Rules of tissue packing involving different cell types: human muscle organization

Natural packed tissues are assembled as tessellations of polygonal cells that do not leave empty spaces between them. They include the epithelial sheets and the skeletal muscles. Epithelia are formed by equivalent cells that change shape and organization through development. The skeletal muscles appear as a mosaic composed by two different types of cells: the slow and fast fibres that are determined by the identities of the motor neurons that innervate them. Their relative distribution is important for the muscle function and can be altered in some neuromuscular diseases. Little is known about how the spatial organization of fast and slow fibres is established and maintained. In this work we use computerized image analysis and mathematical concepts to capture the organizational pattern in two different healthy muscles: biceps brachii and quadriceps. Here we show that each type of muscle portrays a characteristic topological pattern that allows distinguishing between them. The biceps brachii muscle presents a particular arrange based on the different size of slow and fast fibres, contrary to the quadriceps muscle where an unbiased distribution exists. Our results indicate that the relative size of each cellular type imposes an intrinsic organization into the tissue. These findings establish a new framework for the analysis of packed tissues where two or more cell types exist.