The skin is the largest organ of the body and varies greatly in different regions. It has five main functions: protection, thermoregulation, sensation, metabolic functions (vitamin D, adipose metabolism), and sexual attraction.
Skin is composed of several subunits. The three main divisions are:
The epidermis is a stratified squamous epithelium that contains discrete layers of proliferating, differentiating, and differentiated cells called keratinocytes. It is divided into four layers that have different structural appearances:
Recall from the Laboratory of Epithelia that epithelia differ in their degree of keratinization - those exposed to abrasion and desiccation are heavily keratinized, but those that form mucous membranes do not have much keratin. For example, the skin is highly keratinized, but the esophagus, anal canal,and vagina are not. Instead of protection by keratin, these mucous membranes are kept moist by glandular secretions. Mucous membranes lack a stratum granulosum and stratum corneum.
The epidermis contains several characteristic cell types:
The dermis consists of two layers:
Most blood vessels, nerves, and sensory receptors occur in the papillary layer. This region also contains Meissner's corpuscles, which sense light touch.
The hypodermis is the fatty layer beneath the dermis. It is thickest in the abdominal wall and virtually absent in the eyelid, scrotum, penis, and the dorsal side of the hand. This layer contains a significant number of fibroblasts, which synthesize collagen and elastin. The hypodermis contains Pacinian corpuscles, which sense deep touch.
Many structures are derived from epidermal tissue. Keep in mind that just because a structure is derived from the epidermis does not mean that it is located in the epidermis.
Hair follicles are encased by an invagination of the epidermis into the dermis known as the external root sheath. They contain specially organized keratin built into long tubular structures. Remember that hair follicles have generous blood and nerve supplies. There are three states of hair follicles:
Sebaceous glands are pear-shaped alveolar glands that secrete an oily substance called sebum, which moisturizes and waterproofs hair. They are usually attached to hair follicles near the arrector pili muscle, which allows the hair to "stand up." An extensive capillary plexus characterizes sebaceous glands.
Eccrine sweat glands occur throughout most of the skin. They consist of long tubules extending from the epidermis deep into the dermis or hypodermis. The secretory portion of each gland is tightly wound and appears as a collection of cross-sectioned tubules. It is encased by myoepithelial cells, which contain actin filaments. These cells receive input from nerve fibers, which cause them to contract and expulse the sweat from the gland. Keep in mind the organs that do not have eccrine sweat glands: the glans penis, the inner surface of the foreskin, the clitoris, and the labia minora.
Apocrine sweat glands are much larger than eccrine glands and produce a thicker secretion. They have straight, narrow ducts that run parallel to hair follicles and frequently open into the pilosebaceous canal. These sweat glands occur in the axilla, the areola of the nipple, the labia majora, and the circumanal region.
Mammary glands are one of the most complex epidermal derivatives. These glands are present in both sexes, but only develop fully in females after parturition. They begin to undergo dramatic structural changes at puberty.
The basic structure of the mammary glands involves alveoli that contain two layers of cells: an inner cuboidal epithelium and an outer layer of myoepithelial cells. The alveoli make up tubuloalveolar glands, or lobes, which connect via lactiferous ducts to the base of the nipple. After milk is produced, it is secreted and travels through the ducts into spindle-shaped enlargements beneath the areola known as lactiferous sinuses.
Important structural changes occur in the mammary glands over the course of the female's lifetime:
In your study of Histology, you may hear three different terms to describe exocrine cells. These can often be confusing.
