Developmental fates of shark head cavities in the musculoskeletal system of extraocular muscles. a Left lateral view of the shark embryos at st. 25 (top) and st. 31 (bottom). b, c Schematic drawings of a left lateral view of the inset in (a) and a transverse section at the level of the dashed line in (a) showing HCs arranged along the anteroposterior axis (b) and the dorsoventral axis (c) in the embryonic head at st. 25. d, e Schematic drawings of a left lateral view of the inset in (a) and a transverse section at the dashed line level in (a) showing musculoskeletal connections in EOMs via tendons at st.31. The colors of each musculoskeletal component correspond to its developmental origin, shown in (b and c). Yellow dotted arrows represent the dynamic migratory pathways of mnc-derived cells reconstructed based on the results of the present study. The developmental fate of CNC cell-derived tendons (light blue) and mesodermal chondrocranium (pink) are based on the prediction. II, optic nerve; dic, diencephalon; gV, trigeminal ganglion; hyc, hyoid head cavity; insLR, a tendon at the insertion of the lateral rectus muscle; IO, inferior oblique muscle; IR, inferior rectus muscle; LR, lateral rectus muscle; mam, mandibular arch mesoderm; mnc, mandibular head cavity; MR, medial rectus muscle; nt, notochord; op, optic cup; orb, orbital cartilage; oriRcMs, a tendon at the origin of four-rectus muscles; otc, otic capsule; pa, pila antotica; pcc, parachordal cartilage; pm, PmE, Platt’s ‘muscle E’; prmc, premandibular head cavity; rhc, rhombencephalon; Rp, Rathke’s pouch; SO, superior oblique muscle; SR, superior rectus muscle; trb, trabecular cartilage. Scale bars = 1 mm. Not to scale (b-e).