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Cuticle - Inner Root Sheath

 

Cuticle

It is a structure consisting of flat cells stacked upon each other from the hair end to the hair root. Protein synthesis increases in the hardening and keratinization stage. This structure forms 10% of hair fibril in thin hairs. Cuticle cells are stacked like tiles so that their peripheral sides look upwards. Adjacent cuticle cells and cell connections between cuticle and cortex below are generally flat. There are regular yet rare curls. This ensures the mechanical power of cuticle.


Inner root sheath

Inner root sheath consists of three concentric layers. These layers are from inside out inner root sheath cuticle, flat, a one rowed layer formed of stacked cells like tiles so that their ends will point to bulbus. As ends of hair cuticle cells are pointed upwards, these two layers are strongly connected. None of these three layers contains melanin. These layers do not keratinize like cortex and cuticle. Yet these are dyed basophilic like keratohyaline.

 

Outer root sheath

Outer root sheath surrounds the hair follicle like a glove formed of a few rows of cells by extending upwards from lower levels of hair bulbus to entrance of sebaceous ductus. It can be divided into two parts: short lower part surrounds outer part of bulbus; upper part is extended from neck of bulbus to sebaceous channel level. There it changes into superficial epidermis and covers hair follicle infundibulum.

Outer root sheath is thinnest in the bulbus. Its thickness increases gradually. Suprabulbar lane is thickest in the follicle’s middle parts (isthmus). At lower parts below isthmus outer root sheath is covered by inner root sheath and does not keratinize. In the outer root sheath basal layer there are inactive amelanotic melanocytes. These inactive melanocytes may start to produce actively melanin in skin damages such as dermabrasion.

In the middle parts of hair follicle that is located between the attachment place of erector pili muscle and entrance of sebaceous ductus, outer root sheath is not covered by inner root sheath. Thereafter they keratinize and separate.

Upper parts of hair follicle located in the upper parts of the entrance of sebaceous ductus are covered by superficial epidermis. As in the case of sebaceous ductus these are keratinized with the formation of keratohyaline granules.

The function of outer root sheath is not completely clarified. In early stages of anagen phase they elongate due to high mitotic activity and when follicle has reached its complete length, mitotic activity stops and mitotic activity following this is only equal to cell death or cytoplasmic cessation ratio. In general outer root sheath is deemed as a relatively static region that plays a minimum role in cell mobility.