Overview – Skin
Plate 9-1. Thick Skin: The Epidermis
Plate 9-2. Thin Skin: The Scalp
Plate 9-3. Skin: Epidermal Derivatives
The skin, or integument, is a highly complex organ that performs many necessary functions. Skin is in a sense a container; it is the outer boundary of the physical being, the major material interface between a human being and the outside world.
Human beings are made mostly of water and, being warm-blooded creatures, must operate at a relatively constant body temperature. Because temperature and relative humidity of the atmosphere can vary greatly, human skin – which has an extremely large surface area-has many major tasks, including retaining water and regulating temperature. In addition, skin provides protection against the damaging effects of ultraviolet radiation from the sun, serves as a site of synthesis of vitamin D, and provides an exquisitely sensitive sensory surface that keeps an individual “in touch” with his or her immediate surroundings. All the while, the skin serves to protect delicate underlying soft tissues from physical damage imposed by contact with hard objects among which land-dwelling vertebrates such as man must move.
How can a single organ perform such a variety of important, life-sustaining functions? To answer this question, we must look to the carefully designed architecture of the skin and its many integumentary derivatives. The basic design of the skin centers around its subdivision into two major components: the superficially located Epidermis, which is an epithelium, and the elaborate connective tissue layer beneath it called the Dermis, or the Corium. The structure of the Epidermis and Dermis varies considerably in different parts of the body. In areas of thick skin such as the soles of the feet, both Epidermis and Dermis are much thicker and tougher than they are in the thin skin that covers the eyelids.
The Epidermis is a keratinized stratified squamous epithelium made up of four histologically distinct regions. At the surface, the Stratum Corneum, which is in direct contact with the environment, consists of flat, lifeless plates of Keratin. These keratinized plates, called Squames, are periodically lost from the surface and replaced by mitotic activity of cells in deeper layers. The Keratin that fills the Squames is a tough, durable, waterproof protein that is formed within cells of the Stratum Granulosum, the region just beneath the stratum corneum. Although the precise intracellular mechanisms of keratinization await discovery, two conspicuous classes of cytoplasmic inclusions – Keratohyalin Granules and tonofilaments-are evident in the cells of the stratum granulosum (called keratinocytes) that are undergoing keratinization. Beneath the Stratum Granulosumlies the Stratum Spinosum, also called the “spiny layer” or the “prickle cell layer.” The large, polygonal cells of the Stratum Spinosum are attached to one another by desmosomes – cell junctions that form many tiny “spot welds” between the Plasmamembranes of adjacent cells. Beneath the Stratum Spinosum lies the deepest region of the Epidermis, the Stratum Germinativum. The large cells of the Stratum Germinativum proliferate by Mitosis at a relatively rapid rate and supply cells that serve to replace those lost at the surface of the skin. Daughter cells produced within the Stratum Germinativum move upward through the Epidermis, undergo the process of keratinization, and take up residence in the stratum corneum as surface Squames.
The Epidermis, like all epithelia, is avascular. The Dermis, on which it rests, is a highly vascular bed of connective tissue that is subdivided into several regions. The uppermost region, called the papillary layer, makes direct contact with the Basement Membrane that underlies the Epidermis. Made of loose connective tissue, the papillary layer in thick skin is thrown into conspicuous folds called dermal papillae and is responsible for the ridges that make fingerprints. Beneath the thin papillary layer lies the reticular layer of the Dermis, an extensive feltwork of Collagen and elastic fibers classified as dense irregular connective tissue.
In addition to blood vessels and nerves, the Dermis houses a number of important integumentary derivatives such as hair follicles, sebaceous glands, and sweat glands. These will be depicted and described in more detail in the remainder of this chapter. In addition, the skin is generously supplied with sensory receptors such as pacinian corpuscles, Meissner’s corpuscles, and Merkel’s cells, all of which serve important and specific mechanoreceptive functions.
Thick Skin: The Epidermis
Although often thought of as a simple sheet of tissue, the skin is truly an organ, one of the largest and most complex of the organs in the human body. The skin is a covering and as such performs many functions. Of all the skin covering the body, none is tougher or thicker than that covering the palms of the hands and the soles of the feet. The toughness and thickness of palmar and plantar skin makes its inclusion in a single thin section for electron-microscopic investigation difficult.
Fortunately, the skin of the fingertip is much thinner and more delicate and yet retains all the histologic landmarks of thick skin.
Figures A and B at right are a matched pair of light and electron micrographs of serial cross sections taken through the skin of the fingertip of the squirrel monkey. Like all skin, it is made up of two major regions-the Epidermis, a keratinized stratified Squamous Epithelium, and the Dermis, a thick underlying layer of connective tissue. Here, the Epidermis is so extensive and thick that it nearly fills the field. Only a small part of the Dermis in the form of the edge of a dermal papilla (DP) is visible. (Dermal papillae are ridges that project into the Epidermis, push it up, and are responsible for the complex patterns of whorls and lines in fingerprints). Skin has many functions, one of which is protecting the underlying soft, wet tissues from abrasion and dehydration. The protective function of the skin resides in the physical properties of the flattened, horny cells, or Squames, that cover its surface. The superficial layer of the Epidermis, the Stratum Corneum(SC), consists of dead cells very much like flat scales that are filled with the fibrous protein Keratin.
These cells are periodically shed from the surface of the skin. In some cases, they are rapidly worn away by physical contact with tennis rackets, lawn mowers, or, in the case of the squirrel monkey, branches. The tough, expendable surface cells would be of little use unless they were regularly replaced, and it comes as no surprise that they are, due to the special properties of the remainder of the Epidermis upon which the stratum corneum sits.
Replacement of worn-out surface Squames is ultimately a function of mitotic activity in the deepest layer of the Epidermis, the stratum germinativurn (SG). The stratum germinativurn lies atop the Dermis and consists of a single layer of large, polygonal cells. As mitotic activity occurs, daughter cells migrate upward into the next layer, the Stratum Spinosum (SS). The cells of the Stratum Spinosum have a spiny appearance because of the large number of small desmosomes that bind them together. As cells move upward through the stratum spinosum, they synthesize Keratin. When their Cytoplasm contains a noticeable number of dark, electron-dense Keratohyalin Granules, the cells form a layer known as the granular layer, or Stratum Granulosum (SGr). As the cells move upward through the Stratum Granulosum, their cytoplasmic organelles degenerate and the cells become packed with Keratin, until, in the clear layer, or Stratum Lucidum (SL), they have no nuclei and are completely devoid of organelles. The keratinized cells then migrate (or rather are pushed) up into the horny layer, or Stratum Corneum, where they serve to protect the body until lost to the outside world.
Plate 9-1, Figures A and B. Matched pair of light and electron micrographs of serial thick and thin cross sections through the skin of the fingertip of the squirrel monkey. DP, dermal papilla; E, duct of Eccrine Sweat Gland; SC, Stratum Corneum; SG, stratum germinativurn; SGr, Stratum Granulosum; SL, Stratum Lucidum; SS, Stratum Spinosum; arrow, surface opening of Eccrine Sweat Gland. 1,900 X
Thin Skin: The Scalp
Figures A and B, a matched pair of light and electron micrographs of serial cross sections taken through the monkey scalp, portray the microanatomy of thin skin. To observe the vast difference between thick and thin skin, compare the images at left to those of thick skin shown in Plate 9-1. What strikes the eye at once is how delicate the Epidermis of thin skin appears when compared with that of thick skin. In the scalp at right, for example, the Epidermis (E) measures 25 Â µm at its thickest point. The Stratum Corneum (SC), well over 1 mm thick in the sole of the foot, is only 2.5 Â µm deep in the scalp. In thin skin, the layers of the Epidermiscan be difficult to distinguish from one another. The Stratum Corneum, here composed of 10 to 12 layers of flattened, keratinized scales, is distinct and easy to recognize. The deeper layers, however, appear more homogeneous and tend to blend in with one another. The cells of the stratum granulosurn (SGr), just beneath the Stratum Corneum, are long, thin cells with oval nuclei. Although their Cytoplasm contains tonofilaments and Keratohyalin Granules, these are not as conspicuous as in thick skin. Beneath the stratum granulosum lies the stratum spinosurn (SS), filled with large, polyhedral cells that contain round, centrally located nuclei. The many desmosomes (arrow) that serve to conjoin adjacent cells appear as dark dots under the light microscope (Figure A) and as electron-dense bars in the electron microscope (Figure B).
The Basal Cells-the cells of the stratum germinativurn (SG) – are large, isodiametric cells that rest upon the finely fibrillar Basement Membrane (B). Their round, euchromatic nuclei possess prominent nucleoli; both of these features of nuclear structure indicate that basal cells are active in the Transcription of messenger RNA. The Basal Cells are mitotically active as well. Following cell division, some of the daughter cells remain in the Stratum Germinativum, where they continue to serve as stem cells. Others, however, migrate upward through the Epidermis, become keratinized, and ultimately take station as Squames in the Stratum Corneum. Surface Squames are periodically shed from the skin. Consequently, the rate of turnover of epidermal cells is high and varies with location in the body. Whereas thin skin may experience complete renewal of its stratum corneum in a week or two, thick skin may require upward of 1 month to undergo complete cellular replacement. The mitotic potential of the Basal Cells is manifest not only in the generation of cells to replace those lost in the normal course of events, but also in the regeneration of cells lost by injury. The healing power of skin is truly remarkable.
The basal layer of the Epidermis sits atop a thin Basement Membrane (B) that, in turn, rests upon the Dermis (D). As described briefly in the overview, the Dermisconsists of two regions: the papillary layer, just beneath the Epidermis, and the deeper reticular layer. In the figures at right, only part of the papillary layer is shown. In the scalp, the papillary layer is a 50 Â µm – thick region of loose connective tissue made up of a diffuse network of Collagen fibrils (CF) that support small blood vessels, nerves, and various cells of the immune system. Here, a small bundle of unmyelinated nerves (N) is evident. Although small nerve bundles are initially quite difficult to recognize in the light microscope, you can learn to identify them by comparing their images as shown in light and electron micrographs of the same magnification.
Plate 9-2, Figures A and B. Matched pair of light and electron micrographs of serial thick and thin sections taken through the scalp of the squirrel monkey. B, Basement Membrane; CF, Collagen fibrils; D, Dermis; E, Epidermis; F, Fibroblast; N, small nerve bundle; SC, Stratum Corneum; SG, stratum germinativurn; SGr, Stratum Granulosum; SS, Stratum Spinosum; arrow, desmosomes in stratum spinosum. Figure A, 3,000 X; Figure B, 3,400 X
Skin: Epidermal Derivatives
Many of the important functions of skin are carried out not only by the Dermis and Epidermis, as described earlier in this chapter, but also by highly specific skin structures collectively known as epidermal derivatives. Epidermal derivatives include such structures as hairs, sebaceous glands, Apocrine sweat glands, and eccrine sweat glands. The first three of these are captured in a single plane of section in the photomicrographs at right.
Figures A and B are a matched pair of light and electron micrographs of serial horizontal sections taken through the skin of the scalp of the squirrel monkey. Here, the knife has passed through the Dermis and has caught a hair Follicle (HF) in cross section. Closely associated with the hair Follicle are a Sebaceous Gland(SG), the duct of an Apocrine sweat gland (A), an Arrector Pili muscle (M), and a pigment cell, or Melanocyte (MC). This cluster of structures is embedded in the feltwork of Collagenous Fibers (CF) and elastic fibers, which are woven together to form the connective tissue that gives strength and substance to the Dermis. The sebaceous gland, Apocrine gland, and Arrector Pili muscle are all located next to the hair Follicle for one simple reason: their functions are closely related to the proper biologic functions of hair.
The hairs that cover your scalp and much of your body are derived from invaginations of the Epidermis called hair follicles. As seen in the figures at right, the hair Follicle is a multilayered tube of cuboidal cells that surrounds a central hair shaft (HS). During the growth of a hair, cells of the hair Follicle become keratinized. As they differentiate, they are packed into the shaft of the hair in a very precise manner. Continuous addition of new cornified cells to the base of the hair shaft pushes the hair outward through the surface of the skin. Since hair, like the Stratum Corneum of the skin, consists of keratinized cells, it is essentially lifeless. To maintain its elasticity and water-repellant properties, it must be constantly coated with oily secretions. These are provided by the sebaceous glands. As shown at right, sebaceous glands consists of cells that fill themselves with lipid droplets. During secretion, whole cells filled with lipid are released by Holocrine Secretion from a short duct that opens at the base of the hair. During times of fright, excitement, or low temperature, hairs quite literally stand on end. Movement of hair is brought about by muscles associated with the base of the hair called Arrector Pili muscles. It is believed that movement of hair by the Arrector Pili muscles compresses the Sebaceous Gland, causing its oily secretion to be expressed onto the base of the hair at the surface of the skin.
Two major types of sweat glands – Apocrine glands and eccrine glands-are present in skin. Eccrine glands, not depicted here, are responsible for the watery sweat used for temperature regulation. Apocrine glands are quite different. They secrete a highly proteinaceous material, chemically specific for each individual, that provides a chemical “fingerprint” that can readily be detected by the olfactory system. Apocrine glands release their secretion onto the skin surface at the base of the hair shaft. Consequently, the hair serves as a carrier for Apocrine secretions, which are thought to contain, among other things, pheromones. (Pheromones are chemical attractants, mediated by olfaction, that affect members of the opposite sex). The duct of the Apocrine gland at right consists of large, cuboidal cells, rich in lipid droplets, that have a brush border. The entire complex of epidermal derivatives depicted here is richly supplied by capillaries (C).
Plate 9-3, Figures A and B. Matched pair of light and electron micrographs of serial horizontal sections taken through the scalp of the squirrel monkey. A, duct of Apocrine sweat gland; C, capillary; CF, collagenous Fiber; HF, hair Follicle; HS, hair shaft; M, Arrector Pili muscle; MC, Melanocyte; SG, Sebaceous Gland. 1,000 X