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Objectives:
- Be able to identify principal layers of the skin (epidermis, dermis and hypodermis) at the light microscope level and know the principal functions of each layer.
- Be able to identify the layers of the epidermis in thick and thin skin and describe the major cellular events that take place in each layer in the process of keratinization.
- Identify melanocytes and explain the process of pigment formation in the skin.
- Be able to identify eccrine and apocrine sweat glands at the light microscope level.
- Identify the components of the pilosebaceous apparatus and know the structural and developmental relationship between each component and the epidermis of the skin.
- Be able to:
a. identify the mammary gland and its structural components:
i. Lobules
ii. Lobes
b. Recognize the histological differences in active and inactive glands
The skin and its associated structures, hair, sweat glands and nails make up the integumentary system. The mammary glands are modified sweat glands.
I. SKIN
!! If you are a WINDOWS USER on campus, remember to map a drive to the file server (click here), and USE THE [WinLab] LINKS !!
A. Thick skin (W pgs 167-172, 9.2-4)
Slide 106 (plantar skin, H&E) [WinLab] [Mac] [WinHome]
Slide 112 (plantar skin, H&E) [WinLab] [Mac] [WinHome]
Slide 112N (plantar skin, H&E) [WinLab] [Mac] [WinHome]
In this slide the structure of skin, especially the epidermis, is exaggerated in response to the continued stress and abrasion applied to the plantar surface of the foot. Study the epidermis in slide #106, #112, and #112N. Identify the various strata:
1. Stratum basale (also known as S. germinativum): A single layer of cuboidal to columnar cells resting on and separated from the underlying dermis by a basal lamina. Mitotic figures occur in this layer.
2. Stratum spinosum: Several layers in thickness. In reduced light, the cells appear interconnected by "spinous" processes.
3. Stratum granulosum: A few layers of cells that are characterized by numerous, dense, basophilic granules. These are keratohyaline and membrane coating granules.
4. Stratum corneum: Note the striking change in cellular morphology. The cells are flattened, devoid of nuclei or cytoplasmic granules, and filled with mature keratin (#112N). In #106 or #112, however, nuclei are still present in many cells of this layer, which are not normal. Because of differential dye penetration, the staining of the stratum corneum is variable and unpredictable. Sectioning artifacts are common.
The principal cell type of the epidermis is termed a keratinocyte and you will see this term used as a general descriptor for the epithelial cell found in any stratified squamous epithelium. Note the absence of blood vessels in the epidermis. Nourishment is obtained by diffusion from capillaries in the underlying dermis.
The interface of the epidermis and dermis is uneven. A pattern of ridges and grooves on the deep surface of the epidermis fit a complementary pattern of corrugations of the underlying dermis. The projections of the dermis are called dermal papillae and those of the epidermis, epidermal ridges (pegs), because of their appearance in vertical sections of the skin. However, these terms are not always accurately descriptive of the three dimensional configuration of the region of interdigitation. With low power, identify the epidermal ridges and dermal papillae. (W pg 158, 9.2). What is their function? (IN1) Note the finer arrangement of collagen fibers in the papillary dermis [example] as opposed to that of the reticular dermis [example] (refer back to slide 33 [WinLab] [Mac] [WinHome] in order to review the morphology and distribution of elastic fibers in the dermis). The fatty layer beneath the dermis, the subcutaneous connective tissue, is often called the hypodermis or superficial fascia. It is this layer that allows the skin to "move".
B. Thin Skin
Slide 105-1 (thin skin, H&E) [WinLab] [Mac] [WinHome]
Slide 105-2 (thin skin, H&E) [WinLab] [Mac] [WinHome]
Slide 104-1 (thin skin, H&E) [WinLab] [Mac] [WinHome]
Slide 104-2 (thin skin, H&E) [WinLab] [Mac] [WinHome]
The epidermis is much thinner and simpler in structure. Each stratum is thinner and the stratum granulosum may be absent. Melanocytes [example] (derived from neural crest cells) capable of producing the pigment melanin are numerous in the deeper (toward the base) layers of the epidermis. They can be identified by the presence of a nucleus surrounded by a clear space. The cells with brownish pigments are actually keratinocytes that have received melanin granules from the melanocytes by pigment donation. The slides 104-1 and 104-2 are skin samples from light and darker skinned individuals. It is not difficult to tell which sample is from which individual. Note the presence of portions of hair follicles and sebaceous glands in the dermis.
II. SCALP AND HAIR (W pg 176-8, 9.8 -9)
Slide 107 (scalp, H&E) [WinLab] [Mac] [WinHome]
Underneath the thin epidermis, there are numerous circular to oblong structures with a hollow center and surrounding cellular layers. These structures are hair follicles [example] sectioned transversely or tangentially at different levels. The hollow center is occupied by the keratinized component of the hair which has been removed during the slide preparation. The surrounding layers of clear cells form the external root sheath of the hair, which is a downgrowth of the epidermis. Note the presence of sebaceous glands [example] (W pg 168, 9.14) and the arrector pili muscle [example] near the hair follicle. In most instances, you will not find complete pilosebaceous units in a single section, so a bit of mental reconstruction will be required.
III. SWEAT GLANDS
A. Eccrine sweat gland
Slide 106 (plantar skin, H&E) [WinLab] [Mac] [WinHome]
Slide 112 (plantar skin, H&E) [WinLab] [Mac] [WinHome]
Slide 112N (plantar skin, H&E) [WinLab] [Mac] [WinHome]
Slide 105-1 (thin skin, H&E) [WinLab] [Mac] [WinHome]
Numerous coiled eccrine sweat glands are located at the junction of dermis and hypodermis (W pg 179, 9.10). The coiled morphology can be particularly appreciated in the example shown in slide 105-1. Distinguish between the secretory portions of the gland (secretory cells and myoepithelial cells; the latter are best seen in the apocrine gland, see below, slide 111 and 104-2) and the stratified (two layers) cuboidal epithelial cell lined duct. Where do the ducts empty? (IN2) How does sweat get to the surface? (IN3)
B. Apocrine glands (W pg 180, 9.11)
Slide 109-1 (perianal region, trichrome) [WinLab] [Mac] [WinHome]
Slide 109-2 (perianal region, PAS) [WinLab] [Mac] [WinHome]
Slide 111 (axilla, subcutaneal region, trichrome) [WinLab] [Mac] [WinHome]
Slide 104-2 (thin skin, H&E) [WinLab] [Mac] [WinHome] (this slide has both eccrine AND apocrine sweat glands; see if you can identify them)
Look in the deep dermis or hypodermis for secretory tubules with a wide lumen [example]. The epithelium is cuboidal to columnar with distinct apical secretory granules. What should be apparent in your section is the apical "blebbing" of the secretory cells that was responsible for designating these cells as "apocrine" secretory cells. These glands, present in the axillary, areolar, and anal regions, represent the second type of sweat glands. These glands produce a viscous secretion which acquires a distinctive order as a result of bacterial decomposition.
In complete transverse sections of the glands (e.g. slide #111), look for oblong nuclei just inside the basement membrane. These are the nuclei of myoepithelial cells (W pg 167, 9.10b). In some planes of section, the nuclei may appear round. Now look for a tangential section of the gland. Look for regularly spaced, elongate strands of cytoplasm investing the outside of the secretory epithelium. With a bit more looking you should be able to see that these elongate bits of cytoplasm contain the above mentioned oblong nuclei. Slide 104-2 is another good place to see myoepithelial cells [example]; look for eosinophilic strands of cytoplasm and small nuclei just peripheral to the secretory epithelial cells. In PAS stained slides (e.g. #109-2), densely staining granules are obvious in the apical cytoplasm the secretory cells, but the cytoplasm of myoepithelial cells is not well seen. Instead you see darker pink-stained basement membrane between the projections of myoepithelial cells. The relatively great number of these cells in sweat glands (and mammary glands) can only be appreciated by studying such tangential cuts.
IV. MAMMARY GLAND
A. Nipple and Areola (W, pg. 386, 19.37)
Slide 265 (nipple and areola, H&E) [WinLab] [Mac] [Winhome] .
The 16-20 lactiferous ducts [example], one from each lobe, open at the summit of the nipple. These ducts are lined by stratified squamous epithelium near the opening and the lumens are frequently filled with desquamated cells. Deeper in the connective tissue, the ducts acquire a stratified columnar appearance that is really a cuboidal duct cell sitting on a myoepithelial cell as in the sweat gland.
Note the epithelium covering the nipple and areolar region contains a great deal of melanin that is responsible for the macroscopic pigmentation of these areas. Sebaceous glands [example] are present to a variable extent, especially in the areola. Note that the dense irregular connective tissue of the dermis is interrupted by numerous fascicles of smooth muscle [example] that insert into the dermal connective tissue (much like arrector pili muscles). These muscle bundles are responsible for erection of the nipples. Occasional nerves are also present in the dermis (this is another good slide to test your ability to recognize these different tissues).
B. Mammary Gland (W, pg. 387-8, 19.38-40)
Slide 259 (inactive mammary gland) [WinLab] [Mac] [WinHome]
Slide 258 (active mammary gland, H&E) [WinLab] [Mac] [WinHome]
Like the other tissues in the female reproductive system, alterations in circulating
hormone levels result in histologically demonstrable changes in the mammary gland.
Compare the examples of an inactive (Slide #259) and active (#258) gland, noting the differences in the amount of glandular tissues. In slide #259 (inactive gland) note the dense irrengular interlobular connective tissue found between quiescent glandular lobules that consist of only a few clusters of small ducts surrounded by a mass of less dense intralobular connective tissue. Many ducts appear to be composed of 2 layers of cuboidal epithelium. The inner layer are the actual ductal epithelial cells whereas the outer layer of cells is, in fact, a layer of myoepithelial cells.
In slide #258 (active gland), you can see that the amount of the glandular tissues has increased, while that of the connective tissue has decreased. This increase involves the numbers of both the epithelial cells and myoepithelial cells. The proliferation of these cells lead to the formation of secretory alveoli. Note also the increased cellularity (especially, the plasma cells) of the intralobular connective tissue.
Electron Micrograph Wall Charts
#18 EPIDERMIS [WinLab] [Mac] [WinHome]
Review the layering of the epidermis. Remember that there is a
continuous process of cell migration and differentiation from
the basal cell layer to the most superficial layer. Review the
features of the epidermal-dermal junction. (Rhodin, Fig. 25.4).
#83 EPIDERMIS - BASAL CELL [WinLab] [Mac] [WinHome]
Observe the abundance of tonofibrils (= keratin intermediate filaments) and ribosomes and the small number of mitochondria and absence of Golgi apparatus and granular endoplasmic reticulum. Epidermal cells do contain these organelles but in reduced amount. The bulk of synthesis is for structural proteins, not exportable ones. What is the function of the numerous desmosomes? The function of the tonofibrils? (IN4) (No Rhodin counterpart) (alt. Fig. 25-7).
#84 EPIDERMIS - SUPERFICIAL CELL LAYERS [Winlab] [Mac] [WinHome]
Note the keratohyaline granules in the cells of the stratum granulosum. The keratinization process is completed in the cell layers above the stratum granulosum, indicated by the disappearance of nuclei and cell organelles. Note that the cornified cells are of variable appearance (some "dark" and some "light") a reflection of processing rather than from a functional difference. (Rhodin, Fig. 25-9).
#85 MELANOCYTE [WinLab] [Mac] [WinHome]
These cells are located within the basal cell layer of the epidermis. Note absence of tonofibrils within the melanocyte. Find the premelanosomes. The melanosomes of the melanocyte are passed to the adjacent basal cells of the epidermis. (No Rhodin counterpart) (alt. Fig. 25-18).
#86 SEBACEOUS GLAND [WinLab] [Mac] [WinHome]
The purpose of reviewing this micrograph is to make sure that you realize that sebum is composed of cell fragments and lipid (sebaceous) droplets, that are discharged by a process referred to as holocrine secretion. The accumulation of sebaceous (lipid) droplets in the cell is a gradual process, starting in the peripheral cells of the sebaceous alveolus. (No Rhodin counterpart) (alt. Figs. 25-42, 25-43).
#87 SWEAT GLANDS [WinLab] [Mac] [WinHome]
Note the difference in size between ordinary and odoriferous (apocrine) sweat glands. What is the function of the numerous myoepithelial cells? (IN5) (No Rhodin counterpart) (alt. Figs. 25-47, 25-48).
Review Question Answers
IN1: What is
the function of the epidermal ridges and dermal papillae?
answer
IN2: Where do the ducts (eccrine
sweat glands) empty?
answer
IN3: How does sweat (from eccrine
sweat glands) get to the surface?
answer
IN4: What is the function of
the numerous desmosomes (in the basal cell)? The function of the
tonofibrils?
answer
IN5: What is the function of
the numerous myoepithelial cells (in sweat glands)?
answer
Practice Exam (Basic Tissues & Integument)
1. Actin filaments are NOT associated with:
A. microvilli
B. hemidesmosomes
C. zonula adherens
D. Z-lines in skeletal muscle
E. dense bodies in smooth muscle
F. intercalated disks in cardiac muscle
click here for the answer
2. In this tissue section stained with iron hematoxylin and eosin, the the two primary protein components of the structure indicated by the arrow are:
Click here to see the image
A. type III collagen and proteoglycans
B. type I collagen and proteoglycans
C. type IV collagen and laminin
D. elastin and fibrillin
E. actin and myosin
answer
3. The tissue shown is mostly:
Click here for the low mag view
Click here for a high mag view
Click here for an alternate high mag view
A. cardiac muscle
B. skeletal muscle
C. smooth muscle
D. dense irregular connective tissue
E. dense regular connective tissue
answer
4. The tissue indicated is:
click here for the image
A. simple cuboidal epithelium
B. dense regular connective tissue
C. cardiac muscle
D. smooth muscle
E. myelinated nerve
answer
5. The neurons in the ganglia shown:
click here for the low mag image
click here for the high mag image
A. innervate muscle spindles
B. innervate smooth muscle
C. innervate skeletal muscle
D. are bipolar
E. are pseudounipolar
answer
6. The structure enclosed by the brackets indicated by the arrow is a:
click here for the image
A. Microtubule
B. Microvillus
C. Collagen fibril
D. Collagen fiber
E. Nerve fiber
F. Nerve fascicle
G. Myofiber (muscle fiber)
H. Myofibril
answer
7. The cell shown in the electron micrograph:
click here for cross section image
click here for the longitudinal section image
A. is a stretch receptor.
B. is innervated by alpha motor neurons.
C. has almost no regenerative capacity.
D. is metabolically inactive.
E. may adopt a latched contractile state via dephosphorylation of myosin.
answer
8. The structure indicated is a/an:
click here for the low mag image
click here for the high mag image
A. eccrine sweat gland
B. apocrine sweat gland
C. active mammary gland lobule
D. inactive mammary gland lobule
E. parasympathetic ganglion
answer
9. Identify the cell indicated by the pointer.
click here for the low mag image
click here for the high mag image
A. Schwann cell
B. satellite cell
C. dorsal root ganglion (sensory) neuron
D. autonomic ganglion neuron
E. motor neuron
F. macrophage
G. muscle spindle cell
answer
10. Which of the following regarding the cell indicated is NOT true?
click here for the low mag image
click here for the high mag image
A. It synthesizes pigment that protects against damage by UV radiation.
B. It divides to give rise to keratinocytes.
C. It transfers melanosomes to keratinocytes in a cytocrine manner.
D. It is typically found in the stratum basale of the epidermis.
E. It is present in both thick and thin skin.
answer
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