Embryology

• What is fertilization?

  It is the fusion of haploid sperm nucleus and haploid secondary oocyte nucleus
• Where does fertilization occur and when does the oocyte die?

  Fertilization occurs in the fallopian tubes within 24 hours after ovulation, when the oocyte dies.
• What must happen for sperm and oocyte to meet before fertilization?

  The fimbriae of the fallopian tube brush over the ovary to release the secondary oocyte into the lumen of the fallopian tubes.
• How does the oocyte move towards the uterus?

  Oocyte movement occurs through peristalsis and cilia transport towards the uterus.
• How do sperm locate the oocyte?

  Sperm swim towards the oocyte by means of flagella and are attracted by chemical signals released by the oocyte.
• What role do prostaglandins play in fertilization?

  Prostaglandins in semen stimulate uterine contractions that help move sperm towards the oocyte.
• What is capacitation in sperm maturation?

  Capacitation is the final maturation of the sperm that occurs within the female.
• What happens to the acrosomal membrane during capacitation?

  The acrosomal membrane of the sperm head becomes fragile during capacitation.
• What triggers the acrosomal reaction in sperm?

  The binding of zona pellucida protein 3 (ZP3) to the sperm head triggers the acrosomal reaction.
• What occurs during the acrosomal reaction?

  Enzymes are released from the acrosomal region of the sperm, penetrating the corona radiata and digesting the zona pellucida.
• What triggers the fast block to polyspermy?

  The fusion of the first sperm with the oocyte membrane triggers the fast block to polyspermy.
• How does the fast block to polyspermy work?

  The oocyte membrane depolarizes, preventing more sperm from binding to the oocyte.
• What is the slow block to polyspermy?

  The slow block to polyspermy involves the release of Ca$^{2+}$ that triggers exocytosis of secretory vesicles, blocking sperm receptors and hardening the zona pellucida.
• What happens when sperm enters the oocyte?

  Sperm entry triggers the oocyte to complete meiosis II and produce a second polar body.
• What is formed when the male and female haploid pronuclei fuse?

  The fusion of male and female haploid pronuclei forms a diploid zygote.
• What occurs 30 hours post-fertilization?

  The zygote undergoes mitotic division, known as cleavage.
• What are the initial products of cleavage?

  Cleavage produces two identical daughter cells called blastomeres, surrounded by the zona pellucida.
• What happens to the blastomeres after the first division?

  In the next 10-12 hours, the blastomeres divide and increase in size, progressing from 2 cells to 4 cells, then to 8 cells.
• What is the morula stage?

  By day 3, there are 16 cells, and this structure is called a morula.
• What is an advanced morula?

  Once the blastomere has more than 16 cells, it is called an advanced morula.
• What happens at days 5 and 6 of development?

  Uterine fluid passes into the center of the advanced morula, creating a blastocele and forming a blastocyst.
• How do the cells of the blastocyst differ?

  The cells of the blastocyst are no longer identical in size or shape.
• What is the trophoblast?

  The trophoblast is the outer case formed around the blastocyst that differentiates into the chorion, a portion of the placenta.
• What happens during implantation at day 6?

  The blastocyst attaches to the endometrium, and the trophoblast develops into two distinct layers: syncytiotrophoblast and cytotrophoblast layers.
• What role does the syncytiotrophoblast play during implantation?

  The syncytiotrophoblast secretes enzymes that digest the endometrial cells.
• What is the function of the cytotrophoblast layer?

  The cytotrophoblast layer defines the shape of the embryo.
• What hormone do the trophoblast layers secrete, and what is its role?

  The trophoblast layers secrete human chorionic gonadotropin (hCG), which helps maintain the corpus luteum and uterine lining during pregnancy.
• How is hCG used in pregnancy tests?

  Human chorionic gonadotropin (hCG) in urine is measured in a pregnancy test.
• When is the placenta fully formed during pregnancy?

  The placenta is not fully formed until the third month of pregnancy.
• Why is quick implantation important for the developing embryo?

  Quick implantation allows the embryo to tap into maternal nutrients and blood supply.
• what is the structure and function of the umbilical cord?

  links fetus to mother
• how many arteries does the umbilical cord have?

  2 arteries that carry blood from fetus to the placenta
• why does the umbilical cord have 1 umbilical vein?

  to carry oxygenated blood from placenta to fetus
• when is the amniotic cavity and yolk sac formed?

  day 8-12
• how is the amniotic cavity and yolk sac formed?

  cell mass seperates from trophoblasts creating fluid filled amniotic cavity
• what lines the amniotic cavity?

  embryonic discs called ectoderm
• what is the yolk sac? 


  the second cavity within the blastocele
• what are the three germ layers?

  ectoderm, mesoderm, endoderm
• what is the formation of the three germ layers known as?

  gastrulation
• what is the ectoderm?

  lining of the amniotic cavity