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

Recognize skin as an organ understanding that it is made up of many subcomponents. Be able to identify these components and know their function.
Identify the layers of the epidermis and understand the transformation of cells from the germinal layer to the surface.
Identify specialized nerve endings (i.e., Meissner's and Pacinian corpuscles) and understand their function.
Identify epithelial derivatives (i.e. sweat glands and hair follicles) and understand their structure and function.
Identify the different functional states of the mammary glands and relate them to changes in the parenchymal and stromal components.

Observe the entire integument on these specimens: 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.

Skin, sole of the foot
This specimen 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.
Identify melanocytes within the stratum basale, which synthesize melanin and transfer it to the regular epithelial cells of the stratum basale and stratum spinosum. Since the melanocytes are not attached to neighboring cells by desmosomes they shrink away during tissue preparation and appear to be surrounded by a clear space.
Dermis. The dermis is composed of two layers: the papillary layer and reticular layer. The papillary layer is closest to the epidermis. Connective tissue (loose CT) here is less dense than in the reticular layer (dense irregular CT). 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 cells or simple 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 of the glands usually stain lighter than ducts (conducting portions) 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.

Skin, thin
Use this specimen to locate Pacinian corpuscles (pressure receptors) in the deeper parts of the dermis. These are very large with an onion skin pattern. Also find Meissner's corpuscles within the dermal papillae. They are fine touch receptors with a horizontal or spiral arrangement of tissue and cell nuclei. This specimen also contains many eccrine sweat glands.

Finger, monkey
This specimen is an example of thick skin. Observe the layers and structures mentioned in the previous specimen. Observe the various layers of the epidermis; stratum basale (note the melanocytes in this layer), stratum spinosum, (note the small spinous processes between the cells; these represent the many desmosomes between the tightly-adhering cells), stratum granulosum (note the keratohyaline granules in the cytoplasm), stratum lucidum, and stratum corneum. Note that because of the oblique orientation of the epidermis in this section; the appearance of the layers may differ from what is observed in typical transverse orientations. This specimen has many Meissner's corpuscles within the dermal papillae.

Scalp, thin skin
This specimen 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 melananosomes and transfer them 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.
Observe sebaceous glands associated with hair follicles.

Integument