September 12 - Embryology.docx

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Anatomy and Cell Biology
Anatomy and Cell Biology 3319

September 12, 2013 Embryology Pp. 48-57 - Describe where fertilization and implantation normally occur o See Week 1 o Haploid gametes come together and form a fertilized egg that is diploid - Describe the fate of the inner cell mass and trophoblast during the second week of development - Define gastrulation - Identify the 3 germ layers and describe 2 anatomical structures derived from each layer - Describe the formation of the neural tube The Embryonic Period - 25 days – 8 weeks - fetal period = 20 weeks-38 weeks Week 1: From Zygote to Blastocyst (fertilization and the first 8 days) - an egg (oocyte) is released from the ovary into the peritoneal cavity (ovulation) and taken up by the uterine tube/fallopian tube, where it undergoes fertilization (sperm meets and enters egg) to become a zygote (fertilized egg) o at this time, hormones are also being released to help prepare uterus for possible implanation o uterus body is lined by endometrium (this expands, and if no implantation, the endometrium gets released each month as menstrual cycle) - then, as it moves through the uterine tube and into the uterus/womb, it passes through the four-cell stage (2 days) by dividing (mitosis) - and the morula (a solid ball of 12-16 cells) (3 days) - and early blastocyst (morula hollows out and fills with fluid) stage (4 days) o coated with a layer called the zona pellucida (it breaks down and fluid enters into the ball of cells and forms blastocyst cavity) - finally, the blastocyst implants into the wall of the uterus (blastocyst sphere consists of trophoblast cells and the inner cell mass) (7 days) o inner cell mass differentiates into the embryo o trophoblast will help form the placenta (structure that transfers nutrients from mother to fetus) - implantation begins as trophoblast invades into uterine/endometrium wall Clinical Correlation: Ectopic Pregnancy - Possible for the egg not to make it into the fallopian tube and still get fertilized - Ectopic pregnancy: fertilized egg that is implanted outside the uterus - Common areas: ampulla, tubal pregnancy (very dangerous because can rupture or cause bleeding), outer surface of ovary or uterus, in abdominal cavity, placenta previa (embryo implants at opening of cervix) Morula & Blastocyst Formation - Embryoblast: derived from inner cell mass o Make up the embryo - It will progress and then attach to endometrium - Trophoblast cell are invasive and will invade endometrial wall o These cells will go on to form the placenta Week 2: The Two-Layered Embryo (formation of bi-laminar embryo) - day 9: implantation continues; o inner cell mass divides into two sheets; the epiblast and the hypoblast, forming the bi-laminar embryonic disc - day 11: implantation complete; amniotic sac and yolk sac form o extension of the epiblast forms amniotic sac (outer membrane == amnion, inner sac = amniotic sac cavity filled with amniotic fluid) o yolk sac: extension of hypoblast that later forms digestive tube Clinical Correlation: Twins - Monozygotic: derived from ONE fertilized egg (identical twins) o Cells will split o If split at 2-cell stage, will form 2 different blastocysts and 2 different amniotic sacs o If split later and inner cell mass splits, 2 amniotic cavities that share a placenta o Problems occur when split is even later, blastocyst is already implanted and amniotic cavity formed and then splits, then 2 embryos share the same amniotic sac  Babies could be a conjoined twin (sharing body parts)  Could get tangled in umbilical cord - Dizygotic: 2 oocytes released and both fertilized (fraternal twins) Week 3: The Three-Layered Embryo - the primitive streak and the three germ layers o embryo grows from two-layered to three-layered disc o gastrulation: forms 3 primary germ layers (ectoderm, mesoderm and endoderm)  body tissue develops from these
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