Histology 232ANAT Lecture Notes PDF

HISTOLOGY 232ANAT  Lecture Objectives Describe the histological features of: 1.Epidermis. 2.Dermis. Differences between thick and thin skin. Skin receptors.  It is formed of skin and its appendages. The skin is the largest organ in the body forming about 16% of total body weight....

HISTOLOGY 232ANAT  Lecture Objectives Describe the histological features of: 1.Epidermis. 2.Dermis. Differences between thick and thin skin. Skin receptors.  It is formed of skin and its appendages. The skin is the largest organ in the body forming about 16% of total body weight. It is composed of: 1.Epidermis: is the superficial avascular layer formed of stratified squamous keratinized epithelium. 2.Dermis: is the deep layer formed of C.T. Types: Skin is classified according to the thickness of epidermis into: 1- Thick skin: is found on palms and soles. is 2- Thin (Hairy) skin: is found on the rest of the body.  THICK SKIN EPIDERMIS: It is consists of: EPIDERMAL CELLS : 1- Keratinocytes: are 85% of the keratin forming cells and form several layers. 2- Melanocytes: are melanin producing cells. 3- Langerhans cells: are members of antigen presenting cells (2-8%). 4- Merkel's cells: are sensory receptor. KERATINOCYTES: arranged in five layers. A- Stratum Basale (Stratum Germinativum):single layer of columnar resting on the basal lamina and has intense mitotic activity. B- Stratum Spinosum (Prickle Cell Layer): 4-8 polyhedral cells. C- Stratum Granulosum (Granular layer): 3-5 layers of flattened polygonal cells, cytoplasm is filled with Keratohyalin granules. D-Stratum Lucidum (Clear layer): homogenous line separating granular and horney layers, formed of 1-5 rows of acidophilic dead cells (No organelles and nuclei) and filled with keratohyaline and eleidin granules. E- Stratum Corneum (Horney Layer): 15-20 layers of flattened acidophilic non nucleated cells whose cytoplasm is filled with soft keratin. In psoriasis, a common skin disease, there is an increase in the number of proliferating cells in the basal and prickle cell layers as well as a decrease in the cycle time of these cells.  2- MELANOCYTES 2 Present in between the cells of the stratum basale. They are derived from neural crest cells. They have long irregular branching processes. They synthesize melanin by the aid of tyrosinase enzyme. They are DOPA +ve. Melanophores: are melanin containing cells having it by phagocytosis, do not synthesis it. They have not tyrosinase enzyme thus they are DOPA –ve. Albinism: a hereditary inability of the melanocytes to synthesize melanin, is caused by disorders of tyrosinase activity. Vitiligo:occurs due to degeneration and disappearance of entire melanocytes causes a patchy loss of pigment in the skin disorder. 3- LANGERHANS CELLS They are star-shaped cells found mainly in the stratum spinosum. Represent 2-8% of the epidermal cells. They are antigen presenting cells responsible for binding, processing and presenting antigens to T lymphocytes. Derived from bone marrow. 4- MERKEL’S CELLS Free nerve endings form expanded terminal disks at the bases of Merkel’s cells. The cells serve as mechanoreceptors.  2) Dermis : A.Papillary layer: -Formed of areolar connective tissue rich in blood capillaries. -Contains touch receptors called Meissner’s corpuscles. B.Reticular layer: -Formed of a dense reticular and collagenous C.T. In old age, extensive cross-linking of collagen fibers, the loss of elastic fibers due to excessive exposure to the sun (solar elastosis) cause the skin to become more fragile and develop wrinkles. The flow of sebum is one of the reasons for the development of acne, a chronic inflammation of obstructed sebaceous glands common during and after puberty. Has few capillaries and connective tissue cells. Contains: Sweat glands, Pacinian corpuscles, Ruffini and Krause- end bulb.  Thin (hairy) Skin Covers the body except palms and soles. Has a thin epidermis, thin prickle cell layer, thin lucidum and horny layers. The dermal papillae are irregular. Contains hairs, sebaceous and sweat glands but these are not present in the skin of glans penis, clitoris and labia minora. Sweat Glands Of two types: merocrine and apocrine. 1. Merocrine sweat glands: Present all over the body except glans penis and nail beds. Lined with clear and dark cuboidal cells Secrete watery sweat. 2. Apocrine sweat glands: Present in the axilla, pubic and perianal regions. Lined with clear and dark cuboidal cells Secrete unpleasant viscous sweat which starts after puberty.  Hairs Two types of hairs are present Infantile (vellus hair). Adult (terminal hair). - Each hair is formed of shaft, a root and a hair follicle. Hair shaft is formed of: central medulla (keratinized cells), middle cortex (pigmented keratinized cells outer cuticle (scales of flat cells). 2. Root is surrounded by inner and outer sheaths of hair follicles. 3. Hair follicle surrounds the root of the hair and formed of inner and outer root sheath and C.T. sheath. Sebaceous Glands: are branched alveolar glands present in the skin and secrete sebum.  Difference between thick skin and thin skin Thick skin Thin (hairy) skin 1. It covers palms of hands and soles of feet. 1. It covers the whole surface of the body. 2. Thicker with thicker epidermis. 2. Thinner with thinner epidermis. 3. Thick Malpighian layer. 3. Thinner. 4. Thick granular layer. 4. Thinner (one layer). 5. There is a clear layer. 5. No clear layer. 6. Thick horny layer. 6. Thinner horny layer. 7. Numerous dermal papillae. 7. Fewer dermal papillae. 8. Dermis contains numerous sweat glands (merocrine)8. It contains fewer sweat glands 8. Dermis contains numerous sweat glands (merocrine)8. It contains fewer sweat glands (mero-& (mero-& apocrine). apocrine). 9. Dermis contains no hair follicles, no sebaceous glands and no arrector pili muscle. 9. It contains hair follicles, sebaceous glands and arrector pili muscles.