Mikroskopische Untersuchungen zur Struktur des Primitivknotens bei der Ente und dem Huhn
by Johanna Reinermann
Date of Examination:2025-10-06
Date of issue:2025-10-02
Advisor:Prof. Dr. Christoph Viebahn
Referee:Prof. Dr. Dr. Henning Schliephake
Referee:Prof. Dr. Dieter Kube
Persistent Address:
http://dx.doi.org/10.53846/goediss-11538
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Abstract
English
The Hensen’s node plays a central role during gastrulation, as it exhibits asymmetric development shortly after its formation and thereby establishes orientation along the transverse axis as part of the body plan. Acting as an organizer, it serves as a passageway for cells undergoing epithelial-to-mesenchymal transition (EMT) and expresses genes that regulate this process. From stage HH 3+ onward, microcavities can be detected in the primitive node of the chick, appearing as light-blue, matrix-filled spaces usually located near disrupted regions of the basal membrane. These cavities vary in size and shape, are heterogeneously filled with globular, filamentous, and amorphous components, and contain fibronectin. The aim of this study was to determine whether the occurrence of microcavities represents a general phenomenon in avian embryos and to explore their potential function. For this purpose, duck and chicken embryos were explanted, fixed, and prepared for semithin sectioning. Microscopic imaging and systematic documentation were then performed. In all three duck embryos examined, microcavities of various shapes and dimensions were observed, with cavity number increasing with developmental stage and with a predominant localization within or beneath the right node shoulder. While most cavities were embedded in the cell-dense region of the node, a few were found in the looser mesoderm near the hypoblast. In ducks, the cavities appeared small, spindle-shaped, and numerous. Duck embryos at stages HH 5+ and HH 6 displayed a remarkably deep primitive groove, more pronounced than previously described in the literature, and distinct from the subtler groove of the chick. Additionally, the enlarged right node shoulder at stage HH 6 in ducks was particularly striking, protruding dorsally to form a marked epiblast bend. Adjacent to this, a conspicuous epiblast protrusion was observed, seemingly releasing numerous cells. In chicken embryos, no cavities were detected at stage HH 3, but they appeared from stages HH 4–4+, varying widely in size and shape and presenting larger and more vesicular compared to those in ducks. Possible homologous structures may include the notochordal and neurenteric canals, which, unlike the cavities, are lined by differentiated epithelial cells. The Brachet’s cleft of gastrulating Xenopus, characterized by a loose fibronectin-rich matrix, may also represent a homologous structure. Potential functions of these microcavities include the maintenance of asymmetry, stimulation of cell migration, contribution to notochord formation, or regulation of morphogen activity via fibronectin content. Alternatively, they may simply represent remnants of basal membrane disruption caused by EMT.
Keywords: Hensen’s node; gastrulation; primitive streak; primitive groove; microcavities; node shoulder; fibronectin; chick embryo; notochord formation; node shoulder; duck embryo
