Friction Skin Growth
Development of Friction Ridges (as described by William J. Babler, PHD in his article " Embryologic Development of Epidermal Ridges and Their Configurations"- Birth Defects: Original Article Series 27(2): 95-112, 1991)
The initial regression of volar pads around 10-11 weeks corresponds to the initial formation of epidermal ridges. At this time the epidermis is a smooth, thin layer of tissue both at the skin surface and on its deep surface at the epidermal-dermal junction. Epidermal ridges first appear as localized cell proliferations in the basal (deep) layer of the epidermis during the
10th week postfertilization. These cell proliferations form shallow primary ridges that project into the superficial layer of the dermis. The number of primary ridges increases as new ridges are formed between or at the lateral surface of existing ridges. Primary ridges proliferate rapidly to keep pace with the increasing separation of adjacent ridges due to general growth of the hand. This proliferation produces the branchings and islands, the minutiae.
10th week postfertilization. These cell proliferations form shallow primary ridges that project into the superficial layer of the dermis. The number of primary ridges increases as new ridges are formed between or at the lateral surface of existing ridges. Primary ridges proliferate rapidly to keep pace with the increasing separation of adjacent ridges due to general growth of the hand. This proliferation produces the branchings and islands, the minutiae.
As primary ridges begin to develop, they define the basic ridge configurations of the volar skin surfaces. However, these configurations develop at the epidermis-dermis interface and not on the skin surface. As the number of primary ridges increases, the ridges continue to increase in dimension. Primary ridges increase in width and penetrate deeper into the underlying dermis.
Around 14 weeks, sweat glands appear at uniform intervals along the ridges. They continue to grow and penetrate deeper into the dermis. The association between primary ridges and sweat glands has resulted in the term glandular fold being used often in reference to primary ridge. The primary ridge, or glandular fold, corresponds to the surface ridge that we see.
At approximately 15 weeks secondary ridges, lacking sweat glands, appear. The secondary ridges, or furrow folds, correspond to the furrow of the surface ridge. Primary ridges cease proliferation as soon as secondary ridges begin to form. By 17 weeks the human fetus has an epidermal ridge configuration that is comparable to that of an adult. At the end of this period of time, epidermal ridges become visible on the volar surface as fingerprints.
From 17 - 24 weeks, secondary ridges continue to proliferate until they are in a one to one correspondence with primary ridges. They develop similar to primary ridges. At 24 weeks, the epidermal ridge system has an adult morphology.
Dermal papillae begin to develop around 24 weeks. Until this time the morphology of primary and secondary ridges is a smooth ridge of tissue. As the depth of the secondary ridges approximates that of the primary ridges, additional changes can be seen. At this time bridging or anastomoses between primary and secondary ridges begin to appear. The dermis between anastomitic epidermal bridges progressively forms peg-like structures, the dermal papillae characteristic of the definitive dermal ridge.
The process of primary ridge formation is not a generalized event that occurs simultaneously across the volar aspect of the hand. Rather, ridge formation initiates at several points and spreads out such that developing ridge "fields" ultimately meet. The fingers are the earliest sites of ridge formation. Ridge differentiation spreads proximally from fingertip to palm and in a radioulnar (thumb to little finger) gradient. Observational data suggest that on the finger, the centre of the apical pad is the first region to demonstrate primary ridge formation. Once ridge formation begins, the number of ridges proliferate from these sites to meet adjacent ridge "fields".
Factors That Influence Ridge Configuration (Differential Growth)
Growth Stress
Kollmann was the first to examine the question of what determines the alignment of epidermal ridges. He suggested that ridge direction was greatly influenced by growth stresses and compressions in the developing skin. L.S. Penrose (Medical significance of fingerprints and related phenomenon. Br Med J 2:321-325, 1968) extended this hypothesis to state that ridges align at right angles to the compression forces acting on the growing volar surface. This concept has received considerable attention from a mathematical perspective. These include measurement of the curvature of epidermal ridges and their configurations.
Volar Topography
K. Bonnevie (Studies on papillary patterns of human fingers.J Genet 15:1-111, 1924) first noted the correspondence between the height of a volar pad and the special ridge configuration of its pattern. She also proposed that differences in pattern type were related to variations in thickness of the epidermis and/or cushioning, a water-logged state of the epidermis. Hale found that the critical thickness in the epidermis was indeed necessary for ridge morphogenesis to initiate but no evidence for cushioning. More importantly, he found that ridges formed not from a mechanical folding of the epidermis but from actual cell proliferation.
Cummins, having observed the ridge configurations of congenitally malformed hands, proposed that direction of epidermal ridges was determined by growth forces and the contour of volar skin at the time of ridge formation. Mulville and Smith ("The Genesis of Dermatoglyphics.J.Pediatr 75:579-589, 1969) synthesized data from a number of earlier workers to further develop the Cummins' topographic model for ridge configuration. They stated that "ridge configurations are the immediate result of physical and topographical forces affecting the volar skin" during ridge formation. Therefore ridge configuration is dependent on the shape of the volar pad at the time of initial primary ridge formation. A high, round pad would result in formation of a whorl while a low pad would result in an arch. An intermediate pad height offset to one side of the digit would result in a loop formation.
W. Babler ("Prenatal selection and dermatoglyphic patterns."Am J Phys Anthropol 48(1):21-27, 1978) has provided evidence to suggest that volar pad shape is indeed associated with ridge configuration. Results of his studies indicated that the timing of primary ridge formation was associated with the type of ridge configuration. Early ridge formation was associated with a whorl-type pattern. Late ridge formation was associated with an arch configuration and intermediate ridge formation with a loop. Since apical volar pads (on tip of digits) initiate their involution around 10.5 weeks, the relative degree of pad regression at time of ridge formation apparently is associated with ridge configuration. Additionally, Babler reported that rather than pad height being a key factor in ridge configuration, pad width relative to height was the associated factor.
Skeletal Factors
Babler (Prenatal development of dermatoglyphic patterns: Associations with epidermal ridge, volar pad and bone morphology.Coll Antropol II:297-304, 1987.) has examined the developmental relationships between epidermal ridges and the developing bone skeleton of the hand. He has shown a significant prenatal relationship between epidermal ridge dimension and bone dimension of the hand. Whorl patterns tend to be associated with shorter distal phalanges. Whorl patterns also tend to be associated with less ossification, suggesting either early ridge development relative to bone maturity or delayed bone development relative to ridge formation.
Friction Skin Growth continued next page...
