OBJECTIVES: At the end of this laboratory, you should be able to:

1. Recognize skin as an organ understanding that it is made up of many subcomponents. Be able to identify these components and know their function.

2. Name the layers of the epidermis and understand the transformation of cells from the germinal layer to the surface.

3. Describe the melanocyte system and its function.

4. Identify specialized nerve endings (ie Meissner's and Pacinian corpuscles) and understand their function.

5. Identify epithelial derivatives (ie sweat glands and hair follicles) and understand their structure and function.

6. Identify the different functional states of the mammary glands and relate them to changes in the parenchymal and stromal components.

SLIDES FOR THIS LABORATORY: 43, 44, 92 and Supplemental Slides 101 and 133.

Observe the entire integument on these slides: epidermis, dermis, and hypodermis (if present). Recall that two locations exist where skin is different in both gross and histological preparation: the palms of the hands and soles of the feet are examples of thick skin and elsewhere the integument is made up of thin skin. Thick skin has a thicker epidermal layer that includes the stratum lucidum and thick skin lacks hair follicles.

Slide 43 Skin, sole of the foot.

This slide is an example of thick skin.

EPIDERMIS . Observe the layers of the epidermis : the stratum corneum (outermost), stratum lucidum (limited to thick skin), stratum granulosum, stratum spinosum, and stratum basale. Recall that the cells of the stratum spinosum are mechanically attached by desmosomes. Keratohyaline granules give the stratum granulosum its dark appearance. The stratum basale is the germinal layer of the epidermis and often shows mitotic figures.

DERMIS . The dermis is composed of two layers: the papillary layer and reticular layer . The papillary layer is closest to the epidermis. Connective tissue here is less dense than in the reticular layer. There are numerous sections of blood vessels (arterioles, venules, and capillaries) in the dermis.

ECCRINE SWEAT GLANDS . Look in the deeper part of the dermis for these glands. Observe the simple cuboidal or columnar cells of the secretory portion lying on a distinct basement membrane. Find a myoepithelial cell (with elongated nuclei, located between the basement membrane and the base of the secretory cells). Secretory portions usually stain lighter than ducts in H&E preparations. Ducts are made of stratified cuboidal epithelium (a rare type). The ducts project through the dermis to enter the epidermis and open to the surface. Find the ducts as they travel through the dermis.

Supplemental Slide 133 Skin.

Use this slide to locate Pacinian corpuscles (pressure receptors) in the deeper parts of the dermis. These are very large with an onion skin pattern.

Slide 92 Finger, monkey.

Another example of thick skin. Observe the layers and structures mentioned in Slide 43. This slide should have many Meissner's corpuscles (fine touch receptors); look for a horizontal or spiral arrangement of tissue within the dermal papillae.

Slide 44 Scalp.

This slide demonstrates thin skin. Note that the various layers of the epidermis are not as clearly seen in thin skin. Find the pigment in the region of the stratum basale. This pigment comes from the melanin producing melanocytes within the stratum basale, which synthesize melanin and transfer it to the regular epithelial cells of the stratum basale and stratum spinosum. The melanocytes are not attached to neighboring cells by desmosomes and so tend to shrink away with preparation and appear to be surrounded by a clear space. You may also see the gold-brown pigment in the epithelial cells.

Also observe hair follicles and hair shafts with associated sebaceous glands . The hair follicle (an invagination of the epidermis) has a terminal dilation, the hair bulb , composed of matrix cells and the dermal papilla. The outermost layer of the follicle is composed of a connective tissue sheath with the underlying external root sheath The external root sheath is continuous with the epithelium. Inside the external root sheath is the internal root sheath which is composed of Henle's layer , Huxley's layer , and a cuticle . Each hair shaft is composed of a cuticle , cortex , and medulla . Find the arrector pili muscle , smooth muscle that attaches to the connective tissue sheath of the hair follicle and erects the hair upon contraction.

Supplemental Slide 101 Skin of palm, human.

This slide is sectioned very thin (1.5 mm) and stained with H&E. Observe the layers of the epidermis and try to find Meissner's corpuscles within the dermal papillae.

Slide 24 Mammary gland, active, adult.

This slide is a lactating or active gland. Note that the alveoli are well developed and branched from the duct system . The alveoli and lobules expand at the expense of the connective tissue; thus, in a lactating gland one should see a reduction in the amount of connective tissue. Most alveoli are distended and partially to wholly filled with milk. The alveoli consist of a single layer of cuboidal epithelium surrounding the milk-filled cavity . Very active alveoli may be lined with squamous epithelial cells.

Slide 25 Mammary gland, inactive, juvenile.

This slide is a prepubescent nonlactating (resting) gland. Note the lack of development of alveoli ; the lobules consist of segments of the terminal duct system surrounded by connective tissue. The gland consists largely of fibrocollagenous connective tissue .

Slide 26 Mammary gland, inactive, adult.

This slide is an adult nonlactating gland. Compare the amount of adipose tissue with that seen in Slide 25. Also, the alveoli and the relative amount of glandular tissue appear to be more developed in the gland on this slide compared to that on Slide 25 .