The development of the human motor trigeminal complex and accessory facial nucleus and their topographic relations with the facial and abducens nuclei.

Abstract

AbstractThe development of the motor trigeminal complex and accessory facial nucleus was traced in 23 human fetuses (seven and one‐half to 21 weeks, menstrual age; 20.7–p177.5 mm CR length) and a 75‐day infant both by graphic reconstructions and cytologic methods. Positional relationships with the facial and abducens nuclei were determined.Differentiation begins at the junction of the posterior trigeminal and accessory facial nuclei (level of pontine flexure) and proceeds rostralward in the motor trigeminal complex and caudalward in the facial complex. The neurons that differentiate earliest complete their migration first and migrate the least from their original position. Thus the posterior trigeminal nucleus is most dorsally and the accessory facial cell groups are most medially located. Migration of the trigeminal complex is completed by eitht weeks and of the facial complex by ten to ten and one‐half weeks.Correlations are made between the differentiation of the motor neurons of the trigeminal and facial nerves and: (1) the development of the muscles that they have been postulated to supply; (2) the known function of these muscles; (3) the development of reflex activity in these muscles; (4) the mammalian experimental and human pathologic evidence for their innervation. These correlations support the view that the posterior trigeminal nucleus supplies the anterior belly and the accessory facial nucleus the posterior belly of the digastric muscle and that the posterodorsal part of the motor trigeminal nucleus innervates the mylohyoid muscle. Furthermore, the sequence of differentiation of the trigeminal complex indicates that the ventral trigeminal muscles (floor of the mouth) are represented primarily dorsally and posteriorly in the trigeminal complex and the progressively more dorsal muscles increasingly farther ventrally and anteriorly.The three subdivisions of the motor trigeminal nucleus are probably related to the three major embryonic muscle masses that they supply early in development. Because the inconstant subgroups in the three subdivisions appear when the complixity of reflex activity increases, perhaps they represent functional units rather than single muscles.The position of the abducens nucleus varies widely and no consistent migration cephalad occurs. It may reach caudal motor trigeminal nuclear levels early in fetal life or remain at caudal levels of the facial nucleus postnatally. No accessory abducens nucleus was identified. The neurons in the location allocated to it in some mammals develop from the common cell column between the motor trigeminal nucleus and the facial nucleus and constitute the accessory facial nucleus. Possible explanations for the widely different number of cells in the posterior trigeminal and accessory facial nuclei are discussed.