This article is from the Spring 1999 AFRMA Rat & Mouse Tales news-magazine.
By Debra Mauzy Melitz, September 1998
Hairs are derived from the epidermis and are composed of keratin. According to their size, hairs are classified as overhairs, underhairs, or vellus hairs (Kobori and Montagna 1975, Goldsmith 1983). The underhair consists of numerous soft thin down-hairs which are wavy or crimped and are shorter than the overhairs. Overhairs are usually long and sometimes coarse. The overhairs normally contain several hair types. The most prominent is the long thick guard hairs or monotrichs. Other overhair types are shorter and thinner, sometimes with a bend at the distal tip. In hedgehogs, the overhair is the prominent hair type. Vellus hairs are short, very fine, soft, silky, and usually unpigmented. They are usually in regions where at first glance there appears hairless. Quills are modified hairs, probably modified overhairs (Carlier 1916).
Hair consists of an outer cuticle, a central medulla, and a cortex between the two. The hair is formed within a follicle. The parent Latin word for follicle means ‘little bag’.
The cuticle is a single layer of scale-like cells. The cuticle cells are non-pigmented. They function as a protective other layer. Inside the follicle, the cuticle cells are interlocked with cells of the inner root sheath. This firmly locks the hair within the follicle.
The central medulla is composed of large loosely arranged cells that are filled with keratin. In human hair, this region is only about one or two cells wide. In other mammals (including hedgehogs) this region comprises the bulk of the hair or quill.
The layer between the cuticle and the medulla is the cortex and this is generally where the melanin granules are. The granules are aligned with the length of the hair. Air spaces (fusi) are interspersed. Air spaces are important in reflecting light. When no melanin granules are present, the hair is white due to the fusi reflecting the light (Rogers and Harding, 1956).
The diagram below is representative of what a hedgehog quill looks like under a dissecting microscope.
Each hair is formed in a hair follicle. A follicle is an epidermal invagination or pocket into the dermis. The base of the follicle thickens to form a cylinder on top of a papilla. The papilla is a thickening of the dermal layer and is important in influencing the hair to grow. The epidermal cells multiply to form the matrix or scaffolding of the hair follicle. Pigment cells (follicular melanocytes) migrate into the matrix and inject pigment granules into the hair cortex and medulla as they develop. The follicular melanocytes differ from the melanocytes found in the epidermis in that they only produce melanin during a specific phase of hair development, namely anagen III–VI. The follicular epithelium at the sides of the hair shaft grows out into the dermis to form a sebaceous gland. The sebaceous gland’s secretion lubricates the hair and keeps it water repellent. In addition, arrector pili muscles may be attached to the follicle to erect the hair or quill on contraction. The hair grows in 3 phases: anagen, catgen, and telogen. Anagen is the growing phase of the hair, melanin will be deposited in the hair during this time. Anagen is subdivided into six substages, I–VI. Catagen is when the matrix cells cease to divide and melanocytes cease to produce melanin but are still dendritic. Telogen is the resting stage. There are reduced number of melanocytes and they are no longer highly dendritic.