Development of Descending Supraspinal Pathways in Man

Abstract

In the spinal cord of pyridine silver stained human embryos of 5–8 weeks of estimated menstrual age, i.e., about 3–6 postovulatory weeks, Windle and Fitzgerald (1937) noted that motoneurons are the first neurons to develop (Fig. 42). They appear in the uppermost spinal segments at approximately E27 (about Carnegie stage 13/14). At this time of development also dorsal root ganglion cells are present. Central processes of the bipolar ganglion cells reach the spinal cord, where they initiate the formation of the dorsal funiculi. At first, the dorsal funiculi are found only in the cervical spinal cord, and are composed of short fibers, but at stage 15 dorsal funiculi are found throughout most of the spinal cord. At stage 18, collateral branches of primary afferent fibers emerge from the lateral aspect of each dorsal funiculus in the brachial region. A few long collateral branches pass into the lateral division of the ventral horn at stage 20. At this stage of development the ventral funiculus may contain descending axons from the brain stem passing via the flm. Most of its other fibers are probably ascending, however (Rhines and Windle 1941). Interneurons with ascending projections send their axons to the floor plate where they cross in the ventral commissure and form contralateral ascending tracts in the ventral funiculus. Therefore, three components of cutaneous reflex pathways (primary afferent fibers, interneurons and motoneurons) are already found in a human embryo of 4 postovulatory weeks. A rapid differentiation of these components takes place in embryos of 6 postovulatory weeks. The dorsal funiculus has reached the caudal brain stem at stage 16, i.e., about 37 postovulatory days (Müller and O’Rahilly 1989a). Cuneate and gracile decussating fibers forming the medial lemniscus are present at stage 20 (Müller and O’Rahilly 1990a,b). Three types of primitive sensory neurons, presumably transient structures, were described by Humphrey (1944, 1947). Human embryos are capable of movement by the time they have attained a length of 20–21 mm, i.e., at about 7.5 weeks estimated menstrual age or about 5.5 postovulatory weeks (Fitzgerald and Windle 1942; Hooker 1952; Humphrey 1964).