Humain Hair - Terminal Hair

Terminal Hair

Terminal hair is thick, long and pigmented. It is some 50 to 150 microns thick, terminal hairs are the dominant hairs on the scalp, eyebrows, lashes, axillae, and genital areas. In men, terminal hairs are variably found on the trunk and legs. There is great regional difference in terminal body hairs. Cross Section of the terminal hair shaft reveals three major components:

 

Cuticle (The Outer protective Layer)

Cortex (The core of the Hair)

Medulla (A central soft protein core which is more common in thicker hair and particularly so in white hairs)

The main constituents of these structures are sulfur rich proteins, lipids, water, and melanin and trace elements. 

 

 

Cuticle (The Outer protective Layer)

The cuticle is composed of specialized keratins and consists of six to eight layers of flattened overlapping cells with their free edges directed upward to the tip of the hair shaft. There are several layers to each cell. The innermost end cuticle is covered by the executable, which lies closer to the external surface and is comprised of three parts: the B-layer, the A-Layer and the epicuticle. The B-Layer and the A-Layer are largely proteinaceous, while the epicuticcle is a hydrophobic lipid layer of 18-methylecosanoic acid ( a main component of the outer layer of the epicuticle of human hair that endows hydrophobicity to the outer hair surface) attached via a covalent chemical bond to the surface of the fiver. This is commonly known as the F-Layer.

 

 

The F-layer is of critical importance to hair health. The cuticle’s complex structure allow it to slide as the hair swells, and the F-layer imbues a considerable degree of hyrdrophobicity. It is critical in protecting the hair rendering it resistant to the influx and outflow of moisture.

The undamaged cuticle has a smooth appearance and feel. It is primarily responsible for the luster and texture of the hair. The cuticle may be damaged by any of four major insults i.e. environmental, mechanical, chemical and heat. Chemical removal of the F-layer, particularly by oxidation during bleaching or perming, eliminates the first hydrophobic defense of the shaft and leaves the hair more porous and vulnerable. If the cuticle is damaged, there is little change in the tensile properties of hair; however, its protective function is diminished. 

 

 

Cortex (The core of the Hair)

The cortex consists of closely packed spindle-shaped cortical cells rich in keratin filaments that are oriented parallel to the longitudinal axis of the hair shaft, and an amorphous matrix of high sulfur proteins. The intermediate filament hair keratins (40-60 kDa), comprising 400 to 500 amino acid residues in heptad sequence repeats, from hard keratin polypeptide chains that pair together to from protofilaments, which make up a keratin chain. The keratin chains have large number of sulfur containing cysteine residues. Cysteine residues in adjacent keratin filaments from covalent disulfide bonds, which create a strong crosslink between adjacent keratin chains. The disulfide bonds confer shape, stability, and resilience to the hair shaft. Other weaker bond link the keratin polypeptide chains together, such as Van der Waal interactions, hydrogen bonds, and columbic interactions known as salt links. These weaker bonds can be overcome with water.  The disulfide bonds are critical in conferring shape, stability, and resilience to the hair shaft, and can only be broken by external oxidative chemical agents such as occurs with perming or relaxing. Weak hydrogen bonds link the keratin polypeptide chains. These weaker bonds are easily overcome by water, rendering curly hair temporarily straight.  

The cortex contains melanin granules, which color the fiber based on the number, distribution and types of melanin granules. 

 

 

Medulla (A central soft protein core which is more common in thicker hair and particularly so in white hairs)

The medulla is a soft, proteinaceous core present in thicker and white hair. It has no known function in Humans.