Specimen Handling

Once the biopsy is done, the specimen should be placed in formalin fixative immediately for the purpose of routine histological examination. Specimens needed for direct immunofluorescent study ideally should be placed in Michel's medium or, alternatively, put in saline-moistened gauze if it is going to be processed within 24 hours. Specimens required for flow cytometry, molecular studies, and electron microscopy are sent fresh in saline-soaked gauze; they should be processed as soon as possible. If the specimens are excisional biopsies or larger surgical material, proper sharing of the specimen is done, always with consideration that histology has priority if no prior diagnosis exists for that particular patient.

Punch and shave biopsies are described grossly and either embedded intact or sectioned, depending on the size of the biopsy. Then the specimen is embedded on edge (vertical). Five levels are usually obtained for histologic examination.

Excisional biopsies and surgical specimens obtained for neoplasm are described grossly, and the entire deep and lateral margins of the specimen are inked prior to sectioning. The margins are evaluated by cutting along all margins or, most commonly, by entirely bread loafing the specimen. The entire neoplasm is also evaluated using the bread-loafing technique.

Artifacts

Poor histologic preparation as a result of artifacts will hamper the evaluation of slides by the pathologists. These artifacts can be the result of a variety of factors.

• Fixation problems such as poor or no fixation of the specimen prior to cutting, old solutions being used, insufficient fixation time, or inadequate volume of fixative (ideally, the specimen must be properly fixed in a solution 15 to 20 times the volume of the specimen).

• Improper monitoring of the multiple steps involved in the preparation of a slide, such as cutting, temperature of the water bath, freshness of the staining solutions employed, and other factors.

• Artifacts produced at the time of excision, such as cautery and excessive squeezing of the specimen.

• Specimens stored at low temperatures, giving freezing artifacts.

• Artifacts characteristically seen in certain pathologic processes, such as tissue holes in basal cell carcinoma and the lack of epidermis in sections from toxic epidermolytic necrolysis.

Staining Methods

The majority of the skin lesions can be diagnosed with well-prepared H&E-stained sections. However, they will not provide an adequate answer in all cases. A comprehensive review of special stains is beyond the scope of this chapter because every case is different and may require a specific approach. The following are the most common stains used in our laboratory in the study of cutaneous tissue.

Histochemical Stains

• PAS: to study the thickness of the basement membrane, for glycogen (diastase liable) and fungal organisms (diastase resistant)

• Gomori's methanamine silver: for fungal organisms and cutaneous Pneumocystis carinii

• Ziehl-Nielsen and Fite stains: for acid-fast organisms

• Gram's stains: for bacteria

• Steiner and Warthin-Starry stains: in cases of bacillary angiomatosis and for spirochetes

• Giemsa: for mast cells and protozoan organisms, such as Leishmania

• Mucicarmine: for mucin

• Alcian blue: for acid mucopolysaccharides (pH 2.5) and sulfated mucopolysaccharides (pH 0.5)

• Congo red: for amyloid

• Elastic van Gieson (EVG): for elastic fibers

• Fontana-Masson: for melanin

• Von Kossa: for calcium

Immunofluorescence

Immunofluorescence plays an important place in the diagnosis and evaluation of skin disorders such as lupus erythematosus and autoimmune blistering diseases. Either direct immunofluorescence using tissue or indirect immunofluorescence using serum of patients is available for diagnosis of skin diseases. Direct immunofluorescent studies are performed on cryostat sections of skin specimen using fluorescein isothiocyanate conjugated antisera to examine for the presence of immunoglobulins A, G, and M, as well as fibrinogen and complement.

Immunohistochemical Stains

Immunohistochemical stains are used widely now in combinations of H&E-stained sections for diagnosis of difficult cases, such as poorly differentiated malignant tumors, spindle cell neoplasms, and lymphoproliferative malignancies. Most of the times, these stains are used in panels and not as single preparations. The most commonly used in our laboratory are:

• Epithelial markers: Cytokeratin CAM 5.2 (low-molecular weight cytokeratin), a combination of cytokeratins AE1/AE3 (low- and intermediate-molecular weight keratin, carcinoembryonic antigen (CEA), and epithelial membrane antigen (EMA): They are used in the differential diagnosis of epithelial tumors, adnexal tumors, and Paget's disease.

• Melanocytic markers: S-100 protein, Melan A/MART-1, and HMB-45.

• Mesenchymal markers:

• Vimentin: dendritic cells, macrophages;

• Factor XIIIa: fibroblast-like mesenchymal cells, dermal dendrocytes;

• Factor VIII-associated antigen, CD31 and CD34: endothelial cells; CD34 in combination with Factor XIIIa are useful markers in the diagnosis of dermatofibrosarcoma protuberans;

• HHF35 (muscle specific actin), smooth muscle actin and desmin: muscle differentiation;

• S-100 protein: nerve, fat, and cartilage.

• Lymphoid markers: all available CD markers for the characterization of lymphoid proliferations and other hematologic conditions are used.

• Histiocytic markers: CD68 (KP-1) and lysozymes

• Langerhans cells and Langerhans cell granulomatosis: CD1a and S-100 protein.

• Neuroendocrine cells: neurospecific enolase (NSE), chromogranin and synaptophysin. These markers are also expressed in Merkel cell carcinoma; however, cytoplasmic stains for cytokeratin CK20 is most characteristic.

• Cell proliferation marker: Ki-67 is an excellent marker for determining the so-called growth fraction of a given cell population. Higher staining intensity in malignant lesions might be correlated with significant proliferative capabilities in tumor; Ki-67 labeling may have some value in the differentiation between benign and malignant lesions.

Histologic Differences of Skin with Age

Newborns and Children

The epidermis of newborns and children is usually of the same thickness as in adults, with the exception of the acral skin. There is a greater density of melanocytes and Langerhans cells.

The dermis is more cellular than in the adult with a higher concentration of ground substance. The number of eccrine glands is higher at birth, while apocrine glands are not well developed until after puberty. The sebaceous glands are developed in children, but sebaceous secretion begins at puberty under the influence of androgen stimulation.

The adipocytes of the subcutaneous tissue in newborns and children are thin-walled and larger than the adult adipocytes. In addition to white fat as seen in adults, infants possess brown fat, which initially comprises up to 5% of body weight then diminishes with age to virtually disappear by adulthood. Brown adipocytes are rich in mitochondria and contain multiple lipid droplets of varying size in the cytoplasm with centrally located nuclei. Brown fat contains an abundance of blood-filled capillaries and is of particular importance in neonates because it has the ability to produce heat (thermogenesis) by degrading fat molecules into fatty acids.

Elderly

In the elderly, the histologic differences are mainly due to atrophy and to reduction of most cutaneous elements. The cells of the epidermis are arranged haphazardly because of aberrant proliferation of the basal cell layer, which may predispose to the development of neoplasms. There is a marked decrease in the number of melanocytes and in the number of melanosomes, leading to reduced pigmentation and, consequently, more exposure to the damaging effects of ultraviolet light. The Langerhans cells also decrease in number and function with advanced age, which increases the damaging effects of contactants and partially contributes to age-associated deterioration of immune function.

In the elderly, the dermis is thinned, relatively acellular, and avascular. The dermal collagen, elastin, and ground substance are altered and reduced. Elastic fibers show structural and biochemical alterations that change the elasticity of the skin. Collagen bundles are thicker but stiffer. The net effect is that age-associated alterations make the dermis less stretchable, less resilient, and prone to wrinkling. Fibroblasts, dendritic cells, and mast cells are also reduced in number.

Because of the reduction in the cutaneous vascular supply, there is a decrease in inflammatory response, absorption, and cutaneous clearance.

Both eccrine and apocrine glands are also reduced, with diminished secretions in the elderly. Sebaceous glands increase in size and manifest clinically as sebaceous hyperplasia, but paradoxically their secretory output is lessened by decreased activity.

With age, the number and rate of growth of hair follicles decreases, vellus hair will develop into terminal hairs in unusual sites, such as the ear, nose, and nostrils, resulting in possible cosmetic problems. There is also a decreased functioning of Meissner's and Vater-Pacini corpuscles. Finally, there is diminished subcutaneous tissue especially in the face, shins, hands, and feet, but it increases in other areas, particularly the abdomen in men and the thighs in women.

The pathologic hallmark of extrinsic aging is solar elastosis, whereas wrinkling is due to the intrinsic factors mentioned previously.

Histologic Variations According to Anatomic Sites

Regional variations of the normal histomorphology are important to recognize so as to avoid misinterpretation of variation as abnormality.

The normal scalp and other densely hair-containing regions show hair follicles extending through the dermis into the subcutaneous fat. This is usually not seen in areas with less concentration of hair. Abundant vellus hair is seen in sections taken from the skin of the ear. The skin of the face shows characteristically many pilosebaceous units, and large sebaceous glands are seen on the nose.

The squamous layer of the eyelid epidermis is thin and composed of two to three layers of cells and basoloid epithelial buds. Modified apocrine glands (Moll's glands) and vellus hairs are seen in the dermis.

Sections taken from the skin of the trunk, especially the back, show a normally thickened reticular dermis when compared to other sites. Unawareness of this normal variation may lead to the erroneous diagnosis of processes producing thick collagen, such as scleroderma. The skin around the umbilicus also shows thick and fibrotic dermis.

The palms and soles contain stratum lucidum and show a thick, compact cornified layer with loss of the characteristic basket-weave pattern. In addition, there are numerous eccrine units, nerve end organs, and glomus structures seen in the dermis. There are no pilosebaceous units. Sections of the skin of the lower leg may show thicker blood vessels in the papillary dermis as a result of gravity and stasis. Smooth muscle fibers are seen in the dermis of the skin of external genitalia and areola of the nipple. Cutaneous-mucosal junctions may lack granular and cornified layers, and cells of the squamous layers are larger, with higher glycogen content.

Pathologic Changes Found in Biopsies and Interpreted as Normal Skin

Biopsies taken from clinically abnormal skin lesions may be interpreted histologically as normal because of the presence of subtle changes. The following are some examples.

• Dermatophytosis is seen in the cornified layer of an otherwise normal skin.

• A thick or absent granular layer may indicate an abnormal process of keratinization like psoriasis or an ichthyosiform dermatosis.Vitiligo may give the histologic impression of normal skin unless one searches for melanocytes.

• Macular and lichen amyloidosis may be overlooked because the pink globules of amyloid seen in the papillary dermis can be mistaken for normal dermis.

• Urticaria produces only edema, which in routine sections is seen as separation of the collagen bundles in the dermis. Similar changes are seen in the case of dermal mucinosis, in which deposition of mucinous material may be inconspicuous in routine sections. Special stains for mucin will be helpful.

• In telangiectasia macularis eruptive perstans, a subtype of cutaneous mastocytosis, the changes may be quite subtle and are composed of dilated blood vessels in the upper dermis with a scant infiltrate of mast cells. The infiltrate must be confirmed with appropriate stains for mast cells.

• Trichotillomania is a hair pulling habit resulting in areas of alopecia. Although histologic changes can be numerous, at times hair follicles devoid of hair are the only changes seen, which give an impression of normal skin in the biopsy material.

• Some degenerative disease of skin, such as anetoderma, can represent only as partial loss of elastic fibers in the dermis, which will be demonstrated by special stains of elastic tissue.

• The so-called connective tissue nevus representing a hamartoma with an overproduction of collagen bundles and increased, normal or decreased elastic tissue in the dermis is another condition that can be erroneously interpreted as normal skin.

Other conditions that might be missed as normal skins include cafe-au-lait spots, cutis laxa (elastolysis), myxoedema, scleromyxedema, and more. Therefore, the clinical information combined with careful histological examination, sometimes special stains, or immunohistochemical studies of the biopsy material is crucial to avoid misinterpretation of skin disorders as normal tissue.

Dermis

The dermis is a dynamic, supportive connective tissue harboring cells, fibrous tissue, and ground substances with adnexal structures and vascular and nerve plexuses running through it. The dermis consists of two zones: the papillary dermis and reticular dermis. The adventitial dermis combines the papillary and the periadnexal dermis.

The papillary and periadnexal dermis can be recognized by a loose meshwork of thin, poorly organized collagen composed of predominantly type III collagen mixed with some type I collagen and a delicate branching network of fine elastic fibers. The papillary dermis also contains abundant ground substance, fibroblasts and the capillaries of the superficial arterial and venous plexuses.

The reticular dermis is thicker than the papillary dermis and is composed of multiple layers of well-organized thick bundles of collagens, predominantly type I collagen, mostly arranged parallel to the surface. These layers are built from overlapping of individual fibers of uniform size. The plates are oriented randomly in different directions. There are also thick elastic fibers with fragmented appearance detected by special elastic tissue stains. Some ground substance and the vessels of the deep plexuses are also present in the reticular dermis.

The resident cells in the dermis mainly include dermal dendritic cells, fibroblasts, and mast cells. Dermal dendritic cells are a group of cells with immunophenotypic and functional heterogeneity located in the dermis and possessing a dendritic morphology. There are multiple subsets of dendritic cells. At least three types of dermal dendritic cells are recognized as distinct cell types with unique immunophenotype in vivo.

• Factor XIIIa+ dermal dendrocytes are in the perivascular distribution in the papillary dermis and around sweat glands. Dermal dendrocytes, also known as dendrophages, express some markers of mononuclear macrophages and have phagocytic function.
• CD34+ dendritic cells are present in the mid- and deep dermis around adnexae.
• The dermis harbors a true dendritic cell population, also in a perivascular distribution. These are Langerhans' cell-like dendritic cells involved in dermal antigen presentation, expressing HLA-DR and CD1a except for lack of Birbeck granules.

Fibroblasts are the dynamic and fundamental cells of the dermis, synthesizing all types of fibers and ground substances. They appear as spindle-shaped or stellate cells, which are not reliably differentiated from other dermal spindle-shaped cells and dendritic cells in H&E-stained sections. Ultrastructurally, they contain prominent, well-developed rough endoplasmic reticulum.

Mast cells are derived from bone marrow CD34+ progenitor cells and are sparsely distributed in the perivascular and periadnexal dermis. They are recognized by a darkly stained ovoid nucleus and granular cytoplasm, which is highlighted by Giemsa and toluidine blue stains. Mast cells are positive with tryptase and c-kit (CD117) immunohistochemical stains. Mastocytosis is characterized by abnormal growth and accumulation of mast cells in various organs with heterogenous manifestation. Urticaria pigmentosa is the most common cutaneous manifestation of mastocytosis.

Macrophages are also seen in the normal dermis; they become visible when pigments or other ingested material is present in the cytoplasm of the cells.

Besides fibrous tissue and cellular components, the dermis also contains amorphous ground substance filling the spaces between fibers and dermal cells. It mainly consists of glycosaminoglycans or acid mucopolysaccharides (the nonsulfated acid mucopolysaccharides [predominantly hyaluronic acid] and, to a lesser degree, sulfated acid mucopolysaccharide [largely chondroitin sulfate)]). The ground substance is present in small amounts and is seen as empty spaces between collagen bundles in routine H&E-stained sections; it also is hardly identified with Alcian blue and toluidine blue special stains. In pathologic conditions such as lupus erythematosus, granuloma annulare, and dermal mucinosis, the excessive quantity of ground substance produced can be seen without the aid of special stains as strings of bluish material.

Subcutaneous Tissue

Subcutaneous tissue, also called subcutis or hypodermis, is crucial in thermal regulation, insulation, provision of energy, and protection from mechanical injuries. It is composed of mature adipose tissue arranged into lobules. The mature adipocytes within the lobules are round cells rich in cytoplasmic lipids, which compress the nucleus to the side of the cell membrane. The adipocytes express S-100 protein and vimentin in immunohistochemical stains. These lobules of mature adipocytes are separated by the thin bands of dermal connective tissue that constitute the interlobular septa. Thus, inflammatory changes involving the subcutaneous tissue can be divided into septal panniculitis (e.g., erythema nodosum) and lobular panniculitis (e.g., panniculitis associated with pancreatitis).

Blood Vessels, Lymphatics, Nerves, and Muscle

The large arteries that supply the skin are located in the subcutaneous tissue, usually within the interlobular septa, and are accompanied by large veins. Smaller arteries, venules, and capillaries constitute the main vasculature seen in the dermis and within the lobules of the subcutaneous fat.

A network of these smaller vessels is located in the papillary dermis (superficial plexus) and in the deep reticular dermis (deep plexus). Superficial vascular plexuses separate the papillary dermis from the reticular dermis, whereas the deep vascular plexuses define the boundary between the reticular dermis and subcutaneous tissue. The division of superficial and deep plexuses is important in the classification and recognition of many inflammatory diseases of the skin in which characteristic infiltrates are located around the superficial, deep, or superficial and deep plexuses. Blood endothelial cells express von Willebrand factor (factor VIII-related antigen), vimentin, CD34, and CD31 antigens.

Vasculitis is the inflammatory process that involves the blood vessels. It is important to remember that strict criteria are applied for the diagnosis of cutaneous vasculitis, and they include: (a) the presence of inflammatory cell infiltrate within the vessel wall, and (b) the presence of vascular injury, in a spectrum from edema and extravasations of red blood cells, leukocytoclasis, thrombi within the lumina of these blood vessels to fibrinoid necrosis and/or destruction of the blood vessel wall. The presence of fibrinoid necrosis of the vessel wall is essential for diagnosis of true vasculitis. Perivascular inflammation alone is not a sign of vasculitis.

Mainly in the acral skin, special arteriovenous anastomosing structures known as glomera are present in the reticular dermis. Each glomus is composed of an arterial segment (the Sucquet-Hoyer canals) connected directly with venous segments. Each Sucquet-Hoyer canal is surrounded by four to six layers of glomus cells, which are considered as vascular smooth muscle cells serving as a spincter. Glomus is involved in thermal regulation.

The lymphatics of the skin accompany the venules and are also located in the deep and superficial plexuses. Unless valves are seen within these vessels, their recognition in routine sections is impossible. Under normal conditions, they are surrounded by a cuff of elastic fibers. In contrast to blood endothelial cells, lymphatic endothelium do not react with antibodies to von Willebrand factor (factor VIII-related antigen) and CD34.

Large nerve bundles are seen in the subcutaneous fat and in the deep reticular dermis; however, small nerve fibers are present throughout the skin, reaching the papillary dermis. As mentioned earlier, recent immunohistochemical studies have demonstrated that the epidermis contains free nerve axons in association with Langerhans cells.

In sections of the palm and sole, some sensory nerves form nerve ending organs. Meissner corpuscles are seen in the papillary dermis, which is composed of several parallel layers of Schwann cells containing an axon; they function as rapid mechanical receptors for the sense of touch. In weight-bearing areas, the Vater-Pacini corpuscles consist of concentrically arranged Schwann cells with an axon and are located in the deep dermis and subcutaneous fat. They serve as receptors for sense of deep pressure and vibration.

Smooth muscle is represented in the skin by the arrector pili muscles, which arise in the connective tissue of the dermis and insert into hair follicles below the sebaceous glands. Melanocytes of congenital nevus are often seen within the arrector pili muscle. Smooth muscle is also seen in the skin of external genitalia (tunica dartos) and in the areolae.

Strands of striated muscle are found in the skin of the neck, face, and particularly the eyelids as muscle of expression.

Sebaceous Glands

The sebaceous glands are holocrine glands associated with hair follicles. Their secretions are made of disintegrated cells. The palms and soles are the only regions devoid of sebaceous glands. Sebaceous glands are prominent in facial skin. They are also seen in the buccal mucosa, vermilion of the lip (Fordyce's spot), prepuce, labia minora, and, at times, in the parotid gland.

The sebaceous glands are lobulated structures composed of multiple acini in some locations like the head and neck; in other sites, such as chest, they are composed of a single acinus. The periphery of the lobules contains the germinative cells, which are cuboidal and flat with large nucleoli and basophilic cytoplasms without lipid droplets. As differentiation occurs, several inner layers show lipid droplet accumulation in the cytoplasm until they fill the cell.

The more differentiated cells (sebocytes) have a characteristic multivacuolated cytoplasm . The nucleus is centrally located and scalloped due to the lipid imprints. The more differentiated cells disintegrate and discharge the cellular debris (sebum) into the excretory duct, which opens into the hair follicle in the lower portion of the infundibulum. The excretory duct is short, shared by several lobules, and lined by keratinized squamous epithelium.

Within sebaceous glands, the germinative cells express appreciable quantities of keratins. Mature sebocytes demonstrate cytoplasmic reactivity for high-molecular weight keratins and epithelial membrane antigen.

Eccrine Glands

The eccrine glands are the true sweat glands responsible for thermoregulation. They are found in higher concentration in palms, soles, forehead, and axillae and have dual secretory and excretory functions.

The secretory portion of an eccrine gland is a convoluted tube located in the dermis, in the interface with the subcutaneous tissue, and, rarely, within the subcutaneous tissue. In cross-sections, it appears that several glandular structures with a central lumen form the secretory coils. These are seen as lobular structures often surrounded by fat even when located within the dermis.

Three types of cells are identified in the eccrine coil: clear cells, dark cells, and myoepithelial cells. The clear cells are easily seen H&E-stained sections. They rest directly on the basement membrane and on the myoepithelial cells. Clear cells are composed of pale or finely granular cytoplasms with a round nucleus usually seen in the center of the cell. Deep invaginations of the luminal membranes of adjacent clear cells form intercellular canaliculi lined with microvilli. The intercellular canaliculi often persistent in neoplasms derived from eccrine glands. The clear cells contain abundant mitochondria and variable amount of PAS-positive, diastase-labile glycogen.

The dark cells border the lumen of the glands. Electron microscopy shows that they contain abundant secretory granules that have glycogen-staining characteristics. They contain sialomucin (PAS-positive, diastase-resistant (PASD) mucopolysaccharides) and high concentration of proteins. The dark cells are difficult to identify in routine H&E-stained sections. However, the acid-fast, PASD, and S-100 protein stains will highlight the granularity of the cells.

The myoepithelial cells are contractile spindle cells that surround the secretory coil. In turn, they are surrounded by a PAS-positive basement membrane. Elastic fibers, fat, and small nerves are present in the adjacent stroma.

The excretory component of the eccrine gland is composed of three segments: (a) a convoluted duct in close association with the secretory unit; (b) a straight dermal component; and (c) a spiral intraepidermal portion, the acrosyringium, which opens onto the skin surface. The transition between the secretory and excretory component is abrupt. Both convoluted and straight dermal ducts are histologically identical. They are narrow tubes with a slitlike lumina lined by double layers of cuboidal cells. The luminal cells have a more granular eosinophilic cytoplasm and a larger round nucleus than the peripheral row of cells. The peripheral cells are rich in mitochondria.

The luminal cells produce a layer of tonofilaments near the luminal membrane that are often referred to as the cuticular border, which is a PASD eosinophilic cuticle. This cuticular border often persists in the eccrine neoplasm (e.g., eccrine poroma). There are no myoepithelial cells and peripheral hyalin basement membrane zone in the eccrine ducts .

The intraepidermal segment of eccrine duct, known as acrosyringium has a unique symmetrical and helicoidal course in the epidermis with its length correlated to the thickness of the epidermis. It consists of a single layer of luminal cells and two or three rows of concentrically oriented outer cells. The presence of keratohyalin granules in acrosyringium in the lower levels of the squamous layer indicates that they keratinize independently. The intraepidermal lumen is lined by acellular eosinophilic cuticle before keratinization. Melanin granules are absent.

Apocrine Glands

The apocrine gland has a coiled secretory portion and an excretory (ductal) component. The secretory portion is much longer than its eccrine counterpart; and it may reach 200 nm in diameter, compared to nm for the eccrine glands. The secretory glands are located in the subcutaneous fat or in the deep dermis. They are lined by one layer of cuboidal, columnar, or flat cells (luminal cells), and an outer layer of myoepithelial cells, which is surrounded by a PAS-positive basement membrane. The luminal cells are composed of eosinophilic cytoplasm, which may contain lipid, iron, lipofuscin, PASD granules , and a large nucleus located near the base of the cell. Detached fragments of apical cytoplasm are found in the lumen of the glands. The secretion from apocrine glands releases secretory materials accompanied with loss of part of cytoplasm, although other forms of secretion have been observed, including melocrine (granular contents within numerous vesicles are released without loss of cytoplasm) and holocrine type (the entire cell is secreted into the glandular lumen).

Similar to the eccrine duct, the excretory (ductal) component of the apocrine gland has a double layer of cuboidal cells. Microvilli are identified on the surface of the luminal cells and keratin filaments are in their cytoplasms, the latter giving the eosinophilic hyalin appearance to the inner lining of the duct. No myoepithelial cells and peripheral basement membrane are identified in the excretory duct. Apocrine glands are always connected to a pilosebaceous follicle. The intrafollicular or intraepidermal portion of apocrine duct is straight other than the spiral as seen in acrosyringium.

Apocrine glands are mostly located in the axillary, anogenital areas, mammary region, eyelids (Moll's glands), and external ear canal (ceruminous glands), and their presence is characteristic in Nevus sebaceous Jadassohn.

A third type of sweat gland, so-called apoeccrine glands of the human axillae, are composed of a dilated secretory portion that, by electron microscopy, is indistinguishable from the apocrine glands; however, they retain the intercellular canaliculi, as well as the dark cells of the eccrine glands. The duct does not open in the hair follicle but in the epidermis. These glands, which develop from eccrine glands during puberty, account for as much as 45% of all axillary sweat glands in a young person. Recently, it was reported that the obstruction of intraepidermal apoeccrine sweat ducts by apoeccrine secretory cells might be the possible causes of Fox-Fordyce disease.

The most useful marker of sweat gland differentiation is carcinoembryonic antigen (CEA), found mainly in the luminal borders of secretory cells of eccrine glands and excretory eccrine ducts and, to a less extent, on apocrine glands. Gross cystic disease fluid protein-15 (GCDFP-15) and epithelial membrane antigen (EMA) are also detected in both eccrine and apocrine sweat glands. Myoepithelial cells lining the secretory sweat glands express smooth muscle actin and keratin K17 .

Hair Follicle

 来自筷子的翻译：

胚胎学

表皮

从胚胎学上说，外胚层生成表皮及其附属物。中胚层生成真皮及皮下脂肪组织的间充质成分。

最初，胚胎由单层外胚层细胞覆盖。到发育的第6到8周，分化成为两层，基底层和覆于上面的称为周皮的第二层。周皮的表面被微绒毛覆盖，与羊水相接触。基底层的有丝分裂活性超过周皮的有丝分裂活性，很快基底层成为生发层。从该增生活跃的一层另外形成多层细胞，形成外胚层和周皮之间的多层细胞。到第23周，在上层发生角质化，周皮的细胞已经脱落。有趣的是，多种细胞连接蛋白已经发现表达于早期的两层胚胎表皮，甚至早至推测妊娠期的第八周。到妊娠前三个月的末期，皮肤表皮与其成分的连接超微结构上已经类似成熟皮肤的连接。因此，特征性的新生儿表皮在第四个月得到充分发育。

表皮中的细胞大部分是角质化细胞（90-95%）。表皮细胞的其他成分为非角质化细胞（5-10%），包括黑色素细胞、Langerhans细胞、Merkel细胞。非角质化细胞可见于胚胎8-10周的表皮中。黑色素细胞的前体从神经嵴移行至真皮，然后到达表皮，在此处在发育的前三个月分化成为黑色素细胞。在此前一过程中，黑色素细胞可残留在其他器官和组织。.超微结构上，黑色素细胞中可识别的黑色素小体可见于妊娠期8-10周的胚胎表皮。

Physiology

 SOS老师是用的翻译软件还是在线翻译？

这个东西是最靠不住的