The skin has three main layers:

2. The skin has three main layers: • Epidermis - a self-regenerating stratified squamous epithelium which produces a surface layer of the protein, keratin, which is the component of skin in direct contact with external environment. • Dermis - a layer of fibrocollagenous and elastic tissue which contains blood vessels, nerves and sensory receptors. • Subcutis or hypodermis - the deepest layer of skin which is mainly adipose tissue, but also contains the larger vessels which supply and drain the dermal blood vasculature. • skin appendages, specialised structures such as hair follicles, sweat glands and sebaceous glands which arise as downgrowths into the dermis from the epidermis during embryological development. These skin appendages mainly occupy the dermis and, occasionally, the upper subcutis.

4. basal layer (stratum basale) B is the layer responsible, by repeated mitotic division, for the constant regeneration of the other layers of the epidermis. The cells are arranged as a regimented single layer of cuboidal or low columnar cells which connect to the cells of the prickle cell layer • The prickle cell layer (stratum spinosum) is composed of polyhedral keratinocytes with large pale staining nuclei and prominent nucleoli, an indicator of their active protein synthetic function • The cells of the granular layer (stratum granulosum) G begin to lose their polyhedral shape and become progressively more flattened nearer the surface • The keratin layer (stratum corneum) Cis normally composed of flat flakes and sheets of keratin, coated with an anti-wetting agent synthesised by the cells of the granular layer.

8. The main functional cell in the liver is a form of epithelial cell called the hepatocyte. These cells are arranged as thin plates separated by fine vascular sinusoids through which blood flows. The close association of liver cells and the circulation allows absorption of nutrients from digestion as well as secretion of products into the blood. • The outer surface of the liver is covered by a capsule composed of collagenous tissue C called Glisson's capsule over which is a layer of mesothelial cells M from the peritoneum. • The sinusoids can just be seen as pale-stained spaces between the plates of liver cells. The hepatic sinusoids form a very low-resistance system of vascular channels that allows blood to come into contact with the hepatocytes over a huge surface area.

11. Hepatocytes are large polyhedral cells with round nuclei with peripherally dispersed chromatin and prominent nucleoli. • A network of bile canaliculi is located within each plate of hepatocytes but these are far too small to be seen at this magnification. These drain into bile collecting ducts lined by simple cuboidal or columnar epithelium, known as the canals of Hering, which in turn drain into the bile ductulesB.

13. The structural integrity of the liver is maintained by a delicate meshwork of extracellular matrix in the form of a fine meshwork of reticulinfibres (collagen type III). • The reticulin meshwork supports both the hepatocytes and the sinusoidal lining cells (endothelial cells). These micrographs have both been stained by a silver method that shows reticulin as a black stained material.

15. The gall bladder is a muscular sac lined by a simple columnar epithelium; it has a capacity of about 100 mL in humans. • The presence of lipid in the duodenum promotes the secretion of the hormone cholecystokinin-pancreozymin (CCK) by neuroendocrine cells of the duodenal mucosa, stimulating contraction of the gall bladder and forcing bile into the duodenum

17. CholelithiasisBody_ID: B015003 Abnormal concentration and precipitation of the constituents of bile may form stones (calculi) within the gall bladder or the extrahepaticbiliary system. • If the gall bladder is affected by stones then it may become inflamed leading to pain (chronic cholecystitis). The term cholelithiasis is used to refer to formation of stones within the biliary system.

18. Many lymphocytes are, however, located in encapsulated, highly organised structures called lymph nodes, which are interposed along the larger regional vessels of the lymph vascular system. Lymph nodes tend to occur in groups, particularly in areas where the lymphatics converge to form larger trunks as in the neck, axillae, groins, lung hila and para-aortic areas.

21. lymph node is surrounded by a collagenouscapsule from which trabeculae extend for a variable distance into the substance of the node. • Afferent lymphatic vessels divide into several branches outside the node, then pierce the capsule to drain into a narrow space called the subcapsular sinus that encircles the node beneath the capsule. • The dominant feature of the medulla is the network of broad interconnected lymphatic channels called medullary sinuses that converge upon the hilum in the concavity of the node. Lymph drains from the hilum into one or more efferent lymphatic vessels

22. The cortex consists of densely-packed lymphocytes. Cellular medullary cords project into the medulla between the medullary sinuses. In the outer cortex, lymphocytes form a variable number of densely packed lymphoid follicles, many of which show less dense germinal centres. The deep cortex (paracortical zone) is devoid of lymphoid follicles.