Friction ridges are the fine lines produced by the arrangement of tiny sweat pores on the ventral surface of the palm of the hands and soles of the feet. The alignment of these friction ridges leads to the formation of fingerprints, palm prints, and footprints.
But, how do these friction ridges appear when observed through a microscope and anatomical dissection, and why it is important to study them? Let’s explore the morphology, anatomy, and physiology of friction ridges to thoroughly understand the formation of fingerprints.
The anatomy and physiology of the friction ridge skin form the basis for several critical elements that underlie the examination process. The anatomy and physiology explain how the features of the skin persist, how the features of the skin age, how the skin responds to injury, and why scars that form are unique.
Another element explained by the skin’s structure is the mechanics of touch. Understanding how the friction ridge skin reacts when it contacts a surface can provide valuable assistance during the examination of friction ridge impressions.
Morphology of Friction Ridges
The morphology or the external features of the friction ridge skin is a direct reflection of its function. When observed through a microscope, tiny sweat pores arranged in a linear fashion are seen that form the ridges on the fingers of hands and feet.
The ridges and sweat pores allow the hands and feet to grasp surfaces firmly, and the creases allow the skin to flex. Ridges, creases, and mature scars of the friction ridge skin are durable morphological features.
Warts, wrinkles, blisters, cuts, and calluses may also appear on the friction ridge skin and are frequently transient morphological features. The anatomy and physiology of a feature determine whether the feature is durable or transient.
General Anatomy of Skin
Skin is part of the integumentary system which forms the external covering of the human body and contributes as one of the sense organs that is responsible for the sensation of touch.
The skin is an organ composed of three anatomical layers: epidermis, dermis, and hypodermis. These anatomical layers together function to provide the body with a protective barrier, body temperature regulation, sensation, excretion, immunity, a blood reservoir, and synthesis of vitamin D.
1. Epidermis
The outer layer of skin is the epidermis. The epidermis prevents water loss through evaporation, acts as a receptor organ, and provides a protective barrier for the underlying tissues.
The epidermis is described as a “stratified, continually renewing epithelium that exhibits progressive differentiation in a basal to superficial direction. In other words, the epidermis is a layered tissue that must constantly replace the cells leaving the surface.
New cells are generated in the basal layer and pushed toward the surface. As the cells move toward the surface, they undergo sequential changes in chemical composition.
The epidermis is composed of several different types of cells: keratinocytes, melanocytes, Langerhans cells, and Merkel cells.
Melanocytes, the pigment-producing cells of the epidermis, play a key role in the formation of a protective barrier. The pigmentation produced by the melanocytes shields the DNA of the keratinocytes (the primary cell type of the epidermis) from the sun’s harmful rays. Additionally, the melanocyte is responsible for the synthesis of vitamin D.
Layers of Epidermis
The epidermis is the thinnest layer of the skin that consists of five different sub-layers. These layers are mentioned below according to their position from bottom to top in the epidermis:
- Stratum Basale– It is the lowermost layer of the epidermis containing keratinocytes stem cells and melanocytes. The new skin cells are generated here which are transferred to the topmost layer through multiple transitions.
- Stratum Spinosum– As the name suggests, this layer has the spiny projections of polyhedral-shaped keratinocytes held together by sticky desmosomes, and forms the “prickly layer” between stratum basale and stratum granulosum. Langerhans cells (cells that prevent pathogens to enter the skin) are present in this layer.
- Stratum Granulosum– This layer consists of keratinocytes that have basophilic keratohyalin granules inside them. The granules contain glycoproteins, polysaccharides, and lipids which form the cement that holds the stratum corneum layer.
- Stratum Lucidum– It is a thin transparent layer of flattened keratinocytes. This layer represents the transition of granular keratinocytes from stratum granulosum into the flat one.
- Stratum Corneum– It is the topmost layer of the epidermis and consists of dead and hard keratinocytes called corneocytes that protect the skin from the external environment.
2. Dermis
Dermis is the elastic layer that provides tough support to the skin. It is the home of nerve cells, blood and lymphatic vessels, and cutaneous appendages containing various glands. The dermis is the connective tissue that supports the epidermis and binds it to the hypodermis.
The dermis is composed of two layers i.e., the papillary and the reticular layer. The outer papillary layer is a loose connective tissue containing anchoring fibrils and numerous dermal cells. The anchoring fibrils secure the dermis to the epidermis via the basement membrane zone. The papillary layer of the dermis forms the dermal papillae.
Dermal Papillae
Dermal papillae are malleable, peg-like projections of the papillary dermis between the primary and secondary ridges. The malleable nature of the dermal papillae is important because the epidermal-dermal junction remodels with age and in response to sheering stress on the surface of the skin.
Reticular Dermis
The reticular dermis is a compact connective tissue containing large bundles of collagen and elastic fibers. The organization of these fibers provides the dermis with strength and resilience. The reticular dermis is connected to the hypodermis by a network of fibers.
Sweat Glands
Although the skin produces several appendages (e.g., hair, nails, sebaceous glands), the eccrine sweat gland is the only appendage of the friction ridge skin. Eccrine sweat glands are found all over the body surface and function primarily in thermoregulation.
Eccrine sweat glands are classified as simple tubular glands whose ducts open at the skin surface the coiled secretory portion of the gland is embedded in the dermis or hypodermis, and the duct extends through the epidermis.
The fluid secreted by the eccrine sweat glands is predominantly water (99.0–99.5%). The remaining constituents of sweat include sodium chloride, potassium, ammonia, urea, lactate, uric acid, creatinine and creatine, amino acids, sugars, immunoglobulin A, epidermal growth factor, and select
3. Hypodermis
Beneath the fibrous reticular dermis, there is an abrupt transition to the adipose tissue of the hypodermis. Adipose (fat) tissue serves as an energy reserve, cushions the skin, contours the body, and allows for mobility of the skin over underlying structures.
The dermis and hypodermis are physically connected through interlocking fibers and share blood vessel and nerve networks. The primary cell of the hypodermis is the adipocyte. Adipocytes are organized in lobules by fibrous connective tissue and store the subcutaneous fat.
Conclusion
Friction ridges are the fine raised lines present on the ventral surface of the palms of the hands and soles of the feet that provide friction to the hands and foot as well as form the unique patterns of fingerprints, palm prints, and footprints respectively. The formation of fingerprints is related to the biological origin of the friction ridges.
When these ridges are observed carefully, it can be seen that tiny sweat pores are responsible for the formation of ridges. Also, the anatomy of the skin also explains the basis of fingerprint formation. The knowledge of morphology, anatomy and physiology of the friction ridges helps forensic analysts to understand and analyse the fingerprints found at the crime scene.
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