Development of the springbok skin — colour pattern, hair slope and horn rudiments in Antidorcas marsupialis
In the foetal springbok skin, identifiable melanocytes are present before colour patterns are established. These pigment cells fail to appear in the pouch and belly areas, indicating that these zones are piebald and not albinotic. Areas of regional colour differences are sharply separated. Their distinctiveness is caused by a discontinuous change in brightness of the hairy coat. This is due to a stepwise shift in the average quanta of melanin in the hairs. The pigment itself does not alter its colour from one colour-zone to another, but the density of the pigment particles merely changes. It is suggested that the steps could correspond to a discontinuous difference in tissue dosage, resulting perhaps from differing activation among multiple melanizing genes. Anatomical peculiarities of the pouch area and horn rudiment are described. A proposal is made that hair slope is brought about by traction from mobile superficial fibroblasts. These cells travel preferentially in the direction of the greatest tissue strain. By dragging the papillary layer components along behind them, such as the hair buds and arrector pili muscles, the hair streams could arise. The tips of the hair shafts will then point away from each direction of greatest internal expansion, which the fibroblasts strive to cover.