SKIN AND SENSORY RECEPTORS

2006 Unit reviewed by: Victoria Ort, Carmen De Lemos-Chiarandini, Edith Robbins, and Virginia Black

Link to the Hippocrates Module on the SKIN


INTRODUCTION

The skin (integument) and its derivatives (adnexa) comprise the largest organ in the body. Skin is composed of two layers: (1) the ectodermally derived surface layer called the epidermis and (2) the underlying mesodermally derived connective tissue layer which is called the dermis. The epidermis is a stratified squamous epithelium with a different number of cell layers in different areas of skin. As is true of almost all epithelia, it is avascular. It does, however, contain cells belonging to the immune system and is, therefore, important in immune response. The epidermis rests on a basement membrane that delimits it from the dermis. The connective tissue of the dermis most often indents the basal surface of the epidermis, forming dermal papillae. Blood and lymphatic vessels as well as most nerves and sensory receptors (for pain, pressure, touch etc.) are found in the dermis. There are some "naked" nerve endings, which can only be seen with specific stains, that penetrate the lower layers of the epidermis, but for the most part nerve structures are located in the dermis. The subcutaneous tissue (superficial fascia in gross anatomy) anchors the skin to underlying tissues. Often it has large amounts of adipose tissue and it may contain the deeper elements of sweat glands as well as sensory receptors.

Based on the thickness of the epidermal layer only, skin is classified as thick or thin. This can be somewhat misleading because there are regions where the skin is quite thick due to a robust dermis, yet these areas are called thin skin because the epidermis is thin.

The adnexa include teeth, hair, glands (sebaceous, sweat, mammary) and nails. Embryologically they begin as down growths into the connective tissue and they require epithelial-mesenchymal interaction for development. You will examine examples of each of these and be responsible for them, except for developing teeth, which are optional (and found in slides 8 and 42). The mammary glands will be examined with the female reproductive system.

The sensory receptors in the skin, such as Meissner's and Pacinian corpuscles, as well as free nerve endings, are classified as general sensory receptors. Other members of this group of receptors include free nerve endings in the connective tissue, as well as the muscle spindle and Golgi tendon organ found in the musculoskeletal system. Special sensory receptors have specific anatomic locations. They include the retina of the eye (see Nerve Tissue) and portions of the inner ear, the Organ of Corti in the cochlea and the maculae and cristae ampullares in the vestibular apparatus (included in Nerve Tissue, but to be studied in Brain and Behavior), as well as the olfactory mucosa (see Respiratory System) and taste buds (see Digestive System).

Key Words

Skin, Integument:
epidermis
stratum basale
stratum spinosum
stratum granulosum
stratum lucidum
stratum corneum
keratinocyte
melanocyte
Langerhans cell
Merkel cell
dermis
papillary dermis
reticular dermis
dermal papillae
Adnexa:
hair
hair matrix
dermal papilla
hair papilla
inner root sheath
outer root sheath
arrector pili muscle
nail
nail groove
nail plate
eponychium
hyponychium
nail matrix
sebaceous glands
holocrine
sweat glands
eccrine, merocrine
"apocrine", also merocrine

General Sensory Receptors:
Meissner's corpuscle
Pacinian corpuscle

Be sure to review the SKIN study unit.


OBJECTIVES

To be able to:

  1. Recognize and describe the structure and function of the major components of skin: the epidermis and dermis.

  2. Compare the general morphological features, locations and functions of thick and thin skin.

  3. Distinguish and describe the structure, function and origin of sebaceous glands and sweat glands (eccrine and apocrine).

  4. Recognize the general morphological features and describe the origin and growth process of hair and nails.

  5. Describe the morphological and biochemical features of the keratinization process.

  6. Describe the turnover and shedding of cells of the epidermis.

  7. Describe the origin, function, and location of melanocytes and their differences in different types of skin.

  8. Describe briefly the processes which occur during wound healing and after skin grafting.

  9. Describe the blood and lymphatic circulations of skin.

  10. Describe the structure and function of the different sensory receptors associated with skin.


LAB GUIDE

  1. Skin

    1. Thick skin. Slides 35 and 21

    In both slide 35 (thick skin) and slide 21 (moderately thick skin) delineate the approximate border between epidermis and dermis. Analyze the layers of the epidermis from the point of view of changes occurring as the cells progress toward the free surface. The deepest layer of the epidermis is the stratum basale or the stratum germinativum. Also note in this layer the cells with clear cytoplasm. Most of them are melanocytes, but some could be Langerhans cells or Merkel cells, which are dendritic cells of the immune system and sensory system, respectively, that also have clear cytoplasm. These three clear cell types cannot be distinguished in routine paraffin sections. The next several layers belong to the stratum spinosum. If you focus up and down, you may be able to locate the so-called "intercellular bridges," created by desmosomal contacts between cells that are maintained despite the artifactual shrinkage of the cells during tissue preparation. They give this layer a spinous appearance. The next two more superficial layers are transitional in character, representing stages in the process of keratinization. Identify keratohyalin granules in the stratum granulosum. The stratum corneum represents the end stage of keratinization. Its outer layers desquamate.

    You should find portions of sweat gland ducts in the subcutaneous connective tissue, the dermis and epidermis. In the subcutaneous tissue and dermis, the ducts are lined by a stratified cuboidal epithelium. In the epidermis, the cells lining the ducts undergo the same keratinization changes as the surrounding keratinocytes. The ducts follow a tortuous course through the layers of the epidermis. Even if they are present in your section you should not expect to see the entire course of any one duct.

    Delineate the two layers of the dermis: papillary (the portion of the dermis associated with the indentations of the epidermis) and reticular (the deeper more fibrous layer). Compare the thickness, fiber components and vasculature of the two layers. Study the subcutaneous fascia.

    2. Thin skin. Slides 9 and 36

    Compare the various layers of the skin on the dorsal surface of the fetal finger (slide 9) and of the scalp (slide 36) with the slides above, and with the skin of the ventral surface of the finger (slide 9). In some boxes slide 9 is from a late fetus.

    B. Hair and Nails (see diagrams on SS-4 and SS-5)

    1. Nail. Slide 9

    Using the diagram on SS-4, identify on your own finger, the nail wall, the fold of skin around the nail (paronychium), proximally and laterally. Also identify the groove lying between it and the nail (nail groove), the nail plate and the nail bed, which lies beneath the nail plate. Identify the eponychium (cuticle), hyponychium and nail matrix. Finally, identify the nail and underlying stratum germinativum. Compare the stratum corneum of the skin with the nail.

    2. Hair. Slides 36 and 42

    Analyze the hairs that are cut in various planes in the scalp (slide 36) and on the outer side of the lip (slide 42). Identify the hair shaft, hair bulb, hair matrix and dermal papilla. In the follicle, two sheaths surround the hair shaft, one derived from the epidermis, the external root sheath, and one from the connective tissue, the dermal sheath. The connective tissue sheath or dermal sheath gives rise to the dermal papilla, which projects intothe base of the follicle.

    Locate the arrector pili muscle and note its relation to the follicle and to the sebaceous gland. If your slide 36 is from fetal rather than adult scalp, compare with that of your neighbor.

    C. Cutaneous glands

    1. Sebaceous glands. Slide 36 and 42

    Identify the sebaceous glands on these slides of scalp (slide 36) and lip (slide 42). Note their close relationship to hair follicles. As the cells of the glands move from the base toward the center they become filled with lipoidal material (sebum). Sebaceous glands secrete their material in a holocrine fashion; the cells undergo apoptosis and cell debris as well as secretory product are released into the duct. The short ducts of the sebaceous glands empty into hair follicles and their secretion reaches the surface via the hair follicle. Try to find a section through the juncture between gland duct and follicle and note the continuity of its cells with the epidermally-derived external root sheath of the follicle, which is continuous with the stratum germinativum of the skin.

    2. Eccrine sweat glands. Slides 21 and 35

    The intraepidermal course of the ducts of these glands has already been observed. Follow their intradermal course to the coiled glands. The secretory portions of the glands and the proximal duct segments may be found in the dermis, or in the more superficial areas of the subcutaneous tissue or both. Distinguish secretory portions from the ducts. The duct cells stain more intensely than do the secretory cells, but the ducts lack myoepithelial cells. Look for myoepithelial cell nuclei between the secretory cells and the basement membrane. The eosinophilic cytoplasm of the myoepithelial cells adjacent to the basement membrane adds to the apparent thickness of the basement membrane.

    3. Salivary glands and mammary glands

    These are also epidermal derivatives, but will be studied with the digestive system and female reproductive system, respectively.

    4. Fetal skin. Slides 8 and 36

    Look briefly at the epidermis. The cells stain very lightly because they contain large glycogen deposits which are often extracted during tissue preparation. Note the developing dermis. Your slide 8 may show developing facial hair (vibrissae, of the pig). Half of your boxes have fetal scalp as slide 36, so compare with your neighbor's slide.

    D. General Sensory Receptors. Slides 21 and 35

    In slides 21 and 35 of the skin, sensory nerve endings are inside Pacinian and Meissner's corpuscles. Pacinian or lamellar corpuscles, which are encapsulated receptors for deep pressure, are found in the dense connective tissue of the dermis below the epithelium in slide 21. They are large and the concentric layers of the connective tissue capsule surrounding the nerve ending gives them an appearance similar to an onion cut in section. Fibroblasts in the connective tissue lamellae of the corpuscles have an epithelial character similar to that of the layers of the perineurium of peripheral nerves. This is another example of lamellated connective tissue. Meissner's corpuscles are encapsulated receptors for touch, which are found in the loose connective tissue papillae just beneath the epidermis in slides 21 or 35. The connective tissue surrounding these nerve endings is helically wound and the corpuscle appears as an ovoid body consisting of layers perpendicular to the surface of the skin. This orientation will help you to find them, but remember that they are scattered and my be lacking from some sections.


    STUDY QUESTIONS

    1. Compare thick and thin skin as to structure, function and location.

         

    2. What is the embryonic origin of melanocytes? Where are functioning melanocytes found? What is their relationship to the basal lamina of the epidermis? Are melanocytes present in the dermis?

    3. How do melanosomes form? How do they get into keratinocytes?

    4. How do black and white skin differ?

    5. Compare the secretory portions and ducts of apocrine, eccrine and holocrine (sebaceous) glands as to structure, function and location.

         

    6. What is the role of the Langerhans cell?

    7. Name, in order, the layers of the epidermis in thick skin from basement membrane to free surface. Describe the ultrastructural changes occurring in keratinocytes as they move from the basal layer to the cornified layer. Approximately how long does this process take?

         

    8. What are the following believed to do, be or become:
      • tonofilaments
      • keratohyalin granules
      • membrane coating granules

    9. Describe the morphology of the healing process after skin is cut.

    10. Compare keratinized and nonkeratinized stratified squamous epithelia as to structure and location.

    11. Describe the process of hair growth. If hairs fall out or are plucked out, where does regrowth come from?

    12. Compare the location, structure and function of the various sensory receptors associated with skin.

         


    The following POP-QUIZ has questions reminiscent in style and difficulty to those to be asked in the exam.
    Don't look at it before you are ready.

    Skin and Integument POP-Quiz


    Remember to link to the Hippocrates QUIZ on SKIN

    Copyright 2006 New York University