DEVELOPMENTAL EXPRESSION OF THE CD15-EPITOPE IN THE BRAIN-STEM AND SPINAL-CORD OF THE MOUSE

The expression of the 3-fucosyl-N-acetyl-lactosamine (FAL, CD15, Le(x) ) epitope has been examined immunohistochemically in paraffin sections of the brainstem df the mouse. The initial appearance of labelling wa s at embryonic day (E)11, when immunoreactivity of radial glial fibres was noted in the medulla, pens and dorsal isthmus, while the spinal c ord was immunoreactive from E12. Labelling remained faint until E14, w hen a distinctive radial pattern appeared in the medulla, pens and spi nal cord. Immunoreactivity at E14 in both the spinal cord and medulla was strongest in a band in line with the sulcus limitans, passing vent rolaterally through the nucleus of the solitary tract in the case of t he medulla. In both the spinal cord and the medulla, CD15 immunoreacti vity divided the brainstem into radially arranged compartments, with a n immunonegative paramedian region, strongly labelled ventrolateral se gments, and moderately labelled lateral regions. Labelling of midline radial fibres was also apparent in the ventral mesencephalon at E14. A fter E18, labelling appeared in the paramedian region of the medulla, in particular around the inferior olivary nucleus, and gradually adopt ed a tufted appearance throughout the brainstem. Several regions of th e developing brainstem showed specific labelling during fetal life. Di stinct labelling of the developing red nucleus was visible from E15 to the time of birth, while some longitudinal bands of labelling were no ted in the deeper layers of the superior colliculus from E17 until pos tnatal day (P)2. The adult pattern of immunolabelling was achieved by the end of the second postnatal week. The striking concentration of CD 15 along the sulcus limitans of both the spinal cord and brainstem may serve to demarcate and separate dorsal (sensory) and ventral (motor) columns in a similar fashion to Pax gene expression. The precise timin g of transient CD15 expression in the developing inferior olive and re d nucleus is suggestive of a role for this epitope in developmental ev ents of those structures.