Development and growth of the foot lumbricalis muscle: a histological study using human foetuses

Background: Our group has shown early development of the hand lumbricalis and hypothesized that, at midterm, the lumbricalis (LU) bundles flexor tendons to provide a configuration of “one tendon per one finger” (Cho K.H. Folia Morphol. 2012; 71, 3: 154–163). However, the study concentrated on the hand and contained no sections of near-term foetuses.Materials and methods: The present examination of paraffin-embedded tangential sections along the planta from 25 embryos and foetuses at 6–40 weeks (15–320 mm crown-rump length) demonstrated that, at 8 weeks, the initial foot LU appeared in the proximal side of the common tendinous plate of all five deep tendons.Results: After midterm, a drastic three-phase change occurred at the muscle origin: 1) the LU originated from each of the flexor digitorum longus tendon (FDLT), but abundant tenocyte candidates separated the muscle fibre from the tendon collagen bundle; 2) the LU arose from the covering fascia depending on increased thickness of the muscle; and 3) the LU muscle fibres intermingled with tendon collagen bundles and partly surrounded the tendon. Simultaneously, a dividing site of the FDLT migrated distally to accelerate the changes at the LU origin. These phases did not always correspond to the size of foetus after 30 weeks.Conclusions: Consequently, in contrast to the hand LU, the delayed changes in the foot were characterised by involvement of the LU origin into a single common part of the FDLT. The quadratus plantae muscle fibres did not attach to the LU at any phase, and connected with the fourth and fifth toe tendons