The Integumentary System

The integumentary system is made up of the skin and its accessory structures which include the sweat glands, oil glands, hair, and nails. The skin consists of two major regions – the epidermis (superficial, -epi means above or upon) and the dermis (deep). The epidermis is made up of keratinized stratified squamous epithelium broken into several layers [strata (plural) stratum (singular)].  The dermis is made up of two layers of connective tissue that provides a strong anchorage of the epidermis to deeper structures. Deep to the skin (epidermis and dermis) is the hypodermis (-hypo means below or lowest). The hypodermis (pockets of adipose tissue) is not technically part of the cutaneous layers of your skin. We refer to the hypodermis as the subcutaneous layer (the skin is our cutaneous membrane).

Figure 1: Regions of the skin with and without illustration overlay

Interesting note: insulin shots and some vaccines are subcutaneous injections

The skin – a General overview

The epidermis is divided into 4-5 distinct layers/stratum (from superficial to deep): 1. stratum corneum, 2. stratum lucidum (only in thick skin), 3. stratum granulosum, 4. stratum spinosum, and 5. stratum basale. The superficial layer/strata of the epidermis (stratum corneum) often looks like it is peeling away from the underlying layers. This is because it is dead cells that are constantly sloughed off as we move about are day and not strongly attached to underlying layers (Figure 2). In this specific example we can also view brownish melanocytes in the deepest stratum (basale). These cells produce melanin to help protect us from harmful UV radiation.

Figure 2: Thin skin with pigmented deposits (melanin) in the stratum basale and loose attachment of sloughing keratinocytes in the stratum corneum with and without illustration overlay

The dermis is broken down into two layers. The papillary layer is made of areolar connective tissue, although it looks different from what we observed in earlier chapters due to a different staining technique. The reticular layer is made of dense irregular connective tissue, easy to visualize with multiple collagen fibers (pink) running in different directions (Figure 3).

From superficial to deep, the major regions of the integumentary system are:

• Epidermis
• Dermis
• Hypodermis (layer underlying skin)

The layers of the epidermis

We have two general types of skin: thick and thin. Thick skin is tough and protective, found on the palms of our hands and soles of our feet. Thin skin is found covering the rest of the body. Thin skin contains only 4 layers, lacking the stratum lucidum, and typically contains many accessory structures that we will discuss later in the chapter.

In thick skin, the epidermis is divided into 5 distinct layers/stratum (from superficial to deep): 1. stratum corneum, 2. stratum lucidum, 3. stratum granulosum, 4. stratum spinosum, and 5. stratum basale (Figure 3). For thick skin, the outer stratum corneum is several layers thick and consists of dead cells. Because the cells are dead, the stratum corneum has no darkly-staining nuclei. The stratum lucidum typically does not stain well so appears as a slightly translucent layer. The stratum granulosum appears dark and speckled/granular because it is filled with pre-keratin granules. The stratum spinosum is also usually several layers thick. The layers typically has many desmosomes (cell-to-cell junctions) making the cells look spiky at higher magnifications. The deepest layer of the epidermis is the stratum basale. In this layer, the cells also typically appear more cube-shaped and their nuclei appear darker (condensed chromosomes) as these cells are mitotically active (continuously dividing). Cells in the stratum basale also have scattered melanin granules deposited by melanocytes, which contribute to the darker appearance of the stratum basale.

From superficial to deep, the individual strata/layers of the epidermis are:

• Stratum corneum
• Stratum lucidum
• Stratum granulosum
• Stratum spinosum
• Stratum basale

Figure 3: Layers of the dermis and epidermis with and without illustration overlay

Check out our YouTube video to help you understand the layers of the epidermis and dermis:

YouTube Video – Thick Skin

Helpful Hint: To help you remember the layers of the epidermis, you can use the mnemonic “can lucy give some blood”

The Layers of the dermis

The papillary layer of the dermis is made of loosely-arranged areolar connective tissue, although it looks different from what we observed in earlier chapters due to a different staining technique. The collagen fibers look like wispy thin strings that have a lot of ground substance in-between (white space). The reticular layer is made of dense irregular connective tissue, easy to visualize with many patches of thick collagen fibers (pink) running in different directions (Figure 3).

From superficial to deep, the individual layers of the dermis are:

• Papillary layer
• Reticular layer

Integument accessory structures of the scalp

In addition to its basic tissue layers, the skin contains many accessory structures. Anchored within the connective tissue of the dermis are structures such as blood vessels, nerves, hair follicles, sweat glands, sebaceous glands, and more. The accessory structures provide additional functions to the skin. The hair (root and follicle) with its hair follicle receptor helps provide sensory information along with some protection of the skin (Figures 4 and 5). Anchoring the hair follicle to the superficial regions of the skin is the arrector pili muscle. When this muscle contracts, you see goose bumps on your skin! The sensory nerve fibers along with their receptors such as the Pacinian/lamellar corpuscle also allow for sensory information (Figure 6). The sebaceous glands secrete sebum (oil) that helps lubricate the skin and protect against microorganisms (Figures 4 and 5). The eccrine sweat glands secrete watery sweat that assists in thermoregulation (homeostasis of body temperature) and additionally secrete some waste products and provide some protection against microorganisms (Figures 4 and 5) (note: these are also seen in thick skin). The blood vessels of the cutaneous plexus supplies the hypodermis, skin, and several other accessory organs with blood (nutrients and oxygen). The blood vessels of the skin also play a role in thermoregulation.

Figure 4: Accessory structures of the skin (10x objective magnification) with and without illustration overlay

Figure 5: Accessory structures of the skin (40x objective magnification) with and without illustration overlay

Figure 6: Pacinian corpuscle (top – cross section, bottom – longitudinal section) with and without illustration overlay

Check out our YouTube video to help you understand the accessory structures of skin (the scalp):

YouTube Video – Accessory Structures in the Scalp

Chapter Illustrations By:

Juan Manuel Ramiro-Diaz, Ph.D.

Georgios Kallifatidis, Ph.D