac membranes!- 1st migration wave= day 8!- Forms primary yolk sac (the exocoelomic membrane= Heuser's membrane)!!!Day 10-11: simultaneously the outer layer of cells from the primary yolk sac forms a loosely arranged connective tissue= extraembryonic mesoderm, fills the remainder of the blastocyst cavity!!!!!!!!Extraembryonic mesoderm splits into 2 layers forming !chorionic cavity, which sep. embryo, amnio, and yolk sac, from the outer wall of the blastocyst, which is now the chorionDay 12: primary yolk sac displaced and later degen. by second wave of migrating hypoblast cells, which form the secondary yolk sac!!!!!!!!!!!!!!!!!!!!By day 13, embryonic disc with dorsal amnion and ventral yolk sac are suspended in the chorionic cavity solely by connecting (body) stalk. The embryo is now inside.!!Full Implantation - Happening as the cavities develop!- Embryo becomes fully implanted by the continued erosion of endometrium caused by the syncytiotrophoblast--> sync. produces hormone human chorionic gonadotropin (hCG) -> maintains vessel formation in the myometrium !!Uteroplacental Circulation!!- As amnion, embryonic disk, and primary yolk sac form, isolated cavities = lacunae appear in the syncytio.!- Adjacent lacunae fuse to form lacunar networks!- Endometrial capillaries around the embryo become congested and dilated-> form sinusoids !!!!!!!!!!!!!!Syncytio erodes the sinusoids and maternal bloods flows into the lacunar networks. !The cytotrophoblast prolif. locally to form extensions= primary chorionic villi that grow into the blood-filled lacunae. !!!!!!!!! Extraembyronic mesoderm assoc. with the cytotrophoblast penetrates the core of the primary c villi-> become secondary chorionic villi End of 3rd week= villous mesoderm has given rise to blood vessels that connect with the vessels forming in the embryo-> est. a working uteroplacental circulation= tertiary chorionic villi!!!!!!!!!First week of dev. outline!Fert: triggers completion of meisosis--male and female pronuclei fuse to form a zygote!•Day 1: zygote undergoes cleavage as it travels down Fallopian tube!•Day 2: formation of morula!•Day 3: Morula reaches uterus!•Day 4: Formation of blastocyst!•Day 5: Blastocyst hatches!•Day 6-7: Implantation begins!•!Second week of dev. outline!Day 8: Amniotic cavity forms, syncytiotrophoblast expands!•Day 9: cells migrate from the hypoblast to form primary yolk sac, implantation is complete, syncytio •surrounds embryo!Day 10-11: Extraembryonic mesoderm forms and splits to form chorionic cavity, beginnings of the •maternal/embryonic blood connection!Day 12: Cells migrate from the hypoblast to form the secondary yolk sac and the
View Full Document
Unlocking...