Microarchitecture and innervation of the human latissimus dorsi muscle

Re-investigation of the adult human latissimus dorsi muscle (HLD) by microdissection of acetylcholinesterase (AChE)-stained material revealed a complicated microstructure of this muscle. Motor endplates distribute over the entire muscle; numerous AChE-stained myomyonal and myotendinous junctions are interspersed. After teasing muscle fascicles from selected representative areas, the following results were obtained. Most of the single muscle fibers constituting the fascicles are shorter than the length of the fascicles. They are linked together by myomyonal junctions or by myotendinous intersections end-to-end, end-to-side, or via muscular crossbridges side-to-side, thus forming chains and nets of varying appearance and length. Additionally, many fiber furcations were found. These observations throw light on the microarchitectural basis of local mass changes of the triangular HLD occurring during its convergence from its wide lumbar and thoracic attachment line to its narrow tendon attached to the humeral crista tuberculi minoris. The microstructural observations also explain why motor endplates spread over the whole muscle, instead of being restricted to a single central endplate-band, as is found in the majority of other muscles. To clarify postnatal development, the topography of endplates in adult muscles was compared with fetal and infantile muscles. Primarily, the endplate bands were clearly demarcated; their width then broadened, and they gradually scattered into groups of single endplates. Simultaneous with these developmental changes, a corresponding branching of the thoracodorsal nerve occurs.