Objectives:

1. 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.



2. 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.



3. Identify melanocytes and explain the process of pigment formation in the skin.



4. Be able to identify eccrine and apocrine sweat glands at the light microscope level.



5. Identify the components of the pilosebaceous apparatus and know the structural and developmental relationship between each component and the epidermis of the skin.



6. 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

A. Thick skin  (W pgs 167-172, 9.2-4)

!! If you are a WINDOWS USER on campus, remember to map a drive to the file server (click here), and USE THE [WinLab] LINKS !!

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 slides #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 the function of the epidermal ridges and dermal papillae  (IN1) Note the finer arrangement of collagen fibers in the papillary dermis [see example] as opposed to that of the reticular dermis [see 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 in thin skin much thinner and simpler in structure.  Each stratum is thinner and the stratum granulosum may be absent. Melanocytes [see 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 or yellow-brown center and surrounding cellular layers.  These structures are hair follicles [see example] sectioned transversely or tangentially at different levels.  The keratinized component of the hair occupies the central cavity of the follicle, and appears yellow-brown when present. However, the hair often falls out during tissue processing, in which case the central cavity will appear to be occupied by just empty space.  The surrounding layers of clear cells form the external root sheath of the hair [see orientation], which is a downgrowth of the epidermis. In fact, in cases where most of the epidermis is removed (such as severe abrasions or when taking skin graft), it is cells of the external root sheath that will divide and spread over the exposed surface to re-establish the epidermis.   In some sections, you may also see an internal root sheath of darker staining cells right up against the hair follicle --this is the layer of cells that actually produce the keratinized hair shaft. Note also the presence of sebaceous glands [see example] (W pg 168, 9.14) and the arrector pili muscle [see 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 linedduct.  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 [see 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 [see 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 [see 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. 

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.

#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) (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.

#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.

#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.

#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).

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 Questions

Click on the MAC link or PC link to view image.
1. The cluster of cells enclosed by the outline on the slide:

1. produce melanin.
2. produce ceramide.
3. secrete in an apocrine manner.
4. secrete in an eccrine (or merocrine) manner.
5. NONE of the above.
6. ALL of the above.

ANSWER

Click on the MAC link or PC link to view image.
2. The layer indicated by the black bar:

1. is where melanocytes usually reside.
2. contains cells densely packed with keratin tonofilaments.
3. consists primarily of type III collagen fibers.
4. consists primarily of type I collagen fibers.
5. NONE of the above.
6. ALL of the above.

ANSWER

3. Myoepithelial cells are found in:

1. active mammary gland acini.
2. inactive mammary gland ducts.
3. eccrine sweat gland acini.
4. apocrine sweat gland acini.
5. NONE of the above.
6. ALL of the above.

ANSWER

Click here to view image.
4. The layer indicated by the bracket is:

1. reticular layer of the dermis
2. papillary layer of the dermis
3. stratum basale of the epidermis
4. stratum granulosum of the epidermis
5. stratum corneum of the epidermis

ANSWER