Human Histology Lecture 4 PDF

Human Histolog Dr.Buthainah Al.Ezzi Integumentary system The integumentary system is the largest organ of the body that forms a physical barrier between the external environment and the internal environment that it serv...

Human Histolog Dr.Buthainah Al.Ezzi Integumentary system The integumentary system is the largest organ of the body that forms a physical barrier between the external environment and the internal environment that it serves to protect and maintain. The integumentary system includes the epidermis, dermis, hypodermis, associated glands, hair, and nails. The two main components of the integumentary system are the skin and the accessory structures Layers of the Skin Skin consist of two distinct regions 1. Epidermis – outer Is non vascular and is lined by keratinized stratified squamous epithelium 2. Dermis – inner Characterized by dense irregular connective tissue, blood vessels, nerves, and different gland Beneath the dermis is the hypodermis or subcutaneous layer of connective tissue and adipose tissue that forms the superficial fascia seen in gross anatomy. Epidermis ❖ Composed of stratified squamous epithelium ❖ Avascular as it has no blood supply of its own ❖ Oxygen and nutrients diffuse from the underlying dermis ❖ The most abundant epithelial cells, the keratinocytes form different layers. ❖ Thick skin (5) and thin skin (4)-refer only to the epidermis. Five structurally different layers can be identified: EPIDERMAL LAYERS 1-Stratum Germinativum (basale) 2-Stratum Spinosum 3- Stratum Granulosum 4-Stratum Lucidum 5-Stratum Corneum. The stratum basale (First Layer) ❖ The deepest layer of the epidermis (closest to the dermis). It consists of a single layer of columnar or cuboidal cells which rest on the basement membrane ❖ It is found close to the dermal blood supply. ❖ Basal cells serve as stem cells for the epidermis. Their mitotic activity replenishes the cells in more superficial layers as these are eventually shed from the epidermis ❖ Cells mature and migrate upward in the epidermis and produce intermediate keratin filaments. ❖ The accessories arise from this layer and also home to melanocytes that produce melanin (the pigment responsible for skin color Stratum Spinosum(second layer) This consists of 4-6 rows of cells During histological Preparation ,cells shrink and intercellular space appear as spines The cells are often separated by narrow, translucent clefts. These clefts are spanned by spine-like cytoplasmatic extensions of the cells (hence the name of the layer and of its cells: spinous cells) Cells synthesize keratin filaments that become assembled into tonofilaments. Also contain Langerhans cells, part of the immune system (defense against microorganisms and cancers). The stratum granulosum(Third Layer) Consists of 3-5 cell layers of flattened cells. Cells filled with dense keratohylaine granules(hence the name of the layer). and membrane-bound lamellar granules.  Keratohyaline consist of the protein that crosslinks with keratin filaments. Combination of keratin tonofilaments with keratohylaine granules produce soft keratin Dendritic cells found throughout this stratum Nuclei already begin to degenerate in the outer part of the stratum granulosum. stratum lucidum (fourth Layer) lucidum can usually not be identified in thin skin is translucent and barely visible (in thick skin of the palms and soles.) It lies superior to stratum granulosum and inferior stratum corneum. The tightly packed cells lack nuclei or organelles and are dead. The flatted cells contain densely packed keratin filaments Stratum corneum (the fifth layer) All nuclei and organelles have disappeared from the cells. Consist of flatted , dead cells filled with soft keratin filaments Superficial cell from this layer are continually shed or replaced by new cells arising from the deep stratum basale. The hydrolytic enzymes distrupt the nucleus and all cytoplasmic organelles ,which disappear as the cells fill with keratin. The protection of the body by the epidermis is due to the functional features of the stratum corneum. Dermis Attachment of epidermis to dermis is enhanced by an increased interface between the tissues, one can recognize a division into two structully distinct layers. The papillary layer 1. Basement membrane separate the dermis from the epidermis 2. Is the superficial layer in the dermis and contain loose irregular C.T. 3. Dermal papillae and epidermal ridges form evagination and interdigitations. 4. Connective tissue filled with fiber ,capillaries,blood vessels ,fibroblasts , macrophages ,and another loose C.T. 5. Sensory receptor are resent in the dermal papillae. 6. dermal papillae, that project into the under surface of the epidermis.When it contain a blood vessel then called vascular papillae, but when it contain a nerve ending (Meissner corpuscle) then it called nervous papillae. The reticular layer ❖ Liesdeep to the papillary layer; it characterized by thick , irregular bundles of collagen and the presence of more coarse elastic fibers. ❖ Less cellular than papillary layer. ❖ No distinct boundary between the papillae and reticular layers. ❖ Blends inferior with the hypodermis or subcutaneous layer(hypodermis)of superficial fascia. ❖ Accessory organs of epidermal origin, such as hair follicles and sweat glands extend into the dermis. ❖ contain Sensory receptors Meissner’s and Pacinian corpuscles ❖ The hypodermis contains variable amounts of adipose tissue. Other skin cells Keratinocytes: are present in all four layers of the epidermis. Under physiological conditions, keratinocytes proliferate in the stratum basalis and over a 30–50-day period migrate through the epidermis to the stratum corneum. Keratinocytes are the structural cells of skin as well as the most common type of skin cells. In fact, over 90 percent of our skin tissue is made of keratinocytes. They get their name from the keratin they produce, the protein that provides skin with strength and flexibility. As the most dominant cell type constituting the epidermis, keratinocytes play multiple roles essential for skin repair. They are the executors of the re-epithelialization process, whereby keratinocytes migrate, proliferate, and differentiate to restore the epidermal barrier. Keratinocytes. do much more than produce keratin, surface lipids, and intercellular substances , the epidermal melanin unit, which comprise keratinocytes and melanocytes, has a key role in regulating pigmentation and homeostasis of the epidermis keratinocytes of the epidermis are unique in their ability to produce vitamin D3. keratinocytes contribute to protecting the body from ultraviolet radiation (UVR) by taking up melanosomes, vesicles containing the endogenous photoprotectant melanin, from epidermal melanocytes. Each melanocyte in the epidermis has several dendrites that stretch out to connect it with many keratinocytes. Melanocytes:derived from neural crest cells,have long ,irregular cytoplasmic or extension that branch into the epidermis ,located between the stratum basal and stratum spinosum of epidermis ,and synthesize the dark brown pigment melanin(from trypsin),melanin transferred from cytoplasmic extension to Keratinocytes in basal cell layer,and melanin darkens skin color and protects it from U.V radiation. The difference in skin color between lightly and darkly pigmented individuals is due not to the number (quantity) of melanocytes in their skin, but to the melanocytes' level of activity (quantity and relative amounts of eumelanin and pheomelanin). Langerhans cells:originated from bone marrow ,migrate via blood stream to the skin in stratum spinosum Langerhans cells (LC) are a unique population of tissue-resident macrophages a specialized subset of dendritic cells (DCs) that form a network of cells across the epidermis of the skin, but which have the ability to migrate from the epidermis to draining lymph nodes (LN). Their location at the skin barrier suggests a key role as immune barrier. Merkel cells:found in stratum basal layer of epidermis.Merkel cells are no dendritic, nonkeratinocytic epithelial cells located primarily in or near the basal layer of the epidermis. A few of these cells are also found in the dermis and portions of ectodermal derived mucosa.They are commonly found in innervated clusters around hair follicles. These cells are thought to function as slowly adapting mechanoreceptors that mediate the senses of touch and hair movement. Thick versus thin Skin: ❑ The terms thick and thin refer to the thickness of the epidermis. ❑ Most of the body is covered by thin skin. ❑ Thin skin contains hair follicles, sebaceous glands, and arrector pili muscles. ❑ The epidermis in thick skin may be six times thicker than the epidermis that covers the general body surface. ❑ Thick skin does not have hair, smooth muscles, or sebaceous glands but contains sweat glands. Thick skin on the palms of the hands, the fingertips, and soles of the feet may be covered by many layers of keratinized cells that have cornified. Major function of skin 1. Protection through the keratinized epidermis from abrasion and the entrance of pathogen 2. Impermeable to water ,owing to lipid layer in the epidermis 3. Body temperature regulation as a result of sweating and changes in the vessel diameters. 4. Sensory perception of touch ,pain ,pressure ,and temperature changes because of nerve endings 5. Excretions through sweat of water, sodium salts, urea, and nitrogenous waste 6. Formation of vitamin D from precursors molecules produced in the epidermis when exposed to the sun. Accessory structures The derivatives of the skin develop from the epidermis, but are located within the dermis include sweat glands, sebaceous glands, and mammary glands, hair, hair follicles, and nails. Derivatives of the skin- Hair Hairs provide a sensation of touch. A hair consists of three parts: Hair shaft- It projects from the skin and may extend slightly below the surface of the epidermis. Hair root- It is embedded within the dermis. Hair bulb- It contains hair papilla with capillary blood supply. Hair follicle is a structure enclosing the hair root, that is composed of epithelial and connective tissue sheath. The hair bulb and hair follicle are responsible for growth and elongation of hair. Derivatives of the skin- Sebaceous glands Sebaceous glands are branched glands that give out an oily secretion, which makes hair and the outer surface of the skin oily and waterproof. Common problems related to sebaceous glands are as follows: Pimples- Excess sebum accumulation causing bacterial growth. Acne- Inflammation of sebaceous gland. Black head- Enlargement of sebaceous gland. Derivatives of the skin- Sweat glands Sweat gland is a coiled tube that produce sweat consisting of a secretory part and an excretory part. The outer opening are called as sweat pores. They vary from 60-80 per sq cm on the back to 400 per sq cm on the palms of the hands. The secretory part of the sweat gland absorbs fluid and passes it to the excretory duct. Major function of sweating is to lose body heat by evaporation. Sweat consists of 99% of water, 0.2 to 0.5% salts and traces of urea (0.08%). Derivatives of the skin- Mammary glands The mammary (milk) glands are modified sweat glands, present in both male (rudimentary state) and female (breasts). The activity of the mammary glands is related to the reproductive hormones (prolactin) and pregnancy. Derivatives of the skin- Meibomian glands Meibomian gland is a modified sebaceous gland, present at the margin of the eyelids.It serves to lubricate and prevent overflow of tears. Albinism Albinism is a group of inherited disorders that results in little or no production of the pigment melanin, which determines the color of the skin, hair and eyes. Melanin also plays a role in the development of certain optical nerves, so all forms of albinism cause problems with the development and function of the eyes. Other symptoms can include light skin or changes in skin color; very white to brown hair; very light blue to brown eye color that may appear red in some light and may change with age; sensitivity to sun exposure; and increased risk of developing skin cancer.Albinism is caused by mutations in one of several genes, and most types are inherited in an autosomal recessive manner.Although there's no cure, people with the disorder can take steps to improve vision and avoid too much sun exposure.A defect in one of several genes that produce or distribute melanin causes albinism. The defect may result in the absence of melanin production, or a reduced amount of melanin production. pg. 10