Extended Data Fig. 10: Development of the forelimb musculature of amniotes, with focus on the ventral hand musculature deriving from the Hand flexor division.
From: Embryonic muscle splitting patterns reveal homologies of amniote forelimb muscles
Development of the forelimb musculature of amniotes, with focus on the ventral hand musculature deriving from the Hand flexor division (dark blue). The hand flexor musculature derives from the distal portion of the Flexor division and stratifies into layers; a superficial one (white bordered blue arrow) forms two layers of muscles, usually grouped as the Mm. flexores digitorum breves. M. lumbricales (*) of the mouse seem to develop from this portion of the Hand flexor division. A deeper layer (light-blue bordered dark blue arrow), also stratified, forms the deepest sets of muscles, termed lumbricales and interossei dorsales and ventrales. In the mouse, the most-superficial set of hand flexor muscles (coloured white in E15.5) elongates and translocates proximally into the forearm to form M. flexor digitorum superficialis. Deriving from the HF, a group of muscles extends dorsally in between the metacarpals in Monodelphis stages MC 33 + and 34 (red bordered blue arrows), likely precursors of M. flexor digitorum breves profundi and/or M. intermetacarpales. Note that the Hand extensor musculature fails to develop as observed in a dorsal view of the mouse forearm in E14.5 and of the short-tailed opossum MC stages 33+ and 34. Also, both the dorsal and ventral hand musculature is conspicuously reduced in Coturnix, developing from a non-stratified muscle division and forming considerable fewer muscles than in other reptiles. DL: deep layer of the Hand flexor subdivision, fbp: M. flexores digitroum breves profundi, fdl: humeral head of M. flexor digitorum longus, fdlu: ulnar head of M. flexor digitorum longus, fds: M. flexor digitorum superficialis, SL: superficial layer of Hand flexor subdivision, *: M. lumbricales. All scale bars are 500 µm.
