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Lab 3. Skin

Objectives

  1. to recognize the structural elements of the skin which are responsible for its physiological functions.
  2. to understand how the various components of the integument fulfill their physiological roles.

The integument consists of the skin and its appendages (e.g., glands, hair and nails). Skin, one of the largest organs in the body (16% by weight), is composed of an epithelium, the epidermis, and an underlying connective tissue, the dermis. Internal to the dermis is areolar connective tissue called the hypodermis (subcutaneous connective tissue).

The thickness of the epidermis determines whether skin is classified as either thick or thin. The epidermis contains keratin and pigment which protect the underlying tissue. The epidermal cells are called keratinocytes because keratin is formed as the epidermal cells differentiate. Sweat glands, sebaceous glands and hair follicles are derived from the epidermis.

Sweat glands do not have a uniform distribution and are more abundant in certain areas of the body; e.g., axilla. There are two general types, eccrine and apocrine. Eccrine sweat glands are long tubular extensions from the epidermis which coil into a ball-shaped mass in the dermis or hypodermis. Apocrine glands are in the axilla, perianal and pubic areas, scrotum, labia majora and around the nipples. They lie in the subcutaneous tissue and their ducts terminate in hair follicles. There are myoepithelial cells between the secretory cells of eccrine and apocrine glands and their basement membrane.

Sebaceous glands are pear-shaped glands which empty their oily product, sebum, into the upper portion of hair follicles. Even where several glands open into the same follicle, they are situated at the same level, in the superficial region of the dermis. Some sebaceous glands exist independently of hair follicles, opening directly on the skin surface: the lips, the eyelid, the glans penis, the internal fold of the prepuce, the labia minora, and the nipple are areas.

Hair follicles arise as a proliferation and downgrowth of narrow columns of epithelial cells. Greater mitotic activity in the terminal part of this column leads to enlargement and formation of the hair bulb, a rounded mass of epithelial cells. The downward growing tip becomes indented by loose connective tissue and blood vessels from the dermis, forming the dermal papilla of the bulb. From the bulb, an inverted cone of keratinizing cells proliferates and differentiates, pushing up into the middle of the column of cells, to form a tube, the inner or internal root sheath, a thin-walled keratinized canal through which the hair will grow. The cells of the original column become the outer or external root sheath continuous with the epidermis.

Thin Skin

Examine thin skin (abdominal) in slide #45. The epidermis at the free surface under low magnification is seen as a dark line thrown into irregular folds. The dermis is a mass of dense connective tissue with only a few sweat glands and few or no hair follicles. (Hair and glands will be studied later in another slide). Blood vessels are present, but only capillaries are numerous. The hypodermis consist of adipose tissue beneath the dermis.

Examine the epidermis under high magnification: the sections were taken from black subjects and almost all the cells of the stratum germinativum (the basal layer) contain melanin. Along the basal layer, sometimes projecting towards the dermis, there are small, round cells whose cytoplasm is unstained so that the nucleus appears to be surrounded by a clear area. These are the cell bodies of the melanocytes whose processes pass between the epithelial cells and present melanosomes (pigment granules) to cells of the basal layer which ingest them. Melanosomes can occur in all cell layers, but are most common in the basal region. The stratum spinosum is several cells thick. Its name derives from the spiney appearance caused by desmosomes adhering adjacent cells to one another. As the more superficial cells of the spinosum become squamous, their nuclei flatten and a few cells are seen to contain dark blue-staining keratinohyalin granules, forming the stratum granulosum. Finally, the nucleus (and most cytoplasmic organelles seen with E.M.) disappears and the cytokeratin precursors are transformed into keratin. The keratinized cells form the stratum corneum, several layers of tightly adhering cells at the epithelial surface that may be detached during histological processing.

 

Examine the dermis under low magnification. It is divided into 2 parts: the deeper reticular layer containing coarse, irregularly arranged collagen bundles and the thinner papillary layer adjacent to the epidermis. Connective tissue cells (mast cells, fibroblasts, macrophages, leukocytes) may be encountered in either layer.

Examine slide #42 (axillary skin). Under low magnification, the general features of the epidermis and superficial part of the dermis are similar to those of abdominal skin, except for an increase in the number of gland ducts and hair follicles. Examine the epidermis under high magnification and note the absence of any visible melanin; these sections are from white subjects. Locate some melanocytes.

Groups of glandular structures are seen in the deeper parts of the dermis and extend into the hypodermis.Examine the glands on both even and odd-numbered slides. Look for large glandular structures with a wide lumen. These are the apocrine sweat glands which consist of a simple cuboidal to columnar epithelium. Under high magnification, the apical region of secreting cells contains numerous eosinophilic secretory vacuoles. Find elongated nuclei between the basement membrane and the glandular epithelial cells. These belong to myoepithelial cells whose fine, long processes encircle the gland. Long, dark fibroblast nuclei are seen along the base of the glandular cells which, unlike the myoepithelial cells, are outside the basement membrane and are part of the surrounding c.t. Ducts of the apocrine glands empty into the upper portion of Next, locate the eccrine (ordinary) sweat glands which occur as groups of twisted tubular acini, between groups of apocrine sweat glands. The lumen of eccrine glands is much smaller than in apocrine glands and the nuclei do not occur all at the same level, giving a pseudostratified appearance. Unlike apocrine glands, eosinophilic secretory vesicles are not clearly seen. Under high magnification, try to find more peripheral, darker, and roughly triangular myoepithelial cell nuclei. The ducts of eccrine glands are coiled and appear as groups of sectioned tubes in the vicinity of the glands. The ducts are more darkly stained, and the cells are smaller so that the nuclei are more crowded together. The lumen is smaller with a sharp, acidophilic border. There are really two cell layers, but due to the relationships of duct and cell size to section thickness, there seem to be more. It is difficult to distinguish between the ducts of apocrine and ecrine glands. Returning to lower magnification, examine ducts crossing the dermis towards the skin surface. Sections through hair follicles from which the hair shaft has fallen, can superficially resemble sweat gland ducts, but they are larger in diameter and have more cell layers.

Thick Skin

Thick skin (slide #43) contains a much more extensive cornified layer than thin skin. There are eccrine sweat glands (more than the abdominal skin examined earlier) but no hair follicles or sebaceous glands. The stratum corneum may be split, broken or partially missing; it is prone to sectioning artifacts because its harder consistency causes it to loosen from adjacent softer tissues.

Study the epidermis under 10x and locate the various layers: stratum germinativum, (or basale) adjacent to the basement membrane, stratum spinosum with intercellular bridges, stratum granulosum with basophilic keratohyalin granules, the stratum lucidum a pale layer just above the granulosum where nuclei become less prominent, stratum corneum. In the st. corneum, the cells no longer contain nuclei flakes of keratin. Their cell membranes have thickened and adhere tightly at the desmosomes. Intercellular material has been secreted by the cells of the st. granulosum and appears to contribute in a major way to the impermeability of the skin. Identify the dermis under low, then high magnification.

Hair Follicles

Slide #44, odd is scalp with hair follicles in longitudinal section and on slide #44, even, the hair follicles are in cross-section. In both slides, the follicles maybe cut somewhat obliquely and part of the central hair shaft may be damaged or lost in sectioning. Examine the hair follicles at low magnification near the epithelial surface. Large glandular structures with big cells having rounded nuclei can be seen. These are sebaceous glands. Nearby, bundles of smooth muscle (arrector pili) may be seen attaching to the hair follicle and running up to the papillary layer of the dermis. The most obvious components of the hair follicle at this level are the hair shaft (if present) and the external root sheath which is several layers of epithelial cells continuous with and resembling the epidermis. No internal root sheath is seen at this level.

   

Below the level of the sebaceous glands, the hair follicle wall consists of a collagenous connective tissue sheath, the external root sheath, and the internal root sheath. The internal root sheath is only two cells thick and is largely keratinized except near the bulb. The structure of the hair bulb at the deepest part of the hair follicle is best seen in slide #44, odd, but only one or two follicles may be sectioned that show the bulb clearly. It extends well into the fatty hypodermis and there is a dermal papilla indenting the epithelial cells which contains the capillary blood supply. The epithelial cells of the bulb are very basophilic and may also contain abundant melanin. These cells proliferate upward, the central mass forming the hair shaft and a rim of two cells thick forming the internal root sheath. The hair shaft and internal root sheath are seen to differentiate from each other a little above the bulb, and the cells keratinized still further up. In slide #44 even, cross-sections through the various levels can be recognized. Below the level of the sebaceous glands, the inner root sheath is clearly visible. The central hair shaft is retained in the sections of the deeper regions, before it keratinizes. The bulbs do not occur all at a given level; several sections of bulbs are found scattered among higher sections of other follicles, but still in the hypodermis. Sensory nerve endings reach the hair follicles and spread out through the dermal sheath and between the cells of the outer root sheath, but you will not be able to identify them in these preparations.