Skin Structure & Development - PDF

Skin Dr Sapna Shevade October 4, 2024 www.gmu.ac.ae COLLEGE OF ALLIED HEALTH SEIENCES STRUCTURE OF THE SKIN ▪ It consists of two layers: 1. The epidermis, a superficial cellular layer (stratified squamous epithe...

Skin Dr Sapna Shevade October 4, 2024 www.gmu.ac.ae COLLEGE OF ALLIED HEALTH SEIENCES STRUCTURE OF THE SKIN ▪ It consists of two layers: 1. The epidermis, a superficial cellular layer (stratified squamous epithelium). 2. The dermis, a deep connective tissue layer. N.B : Hypodermis : is a layer of areolar and adipose connective tissue that lies deep to the dermis. It also called the subcutaneous layer. It is not a part of the skin. EPIDERMIS From deep to superficial they are the: Stratum basale Stratum spinosum Stratum granulosum Stratum lucidum Stratum corneum Epidermis The epidermis contains four major layers (thin skin) or five major layers (thick skin) Stratum basale (deepest layer) or stratum germinativum, where continuous cell division occurs which produces all the other layers Stratum spinosum, 8-10 layers of keratinocytes Stratum granulosum, which includes keratohyalin and lamellar granules Stratum lucidum is present only in thick skin (the skin of the fingertips, palms, and soles) Stratum corneum: composed of many sublayers of flat, dead keratinocytes called corneocytes or squames that are continuously shed and replaced by cells from deeper strata; constant friction can stimulate formation of a callus. Epidermis - It contains four major types of cells: Keratinocytes (90% of the cells) produce keratin which is a tough fibrous protein that provides protection Melanocytes: which produce the pigment melanin that protects against damage by ultraviolet radiation Langerhans cells: involved in immune responses, arise from red bone marrow Merkel cells: which function in the sensation of touch along with the adjacent tactile discs  Types of skin (Based on the thickness of epidermis) A. Thick Skin ▪Thick epidermis is found on the palms of the hands, the soles of the feet, and corresponding surfaces of the fingers and toes. ▪All five epidermal strata occur in thick skin. ▪None hairy (Hairless)  Thin Skin ▪ Thin epidermis covers most of the body. ▪ Has thin stratum corneum ▪ Lacks the stratum lucidum. ▪ Has only four specific layer  Dermis ▪ Is a dense layer of irregular connective tissue (collagen & elastic fibers) ▪ Variable in thickness ▪ Rich in nerves, blood vessels. ▪ The deep layer of the dermis contains hair follicles,with associated smooth arrector pili muscles and sebaceous & sweat glands. Hairs are filaments that arise from hair follicles (modified part of stratum corneum) Hair follicles are tubular invaginations of epidermis. Arrector pili smooth muscle, connects the follicle to the superficial part of the dermis Sebaceous glands ▪ Are holocrine glands (cells of gland rupture during secretion). ▪ Secrete sebum which prevent dryness  Sweat glands ▪ Each gland is a single long tube, with highly coiled end while the body is the secretory part. ▪ It is lined by cuboidal epithelium Typical Glands: are merocrine glands (watery secretion & secrete by exocytosis). Fascia Collection of connective tissue Superficial fascia Deep fascia  Lymphatic System Dr Sapna Shevade Assistant Professor of Anatomy October 4, 2024 www.gmu.ac.ae COLLEGE OF ALLIED HEALTH SEIENCES Definition: It is a combination of cells, tissues and organs that function to protect body from invasion and damage by foreign cells, microbes, viruses and parasites. Lymph :Colorless fluid (proteins, water, WBCs), derived from body tissues and circulate the lymphatic system Spleen 3L 27 L 30 L Types of lymphoid organs: Primary (CENTRAL) lymphoid organs: − The organs at which lymphoid cells undergo antigen dependent proliferation and differentiation e.g. 1. T lymphocytes in thymus. 2. B lymphocytes in bone marrow. Secondary (PREPHIRAL) lymphoid organs: – Where functional lymphocytes reach (final destination). 1. Lymph nodes 2. Spleen. 3. Tonsils. 4. Payer ‘s patches. 1. Thymus: – It is central lymphoid organ. – It is located behind sternum. Functions strictly in lymphocyte maturation – It is covered by thin capsule which send septa that divide the gland into lobules. – Each lobule is characterized by: a. Darkly stained peripheral cortex, which is composed of densely packed lymphocytes with little epithelial reticular cells. b. Lightly stained, central medulla, which is composed of epithelial reticular cells more than lymphocytes. Hassall's corpuscles: concentric layers of degenerated epithelial reticular cells, with unknown significance. – ERC secrete, thymosin hormone (stimulates the development of T lymphocytes).  2. Lymph Nodes: – Bean shaped structure. – Hilum: concave side through which arteries, nerves enter, and veins leave. –. – Each lymph node is covered by a connective tissue capsule. – From the capsule many septa arise. – The gland is divided into: 1. Cortex: Its outer part contain lymph nodules (B – lymphocytes) 2. Medulla: Its deeper part is rich in T- It contains cords of B – lymphocytes & is called lymphocytes & paracortex. plasma cells. 2. Medulla: It consists mainly of the medullary sinuses (MS) separated by intervening medullary cords (MC). spleen: − The spleen is covered by a connective tissue capsule. − From the capsule many trabeculae arise. − The spleen is composed of white pulp and red pulp. 1. The white pulphas: Splenic nodule (lymphoid nodule → B lymphocytes). ▪ Made up of splenic cords (all types ofWBCs) and blood 2. The red pulp: sinuses. 3. Tonsils: – Are Incompletely encapsulated lymphoid nodules. a. Palatine: located in oral cavity, covered by stratified squamous non keratinized epithelium; crypts and underlying connective tissue barrier. b. Pharyngeal: single , at midline, roof of naso -pharynx, covered by ciliated pseudo -stratified columnar epithelium, no crypts.  Development of Limbs October 4, 2024 www.gmu.ac.ae COLLEGE OF ALLIED HEALTH SEIENCES I. Development of cartilage, bone & muscle: Germinal layers of the embryo: 1. Ectoderm. 2. Endoderm. 3. Mesoderm. a. Paraxial mesoderm (somites). b. Intermediate mesoderm. c. Lateral mesoderm which is divides into somatic and splanchnic mesoderm. – Somatic mesoderm gives rise to cartilages, bones, joints and muscles of the limbs. Somites: A. Ventro - medial part which is called the sclerotome. B. Dorso - lateral part which is called dermatome. C. Middle part which is called myotome Sclerotome will form: i. The axial skeleton e.g. (vertebral column ,ribs, sternum and skull). ii. Annulus fibrosus of intervertebral disc. Myotome will form the striated muscles of the trunk, shoulder girdle & pelvic girdle. – The upper limb appears first, followed by the lower limb 1 - 2 days later. Upper limb buds develop opposite lower cervical segments & lower limb buds opposite lumbar and sacral segments. − Initially, the limb buds consist of a mesenchymal core derived from the somatic layer of lateral mesoderm that will form the bones and connective tissues of the limb, covered by a layer of ectoderm. − At the 6th week, the terminal portion of the limb buds becomes flattened to form the hand and footplates. − Then the mesenchymal tissue in the plates has condensed to form digital rays to form bones of digits. − The hand & footplates are separated from the proximal segment by a circular constriction. − Later, a second constriction divides the proximal portion into 2 segments. − During the 7th week of gestation, the limbs rotate in opposite directions. − Upper limbs rotate laterally through 90 degrees on the long axis (extensors are posterior). − Lower limbs rotate medially through 90 degrees (extensors are anterior). Congenital anomalies: 1. Amelia: is complete absence of a limb (suppression of limb bud development during the early part of the 4th week). 2. Meromelia: is partial absence of a limb (arrest or disturbance of the differentiation or growth of the limbs during the 5th week). Causes: genetic, chromosomal abnormalities (trisomy 18), environmental (thalidomide). Congenital clubfoot (Talipes equinovarus) – More in males. – The sole of the foot is turned medially, and the foot is inverted. – Cause: is multifactorial. References https://www.clinicalkey.com/#!/content/book/3-s2.0- B9780323393041000014?scrollTo=%23hl0001011 Marieb EN, Hoehn K. Human Anatomy and Physiology,9th Edition, Pearson; 2014 ISBN- 10:129-2026-499, ISBN-13: 978-129-2026-497