Histology BMS (I) Skin (Part 1) PDF

HISTOLOGY BMS (I) Module Skin (Part 1) Prepared by Dr. Haitham Sewilam Lecturer of Histology Faculty of Medicine - Helwan University - Egypt  THE SKIN The skin is the outer covering of the body. Largest organ of the body whi...

HISTOLOGY BMS (I) Module Skin (Part 1) Prepared by Dr. Haitham Sewilam Lecturer of Histology Faculty of Medicine - Helwan University - Egypt  THE SKIN The skin is the outer covering of the body. Largest organ of the body which forms about 16% of body weight. Histological Structure: 1- The epidermis: (thin superficial layer) - Formed of stratified squamous keratinized epithelium. - Ectodermal in origin. 2- The dermis: (thick deep layer) - Formed of connective tissue. - Mesodermal in origin. Types of the skin: 1- Thick skin: Found in the palms and soles. 2- Thin skin: It covers the rest of the body. The Thick Skin I- The epidermis: It consists of 4 types of cells: A- Keratinocytes: (85 % of the cells). - They are responsible for keratin formation. - They form several layers, which are continuously shed at the top layer and repaired by cell division of deeper layers. - They are arranged in 4 layers. 1- Malpighian layer (stratum Malpighii): a- Basal cell layer or stratum germinativum: - Single layer of columnar cells resting on thick wavy basement membrane. - Have basal oval deeply stained nuclei with deep basophilic cytoplasm. b- Prickle cell layer or stratum spinosum: - Formed of 4-8 layers of polyhedral cells. - Have central rounded nuclei with less basophilic cytoplasm than that of the basal layer. - Cells appears connected by processes giving spinous appearance. 2- Granular layer or stratum granulosum or skin barrier layer: - Formed of 2-4 layers of flattened cells. - Have central flat nuclei with basophilic granules in their cytoplasm act as skin barrier. 3- Clear layer or stratum lucidum: - It appears as a thin wavy acidophilic line formed of dead non-nucleated flattened cells “ Scales”. 4- Horny layer or stratum corneum: - It consists of several layers of acidophilic horny scales called squames contains soft keratin with no nuclei. B- Melanocytes: - Found just under or in between the cells of the basal layer of the epidermis and at the basal parts of the hair follicles. - They are branched cells with rounded central nuclei. - Their cytoplasm contains granules containing melanin pigment with abundant rER, Golgi apparatus and mitochondria. C- Langerhans cells: - Found in the upper layer of the stratum spinosum. - They are branched cells with deeply stained nuclei. - They are considered as macrophage cells. D- Merkel’s cells: - Found at the basal layer of the epidermis. - Large cells with underlying sensory nerves and considered as receptors for touch sensations. II- The dermis: A- The papillary layer: (the thinner superficial layer) - It is called papillary because it forms dermal papillae which fit into the concavities on the under surface of the epithelium. - It is formed of loose C.T. - It is more cellular i.e. rich in all types of C.T cells. - It is more vascular i.e. rich in blood capillaries. B- The reticular layer: (the thicker deep layer) - It is called reticular because of its high content of interlacing collagen fibers forming a network. - It is formed of dense C.T. - It is less cellular i.e. with few C.T cells. - It is less vascular i.e. with few blood capillaries.  The Thin Skin It differs from the thick skin in the following: 1- Site: whole body areas excepts palms, soles. 2- Epidermis: a- Thickness: thinner. b- Prickle cell layer: 2-4 layers. c- Granular layer: thinner. d- Clear layer: absent. e- Horny layer: thinner. 3- Dermis: a- Dermal papillae: irregular, fewer and narrower. b- Sweat glands: less numerous. c- Hair follicles: present d- Sebaceous glands: present e- Arrector pili muscle: present