Histology is the study of the microscopic anatomy of cells and tissues of plants and animals. It is performed by examining a thin slice (section) of tissue under a light microscope or electron microscopy. The ability to visualize or differentially identify microscopic structures is frequently enhanced through the use of histological stains.

Histopathology, the microscopic study of diseased tissue, is an important tool in anatomic pathology, since accurate diagnosis of cancer and other diseases usually requires histopathological examination of samples.

Introduction - Vertical sections of scalp biopsy samples have provided the traditional view of hair follicles; indeed, most anatomical and histopathological features of hair follicles have been described in vertical sections. However, recently, the value of horizontal (transverse) sectioning has been recognized. In fact, a thorough knowledge of follicular anatomy in both planes is needed to obtain maximum information from scalp biopsy samples.

History - Considerable knowledge of hair follicle anatomy was amassed by histologists in Europe in the nineteenth and early twentieth centuries. Further progress was made later in the twentieth century in research and symposia directed by William Montagna. The concept of horizontal sectioning was introduced by Headington in 1984 and an increasing number of dermatopathologists now interpret horizontal sections.

Structure and Function - Hair follicles comprise a permanent upper segment of follicular infundibulum and isthmus, and an impermanent lower segment of lower follicle and bulb. They cycle continuously through periods of growth and rest, namely anagen and telogen. The lower segment is responsible for generation of the growing hair and disappears during the resting phase. The cylindrical hair fiber is extruded continuously during the growth phase. The hair fiber cortex consists of keratin filaments embedded in a sulfur-rich matrix, enclosing the medulla and surrounded by the cuticle of the hair shaft. It is generated by transient amplifying matrix cells in the hair bulb, which surround the dermal papilla. The hair fiber diameter remains uniform during a single growth phase under normal conditions. The entire hair follicle is enclosed by the outer root sheath or trichilemma, which extends from the hair bulb to the skin surface epidermis lining the infundibulum. The inner root sheath invests the growing hair fiber from bulb to mid-isthmus level.

Human hairs grow to a specific length, which varies with the individual. Hair length depends on genetic influences, body site, climate, age, and nutritional, hormonal and other factors. Hair length is determined by the rate and duration of anagen. Scalp hair grows an average of 1 cm per month and the anagen growth phase lasts 1-7 years; therefore, scalp hairs can grow 12-84 cm in length. Hairs of different diameters are commonly found in the same scalp.

On a normal scalp there are 100,000-150,000 scalp hairs, of which 90%-95% are growing and 5-10% are resting. The telogen or resting phase lasts 3 months (range 2-4 months). Adjacent hairs cycle independently so that shedding is scattered and inconspicuous rather than localized and obvious. Hair is subject to weathering from normal wear and tear, and keratinized hair cannot repair itself. However, hair will replace itself, with new hair growing in after the old hair is shed. Therefore, hair cycling is of paramount importance. Hair development starts after 8 weeks of fetal life with the appearance of placodes in the epidermal basal layer, above underlying dermal condensates of mesothelial cells. Epidermal pegs grow down to enclose these dermal papillary cells. When downward extension and follicular formation are complete, hair growth ensues and the initial hair population is complete by 22 weeks.

Fine lanugo hair develops in an advancing wave from the frontal to the occipital scalp and is shed by 36 weeks gestation. A second coat of lanugo hair appears and it is shed in a synchronized wave pattern at 3-4 months of life. The bare occipital patch often seen in infants is usually physiological, resulting from synchronized shed ding of the final wave of lanugo telogen hairs prior to their replacement by normal scalp hairs. The maxi mum number of scalp hair follicles during the human life span is present at birth; thus, hair follicle density greatest in neonates and lessens progressively during childhood and adolescence as the scalp stretches over the growing skull until it stabilizes in adults (250-3: hairs per cm2") .