Human fleshy muscle-bone attachments are histologically distinct from classical entheses: A comparative exploratory...

Tendinous muscle-bone interfaces (entheses) are well characterized histologically, but fleshy attachments, where skeletal muscle anchors to bone without a macroscopic tendon, remain poorly defined. This exploratory study described the histology of fleshy attachments, compared them directly with tendinous entheses processed identically, and tested whether a reproducible morphometric framework was feasible.

Muscle-bone blocks from seven sites were taken from six formalin-fixed elderly male donors, spanning tendon-to-prominence entheses (biceps brachii, tendocalcaneus), linear fleshy attachments (external intercostal, brachioradialis, external oblique) and broad fleshy attachments (infraspinatus, brachialis). One block per site was decalcified, sectioned at 8μm, stained with hematoxylin-eosin or Masson trichrome and examined by light microscopy. Qualitative zonal description was the primary aim; pilot automated morphometry was a feasibility exercise.

Entheses showed the classical four-zone fibrocartilaginous gradient (dense tendon, uncalcified and calcified fibrocartilage, lamellar bone) separated by a serrated tidemark. Fleshy attachments showed a three-zone fibrous pattern (muscle, dense connective tissue, bone) without fibrocartilage; collagen bundles met bone at acute angles through Sharpey-fiber-like interdigitations, and the external oblique was mixed. Pilot morphometry showed cell/lacuna-like void density rising from cell-sparse entheses to cell-dense fleshy attachments.

Across fibrocartilaginous entheses, fleshy fibrous attachments and transitional forms, the findings support a provisional three-pattern framework in which fleshy attachments transfer force through a specialized periosteal continuum rather than fibrocartilage. Given the small single-sex cohort and pilot morphometry, the results are hypothesis-generating and need confirmatory validation.