Page Cell - Model # C1 & 2 A&P Numbers correspond to embryo Model Numbers 1. Zygote - a fertilized oocyte - note that the sperm (male) pronuclei has not yet fused with the oocyte (female) pronuclei. - the oocyte is fertilized in the distal fallopian tube and is in Metaphase II of Meiosis - fertilization can occur during day 14 of a 28 day reproductive cycle. The oocyte can only be fertilized for a very short period of time (as little as 15 hours after ovulation) - Zona Pellucida - NOT SHOWN ON MODELS - glycoprotein shell surrounding the embryo until the blastula stage. The human embryo must "hatch" out of this "shell" in order to implant (nidate). - Zona Radiata – - Male and Female Pronuclei 2. Two-cell embryo - occurs approximately 30 hrs after fert - each cell is a Blastomere 3. Four-cell embryo 4. Eight-cell embryo - (actually, I only count 7, but call it 8) 5. Morula - about 16 blastomeres and occurs around day 3 after fertilization. - blastomeres still totipotent - each has the potential to develop into a complete individual, identical twins or "clones" 6. Blastula/blastocyst - hollow ball of cells - begins around day 4 (the time the embryo reaches the uterus) a. Inner Cell Mass - will become the embryo proper b. Trophoblasts - outer wall of the embryo and will contribute to the placenta & produce hCG (human chorionic gonadotropin) which keeps the corpus luteum functioning (it produces progesterone to maintain the uterus for pregnancy) c. Blastocoel - this is the fluid filled space - implantation or nidation will occur around day 6 after fertilization at blastula stage 7. Bilaminar Blastula - Day 7, the Inner Cell Mass differentiates into two layers. a. Epiblast (yellow) b. Trophoblasts c. Blastocoele - will be lined by mesoderm to become the Primary Yolk Sac d. Hypoblast - will give rise to yolk sacs when its cells line the blastocoel 8. Late Blastula - around day 8 b. Trophoblasts - differentiate into: 1. Cytotrophoblasts - surround the entire embryo and give rise to #2 below 2. Syncytiotrophoblasts - "Syncytio" refers to the fact that these are Multinucleated. They are aggressively invasive cells that digest into the uterus to establish placenta 3. Lacunae - spaces eventually filled with Maternal blood which will bath the chorionic villi c. Blastocoele - once lined by cells from the Hypoblast becomes the Primary Yolk Sac e. Amnion - surrounding the amniotic space - between inner cell mass/epiblast & trophoblasts |