The Puppeteers have a pattern of hair growth derived from the Sonic Hedgehog (SHH) expression. The hair is induced by the interaction between underlying mesoderm with competent epithelium. Initiated by reciprocal inductions, SHH facilitates contraction of the overlying epithelium, while BMP proteins restrict the condensation of epithelium and stop the formation of hair follicles. SHH is expressed in the region overlying the braincase and extends posterior over the spinal cord. However, SHH is necessary for the formation and preservation of the condensed mesenchyme, called dermal papilla; hence, it does not diffuse very far because it acts locally by diffusion in high concentrations in the hair germ around the braincase and along the torso to induce hair follicle formation in the alien’s developing embryo.

Above image created by Sue Kim

The dermal papilla induces local thickenings, called the epidermal placode, to grow and multiply, eventually becoming a hair germ precursor. The primitive hair germ proliferates down into the mesenchyme, encompassing the dermal papilla and forming the hair shaft. The inner hair root develops directly above the papilla, and the dermal papilla pushes upward against the hair root cells, promoting further division. Melanocytes then transfer their pigment to the growing hair, resulting in the brown color of the mane.