Review: The evolving placenta: different developmental paths to a hemochorial relationship

Area of Science:

• Comparative Embryology
• Reproductive Biology
• Developmental Anatomy

Background:

• The maternal blood-trophoblast interface is fundamental to placental formation.
• Hemochorial placentation, where fetal villi are bathed in maternal blood, exhibits diverse developmental origins.
• Understanding these origins is key to interpreting placental structures in evolutionary contexts.

Purpose of the Study:

• To categorize and describe the primary developmental patterns of maternal blood association with trophoblast before hemochorial placenta formation.
• To analyze variations in these patterns across different mammalian groups.
• To emphasize the importance of developmental pathways when inferring placental evolution from structure.

Main Methods:

• Comparative analysis of placental development across various species.
• Classification of observed maternal blood-trophoblast interactions into distinct patterns.
• Review of existing literature on placental morphogenesis.

Main Results:

• Identified four main patterns: transitory endotheliochorial, blood released into trophoblast mass, blood confined to lacunae, and villi entering maternal blood sinuses.
• Noted that the endotheliochorial stage is transient in some species, not a universal precursor.
• Described species-specific variations, including lacunar stages in Old World primates and trabecular arrangements in New World monkeys and armadillos.

Conclusions:

• Placental development follows diverse routes to achieve a hemochorial state, challenging a single evolutionary progression.
• The developmental origin of placental structures significantly impacts their final form.
• Considering developmental patterns is essential for accurate evolutionary extrapolations based on placental morphology.