Week 1 of Human Development

Question 1

Phases of Fertilization

Answer 1

Phase 1: Sperm penetration of corona radiata
- aided by mucosal enzymes
Phase 2: Sperm binding and penetration of zona pellucida
- sperm binding trough interaction of glycosyltransferase + ZP3 receptors ⇒ triggers acrosome reaction + fusion ⇒ release of acrosomal enzymes
- Penetration of zona pellucida with acrosin⇒ makes membrane impermeable to other sperm
Phase 3: Fusion of sperm and oocyte cell membranes
- membranes break down
- entire sperm enters oocyte ⇒ mitochondria + tail degenerates
⇒⇒ secondary oocyte completes meiosis2
⇒⇒ pronuclei fuse ⇒ zygote

Question 2

Cleavage

Answer 2

1. Zygote cutoplasm cleaved to form blastula ⇒ many blastomeres (totipotent)
2. Blastomere ⇒ morula by compaction with uvomorulin

Question 3

Blastocyst formation

Answer 3

• fluid secreted within morula forms blastocyst cavity
  1. inner cell mass at one pole of blastocyst
  2. inner cell mass ⇒ embryo blast
  3. outer cell mass ⇒ trophoblast

Question 4

Zona pellucida degredation

Answer 4

• 4 days after conception

- must degenerate for implantation to occur

Question 5

Implantation

Answer 5

• usually within posterior superior wall of uterus
• day 7 after fertilization
• in functional layer of endometrium during progestational phase
• trophoblast ⇒ cytotrophoblast + syncytiotrophoblast
• involves toleration of antigenic ally different cell types

Question 6

Ectopic tubal pregnancy

Answer 6

• blastocyst implants within uterine tube
• can also happen in ovary, abdomen, internal osmium, cervix
• due to delayed transport
• produces abdominal pain