Placental Function and Implantation Notes

IMPLANTATION & EARLY EMBRYONIC DEVELOPMENT
Timeline of Early Development:

• Day 0: Fertilisation (sperm meets egg in the ampulla [part of the fallopian tube])

• Day 3–4: Morula [solid ball of ~16 cells]

• Day 5: Blastocyst forms [a hollow structure with an inner cell mass (becomes the baby) and trophoblast (becomes placenta)]

• Day 6–7: Implantation begins

• Day 9–10: Fully embedded into the endometrium [uterus lining]

4 Stages of Implantation:

1. Hatching

   • Blastocyst breaks out of the zona pellucida [protective shell]

   • Needed so it can interact with the uterus lining

2. Apposition

   • Blastocyst loosely aligns with the endometrium

   • Inner cell mass faces the uterine wall

3. Adhesion

   • Blastocyst sticks firmly to the endometrial cells

   • Uses sticky molecules like integrins, L-selectin, laminin, and fibronectin

4. Invasion

   • Trophoblast cells invade the uterine lining

   • Syncytiotrophoblast [outer layer] secretes enzymes to digest tissue and form spaces (lacunae)

   • Cytotrophoblast [inner layer] keeps dividing

MOLECULAR & CELLULAR MECHANISMS OF IMPLANTATION

• Integrins & Selectins: Sticky proteins for attachment

• MMPs (matrix metalloproteinases): Enzymes that break down tissue

• Cytokines (e.g., LIF): Tell the uterus to be “receptive”

• Progesterone: Prepares and maintains the uterine lining

FORMATION & STRUCTURE OF THE PLACENTA

• Fetal side: From the chorion [trophoblast + supporting tissue]

• Maternal side: From decidua basalis [part of the endometrium]

Villi (finger-like structures):

• Primary villi: Trophoblast only

• Secondary villi: Now have mesoderm (supportive tissue)

• Tertiary villi: Blood vessels develop for nutrient exchange

FUNCTIONS OF THE PLACENTA

1. Gas Exchange: Oxygen in, CO2 out

2. Nutrient Delivery: Glucose, amino acids, fats

3. Waste Removal: Urea, bilirubin

4. Hormone Production: To maintain pregnancy

5. Immune Role: Passes IgG antibodies to baby

6. Barrier: Protects against some harmful substances (but not all — drugs, alcohol can cross)

PLACENTAL ENDOCRINE FUNCTION

Placenta takes over hormone production around week 10–12.

IMMUNOLOGICAL & PLACENTAL COMPLICATIONS

• Rh incompatibility: Mom is Rh–, baby is Rh+ → mom makes antibodies against baby’s red cells

• Pre-eclampsia: High BP + protein in urine → shallow implantation = poor blood flow

• Placenta previa: Placenta covers cervix → dangerous in labour

• Placental abruption: Placenta detaches early → bleeding + oxygen loss

• Gestational trophoblastic disease (e.g., molar pregnancy): Abnormal growth of placental tissue.