Skin Anatomy PDF

THE SKIN (INTEGUMENTARY SYSTEM) Assoc. Prof. Dr. Seda Karabulut  The skin and its derivatives (appendages), including sebaceous glands, sweat glands, hair, nails form the integumentary system The integument is the largest and the heaviest organ It is about 1.2...

THE SKIN (INTEGUMENTARY SYSTEM) Assoc. Prof. Dr. Seda Karabulut  The skin and its derivatives (appendages), including sebaceous glands, sweat glands, hair, nails form the integumentary system The integument is the largest and the heaviest organ It is about 1.2 - 2.3 m2 Constitutes about 16% of the total body weight. The skin: Protects the body against injury, desiccation, and infection Regulates body temperature Absorbs ultraviolet (UV) radiation for protection and for vitamin D synthesis Contains receptors for touch, temperature, and pain stimuli. Mammalian skin is composed of two primary layers: 1. Epidermis: is ectodermal origin; is a keratinized stratified squamous epithelium; provides waterproofing serves as a barrier to infection. 2. Dermis: is mesodermal origin; is a dense fibrous connective tissue serves as a location for the appendages of skin.  The hypodermis, is a sheath of tissue between skin and deeper structures is not part of the skin. Its purpose is to attach the skin to underlying bone and muscle, supplying it with blood vessels and nerves. It is rich in adipose tissue and consists of loose connective tissue and elastin.  Sweat gland duct The epidermis and dermis interdigitate with each other by the formation of epidermal ridges and dermal ridges (dermal papillae). Where the epidermis overlies the dermal ridges, a series of epidermal ridges is produced that are visible as fingerprints on the fingertips. (E) epidermis, (D) dermis (D), (ER) epidermal ridges, (DR) dermal ridges (=dermal papillae), (SC) stratum corneum, (d) the duct of the sweat gland, (RL) the reticular layer (RL), (BV) blood vessels  Sweat gland duct What is the function of these raised strips of epidermis which are located on the palms of the hands and soles of the feet? They increase friction Traction of the foot and gripping ability of the hand increases  Thick Skin Thin Skin Skin can be classified into two depending on the thickness of its epidermis: thick, as on the sole of the foot and the palm of the hand, or thin, as over the remainder of the body. Thick skin: has an epidermis that is 400-600 µm thick; has five well-developed layers: stratum basale, stratum spinosum, stratum granulosum, stratum lucidum, and stratum corneum (well-developed); contains sweat glands; lacks hair follicles, sebaceous glands, and arrector pili muscles. Thin skin has an epidermis that is 75-150 µm thick; has three layers The stratum granulosum and stratum lucidum are absent Contains individual cells that are similar to the cells of these layers; contains sweat glands, hair follicles, sebaceous glands, and arrector pili muscles.  The epidermis is composed of four cell types, keratinocytes (~95% of the cells of the epidermis), melanocytes, Langerhans cells, and Merkel cells. Special Feature of these cells Intact nucleus Segmented nucleus  Keratinocytes Keratinocytes make up the characteristics of the five layers of epidermis. They regenerate the skin ~every 30 days, by mitosis, which typically happens at night. They produce keratin, which is a family of fibrous structural proteins It is the key structural material making up the outer layer of human skin, hair and nails.  Keratin monomers forms bundles Form intermediate filaments Hydrogen bonds are present Keratins have large amounts of the sulfur-containing amino acid cysteine It is required for the disulfide bridges It confer additional strength and rigidity by permanent, thermally stable crosslinking.  Epidermis Epidermis of the finger 1 Stratum corneum 2 Stratum lucidum 3 Stratum granulosum 4 Stratum spinosum 5 Stratum basale Dermis 6 Stratum papillare 7 Stratum reticulare  The stratum basale (stratum germinativum) is the deepest layer of the epidermis Composed of keratinocytes that are cuboidal to columnar in shape and are mitotically active. These cells are responsible for cell renewal Newly formed cells are pushed surface ward.  Keratinocytes are attached to the basal lamina (basement membrane) It separates them from the dermis, by hemidesmosomes. They form desmosomal contacts with each other and with the cells of the stratum spinosum. This layer also contains melanocytes (M) and Merkel cells. Keratinocytes of this layer form keratins 5 and 14. Epidermis 1 Stratum corneum 2 Stratum lucidum 3 Stratum granulosum 4 Stratum spinosum 5 Stratum basale Dermis 6 Stratum papillare 7 Stratum reticulare  The stratum spinosum is is composed of polyhedral keratinocytes (prickle cells) Prickle cells extend numerous processes (intercellular bridges) that form desmosomes with processes of surrounding cells. Cells in the deeper layer of the stratum spinosum, also display mitotic activity. Stratum spinosum and stratum basale constitute the malpighian layer (stratum malpighii). Nearly all the mitotic activity in the epidermis occurs in this region.  The keratin filaments are abundant in the cytoplasm of keratinocytes The keratin filaments are inserted on the desmosomes of keratinocytes and form spindles or spines at the interfaces. These spines seen at the interfaces of polyhedral keratinocytes are responsible for the term prickle cell.  Prickle cells form keratin 1 and keratin 10.  Red arrows: secreted lamellar bodies Green arrows: lamellar bodies in the cytoplasm. In the superficial regions of the stratum spinosum are keratinocytes that contain membrane-coating granules (Odland bodies, lamellar bodies). The contents of these granules are released into the intercellular spaces in the form of lipid-containing sheets that are impermeable to water and many foreign substances. Play a role in the barrier function of the stratum corneum. The contents include lipids (e.g. glucosylceramides, phospholipids, and glycosphingolipids), hydrolytic enzymes (e.g. proteases, acid phosphatases, glucosidases, lipases) and proteins (e.g. corneodesmosin). Epidermis 1 Stratum corneum 2 Stratum lucidum 3 Stratum granulosum 4 Stratum spinosum 5 Stratum basale Dermis 6 Stratum papillare 7 Stratum reticulare  Continuous migration of the cells of the stratum spinosum forms the next layer, the stratum granulosum. It comprises three to five layers of flattened keratinocytes. It is the most superficial layer in which nuclei are still present. Cells of this layer is flattened and therefore are squamose  Cells of the stratum granulosum contact each other via desmosomes Also form claudin-containing occluding junctions (tight junctions) with each other as well as with cells of the stratum lucidum (or with the stratum corneum).  Keratinocytes of stratum granulosum accumulate keratohyalin granules (*), It overfill the cells, destroying their nuclei and organelles. Keratohyalin granules are non– membrane-bound structures that are composed of trichohyalin and filaggrin. These two proteins promote the aggregation of keratin by cross- Note that several cells show brown linking the keratin filaments into melanin pigment granules, which will thick bundles of tonofilaments. be discussed later next to their nucleus (arrows). Epidermis 1 Stratum corneum 2 Stratum lucidum 3 Stratum granulosum 4 Stratum spinosum 5 Stratum basale Dermis 6 Stratum papillare 7 Stratum reticulare  When evident in thick skin (is found only in palmar and plantar skin), the stratum lucidum usually appears as a thin, translucent region, interposed between the stratum granulosum and the corneum. The cells of the stratum lucidum have no nuclei or organelles but contain a large amount of keratin filaments and eleidin, a transformation product of keratohyalin. It is often difficult to distinguish in histologic sections. Epidermis 1 Stratum corneum 2 Stratum lucidum 3 Stratum granulosum 4 Stratum spinosum 5 Stratum basale Dermis 6 Stratum papillare 7 Stratum reticulare  The stratum corneum is the most superficial layer of the epidermis. It may consist 15- 20 layers of flattened, 14-sided polygon shaped, nonnucleated, dead "cells“ (hulls) filled with keratin. These scale-like structures are called squames (or horny cells).  The most superficial layer of the stratum corneum is continuously shed by desquamation. Squames (sheets of keratin) are desquamated at the same rate as they are being produced by stratum basale and spinosum. Thin skin epidermis D: Stratum corneum (thinner) (Stratum lucidum is absent) C: Granular cells B: Stratum spinosum (thinner) A: Stratum basale  Melanocytes, derived from neural crest cells, are present in the stratum basale. These cells synthesize melanin, the pigment that gives skin its color. They form long processes, dendrites, that penetrate the stratum spinosum.  Each melanocyte forms an association, via its dendrites, with a number of keratinocytes, referred to as epidermal-melanin unit. The number of keratinocytes per melanocyte varies with regions of the body but is relatively constant across the races Approximately 3% of the cells of the epidermis consist of melanocytes.  Melanin is derived from the amino acid tyrosine, which is transported into specialized tyrosinase-containing vesicles known as premelanosomes. While these vesicles are transported from the melanocyte cell body towards the keratinocytes through the dendrites, tyrosine is converted to melanin via a series of enzymatic reactions. As the amount of melanin increases within the premelanosomes, they mature into much darker structures known as melanosomes.  Melanosomes are exocytosed by the melanocytes and are phagocytosed by keratinocytes of the stratum spinosum. This unique mechanism is called cytocrine secretion. The melanosomes migrate to the nuclear region of the keratinocyte and form a protective umbrella, shielding the nucleus from UV. When we are exposed to UV rays : increase the rates of darkening of melanin increase endocytosis of the melanosomes enhance tyrosinase activity and thus melanin production.  Skin pigmentation is related to the location of melanin rather than to the numbers of melanocytes. Melanosomes are fewer and congregate around the keratinocyte nucleus in light-skinned individuals In dark-skinned individuals they are larger and are more dispersed throughout the keratinocyte cytoplasm.  There are two types of melanin, eumelanin, which is a dark brown to black pigment Is present in individuals with dark hair Pheomelanin, which is a red to rust-colored compound Is found in individuals with red and blond hair.  Langerhans cells (also known as dendritic cells because of their long processes) Derived from bone marrow Located mostly in the stratum spinosum. They do not make desmosomal contact with the cells of the stratum spinosum.  They contain characteristic paddle-shaped Birbeck granules Function as antigen-presenting cells in immune responses. They phagocytose antigens entering the epidermis They express CD1, a surface marker and MHC I and MHC II receptors.  Merkel cells are present in small numbers in the stratum basale, near areas of well-vascularized, richly innervated connective tissue. Their origin is uncertain, although mostly believed to be epithelial origin because they possess desmosomes and keratin filaments. They are most abundant in the fingertips.  Their pale cytoplasm contains small, dense-cored granules that are similar in appearance to those in some cells of the diffuse neuroendocrine system. Afferent nerve terminals approximate these cells, forming complexes, known as Merkel discs (tactile disk). Merkel discs are believed to function as mechanoreceptors (touch receptors).  The epidermis and dermis are separated by a thin sheet of fibers called the basement membrane, Made through the action of both tissues. Its primary function is to anchor down the epidermis to the dermis The basal lamina layer is divided into two layers. The clear layer closer to the epidermis is called the lamina lucida, while the dense layer closer to the dermis is called the lamina densa. The basement membrane controls the traffic of the cells and molecules between the dermis and epidermis.  The dermis of the skin lies directly under the epidermis, Derived from mesoderm.  The dermis is composed of dense, irregular collagenous connective tissue containing mostly type I collagen and numerous elastic fibers  The dermis is divided into a superficial papillary layer and a deeper, more extensive reticular layer No distinct boundary exists between these layers  The papillary layer is uneven and forms dermal papillae (dermal ridges), which interdigitate with the epidermal ridges. This dermal layer is composed of thin, loosely arranged fibers and cells Contains capillary loops, which bring nourishment to the avascular epidermis and Meissner corpuscles, which are fine-touch receptors.  The dermal reticular layer constitutes the major portion of the dermis. It is composed of dense bundles of collagen fibers and thick elastic fibers May contain Pacinian corpuscles (pressure receptors) and Krause end-bulbs (cold receptors) in its deeper aspects. Cutaneous (Skin) sensory receptors Encapsulated and nonencapsulated (free) nerve endings Free (nonencapsulated) nerve endings  Tactile corpuscles (Meissner's corpuscles) are a type of mechanoreceptor. Touch, pressure, vibration Tactile corpuscles are encapsulated unmyelinated nerve endings Consist of flattened supportive cells (modified Schwann cells) arranged as horizontal lamellae surrounded by a connective tissue capsule.  Lamellar corpuscles (Pacinian corpuscles) are one of the four major types of mechanoreceptor. They are responsible for sensitivity to vibration and pressure. The lamellae are very thin, flat  A neurite refers to any projection from the cell body of a neuron. This projection can be an axon or a dendrite. Epithelial cells inside the capsule and modified Schwann cells inside the inner core of the corpuscle. The center of the corpuscle is a neurite of single afferent with oval cross-section and unmyelinated at the receptive region.  The Bulbous corpuscle or Ruffini ending or Ruffini corpuscle is a slowly adapting mechanoreceptor. Enlarged dendritic endings with elongated capsules. This spindle-shaped receptor is sensitive to skin stretch Contributes to the control of finger position and movement.  The bulboid corpuscles (end-bulbs of Krause) are cutaneous receptors They are cylindrical or oval bodies Consists of a capsule formed by the expansion of the connective-tissue sheath of a medullated fiber, and containing a soft semifluid core in The end-bulbs of Krause are thermoreceptors, sensing cold temperatures. CLINIC CORRELATIONS Collagen Collagen Elastin Elastin BURNS WARTS Yeast infections