The Role of the Basement Membrane in Basal Tissue Folding

The ability of organs and tissues to function properly is reliant on the correct formation of shape. Proper morphogenesis is carefully regulated by both intracellular dynamics and extracellular mechanical forces. Using the highly conserved zebrafish midbrain-hindbrain boundary (MHB), we investigated the role of the basement membrane, a specialized extracellular matrix that basally lines the tissue, in mediating cell shapes and mechanical tissue stresses during basal tissue folding. A primary component of the basement membrane is laminin-111, a heterotrimer composed of the alpha1, beta1 and gamma1 chains, encoded by lama1, lamb1a, and lamc1 genes respectively. We investigated the role of each laminin-111 gene in mediating cell and tissue shape during MHB morphogenesis and discovered a previously unidentified hierarchy between laminin-111 genes, revealing lamc1 as the most critical for basal tissue folding. The same hierarchy was observed when we analyzed myosin localization within the tissue. In addition to functions in cell and tissue shape, the basement membrane, and laminin-111, have known roles in mediating mechanical tissue stresses. Our study characterized mechanical stresses during basal tissue folding using a novel and innovative microdroplet-based force sensor technique. We identified a region specific increase in mechanical tissue stress directly at the fold of the MHB compared to surrounding regions of the tissue. Stresses were oriented towards the midline of the embryo in the same direction as the tissue fold. Lastly, we determined laminin-111 is required for the mechanical tissue stresses and their orientation during basal tissue folding. Together, these studies advance our understanding of tissue morphogenesis, critical for furthering the field of therapeutics and tissue engineering