Integument-Singh_2023-Notes.pdf

Microanatomy of Integumentary System Dr. Lalit P. Singh Page 1 of 31 MICROANATOMY OF THE INTEGUMENTARY SYSTEM Session Learning Objectives: 1. Describe the microanatomy of skin.  Describe the character...

Microanatomy of Integumentary System Dr. Lalit P. Singh Page 1 of 31 MICROANATOMY OF THE INTEGUMENTARY SYSTEM Session Learning Objectives: 1. Describe the microanatomy of skin.  Describe the characteristics of each layer of the skin.  Compare and contrast thick and thin skin in terms of their histologic appearance and function.  Describe the histological appearance of the epidermal layers, noting unique features of each.  Describe the histological appearance of the dermis and distinguish the two dermal layers.  Describe the features and location of the four cell types in the epidermis.  Describe the functions of keratinocytes in the epidermis.  Describe the process of keratinization beginning in the stratum basale.  Describe the production of melanin and its fate in the skin.  Describe the factors that determine skin color.  Describe the functions of Langerhans’ cells and Merkel’s cells. 2. Describe the microanatomy of hair and hair follicles.  Describe the histological organization of the hair follicle, including the hair bulb.  Describe the derivation and relative location of each layer of a hair follicle.  Describe the location and functions of the arrector pili muscle.  Describe the structure and formation of hair.  Compare and contrast soft and hard keratin. 3. Describe the microanatomy and function of the integumentary glands.  Describe the microanatomy and function of the eccrine sweat gland.  Describe the morphology and distribution of the eccrine sweat gland.  Describe the histological appearance of the secretory and ductal components of the eccrine sweat gland.  Describe the product and function of the eccrine sweat gland.  Describe the nature and control of the secretion of the eccrine sweat gland.  Describe the 3 cell types in the secretory segment of the eccrine sweat gland.  Describe the microanatomy and function of the apocrine sweat gland.  Describe the morphology and distribution of the apocrine sweat gland.  Describe the histological appearance of the secretory and ductal components of the apocrine sweat gland.  Describe the product and function of the apocrine sweat gland.  Describe the nature and control of the secretion of the apocrine sweat gland. Microanatomy of Integumentary System Dr. Lalit P. Singh Page 2 of 31 4. Describe the sensory structures of the integumentary system.  Describe the structure, location, and function of Meissner’s corpuscles. Identify them in H&E sections.  Describe the structure, location, and function of Pacinian corpuscles. Identify them in H&E sections. Lecture Content Outline I. Integumentary system A. Components B. Functions II. Skin A. Layers of skin B. Two types (categories) C. Layers of epidermis D. Layers of dermis E. Cells of epidermis III. Hair and hair follicles A. Hair follicle B. Hair IV. Glands A. Eccrine sweat glands B. Apocrine sweat glands C. Sebaceous glands V. Sensory structures A. Free nerve endings B. Encapsulated nerve endings VI. Nail (not included) VII. Some clinical correlations of Dermatology Microanatomy of Integumentary System Dr. Lalit P. Singh Page 3 of 31 I. INTEGUMENTARY SYSTEM A. COMPONENTS (Figure 1) 1. Skin: cutis or integument 2. Epidermal derivatives of skin: hair follicles and hair, sweat glands, sebaceous glands, and nails Figure 1. Integumentary system and its components. Graphic 11-1a from Color Atlas of Histology (Gartner & Hiatt, 3rd ed., 2000). Microanatomy of Integumentary System Dr. Lalit P. Singh Page 4 of 31 B. FUNCTIONS 1. Barrier function: protects body against physical, chemical, and biologic agents in environment 2. Immunologic function: site of antigen processing; provides information to appropriate effector cells in lymphatic tissue 3. Homeostasis: involved in regulating body temperature and water loss 4. Sensory function: conveys sensory information about external environment 5. Endocrine function: a. Secretes hormones cytokines, and growth factors b. Converts 7-dehydrocholesterol into vitamin D precursor following exposure to UV light 6. Excretory function: exocrine secretion of sweat, sebaceous, and apocrine glands II. SKIN A. LAYERS OF SKIN (Figure 2) 1. Epidermis - outermost layer; epithelium exposed to external environment 2. Dermis - layer of dense connective tissue beneath epidermis that imparts mechanical support and strength to skin. Microanatomy of Integumentary System Dr. Lalit P. Singh Page 5 of 31 3. Hypodermis (subcutaneous tissue or fascia) - lies deep to dermis and contains varying amounts of adipose tissue Figure 2. LM section of thick skin. Plate Figure 3. LM section of thick skin. Note thick 42-1 from Histology: A Text and Atlas (Ross epidermis. Epi: epidermis; Derm: dermis; PL: & Pawline, 5th ed., 2006). Ep: epidermis; D: papillary layer; RL: reticular layer; D: sweet duct; sweet ducts; De: dermis; Hy: hypodermis; SB: stratum basale; SS: stratum spinosum; SGr: SG: sweet gland; PC: Pacinian corpuscles. stratum granulosum. Figure 15.2 from Histology. A Text and Atlas (Ross & Pawlina, 5th ed., 2006). B. TWO TYPES (CATEGORIES): Based on thickness of epidermis 1. Thick skin (Figure 3) a. Found on palms of hands and soles of feet b. Thick epidermis (mainly due to thick stratum corneum) c. Key features: hairless with no sebaceous glands Microanatomy of Integumentary System Dr. Lalit P. Singh Page 6 of 31 2. Thin skin (Figure 4) a. Found everywhere but palms of hands and soles of feet b. Thin epidermis c. Key features: contains hair and sebaceous glands C. LAYERS OF EPIDERMIS (Figure 5) Figure 4. LM section of thin skin. E: 1. Stratum basale (stratum epidermis; D: dermis; hD: hypodermis; H: germinativum) (Figures 6 hair; HF: hair follicle; AP: arrector pili muscle; sG: eccrine sweet gland. Plate 11-2 and 7) Figures 1 and 2 from Color Atlas of Histology (Gartner & Hiatt, 3th ed., 2000). a. Single layer of small cuboidal to low columnar cells resting on the basal lamina at the basal surface of the epithelium b. Contains mitotically active cells (basal cells) that give rise to keratinocytes. Also contains melanocytes (Figure 7). c. Basal cells have closely spaced nuclei with basophilic cytoplasm and contain varying amounts of melanin transferred from neighboring melanocytes (M). Microanatomy of Integumentary System Dr. Lalit P. Singh Page 7 of 31 Figure 5. Schematic showing layers of epidermis. Graphic 11-1b from Color Atlas of Histology (Gartner & Hiatt, 3rd ed., 2000). Figure 6. Masson’s trichrome- Figure 7. LM section of lower stained section through epidermis. D: (basal) epidermis. PL: papillary layer dermis; B: stratum basale; S: stratum of dermis; M: melanocyte; SGe: spinosum; G: stratum granulosum; C: stratum granulosum; SS: stratum stratum corneum. Figure 9.4a in spinosum; Plate 11-1 Figure 3 from Wheater’s Functional Histology Color Atlas of Histology (Gartner & (Young and Heath, 4th ed., 2000). Hiatt, 3th ed., 2000). Microanatomy of Integumentary System Dr. Lalit P. Singh Page 8 of 31 d. Cells attached to each other and to keratinocytes by desmosomes; anchored to basal lamina by hemidesmosomes. e. After mitosis the resultant keratinocytes move up into next layer of epidermis, the stratum spinosum. Figure 8. LM section through stratum spinosum. Figure 9. EM section of keratinocytes in stratum Note cytoplasmic spines or prickles between cells. spinosum. Note the cytoplasmic connections Figure 9.4c in Wheater’s Functional Histology between cells joined by desmosomes. Figure 9.6a (Young and Heath, 4th ed., 2000). in Wheater’s Functional Histology (Young and Heath, 4th ed., 2000). 2. Stratum spinosum (spinous or prickle cell layer) (Figures 8 and 9) a. Composed primarily of keratinocyte that is producing keratin (see Section II E). b. At least several cells thick. c. Exhibit numerous cytoplasmic processes connected to processes from adjacent cells by desmosomes; cell shrinkage during fixation results in increase in intercellular space and the appearance of spines or prickles – hence this is sometimes called the prickle cell layer (Figures 8 and 9). Microanatomy of Integumentary System Dr. Lalit P. Singh Page 9 of 31 d. As cells mature and move toward surface, cells increase in size and flatten out. 3. Stratum granulosum (Figure 10a) a. Most superficial layer of non-keratinized portion of epidermis b. Cells contain numerous keratohyalin granules. Granules contain protein precursors of filaggrin and trichohyalin. Filaggrin and trichohyalin aggregate keratin filaments in stratum corneum. c. Keratohyalin granules are irregular in shape and variable in size. They stain intensely basophilic and are readily visible in H&E sections (Figure 10a). (a) (b) Figure 10. (a) LM section of upper epidermis showing stratum granulosum (SG, arrows). (b) stratum lucidum (SL). SS: stratum spinosum; SC: stratum corneum; d: sweet ducts. Plate 11-1 Figure 4 from Color Atlas of Histology (Gartner & Hiatt, 3th ed., 2000). 4. Stratum lucidum (Figure 10b) a. Normally only well seen in thick skin. Microanatomy of Integumentary System Dr. Lalit P. Singh Page 10 of 31 b. Highly refractile, glassy layer containing eosinophilic cells in which keratinization is well advanced. c. Sometimes now considered a subdivision of stratum corneum. 5. Stratum corneum (Figures 10a and b) a. Outermost layer of epidermis, exposed to external environment b. Contains flattened, desiccated, anucleate cells. c. Cells lose their nucleus and cytoplasmic organelles and become filled almost entirely with keratin filaments. d. These cornified, keratinized cells have thick plasma membranes that are coated with an extracellular layer of lipids - major constituent of epidermal water barrier. e. Extremely thick in thick skin - accounts for most of thickness difference between thin and thick skin. D. LAYERS OF DERMIS 1. Papillary layer (Figures 11 and 12) a. More superficial layer consisting of loose connective tissue with delicate collagen network (type I and type III collagen) and threadlike elastic fibers. Microanatomy of Integumentary System Dr. Lalit P. Singh Page 11 of 31 b. Attachment of epidermis to dermis enhanced by increased interface formed by dermal papillae, which are fingerlike dermal protrusions into the epidermis. The complementary epidermal protrusions that project into the papillary dermis are called epidermal ridges or rete ridges. Dermal papillae and epidermal ridges are most prominent in thick Figure 11. LM section of thin skin. PL: papillary layer; RL: skin of palms and soles. reticular layer; E: epidermis; IP: Distinctive parallel arrangement interpapillary peg; AP: arrector pili; SC: stratum corneum; N: of dermal ridges in fingers is nuclei; CF: collagen fibers; BV: blood vessels. Plate 11-2 Figure 2 responsible for fingerprints. from Color Atlas of Histology (Gartner & Hiatt, 3th ed., 2000). c. Papillary layer contains blood vessels that supply nutrients, but do not enter epidermis. d. Papillary layer contains many of the nerve processes that terminate in the dermis or penetrate the basal lamina and enter the epithelium (see Section V). 2. Reticular layer (Figures 11 and 12) a. Layer of dense irregular Figure 12. LM section of connective tissue underlying the dermis. P: papillary layer; R: reticular layer. Figure 9.4c in papillary layer. Wheater’s Functional Histology (Young and Heath, 4th ed., 2000). Microanatomy of Integumentary System Dr. Lalit P. Singh Page 12 of 31 b. Characterized by thick, irregular bundles of Type I collagen and coarse elastic fibers. E. CELLS OF EPIDERMIS 1. Keratinocytes (Figure 13) a. Predominate cell type in epidermis b. Originate from mitosis of stem cells in stratum basale. Figure 13. Cell types in epidermis. Modified and adapted from Figure 11-3 in Histology and Cell Biology (Kierszenbaum, 1st ed., 2002). c. Two functions of keratinocytes: i. Produce keratin, major structural protein of epidermis; keratin makes up ~85% of fully differentiated keratinocytes. ii. Participate in formation of epidermal water barrier. Figure 14. EM section of keratinocytes in stratum spinosum showing many desmosomal d. Keratinocytes are held together connections. Figure 2-11 in The Structure and Function of Skin by desmosomes (Figure 14). (Montagna & Parakkal, 3rd ed., 1974). Microanatomy of Integumentary System Dr. Lalit P. Singh Page 13 of 31 e. Cytoplasm of immature keratinocytes in and near stratum basale appears basophilic, because of numerous free ribosomes engaged in synthesizing keratin. f. Changes in keratinocytes (Figure 15): As keratinocytes move through the stratum spinosum, keratin filaments (tonofilaments) are continuously produced and become grouped into thick bundles called tonofibrils. Cells become more eosinophilic as tonofibrils take up more of cytoplasm. Cells also begin to produce Figure 15. Changes in keratinocytes in epidermis. Modified and adapted from lamellar bodies. In upper part Figure 11-5 in Histology and Cell of stratum spinosum the free Biology (Kierszenbaum, 1st ed., 2002). ribosomes begin to synthesize keratohyalin granules, and the cells move into the stratum granulosum. Keratohyalin granules contain two major intermediate filament-associated proteins, filaggrin and trichohyalin, which function as promoters in the aggregation of keratin filaments into tonofibrils. The conversion of granular cells into cornified cells is called keratinization. The keratin fibril formed in this process in skin is called soft keratin. The cornified cells are eventually sloughed off by desquamation (shedding). Microanatomy of Integumentary System Dr. Lalit P. Singh Page 14 of 31 g. Epidermal water barrier in stratum corneum prevents water loss and maintains body homeostasis (Figure 15). Two components: i. Cell envelope: 15 nm thick layer of insoluble proteins on inner surface of plasma membrane; imparts strength. ii. Lipid envelope: 5 nm thick layer of lipids extruded from lamellar bodies in stratum granulosum; imparts hydrophobicity. 2. Melanocytes a. Called dendritic cells of the epidermis because of the presence of long cytoplasmic processes that percolate among keratinocytes but do not have any immune function. They are scattered among basal cells of the stratum basale. In H&E, cytoplasm of rounded cell body appears clear in stratum Figure 16. LM section of epidermis, basale (Figure 16). They showing melanocyte and keratinocytes extend long processes containing melanin. Figure 11-8 in Histology and Cell Biology between keratinocytes of (Kierszenbaum, 1st ed., 2002). stratum spinosum (Figure 17). b. Melanocytes do not form desmosomal attachments with neighboring keratinocytes. Microanatomy of Integumentary System Dr. Lalit P. Singh Page 15 of 31 c. Ratio of melanocytes to keratinocytes ranges from 1:4 to 1:10 in different parts of body; constant in all races. d. Melanocytes can replicate, but at much slower rate than keratinocytes. Figure 17. TEM of a melanocyte. P: processes K: keratinocytes. Dark e. Function: produce granules are melanosomes. Figure 15.8 melanin, pigment that from Histology: A Text and Atlas (Ross & Pawlina, 5th ed., 2006). protects cells against damage from nonionizing ultraviolet irradiation. f. Production of melanin (Figure 18a): Melanin produced by oxidation of tyrosine to 3,4- dihydroxyphenylalanine (DOPA) by tyrosinase and subsequent Figure 18a. Formation of melanin and transformation of secretion of pigment into keratinocytes. Left: lighter skin, Right: darker skin. Figure DOPA into melanin. 15.9 from Histology: A Text and Atlas (Ross & Pawlina, 5th ed., 2006). Reactions initially occur in premelanosomes and early melanosomes (low melanin content) that both contain a lot of tyrosinase. As more melanin is produced, internal structure becomes obscured and this mature melanin granule becomes electron-opaque and is called a Microanatomy of Integumentary System Dr. Lalit P. Singh Page 16 of 31 melanosome. This maturation occurs as the granule moves along the processes, so premelanosomes are near the Golgi apparatus, while melanosomes are near the end of the processes. Melanosomes and the melanin within are transferred to keratinocytes by pigment donation, which involves phagocytosis of tips of melanocyte processes by keratinocytes. g. Skin color (Figure 18b) determined by two factors: i) melanin content - degradation of melanin more rapid in individuals with light skin (Figure 18a); ii) two genetically determined forms of melanin: eumelanin (brownish black pigment) and pheomelanin (reddish yellow pigment). Figure 18b. Skin color in different races according to different pigmentation. Note: The images in the upper and lower panels are not matched. Microanatomy of Integumentary System Dr. Lalit P. Singh Page 17 of 31 3. Langerhans’ cells (Figure 19) a. Antigen presenting dendritic-like cells of the epidermis that encounters and process antigens entering through the skin. b. Reside primarily in stratum spinosum and are most easily detected by special techniques or immunostaining. No desmosome formation with Keratinocytes. c. Function: Langerhans’ cell ingests antigens by phagocytosis after they bind to cell surface receptors. They Figure 19. Function of Langerhans’ cell in epidermis. leave epidermis and travel to Modified and adapted from Figure 11-10 in Histology and Cell Biology regional lymph node via (Kierszenbaum, 1st ed., 2002). lymphatics. In lymph node, they become dendritic cells, which when activated stimulate T cells. 4. Merkel’s cells (Figure 20) a. Modified epidermal cells located in stratum basale that function in cutaneous sensation (touch). b. Most abundant in fingertips and other areas of acute sensory Figure 20. Merkel’s cell and disk. perception. Modified and adapted from Figure 11- 13 in Histology and Cell Biology (Kierszenbaum, 1st ed., 2002). Microanatomy of Integumentary System Dr. Lalit P. Singh Page 18 of 31 c. Closely associated with expanded terminal bulb of afferent myelinated nerve fibers. Neuron terminal loses its Schwann cell covering, penetrates basal lamina, and expands into a disk-shaped ending (Merkel’s disk) that lies in close apposition to Merkel’s cell. Combination of neuronal nerve and Merkel’s cell = Merkel’s corpuscle. III. HAIR AND HAIR FOLLICLES A. HAIR FOLLICLE (Figure 21) 1. Invagination of the epidermis in which a hair is formed. Extends deep into hypodermis. 2. Responsible for production and growth of a hair. 3. Associated with sebaceous glands and found only in thin skin. Figure 21. Hair follicle and associated structures. Figure 15.14a from Histology: A Text and Atlas (Ross & Pawlina, 5th ed., 2006). 4. Base of hair follicle is called the bulb (Figure 22). a. Base of the bulb is invaginated by vascularized loose connective tissue, the dermal papilla. b. Papilla is surrounded by matrix cells. c. Those matrix cells adjacent to papilla are the germinative layer of the follicle and are responsible Microanatomy of Integumentary System Dr. Lalit P. Singh Page 19 of 31 for hair growth. Layer also includes melanocytes that contribute melanosomes to developing hair cells. d. Dividing matrix cells in germinative layer differentiate into keratin- producing cells and the internal root sheath. e. In cases where epidermis Figure 22. LM section through a hair follicle. Adapted from Figure 11-14 in is lost by trauma or Histology and Cell Biology (Kierszenbaum, 1st ed., 2002). surgery, germinating matrix cells in deepest part of hair follicles will divide and migrate to reestablish the epidermis. 5. Layers of hair follicle (Figure 23) a. Internal root sheath: multilayered cellular covering that surrounds the deep part of the hair; it contains cells with soft keratin formed from Figure 23. LM cross-section of hair germinative layer of bulb follicles. Co: cortex, Cu: cuticle, IRS: internal root sheath, ERS: external root and is broken down at sheath, CTS: connective tissue sheath, BM: glassy basement membrane, AP: arrector pili about the level of the muscle. Plate 11-3 Figure 2 from Color Atlas of Histology (Gartner & Hiatt, 3rd ed., sebaceous gland. 2000). Microanatomy of Integumentary System Dr. Lalit P. Singh Page 20 of 31 b. External (outer) root sheath: down growth of the epidermis that extends the entire length of the follicle. c. Connective tissue sheath: collar of dense irregular connective tissue that surrounds follicle. 6. Arrector pili muscle attached to connective tissue sheath (Figure 21) a. Contraction responsible for “goosebumps”, upon thermal regulation or fright reactions. b. Its contraction aids sebaceous gland secretion (Figure 21, also in Figure 30). B. HAIR (Figure 21) 1. Elongated filamentous structures composed of keratinized cells that develop from the matrix of hair follicles. 2. Hair consists of two or three concentric zones of keratinized cells (Figures 23 and 24): a. Cuticle: outermost layer b. Cortex: middle layer in thick hair or inner layer in thin hair c. Medulla: inner most layer; present in thick hair only Microanatomy of Integumentary System Dr. Lalit P. Singh Page 21 of 31 3. Formation of hair (Figure 24) a. Cells divide in the germinative layer of the matrix in the bulb of the hair follicle. Cells from the apex of the papilla give rise to the medulla of the hair. Basal stem cells produce cells of the hair cortex and Figure 24. Formation of hair. Adapted and cuticle. modified from Figure 11-14 in Histology and Cell Biology (Kierszenbaum, 1st ed., 2002). b. Keratinization of hair occurs shortly after cells leave the matrix in the keratogenous zone. c. Cells contain melanin pigment from melanocytes resident in germinative layer. d. The hair cells contain hard keratin, while cells of the epidermis and internal root sheath contain soft keratin. 4. Hard keratin a. Consists of densely packed keratin filaments embedded in a matrix of keratin with high sulfur content, which is responsible for the hardness. b. Unlike soft keratin, the formation of hard keratin does not involve keratohyalin granules and it does not desquamate. Microanatomy of Integumentary System Dr. Lalit P. Singh Page 22 of 31 IV. GLANDS (Figure 21) A. ECCRINE SWEAT GLANDS (Figures 25 and 26) 1. Distributed over entire body surface except for lips and part of external genitalia. Found in both thin and thick skin. 2. Classification: simple coiled tubular gland (Figures 21, 25, and 26) a. Secretory unit: Coiled tubular secretory unit located deep in Figure 25. LM section of thick skin, dermis or in upper hypodermis. showing eccrine sweat glands (SG). Ep: epidermis, D: duct, BV: blood vessel, Epithelium is pseudostratified AT: adipose tissue. Plate 38 Figure 1 from Histology: A Text and Atlas (Ross columnar. & Pawlina, 5th ed., 2006). b. Duct portion: Less coiled duct segment with narrow lumen that leads to epidermal surface. Multiple profiles are often visible in section. Epithelium is stratified cuboidal. 3. Secretory cells produce secretion similar in composition to an ultrafiltrate of blood. Some sodium and water are resorbed in duct segment to produce a hypotonic Figure 26. LM section of eccrine sweat sweat that is released onto gland. Figure 15.16 from Histology: A Text epithelial surface. and Atlas (Ross & Pawlina, 5th ed., 2006). Microanatomy of Integumentary System Dr. Lalit P. Singh Page 23 of 31 4. Function: play major role in temperature regulation – sweat evaporates and cools skin. 5. Eccrine glands respond to heat (thermoregulatory sweating) and stress (emotional sweating). Controlled by sympathetic portion of autonomic nervous system. 6. Three cell types in secretory segment (Figure 27) a. Clear cells: secretory epithelial cells characterized by abundant glycogen (poorly stained in H&E). Produce the watery component of sweat. Product travels to lumen via intercellular canaliculi. b. Dark cells: secretory epithelial cells characterized by abundant rough ER and secretory granules. Produce proteinaceous component of sweat. They have direct plasma membrane exposure to lumen. Figure 27. EM of an eccrine sweat gland. C: clear cells, D: dark cells, My: myoepithelial cells, IC: intercellular canaliculus, L: lumen. Figure c. Myoepithelial cells: 15.17a from Histology: A Text and Atlas (Ross & Pawlina, 5th ed., 2006). contractile cells that lie between the secretory cells and the basal lamina. The cytoplasm contains actin filaments that stain deeply with eosin. Contraction of these cells results in rapid expulsion of sweat from gland. Microanatomy of Integumentary System Dr. Lalit P. Singh Page 24 of 31 B. APOCRINE SWEAT GLANDS (Figures 28 and 29) 1. Limited to axilla, areola and nipple of the mammary gland, skin around the anus, and the external genitalia. Apocrine gland development is dependent on sex hormones, Figure 28. LM of an apocrine sweat gland. HF: hair follicle, aSG: apocrine sweat gland, and they become functional at SG: eccrine sweat gland, G: granules, C: capillaries, arrows: myoepithelial cells. puberty. Plate 40 Figure 1 from Histology: A Text and Atlas (Ross & Pawlina, 5th ed., 2006). 2. Classification: simple coiled tubular gland. They may be branched. a. Secretory unit: Coiled tubular secretory unit with very wide lumen located deep in dermis or more often in upper hypodermis. Epithelium is simple cuboidal or columnar. Figure 29. LM section of apocrine sweat gland. Figure 11-18 in Histology and Cell b. Duct portion: Straight Biology (Kierszenbaum, 1st ed., 2002). duct segment with narrow lumen that leads to canal of hair follicle just above duct from sebaceous gland. From the hair follicle, the product reaches the skin surface. Duct profile is often not present in section of secretory unit. Epithelium is stratified cuboidal. Microanatomy of Integumentary System Dr. Lalit P. Singh Page 25 of 31 3. Associated with hair follicle – duct empties into it. They develop from same down growth of epidermis that gives rise to hair follicle. Like cells in hair follicle, these gland cells can also reestablish absent epidermis. 4. Secretory cells produce a protein-rich secretion containing pheromones. Unlike eccrine sweat gland, no resorption occurs in duct segment. Product is not altered in duct. 5. Function: Pheromones are chemical signals important in sexual, maternal, and social behaviors in animals. Apocrine secretions may function similarly in humans 6. Gland received name, since it was once thought that secretion was apocrine, i.e., a piece of cytoplasm was released with product. It is now known that product is released by merocrine secretion 7. Apocrine glands respond to emotional and sensory stimuli, but not heat. Controlled by sympathetic portion of autonomic nervous system 8. Two cell types in secretory segment (Figure 29) a. Secretory cells: secretory epithelial cells contain secretory granules at apical surfaces, discharged by exocytosis, and have numerous mitochondria. b. Myoepithelial cells: Contractile cells that lie between the secretory cells and the basal lamina. The cytoplasm contains actin filaments that stain deeply with eosin. Contraction of these cells results in rapid expulsion of product from the gland. Microanatomy of Integumentary System Dr. Lalit P. Singh Page 26 of 31 C. SEBACEOUS GLANDS (Figures 21 and 30) 1. Distributed in thin skin over entire body surface. 2. Classification: simple saccular or acinar gland a. Secretory unit: Small, undifferentiated basal cells are dividing cells and produce the secretory cells. The secretory cells move toward the opening of the duct and produce a fatty product called sebum. Figure 30. LM section of sebaceous gland (G) and hair follicle (F). M: arrector pili muscle. Figure 9.14a in Wheater’s Functional Histology (Young b. Duct portion: Short duct is & Heath, 4th ed., 2000). lined by stratified squamous epithelium continuous with the external root sheath. It opens to the hair follicle, through which sebum reaches the skin surface. 3. Associated with hair follicle, duct empties into it (Figure 30). They develop as outgrowth of external root sheath of hair follicle. Figure 31. Structure and cells of sebaceous gland. Modified from Figure 11-16 in Histology and Cell Biology 4. Secretory cells produce an oily product (Kierszenbaum, 1st ed., 2002). called sebum, which is released via a holocrine method. The cell becomes filled with sebum and undergoes apoptosis, releasing sebum and debris into the hair follicle. (Figure 31) Microanatomy of Integumentary System Dr. Lalit P. Singh Page 27 of 31 5. Two cell types in secretory segment (Figure 31) a. Secretory cells: As cells move from periphery toward opening of gland into hair follicle, they become filled with more and more lipid. They display a washed out cytoplasm in H&E. b. Basal cells: Small, undifferentiated cells located at basal surface of secretory unit. They are the source of the secretory cells. 6. No myoepithelial cells in sebaceous gland. The arrector pili muscle accompanies the gland, and its contraction assists in gland secretion (Figure 30). 7. Function: Role of sebum is not clearly defined. May be bacteriostatic, have a barrier function, or have a pheromone function. Sebum appears to play a role in development of acne, since its secretion increases at puberty, and it has been linked to formation of acne lesions. V. SENSORY STRUCTURES A. FREE NERVE ENDINGS (Figure 32) 1. Neuronal processes in the epidermis or dermis that lack a connective tissue or Schwann cell investment. 2. Most numerous neuronal receptors in epidermis, where they function in sensing fine touch, heat, cold, and other sensations. Microanatomy of Integumentary System Dr. Lalit P. Singh Page 28 of 31 Figure 32. Sensory structures in skin. Modified and adapted from Figure 11-13 in Histology and Cell Biology (Kierszenbaum, 1st ed., 2002). There are both free and encapsulated nerve endings in the skin. 3. In dermis they can surround hair follicles and attach to outer root sheath, where they are sensitive to hair movement and function as mechanoreceptors. B. ENCAPSULATED NERVE ENDINGS 1. Meissner’s corpuscle (Figure 33) a. Tapered cylindrical structures (~150 m size) found in dermal papillae of papillary layer of hairless skin, e.g., the lips and the palmar and volar surfaces, especially fingers and toes. Figure 33. Left: LM of Meissner’s corpuscles (MC). Right: higher mag of a single corpuscle. Plate 42 Figures 3 and 4 from Histology: A Text and Atlas (Ross & Pawlina, 5th ed., 2006). Microanatomy of Integumentary System Dr. Lalit P. Singh Page 29 of 31 b. Composed of one or two unmyelinated nerve endings following spiral paths within a cellular capsule. The capsule consists of flattened specialized Schwann cells that form several irregular lamellae through which the axons course to the pole. It has the appearance of a loose, twisted skein of wool or yarn. c. Function: Touch receptors particularly responsive to low-frequency stimuli. Most likely respond through displacement of capsule. 2. Pacinian corpuscle (Figure 34) a. Large ovoid structures (>1 mm long) found in deeper dermis and hypodermis, especially in fingertips. Also found in connective tissue in general, e.g., in association with joints and internal organs. Figure 34. LM of Pacinian corpuscle. Arrowhead: centrally located nerve fiber. Plate 42 Figure 2 from Histology: A Text and Atlas (Ross & Pawlina, 5th ed., 2006). Microanatomy of Integumentary System Dr. Lalit P. Singh Page 30 of 31 b. Composed of a myelinated nerve ending surrounded by a capsule. The nerve enters the capsule with myelin sheath intact for one or two nodes. The unmyelinated portion of the axon extends toward the opposite pole. It is wrapped by several concentric Schwann cell inner lamellae and multiple outer lamellae. It has the appearance of a toothpick (axon) stuck into a cocktail onion (capsule). c. Function: Deep pressure sensors for mechanical and vibratory pressure. Respond through displacement of the capsule lamellae, which effectively causes depolarization of the axon. Clinical Correlation: (some skin disorders)  Acne (pimples)  Melanoma, basal cell carcinoma (skin cancers)  Vitiligo (loss of pigmentation)  Scleroderma (autoimmune disease of the connective tissue leading to skin thickening)  Rosacea (thickening of the skin in the face covered with pimples and bumps, frequent in adults - ages 30-60 in fair-skinned families) (Figure 35 left)  Rosacea also occur in dark- and brown-skined people; however, initial diagnosis may be difficult due to skin pigmentation. The skin on the forehead, nose, cheeks and chin may appear rough, swollen, thickened and malformed (Figure 35, middle and right) Microannatomy of In ntegumenta ary System Dr. Lalitt P. Singh Page 31 of 31 Figure 35. Rosacea in n different skin n types. Referencces: Cui, D., Atlas A Correlation, 1st ed., Lippiincott, Williaams of Histtology with Functional & Clinical C & Wilkin ns, Philadelp phia, PA, 2011. Gartner and a Hiatt, Co olor Atlas off Histology, 3rd ed., Lipppincott Williiams & Wilkkins: Philadelp phia, 2000, Ch. C 13. Kierszennbaum, Histo A Introducction to Pathoology, 1st edd., Mosby: Stt. ology and Ceell Biology: An Louis, 20 002, Ch. 15. Ross, M.H. & Pawlin na, W., Histo xt and Atlas , 5th ed., Lipppincott Willliams & Willkins: ology: A Tex phia, 2005, Ch. Philadelp C 15. T and Atlaas, 6th ed., Lippincott, W Ross, M.H. and Pawllina, W., Hisstology: A Text Williams, & Wilkins: Philadelphiaa, 2011, Ch. 15. T and Atllas, 7th ed., L Ross, M.H. and Pawllina, W., Hiistology: A Text Lippincott, W Williams & Wilkins, WoltersKlu uwer Health: Philadelphia, 2016, Ch.. 15. Young annd Heath, Wheater’s W Fun Text and Collour Atlas, 4tth ed., Churcchill nctional Histtology: A T Livingsto one: Edinburrgh, 2000, Ch. C 13. LPS: 5/17 7/2023