Clinical Applications of Human Anatomy and Physiology for Healthcare Professionals. Jassin M. Jouria

– This layer of skin is known as the clear layer, which under microscopic evaluation appears as a translucent layer or band of skin that serves as a boundary of sorts between the stratum corneum and the next layer of skin, the stratum granulosum. It’s much narrower than that of stratum corneum, maybe three cells thick.

      Stratum granulosum – Known as the granular layer, stratum granulosum is also relatively thin and averages one to five layers in cell thickness. Within this skin layer, keratinocytes are constantly morphing due to a process known as keratinization. During this process, the cells change shape; they flatten while their organelles and nuclei disintegrate. During this process, they collect two types of granules. One type is the keratohyalin granule and the other is the lamellar granule. The keratohyalin granules are involved in the production of keratin. The lamellar granules contain glycolipid – a water-resistant lipid that aids in water retention of the epidermis. The function of the stratum granulosum layer is to inhibit loss of water from the epidermis, and ergo serves as an effective water barrier.

      Stratum spinosum – This layer of the skin is known as the prickly layer, again several cell layers thick. Its primary function is to aid in flexibility and add a layer of strength and protection to areas of the skin that may experience greater friction or abrasion, such as the palms or the soles of the feet. The cells of the stratum spinosum actually interconnect into a network much like a spider’s web. They appear under a microscope as round balls with spikes extending from them. In this layer of skin, keratinocytes appear to have prickly ridges or spines, hence their name.

Stratum basale – The basal cell layer can be thought of as the “basement” membrane or layer of the epidermis. It’s also known as the stratum germinativum. This layer is about one cell thick and is mainly responsible for replenishing or replicating through mitosis. For this reason, this layer of skin is often nicknamed the germinating layer. Following cellular division in this layer, one basal cell from each cell division remains behind while the other migrates outward. That migrating cell passes through what are known as intermediate layers of the epidermis. As the migrating cell passes through each of those intermediate layers, it flattens still further and differentiates. It takes an average of 28 days for a mature keratinocyte to reach the stratum corneum or the outer layer of the epidermis.11

      In a healthy, natural, and uninjured state, the epidermis protects the body against invasion by germs. It also provides padding and protects the body against potentially dangerous heat or water loss.

      The dermis is the second layer of skin, found directly beneath the epidermis.

      The dermis, also known as the cutis, measures anywhere from 1 mm to 4 mm in thickness and serves as a foundational support to the skin, as well as to its mass. The dermis itself comprised of numerous components of connective tissues that include:

      •Elastin

      •Collagen

      •Reticulum fibers

      •Water

      •Sensory nerves

      •Postganglionic nerves

      •Blood vessels

      •Lymphatics

      The dermis is also known as the corium. Two distinct layers of the dermis include the papillary layer and a reticular layer.

The papillary layer contains blood vessels and connective tissues that provide nutrients to the outer skin layer (epidermis). The papillary layer also helps to maintain temperature of the skin.12

The reticular dermis is a thicker, bottom layer of the dermis, which also contains blood vessels and connective tissues in support of the skin, but in addition also contains sweat glands, oil, and hair follicles.13

      This layer of skin contains a network of fibrous, elastic-like connective tissues as well as other components including:

      •Sebaceous glands

      •Sweat glands

      •Nerves

      •Lymphatics

      •Blood vessels

      •Involuntary muscle fibers

      Not all layers of dermis are the same thickness. Compare the difference between the skin of the eyelid and the skin on the palm of your hand or the sole of your foot.

      The third layer of skin, the subcutaneous layer, typically consists of lipocytes (aka adipocytes) or fat cells. These fat cells are separated by fiber-like walls of the blood vessels and collagen. Adipose or fat tissue serves as the body’s insulator against heat. Subcutaneous tissues also serve as a/an:

      •cushioning barrier between bones and the outside environment

      •storage site (for calories)

      •an aid for anchoring of bone, muscle, and tendons

      Figure 3-3 Skin glands.

      Thickness or density of subcutaneous (adipose) tissues varies depending on location in the body. This is why humans tend to have more “cushioning” around the waist or the hips and very little in the eyelids for example. A number of factors can influence subcutaneous tissue distribution such as genetics, age, diet, and hormones.

       ■Your Sweat Glands

      Three types of glands are also found in the dermis:

      •Apocrine sweat glands

      •Eccrine sweat glands

      •Sebaceous glands

      Sweat glands are known as sudoriferous glands.

      We have two different types of sweat glands: apocrine and eccrine.

The body contains an estimated 2 million sweat glands.14 Two major types of sweat glands are identified here: apocrine and eccrine.

      Figure 3-4 Apocrine sweat gland.

      Apocrine glands are those found in mammary, anal, and pubic areas of the body and typically provide access to hair follicles. Apocrine glands are found around the:

      •Umbilical area

      •Axilla

      •Genitals

      •Breasts

      •Eyelids

      •External ear canal

      Interestingly, apocrine glands are quite mysterious, with no specific biological function although they can be triggered by epinephrine and norepinephrine to secrete. On the surface of the skin, these secretions make contact with bacteria found on the skin or in hair follicles, producing body odor.

      Two additional glands are associated with apocrine sweat glands: ceruminous glands and mammary glands, although mammary glands secrete milk rather than sweat. We’ll discuss functions of the mammary glands in greater detail in a later chapter. For now, we consider the mammary glands as part of the integumentary system.

      Ceruminous glands line the outside of the external ear canal. Secretions of the ceruminous glands combine with sebum from nearby glands and form cerumen, more commonly known as earwax. Earwax is a major deterrent when it comes to protecting the ear canals from dirt, foreign materials, and even insects.

      Eccrine glands are found on the trunk, the extremities, the head, and on the hands as well as the soles of the feet.

      These glands are involved in heat regulating functions and mechanisms, activated and controlled by the sympathetic nervous system.

      Such functions will be discussed later