h₃ost-
“bone, skeletal element”bone, skeletal structure
Root for bone, yielding Latin os/ossis, Greek osteon, Sanskrit asthi.
Discussion
The PIE noun *h₃ost- (bone, skeletal element — genitive *h₃estós or *h₃sth₁-ós) produced the word for bone across the major branches with a regularity that places it among the most secure body-part reconstructions. The initial laryngeal *h₃ is evidenced by the o-colouring visible in several reflexes.
Latin os (bone — genitive ossis, from an earlier *oss < *ost-s) continues the root and generated the English medical and scientific vocabulary: osseous (pertaining to bone), ossify (to turn into bone — used both literally and figuratively, as in "ossified thinking"), ossuary (a repository for bones), and periosteum (the membrane "around the bone"). The word osteopath (Greek ostéon + páthos) and the combining form osteo- in medical terminology derive from the Greek cognate rather than Latin.
Greek ostéon (ὀστέον, "bone") preserves the root with a thematic extension and is the more productive form in modern English borrowings: osteoporosis ("porous bone" disease), osteology (the study of bones), osteopathy (bone-treatment), and osteocyte (bone cell). The Greek form displaced the Latin form in most scientific coinages.
Sanskrit ásthi (bone — genitive asthnás) continues the root in the Indo-Iranian branch with the expected alternation between full-grade and zero-grade in the paradigm. The Vedic texts mention bones in ritual and medical contexts.
Hittite ḫastāi (bone) provides Anatolian attestation with the initial laryngeal preserved as ḫ — crucial evidence for the *h₃ reconstruction.
The Germanic branch does NOT continue this root for "bone" — English bone (OE bān) is from a different PIE root, and German Knochen is also unrelated. This replacement of the inherited bone-word in Germanic parallels the replacement of the inherited horse-word: a core vocabulary item unexpectedly abandoned. English recovers the PIE root only through the Greek and Latin learned borrowings (ossify, osteoporosis).
Notes
LIV h3esth1-; Pokorny 783. Widely attested across branches.