This work presents ψ-Biocosmos, a retentive cross-domain framework exploring potential structural continuities between cosmological persistence and biological tissue stability. Drawing on observational analyses from Euclid pre-DR1 environments, Rubin LSST lensing fields, NASA SciX structural clustering, and emerging biophysical modelling, the study examines whether retention—understood as the preservation of structural difference—may function as an organising principle across distinct physical scales. A bio-retentive lagrangian is introduced, formally parallel to the Λψ-based cosmological retentive models. From this formulation, two structural parameters are defined: ψB (biological retentive rigidity) and ψbio (bio-topological retentive density). These parameters reflect conceptual analogues of the cosmological Δψ-floor and Ξ-node topology, suggesting a possible shared architecture underlying the persistence of structure in both cosmic and biological systems. The work does not claim empirical universality. It is offered with scholarly humility as a theoretical contribution, proposing a structured formalism for future investigation into retention as a potential scale-invariant feature of nature.