Histology - Important Notes

Hyaline Cartilage, H&E Stained Hyaline cartilage consists of small cells with blue staining nuclei embedded in an amorphous matrix. Note how the cells are separated from each other. The matrix does not stain very well, and in this case appears mostly a pale blue colour.

Lymph Node H&E Stained This image shows part of a lymph node. Most of the cells are small immature lymphocytes. Their nuclei stain dark blue but their cytoplasm is very small and can not be rersolved in this photograph.

Skeletal Muscle, H&E Stained This image shows skeletal muscle fibres cut in longitudinal section. The contractile proteins stain pink whereas the nuclei (oval in shape) stain blue and lie alongside the fibres. The cells are long and thin (fusiform) in shape.

Spinal Cord - Silver Stained Spinal cord stained with a silver stain showing a large motor neurone with a pale nucleus and 3 long processes. Parts of other cells and processes can be seen surrounding it.

Cilia This series of pictures are of cilia. They illustrate the arrangement of the micro-tubules into 9 peripheral doublets and a pair of central mictotubules. These microtubules are composed mainly of tubulin links together with dynein. These two proteins interact to cause bending of the cilia.

Columnar Epithelium of the Intestine This micrograph shows a row of enterocyte from the lining of the small intestine. These cells are columar in shape and thier uper surface (apical surface) shows a regular arrangement of fingerlike processes called microvilli. These increase the surface area of the cell, which is important for the process of absorbtion.

Endothelial Cell High magnification view of a capillary. Its wall is formed by a flattened (squamous) endothelial cell. This cells can be seen to contain many small pinocytotic vesicles. These transport fluid from one side of the cell to the other.

Thyroid Gland, H&E Stained This slide shows part of the thyroid gland. The cells are arranged as hollow spheres (follicles) in the centre of which is some amorphous pink-staining colloid. This colloid contains inert (storage) forms of the hormones (e.g. throxine) that are eventually secreted by the gland. Note the cells of the folliclesare cuboidal in shape .

Intestinal Cell Showing Microvilli This micrograph shows part of an gut enterocyte. The finger-like processes are microvilli on the apical surface of the cell. These greatly increase the surface area of the cells and facilitate the absorbtion of food-stuffs. Each of these microvilli contains a core of contractile filaments. The rest of the cells can be seen to contain many mitochondria..

Liver cell This micrograph shows part of a liver cell. The rough endoplasmic reticulum is studdied with small dark-staining ribosomes. The smooth endoplasmic reticulum appears as small circular vesicles in the bottom right-hand corner. Some mitochondria can be sen in the top left hand corner

Mesenteric Nerve This image shows fine unmyelinated nerves from the gut (there are no gut epithelial cells present). The nerves contain very many small membrane vesicles that transport neuro-tranmitter substances to the nerve endings (not evident). In between the axons are some fibres of collagen that can be seen to be finely banded.

Myelinate Nerve This image shows a transverse section through a myelinated nerve with some skeletel muscle below. It is part of a nerve that supplies the muscle. Within the nerve can be seen a number of darkstaining rings. These dark rings are myelin sheaths that surround the pale-staining nerve axons. The nuceli are those of Schwann cells that manufacture the myelin sheath. No nerve cell nuclei can be seen in this micrograph. At the bottom of the picture is some skeletal muscle

Nerve in Transverse Section This image shows several myelinated axons and unmyelinated axons cut in transverse section. The nuclei visible in the section are Schwann cell nuclei. Note how each spiral of myelin has a single axon at its core. In the case of the unmyelinated nerves however, several (2 to 6) axons can be seen embedded within the cytoplasm of the Schwann cells. Note that both myelinated and unmyelinated nerves have Schwann cells associated with them.

Nerve in Transverse Section This image shows several myelinated axons and unmyelinated axons cut in transverse section. The nuclei visible in the section are Schwann cell nuclei. Note how each spiral of myelin has a single axon at its core. In the case of the unmyelinated nerves however, several (2 to 6) axons can be seen embedded within the cytoplasm of the Schwann cells. Note that both myelinated and unmyelinated nerves have Schwann cells associated with them.

Secretory Cell This high magnification image shows part of a secretory cell. In the middle is a Golgi apparatus. This organelle is always well developed in secretory cells. There are also a number of dark staining secretory granules. The pale staining granule-like structure next to the Golgi apparatus is a condensing vacuole. This appears damaged but would normally mature into a fully formed secretion granule. Some rough endoplasmic reticulum and some small mitochnodria can also be seen in this picture. The surface of the cell (top right hand corner) is elaborately folded.

Skeletal Muscle in Longitudinal Section This microgrpah show skeletal (voluntary) muscle cut in longitudinal section. It shows alternating dark (A) and light (I) bands in register. In the middle of the I band is a dark Z line.

Skeletal Muscle in Transverse Section This microgrpah shows skeletal muscle myofibrils cut in transverse section. The polygonal shapes are myofibrils and within each it is possible to see thick and/or thin filaments. In a few places it is possible to see thick filaments surrounded by six thin filaments. The membrane vesicles between the myofibrils are part of the sacroplamsic reticulum

Tubules and sarcoplasmic reticulum This micrograph of skeletal muscle shows 3 sarcomeres. The ones on either side are sectioned thruogh the myofibrils whereas the one sandwiched in the middle has been slices to reveal the t tubules and sacroplasmic reticulum that surrounds each myofibril.

Bladder H&E stained The image shows a fold of the lining of the bladder. The epithelium lining the bladder is a specialised (urine-proof) epithelium. It appears to composed of many layers of cells but as the bladder fills with urine the epithelium is stretched and becomes thinner. However, the surface is always covered by a surface layer of multi-nucleate cells sometimes referred to as umbrella cells that are abel to resist the toxic deleterious effects of urine.

Bronchus H&E stained This image shows a pseudo-stratifide epithelium lining the bronchus of the lung. In places the epithelium appears to be composed of more than one layer of cells but when the brochus expands the surface streches out to form a single layer of cells.

Gall Bladder Epithelium, H&E stained This image shows a thin finger-like fold of the lining of the gall bladder. On the surface is a simple columnar epithelium consisiting of a single row of identical cells with oval shaped nuceli and pink cytoplasm..

Intestinal Villi, H&E stained This picture shows parts of 5 intestinal villi. The surfaces of each of these is covered by a simple columnar epithelium. This epithelium appears to have 2 main cell types, dark staining enterocytes (the majority) and pale staining goblet cells. Inside the villi (villus core) is a mixture of blood vessels, lymphatics and connective tissues.

Nasal Septum stained with Alcian Blue and H&E This picture show a bony septum from the nose. On each side (surface) is a simple cilated epithelium. The blue sataining cells are gobelt cells that are interspersed with the cilated cells. The cila are very fine and can only be seen in places.

Non-hairy Skin, H&E stained This image shows a purple staining stratified squamous epithlium with layers of pink-staining keratinised squames on top. The cells of the deeper purple staining layers are living cells whereas the surface squames that are full of keratin are dead. No nuclei are present in the surface squames.

Oesophagus, H&E stained This image shows a fold in the lining of the oresophagus. On each surafe is a statified squamous epithelium. The deeper layers of cells of this compound epithlium are round and purple staining but the surface layers are flattened and palely stained. All the cells of this epithelium are living and contain nuclei.

PAS Stained Intestinal Villi This image shows parts of 2 inmtestinal villi. The tissue has been stained by the PAS procedure (periodic acid Schiff)and with haematoxylin. The PAS procedure demonstrates the distribution of carbohydrates containing hexose sugars. Here the mucin containing goblet cells, the brush border of the enterocytes (absorbitive gut cells)and the basement membranes of all the tissues stain dark pink (magenta). The tissue has been counter stained with haematoxylin so the cell nuclei all stain blue.

Small Intesine H&E stained This image shows mainly smooth muscle from the wall of the small intestine. The outside surface of the intestine is covered in many places by a simple squamous epithelium (serosa). This serosa pervents the intestinses from sticking to one another and allows them to slide smoothly over each other. The serosal layer is composed of flattened cells with oval nuclei and here forms the uppermost layer of cells.

Sublingual Salivary Gland , H&E stained This image shows a duct inside the sublingual salivary gland. The duct lies in the middle of the picture and is lined by a cuboidal epithelium. The duct is branched and the cells have a clear pinkstaining cytoplasm and a round blue-staining nucleus. Surrounding the duct are many pale-staining mucus producing cells

Aorta stained with elastic stain This picture shows mainly the medial layer of the aorta. The dark brown lines are concentric sheets of elastic tissue that occupy the medial layer.

Aorta stained with H&E This picture shows mainly the medial layer of the aorta. The aorta is an elastic artery and the medial layer consists mainly of concentric sheets of elastic tissue. These sheets are perforated to permit the diffusion of oxygen and nutrients and the migration of cells though them.

Fatty connective tissue stained with H&E This picture shows of white fat permeated by capillary blood vesels. The triglyceride contained in the fat cells as a single large droplet is extracted duringg tissue processing, leaving only a hollow shell. The fat cells appear as large open circles with the occasional blue staining nucleus. The small capillaries can be seen to contain brightly stained red blood cells.

Liver stained with a silver stain This picture shows part of the liver. The black strands are fibres of reticulin (type III collagen) that form a delicate framework between the liver cells (poorly stained) to which the liver cells are loosely attached. Similar reticular meshworks are present in many parenchymatous organs including liver, lung, spleen and lymph nodes

Oesophagus stained with H&E This picture shows the lamina propria of the oesophagus. This layer is composed of loose connective tissue. Small blood vessels and fine strands of collagen or elastic tissue can beseen. The clear spaces between the fibres isoccupied by a highly hydrated jellylike substance rich in glycosaminoglycans (GAGS). The epithelium at the top and left hand side of the picture is a stratified squamous, non-keratinising epithelium.

Penis stained with H&E This picture shows part of a transverse section through the penis. The broad dark pink staining band in the centre of the picture is part of the capsule that surrounds one of the erectile compartments of the shaft of the penis. It is a good example of dense regularr connective tissue and is composed mainly of collagen. The inelastic properties of collagen means that when the blood vessels become engorged with blood the penis becomes erect.

Skin stained with H&E The slide shows the dermis of the skin of the scalp. This is a good example of dense irregular connective tissue. Braod strands of dark pink staining collagen fibres are clearly evident.

Tendon Plastic section stained with H&E showing part of a tendon. This is an example of dense regular connective tissue. Extracellular fibres of type I collagen are aligned parallel with one another to form a strong inelastic tendon. The fibroblasts that synthesis the tropocollagen subunits from which the tendon is constructed are sandwiched between the fibres. Their elongated nuclei can be clearly seen

Heart stained with H&E This picture shows cardiac muscle cells with some pale staining connective tissue is between. The cells appear rectangular in shape and have a prominent nucleus at their centre. The cells are joined end to end by intercalated discs (pale staining) to form fibres. Note how the fibres branch.

Intestine stained with H&E Picture showing parts of the two layers of smooth muscle that make up the muscularis externa of the intestine. The inner (circular) layer appears in the bottom right hand corner. The smooth muscle cells are cut longitudinally and can be seen to have cigar-shaped nuclei. The outer (longitudinal) layer runs diagonally across the picture and can be seen to contain a small arteriole and venule. The muscle cells of this layer are cut transversely and appear circular in outline. Those cells sectioned through their equator show round nuclear profiles.

Teased skeletal muscle fibres stained with iron haematoxylin This picture shows a number of whole skeletal muscle fibres stained with iron haematoxylin. The banding (striations)of the fibres can be clearly seen

Tongue stained with H&E This picture shows bundles of skeletal (voluntary) muscle cut in transverse section. The polygonal shaped pink blocks are muscle fibres with blue staining nuclei located at their periphery. These fibres are grouped together into fasciculi.

Tongue stained with H&E (2) This picture shows skeletal (voluntary) muscle fibres sectioned longitudinally. The muscle appears as separate fibres each of which is composed of numerous microfibrils. The nuclei lie at the periphery of the fibres. Each fibre is a syncitium derived from many fused myoblasts.

Dorsal root ganglion stained with H&E Dorsal root ganglion showing numerous mauve staining nerve cell bodies. The small cells surrounding the nerve cell bodies are satellite cells. A tract of pink staining nerve axons pass obliquely across the centre of the picture.

Dorsal root ganglion stained with H&E (2) Dorsal root gandglion showing large pseudo-unipolar nerve cell bodies and small dark blue staining satellite cells. At the bottom of the picture is a tract of pink staining axons.

Myelinated and unmyelinated axons An electron microscope image showing several myelinated and unmyelinated axons. The dark staining myelin sheaths each surround a single large axon. Many smaller unmelinated axons can be seen partially embedded in Schwann cell cytoplasm. A large Schwann cell nucleus is present lower right (A)

Nerve stained with H&E Large peripheral nerve bundle showing obliquely sectioned axons contained surrounded by perineurium (connective tissue).

Neuromuscular junction Electron microscope image of a neuro-muscular junction. A nerve ending containing several dark staining mitochondria and numerous small neurosecretory vesicles lies mid left. To the right is the muscle fibril with which it synapses.

Peripheral nerve stained with osmium tetroxide Part of a nerve bundle surrounded by pink staining perineurium. Numerous brown staining myelinated axons can be seen inside the nerve bundle, together with fine strands of connective tissue (endoneurium).

Peripheral nerve stained with osmium textroxide Peripheral nerve stained with osmium tetroxide. Axons cut longitudinally can be seen passing from side to side. Several of these axons show constrictions known as nodes of Ranvier.

Small nerve stained with H&E Small nerve surrounded by connective tissue. Individual myelinated axons can be seen within the nerve bundle. The blue staining nuclei within the nerve bundle are those of fibroblasts (flattened, endoneurium) or of Schwann cells (rounded). The bundle is enclosed by a connective tissue sheath of perineurium.

Small nerve stained with H&E Small nerve surrounded by connective tissue. Individual myelinated axons can be seen within the nerve bundle. The blue staining nuclei within the nerve bundle are those of fibroblasts (flattened, endoneurium) or of Schwann cells (rounded). The bundle is enclosed by a connective tissue sheath of perineurium. .

Respiratory Epithelium This type of epithelium lines the nose, nasopharynx, larynx. trachea, bonchi and bronchioles. It consists of a simple or pseudo-stratified columnar ciliated epithelium punctuated by goblet cells. The underlying connective tissue often contains a rich plexus of thin-walled blood vessels and additional sero-mucous (mixed) glands that secrete via ducts adding to the protective mucous layer that covers the surface. This mucous prevents dehydration of the epithelium and traps particulate matter that is eventually expelled from the system by the beating of the cilia. Cilia beat towards the throat and contaminated mucous is swallowed or expectorated.

Nose Except at its opening (hairy skin), the nose is lined by a simple ciliated respiratory epithelium. Three bony projections extend from the lateral walls of nose (concha) and these too are covered by a respiratory epithelium. A similar epithelium lines the nasal sinuses that connect with the nasal cavity. A rich plexus of thin-walled arterioles and venules lie deep to the epithelium (swell bodies). These are the source of many nose bleeds. They help to warm and humidify the inspired air.

Olfactory Epithelium A small area of epithelium in the roof of the nose, below the cribiform plate, is adapted for the detection of smells. Unmyelinated olfactory nerves pass through holes in the cribiform plate to connect to the olfactory bulb of the brian. The columnar pseudo-stratified epithelium of the olfactory mucosa consists of basal and sustentacular(supporting) cells. embedded in this are bipolar neurones whose dendritic processes reach as far as the apical surface. Deep to the epithelium are serous glands whose ducts open on to the epithelial surface. The watery secretion they produce acts as a solvent for odorous substances. They also irrigate the surface and help to refresh the epithelium.

Larynx The larynx is lined by a respiratory epithelium except over the vocal folds. The vocal folds are covered by a stratified squamous epithelium that can withstand the vibrations of the folds. The folds contain the free upper margin of a cone of elastic tissue (conus elasticus). This cone is tensioned by the voluntary (skeletal) muscle fibres of the vocalis muscle. Further manipulation of the folds is undertaken by the extrinsic muscles of the larynx including the crico-thyroid muscle illustrated here.

Trachea The trachea is lined by a pseudo-stratified cilated epithelium (respiratory epithelium). It is held open 12-15 incomplete ('C' shaped) rings of hyaline cartilage. The ends of the 'C' lie posteriorly, towards the oesophagus and are joined by smooth muscle (trachealis muscle). Deep to the epithelium are numerous sero-mucous (mixed) glands and lymph nodules.

Bronchus Bronchi are lined by a pseudo-stratified cilated epithelium (respiratory epithelium). They are held open by an external framework of hyaline cartilage. This often appears as isolated islands of cartilage in thin section. Smooth muscle underlies the epithelium Lymph nodules lie in the underlying connective tissue.

Part of the wall of a bronchus stained with H&E. To the left is part of the supporting plate of hyaline cartilage and the lumen is to the right. The bronchus is lined by a pseudo-stratified ciliated epithelium (respiratory epithelium).

Branches of the vascular tree - pulmonary arteries and veins usually lie close by.

Pulmonary Vessels Large pulmonary blood vessels accompany the bronchi. These have longitudinally running elastic fibres in their walls. Large pulmonary veins often have a clearly defined medial layer consisting of muscle and elastic fibres and a well marked internal elastic lamina. Smaller pulmonary veins that accompany the brochioles are difficult to distinguish from their equivalent arteries since they often have similar amounts of smooth muscle in their walls.

Lung tissue stained with H&E. This picture shows part of the wall of a large pulmonary vein. The wall contains a large number of pink staining elastic fibres.

Bronchiole Bronchioles divide repeatedly and are typically less than 1 mm in diameter. They are lined by a simple ciliated epithelium (respiratory epithelium). They are not supported by hyaline cartilage but display a prominent band of smooth muscle. Lymph nodules lie in the underlying connective tissue.

Picture of lung tissue stained with H&E showing a large bronchiole. Unlike bronchi, bronchioles have no cartilage in their walls and can be constricted by the smooth muscle in their walls.

Branches of the vascular tree - pulmonary veins and arteries usually lie close by.

Terminal and Respiratory Bronchioles The conducting system of the lungs (bronchial tree) ends with small diameter terminal bronchioles. In these bronchioles the lining is a simple cuboidal epithelium that is only sparsely ciliated but includes Clara cells whose precise role remains unresolved. A prominent sphincter-like ring of smooth muscle surrounds the airway at this point.

Picture of lung tissue showing a terminal bronchiole opening into an alveolar duct. The terminal bronchiole is the last part of the bronchial tree. Beyond this point gasseous exchange takes place.

Next come respiratory bronchioles. These are lined by a similar epithelium, but are usually larger in diameter than terminal bronchioles. They mark the beginning of the respiratory portion of the lung.

Alveolus Alveoli occupy most of the volume of the lung. These thin walled sacs have capillaries embedded in their walls. The walls also contain fibroblasts that produce a framework of reticulin (collagen III) and large amounts of elastic tissue. Adjoining alveoli that arise from the same terminal bronchiole are interconnected by pores that ensure equal inflation.

High magnification picture of lung alveoli stained with H&E. The walls of the alveolar sacs are composed of flattened capillary endothelial cells and Type I pneumocytes sandwiched together to form a thin air-blood barrier normally no more than 0.6 microns thick.

Air-Blood Barrier Air that is drawn into the alveoli comes in close contact with blood circulating in the capillaries. The two are separated by the air-blood barrier that is only 0.2 and 0.6 micrometres thick. The barrier consists of flattened Type I pneumocyte on the alveolar side and flattened capillary endothelial cell on the capillary side. The two cells are fused to an intervening basement membrane. The surface of the Type I pneumocytes is covered by a thin layer of lipid-rich surfactant.

Type II Pneumocytes Type II pneumocytes are globular cells that are interspersed with the flattened Type I pneumocytes of the alveolar wall. They are normally less numerous than the Type I cells. Evidence suggests that they are stem cells from which Type I pneumocytes arise. They synthesise, store and secrete surfactant, a phospholipidbased secretion that spreads out over the whole lining of the alveolus. It reduces surface tension within the alveolus, facilitating expansion and collapse of the air sacs during respiration. It also helps to prevent desiccation of the air sacs.

Alveolar Macrophages Macrophages roam freely within the alveoli phagocytosing particulate material and cell debris. These differentiate from monocytes that migrate from the blood stream. Other resident macrophages remain within the substance of the lung and engulf material that penetrates the alveolar wall.

High magnification picture of lung alveoli. Alveolar macrophages containing black injested dust particles can be clearly seen.

Blood Vessels The walls of arteries and veins have 3 histological layers, an intima, media and adventitia. The innermost layer is the intima; a layer of endothelial cells resting on loose connective tissue. The medial layer contains smooth muscle, elastic and collagen fibres in variable proportions. The outermost layer of adventitia contains densely packed collagen and elastic fibres. Usually the wall of an artery is much thicker than that of the accompanying vein.

Lymph Vessels Small lymphatics are similar in structure to capillaries. Larger ones have valves and a wall composed mainly of connective tissue. The lumen is devoid of blood but contains plasma and some white blood cells, mainly lymphocytes. It must be appreciated however that most lymphocytes migrate around the body via blood vessels and not via lymphatics.

Elastic Arteries Slide M76 is of an aorta. The aorta and great vessels close to the heart are elastic arteries. The medial layer of these vessels is thick and consists of alternating layers of smooth muscle cells and perforated sheets of elastic tissue. Elastic recoil of these vessels helps to smooth out the surge in blood coming from the heart and to drive blood around the coronary circulation. The walls of these vessels may have a blood supply (small vessels) of their own, the so-called vasa vasorum.

Muscular Arteries Slide M17 contains good examples of arteries and veins. In muscular arteries the medial layer is well defined and composed mainly of smooth muscle interspersed with fine strands of elastic tissue. The media is separated from the intima by a "wavy" internal elastic lamina - a perforated sheet of elastic tissue. The profile (outline) of muscular arteries is more regular and the lumen smaller than that of the accompanying vein.

Wall of a large muscular artery with its lumen to the right. Note the internal and external elastic laminae (blue) either side of the medial layer. A small nerve lies mid-left.

The adventitia consists of thick strands of collagen and elastic fibres and is separated from the media by an external elastic lamina.

Arterioles Arterioles are found within the substance of organs and tissues. They are arbitrarily defined as small diameter arterial vessels with fewer than 3 layers of smooth muscle in their medial layer. They often lack an internal elastic lamina. This allows them to be completely closed off. They are highly responsive to vasoactive stimuli allowing significant regulation of peripheral resistance. The adventitial layer is often poorly developed.

Capillaries Capillaries are small diameter vessels formed from endothelial cells together with thier underlying basement membrane. Contractile pericytes often lie alongside but do not form a complete layer. Most capillaries are continuous (closed) so that solutes must be transported across them. However, capillaries of the gut, kidney glomerulus and endocrine glands are fenestrated (open). This allows solutes to pass freely though macroscopic pores. Sinusoids are thin walled, large diameter vessels with a fenestrated endothelium. These have a poorly developed basement membrane but are supported by surrounding tissue. They commonly occur in the liver and spleen.

Endothelial Cells Endothelial cells are highly specialized cells that release a variety of vasoactive agents. They actively transport molecules across their cytoplasm. They influence the tone of the surrounding layers of muscle. They modulate blood coagulation. When activated they produce adhesion molecules that can influence lymphocyte and neutrophil migration.

Venules Small diameter post capillary vessels have an almost complete layer of pericytes or 1 or 2 layers of smooth muscle (muscular venules). Larger venules have a surrounding adventitial layer.

Veins Examine slide M17. The medial layer of a vein is thinner than that of the accompanying artery. The medial layer is composed of a mixture of smooth muscle and collagen fibres. Its boundaries with the intima and adventitia are often poorly defined. The profile (outline) of veins is irregular and the lumen is larger than that of the accompanying artery.

Veins Many veins contain flap-like valves composed of collagen fibres covered by a layer of endothelium. Valves are absent from the largest veins such as the jugular veins and the vena cavae. Large veins have a thick wall with a distinct intima separated from the media by an internal elastic lamina. In large veins the medial layer is composed of longitudinally running bundles of smooth muscle surrounded by collagen fibres.

Myocardium Slide M95. This is composed of branching chains of discrete cardiac myocytes joined end to end by complex junctions called intercalated discs. Like voluntary (skeletal muscle) these cells contain myofibrils with repeat sarcomeres that give rise to visible striations. Unlike voluntary muscle,the cells remain separate and contain a single centrally place nucleus. Cardiac muscle fibres showing striations and dark staining intercalated discs.

Unlike voluntary muscle the fibres branch giving a characteristic appearance.

Myocytes The myocytes of the atria are normally smaller than those of the ventricles due to differences in resistance and workload. Atrial myocytes, especially those of the right atrium contain small secretion granules close to their nucleus. These contain atrial natruretic hormone that is released when the cells are stretched excessively. This hormone increases the excretion of water and sodium and potassium ions by the kidney. It also inhibits renin secretion causing a lowering of blood pressure. Cardiac myocytes showing branched fibres and pale staining intercalated discs. Note the centrally placed nuclei.

Intercalated Discs. Individual myocytes are joined together by intercalated discs. These complex junctions contain desmosomes and adherent junctions that bind the cells together. They also contain gap junctions that allow the cells to become electrically coupled.

Diagram of an intercalated disc. These join cardiac myocytes together and include desmosomes, gap junctions and adherent junctions. These provide physical linkage and electrical coupling between the myocytes

Sino-atrial and Atrio-ventricular nodes These nodes are composed of very small, irregularly arranged myocytes adapted for impusle conduction rather than contraction. They are surrounded by numerous blood vessels and nerve plexuses. The SA node is located on the medial side of the superior vena cava at its junction with the right atrium. The AV node is located at the base of the inter-atrial septum, anterior to the opening of the cardiac sinus. Diagram of the conducting system of the heart. The SA node is located in the wall of the right atrium, adjacent to the superior vena cava. This is linked by poorly defined pathways to the AV node from which fast impulses are relayed to the apex of the heart via the right and left bundle branches.

There are no clearly defined anatomical fibres linking the SA and AV nodes although large conducting fibres (Purkinje fibres) arise from the right and left bundle branches.

Purkinje Fibres These are large modified muscle fibres lying mostly just beneath the endocardium in the interventricular septum. The cells have large vacuoles but have few myofibrils. Thus appear pale in H&E stained sections. They contain stores of glycogen that allows them to be easily visualised with the PAS staining procedure. Myocardium showing large pale-staining Purkinje Fibres.

Valves All 4 valves have a similar histological structure. They consist of thick collagen fibres with occasional strands of elastic tissue. Both surfaces are covered by a layer of endothelial cells. The free margins of the atrio-ventricular and mitral valves are connected to papillary muscles by fibrous chordae tendinae. Damage frequently take the form of excessive collagen (scar) deposition or calcification. This results in reduced flexibility and can lead to stenosis or incompetence.

Photograph showing the atrial surface of the tricuspid valve. The pin marks the posterior cusp.

Gut Tube The gastro-intestinal tract has 4 layers

• • • •

an innermost mucosa an underlying submucosa an external muscle coat (muscularis externa) a serosa The mucosa has 3 components, a lining epithelium, a layer of connective tissue (lamina propria) and a ring of smooth muscle (muscularis mucosa).

The submucosa is composed of loose connective tissue. Glands and lymphoid tissue are often contained within it. Many of the blood vessels and nerves that supply the gut run it this layer. The muscularis externa usually consists of 2 layers of smooth muscle whose peristaltic contractions help to propel foodstuffs through the alimentary tract. A serosal layer consisting of a simple squamous epithelium separates the gut tube from the peritoneal cavity.

Oesophagus This is lined by a thick stratified squamous non-keratinized epithelium. The submucosa contains many sero-mucous glands that help to lubricate the oesophagus. At the distal end of the oesophagus the submucosal layer contains many large, thin-walled veins. These may become the site of oesophageal varicosities. At the proximal end of the oesophagus the outermost layer of muscle (muscularis externa) is composed mainly of skeletal muscle but at the distal end it is composed of smooth muscle.

Stomach The stomach can be divided into 3 regions

1. cardiac region 2. body 3. pyloric region Each of these regions has a distinct histological appearance to its mucosal layer. The mucosa is thrown into macroscopic longitudinal folds known as rugae. The muscular coat (muscularis externa) which is 3 layers thick has an extra oblique layer of fibres closest to the mucosa that helps to churn up the food within the stomach.

Gastric Mucosa The stomach is lined by a simple columnar epithelium. The tall columnar cells produce a special acid-resistant mucin that protects the lining of the stomach. The surface epithelium also has deep invaginations (gastric pits) at the bottoms of which are the openings of the gastic glands. These glands occupy the lamina propria. Several tall straight or branched glands open into each gastric pit. The glands contain different types of secretory cell.

Low magnification picture of the stomach. The mucosa (top of picture) is thrown into folds (rugae). Below this are closely packed gastric glands.

Acid Producing Parietal (Oxyntic) Cells These cells are found in the gastric glands in the body of the stomach. They occur mostly in the upper parts of the glands, close to the surface epithelium. The cells are globular in shape and stain bright pink with H&E. The surface of these cells is deeply invaginated to form intracellular canaliculi and the cells contain many mitochondria. The cells are rich in the enzyme carbonic anhydrase. They secrete hydrochloric acid. They are also believed to be the source of intrinsic factor essential for the absorption of vitamin B12 High magnifaction picture of gastric glands from the body of the stomach. This slide has been specially stained and the parietal cells appear rounded and dark pink. The chief cells appear pale.

Regional Variations and Endocrine Cells The glands of the cardiac region and pylorus do not normally have acid-producing nor chief cells. Instead their cells produce mainly mucus (mucous neck cells). The glands of the pylorus do, however, contain cells that produce gastrin and a bombesin-like peptide. The glands of the stomach also contain cells that produce serotonin, somatostatin and vasoactive intestinal peptide (VIP). These pale staining cells are scattered throughout all of the glands of the stomach.

hief (Zymogenic) Cells The cells are pyramidal in shape and normally occupy the deeper parts of the glands. Part of their cytoplasm stains blue with H&E and the cells can be seen to contain secretion granules. They synthesise and secrete pepsinogen (pepsin) and lipases. Like the acid producing cells, these enzyme-secreting cells are found mostly in the glands in the body of the stomach and not in the glands of the cardiac or pyloric regions.

Villi The small intestine is thrown into macroscopic circular folds known as plicae circulares. These increase the surface area available for absorption. The surface area is further augmented by finger-like projections of the mucosa known as villi. Villi are covered by a simple columnar epithelium with 2 main cell types, enterocytes and goblet cells. Cells of the villi are short lived and are replaced by cells from tubular glands (crypts of Lieberkuhn) that lie between the villi. Each villus contains a small arteriole, a thin walled venule and a blind-ended lymphatic (lacteal). The venules and lacteals carry absorbed nutrients to the liver.

Duodenum The duodenum has a relatively smooth lining with few plicae circulares. The villi are broad leaf-like structures and the epithelium has relatively few goblet cells. The crypts contain stem cells for the replacement of villus cells and Paneth cells that secrete lysozyme, an enzyme important for the breakdown of bacterial cell walls. The submucosa of the duodenum is filled by a mucous secreting Brunner's gland. Its alkaline secretion helps to neutralise the chyme emerging from the stomach.

Jejunum The jejunum has close packed plicae circulares and the villi are long and narrow. Many goblet cells are present within the epithelium. The submucosa is normally broad and composed of loose connective tissue. Lymph nodules are present in the lamina propria but not normally in the submucosa. Jejunal villi stained with H&E. The villi are narrower Fingerlike) than those of the duodenum and there are more palestaining goblet cells in the epithelium.

Ileum Here the plicae circularis are not as closely packed as in the jejunum and the villi are not as tall. Goblet cell numbers increase towards the distal end. Large patches of lymphoid tissue (Peyer's patches) are present. These occupy not only the lamina propria but also penetrate the muscularis mucosa to occupy the submucosa.

Low magnification view of the ileum. Note the large, round, blue-staining lymph nodules within the submucosa.

Vermiform Appendix The vermiform appendix is lined by a simple columnar epithelium with goblet cells but with no villi and only a few rudimentary crypts. The lamina propria and submucosa are filled with lymphoid tissue. The amount of lymphoid tissue present is large in children but declines with age.

Low magnification picture of the appendix. Note that the surface epithelium is smooth and has no villi and that there are very few crypts. The lamina propria and submucosa are filled with lymphoid material.

Colon and Rectum There is very little macroscopic folding of the lining of the large bowl. Furthermore, there are no villi. The epithelium consists of close packed intestinal crypts occupied mainly by goblet cells. The muscularis external consists of a thickened inner circular layer of smooth muscle and an outer layer that is drawn up into 3 longitudinal bands known as taeniae coli.

Low magnification picture of the colon. The mucosa has numerous crypts but no villi. The crypts have large numbers of palestaining goblet cells.

Anus The anal canal in lined mainly by a stratified squamous keratinizing epithelium that arises abruptly from the simple columnar epithelium of the rectum. The submucosa contains fat and a prominent (internal) plexus of veins that may give rise to anal varicosities. The smooth muscle of the mucularis externa is thickened to form the internal anal sphincter and is surrounded by striated muscle of the external anal sphincter.

Low magification view of the anal canal. The epithelium is a stratfied squamous one. Deep to this is pale staining fatty tissue and muscle of the anal sphincter.

Overall Organisation The liver is divided into 4 macroscopic lobes (left, right, caudate, quadrate). Within these, the cells are organised into polygonal lobules separated from each other by delicate connective tissue septa. Each lobule consists of cords of tightly joined hepatocytes that radiate from the centre. Between the cords are thin walled sinusoidal blood vessels that bathe the hepatocytes in mixture of arterial and venous blood. Low magnification picture of a liver lobule showing cords of hepatocytes (liver cells) converging on a central vein.

The sinusoids are fed with blood by the terminal branches of the hepatic artery and hepatic portal vein that are distributed around the periphery of each lobule. Blood drains from each lobule via the central vein that coalesces with others to form the hepatic veins.

Biliary Canaliculi and Ducts A network of bile canaliculi (singular. canaliculus) run between the hepatocytes, in the plane of the cords. These are sealed channels between adjacent hepatocytes created by the presence of occluding junctions on either side. The hepatocytes secrete bile (bilirbubin and bile acids) into these channels (canaliculi).

Picture of liver showing a portal triad. These are located around the periphery of each lobule and contain three small vessels, an arteriole (branch of hepatic artery), a venule, here containing red blood cells (branch of the hepatic portal vein) and a intrahepatic bile duct lined by a cuboidal epithelium

The canaliculi coalesce and on leaving the lobule (at its periphery) drain into small intra-hepatic bile ducts that are lined by a simple cuboidal epithelium. These can be seen at the 'corners' of the lobules along with terminal branches of the hepatic artery and hepatic portal vein where the form a "portal triad".

Liver Sinusoids The sinusoids of the liver are thin walled fenestrated vessels. These vessels sit on a delicate meshwork of reticulin (collagen III fibres) and are separated from the cords of hepatocytes by the space of Disse, from which blood cells, but not plasma are normally excluded. Included within the lining endothelium are specialised macrophages known as Kupffer cells. One of their functions is the production of bilirubin that is taken up and excreted by the hepatocytes.

Gall Bladder and Biliary Tree The biliary tree is lined throughout by a cuboidal epithelium. In the smaller vessels it is a single layered (simple) epithelium but towards the duodenal papilla it often becomes stratified. The gall bladder is lined by a simple columnar epithelium cells of which are all of the same type. These cells have a poorly developed brush border (microvilli). They are adapted for the re-absorption of water and salts and thereby serve to concentrate the bile. Low magnification picture of the gall bladder. The lining is a simple columnar epithelium that is thrown into folds (not villi).

The epithelium, which sits on a submucosa of connective tissue, is thrown into folds. Outside of this is a smooth muscle coat. This contracts, particularly in response to CCK and causes bile to be expelled into the common bile duct. Concentrated and compacted bile products (gall stones) may be present in the lumen of the gall bladder.

Exocrine Pancreas The exocrine pancreas constitutes over 90% of the whole gland and is responsible for the synthesis and secretion of a wide range of digestive enzymes. These are discharged into the 2nd part of the duodenum. The endocrine pancreas that produces insulin, glucagon and other hormones is examined elsewhere. Low magnification picture of the pancreas. The exocrine cells are arranged into circular (spherical) clumps known as acini. The nuclei and cell cytoplasm at the periphery of each acinus stains dark blue but the secretion granules at the centre of each acinus stain bright pink. Blue staining nuclei can also be seen at the centres of the acini to the left of the picture. These are the nuclei of the bicarbonate-producing centro-acinar cells. Note also the small duct at the top of the picture.

The exocrine gland is a wholly serous gland whose cells each produce a mixture of enzymes that are packaged and released together. The fluid component of the secretion is mainly produced by the duct cells, including the proximal cells of the duct that arise at the centre of each acinus (centro-acinar cells).

Secretory Acinus The pyramidal cells are arranged into spherical clumps each with their apex pointing to the centre of the acinus. The cells have a large centrally placed nucleus and the cytoplasm is strongly polarised. The base of the cells (periphery of the acinus) is occupied by rough endoplasmic reticulum whereas the apex of the cells (centre of acinus) contains large strongly staining zymogen granules. Stimulation with CCK promotes the fusion of the granules with the apical membrane and the release of their contents by exocytosis. Stimulation with secretin, on the other hand, promotes the flow of abundant fluid, released mainly by the ducts.

Mouth This is lined mostly by a stratified squamous non-keratinizing epithelium but keratinized areas are common at sites of abrasion. It sits on a connective tissue layer (submucosa) containing collagen and elastic fibres. The deeper layers often contain skeletal muscle fibres that help to change the shape of the oral cavity. Many small blood vessels lie in the submucosa and fluid from these helps to keep the epithelium moist.

Low magnification picture of the lip showing the junction between skin - stratified squamous keratinizing epithelium to the left (outside of lip) and the lining of the mouth - stratified squamous non-keratizing epithelium to the right. Deep to the epithelia are connective tissue, blood vessels and bundles of muscle fibres.

There is an abrupt transition to stratified squamous keratinising epithelium at the margin of the lips.

Tongue The tongue is covered by a stratified squamous epithelium that remains non-keratinized on its ventral surface but is heavily keratinized on its dorsal surface. Inside, it is composed of coarse bundles of skeletal muscle fibres that run in many different directions. Some of these insert onto the jaw but others attach to the fibrous connective tissue underlying the mucosa (intrinsic fibres). Many mixed sero-mucous salivary glands are embedded within the substance of the tongue. So too are lymph nodules, particularly in the posterior 1/3rd of the tongue (e.g. lingual tonsils). Picture of the lower surface of the tongue showing a sgtratified squamous epithelium. Below this is connective tissue and glandular tissue from the sublingual salivary glands.

Papillae The dorsum of the tongue is thrown into complex folds known as papillae. Tall, pointed filiform papillae are the most common and cover the whole of the anterior 2/3rd of the tongue, producing a rasp-like surface. Less numerous mushroom shaped fungiform papillae are found at the tip and sides of the tongue. Pale-staining taste buds are embedded in the epithelium on their undersides of these papillae. Here they are protected from the oral cavity.

High magnification picture of part of the surface of the tongue showing small pale-staining taste buds embedded in the surface epithelium.

A V-shaped row of dome-shaped circumvallate papillae separate the anterior 2/3rd and the posterior 1/3rd on the tongue's surface. These too bear taste buds.

Parotid Salivary Glands These wholly serous glands lie anterior to the ear on the side of the face and enter the mouth via ducts adjacent to the 1st upper molar teeth. They are penetrated but not innervated by branches of the facial nerve and have a large lymph node contained within their capsule. The secretory cells produce mainly digestive enzymes and fluids.

Part of the parotid salivary gland. This is a wholly serous gland. The secretory acini are all of the same kind. This picture shows a number of circular clumps of secretory cells. The secretion granules stored in the cells stain dark pink.

The secretory cells are organised into spherical clumps called acini (singular, acinus) that show all the characteristics of protein secreting cells. The cells are pyramidal in shape with a spherical nucleus. The basal cytoplasm is filled with rough endoplasmic reticulum and prominent secretion granules lie at the apex of the cell. The acini drain into ducts lined by a simple cuboidal epithelium that may become stratified at its distal end.

Sublingual Salivary Glands These lie in the floor of the mouth, beneath the tongue and discharge by a series of separate ducts either side of the frenulum of the tongue. These branched tubular-acinar glands are almost wholly mucous secreting. The swollen cells are filled with pale staining mucus granules and the oval nucleus is squashed to the base of the cell. Part of sublingual salivary gland. This is a wholly mucous secreting gland and the picture shows pale staining secretory cells and a pinker staining duct with a simple cuboidal epithelium to the right. Note that the secretory cells have characteristic flattened nuclei that are pushed to the base of the cells.

Secretion from these glands drain into ducts lined by a simple or stratified cuboidal epithelium.

Submandibular Glands These well-defined globular glands lie subcutaneously, below the mandible, either side of the tongue. They discharge by a pair of ducts on to the frenulum of the tongue. This branched tubulo-acinar gland is often infiltrated by fat cells. The enzyme secreting serous cells form acini or lie at the closed ends of the tubules where they form characteristic crescentshaped "demilunes". The pale-staining mucous secreting cells line the tubular portions of the glands and drain into a prominent system of ducts lined by a simple or stratified cuboidal epithelium.

Picture of the submandibular salivary gland. This is a mixed, seromucous gland. The lower half of the picture shows mostly serous acini but at the top of the picture some pale-staining tubular mucous glands (middle and right) and a duct (left) can be seen.

Like the parotid, this gland is a major source of epidermal growth factor that promotes the growth of the epithelium of the tract.