Electronmicroscopic observations on Auerbach's plexus in a 12 mm human embryo.

Abstract

In a human embryo of 12 mm crown-rump length, the morphology of the primordial plexus of Auerbach in the small intestine was studied by the aid of the electronmicroscope. At this stage of development, no muscle layers have as yet differentiated. In the outer half of the intestinal wall, small disseminated cell clusters appeared, associated with blood capillaries and encapsulated by a single layer of spindle-shaped cells presumably corresponding to the capsular layer of Auerbach's plexus in the adult. This capsule is separated from the cell cluster by a subacpsular space. The cell clusters are made up of neuroblasts, developing Schwann cells, and processes emitted by both cell types. The cytoplasm of the neuroblast contains a Golgi complex, mitochondria, endoplasmic reticulum with or without osmiophilic particles, free RNA-particles, tubular structures and densecore bodies. In the cytoplasmic area at the base of the neural process, the cytoplasmic elements mentioned above are especially abundant. The axoplasm of the neuronal process contains dense-core bodies as well as tubular structures having a fairly uniform diameter of 200 to 250 A and extending into the neural process from the perikaryon. The dense-core bodies found in both the perikaryon and the axoplasm of the neuroblasts are similar in size and in shape to the dense bodies described by many authors in mature neurons and assumed to contain neurotransmittors. In addition to the dense-core bodies, the neuroblastic processes occasionally exhibit accumulations of small clear vesicles comparable to those commonly found in synaptic structures in adult animals. None of other structural details characterizing synaptic junctions could be identified. As yet, there is no evidence that the first appearance of either dense-core or small clear vesicles coincides with the onset of neuronal activity and interneuronal impulse transmission.