Lecture Notes: Zygote to Implantation

Key Concepts

1. Fertilization and Cleavage

   • After fertilization, the egg undergoes rapid mitosis called cleavage.
   • The sequence of cell division: 2 cells → 4 cells → 8 cells.
   • Blastomeres: The progressively smaller cells formed by cleavage.
   • Morula: A solid mass of 32 cells, formed by Day 4, moving through the uterine tube.

2. Transition to Blastocyst

   • As the morula enters the uterine cavity, it is exposed to uterine milk rich in glycoproteins, prompting differentiation.
   • Differentiation: The process by which cells become more specialized.
   • Blastocyst Structure:
     • Trophoblast: Becomes the embryonic part of the placenta.
     • Inner Cell Mass (ICM): Develops into the embryo.
     • Blastocele: Fluid-filled cavity within the blastocyst.

3. Implantation Process

   • Implantation: The blastocyst attaches to the uterine wall and embeds itself.
   • Trophoblast Layers:
     • Syncytiotrophoblast: Secretes enzymes to burrow into the endometrium.
     • Cytotrophoblast: Contributes to the embryonic structures.
   • Human Chorionic Gonadotropin (HCG): Maintains the corpus luteum, ensuring progesterone production to support the uterine lining.

4. Decidua Formation

   • Post-implantation, the endometrium becomes the decidua with three regions:
     • Decidua Basalis: Between chorion and stratum basalis, maternal placenta part.
     • Decidua Capsularis: Covers the embryo.
     • Decidua Parietalis: Lines non-involved areas.

5. Ectopic Pregnancy

   • Occurs when an embryo develops outside the uterine cavity, often in the uterine tube.
   • Causes: Blockages, often from STDs causing scarring.
   • Symptoms: Missed menstrual cycles, bleeding, acute pain.
   • Risks: Tube rupture leading to internal bleeding.
   • More common in smokers due to nicotine effects on cilia.

6. Placenta Previa

   • Implantation of the placenta near or covering the cervical os.
   • Can lead to miscarriage, premature birth, or increased maternal mortality.
   • Symptoms: Sudden, painless bright red vaginal bleeding in the third trimester.
   • Often managed by cesarean delivery.

Goals of Lecture

• Define and understand terms: cleavage, blastocyst, morula, trophoblast, Inner Cell Mass, blastocele, syncytiotrophoblast, cytotrophoblast, implantation, uterine milk, decidua, decidua capsularis, decidua parietalis, ectopic pregnancy, placenta previa.