Three-dimensional visualization of testis cord morphogenesis, a novel tubulogenic mechanism in development

Testis cords are specialized tubes essential for generation and export of sperm, yet the mechanisms directing their formation, and the regulation of their position, size, shape, and number remain unclear. Here, we use a novel fluorescence-based three-dimensional modeling approach to show that cords initially form as a network of irregular cell clusters that are subsequently remodeled to form regular parallel loops, joined by a flattened plexus at the mesonephric side. Variation in cord number and structure demonstrates that cord specification is not stereotypic, although cord alignment and diameter becomes relatively consistent, implicating compensatory growth mechanisms. Branched, fused, and internalized cords were commonly observed. We conclude that the tubule-like structure of testis cords arise through a novel form of morphogenesis consisting of coalescence, partitioning, and remodeling. The methods we describe are applicable to investigating defects in testis cord development in mouse models, and more broadly, studying morphogenesis of other tissues.