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

Abstract

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" (Folia Morphologica 2012; 71:154). However, the study concentrated on the hand and contained no sections of near-term fetuses. The present examination of paraffin-embedded tangential sections along the planta from 25 embryos and fetuses at 6-40 weeks (15-320 mm crown-rump length or CRL) demonstrated that, at 8 weeks, the initial foot LU appeared in the proximal side of the common tendinous plate of all five deep tendons. 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 fiber 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 fibers 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 fetus after 30 weeks. Consequently, in contrast to the hand LU, the delayed changes in the foot were characterized by involvement of the LU origin into a single common part of the FDLT. The quadratus plantae muscle fibers did not attach to the LU at any phase, and connected with the fourth and fifth toe tendons.

Ontogenic transformation of the ankle from the initial mediolateral arrangement of the calcaneus and talus: A histological study of human embryos and early fetuses

“…A robust calcaneus is required for the heel-contact phase of bipedal walking, and the nervous system seems to control growth phase-specific muscle activity (Jin et al, 2021;Sugai et al, 2021). At a GA of 9 to 10 weeks or later, the talus "rode over" the calcaneus, but the medial or tibial part still faced the plantar subcutaneous tissue because of the relatively small sustentaculum tali.…”