A morphological and histochemical study of the developing tongue musculature in the mouse: Its relationship to palatal closure

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Abstract
Some question exists concerning the ability of the embryonic tongue to undergo reflex movements at the time of palatal closure (15.5 days of development). Functional motor endplates are prerequisite for such movements to occur. Light and ultrastructural cytochemical methods were employed to elucidate the morphology of neuromuscular relationships in the developing mouse tongue. The A/Jax mice used in the experiments demonstrated a 12–20% incidence (seasonal variation) of spontaneous cleft palate, allowing a correlation between normal and teratological processes. Organized myofibrils were first seen in tongues of normal and spontaneous cleft lip‐cleft palate (SCL‐CP) specimens at 14.5 days of development. The thiocholine technique of Karnovsky and Roots was used to demonstrate acetylcholinesterase (AChE) activity at the light microscope level. The Lewis and Schute method was used for ultrastructural localization of this enzyme. Tissues from normal and SCL‐CP specimens from 12.5 to 20.5 days of gestation failed to show differences in amounts or distribution of AChE activity. AChE activity was seen as early as 14 day's gestation. Electron microscopic studies demonstrated reaction product in the endoplasmic reticulum and nuclear envelope of developing myoblasts. AChE activity at the developing neuromuscular junction and the occurrence of myofilaments preceded palatal closure by several days.Based on these morphological and histochemical findings the tongue of normal and SCL‐CP embryos appears capable of responding to a neurogenic stimulus at the time of palatal closure. The findings suggest that the tongue of animals exhibiting a spontaneous cleft palate is not actively involved in the etiology of this condition.