Epidermis

The epidermis (or epithelium) is separated from the underlying dermis or connective tissue by heating at 60° for 30 sec, laid flat on a petri dish surface, and peeled away with fine forceps.

From: Methods in Enzymology , 2000

The Skin of the Neonate

Richard J. Martin MBBS, FRACP , in Fanaroff and Martin's Neonatal-Perinatal Medicine , 2020

Development of the Epidermis

Human skin has two distinct but interdependent components—the epidermis and the dermis ( Fig. 94.1). The epidermis has marked regional variations in thickness, color, permeability, and surface chemical components. It consists of a highly ordered, compact layering of keratinocytes and melanocytes. Traditionally, the epidermis is segmented into distinct structural and functional compartments called the stratum germinativum (basale), stratum spinosum, stratum granulosum, and stratum corneum (seeFig. 94.1). Intermixed is a third distinct cell type, the Langerhans cell, which is derived from bone marrow precursors and migrates into the primitive epidermis.

The process of cutaneous morphogenesis can be divided into embryonic and fetal periods (Fig. 94.2). 30 This transition, which occurs at approximately 2 months, is an important time in skin development, because many critical morphogenetic events occur during this transitional period. Between 30 and 40 days of development, the embryonic skin consists of a two-layered epidermis: the basal layer, associated with the basal lamina, and the periderm, which serves as a cover and a presumptive nutritional interface with the amniotic fluid. The basal layer includes cells that give rise to the future definitive epidermis, whereas the periderm is a transient layer that covers the embryo and fetus until the epidermis keratinizes at the end of the second trimester. Basal cells join with each other and peridermal cells by a relatively few desmosomes but do not yet form hemidesmosomes with the basement membrane. Small numbers of keratin intermediate filaments are associated with these junctions. Matrix adhesion of the embryonic epidermis is likely mediated by actin-associated α6β4 integrin. 30

Langerhans cells and melanocytes migrate into the embryonic epidermis and are identifiable at 40 and 50 days of gestation, respectively. At this stage, the Langerhans cells do not express CD1 antigen on their cell surfaces, nor are Langerhans cell granules identifiable. Likewise, at this time, the melanocyte lacks the characteristic cytoplasmic organelle, the melanosome. A third immigrant cell, the Merkel cell, does not appear to be present in embryonic epidermis and may differentiate at a later stage from keratinocytes in situ.

At the time of embryonic–fetal transition (60 days' gestation), the epidermis begins to stratify, forming an intermediate layer of cells. These young keratinocytes still contain a high volume of glycogen in their cytoplasm and produce large amounts of intermediate filaments in association with the desmosomes. At this time, new keratins are identifiable as markers of differentiation, as is the pemphigus antigen, which is detectable on the cell surface. These cells, unlike adult spinous cells, remain proliferative and continue to express epidermal growth factor receptors on their surfaces. Two to three additional layers of intermediate cells are added during the second trimester. These cells show a progressive increase in the number of keratin filaments but do not further differentiate until the onset of keratinization in the interappendageal epidermis around 22-24 weeks' gestation.

Skin penetration of nanoparticles

Shohreh Nafisi , Howard I. Maibach , in Emerging Nanotechnologies in Immunology, 2018

3.2.1.1 Epidermis

Epidermis is a superficial layer of stratified epithelium which develops from ectoderm and acts as a physical and chemical barrier between the interior body and exterior environment. The multilayered structure which forms the dermoepidermal junction is called basement membrane. The epidermis is stratified squamous epithelium without any blood vessels. It is entirely nourished by the underlying dermis and wastes disposal via diffusion through the dermoepidermal junctions and skin surface. The epidermis thickness varies from 0.05  mm on the eyelids to 0.8±1.5   mm on the soles and palms. Epidermis largely consists of keratinocytes which are in a constant state of transition from the deeper layers to the superficial. Protein keratins (desmosomes) produced by keratinocytes serve as a bridge and are able to connect the keratinocytes. The four layers of the epidermis are formed by different stages of keratin maturation, moving from the lower layers upward to the surface. The four layers of the epidermis include:

–

Stratum basale (SB) (basal or germinativum cell layer)

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Stratum spinosum (SS) (spinous or prickle cell layer)

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Stratum granulosum (SG) (granular cell layer)

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Stratum corneum (SC) (horny layer) (Fig 3.2).

Figure 3.2. Four layers of the epidermis: Stratum basale (SB), Stratum spinosum (SS), Stratum granulosum (SG), Stratum corneum (SC).

There exists a thin layer of translucent cells in thick epidermis called "stratum lucidum." It represents a transition from the SG and SC and is not usually seen in thin epidermis. Together, the SS and SG are sometimes known as the Malphigian Layer.

SB: The SB, the innermost layer of epidermis located adjacent to the dermis, consists of mainly dividing and nondividing keratinocytes attached to the basement membrane by protein keratin of hemidesmosomes. As keratinocytes divide and differentiate, they move from SB to the surface. Melanocytes, melanin (pigment) producing cells make up a small proportion of the basal cell population and lie between relatively larger numbers of keratinocytes. The produced melanin accumulates in melanosomes and transfers to the adjacent keratinocytes where they remain as granules. Melanin pigment protects skin against ultraviolet (UV) radiation; long-term exposure to UV light increases the ratio of melanocytes to keratinocytes. Merkel cells also occur in the basal layer of epidermis. They can be found with cutaneous nerves and seem to be involved in light touch sensation. A greater number of Mercel cells are in sensitive sites such as the fingertips and lips.

SS: The SS (spinous layer/prickle cell layer) is a layer of the epidermis located between the SB and SG. By reproducing and maturing, basal cells, they move toward the outer layer of skin, initially forming the SS. The cells are connected by intercellular bridges called desmosomes. Langerhans cells, immunologically active cells derived from the bone marrow can be found on all layers of the epidermis, but mainly in the middle of this layer. They are also present in the papillary dermis, especially around blood vessels. The Langerhans cells act as antigen-presenting cells and play a major role in immune reactions of the skin [30].

SG: The SG (or granular layer) is a thin layer of cells located in the epidermis. By continuing the transition of the keratinocytes from the underlying SS to the surface, they lose their nuclei and cytoplasm and appear as granular at this level. These cells possess keratohyalin granules containing histidine—and cystine-rich proteins and are able to bind the keratin filaments together [31,32].

SC: The SC, outermost layer of the epidermis also known as corneocytes, is the final outcome of keratinocyte maturation. It consists of flattened dead cells with no nuclei and cell organelle. The extracellular space of the cells is surrounded by lipidic bilayers. There are around 10±30 layers of stacked corneocytes in the most areas of the skin, with the palms and soles having the most. Each corneocyte is surrounded by a protein envelope and contains water-retaining keratin proteins. The orientation of the keratin proteins and their cellular shapes give strength to the SC. SC mainly contains three lipid components: Ceramides, cholesterol, and fatty acids serve as a natural physiological and water-preserved environment for the skin.

Junctions of dermoepidermal are the place where nutrients feed the skin cells and metabolic waste disposer. This irregular complex structure is made up of two layers. Some rare skin disorders such as bullous pemphigoid and epidermolysis bullosa take place in this structure. This junction is also responsible for expression of aging signs since during the aging its structure is changed and become flattened [28].

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Structure of the Skin and Cutaneous Immunology

A. Wesley Burks MD , in Middleton's Allergy: Principles and Practice , 2020

Cells of the Epidermis

The epidermis is composed primarily of cells. It is approximately 150 µm thick and is schematically represented in Fig. 32.2. Projections of the epidermis into the underlying papillary dermis are referred to asrete ridges. The most abundant epidermal cell type is the keratinocyte (approximately 90% of cells). Keratinocytes are continually renewing cells that are roughly divided into four types: basal (stratum germinativum), spinous (stratum spinosum), granular (stratum granulosum), and cornified (stratum corneum) keratinocytes. Keratinocytes in these epidermal layers characteristically express pairs of various acidic (type 1) and basic-neutral (type 2) keratin proteins, for which an updated consensus nomenclature has been proposed. 1,2 Elements that control keratinocyte proliferation, differentiation, and apoptosis-like cornification include growth factors, cytokines, neuropeptides, adrenergic and cholinergic signaling, calcium, and cell-cell and cell-matrix interactions. Key roles for TP63 (p63) and caspase 14 in epidermal development, proliferation, and differentiation have been identified. 3–5 Some of the critical signals regulating keratinocyte development also originate from the dermis. 6,7 Through their physical and immunologic barrier functions, keratinocytes are key participants in host defense against environmental exposures.

Nonmigrating basal keratinocytes rest on the basement membrane, a structure that divides the epidermis from the dermis. They are tethered to the basement membrane by protein structures calledhemidesmosomes, which are described inFig. 32.3. 8,9 Basal keratinocytes form a single layer of columnar-shaped cells expressing keratins K5 (basic-neutral) and K14 (acidic), and three recognized subtypes are distinguished by mitotic activity. One subtype includes the epidermal stem cells that constitute approximately 5% to 10% of basal keratinocytes, and these multipotent stem cells appear to reside in the bulge region of hair follicles. 10,11 Epidermal stem cells retain radiolabeled thymidine because of infrequent cell division, and they appear to express high levels of β1 integrin. 10,12 Transient amplifying cells are a second subtype and constitute most of the basal keratinocytes. These cells divide to produce the third subtype, postmitotic keratinocytes, which progressively differentiate and migrate toward the surface of the skin. 13 During migration, postmitotic keratinocytes undergo terminal differentiation and gradually flatten, lose their cellular organelles, dehydrate, and modify their protein and lipid composition toward a more impermeable, cornified composition. 14 The postmitotic basal keratinocyte requires 14 days to migrate to the stratum corneum and another 14 days before desquamation from the surface of healthy skin.

Burns and Skin Ulcers

Holger Schlüter , ... Pritinder Kaur , in Essentials of Stem Cell Biology (Third Edition), 2014

34.1 Introduction

The epidermis of the skin is a constantly renewing stratified squamous epithelium. It consists mostly of keratinocytes, but also of Langerhans cells, melanocytes, and Merkel cells resting on a supporting dermis that contains the nerve and vascular networks, which nourish the epidermis. The dermis is also the location of epidermal appendages, fibroblasts, mast cells, macrophages, and lymphocytes. Epidermal stem cells are responsible for the ability of the epidermis to replace itself, both in normal circumstances and in traumatic skin loss, such as from burns and skin ulceration.

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Tumors of the epidermis

James W. Patterson MD, FACP, FAAD , in Weedon's Skin Pathology , 2021

Histopathology 614

Diagnostic biopsy is undertaken in only a small percentage of actinic keratoses diagnosed clinically. 545 The clinical accuracy in the recognition of actinic keratoses varies from 74% to 94%. 615 The usual actinic keratosis is characterized by focal parakeratosis, with loss of the underlying granular layer and a slightly thickened epidermis with some irregular downward buds ( Fig. 32.18 ). Uncommonly, the epidermis is thinner than normal. In all cases, there is variable loss of the normal orderly stratified arrangement of the epidermis; this is associated with cytological atypia of keratinocytes, which varies from slight to extreme. The termbowenoid keratosis may be used when the atypia is close to full thickness. 578 This variant differs markedly from thede novo form of Bowen's disease (see later). Sometimes the dysplastic epithelium shows suprabasal cleft formation (see later). 539,616,617 There is often a sharp slanting border between the normal epidermis of the acrotrichia and acrosyringia and the parakeratotic atypical epithelium of the keratosis. 618 However, dysplastic epithelium may involve the infundibular portion of the hair follicle. 618–620 The parakeratotic scale may sometimes pile up to form a cutaneous horn. 539 Large keratohyaline granules are sometimes present in actinic keratoses. 621

The dermal changes include actinic elastosis, which is usually quite severe, and a variable, but usually mild, chronic inflammatory cell infiltrate. 539 As mentioned previously, the grade of solar elastosis is a marker of epithelial UV damage. 599 Inflammatory keratoses may develop during chemotherapy of malignant disease with fluorouracil and its analogs. 622–625 There is vascular telangiectasia and a moderately heavy mixed inflammatory cell infiltrate in the upper dermis. Inflammation of actinic keratoses has also been reported after therapy with sorafenib, a multitargeted tyrosine kinase inhibitor. 626 Some actinic keratoses progress to squamous cell carcinoma with this therapy. 626 An inflammatory response is also present in actinic keratoses before they progress to squamous cell carcinomas, unrelated to any therapies. 627 The inflammation subsides rapidly after this conversion. 627

Skin Structure and Function

Golara Honari , Howard Maibach , in Applied Dermatotoxicology, 2014

Epidermis

Epidermis is the outermost layer and is about 0.05–1  mm in thickness depending on body part. Three main populations of cells reside in the epidermis: keratinocytes, melanocytes, and Langerhans cells. Keratinocytes are the predominant cells in the epidermis, which are constantly generated in the basal lamina and go through maturation, differentiation, and migration to the surface. As keratinocytes differentiate, they form three layers above the basal layer known as stratum spinosum (SP), stratum granulosum (SG), and stratum corneum (SC) (Figure 1.1). Keratinocyte transit time from basal layer up to SC is about 14 days 1 and turn over time within SC is also around 14 days, 2 certain inflammatory conditions can affect these turn over times.

Figure 1.1. Schematic of epidermis—basal cell layer is the deepest layer of epidermis differentiating to spinous cells then to granular cells and eventually terminally differentiate to SC.

SC is the outer layer of the epidermis and serves as the main functional barrier. A theoretical model is "brick and mortar" like structure where bricks represent terminally differentiated nonviable keratinocytes, also known as corneocytes embedded in intercellular lipid membranes. 3 As corneodesmosomes (protein bridges between corneocytes) degrade, lacunar spaces are created within the SC referred to as aqueous "pore" pathway. These spaces can extend and form continues networks, creating a pathway for penetration across the SC. 4

Major components of the SC lipid membranes are free fatty acids, ceramides, and esterols. 5 Melanocytes are neural crest-derived, pigment synthesizing dendritic cells that reside primarily in the basal layer. Merkel cells are mechanosensory receptors also present in basal layer. Langerhans cells are dendritic antigen-processing and antigen-presenting cells in the epidermis. 6 They form 2–8% of the total epidermal cell population, mostly found in a suprabasal position. The dermal–epidermal junction (DEJ) is a basement membrane zone that forms the interface between the epidermis and dermis. The major functions of the DEJ are to attach the epidermis and dermis to each other and to provide resistance against external shearing forces.

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Common integument

M. Navarro , ... A. Carretero , in Morphological Mouse Phenotyping, 2017

Epidermis

The epidermis is a cornified stratified squamous epithelium. It consists mainly of keratinocytes which multiply in the basal layer of the epithelium, and then leave this layer toward the outer surface. Along the way they undergo the process of keratinization before its dead. In the mouse the average life of a keratinocyte is six days. During the first days of life, the mouse epidermis ( Fig. 16-6 ) under-goes a thinning process, before resulting in the mature epidermis around 8-10 days after birth.

Figure 16-1. Skin of adult mouse. Abdominal region. A) Histological section. Hematoxylin-eosin stain (100X). B) Magnification of epidermis. Semithin section. Methylene blue stain (1,000X). C) Stratum corneum and stratum basale. Transmission electron microscopy image (8,000X). D) Magnification of the epidermal-dermal junction. Transmission electron microscopy image (40,000X).

1: Epidermis; 2: Stratum papillare (dermis); 3: Stratum reticulare (dermis); 4: Hair follicle; 5: Sebaceous gland; 6: M. arrector pili; 7: Stratum corneum; 8: Stratum basale (germinativum); 9: Nucleus (basal cell); 10: Basement membrane; 11: Hemidesmosomes; 12: Lamina lucida; 13: Lamina densa; 14: Collagen fibers (stratum papillare of dermis).

Figure 16-2. Cells of dermis. A) Fibroblast (12,000X). B) Macrophage (10,000X). C) Eosinophil (15,000X). D) Lymphocyte (25,000X). Transmission electron microscopy images.

1: Nucleus; 2: Mitochondria; 3: Rough endoplasmic reticulum; 4: Collagen fibers; 5: Pseudopodia; 6: Filopodia; 7: Vacuoles; 8: Lysosome; 9: Phagolysosomes; 10: Primary lysosomes with crystalloid.

Figure 16-3. Vascularization and innervation of skin. A) Deep vascular plexus. Vascular injection with Indian ink and clearing. B) Confocal laser microscopy image. Blood vessels injected with FITC dextran (green) and staining with phalloidin (red). Nuclei counterstained with DAPI (blue) (630X). C) Capillary of skin. Transmission electron microscopy image (8,000X). D) Nerve of skin. Transmission electron microscopy image (4,000X).

1: Arteriole (deep plexus of dermis); 2: Venule (deep venous plexus of dermis); 3: Capillaries of dermis; 4: Muscle fibers (m. cutaneus); 5: Collagen fibers; 6: Erythrocyte; 7: Endothelial cell; 8: Pericyte; 9: Fibroblast; 10: Myelinated axon; 11: Unmyelinated axon; 12: Schwann cell; 13: Endoneurium; 14: Perineurium.

Figure 16-4. Three characteristic mouse coat colors. Differential interference contrast microscope images of hairs. A) Agouti (C57BL/6). B) White (ICR). C) Black (B6.SJL). Note the absence of melanin granules in white hair and their distinct distribution in agouti and black hairs.

1: Hair medulla; 2: Hair cortex; 3: Melanin granules.

Figure 16-5. Coat hair types. Macroscopic aspect and scanning electron microscopy images (Bar   =   30   μm). A) Type «guard». B) Type «awl». C) Type «auchene». D) Type «zigzag».

1: Cuticular epitheliocyte.

Figure 16-6. Hair follicles in growing anagen stage. Skin of abdomen of one week old mouse. Histological sections. A) Hematoxylin-eosin stain (100X). B) Mallory's stain (100X). C) Transverse section of hair follicle. Hematoxylin-eosin (200X).

1: Stratum corneum (epidermis); 2: Stratum basale (epidermis); 3: Dermis; 4: Subcutaneus adipose tissue; 5: M. cutaneus; 6: Hair follicle; 7: Hair bulb; 8: Hair (dermal) papilla; 9: Hair medulla; 10: Hair cortex; 11: Hair cuticle; 12: Inner root sheath; 13: Cuticle of inner root sheath; 14: Granular epithelial (Huxley's) layer; 15: Pale epithelial (Henle's) layer; 16: Outer root sheath; 17: Glassy membrane; 18: Capillary.

The mature epidermis consists of four cell layers based on their degree of differentiation. The stratum basale or germinativum is the innermost layer and is composed of immature cuboidal cells capable of mitotic division. The cells of the stratum basale essentially differentiate into keratinocytes, but they can also give rise to melanocytes, Langerhan's cells (phagocytic cells) and Merkel cells (neuroendocrine cells). Located superficially to the stratum basale is the stratum spinosum, which is usually poorly defined in the mouse, and the stratum granulosum, which is composed of several layers of fusiform cells containing keratohyalin granules. Finally, the outermost layer is the stratum corneum which is composed of several layers of flattened anuclear cells that are gradually desquamated. The epidermis in haired areas is very thin and consists only of a stratum basale and a narrow layer of stratum corneum ( Fig. 16-1 ). In these regions, spinosum and granulosum strata are indistinguishable and can only be observed in thicker areas of the skin, such as in the foot pads ( Fig. 16-20 ). The epidermis of the external ear is similar to the haired trunk epidermis but slightly thicker, whereas that of the tail is significantly thicker. The epidermis is supported by a basement membrane, which functions regulate the metabolic transfer between dermis and epidermis.

Figure 16-7. Sebaceous gland. A) Histological section of an adult mouse hair follicle. Hematoxylin-eosin stain (100X). B) Semithin section. Methylene blue stain (1,000X). C) Transmission electron microscopy image of a sebaceous cell (5,000X).

1: Sebaceous gland; 2: Nucleus (sebaceous cell); 3: Glandular duct; 4: Epithelial (basal) cell; 5: Lipid droplets; 6: Hair follicle.

Figure 16-8. Specialized hair types. A) Facial hairs. B) Carpal hairs. C) Tail hairs.

1: Tactile hairs of upper lip; 2: Tactile hairs of lower lip; 3: Supraorbital tactile hairs; 4: Zygomatic tactile hair; 5: Buccal tactile hairs; 6: Mental tactile hairs; 7: Tragi (coat hairs); 8: Carpal tactile hairs.

Figure 16-9. Structure of hairs types. Differential interference contrast microscope images. A) Eyelash. B) Tactile carpal hair. C) Coat hair «guard». D) Tactile hair of upper lip.

1: Hair medulla; 2: Hair cortex; 3: Hair cuticle; 4: Medullar air space.

Figure 16-10. Tactile hairs of upper lip (mystacial vibrissae). A) Organization in five rows A to E. B and C) Scanning electron microscopy images.

1: Tactile hair; 2: Follicular orifice; 3: Coat hairs.

Figure 16-11. Longitudinal histological section of a tactile hair of upper lip (mystacial vibrissal follicle-sinus complex). Hematoxylin-eosin stain (40X).

1: Follicular orifice; 2: Hair shaft; 3: Inner root sheath; 4: Outer root sheath; 5: Glassy membrane; 6: Hair matrix; 7: Hair (dermal) papilla; 8: Ring sinus; 9: Ring body; 10: Cavernous sinus; 11: Fibrous capsule; 12: Outer conical body; 13: Inner conical body; 14: Follicular intrinsic muscle; 15: Deep vibrissal nerve; 16: Nerve ending; 17: Ringwulst.

Figure 16-12. Interscapular adipose tissue. A) Dorsal view of a male mouse interscapular region. B) The adipose tissue was removed, showing its ventral aspect. C) Ultrasound image. Transverse section. D) Magnetic resonance image. Transverse section. E) Transverse histological section of a pregnant female mouse. Hematoxylin-eosin stain (40X).

1: White adipose tissue; 2: Brown adipose tissue; 3: Skeletal muscle (m. trapezius). 4: Lactiferous ducts; 5: Skin; 6: Scapula.

Figure 16-13. Structure of white adipose tissue. A and B) Hematoxylin-eosin stain (100X and 1,000X, respectively). C) Transmission electron microscopy image (6,000X). D) Confocal laser microscopy image. Staining with BODIPY (493/503) (green) and immunodetection of collagen IV (red). Nuclei counterstained with TO-PRO 3 (blue) (630X). E) Isolated white adipocyte. Confocal laser microscopy image. Staining with BODIPY (493/503) (yelow). Nuclei counterstainted with TO-PRO 3 (blue).

1: Lipid droplet; 2: Nucleus of adipocyte; 3: Cytoplasm of adipocyte; 4: Capillaries with erythrocytes; 5: Basement membrane.

Figure 16-14. Structure of brown adipose tissue (interscapular region). A and B) Hematoxylin-eosin stain (100X and 1,000X, respectively). C) Transmission electron microscopy image (6,000X). D) Confocal laser microscopy image. Staining with BODIPY (493/503) (green). Nuclei countestained with TO-PRO 3 (blue) (630X). E) Sudan IV and hematoxylin stain (1,000X).

1: Lipid droplet; 2: Nucleus of adipocyte; 3: Cytoplasm of adipocyte; 4: Mitochondrium; 5: Capillary with erythrocytes.

Figure 16-15. Nipples of mammae. A) Ventral view of a female at the end of gestation. Nipples surrounded by black circles. B) Abdominal nipple. C) Scanning electron microscopy image of a nipple in a lactating female.

1: Nipples of cervical mammae; 2: Nipples of cranial thoracic mammae; 3: Nipples of caudal thoracic mammae; 4: Nipples of abdominal mammae; 5: Nipples of inguinal mammae; 6: Papillary orifice; 7: Nipple hair; 8: Areola.

Figure 16-16. Structure of a nipple. A) Sagittal histological section of a nipple in a non-pregnant female mouse. Hematoxylin-eosin stain (100X). B) Sagittal histological section of a nipple at the end of gestation. Hematoxylin-eosin stain (100X). C) Cleared mamma at the end of gestation. Carmin stain. D) Transverse histological section of a nipple at the end of gestation. Hematoxylin-eosin stain (100X).

1: Papillary duct; 2: Papillary part (lactiferous sinus); 3: Glandular part (lactiferous sinus); 4: Lactiferous duct; 5: Mammary gland; 6: Lobules of mammary gland; 7: Papillary venous plexus.

Figure 16-17. Gross anatomy and topography of mammae. A, B and C) Dorsal and ventral views, respectively. D) Isolated mammae of a female at the end of gestation. Nipples surrounded by white circles.

1: Cervical mammae; 2: Cranial thoracic mammae; 3: Caudal thoracic mammae; 4: Abdominal mammae; 5: Inguinal mammae; 6: Interscapular adipose tissue; 7: Mm. cervicoauriculares; 8: M. longuissimus thoracis; 9: Mandibular gland; 10: Mm. pectorales; 11: Mm. abdominis.

Figure 16-18. Mammary structure and evolution. A) Non-pregnant mammary gland. Hematoxylin-eosin stain (100X). B) Mammary gland at 7   days of gestation. Hematoxylin-eosin stain (100X). C) Mammary gland at the end of gestation. Hematoxylin-eosin stain (100X). D) Lactiferous duct (7   days of gestation). Confocal laser microscopy image. Immunodetection of collagen IV (green). Nuclei counterstained with TO-PRO 3 (blue) (630X). E) Cleared mammary gland at the end of gestation. Carmin stain.

1: Epidermis; 2: Dermis; 3: Sebaceous gland; 4: Mammary arteriole; 5: Mammary venule; 6: Deep venous plexus of dermis; 7: Capillary; 8: M. cutaneus; 9: Lactiferous duct; 10: Lobules of mammary gland; 11: Mammary alveoli; 12: Adipose tissue; 13: Epithelial cells; 14: Myoepithelial cells.

Figure 16-19. Foot pads. A) Left forepaw. Palmar view. B) Left hindpaw. Plantar view. C and D) Scanning electron microscopy images. The roman numerals indicate the number of the digits. First digit rudimentary in forepaw.

1: Digital pads; 2: Metacarpal pads; 3: Carpal pads; 4: Metatarsal pads; 5: Secondary metatarsal pad; 6: Tactile carpal hairs.

Figure 16-20. Structure of foot pads. A) Sagittal histological section (fourth digit of forepaw). Hematoxylin-eosin stain (10X). B) Hematoxylin-eosin stain (100X). C) Masson's trichrome stain (200X).

1: Unguicula (claw); 2: Middle phalanx; 3: Proximal phalanx; 4: Metacarpal bone; 5: Proximal sesamoid bone; 6: Fourth and fifth carpal bones; 7: Ulnar carpal bone; 8: Accessory carpal bone; 9: Digital pad; 10: Metacarpal pad; 11: Carpal pad; 12: Stratum corneum; 13: Stratum granulosum; 14: Stratum spinosoum; 15: Stratum basale (germinativum); 16: Dermal papilla (stratum papillare); 17: Stratum reticulare (deep dermal layer); 18: Eccrine (merocrine) sweat gland; 19: Excretory duct; 20: Capillaries.

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Embryonic Versus Adult Stem Cells

Michaela Prochazkova , ... Ophir D. Klein , in Stem Cell Biology and Tissue Engineering in Dental Sciences, 2015

18.3.2.5.1 Epidermal Stem Cells

The epidermis consists of the interfollicular epidermis (IFE), which forms the functional barrier of the skin, and the appendages of the epidermis, which include the hair follicle, sebaceous glands, and the sweat gland [ 71]. The epidermis begins at the most basal layer of the dermis, termed the stratum basale (Figure 18.4, left panel). This layer contains epidermal stem cells that give rise to the rest of the epidermis, which differentiate as they move upwards away from the dermis. The next layer, the stratum spinosum, is further differentiated into keratinocytes. The stratum granulosum contains the last layer of living cells, and these cells contain protein-filled granules that promote keratin crosslinking. The stratum lucidum are layers of dead cells that exocytosed lamellar bodies, which are rich in lipids, and aid in the skin's barrier function. Finally, the stratum corneum consists of a layer of dead cells with no nuclei or organelles, which serve as the skin's barrier to any chemical or mechanical stress. The continual turnover of the epidermis is mediated by epidermal proliferative units, which consist of a stem cell in the stratum basale and several transit amplifying cells. Clonal analysis revealed continuous expansion of a small population of labeled cells within the interfollicular epidermis in a random fashion [72,73]. This is in contrast to the previous long-held belief that IFE stem cells are scattered uniformly throughout the epidermis and are responsible for discrete units of the epidermis. During wound healing, several different populations of the bulge region of the hair follicle can contribute to reestablishment of the epidermis (see Section 18.3.2.5.2 below for details) [74].

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Cutaneous and Transdermal Drug Delivery

Dinesh K. Mishra , ... Rakesh K. Tekade , in Basic Fundamentals of Drug Delivery, 2019

15.3.1.6 Skin Shedding

The epidermis is in a continuous state of regeneration, which undergoes various transformations like development of a new cell layer of keratinocytes at the stratum basale, formation of desiccated, proteinaceous corneocytes and eventually desquamation. This leads to an alteration in the structure of epidermal cells, which changes from stratum basale, through the stratum spinosum, stratum granulosum, and stratum lucidum to the outermost stratum corneum. These situations make epidermis as a greatest barrier in the transport of most of the molecules across it. Hence there is a need of development of various techniques and transdermal systems, which could gain permeability across the full epidermis and not focus only on stratum corneum.

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Development

J.O. Vigoreaux , D.M. Swank , in Comprehensive Molecular Insect Science, 2005

2.2.2.3.1 Role of the epidermis

The epidermis plays an important role in myoblast migration to muscle development sites. VijayRaghavan et al. (1996) followed the paths of imaginal myoblast migration from wing and leg discs. Wing or leg discs were transplanted onto the thorax or abdomen of pupal hosts. In these animals, the myoblasts from both discs migrated over the host epidermis when transplanted onto pupal thorax, but no migration occurred when the transplantation site was on the abdomen. The resulting flies had ectopic wings or legs. The migration path cues from the epidermis may be due to expression of Hox genes (Roy and VijayRaghavan, 1997). Epidermal cells also contribute to the formation of muscle attachment sites (see Section 2.2.3.3.2).

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