5skin gross anatomy and embryology .pptx

The skin is the largest organ in the body, covering its entire external surface. The skin has 3 layers—the epidermis, dermis, and hypodermis, which have different anatomical structures and functions (see Image. Cross Section, Layers of the Skin). The skin's structure comprises an intricate network that serves as the body's initial barrier against pathogens, ultraviolet (UV) light, chemicals, and mechanical injury. This organ also regulates temperature and the amount of water released into the environment.

Skin thickness varies by body region and is influenced by the thickness of the epidermal and dermal layers. Hairless skin in the palms of the hands and soles of the feet is the thickest due to the presence of the stratum lucidum, an extra layer in the epidermis. Regions lacking this extra layer are considered thin skin. Of these regions, the back has the thickest skin because it has a thick epidermis.[1][2][3] The skin's barrier function makes it susceptible to various inflammatory and infectious conditions. In addition, wound healing, sensory changes, and cosmesis are significant surgical concerns. Understanding the skin's anatomy and function is crucial for managing conditions across all medical fields.

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Structure and Function
Epidermis

The epidermis, the skin's outermost layer, is composed of several strata and various cell types crucial for its function.

Layers of the epidermis: From the deepest to the most superficial, the epidermal layers are the stratum basale, stratum spinosum, stratum granulosum, stratum lucidum, and stratum corneum. The stratum basale, also known as stratum germinativum, is separated from the dermis by the basement membrane (basal lamina) and attached to it by hemidesmosomes. The cells in this layer are cuboidal to columnar, mitotically active stem cells that constantly produce keratinocytes. This layer also contains melanocytes. The stratum spinosum, comprising 8 to 10 cell layers, is also called the prickle cell layer. This layer contains irregular, polyhedral cells with cytoplasmic processes, sometimes called spines, that extend outward and contact neighboring cells by desmosomes. Dendritic cells can be found in this layer.[4][5]

The stratum granulosum has 3 to 5 cell layers and contains diamond-shaped cells with keratohyalin and lamellar granules. Keratohyalin granules contain keratin precursors that aggregate, cross-link, and form bundles. The lamellar granules contain the glycolipids secreted to the cell surfaces, functioning as an adhesive to maintain cellular cohesion. The stratum lucidum comprises 2 to 3 cell layers and is present in thicker skin on the palms and soles. This thin and clear layer consists of eleidin, a transformation product of keratohyalin. The stratum corneum has 20 to 30 cell layers and occupies the uppermost epidermal layer. The stratum corneum is composed of keratin and dead keratinocytes (anucleate squamous cells) that form horny scales. This layer has the most variable thickne

1. College of health sciences Unit of Biomedical sciences Department of human anatomy For 2 rd year medical students Anatomy of the integumentary System Set by : Atalo a. ( Msc . in Human Anatomy ) Atalo.A

2. Gross anatomy, Histology & embryology of integumentary system

3. Name the tissue types that compose the epidermis and the dermis, describe the primary functions of skin. Mention the layers of the epidermis and the dermis & describe the structure and functions of each layer. Describe the structure and function of the hypodermis. Describe the factors that contribute to skin color. 3 Objectives : at the end of the lesson the students will able to:-

4. Histology of The Skin 4

5. Skin Is also known as cutaneous membrane (integument) The largest organ of the body in both surface area and weight. In adults, it: Covers an area of about 2 square meters Weighs ~ 13 kg ; 15% of the total body weight. The thickness ranges from 1.5 to 5.0 mm and depends mainly on its location However, over most of the body it is 1–2 mm thick . 5

6. It covers the entire external surface of the body, including the EAM, the lateral aspect of the tympanic membrane and the vestibule of the nose. >> 6

7. Function of skin: As the primary interface between ourselves and our environment, the skin serves several distinct functions. protection Against microbial organisms, and Against mechanical, chemical, osmotic, thermal and UVR damage. It is an important site of immune surveillance against the entry of pathogens and the initiation of primary immune responses. Skin is also a permeability barrier against excessive loss or uptake of water, 7

8. Sensation: Thermoregulation : via skin’s insulating components ( eg , the fatty layer and hair on the head) and (sweat production and a dense superficial microvasculature) Metabolic : synthesize vitamin D3 Excretion Sexual signaling : sex pheromones The microscopic anatomy of skin reflects this functional complexity , with each functional specialization implemented by particular features of cell and tissue 8

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10. The dermal-epidermal interdigitations are of the peg-and socket variety in most skin, but they occur as well-formed ridges and grooves in the thick skin of the palms and soles, which is more subject to friction. These ridges and the intervening sulci form distinctive patterns unique for each individual, appearing as combinations of loops, arches, and whorls, called dermatoglyphs , also known as fingerprints and footprints. Skin is elastic and can expand rapidly to cover swollen areas Skin is self-renewing throughout life. In healthy individuals injured skin is repaired rapidly 10 Function of skin…

11. 11 Integument Epidermis, Dermis Subcutaneous tissue

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13. Skin Consists of two main parts: Epidermis Superficial, thinner portion Composed of epithelial tissue It is avascular Its derivatives include hairs, nails, and sebaceous sweat glands & mammillary glands . 13

14. Skin Dermis Deeper, thicker portion it is Connective tissue Vascular and a cut in it produces bleeding. 14

15. Skin Subcutaneous/hypodermis Deep to the dermis, but not part of the skin It consists of areolar and adipose tissues. 15

16. Types of skin The thickness of each layer of the skin varies depending on body region and categorized based on the thickness of the epidermal layers.  Thin (hairy) skin Has four layers of epidermis, they do not have stratum lucidum Majority of the skin of our body is thin Dermal papillae are few in number, small and scattered 16

17. Types of skin Thick (hairless) skin The greatest contributor to thickness is increased number of layers in stratum corneum—due to greater mechanical stress Has all the five layers of the epidermis Dermal papillae are numerous, large in size and arranged in pattern Examples of areas include sole of foot, palm of hand tips of fingers and toes. Lacks hair, arrector pilli muscle and sebaceous gland 17

18. Epidermis forms the major distinction between thick & thin skin. It is classified as keratinized stratified squamous epithelium The epidermis is constantly being regenerated, Regeneration , which occurs approximately every 15-30 days, depending on age, the region of the body, and other factors. It contains four principal types of cells: Keratinocytes Melanocytes Langerhans cells Merkel cells 18

19. keratinocytes Comprises about 90 percent of epidermal cells They are arranged in four or five layers Produce protein keratin and lamellar granules Th e cytoplasm of immature keratinocytes appears basophilic in histologic sections They are the melanin depot and contain more of this pigment than the cells that make it. 19

20. keratinocytes Keratin Is a tough, fibrous intracellular protein Helps protect the skin and underlying tissues from abrasions, heat, microbes, and chemicals. Lamellar granules Release a water-repellent sealant that decreases water entry and loss and inhibits passage of foreign materials. 20

21. Layers of epidermis Generally epidermis has five strata or layers: Stratum basale Stratum spinosum Stratum granulosum Stratum lucidum Stratum corneum 21

22. Stratum basale The deepest layer it consists of a single row of cells, mostly stem cells from which new cells, the keratinocytes , arise by mitotic division. It is also called the stratum germinativum . The cells are small and cuboidal to low columnar. Mitotically active cells are attached directly to the basal lamina of the basement membrane by hemidesmosomes and to each other by desmosomes . Melanocyte and Merkel cells are interspersed among the basal keratinocytes. 22

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25. Stratum Basale.. An important feature of all keratinocytes in the stratum Basale is the cytoskeletal keratins , intermediate filaments about 10 nm in diameter . Keratin protects the deeper layers from injury. 25

26. S tratum spinosum < is normally the thickest layer, especially in the epidermal ridges Cells in this layer are rounded and larger in living tissue But, shrink and pull apart during preparation appear to be covered with thorn like spines consists of a few layers of polyhedral keratinocytes ( prickle cells ) , arranged in 8–10 layers. These keratinocytes are produced by the stem cells in the basal layer 26

27. S tratum spinosum Keratinocytes in the deeper aspects of the stratum spinosum are also mitotically active This layer also contains Langerhans cells Form Malpighian layer Nearly all of the mitotic activity in the epidermis occurs in this region, and cell division occurs at night.

28. Stratum spinosum The filaments assemble into microscopically visible bundles, tonofibrils . This Bundles insert into desmosomes, Intercellular bridge… holding the cell layers together. This arrangement provides both strength and flexibility to skin. They exhibit numerous cytoplasmic processes or spines, which gives this layer its name 28

29. Stratum granulosum At about the middle of the epidermis Consists of three to five layers Marks the transition between metabolically active strata and the dead cells of strata. Keratinocytes are undergoing apoptosis Because they have moved farther from dermal blood vessels 29

30. Stratum granulosum Keratin intermediate filaments Are no longer being produced by these cells They become more apparent because the organelles are regressing A distinctive feature of cells in this stratum is the presence of keratohyaline granule 30

31. Stratum granulosum Keratohyaline granule Darkly (Basophilic) staining cystine-rich and histidine-rich protein granule, which are the precursors of the protein filaggrin , function as promoters used as a clinical marker for the initiation of the final stage of apoptosis. Involved in assembling the keratin intermediate filaments into keratin within the cornified cells > The combination of keratin tonofilaments with keratohyalin granules in these cells produces keratin 31

32. Stratum granulosum Lamellar granules Membrane-enclosed granules in the keratinocytes of this layer Fuse with the plasma membrane Release a lipid-rich secretion . 32

33. Stratum granulosum The secretion of lamellar granules is: Deposited in the spaces between cells of strata of granulosum, lucidum, and corneum. Acts as a water-repellent sealant, retarding loss of body fluids and entry of foreign materials. 33

34. is responsible for maintaining body homeostasis . >> Thus EWB consists of two structural elements: cell envelope (CE) is a 15-nm-thick layer insoluble proteins that contributes to the strong mechanical properties of the barrier. The thickness of the CE increases in epithelia that are subject to considerable mechanical stress ( lip , sole, palm) Lipid envelope is a 5-nm-thick layer. The major are ceramides , , cholesterol , and free fatty acids . 34 Epidermal water barrier

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36. Stratum lucidum It is present in the thick skin only . is a clear, homogeneous layer It consists of four to six layers of clear, flat, dead keratinocytes that contain keratin filaments and eleidin , Keratinocytes contain large amounts of keratin and thickened plasma membranes. The keratin is more regularly arranged parallel to the skin surface. Provides an additional level of toughness 36

37. Stratum corneum Consists on average of 25 to 30 layers dead keratinocytes Being multiple layered help it to protect deeper layers from injury and microbial invasion 37

38. Stratum corneum Corneocytes / squames Extremely thin, flat, plasma-membrane-enclosed packages of keratin Final product of the differentiation process of the keratinocytes. They do not contain nuclei or any internal organelles. How changes from granule into cornified cell ? 38

39. Desquamation Cells are regularly exfoliated or desquamated from the surface of the stratum corneum. The continuous exfoliation of surface keratinocytes is a regulated proteolytic process that involves degradation of the cells’ desmosomes. The human kallikrein-related serine peptidases such as KLK5 , KLK7 , and KLK14 cause cleavage of desmosomes in a pH dependent manner . lymphoepithelial Kazal -type inhibitor (LEKTI in neutral pH, prevents desmosome cleavage. 39

40. Keratinization and growth of the epidermis New cells are formed in stratum basale They will be pushed through the various layers to the skin’s surface. As the cells move up they accumulate more keratin in a process called keratinization They undergo apoptosis Keratinized cells slough off The cells will be replaced by underlying cells 40

41. the turnover time for keratinocyte compartment (stratum spinosum and granulosum) is about 31 days, with an additional 14 days for the stratum corneum (average thickness in humans, 16 to 20 cell layers). Adding 1 to 2 days for mitotic divisions in stratum basale , the total epidermal turnover time takes approximately 47 days . 41 The epidermis is in a state of dynamic equilibrium in which exfoliated keratinized cells are constantly replaced by a steady flow of terminally differentiated cells

42. Epidermis Burn and abrasion increase the rate of cell division in basale . An excessive amount of keratinized cells shed from the skin of the scalp is called dandruff . 42

43. Nonkeratinocytes in the epidermis M elanocytes Comprises about 8 percent of the epidermal cells Develop from the neural crest Found among the cells of the basal layer and in hair follicles. Produce the pigment melanin . Form & maintain epidermal–melanin unit, each unit is estimated to contain one melanocyte associated with approximately 36 keratinocytes. The ratio varies from 1:4 to 1:40 or even higher, depending on the area of the body. This ratio is constant in all races > 43

44. Melanocytes They have: pale-staining cuboidal cell body attached by hemidesmosomes to the basal lamina, but lack desmosomes Long arm like processes ( so called dendritic cell ) By it, one melanocyte can contact 30 neighboring keratinocytes. And transfer melanin granules to them 44

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46. Epidermis melanocytes Melanin Is a yellow-red or brown-black pigment Is a group of biopolymers that are synthesized from tyrosine in melanocytes, within lysosome-related organelles called melanosomes . Contributes to skin color Shield the nuclear DNA from damage by UV light Their granules cluster to form a protective veil over the nucleus But, themselves are susceptible to damage by UV light 46

47. melanosome biogenesis in melanocytes 47

48. 48 (4) . Keratinocytes phagocytose the tips of the melanocyte processes ( 5) . as pigment donation, melanin is transferred to neighboring keratinocytes (6) . inside the keratinocytes, melanosomes are released into the cytoplasm (7) . Melanosomes more pronounced accumulation in areas over the nuclei, creating “dark umbrellas” (8) that protect the nuclear DNA from the sun’s harmful ultraviolet radiation . (1 ) . premelanosomes involved in melanin synthesis. (2) . As maturation progresses, melanosomes travel toward the ends of the melanocyte processes. (3) . Mature melanosomes have a high concentration of melanin and accumulate at the ends of melanocyte processes

49. Langerhans cells Macrophage cells ( MPS ) , APC in the epidermis arise lymphoid progenitor (CLP) cells in red bone marrow They have long arm like processes located mostly in the stratum spinosum. encounter and process antigens entering through the skin. They participate in immune responses against microbes that invade the skin. They can be easily damaged by uv light. 49

50. Langerhans cells Express CD1a surface marker and MHC I , MHC II , Fc receptors for IgG , C3b receptors, and the transmembrane protein langerin that is associated with Birbeck granules . Langerin and CD1a facilitate the immune defense against Mycobacterium leprae , 50

51. Merkel cells / tactile epithelial cells The origin of Merkel’s cells is unknown ; Located in the deepest layer of the epidermis, The least numerous cells & interspersed among the cells of the stratum basale Abundant t in highly sensitive skin like that of fingertips and at the bases of some hair follicles. where they contact the tactile (merkel) disc 51

52. merkel cells .. Merkel cells and tactile discs detect touch sensations. They possess antigenic markers of both epidermal and neural type. T heir cytplasm have dense-cored neurosecretory granules . 52

53. 53 Merkel’s corpuscles containing Merkel’s cells and disc receptors of afferent myelinated nerve fiber

54. The color of human skin is derived from, and varies with:- T ype ,d distribution and amount of melanin’s, the size, number, and type of melanin-containing compartments within keratinocytes. The ratio of eumelanin (brown/black) and pheomelanin (red/yellow). ( Each is genetically determined ) The amount of blood (and its degree of oxygenation) in cutaneous circulation , The thickness of the cornified layer, and Exposure to ultraviolet radiation , particularly the sun’s rays, is called tanning . ( the response to UV R is gen e tically determined ,,,in darker skin). 54

55. color of human skin .... Addison’s disease Vs albinism Two genes— Bcl2 and Mitf—appear to be responsible for the process of graying . Deficiency in Bcl2 expression causes apoptosis of melanocyte stem cells and a consequent decrease in the number of melanocytes. Individuals with a mutation in the Bcl2 gene may become prematurely gray . degradation products of hemoglobin, Hemosiderin is a golden brown pigment, whereas bilirubin is a yellowish brown pigment. ( jaundice ) 55

56. F actor for appearance of skin e.g. size, shape and distribution of hairs and skin glands (sweat, sebaceous and apocrine), Changes associated with maturation, ageing, metabolism and pregnancy. The general state of health is reflected in the appearance and condition of the skin, and the earliest signs of many systemic disorders may be apparent in the skin. 56

57. Dermis The second, deeper part of the skin < Composed of dense irregular connective tissue containing ( T I ) collagen and elastic fibers. It is much thicker than the epidermis The junction of the dermis with the epidermis is irregular. 57

58. Dermis The thickness of the dermis varies with the region of the body and reaches its maximum of 4 mm on the back Typically thinner in women than in men—dimple 58

59. Dermis The dermis has great tensile strength (resistance to stretching forces). It also has the ability to stretch and recoil easily. As any connective tissues, the cells present within it include: fixed and wandering cells. 59

60. Dermis The predominant fixed cells are fibroblasts Wandering cells include: macrophages, mast cells, eosinophils, neutrophils, and dermal interstitial dendritic cells Blood vessels, nerves, glands and hair follicles are embedded in the dermal layer. 60

61. Dermis Can be divided into: Superficial papillary region Deeper reticular region dermis 4/5 th of total thickness 61

62. papillary Layer Make 1/5 th of total thickness forms dermal ridges (dermal papillae), which interdigitate with the epidermal ridges (and interpapillary pegs) of the epidermis, is filled with loose irregular connective tissue fibers, capillaries, blood vessels, fibroblasts, macrophages, and other loose connective tissue cells. All of them contains capillary loops and some Meissner corpuscles, are fine-touch receptors. 62

63. dermal papillae In thin skin few in number, small, and irregularly scattered. In thick skin numerous, tall, and arranged in patterned rows. 63

64. Reticular region Thicker but less cellular than the papillary layer It is attached to the subcutaneous layer It consists of: Bundles of thick collagen fibers and Some coarse elastic fibers Scattered fibroblasts Various wandering cells Some adipose cells in the deepest part 64

65. Reticular region The collagen fibers are arranged in: A netlike manner A more regular formation than in the papillary region. The collagen and elastic fibers are not randomly oriented but form regular lines of tension in the skin called Langer’s lines . Skin incisions made parallel to Langer’s lines heal with the least scarring 65

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67. R eticular region Combination of collagen and elastic fibers provides the skin with strength, extensibility and elasticity. Extensibility Is the ability to stretch It can be seen around joints and in pregnancy and obesity. Elasticity The ability to return to original shape after stretching > . 67

68. From the deep part of the dermis arise the skin surface markings called flexure lines. for example, the deep skin creases on your palm. These result from a continual folding of the skin, often over joints, where the dermis attaches tightly to underlying structures. Flexure lines are also visible on the wrists, soles, fingers, and toes 68

69. Dermis… Both dermal regions contain a rich network of blood and lymphatic vessels. Nutritive vessels form two major plexuses Subpapillary plexus , from which capillary branches extend into the dermal papillae and form a rich, nutritive capillary network just below the epidermis. A deep plexus with larger blood and lymphatic vessels lies near the interface of the dermis and the subcutaneous layer dermal vasculature also has a thermoregulatory function, which involves numerous arteriovenous anastomoses or shunts . 69

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71. Epidermal ridges Series of ridges and grooves that marks surfaces of the palms, fingers, soles, and toes. Impressions of the ridges create fingerprint patterns, . Each epidermal ridge follows the outline of an underlying dermal ridge. Are produced as the epidermis projects downward into the dermis between the dermal papillae 71

72. Functions of epidermal ridges Increase the surface area of the epidermis: Increase the grip of the hand or foot by increasing friction Increases the number of corpuscles of touch and thus increases tactile sensitivity. Creates a stronger bond between the epidermis and dermis By interdigitating pattern between epidermal ridges and dermal papillae 72

73. Epidermal ridges Fingerprints ( footprints) Formed by sweat pores, the sweat and epidermal ridges Ducts of sweat glands open on the tops of the epidermal ridges Ridge pattern is in part genetically determined, but even identical twins have different patterns. Ridge pattern does not change during life, except to enlarge 73

74. Blood supply of the skin Cutaneous blood flow amounts to approximately 5% of the cardiac output. can be rapidly increased or decreased by as much as 20 times 74

75. In dermal papillae it forms extensive capillary loops And this capillary loops supply the epidermis The supply of which is increased by the increase of surface area by the dermal papillae arteriovenous anastomoses are common, particularly in the extremities (hands, feet, ears, lips, nose), 75

76. 76 Arteriovenous anastomosis

77. hypodermis is a layer of loose and adipose connective tissue of variable thickness that merges with the deep aspect of the dermis . contains sweat glands, superficial blood vessels, lymphatic vessels, and cutaneous nerves It mediates the increased mobility of the skin, and In the head and neck, it also contains striated muscles, such as platysma its adipose component contributes to thermal insulation, acts as a shock absorber and constitutes a store of metabolic energy. The amount and distribution of subcutaneous fat varies according to:- nutritional state, In different region of the same & gender > : 77

78. hypodermis … The total amount of subcutaneous fat tends to increase in both males and females in middle age. At any age, the amount of adipose tissue reflects the quantity of lipid stored in adipocytes rather than a change in the number of cells. The hypodermis is most distinct on the lower anterior abdominal wall, Skin ligaments , numerous small fibrous bands, extend through the subcutaneous tissue attach the deep surface of the dermis to the underlying deep fascia. The length and density of these ligaments determines the mobility of the skin over deep structures > 78

79. Fascias : constitute the wrapping, packing, and insulating materials of the deep structures of the body two type

80. SENSORY RECEPTORS Unencapsulated receptors include the following: Merkel cells , for sustained light touch and for sensing an object’s texture. Free nerve endings in the papillary dermis and into lower epidermal layers, which respond primarily to temperatures, pain, and itching, but also function as tactile receptors. Root hair plexuses , a web of sensory fibers surrounding the bases of hair follicles in the reticular dermis that detects movements of the hairs. 80

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82. Encapsulated receptors are all phasic mechanoreceptors, responding rapidly to stimuli on the skin. Meissner corpuscles Pacinian corpuscles: found deep in the reticular dermis and hypodermis. detect coarse touch, pressure (sustained touch), and vibrations. Krause end bulbs are simpler encapsulated, ovoid structures. in skin of the penis and clitoris where they sense low frequency vibrations. Ruffini corpuscles 82

83. Hairs are elongated keratinized filamentous structures that project from the hair follicles . palms, soles, the flexor surfaces of the digits, the thin skin of the umbilicus, nipples, glans penis and clitoris, the labia minora and the inner aspects of the labia majora and prepuce. There are individual and racial variations in density, form, distribution and pigmentation. In length, hairs range from less than a millimetre to more than a meter, and in width from 0.005 to 0.6 mm. They can be straight, coiled, helical or wavy, differ in colour depending on the type and degree of pigmentation

84. Hairs… They are composed of heavily cross-linked hard keratins. consist of three layers 1. Medulla forms the central part of the shaft contains a column of large, loosely connected keratinized cells containing soft keratin. The medulla is present only in thick hairs.

85. Hairs… 2.Cortex is the largest layer , account 80% of total hair mass. It is composed of cortical cells filled with hard keratin. It determines texture, elasticity, and the color of hair 3.Cuticle thin layer of heavily keratinized, squamous cells covering the cortex. protects the hair from physical and chemical damage and determines its porosity

86. Hair 86 Hair is consisted of two distinct structures: follicle—the living part located under the skin and hair shaft—fully keratinized nonliving part above the skin surface.

87. Hair follicle Is tubular invaginations of the epidermis. is responsible for the production and growth of a hair. Coloration of the hair is attributable to the content and type of melanin that the hair contains. It varies in histologic appearance, depending on whether it is in a growing or a resting phase. The growing follicle shows the most elaborate structure; thus, it is described here .

88. Hair follicle is divided into four regions. Infundibulum: from the surface opening of the follicle to the level of the opening of its sebaceous gland. It is a part of the pilosebaceous canal . Isthmus from infundibulum to the level of insertion of arrector pili muscle. Follicular bulge protrudes from the hair follicle near the insertion of the arrector pili muscle and contains epidermal stem cells . Inferior segment extends from the bulge to base of the follicle. hair bulb is the expanded end portion of the invaginated hair follicle.

89. Hair follicle … The base of bulb is invaginated by a tuft of vascularized loose connective tissue called, a dermal papilla. Other cells forming the bulb that surround dermal papilla, are collectively referred to as the hair matrix , consists matrix cells . Matrix cells represent the population of rapidly dividing and differentiating cells containing epidermal stem cells . These stem cells accounts for the growth of the hair ( hair shaft and internal root sheath ) This zone also contains melanocytes that give color to the hair by passing melanin to the matrix cells.

90. Hair follicle… Hair shaft and bulb surrounded by Internal root sheath is a multilayered cellular covering that surrounds the deep part of the hair. External root sheath covers the internal sheath and extends all the way to the epidermis, where it is continuous with the basal and spinous layers. Glassy membrane , acellular hyaline layer, the thickened basement Separating the hair follicle from the dermis.

91. Hair follicle… The follicular bulge identified as a niche of epidermal stem (ES) cells ES cells are responsible for providing stem cells for the growth of hair follicles (the hair matrix, internal root sheath, cortex, and medulla) as well as sebaceous glands. In case of wound or skin injury, reprogram & wound surface

92. Arrector pili muscle : a small bundle of smooth muscle cells, extends from the midpoint of the fibrous sheath to the dermal papillary layer. Contraction of these muscles pulls the hair shafts to a more erect position, .

93. Hairs grow : asynchronously , cyclically, and at different rates in different regions of the body The hair growth cycle has three major phases: Anagen : a generally long period of mitotic activity and growth. At any time, about  90 % of the hairs on your head are in this phase Catagen() : a brief period of arrested growth and regression of the hair bulb. lasts 2 weeks , regardless of the site and follicle type. Telogen() : a final long period of inactivity , during which the hair may be shed.

94. Club hair : Fully keratinized, dead hair formed at telogen stage Lanugo hair : Fine hairs on the fetus body; shed in utero or within the first weeks after birth Vellus hair : Non-pigmented, and generally non medullated; short hairs> Terminal hairs : Large and pigmented hairs, > In the balding Indv, large terminal follicles are gradually converted into small vellus follicles after several growth cycles.

95. Skin glands Exocrine associated with the skin are: Sebaceous (oil) glands Sudoriferous (sweat) glands Ceruminous glands Mammary glands 98

96. Relationships between the skin, hair follicle, arrector pili muscle, and sebaceous and sweat glands. The arrector pili muscle originates in the connective tissue sheath of the hair follicle and inserts into the papillary layer of the dermis, where it ends. 99

97. Sebaceous glands develop as outgrowths of the external root sheath of the hair follicle, Simple, branched acinar glands With few exceptions, they are connected to hair follicles They are found over all regions of the body except palms and soles 100

98. Sebaceous glands The process of sebum production from the time of basal cell mitosis to the secretion of the sebum takes about 8 days Sebum is a complex mixture of lipids that includes wax esters, squalene, cholesterol, and triglycerides that are hydrolyzed by bacterial enzymes after secretion. 101

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100. Sebaceous glands Vary in size and shape Most numerous where hairs are most numerous Small in most areas of the trunk and limbs Large in the skin of the breasts, face, neck, and upper chest. 103

101. Sebaceous glands Secreting portion: Lies in the dermis Opens into: Usually, neck of the hair follicle. Lips, glans penis, and labia minora, eyelids ,nipples (Montgomery’s glands), areolae of the female breast , directly onto the surface of skin 104

102. Sudoriferous glands Sweat glands distributed over the entire skin surface, except on the nipples and parts of the external genitalia. Humans normally produce about 500 ml of sweat per day, Their secretions are: Released by exocytosis Emptied onto the skin surface through pores or into hair follicles. Depending on structure and type of secretion, classified as either eccrine or apocrine. 105

103. Sudoriferous glands Eccrine/merocrine sweat glands Simple,coiled tubular glands Widely distributed throughout the body, except lips, nail beds, glans penis, glans clitoris, labia minora, and eardrums. Most numerous in forehead, palms, and soles (620/cm2). 106

104. Sudoriferous glands Eccrine sweat glands Secretory portion is located in the deep dermis is more pale-staining than the ducts consists of an unusual stratified cuboidal epithelium with three cell types Excretory duct ends as a pore at the surface of the epidermis On the palms and soles, they open along the apex of epidermal ridges—fingerprint They stimulated by cholinergic NTs 107

105. Sudoriferous glands Functions of eccrine sweat glands Thermoregulation > During fear and embarrassment Excretion 108

106. Apo crine sweat glands Simple, coiled tubular glands Their development depends on sex hormones and is not complete and functional until after puberty. They are found mainly in the skin of the axilla, groin, areolae of the breasts, and bearded regions of the face in adult males. 109

107. Apocrine sweat glands Secretory portion Is located mostly in the subcutaneous layer Surrounded by vascular connective tissue containing numerous nerve endings. Consists of wide and dilated lumina is composed of a single cell type. . their cells show merocrine, not apocrine secretion 110

108. Apocrine sweat glands The ducts of apocrine glands are similar to those of the eccrine glands, but they usually open into hair follicles at the epidermis. and may contain the protein rich product. The slightly viscous secretion is initially odorless but may acquire a distinctive odor as a result of bacterial activity. 111

109. Apocrine sweat glands Are stimulated during emotional stress and sexual excitement--“cold sweat.” Are not active during thermoregulatory sweating. Secretions contains components of eccrine sweat plus lipids and proteins ., odorless or odiferious respond to emotional and sensory stimuli but not to heat. 112

110. Ceruminous glands Modified apocrine sweat glands in the external ear Produce a yellowish waxy lubricating secretion Secretory portions of lie in the subcutaneous layer Excretory ducts open either directly onto the surface of the external auditory canal or into ducts of sebaceous glands. Combined secretion of the ceruminous and sebaceous glands is called cerumen or earwax. 114

111. Ceruminous glands Cerumen Provides a sticky barrier that prevents the entrance of foreign bodies Waterproofs the canal and keeps bacteria and fungi from entering cells. 115

112. Nails

113. Nails (3) They are plates of tightly packed, hard , dead keratinized epidermal cells . are homologous with the cornified layer of general epidermis. variety of mineral elements including calcium are present in N. Fingernails grow by 71mm per week (toenails more slowly) 117

114. Each nail consists of a nail body, a free edge, and a nail root

115. Nails Nail body (plate) Is the visible portion of the nail. It is comparable to the stratum corneum of epidermis, but it has a harder type of keratin and the cells are not shed . Below it is a layer of epithelium and a deeper layer of dermis. It appears pink because of blood flowing through underlying capillaries. 119

116. Nails Free edge Is the part of the nail body that may extend and pass the distal end of the digit. White because there are no underlying capillaries. Nail root— part of the nail that is buried in a fold of skin. Lunula— whitish, crescent-shaped area proximal to nail body 120

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118. Nails Hyponychium represents the union between the nail bed and the nail plate at the fingertip. A thickened region of stratum corneum beneath free edge Its function is to render the nail bed impermeable for protection purposes Secures the nail to the fingertip. 122

119. Eponychium or cuticle is the thick corneal layer extending on the dorsal surface of the nail plate forms a seal between the nail plate and the proximal nail fold. Occupies the proximal border of the nail, overlies the crescent-shaped whitish lunula . 123

120. Nails Nail matrix Proximal portion of epithelium deep to the nail root The superficial nail matrix cells divide mitotically to produce new nail cells. Nail growth occurs by the transformation of superficial nail matrix cells into new nail cells. 124

121. Nails Functions: They protect the distal end of the digits. They provide support and counter pressure to the palmar surface of the fingers: To enhance touch perception and manipulation. Allow us to grasp and manipulate small objects They can be used to scratch and groom the body in various ways. 125

122. Development of Skin 126

123. Development of skin Epidermis is derived from surface ectoderm . Dermis is derived from mesodermal mesenchyme. 127

124. Epidermis Primordium of the epidermis is a layer of surface ectodermal cells. A t 2 nd month, These cells proliferate and form: Periderm—a layer of squamous epithelium A basal layer 128

125. Epidermis Peridermal cells Continually undergo keratinization and desquamation Replaced by cells arising from the basal layer Form part of the white greasy substance- vernix caseosa . 129

126. Epidermis Vernix caseosa a layer of cellular debris that covers the fetal skin Later, it contains sebum Protects the developing skin from constant exposure to amniotic fluid It also facilitates birth of the fetus. 130

127. Epidermis In the eleventh week, proliferation of the basal layer produces a new intermediate layer just deep to the periderm Basal layer becomes stratum germinativum Stratum germinativum the layer of stem cells that will continue to replenish the epidermis throughout life Epidermal ridges also forms. 131

128. Epidermis The type of pattern of epidermal ridges that develops is determined genetically Abnormal chromosome complements affect its development patterns, e.G. Down syndrome 133

129. at 21 st week, Periderm disappears and the intermediate layer replaced by 3 layer:- stratum spinosum, stratum granulosum ,and , stratum corneum . This transformation begins at the cranial end of the fetus and proceeds caudally. 134 are connected by cell-to-cell membrane junctions called desmosomes

130. Other cells of Epidermis .Langerhans cells : are immune cells of bone marrow origin . . Merker cell : arise from epidermal cells .Melanocytes Late in the embryonic period, Neural crest cells migrate into the developing dermis and differentiate into melanoblasts . Later melanoblasts migrate to the dermo-epidermal junction and differentiate into melanocytes . Melanocytes appear in the developing skin at 40 to 50 days, 135

131. Melanin Begins to be produced and distributed before birth by melanocytes Its amount increases in response to ultraviolet light Pigment formation can be observed prenatally in the epidermis 136

132. Epidermis Development of epidermis need inductive interactions of dermis. 137

133. Dermis It develops from mesenchyme, which is derived from the mesoderm is a tissue with a triple embryonic origin. Lateral plate mesoderm supplying cells for dermis in the limbs and body wall, Paraxial mesoderm supplying cells of dermis in the back, and Neural crest cells supplying cells for dermis in the face and neck. 138

134. Dermis Dermal ridges Formed by the projection of dermis into the epidermis Capillary loops develop in some of it Sensory nerve endings form in others 139

135. Ectodermal derivatives develop & depends on epithelial-mesenchymal interactions All are characterized by:- The development of an ectodermal placode Condensation of cells in the underlying mesenchyme, and Then invagination of the epithelium into the underlying dermis to form a bud

136. DEVELOPMENT OF HAIR Hair follicles first form at the end of the second month on the eyebrows, eyelids, upper lip, and chin. Hair follicles do not form in other regions until the fourth month. Most, not all, hair follicles are present by the fifth month, and it is believed that novel hair follicles do not form after birth. About five million hair follicles develop in both males and females, >

137. DEVELOPMENT OF HAIR… The hair follicle first appears as a small concentration of ectodermal cells, called a hair germ , in the basal layer of the two epidermis Hair germs are induced by the underlying dermis. The hair germ recruits dermal cells to form a dermal condensate that promotes further differentiation of the hair germ The hair germ proliferates to form a rod-like hair peg that pushes down into the dermis Within the dermis, the tip of the hair peg expands, forming a bulbous hair peg , and the dermis cells just beneath the tip of the bulb proliferate to form a small hillock called the dermal papilla ( hair papilla )

138. DEVELOPMENT OF HAIR… The epithelial cells within the hair bulb differentiate into the germinal matrix, where cells proliferate, grow toward the surface, keratinize, and form the hair shaft The epidermal cells lining the follicular canal constitute the inner and outer epidermal root sheaths Except in the case of the eyebrows and eyelashes, the dermal root sheath of the follicle becomes associated with a bundle of smooth muscle cells called the arrector pili muscle The stem cells of the follicular epithelium lie in bulge

139. lanugo Are the first generation of hairs that is formed during the twelfth week; are fine, soft and unpigmented hair They are mostly shed before birth and are replaced by coarser hairs during the perinatal period. At puberty, rising levels of sex hormones cause fine body hair to be replaced by coarser hairs on some parts of the body: In boys, similar coarse hairs also appear on the face and often on the chest and back.

140. Arrector muscles of hairs small bundles of smooth muscle fibers, differentiate from the mesenchyme surrounding the hair follicles and attach to the dermal root sheaths of hair follicles and the papillary layer of the dermis , which interdigitates with the epidermis Contractions of the arrector muscles depress the skin over their attachment and elevate the skin around the hair shafts, causing the hairs to stand up.

141. sebaceous glands Over most of the body, these glands form as diverticula of the hair follicle shafts, budding from the side of the root sheath about 4 weeks after the hair germ begins to elongate. In some areas of hairless skin—such as the glans penis of males and the labia minora of females sebaceous glands develop as independent downgrowth's of epidermis. Mature sebaceous glands are present on the face by 6 months of development .>>

142. SWEAT GLANDS Eccrine sweat glands: form in the skin over most parts of the body beginning as buds of the germinative layer of the epidermis. The buds grow into the dermis, and their end coils to form the secretory parts of the glands Apocrine sweat glands : develop anywhere there is body hair, including the face, axillae, and pubic region. as buds of the root sheath of hair follicles, superficial to the buds of sebaceous glands, Its secretion contains lipids, proteins, and pheromones,

143. DEVELOPMENT OF MAMMARY GLANDS In the fourth week, a pair of epidermal thickenings called the mammary ridges develop along either side of the body from the area of the future axilla to the future inguinal region and the medial thigh. Seventh week, These ridges normally disappear, except at the site of the breasts. The remnant of the mammary ridge produces a well-defined primary bud of the mammary gland. This bud grows down into the underlying dermis toward the presumptive fat pad that will induce the duct to branch

144. In the tenth week , the primary bud begins to branch, and by the twelfth week, several secondary buds have formed. These buds lengthen and branch throughout the remainder of gestation, and the resulting ducts canalize > . At birth, the mammary glands consist of 15 to 25 lactiferous ducts, which open onto a small superficial depression called the mammary pit Proliferation of the underlying mesoderm usually converts this pit to an everted nipple within a few weeks after birth ,> The skin surrounding the nipple also proliferates to form the areola

145. Nail development assignments

146. Anomalies of the skin congenital ectodermal dysplasia A group of hereditary disorders involving tissues that are ectodermal in origin. Teeth are completely or partially absent. Often the hairs, nails, and skin are also severely affected. 152

147. Anomalies of The Skin Ectrodactyly-ectodermal Dysplasia- clefting Syndrome Is a congenital skin condition that is inherited as an autosomal dominant trait It involves both ectodermal and mesodermal tissues Consists of ectodermal dysplasia 153

148. Anomalies of the skin ectrodactyly -ectodermal dysplasia- clefting syndrome Associated with: Hypopigmentation of skin and hair, Scanty hair and eyebrows Absence of eyelashes Nail dystrophy Hypodontia and microdontia Cleft lip and palate. 154

149. Thank you for your attention 10/03/2022 by ATALO. A. 155