Integumentary System and Mammary Gland

1. Be able to identify the principal layers of the skin (epidermis, dermis and hypodermis) at the light microscope level and know the composition and general 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 including the process of epidermal turnover and keratinization.
3. Know the cell types present in the epidermis and be able to identify keratinocytes and melanocytes. Explain their roles in pigmentation and their cellular interactions.
4. Be able to recognize Meissner's and Pacinian corpuscles in micrographs and know their functions.
5. Be able to identify eccrine, apocrine, and sebaceous sweat glands (ducts and secretory portions) at the light microscope level.
6. Identify hair follicles and their major components including the pilosebaceous apparatus.

The skin and its associated structures, hair, sweat glands and nails make up the integumentary system. 

A. Meissner's Corpuscles 

Slide UCSF 180 finger tip H&E View Virtual Slide (virtual slide courtesy of the University of California, San Francisco)

Look at slide 112 thick skin, sole of foot H&E View Virtual Slide (note, there's only one corpuscle apparent in this slide) Meissner's corpuscles. Slide UCSF 180 Meissner's corpuscles View Image are touch receptors that are responsive to low-frequency stimuli and are usually associated with hairless skin of the lips and palmar and plantar surfaces, particularly those of the fingers and toes. Generally, these receptors are tapered cylinders located in the undulating connective tissue just underneath the stratified epithelium of the skin. The long axis of the cylinder is perpendicular to that of the overlying epidermis and is usually about 150 um long and is usually tucked within extensions of the underlying connective tissue dermis (called "dermal papillae") that project into the underside of the epidermis. Within these receptors, one or two nonmyelinated endings of myelinated nerve fibers follow a spiral path through the corpuscle. The fibers are accompanied by ensheathing Schwann cells, the nuclei of which are flattened and stacked on top of each other giving the corpuscle its characteristic irregular, lamellar appearance.

B. Pacinian Corpuscles

Slide 108 Fetal fingar H&E, 20X View Virtual Slide

Slide 108-1 Fetal fingar Masson View Virtual Slide

Slide 042 Mesentery H&E View Virtual Slide

Slide 095 Small arteries and vein H&E View Virtual Slide

Slide 095M Mesentary Masson View Virtual Slide

Pacinian corpuscles slide 095M View Image (best seen in slide 108-1) are large, ovoid structures up to 1 mm in diameter found in the dermis and hypodermis of the skin and also in the connective tissue associated with bones, joints, and internal organs. They respond primarily to pressure and vibration and are composed of a myelinated nerve ending surrounded by a capsule. The nerve enters the capsule at one pole (which might be out of the plane of section and therefore not visible) with its myelin sheath intact but then it is quickly lost. The unmyelinated portion of the axon extends toward the opposite pole from which it entered and its length is covered by flattened Schwann cell lamellae that form the inner core of the corpuscle. The remaining bulk of the capsule, or outer core, is comprised of a series of concentric, onionlike lamellae with each layer separated by an extracellular fluid similar to lymph. Each lamella is composed of flattened Schwann cells and endoneurial fibroblasts. In addition the fluid between each layer, delicate collagen fibers may be present as well as occasional capillaries. Displacement of the lamellae by pressure or vibrations effectively causes depolarization of the axon, which sends the signal to the central nervous system.

A. Eccrine sweat gland

Slide 106 Thick skin, sole of foot (Homo) View Virtual Slide

Slide 112 Thick skin (Homo) View Virtual Slide

Slide 112N Thick skin (Homo) View Virtual Slide

Slide 105-1 Thin skin (Homo) View Virtual Slide

Numerous coiled eccrine sweat glands are located at the junction of dermis and hypodermis. The coiled morphology can be particularly appreciated in the example shown in slide 112N. 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?

How does sweat get to the surface?

B. Apocrine glands

Slide 109-1 Perianal region Masson View Virtual Slide

Slide 109-2 Perianal region PAS/Pb hematoxylin View Virtual Slide

Slide 111 axilla, subcutaneal region Masson View Virtual Slide

Slide 104-2 View Virtual Slide [orientation] (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. 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 histologists originally designating these cells as "apocrine" secretory cells, although we now know the cells actually secrete in a MEROCRINE manner just like eccrine sweat glands. The "apocrine" sweat 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 odor 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. 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 slide 104-2 View Image; look for eosinophilic strands of cytoplasm and small nuclei just peripheral to the secretory epithelial cells. In PAS stained slides (e.g. slide 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. Please also look for apocrine sweat gland ducts in these slides. They look similar to eccrine gland ducts described above (small diameter and small lumen with a stratified cuboidal epithelium). Similarly, they also lack lack myoepithelial cells. Make sure you can discriminate the ductal portion of apocrine sweat glands from their secretory portion.

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1. Identify the histological components of the mammary gland, including structures associated with the nipple and the areola, the overall organization into lobes and lobules, as well as secretory alveoli (acini), lactiferous ducts/sinuses, and the intralobular and interlobular connective tissue.

2. Identify and describe the histological differences between the mammary gland in adult females prior to pregnancy (inactive), during pregnancy, and during lactation (active).

3. Know the cellular mechanisms involved in the formation and release of milk.

Slide 259 Mammary gland inactive nulliparous H&E View Virtual Slide
Slide 258 Mammary gland active, pregnant H&E View Virtual Slide
Slide 261 Mammary gland active lactating H&E View Virtual Slide

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 and active glands, 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 lobulesthat 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. This tissue was probably taken from an individual before the last trimester. When compared with the inactive mammary gland, you can see that the intralobular ducts have proliferated to form additional secretory regions. Both the epithelial cells and myoepithelial cells increase in number. Alveoli slide 261 View Image have formed, their epithelial cells have large, clear areas of apical cytoplasm, a region occupied by glycogen and lipid. Note the increased cellularity of the intralobular connective tissue. Note also that not all lobules within the gland have proliferated to the same degree. In many sections, portions of a large excretory, lactiferous duct slide 261 View Image are present. The epithelial lining is again two layered, the bottom layer being principally myoepithelium. Compare the morphology of the inactive and active gland. Observe the intralobular connective tissue and note the abundance of plasma cells slide 261 intralobular connective tissue and note the abundance of plasma cells View Image. These plasma cells are the source of secretory IgA.