This study is concerned with the embryonic development of the hindbrain in the rat (Rattus rattus norvegicus albinus). It concentrates on the contribution of the roof plate of the rhombencephalon in the embryonic development. It covers the period from the embryonic day 15 till birth. The aim of the study is to identify the morphological changes, histological development and the expression of glycoconjugates in this region. Embryonic study of the relevant developmental stages was done on (68) rat embryos. Haematoxylin and Eosin stains were used to study the morphological and the histological features. Molecular probes for the glycoconjugates were used in the form of the fluorescence isothiocyanate conjugated lectins (SBA, WGA& Con-A). This study showed that the caudal part of the roof of the fourth ventricle forms what is called the “Rhombencephalic demilune” forming the caudal and the lateral boundaries of the dorsal metencephalic anlage. This term should replace the term “germinal trigone” because of better morphological and embryological designation. Glycoconjugates’ expression in the Rhombencephalic demilune was examined. N-acetylegalactoseamine was found to be a marker for the developmental events showing a spatiotemporal variation in its expression. It was expressed intracellularly during the embryonic days (17-21), in the
regions of the Rhombencephalic demilune, the external germinal layer and the Purkinje cell migration wave layer. N-acetylegalactoseamine also served as navigation cue for the formation of the primitive cerebellar cortex particularly the external germinal layer. Mannose expression on glycoconjugates was demonstrated intracellularly from the embryonic days (17-21), in the regions of the Rhombencephalic demilune, the external germinal layer and the Purkinje cell migration wave layer. The mannose expression seems to play a role in the neurite outgrowth from the embryonic neurons. Binding of WGA demonstrating the N-acetyleglucoseamine was different from the binding of Con-A and SBA. It didn’t exhibit the variation in the intensity; location or timing that justifies its use as an embryonic marker. This work emphasizes the role-played by the glycoconjugates in the embryonic development as markers of migration, histological differentiation and cell process dynamics in particular during complex embryonic development taking place in the dorsal metencephalic anlage