Embedded Files
Table of Contents
Table of Contents
2.1 Skin
2.1 Skin
The skin, or integument, covers the body surface. In image 2.1a, a superficial epidermal (labeled Ep in image 2.1a) layer is attached to an underlying dermis (labeled De in image 2.1a) by a basement membrane. The skin accounts for approximately 15% of the body weight in humans, and the functions of the skin are closely related to its internal organization. Cells in the basal layer of the epidermis are capable of rapid mitosis and undergo keratinization to form the more superficial layers of the epidermis. A distinction is commonly made between thick skin, which covers the palms of the hands and soles of the feet, and thin skin, which covers most of the remainder of the body. Thick skin (image 2.1b) has a deeper epidermis than thin skin; the stratum corneum (composed of keratin) and stratum granulosum (the cells of which contain keratohyalin) are especially thick. The wavy black layer in the middle of the epidermis is the stratum granulosum, and above it is the stratum corneum. A layer called the stratum lucidum (which contains eleidin) is often absent in thin skin. The dermis of thin skin, however, is deeper than that of thick skin. Early in development the epidermis can, as a result of its ability to proliferate and differentiate, give rise to such appendages as hair (labeled Ha in image 2.1a), nails, sebaceous glands, and sweat glands. In image 2.1b, the structure labeled "a" represents the duct of a sweat gland which ultimately reaches the terminal coiled secretory elements (labeled "b" in image 2.1a).
Image 2.1c is a microscopic view of skin showing both oil-secreting sebaceous glands (labeled "a" in image 2.1c) and the secretory portions of sweat glands (labeled "b" in image 2.1c).
Skin serves a variety of important functions. The tough protein keratin forms the outer layer of an uninterrupted cellular covering. The keratin serves as a waterproofing material and is impermeable to many disease organisms. Melanocytes migrate into the basal layer of the epidermis and synthesize melanin pigment, which serves to protect against the harmful effects of ultraviolet light. Free nerve endings may be present in the epidermis, and various encapsulated nerve endings are present in the underlying dermis. Sweat glands are also located in the dermis of the skin, and through their activity, the integument functions in thermoregulation and the maintenance of water balance. Fat cells (labeled FC in image 2.1a) located in the dermis also provides insulation and food storage for the body.
Image 2.1a: Skin with epidermal layler (Ep), dermis (De), hair (Ha), and fat cells (FC) identified.
Image 2.1b: Thick skin with duct of a sweat gland (labeled "a") and coiled secretory elements (labeled "b").
Image 2.1c: Microscopic view of skin with sebaceous glands (labled "a") and secretory portions of sweat glands (labled "b).
2.2 Epidermis
2.2 Epidermis
The outer layer of the skin, called the epidermis, is stratified squamous epithelium (as seen in image 2.2a). Because it contains no blood vessels, cuts made into this layer usually will not bleed. Also, because this layer renovates most of its cells each month, small nicks and cracks are being continuously repaired. In the thick epidermis of the palms of the hands and the soles of the feet, there are five typical layers (strata).
In image 2.2b, starting with the outermost layer, the layers are stratum corneum (layer "a" in image 2.2b), the stratum lucidum (the light thin orange layer labeled "b" in image 2.2b right above the purple colored cell layer), the stratum granulosum, the stratum spinosum, and stratum basale. The stratum lucidum consists of flat, translucent layer (seen as a light region at the arrow in the middle of "b" in image 2.2b). It consists of dead cells that contain a protein called eleidin. This protein is probably a transitional substance between the soft keratin of the stratum corneum and the precursor of soft keratin, keratohyaline, of the layer below. The stratum lucidum appears only in the palms of the hands and soles of the feet, acting as a protective shield against the ultraviolet rays of the sun and preventing sunburn on the palms and soles. The stratum spinosum and stratum basale together are known as the stratum germinativum because they generate new cells. In all parts of the body, other than the palms and soles, only the stratum corneum and stratum germinativum are regularly present. The stratum granulosum lies below the stratum lucidum (darker purple layer below "b" in image 2.2b). It is usually two to four cells thick. The cells contain granules of keratohyalin and initiate the process of keratinization, associated with the dying process of cells. The stratum spinosum is composed of layers of cells that have delicate "spines" protruding from their surface (see area "a" in image 2.2b), a look that has given them the title: "prickle cells". Some new cells are formed here and are pushed to the surface to replace the cornified cells of the stratum corneum.
Image 2.2c shows the stratum corneum (cells arranged in parallel rows) and the stratum basale (resting on the baement membrane next to dermis with dark melanocytes identifed by the arrow). The stratum corneum is a flat, thick layer of dead cells arranged in parallel rows. This cornified layer of the epidermis consists of soft keratin (as compared with the hard keratin in fingernails and toenails), which helps keep the skin elastic. The soft keratin also protects living cells. The cells in the stratum corneum are constantly being shed through normal abrasion. They are replaced by new cells that are formed by cell division and pushed up from the germinal layers below. The soles of the feet, for example, are completely replaced every month. The stratum basale usually consists of either a single layer of columnar or cuboidal cells. Like the stratum spinosum, it is also capable of undergoing cell division to produce new cells to replace those being shed in the exposed superficial layer.
Image 2.2a: Stratified squamous epithelium of the epidermis.
Image 2.2b: Layers of the epidermis with stratum corneum (labeled "a") and stratum lucidum (labeled "b") identified.
Image 2.2c: Stratum corneum and stratum basale layers of the epidermis.
Created by Marissa Fukunaga, 2022Images by Bob Moeng, 2018Descriptions adapted from Bob Moeng, 2018
Page updated
Report abuse