A) Skin - consists of an outer epithelial layer, the epidermis, and an inner connective tissue layer, the dermis.
Examine the epidermis in Slide #26 of thick skin where the epidermis is very thick. Note the wavy contour of the junction between dermis and epidermis. This is due to projections of the dermis (dermal papillae) into the under surface of the epidermis. Five layers may be recognized where the epidermis is well developed, as in this location. Study the layers with the oil immersion objective. Note the stratum germinativum (stratum basale) and the stratum spinosum next to it. The attachment of adjoining cells is by intercellular bridges (desmosomes) which are conspicuous in these layers.
Do you remember the ultrastructural characteristics of these cell-cell attachments?
New cells are continuously being formed by mitotic activity within the stratum germinativum. Some of these cells are pushed toward the surface, and their morphology is gradually changed. You may find references to the Malpighian layer (living layer) of the skin; some authors group the stratum germinativum and the stratum spinosum together under this heading.
Identify next the thinner stratum granulosum, in which the cytoplasm of the cells contains blue-staining irregular granules of keratohyalin, which at a later stage become associated with tonofilaments to give rise to keratin. At this level, the cells are dying. The next layer is highly refractile, the stratum lucidum. Little or no cell detail is visible in this layer or in the outer, thick, stratum corneum. Here the dead cells are converted into horny keratin and are desquamated from the surface.
Now study the epidermis in a location where it is thinner, such as in Slide #37, Slide #68 (axilla) or Slide #108 (lip). All of the layers are thin, but especially the outer three layers. In certain locations (eyelids) only the stratum germinativum and the stratum corneum are present to any extent.
Can you now outline the basic differences between thick and thin skin?
The epidermis is an example of stratified, squamous, keratinized epithelium. It contains no blood vessels, and nourishment for the cells is received from the dermal vessels. There are numerous fine nerve endings in the epidermis which are concerned with pain reception. Special staining techniques are required for their demonstration.
Now, examine the dermis in Slides #26 & #216. Note that it consists of a superficial papillary layer of loose areolar-type connective tissue and a deeper reticular layer of irregularly arranged coarse collagen. In Slide #26, special neurosensory nerve endings for fine touch - Meissner's Corpusles - are evident within the demal papillae. In Slide #216, the deeper layer rests on, and merges into the fatty subcutaneous (hypodermis) tissue, the superficial fascia. The dermis in addition contains some elastic and reticular fibers and also, particularly in the papillary layer, the various types of connective tissue cells, which you should now review. In Slide #129, dense vertical bands of regular connective tissue fibers serve to anchor the skin to the underlying phalangeal bones.
Examine in a sweat gland and in Slide #68 the special axillary sweat glands. These are simple coiled tubular glands. The secretory portions are located deep in the dermis or even in the hypodermis. It is very unlikely that a single section will show continuity through an entire gland. The darkly staining duct spirals through the dermis and also through the epidermis to open onto the surface of the stratum corneum as a sweat pore. The secretory portion of the gland is lined by a simple columnar eplithelium. Look for myo-epithelial cells between the secretory cells and the basement membrane. These cells have vesicular nuclei in which the chromatin appears evenly dispersed. The cytoplasm is more acidophilic in Slide #216.
From the name, myoepithelial, what do you think the function of these cells is, and what is their origin?
It is now appropriate to review the definition of exocrine and endocrine glands.
Can you outline the different types of exocrine glands? What is holocrine, merocrine and apocrine secretion?
Examine Slide #216, looking for blood vessels in the connective tissue. Look also for Pacinian corpuscles in the hypodermis. These are deep pressure receptors and look like onions in section; some are big enough to be seen with the unaided eye.
B) Hair - the body of vertebrates may have a protective covering, which is derived from the epidermis. This may take the form of scales, hair, or feathers. Humans are covered with hair, although the protective function has to a greater extent been lost.
Examine Slides #21, #37 & 108 which are sections through hairy skin. The hair follicles are cut obliquely so you should look at a number of different follicles and try to build up a mental picture of this structure. Each follicle contains a hair together with an internal and external (epidermal) root sheaths. The external root sheath is a continuation of the epidermis, while the internal root sheath is a lining layer of soft keratin which is analogous to the stratum corneum of the epidermis. An external connective tissue sheath (dermal root sheath) represents a condensation of dermal connective tissue which invests the external root sheath. The portion of the hair which projects above the skin consists of the cuticle (the peripheral layer of the cells are arranged like shingles), and cortex (which forms the greater portion of the shaft). If a medulla is present it consists of the central two or three rows of cells. At the base of the follicle the connective tissue forms a hair papilla which projects into the epithelium. The hair itself, however, is completely epidermal.
A sebaceous gland and a strand of smooth muscle, the arrector pili, are often associated with the hair follicle. The axis of the follicle is never exactly perpendicular to the surface of the skin and the muscle and the gland lie in the wider angle formed between the follicle and the surface. The sebaceous glands are branched alveolar glands of the holocrine type, and are encapsulated by a thin layer of connective tissue. The secreting alveoli are filled with cells. The secretion of these glands is accompanied by the breakdown of the cells and their remains are poured out with the oily secretion. The cells are thus destroyed and are replaced from the peripheral layer.
What is the function of the arrector pili muscle and the secretions of the sebaceous glands?
C) Nail - Examine Slide #33 a section of human finger showing the finger- nail. You may wish to use your text or an atlas as a guide.
The nail plate consists of closely welded keratinized epithelial cells which are firmly attached to the skin below, the nail bed. The root is protected by a fold of skin, the nail fold and is inserted in the nail groove. The nail fold has all of the layers of skin, but as it turns inward at the nail groove it loses its outer layers and only the stratum germinativum plus stratum spinosum remains. New nail is constantly being formed in the region of the root by the proliferation of epidermal cells in the matrix. As the nail glides forward a few cornified epidermal cells from the nail fold remain attached to its upper surface; these constitute the eponychium or the cuticle. Under the free edge of the nail the stratum corneum of the epidermis of the nail bed reforms and constitutes the thickened hyponychium or subunguis. The lunula is an opaque semilunar area at the proximal end of the nail and represents an area where nail keratin is immature. Once the nail keratin has matured it renders the nail transparent to the underlying blood vessels - thus the pink colour.
Why does the nail bed appear blue when the individual is cold?