Close-up of longitudinal section of dermal papilla (DP), which is an invagination of the dermis into the matrix (M).  The DP allows capillaries to gain entrance to the cells of the matrix.  It is the signal transduction and communication between the DP and the matrix that determines how long a hair will grow and how thick a shaft will be produced.  Melanocytes fill the matrix and produce the pigment of the hair.