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The Fingerprint Sourcebook CHAPTER EMBRYOLOGY AND MORPHOLOGY OF FRICTION RIDGE SKIN Kasey Wertheim CONTENTS 3 3.1 Introduction 12 3.7 Pattern Formation 4 3.2 Embryology: Establishing 18 3.8 Genetics Uniqueness and Pattern Formation in the Friction Ridge Skin 5 3.3 Limb Development 21 3.9 Uniqueness: Developmental Noise 7 3.4 Differentiation of the Friction 22 3.10 Summary: Keys to Ridge Skin Uniqueness and Pattern Formation 8 3.5 Primary Ridge Formation 22 3.11 Reviewers 11 3.6 Secondary Ridge 24 3.12 References Formation 3–1 Embryology and Morphology of Friction Ridge Skin C H A P T E R 3 CHAPTER 3 EMBRYOLOGY AND 3.1 Introduction Friction ridge skin has unique features that persist from MORPHOLOGY OF before birth until decomposition after death. Upon contact with a surface, the unique features of friction ridge skin may leave an impression of corresponding unique details. Two FRICTION RIDGE SKIN impressions can be analyzed, compared, and evaluated, and if sufficient quality and quantity of detail is present (or Kasey Wertheim lacking) in a corresponding area of both impressions, a com- petent examiner can effect an individualization or exclusion (identify or exclude an individual). The analysis, comparison, evaluation, and verification (ACE-V) methodology, combined with the philosophy of quantitative–qualitative examinations, provide the framework for practical application of the friction ridge examination discipline. But at the heart of the disci- pline is the fundamental principle that allows for conclusive determinations: the source of the impression, friction ridge skin, is unique and persistent. Empirical data collected in the medical and forensic com- munities continues to validate the premises of uniqueness and persistence. One hundred years of observations and statistical studies have provided critical supporting docu- mentation of these premises. Detailed explanations of the reasons behind uniqueness and persistence are found in specific references that address very small facets of the underlying biology of friction ridge skin. This chapter brings together these references under one umbrella for the la- tent print examiner to use as a reference in understanding why friction ridge skin is unique and persistent. The basis of persistence is found in morphology and physiology; the epidermis faithfully reproduces the three- dimensional ridges due to physical attachments and constant regulation of cell proliferation and differentiation. But, the basis of uniqueness lies in embryology; the unique features of the skin are established between approximately 10.5 and 16 weeks estimated gestational age (EGA) due to developmental noise. 3–3 C H A P T E R 3 Embryology and Morphology of Friction Ridge Skin 3.2 Embryology: Establishing Once specialized, the three primary cell types begin their development into tissue and organs. The process of tissue Uniqueness and Pattern Formation differentiation begins with neurulation, or the formation of in the Friction Ridge Skin the notochord (the precursor to the spinal cord and brain) as well as the neural crest (the precursor to much of the 3.2.1 Introduction to Embryology embryo’s nervous system). Segmented blocks of tissue that become muscles, vertebrae, and connective tissue The uniqueness of friction ridge skin falls under the larger form on either side of the notochord. The remainder of the umbrella of biological uniqueness. No two portions of any mesoderm moves out and around the inner endoderm, living organism are exactly alike. The intrinsic and extrinsic fac- forming a hollow chamber that will ultimately become the tors that affect the development of any individual organ, such lining of the stomach and intestines. as human skin, are impossible to duplicate, even in very small areas. The uniqueness of skin can be traced back to the late During late embryological development, the embryo embryological and early fetal development periods. undergoes “morphogenesis”, or the formation of shape. Limbs rapidly develop from about 4 weeks EGA, and the 3.2.2 Early Embryological Development: arms, legs, knees, elbows, fingers, and toes can all be 0–2 Weeks EGA (Raven and Johnson, 1992, seen in the second month. During this time, the hand pp 1158–1159) changes from a paddlelike form to an adult form, including the formation of the fingers and rotation of the thumb. Also The process of embryological development begins with fer- during this time, swellings of mesenchyme called “volar tilization of the egg and continues through a period of rapid pads” appear on the palms of the hands and soles of the cell division called “cleavage”. In mammalian eggs, an inner feet. Within the body cavity, the major organs such as the cell mass is concentrated at one pole, causing patterned al- liver, pancreas, and gall bladder become visible. By the end terations during cleavage. Although egg cells contain many of week 8, the embryo has grown to about 25 millimeters different substances that act as genetic signals during early in length and weighs about 1 gram. embryological development, these substances are not dis- tributed uniformly. Instead, different substances tend to be 3.2.4 Fetal Growth: 9–12 Weeks EGA clustered at specific sites within the growing embryo. Dur- During the third month, the embryo’s nervous system and ing growth, signal substances are partitioned into different sense organs develop, and the arms and legs begin to daughter cells, endowing them with distinct developmental move. Primitive reflexes such as sucking are noticed, and instructions. In this manner, the embryo is prepatterned to early facial expressions can be visualized. Friction ridges continue developing with unique cell orientation. begin to form at about 10.5 weeks EGA and continue to 3.2.3 Late Embryological Development: mature in depth as the embryo passes into the second tri- mester. From this point on, the development of the embryo 3–8 Weeks EGA (Raven and Johnson, 1992, is essentially complete, and further maturation is referred pp 1160–1164) to as fetal growth rather than embryonic development. The first visible results of prepatterning can be seen immediately after completion of the cleavage divisions 3.2.5 Second Trimester as different genes are activated. Certain groups of cells The second trimester is marked by significant growth to move inward toward the center of the sphere in a carefully 175 millimeters and about 225 grams. Bone growth is orchestrated migration called “gastrulation”. This process very active, and the body becomes covered with fine hair forms the primary tissue distinctions between ectoderm, called lanugo, which will be lost later in development. As endoderm, and mesoderm. The ectoderm will go on to the placenta reaches full development, it secretes numer- form epidermis, including friction ridge skin; the mesoderm ous hormones essential to support fetal bone growth and will form the connective tissue of the dermis, as well as energy. Volar pads regress and friction ridges grow until muscle and elements of the vascular system; and the about 16 weeks EGA, when the minutiae become set. endoderm goes on to form the organs. 3–4 Embryology and Morphology of Friction Ridge Skin C H A P T E R 3 FIGURE 3–1 Growth of the hand progresses from (A) a paddle- like form (magnification = 19.5 X), (B) continues as the fingers separate (magnification = 17.3 X) and (C) the volar pads become prominent (magnifica- tion = 7.7 X), and (D) achieves infantlike appearance by 8 weeks EGA (magnification = 4.2 X). (Reprinted with permission from Cummins (1929).) Sweat glands mature, and the epidermal–dermal ridge 3.3.2 Volar Pad Development system continues to mature and grow in size. By the end Volar pads (Figure 3–2) are transient swellings of tissue of the second trimester, sweat ducts and pores appear called mesenchyme under the epidermis on the palmar along epidermal ridges, and the fetus begins to undergo surface of the hands and soles of the feet of the human even more rapid growth. fetus (Figure 3–3). 3.2.6 Third Trimester The interdigital pads appear first, around 6 weeks EGA, In the third trimester, the fetus doubles in weight several followed closely in time by the thenar and hypothenar times. Fueled by the mother’s bloodstream, new brain cells pads. At approximately 7–8 weeks EGA, the volar pads and nerve tracts actively form. Neurological growth contin- begin to develop on the fingertips, starting with the thumb ues long after birth, but most of the essential development and progressing toward the little finger in the same radio- has already taken place in the first and second trimesters. ulnar gradient that ridge formation will follow. Also at The third trimester is mainly a period for protected growth. about 8 weeks EGA, the thenar crease begins to form in the palm, followed by the flexion creases in the fingers at around 9 weeks EGA (Kimura, 1991). 3.3 Limb Development 3.3.3 Volar Pad “Regression” 3.3.1 Hand Development The pads remain well rounded during their rapid growth During the initial phases of formation, the hand undergoes around 9–10 weeks EGA, after which they begin to demon- significant changes in topography. Until approximately 5–6 strate some individual variation in both shape and position weeks EGA, the hand appears as a flat, paddlelike structure (Babler, 1987; Burdi et al., 1979; Cummins, 1926, 1929). with small protrusions of tissue that will become fingers. During the period from 8 to 10 weeks EGA, thumb rotation From 6 to 7 weeks EGA, these finger protrusions in the is achieved (Lacroix et al., 1984, p 131). Also at about 10 hand plate begin to form muscle and cartilage that will weeks EGA, the flexion creases of the toes begin forma- become bone at later stages of hand growth (Figure 3–1).
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