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Indian Journal of Dermatology, Venereology, Leprology, Vol. 71, No. 1, January-February, 2005, pp. 57-59 Resident's Page Lines of Blaschko Tagra Sunita, Talwar AmritaKaur, Walia Rattan LalSingh Departments of Skin and V.D. Govt. Medical College, Patiala Code Number: dv05020 INTRODUCTION The lines of Blaschko represent a pattern assumed by many different nevoid and acquired skin diseases on the human skin and mucosae.[1] These lines do not correspond to any known nervous, vascular or lymphatic structures but represent the developmental growth pattern of the skin.[2] The epidermis and its appendageal structures such as melanocytes, vascular system, fatty hypoderm, all, separately or in combination, may be involved in the morphological manifestations which follow the Blaschko′s lines. The embryological basis of these lines is not clear.[1] HISTORICAL BACKGROUND The lines of Blaschko were described and drawn in 1901 by Alfred Blaschko (1858-1922), a private practitioner of dermatology in Berlin.[3] He examined more than 140 patients with nevoid and acquired linear skin diseases and carefully transposed the pattern in each patient onto dolls and statues. The lines thus obtained were schematically transferred to the back and front view of the human body. A composite diagram of these distribution patterns was then drawn that has subsequently been referred to as the lines of Blaschko. His original description of the lines has been translated as ′a system of lines on the surface of the human body which the linear nevi and dermatoses follow′.[1] About 75 years later, Jackson provided a detailed review of the 1901 publication and introduced the concept of the lines of Blaschko in the English language medical literature.[1] DESCRIPTION OF THE LINES OF BLASCHKO In Blaschko′s lines [Figure - 1] and [Figure - 2], there is a V-shape over the upper spine, an S-shape on the abdomen, and an inverted U-shape from the breast area onto the upper arm. There are perpendicular lines down the front and back of the lower extremities.[1] As the distribution of the lines is linear, these were earlier thought to represent Koebner′s phenomenon but the curvature of the lesions does not support this hypothesis.[4] The lines of Blaschko are less well-defined on the head and neck. Happle et al[5] added lines to the posterior scalp whereas Bolognia et al[4] further delineated the lines on the lateral aspect of the face and neck. The anatomic equivalent of Blaschko′s lines has been described in the teeth and eyes as well.[4] Blaschko′s lines have been confused with dermatomes most commonly as both the distribution patterns are characterized by a striking demarcation of cutaneous lesions at the midline. But on close comparison the two do not seem to be related, though on the upper extremities there is a resemblance to the anatomical location of the motor nerves.[1],[4] Similarly, Blaschko′s lines can be differentiated from Voigt′s lines (the boundaries of the areas of distribution of the main cutaneous nerve stems) as these lines do not explain the V-shape on the back or the S-shape on the abdomen.[1],[4] BASIS OF DISTRIBUTION PATTERN OF THE LINES OF BLASCHKO The embryological basis of distribution pattern of these lines is so far an enigma, but certainly these are distinct from other known linear patterns. They do not relate to any vascular or lymphatic structures in the skin.[1],[4] Jackson suggested that their localization is partly determined by the dermal tissues.[1] An inborn error of morphogenesis due to a single mutant gene[6] or stretching of the skin during embryogenesis[4] have also been proposed as the underlying mechanisms. Their characteristic distribution pattern points to the presence of two different clones of cells in early embryogenesis. Thus, possibly, these lines represent a form of human ′mosaicism′ where two or more genetically distinct cell populations are present in an individual derived from a single zygote. These different clones may be due to lyonization (random inactivation of one of the two X chromosomes in all cells in females), post-zygotic somatic mutations during early embryogenesis or gametic half chromatid mutation occurring before fertilization.[1],[4] But no single theory as yet clearly elucidates the localization of Blaschko′s lines. DERMATOSES FOLLOWING THE LINES OF BLASCHKO Several congenital and acquired dermatological conditions are known to follow the lines of Blaschko. These dermatoses have been grouped as genodermatoses [Table - 1], congenital and/or nevoid conditions [Table - 2], and acquired conditions [Table - 3]. There are recent reports of the occurrence of the erythematous exanthem of scarlet fever along these lines in a child with hypomelanosis of Ito,[15] and of eosinophilic cellulitis[16] along these lines. Contrary to earlier belief, recently cases of linear scleroderma, both of the fronto-parietal type and those occurring on limbs, have been reported in a Blaschko pattern.[4],[14] Becker′s nevus, segmental neurofibromatosis, zosteriform leukemia cutis, cutaneous metastases and angiolipomas probably do not follow the lines of Blaschko.[4] There have been successive theories to explain Blaschko′s lines each more convincing than the previous one, but till date mosaicism remains the most widely accepted one. However, no definitive conclusion has been reached and therein lies scope for research. REFERENCES
Copyright 2005 - Indian Journal of Dermatology, Venereology, Leprology The following images related to this document are available:Photo images[dv05020t3.jpg] [dv05020f2.jpg] [dv05020t2.jpg] [dv05020t1.jpg] [dv05020f1.jpg] |
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