Making waves with hairs.

The skin is like a canvas. Pigment distribution, skin appendages, skin ridges, etc. are arranged on the skin surface. The arrangement may be random but every now and then they form specific patterns. In some occasions, the specific arrangement is important for organism function or message display. In all occasions, these distinct arrays provide us with windows into the fundamental processes of pattern formation. Skin patterns can be studied by noninvasive visual observations, and dynamic changes can be recorded in vivo for analyses. The hair distribution pattern consists of multiple hair primordia with different developmental stages, directionality, and spacing. Hair primordia on the skin can be randomly distributed, de-synchronized in hair cycling, or coordinated to give rise to a higher level pattern. When there are incremental changes of developmental stages or hair orientation from one primordium to the next, they give the impression of a wave. These waves can be parallel to each other and form stripes or radiate out from focal centers, thus forming whorls as seen in fingerprints and hair patterns on the scalp. Do these patterns result from precise genetic coding or self-organizing cellular events? Patterns such as fingerprints have similar attributes (similar width, organization plan) but they are non-identical (with enough difference to be used as individual identifiers). In fact fingerprints among monozygotic twins, while more similar than nonrelated individuals, are still different (Jain et al, 2002). Thus there is a non-genetic component in the formation of fingerprints. The hair whorls in the scalp of monozygotic twins have not been documented previously and in this issue there is a case report (Paine et al, 2004). Whorls: In human fetuses lanugo hairs form whorl patterns both on the scalp and trunk skin (Gworys and Domagala, 2003). On the thoracic wall there are lanugo whorls that begin bilaterally over the nipples. The whorls collide and merge along the midlines. In adults, whorl patterns are distinct only on the parietal scalp. Ziering and Krenitsky (2003) identified five distinct hair whorl patterns in their study from almost 500 males (Fig 1A): S (75%) and Z (11%) patterns refer to the clockwise and anti-clockwise orientation of the whorl, respectively. The DSZ (Double SZ, 3%) pattern describes the presence of two whorls: one with an S and another with a Z orientation. DSS (Double SS, 0.6%) describes a pattern with two separate S whorls. Hair in the diffusion pattern (9.8%) has no obvious orientation. The ‘‘tightness’’ of the whorl can also vary. When ethnic groups are analyzed, the more interesting difference is that 80% of African-Americans showed diffusive type and 29% of Asian-Americans showed counter-clockwise type whorls. However, in both cases, n is only around 25. Similarly, the number of female subjects is too small to be conclusive, although preliminary data suggest that females predominantly have a diffusion pattern. So, is the whorl pattern genetically controlled? Paine et al (2004) showed that one of the monozygotic twins has an S type pattern, while the other has a DSZ pattern. The monozygosity of the twins was verified by PCR analysis. Therefore, there must be an epigenetic component in the determination of the hair whorl. While conserved molecular pathways underlie all hair follicle formation, local environmental and fortuitous factors can influence the final hair pattern. Skin generally follows inherited linear patterns (Blaschko lines). These are seen in the hairless streaks on the scalp of patients with oral–facial–digital syndrome (Happle et al, 1984), suggesting that the wave of hair follicle formation may be lineage related. There also have been attempts to associate the direction of hair whorls with rightor lefthandedness (Klar, 2003). More cases will have to be analyzed in this interesting area to more fully appreciate this relationship. Waves: Hair waves may represent patterning events in time etched in space (on the skin surface). In adult animals, since hairs go through constant molting and regeneration, the integument reflects the dynamic cell behaviors underlying hair cycles in vivo. The majority of postnatal human hair growth seems to be de-synchronized. In a rare case, a 4-y-old boy was reported to have regions of scalp hairs exhibit shortened anagen and continuous synchronized hair cycles (Thai and Sinclair, 2003). Several recent works show interesting wave-like hair growth in mice. Visualizing hair waves is facilitated by hair cycle dependent changes in integument pigmentation and hair loss in nude mice and Msx2 knockout mice. Militzer (2001) analyzed more than 400 nude mice on albino (NMRI, foxn1) and pigmented (C57BL/6, foxn1) backgrounds for more than one year. Pink skin turns dark when hairs enter anagen and turns pink again when anagen is completed. Skin pigmentation changes progress in a wave-like fashion on the skin surface of these mice. When mice are young, all hairs cycle synchronously, but with increasing age the hair cycles over different regions desynchronize. Thus, the skin pigment pattern breaks into distinct stripes and patches. As mice age, the stripes and patches become narrower/smaller and eventually appear random. Domagala, personal communication.