6 - SKIN GLANDS: SEBACEOUS, ECCRINE AND APOCRINE GLANDS Flashcards
multilobular structures of epithelial origin that consist of acini connected to a common excretory duct, the sebaceous duct (ductus seboglandularis) (Fig. 6-1)
Figure 6-1 Cross-section of a pilosebaceous unit: a multiacinar sebaceous gkland associated with a hair follicle (HF). AP, arrector pili muscle (×20); SD, sebaceous duct, Sebum, sebum and keratin. (Modified with permission from: Zouboulis CC, Tsatsou F. Anatomy of the sebaceous gland. In: Zouboulis CC, Katsambas AD, Kligman AM, eds. Pathogenesis and Treatment of Acne and Rosacea. Berlin: Springer; 2014:27-31. Copyright © 2014.)
Human sebaceous glands
Sebaceous glands are composed of sebocytes, which are lipid-producing uniquely differentiated epithelial cells. 1,2 On the other hand, the sebaceous duct is lined by undifferentiated keratinocytes and is usually associated with a hair follicle which is composed of stratified squamous epithelium. The periphery of the sebaceous gland is a basal cell layer composed of small, cuboidal, nucleated, highly mitotic sebocytes. 1,3 Cells progress toward the middle of the gland and accumulate lipid droplets (LDs) as they transform into terminally differentiated cells, full of lipids.
The latter lack all other cellular organelles, burst, and die, excreting their entire contents to the duct in a holocrine manner (Fig. 6-2). Surrounding the glands are connective tissue capsules composed of collagen fibers that provide physical support.
Figure 6-2 A, Hematoxylin and eosin–stained section of the human sebaceous gland showing the different stages of sebocyte differentiation. Cells progress toward the middle of the gland, lose their nuclei, and organelles, and accumulate lipid droplets. B, Differentiation stages of human sebocytes in tissue (left) 19 and in vitro (right) 3 according to Tosti17 and McEwan Jenkinson and coworkers. 18 Undifferentiated sebocytes are small cells with a high nucleocytoplasmic ratio. Early differentiated sebocytes are larger cells with a decreased nucleocyloplasmic ratio compared with the undifferentiated sebocytes and a few lipid droplets arranged in the perinuclear area. Advanced differentiated sebocytes are cells with further increases in size and decreases of the nucleocytoplasmic ratio. Multiple cytoplasmic lipid droplets are distributed inside the cytoplasm. Fully differentiated sebocytes are cells with abundant, partially large, cytoplasmic lipid droplets. Mature sebocytes are disorganized large cell with denatured nucelei; the lack of cytoplasmic lipids is caused by lysis of the cell blood cell membrane.
Sebaceous glands are associated with hair follicles all over the body. A sebaceous gland associated with a hair follicle is termed ______
pilosebaceous unit
The glands may also be found in certain nonhairy sites, including the eyelids (Meibomian glands, tarsal glands), the nipples (Montgomery glands, areolar glands), around the genitals (Tyson glands), and the mucosa (lips, gums and inner cheeks, and genitals; Fordyce spots).
Fordyce spots open and release their content directly to the epithelial surface. The latter are visible to the unaided eye because of their large size (up to 2 to 3 mm) and the transparency of the oral epithelium (Fig. 6-3).
Figure 6-3 Fordyce spots at the upper lip mucosa.
Areas devoid of sebaceous glands
Only the palms and soles, which have no hair follicles, are totally devoid of sebaceous glands.
In addition, the dorsal surfaces of the hand and foot have sparse sebaceous glands. 5 Sebaceous glands vary considerably in size, even within the same individual and within the same anatomic area. On the external body surface, most glands are only a fraction of a millimeter in size. The largest glands and greatest density of glands are located on the nose (1600 glands/cm2 ) followed by the face and scalp (up to 400 to 900 glands/cm2 ). 4 The hairs associated with these large glands are often tiny, and the total structure is more specifically termed sebaceous follicles, being a pilosebaceous unit variant, the other two being the terminal hair follicle and the vellus hair follicle.
EMBRYOGENESIS AND MORPHOGENESIS of sebaceous glnds
The development of the sebaceous glands is closely related to the differentiation of hair follicles and epidermis. 6-8 At the 10th to 12th weeks of fetal life, a stratum intermedium becomes apparent, and at about the same time, developing hair germs are quite distinct. In the following weeks, the follicles extend downward into the dermis, and the rudiments of the sebaceous glands appear on the posterior surfaces of the hair pegs. By 13 to 16 weeks, the glands are clearly distinguishable, arising in a cephalocaudal sequence from bulges (epithelial placodes) of the hair follicles. The latter contain the epidermal stem cells that generate multiple cell lineages, including epidermal and follicular keratinocytes, as well as sebaceous glands. As daughter cells migrate from the bulge region, changes in the expression patterns of numerous transcription factors determine their final cell lineage. Despite continuous differentiation of its cells, the sebaceous gland can be regenerated by the reservoir of stem cells in the hair follicle bulge. However, retroviral lineage marking has provided strong evidence that the sebaceous gland might arise and be maintained independently of the hair follicle bulge.
Wnt or wingless (Wnt) and Sonic hedgehog (Shh) signaling pathways are intricately involved in embryonic patterning and cell fate decisions. Cells destined to become sebocytes have increased Shh and Myc signaling and decreased Wnt signaling (Fig. 6-4A). 10,11 In human SZ95 sebocyte and transgenic mouse models, whereas intact Wnt signaling promotes hair follicle differentiation, inhibition of Wnt signaling by preventing the Lef1B-catenin interaction leads to sebocyte differentiation.11,12 Loss of function and gain of function in both models demonstrated that blocking Shh signaling inhibited normal sebocyte differentiation, and constitutively activating Shh signaling increased the number and size of human sebocytes and mouse sebaceous glands in skin.
Figure 6-4 Simplified signaling pathways and transcription factors that are involved in cell lineage determinations.9-11 As daughter cells migrate from the bulge region, changes in the expression patterns of numerous transcription factors determine their final cell lineage. Additional pathways and transcription factors play a significant role in determining each cell lineage. Lef1, lymphoid enhancer binding factor 1; Myc, myelocytomatosis oncogene; Shh, Sonic hedgehog; Tcf3, transcription factor 3; Wnt, wingless (wg)/int.
Several important molecular aspects of sebaceous gland development have been identified, mostly with the aid of genetically modified cell lines. The earliest known signal necessary for sebaceous gland development is SOX9, which is in fact essential for the specification of early hair follicle stem cells and therefore for the morphogenesis of both structures (Fig. 6-5).9 Further studies indicate that later in embryonic development, a subpopulation of these stem cells expressing PRDM1 (formerly known as BLIMP1) is established near the entrance of the sebaceous gland. PRDM1 (BLIMP-1) acts as a marker of terminal epithelial cell differentiation. 13,14 Loss of PRDM1 (BLIMP-1) results in increased gene expression of c-myc, an essential player in sebaceous gland homeostasis. Overexpression of c-myc in transgenic mice results in enlarged and more numerous sebaceous gland at the expense of the hair follicle lineage. Moreover, skin-specific deletion of c-myc negatively affects sebaceous gland development. In skin, c-myc and β-catenin exert opposing effects on sebocyte differentiation (see Fig. 6-4). Antagonizing Wnt–β-catenin signaling constitutes an important prerequisite for normal sebaceous differentiation in postnatal skin tissue. Stem cells expressing LRIG1, which has been suggested to be multipotent stem cells giving rise to epidermal lineages, can act under homeostatic conditions as sebocyte progenitor cells.
Sebaceous gland cells at first contain glycogen. This lingers at the periphery of the gland but is quickly lost at the center, where lipid drops are visible at 17 weeks. 13,14 The future common excretory duct, around which the acini of the sebaceous gland attach, begins as a solid cord. The cells composing the cord are filled with sebum, and eventually they lose their integrity, rupture, and form a channel that establishes the first pilosebaceous canal, the duct, through which sebum flows into the follicular canal and subsequently to the skin surface. New acini result from buds on the peripheral sebaceous duct wall. The cell organization of the neonatal sebaceous acini consists of undifferentiated (basal), differentiating (early, advanced and fully differentiated), and mature sebaceous gland cells (see Fig. 6-2). 3,17-19 Undifferentiated cells arranged in a single layer facing the basal lamina, comparable to the epidermal basal layer; they represent the germinative cells of the gland, flattened or cuboidal in shape, showing round and densely basophilic nuclei. 20 These bear characteristics of stem cells because they give rise to a continual flux of proliferating and differentiating cells. The basal cells of the peripheral zone form about 40% of the gland. Growing toward the center of the gland lobules, the basal cells gradually differentiate into an early differentiated cell type, an advanced differentiated cell type, a fully differentiated cell type, and the mature sebocyte. 3,21 The maturation zone also represents about 40% of the sebaceous gland.
The sebaceous glands exude lipids by disintegration of entire cells, a process known as ___________
HOLOCRINE SECRETION
Holocrine secretion by sebaceous gland cells does not occur mechanically via increased cell volume, as considered previously, but rather from a multistep, cell-specific lysosomal DNase2-mediated mode of programmed cell death, which differs from apoptosis, necroptosis, and cornification.22
As sebaceous gland cells are displaced into the center of the gland, they begin to produce lipids, which accumulate in droplets. With approaching the sebaceous duct, they disintegrate and release their content. Only neutral lipids reach the skin surface. Proteins, nucleic acids, and the membrane phospholipids are digested and are apparently recycled during the disintegration of the cells. 2 Sebaceous gland secretion can be enhanced with increased rates of induced terminal sebocyte differentiation.
LIPID COMPOSITION OF SEBUM
Figure 6-6 Human sebaceous gland lipids. The structures of the cholesterol ester, wax ester, and triglyceride are representative of the many species that are present. Two sebaceous-type unsaturated fatty acid moieties are shown: sapienic acid (16:1∆6) (in the wax ester structure) and sebaleic acid (18:2∆5,8) (in the triglyceride structure). Anteiso branching is shown in the alcohol moiety of the wax ester, and iso branching is shown in the triglyceride.
Sebum production is a continuous event. The exact mechanisms underlying its regulation are not fully defined. Complexity and uniqueness are the two terms that best characterize sebaceous lipids. ∆6 desaturation, wax ester synthesis, and squalene accumulation are examples that manifest sebaceous lipid biology.22-24 Genetic knockout animal models of lipid synthesis demonstrate dramatic changes in skin physiology and pathology, resulting from impairment of sebaceous lipid pathways. 25 Human sebum, as it leaves the sebaceous gland, contains a mixture of nonpolar (neutral) lipids, mainly triglycerides, wax esters, squalene, and smaller amounts of cholesterol and cholesterol esters (Fig. 6-6). During passage of sebum through the hair canal, bacterial enzymes hydrolyze some of the triglycerides, so that the lipid mixture reaching the skin surface contains free fatty acids and small proportions of mono- and diglycerides, in addition to the original components. Triglycerides, diglycerides, and free fatty acids form 40% to 60% of total skin surface lipids followed by wax esters (25% to 30%), squalene (12% to 15%), cholesterol esters (3% to 6%), and cholesterol (1.5% to 2.5%). 26,27 The wax esters and squalene distinguish sebum from the lipids of human internal organs, which contain no wax esters and little squalene. However, human sebaceous glands appear to be unable to transform squalene to sterols, such as cholesterol. The patterns of unsaturation of the fatty acids in the triglycerides, wax esters, and cholesterol esters also distinguish human sebum from the lipids of other organs. The “normal” mammalian pathway of desaturation involves inserting a double bond between the 9th and 10th carbons of stearic acid (18:0) to form oleic acid (18:1∆9). However, in human sebaceous glands, the predominant pattern is the insertion of a ∆6 double bond into palmitic acid (16:0). The resulting sapienic acid (16:1∆6) (see Fig. 6-6) is the major fatty acid of adult human sebum. Elongation of the chain by two carbons and insertion of another double bond gives sebaleic acid (18:2∆5,8), a fatty acid thought to be unique to human sebum.22-24
Sebaceous fatty acids and alcohols are also distinguished by chain branching. Methyl branches can occur on the penultimate carbon of a fatty acid chain (iso branching), on the third from the last (antepenultimate) carbon (anteiso branching), or on any even-numbered carbon (internal branching). Examples of these unusual unsaturated and branchedchain moieties are included in the lipid structures in Fig. 6-6.
FUNCTION OF SEBUM
Sebum in humans was initially considered to solely cause acne. 28,29 Subsequently, it has been suggested that sebum reduces water loss from the skin’s surface and functions to keep skin soft and smooth, although evidence for these claims in humans is minimal; however, as demonstrated in the sebaceous gland–deficient (Asebia) mouse model, glycerol derived from triglyceride hydrolysis in sebum is critical for maintaining stratum corneum hydration. 30 Sebum has later been shown to have mild antibacterial action, protecting the skin from infection by bacteria and fungi because it contains antiinflammatory lipids and immunoglobulin A, which is secreted from most exocrine glands. 31-33 Vitamin E delivery to the upper layers of the skin protects the skin and its surface lipids from oxidation. Thus, sebum flow to the surface of the skin may provide the transit mechanism necessary for vitamin E to function. 34 The current concept is that sebum is involved in the multimodal activities of the sebaceous glands (Table 6-2).
INNATE IMMUNITY role of sebaceous glands
Antimicrobial peptides, including cathelicidin, psoriasin, β-defensin 1, and β-defensin 2, are expressed within the sebaceous gland. Functional cathelicidin peptides have direct antimicrobial activity against Propionibacterium acnes but also initiate cytokine production and inflammation in the host organism. 35,36 In addition, free fatty acids in human sebum are bactericidal against gram-positive organisms as a result of its ability to increase β-defensin 2 expression. 31,36 Innate immune Toll-like receptors 2 and 4 (TLR2, TLR4) as well as CD1d and CD14 molecules are also expressed in sebaceous glands and immortalized human sebocytes.37 With the expression of innate immune receptors and antibacterial peptides, the sebaceous gland may play an important role in pathogen recognition and protection of the skin surface.
FACTORS REGULATING SEBACEOUS GLAND SIZE AND SEBUM PRODUCTION
Sebocytes preserve characteristics of stem-like cells despite their programming for terminal differentiation because they present a remarkable potential of bipotential differentiation. 42,43 The sebaceous gland might be maintained by unipotent stem cells that are replenished by multipotent stem cells in the hair follicle bulge. 13 However, it is an emerging view that there might be at least three distinct niches for skin stem cells: the follicle bulge, the base of the sebaceous gland, and the basal layer of the epidermis.44
The average transit time of sebaceous gland cells from formation to discharge, has been calculated as 7.4 days in the human gland, with 4 to 7 days in undifferentiated and 14 to 25 in differentiated lipidproducing cells. 1 Within any one glandular unit, the acini vary in differentiation and maturity. The synthesis and discharge of the lipids contained in the sebaceous cells require more than 1 week. The size of sebaceous glands increases with age. The mean size rises from 0.2 mm 2 ± 0.5 mm 2 to 0.4 mm 2 ± 2.1 mm2 . The sebaceous cells of prepubertal and hypogonadal boys and men are qualitatively similar to those of normal adults, even though the glands are smaller. 45 In general, whereas the number of sebaceous glands remains approximately the same throughout life, their size tends to change with age. 46 The turnover of the sebaceous glands in older adults is slowed down compared with young adults.
A variety of experimental models are used to study the factors involved in sebaceous gland regulation, including cell culture of isolated human sebaceous glands, primary sebocytes, immortalized sebocyte cell lines, and three-dimensional models, as well as mouse and hamster animal models. 47-50 Results from these investigations clearly indicate that sebaceous glands are multifactional (see Table 6-2), 51,52 regulated, among others, by ligands of sebaceous gland cell receptors (Table 6-3), such as androgen and estrogen receptors, peroxisome-proliferator-activated receptors (PPAR) and liver-X receptor (LXR), neuropeptide receptors, retinoid, and vitamin D receptors. 53-56 The ligandreceptor complexes activate pathways involving lipogenesis but also cell proliferation, differentiation, hormone metabolism, and cytokine and chemokine release.57
Role of androgens in sebum production
Sebaceous glands require androgenic stimulation to produce significant quantities of sebum. Individuals with a genetic deficiency of androgen receptors (complete androgen insensitivity) have no detectable sebum secretion and do not develop acne. 58 Although the most powerful androgens are testosterone and its
end-organ reduction product, 5α-dihydrotestosterone (DHT), levels of testosterone do not parallel the patterns of sebaceous gland activity. For example, testosterone levels are many fold higher in males than in females, with no overlap between the sexes. However, the average rates of sebum secretion are only slightly higher in males than in females, with considerable overlap between the sexes. Also, sebum secretion starts to increase in children during adrenarche, a developmental event that precedes puberty by about 2 years.
The weak adrenal androgen, dehydroepiandrosterone sulfate (DHEAS), is probably a significant regulator of sebaceous gland activity through its conversion to testosterone and DHT in the sebaceous gland.59 Levels of DHEAS are high in newborns, very low in 2- to 4-year-old children, and start to rise when sebum secretion starts to increase. In adulthood, DHEAS levels show considerable individual variation but are only slightly higher in men than in women on the average. There is a decline in DHEAS levels in both sexes starting in early adulthood and continuing throughout life; this decline parallels the decline of sebum secretion. DHEAS is present in the blood in high concentration. The enzymes required to convert DHEAS to more potent androgens are present in sebaceous glands.60 These include 3β-hydroxysteroid dehydrogenase, 17β-hydroxysteroid dehydrogenase, and 5α-reductase. Each of these enzymes exists in two or more isoforms that exhibit tissue-specific differences in their expression. The predominant isozymes in the sebaceous gland include the type 1 3β-hydroxysteroid dehydrogenase, the type 2 17β-hydroxysteroid dehydrogenase, and the type 1 5α-reductase.
most potent pharmacologic inhibitor of sebum secretion
Isotretinoin (13-cis-retinoic acid, 13-cis-RA)
Significant reductions in sebum production can be observed as early as 2 weeks after use. 63,64 Histologically, sebaceous glands are markedly reduced in size, and individual sebocytes appear undifferentiated lacking the characteristic cytoplasmic accumulation of sebaceous lipids.3,65
Isotretinoin does not interact with any of the known retinoid receptors. It may serve as a prodrug for the synthesis of all-trans-retinoic acid, which interacts with retinoid receptors expressed in sebaceous gland cells (retinoic acid receptors [RARs; isotypes α and γ] and retinoid X receptors [RXRs; isotypes α, β, γ]). 66 However, it has greater sebosuppressive action than do all-trans- or 9-cis-retinoic acid. 67 13-cis-RA exerts pluripotent effects on human sebaceous gland cells and their lipogenesis.63 Inhibition of androgen synthesis, cell cycle arrest, and apoptosis by 13-cis-RA may explain the reduction of sebaceous gland size after treatment.
members of the nuclear hormone receptor family and act as transcriptional regulators of a variety of genes, including those involved in lipid metabolism in adipose tissue, liver, and skin
PEROXISOMEPROLIFERATOR ACTIVATED RECEPTORS
PPARs are similar to retinoid receptors in many ways. Each of these receptors forms heterodimers with retinoid X receptors to regulate the transcription of genes involved in a variety of processes, including lipid metabolism and cellular proliferation and differentiation. PPARα, δ, and γ receptor subtypes have been detected in basal sebaceous gland cells. 54 PPARγ is also detected within differentiated cells. Pharmacologic PPAR-γ modulation regulates sebogenesis and inflammation in SZ95
human sebaceous gland cells. 68 In patients receiving fibrates (PPAR-α ligands) for hyperlipidemia or thiazolidinediones (PPAR-γ ligands) for diabetes, sebum secretion rates are increased.69
PPAR-γ–RXR-α and LXR–RXRα promoter interactions are of crucial importance for the regulation of key genes of lipid metabolism. Although various fatty acids, eicosanoids, and prostanoids activate PPARs, oxysterols and intermediate products of the cholesterol biosynthetic pathway activate LXRs. PPAR-α agonists and PPAR-γ antagonists may reduce sebaceous lipid synthesis and as such may be useful in the treatment of acne. On the other hand, whereas PPAR-γ agonists may be beneficial in aging skin, PPAR-δ agonists may be involved in sebaceous tumorigenesis.
members of the NHR family, play a critical role in cholesterol homeostasis and lipid metabolism.
LXR
Treatment of SZ95 sebaceous gland cells with the LXR ligands TO901317 or 22(R)hydroxycholesterol enhanced accumulation of LDs in the cells, which could be explained through induction of the expression of the LXRα receptor and known LXR targets, such as fatty acid synthase and sterol regulatory element–binding protein-1 (SREBP-1).
FOXO1
FoxO1 is expressed in most lipid-metabolizing cells, including the prostate, liver, fat tissue, and skin.72 Human sebaceous gland cells also express FoxO1. Acne and increased sebaceous lipogenesis are associated with a relative nuclear deficiency of FoxO1 caused by increased growth hormone–insulin–insulin-like growth factor 1 or fibroblast growth factor 2 signaling.
Structural proteins involved in sebogenesis
During sebogenesis, lipids are stored in LDs. LDs are limited by a membrane containing phospholipids and numerous proteins and enzymes. The most relevant membrane proteins are the perilipin (PLIN) family, which possesses structural and regulatory properties. In particular, PLIN2, the major form expressed during the differentiation process, regulates the gland size in vivo and regulates sebaceous lipid accumulation.73 Experimental downmodulation of the PLIN2 expression significantly modifies the composition of neutral lipids with a significant decrease in the unsaturated fatty acid component caused by a marked decrease in the expression of specific lipogenic enzymes. On the other hand, PLIN3 has currently been shown to modulate specific lipogenic pathways in human sebaceous gland cells. 74 Another structural protein, angiopoietin-like 4, is strongly induced during human sebocyte differentiation and regulates sebaceous lipogenesis.
How many sweat glands does a human have?
A human has 2 to 4 million sweat glands (200 to 400/cm 2 of skin surface).
How much sweat is produced by acclimatized individuals?
Up to 10 L/day of sweat is produced by acclimatized individuals.
In humans, sweat glands are generally classified into ______ and _____
apocrine and eccrine types
strongest stimulus for sweating
Hypothalamic temperature
The secretory coil of eccrine sweat glands contains three distinct cell types:
Figure 6-7 Electron micrograph of the secretory coil of a human eccrine sweat gland. B with arrow, basal lamina; CC, clear cell; DC, dark cell; ICC, intercellular canaliculi; Lu, lumen; MC, myoepithelial cell.
(1) clear (secretory),
(2) dark (mucoid), and
(3) myoepithelial
The clear and dark cells occur in approximately equal numbers but differ in their distribution. Although the dark cells border the apical (luminal) surfaces, the clear cells rest either directly on the basement membrane or on the on the myoepithelial cells. The clear cells directly access the lumen by forming intercellular canaliculi (Fig. 6-7). Spindle-shaped contractile myoepithelial cells lie on the basement membrane and abut the clear cells. An adult secretory coil is approximately 2- to 5-mm long and approximately 30 to 50 µm in diameter. Heat accumulation results in larger sweat glands and ducts, and their dimensions in turn correlate with enhanced sweat output. 84 Clear cells contain abundant mitochondria and an autofluorescent body, called the lipofuscin granule, in the cytoplasm. The clear cell plasma membrane forms many villi. The clear cell secretes water and electrolytes. Dark cells have a smooth cell surface and contain abundant dark cell granules. 83 The function of dark cells is unknown. Myoepithelial cells contain actin filaments and are contractile, 85,86 producing pulsatile sweat.
The duct of the eccrine sweat gland consists of what?
The duct of the eccrine sweat gland consists of an outer ring of peripheral or basal cells and an inner ring of luminal or cuticular cells. It seems that the proximal (coiled) duct is functionally more active than the distal straight portion in pumping Na + for ductal Na + reabsorption because Na+ , K+ -adenosine triphosphatase (ATPase) activity and the number of mitochondria are higher in the proximal portion (Fig. 6-8). 83,85,87,88 In contrast, the luminal ductal cells have fewer mitochondria, much less Na+ , K+ -ATPase activity, and a dense layer of tonofilaments near the luminal membrane, which is often referred to as the cuticular border. The cuticular border provides structural resilience to the ductal lumen, which may dilate whenever ductal flow of sweat is blocked. The entire structural organization of the duct is well designed for the most efficient Na + absorptive function. The luminal membrane serves as the absorptive surface by accommodating both Na + and Cl − channels, and the basal ductal cells serve in Na + pumping by providing maximally expanded Na + pump sites and efficient energy metabolism. The lumen and the duct contain β-defensin, an antimicrobial, cysteine-rich, low-molecular-weight peptide. 80,81 In the epidermis, the duct spirals tightly upon itself.
Figure 6-8 The ultrastructure of the eccrine duct and secretory coil and the localization of Na+ , K+ -adenosine triphosphatase (ATPase). The thick lines indicate the localization of Na+ , K+ -ATPase. BM, basement membrane; C, clear cell; D, dark cells; IC, intercellular canaliculi; M, myoepithelial cell; Mc, mitochondria.
PHARMACOLOGY OF THE ECCRINE SWEAT GLAND AND SWEATING RATE
Sweat glands respond to cholinergic agents, α- and β-adrenergic stimulants, and other periglandular neurotransmitters, such as vasoactive intestinal peptide and ATP. Periglandular ACh is the major stimulant of sweat secretion, and its periglandular concentration determines the sweat rate in humans. 96 When dissociated clear cells are stimulated in vitro by cholinergic agents, they lose K + and Cl− , increase intracellular Ca2+ , and shrink, mimicking actions seen in vivo. Striking individual differences exist in the degree of sweating in response to a given thermal or physical stress. In general, males perspire more profusely than females.97 The sweat rate in a given area of the skin is determined by the number of active glands and the average sweat rate per gland. The maximal sweat rate per gland varies from 2 to 20 nL/min2 . Sweat rate increases during acclimatization, but the morphologic and pharmacologic bases of the individual and regional differences in sweating rate during acclimatization are still poorly understood (Fig. 6-9). In thermally induced sweating, the sweat rate can be mathematically related to the body and skin temperatures in a given subject only in the low sweat rate range. Cholinergic stimulation yields a 5 to 10 times higher sweating rate than does β-adrenergic stimulation. α-Adrenergic stimulation (by phenylephrine) is no more potent than isoproterenol (ISO) (a β-adrenergic agonist) in humans in vivo.98 Whereas cholinergic sweating begins immediately on intradermal injection, β-adrenergic sweating requires a latent period of from 1 to 2 minutes, which suggests that the intracellular mechanism of sweat induction may be different for methacholine and for ISO. Because the sweat rate in response to adrenergic agents is rather low, it may be reasonable to surmise that adrenergic stimulation in periglandular nerves may be involved in the regulation of sweat gland function but not in the induction of sweat secretion. One consequence of dual cholinergic and adrenergic innervation is to maximize tissue accumulation of cyclic adenosine monophosphate (cAMP), which may be instrumental in stimulating the synthesis of sweat and glandular hypertrophy of the sweat gland. The possibility that periglandular catecholamine is directly involved in emotional sweating or sweating associated with pheochromocytoma99 may be ruled out because these sweating responses can be blocked by anticholinergic agents.
COMPOSITION OF HUMAN ECCRINE SWEAT
Inorganic Ions: Sweat is formed in two steps:
(1) secretion of a primary fluid containing nearly isotonic NaCl concentrations by the secretory coil and
(2) reabsorption of NaCl from the primary fluid by the duct. Although a number of factors affect ductal NaCl absorption, the sweat rate (and thus the transit time of sweat) has the most important influence on final NaCl concentration. Sweat NaCl concentration increases with increasing sweat rate to plateau at around 100 mM (Fig. 6-10). Potassium (K+ ) concentration in sweat is relatively constant. It ranges from 5 to 10 mM, which is slightly higher than plasma K + concentration. HCO 3 − concentration in the primary sweat fluid is approximately 10 mM, but that of final sweat is less than 1 mM, which indicates that HCO 3 − is reabsorbed by the duct, presumably accompanied by ductal acidification. 101 Sweat NaCl concentration is increased in individuals with cystic fibrosis. 102 Aquagenic wrinkling of the palms (whitened, wrinkling, and papillation of the palms after brief water exposure) is seen more frequently in carriers and patients with cystic fibrosis (see Chap. 81).
Lactate: The concentration of lactate in sweat usu-
ally depends on the sweat rate. At low sweat rates, lactate concentration is as high as 30 to 40 mM, but it rapidly drops to a plateau at around 10 to 15 mM as the sweat rate increases. Whereas acclimatization is known to lower sweat lactate concentrations, arterial occlusion rapidly raises sweat NaCl and lactate concentrations and reduces the sweat rate. 100 Sweat lactate is probably produced by glycolysis of glucose by the secretory cells.103
Urea: Urea in sweat is derived mostly from serum urea. 104 Sweat urea content is usually expressed as a sweat–plasma ratio (S/P urea). S/P urea is high (2 to 4) at a low sweat rate range but approaches a plateau at 1.2 to 1.5 as the sweat rate increases.
Ammonia and Amino Acids: The ammonia
concentration in sweat is 0.5 to 8 mM, 105 which is 20 to 50 times higher than the plasma ammonia level. The concentration of sweat ammonia is inversely related to the sweat rate and sweat pH. Free amino acids are present in human sweat, 106 although it is not clear what proportion of measured amino acids derive from epidermal contamination.
Proteins Including Proteases: The concen-
tration of sweat protein in the least contaminated, thermally induced sweat is approximately 20 mg/dL, with the major portion being low-molecular-weight proteins (ie, molecular weight <10,000). Because sweat samples collected by simple scraping (and even those collected with a plastic bag) can be massively contaminated with plasma or epidermal proteins, previous reports on the presence of α- and γ-globulins, transferrin, ceruloplasmin, orosomucoid, albumin, 106,107 and immunoglobulin E must be carefully reexamined. The sweat samples collected over an oil barrier placed on the skin (the least-contaminated sweat) contain no or trace of γ-globulin and a very small amount of albumin. Yokozeki and coworkers 108 also reported the presence of cysteine proteinases and their endogenous inhibitors in sweat and the sweat gland. Dermcidin is an antimicrobial peptide produced and secreted in sweat. 79 Other organic compounds reported to be present in sweat include histamine, 109 prostaglandin, 110 and vitamin K–like substances. 111 Sweat also contains traces of pyruvate and glucose. Sweat glucose increases concurrently with a rise in plasma glucose level. Some orally ingested drugs, including griseofulvin,112 ketoconazole, 113 amphetamines, 114 and various chemotherapeutic agents, 115 are secreted in sweat.
MECHANISMS OF SWEAT SECRETION
Several distinct sequential processes lead to eccrine gland sweat production116 : (1) stimulation of the eccrine sweat gland by ACh via increased intracellular Ca2+ ; (2) Ca2+ -stimulated loss of cellular K+ , Cl− , and H2 O, which leads to eccrine gland cell shrinkage; and (3) volume-activated transcellular plus paracellular fluxes of Na+ , Cl− , and H2 O, which leads to net flux of largely isotonic NaCl solution into the glandular lumen. These processes are illustrated in Fig. 6-11.
Sweating initially is stimulated when ACh is released from periglandular cholinergic nerve endings in response to thermal or emotional stimuli. ACh binds to cholinergic receptors on the clear cell plasma membrane, stimulating intracellular Ca 2+ release and influx, and increasing cytosolic Ca 2+ concentrations. Increased intracellular Ca2+ , in turn, opens Ca2+ -sensitive Cl − and K + channels in the clear cell basolateral membrane, which allows Cl − and K + to escape. Because H2 O follows K + and Cl− , to maintain cell iso-osmolarity, the cell shrinks.116,117
This decrease in cell volume sets off a second cascade of cell signaling events. First, decreased cell volume stimulates the NKCC1 118 class of Na/K/2Cl cotransporters, which carry Na+ , K+ , and 2Cl − into the cell in an electrically neutral fashion (ie, two cations and two anions cancel out net charges). The resulting increase in cytosolic Na + activates the Na+ , K+ -ATPase, located in the basolateral membrane, which recycles Na + and K + across the basolateral membrane. The net movement of the negatively charged Cl − ion across the apical membrane into the lumen in turn drives the positively charged Na+ ion into the lumen as well, along a paracellular pathway. Therefore, the final product of glandular secretion is the net movement of Na+ , Cl− , and H2 O into the glandular lumen to form the isotonic NaCl precursor of sweat.
ACh-induced sweating, which constitutes the bulk of sweat production, appears to be mediated by intracellular Ca2+ , as detailed earlier. In contrast, adrenergicinduced sweating appears to be mediated by increased intracellular cAMP.119
MECHANISM OF DUCTAL REABSORPTION
Because the production of large sweat volumes could lead to dangerous losses of NaCl, the sweat duct has evolved to reabsorb NaCl, which minimizes electrolyte loss, even at high sweat volumes (Fig. 6-12). Ductal Na + reabsorption is accomplished through the coordinated activities of intracellular enzymes and plasma membrane ion channels, pumps, and exchangers. These mechanisms not only reabsorb electrolytes but also acidify the sweat, which results in a final sweat product that is hypotonic and acidic. Na+ reenters the duct cells through the apical membrane via amiloride-sensitive 120 epithelial Na + channels (ENaC) 5 and is transported across the basolateral membrane by ouabain-sensitive 88 Na+ , K+ -ATPase pumps. Cl − transport appears to be both transcellular and paracellular, with the cystic fibrosis transmembrane regulator (CFTR) Cl − channels playing an important role in transcellular fluxes. 118 In cystic fibrosis, CFTR Cl − channels are mutated, and eccrine duct Cl − reabsorption is defective but not completely abolished.27 Na + is increased in the duct and the sweat at the skin
surface. 121 Unlike in the lung, CFTR mutations do not lead to increased ENaC-mediated Na + influx, which suggests that the CFTR–ENaC interactions seen in other tissues differ from that in the eccrine duct. Sweat acidification appears to be mediated via the enzyme carbonic anhydrase, the HCO3 − /Cl and Na+ /H+ exchangers, and the V-type H + ATPase. The intracellular enzyme carbonic anhydrase catalyzes HCO 3 − and H + production. Whereas intracellular HCO 3 − is cleared via the HCO3 − /Cl antiporter, H + is pumped into the luminal sweat by the V-type H + ATPase. 122 The Na+ /H+ antiporter NHE1 (Na+ /H + exchanger isoform 1),123,124 found in the basolateral membrane, is important in intracellular pH regulation.
The transfer of sweat to the skin surface without leakage is important for the homeostatic regulation of skin and is impaired in atopic dermatitis; lesional skin presents a decreased claudin-3 expression in sweat glands, which is accompanied by sweat leakage.122
Several drugs are known to modify ductal NaCl reabsorption. When aldosterone is injected systemically or locally, the Na/K ratio in sweat begins to decrease within 6 hours, reaching a nadir at 24 hours and returning to the preinjection level in 48 to 72 hours.121 Na + deprivation stimulates both renin and aldosterone secretion, but high thermal stress per se (a single 1-hour exposure of humans to a temperature of 40°C [104°F]) is a potent stimulator of renin and aldosterone secretion in either the presence or absence of sodium deprivation. In an in vitro sweat gland preparation, neither acetazolamide (a carbonic anhydrase inhibitor) nor antidiuretic hormone changed ductal or secretory function. However, more potent carbonic anhydrase inhibitors, such as topiramate, 125 have been reported to induce oligohidrosis.
