Amniotic Fluid and Ocean Water: Evolutionary Echoes, Chemical Parallels, and the Infiltration of Micro- and Nanoplastics.

Background: Abiogenesis is hypothesized to have occurred in the aquatic environments of the early Earth approximately 3.8-4.0 billion years ago, in oceans containing high concentrations of ions (Na⁺ ≈ 470 mmol/L, Cl^- ≈ 545 mmol/L, Mg²⁺ ≈ 51-53 mmol/L, Ca²⁺ ≈ 10 mmol/L, K⁺ ≈ 10 mmol/L, SO₄^2- ≈ 28-54 mmol/L, HCO₃^- ≈ 2.3 mmol/L). Primitive membranes evolved ion-regulatory mechanisms to sustain electrochemical gradients, enabling metabolic activity. Objectives: This review compares the composition of amniotic fluid (AF) to seawater, framing AF as a "biological ocean" for the fetus, and evaluates the impact of micro- and nanoplastics (MNPs) on this protected milieu. Methods: We synthesized data from published studies on concentrations of and ions and other important substances in AF during pregnancy and compared them with marine values. Reports of MNPs detected in placenta, AF, and human organs were systematically reviewed. Results: AF exhibits high ionic similarity to seawater, although the absolute concentrations of ions are lower, reflecting evolutionary conservation. Recent analytical studies identified MNPs in samples of human placenta (4-10 particles per 1 g of tissue), meconium (median 3-5 particles per g), and AF (detectable in >60% of tested samples). Co-exposure to heavy metals, persistent organic pollutants, and endocrine disruptors were reported in 20-40% of maternal-fetal samples. Conclusions: The analogy between oceans and AF underscores a conserved evolutionary continuum. However, the infiltration of MNPs into intrauterine environments is a novel toxicological challenge with potential implications for neurodevelopment, immune programming, and epigenetic regulation. Within the One Health framework, protecting AF from anthropogenic contaminants is as critical as safeguarding marine ecosystems.