Human hair serves various functions, including protection and sensory perception. At its core, each strand contains a distinct component known as the medulla. This innermost region is part of the hair shaft’s architecture. Understanding its characteristics provides insight into hair’s structure and properties.
Hair’s Core Structure
The visible hair shaft is composed of three primary concentric layers. The outermost layer is the cuticle, which functions as a protective shield with overlapping, scale-like cells. Beneath this protective layer lies the cortex, the thickest part of the hair shaft, providing strength, elasticity, and color due to keratin protein and pigment. Deep within the cortex, the medulla forms the central core of the hair shaft. Not all hair strands possess a medulla, especially finer hair types.
Characteristics of the Medulla
When present, the medulla is the innermost layer of the hair shaft. It often consists of a column of loosely packed, keratinized cells, sometimes containing air spaces. Under a microscope, this layer can appear soft, spongy, or even invisible. Its cellular arrangement is less rigid and organized compared to the denser, tightly packed keratin fibers in the cortex. The medulla’s appearance can vary, sometimes appearing dark or translucent depending on whether it is filled with air or other substances.
Variations and Proposed Functions
The presence and appearance of the medulla can vary significantly among individuals and even within different hair strands from the same person. Medulla patterns are classified into several types:
Continuous, where the medulla forms an unbroken line;
Interrupted or discontinuous, with regular or irregular breaks;
Fragmented, showing unevenly spaced sections;
Entirely absent.
While its exact biological role in human hair is not fully understood, proposed functions include providing structural support and aiding thermal insulation. It may also serve as a pathway for substances within the hair.
Medulla in Forensic Science
Medulla characteristics are valuable in forensic analysis, particularly for distinguishing human from animal hair. In human hair, the medulla is thin, often fragmented or absent, and its width is typically less than one-third of the hair shaft’s diameter. In contrast, animal hair commonly has a continuous, wider medulla, often occupying more than one-half of the hair shaft’s diameter, and may display distinct patterns like ladder or lattice structures not seen in humans. While the medulla’s features can help differentiate species and provide clues about hair type, they typically cannot identify a specific individual.