Lecture on Mammalian Reproduction and Placenta

Introduction

• The placenta is a critical interface in mammalian reproduction.
• Interaction between fetus and mother occurs at the placenta, responsible for fetal nourishment and gas exchange.

Invasiveness of Placentas

• Invasive placentas are common in many mammals and affect fetal development.
  • Invasion gives embryos control over nutrition and growth.
  • Trade-off: Increased risk of preeclampsia and metastatic cancer.
• Invasive stem cells share properties with metastatic cancer cells.

Mechanism of Placental Invasion

• Trophoblast cells from the embryo invade the uterus by secreting enzymes that dissolve tissue.
• These cells become part of the placenta, influencing maternal blood sugar and nutrient delivery.

Placental Morphology

• Determines how fetal blood interacts with maternal blood.
• Types of Placental Attachments:
  • Diffuse: Occupies most of the embryonic sac (e.g., horses, pigs).
  • Cotyledonary: Multiple discrete attachments (e.g., cattle, sheep).
  • Zonary: Forms a band around the fetus (e.g., dogs, cats).
  • Discoid: Attaches at one point (e.g., humans, apes, monkeys).

Historical Evolution of Placenta

• Discoid placentas are ancient, present for 150 million years.
• Human placenta has a villus structure, invading with finger-like projections (villi).

Types of Placenta Based on Layers

• Varies by how many maternal layers are retained:
  • Epitheliochorial: All maternal layers retained (e.g., horses, pigs).
  • Endotheliochorial: Only endothelium retained (e.g., dogs, cats).
  • Hemochorial: No maternal layers retained (e.g., humans, rodents).
• Invasive placentas like hemochorial are ancient and maximize maternal blood supply access.

Evolutionary Conflicts and Placental Types

• Human placentas are a mosaic of ancient and derived traits.
• Epithelial and endothelial corioal placentas are less invasive.
• Invasive embryos typically have shorter gestation periods.

Genetic Aspects

• Genes and Preeclampsia:
  • Positive selection at points of origin for hominid and hominin branches.
  • Genes enriched for preeclampsia risk and trophoblast invasion.

Cancer and Placental Invasion

• Similarities between invasive placental cells and cancer cells:
  • Use similar mechanisms for invasion and angiogenesis.
  • Understanding trophoblast invasion could inform cancer metastasis control.

Summary

• Mammalian placentas vary widely in structure and function.
• Human placentas combine ancient and newly derived features, including deep invasion.
• Trophoblast invasion links to cancer risk, showcasing evolutionary trade-offs (antagonistic pleiotropy).