Special Histology and Embryology: Practical Course - V. K. Naphanyuk 2001

Skin and its Appendages
Hair

Skin Appendages include hair, Sweat Glands, Mammary Glands, Sebaceous Glands, and Nails.

Hair (pili) covers almost the entire surface of the skin. It is densest on the HEAD, where the total count reaches 100,000. Hair length ranges from a few millimeters to 1.5 m, and thickness varies from 0.005 to 0.6 mm.

There are three MAIN TYPES OF hair:

1) long hair — hair of the scalp, beard, mustache, as well as hair located in the axillary and pubic regions;

2) bristle hair — eyebrow and eyelash hair, along with hair growing in the external auditory canal;

3) vellus hair, which covers the remaining areas of the skin.

A hair consists of two parts: the shaft, located above the skin surface, and the ROOT, embedded within the skin thickness and extending down into the subcutaneous tissue. The hair Structure comprises the cuticle (cuticula pili), cortex (cortex), and medulla (medulla).

Vellus hair lacks a medulla.

The cuticle is a single layer of flat Cells overlapping each other like roof tiles, with their free ends pointing upward.

The cortex consists of elongated cells whose Cytoplasm contains hard keratin, pigment, and gas bubbles.

The medulla is built of cubic or polygonal cells containing soft keratin, pigment, and gas bubbles.

Hair grows from the hair bulb (bulbus pili), where epithelial cells proliferate before moving upward and undergoing keratinization. The hair bulb is nourished by the Connective Tissue dermal papilla (papilla pili). The hair root is surrounded by internal and external epithelial root sheaths.

The internal epithelial root sheath (Vagina epithelialis radicularis interna) consists of the following components:

— the cuticle, which resembles tile-like overlapping structures with the free ends of the cells pointing downward;

— the internal epithelial layer containing granules (stratum epitheliale internum);

the external epithelial layer, known as the pale layer (stratum epitheliale externum pallidum).

The external epithelial root sheath (vagina epithelialis radicularis externa) is similar to the basal and spinous layers of the epidermis.

The dermal root sheath (vagina dermalis radiacularis) consists of a basement membrane, an inner circular fiber layer, and an outer longitudinal layer.