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
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).
outer root sheath
arrector pili muscle
"apocrine", also merocrine
General Sensory Receptors:
Be sure to review the SKIN study unit.
To be able to:
- Recognize and
describe the structure and function of the major components of skin: the
epidermis and dermis.
- Compare the general morphological features, locations and functions
of thick and thin skin.
- Distinguish and describe the structure, function and origin of
sebaceous glands and sweat glands (eccrine and apocrine).
- Recognize the general morphological features and describe the origin
and growth process of hair and nails.
- Describe the morphological and biochemical features of the
- Describe the turnover and shedding of cells of the epidermis.
- Describe the origin, function, and location of melanocytes and their
differences in different types of skin.
- Describe briefly the processes which occur during wound healing and
after skin grafting.
- Describe the blood and lymphatic circulations of skin.
- Describe the structure and function of the different sensory
receptors associated with 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
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
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.
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.
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
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.
- Compare thick and thin skin as to structure, function and
- 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?
- How do
melanosomes form? How do they get into keratinocytes?
- How do
black and white skin differ?
- Compare the secretory portions and ducts of apocrine, eccrine and
holocrine (sebaceous) glands as to structure, function and location.
- What is the role of the Langerhans cell?
- 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?
- What are the following believed to do, be or become:
- keratohyalin granules
- membrane coating
- Describe the morphology of the healing process
after skin is cut.
- Compare keratinized and nonkeratinized
stratified squamous epithelia as to structure and location.
- Describe the process of hair growth. If hairs fall out or are
plucked out, where does regrowth come from?
- Compare the location,
structure and function of the various sensory receptors associated with
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