Endothelium Health Dictionary

Drugs are usually administered distant to their site of action in the body; they must then pass across cell membranes to reach their site of action. For example, drugs given by mouth must pass across the gut membrane to enter the bloodstream and then pass through the endothelium of vessel walls to reach the site of action in the tissues. This process is called absorption and may depend on lipid di?usion, aqueous di?usion, active transport, or pinocytosis – a process in which a cell takes in small droplets of ?uid by cytoplasmic engulfment.... drug absorption

The eye is the sensory organ of sight. It is an elaborate photoreceptor detecting information, in the form of light, from the environment and transmitting this information by a series of electrochemical changes to the BRAIN. The visual cortex is the part of the brain that processes this information (i.e. the visual cortex is what ‘sees’ the environment). There are two eyes, each a roughly spherical hollow organ held within a bony cavity (the orbit). Each orbit is situated on the front of the skull, one on each side of the nose. The eye consists of an outer wall of three main layers and a central cavity divided into three.

The outer coat consists of the sclera and the cornea; their junction is called the limbus. SCLERA This is white, opaque, and constitutes the posterior ?ve-sixths of the outer coat. It is made of dense ?brous tissue. The sclera is visible anteriorly, between the eyelids, as the ‘white of the eye’. Posteriorly and anteriorly it is covered by Tenons capsule, which in turn is covered by transparent conjunctiva. There is a hole in the sclera through which nerve ?bres from the retina leave the eye in the optic nerve. Other smaller nerve ?bres and blood vessels also pass through the sclera at di?erent points. CORNEA This constitutes the transparent, colourless anterior one-sixth of the eye. It is transparent in order to allow light into the eye and is more steeply curved than the sclera. Viewed from in front, the cornea is roughly circular. Most of the focusing power of the eye is provided by the cornea (the lens acts as the ‘?ne adjustment’). It has an outer epithelium, a central stroma and an inner endothelium. The cornea is supplied with very ?ne nerve ?bres which make it exquisitely sensitive to pain. The central cornea has no blood supply – it relies mainly on aqueous humour for nutrition. Blood vessels and large nerve ?bres in the cornea would prevent light from entering the eye. LIMBUS is the junction between cornea and sclera. It contains the trabecular meshwork, a sieve-like structure through which aqueous humour leaves the eye.

The middle coat (uveal tract) consists of the choroid, ciliary body and iris. CHOROID A highly vascular sheet of tissue lining the posterior two-thirds of the sclera. The network of vessels provides the blood supply for the outer half of the retina. The blood supply of the choroid is derived from numerous ciliary vessels which pierce the sclera in front and behind. CILIARY BODY A ring of tissue extending 6 mm back from the anterior limitation of the sclera. The various muscles of the ciliary body by their contractions and relaxations are responsible for changing the shape of the lens during ACCOMMODATION. The ciliary body is lined by cells that secrete aqueous humour. Posteriorly, the ciliary body is continuous with the choroid; anteriorly it is continuous with the iris. IRIS A ?attened muscular diaphragm that is attached at its periphery to the ciliary body, and has a round central opening – the pupil. By contraction and relaxation of the muscles of the iris, the pupil can be dilated or constricted (dilated in the dark or when aroused; constricted in bright light and for close work). The iris forms a partial division between the anterior chamber and the posterior chamber of the eye. It lies in front of the lens and forms the back wall of the anterior chamber. The iris is visible from in front, through the transparent cornea, as the ‘coloured part of the eye’. The amount and distribution of iris pigment determine the colour of the iris. The pupil is merely a hole in the centre of the iris and appears black.

The inner layer The retina is a multilayered tissue (ten layers in all) which extends from the edges of the optic nerve to line the inner surface of the choroid up to the junction of ciliary body and choroid. Here the true retina ends at the ora serrata. The retina contains light-sensitive cells of two types: (i) cones – cells that operate at high and medium levels of illumination; they subserve ?ne discrimination of vision and colour vision; (ii) rods – cells that function best at low light intensity and subserve black-and-white vision.

The retina contains about 6 million cones and about 100 million rods. Information from them is conveyed by the nerve ?bres which are in the inner part of the retina, and leave the eye in the optic nerve. There are no photoreceptors at the optic disc (the point where the optic nerve leaves the eye) and therefore there is no light perception from this small area. The optic disc thus produces a physiological blind spot in the visual ?eld.

The retina can be subdivided into several areas: PERIPHERAL RETINA contains mainly rods and a few scattered cones. Visual acuity from this area is fairly coarse. MACULA LUTEA So-called because histologically it looks like a yellow spot. It occupies an area 4·5 mm in diameter lateral to the optic disc. This area of specialised retina can produce a high level of visual acuity. Cones are abundant here but there are few rods. FOVEA CENTRALIS A small central depression at the centre of the macula. Here the cones are tightly packed; rods are absent. It is responsible for the highest levels of visual acuity.

The chambers of the eye There are three: the anterior and posterior chambers, and the vitreous cavity. ANTERIOR CHAMBER Limited in front by the inner surface of the cornea, behind by the iris and pupil. It contains a transparent clear watery ?uid, the aqueous humour. This is constantly being produced by cells of the ciliary body and constantly drained away through the trabecular meshwork. The trabecular meshwork lies in the angle between the iris and inner surface of the cornea. POSTERIOR CHAMBER A narrow space between the iris and pupil in front and the lens behind. It too contains aqueous humour in transit from the ciliary epithelium to the anterior chamber, via the pupil. VITREOUS CAVITY The largest cavity of the eye. In front it is bounded by the lens and behind by the retina. It contains vitreous humour.

Lens Transparent, elastic and biconvex in cross-section, it lies behind the iris and in front of the vitreous cavity. Viewed from the front it is roughly circular and about 10 mm in diameter. The diameter and thickness of the lens vary with its accommodative state. The lens consists of: CAPSULE A thin transparent membrane surrounding the cortex and nucleus. CORTEX This comprises newly made lens ?bres that are relatively soft. It separates the capsule on the outside from the nucleus at the centre of the lens. NUCLEUS The dense central area of old lens ?bres that have become compacted by new lens ?bres laid down over them. ZONULE Numerous radially arranged ?bres attached between the ciliary body and the lens around its circumference. Tension in these zonular ?bres can be adjusted by the muscles of the ciliary body, thus changing the shape of the lens and altering its power of accommodation. VITREOUS HUMOUR A transparent jelly-like structure made up of a network of collagen ?bres suspended in a viscid ?uid. Its shape conforms to that of the vitreous cavity within which it is contained: that is, it is spherical except for a shallow concave depression on its anterior surface. The lens lies in this depression.

Eyelids These are multilayered curtains of tissue whose functions include spreading of the tear ?lm over the front of the eye to prevent desiccation; protection from injury or external irritation; and to some extent the control of light entering the eye. Each eye has an upper and lower lid which form an elliptical opening (the palpebral ?ssure) when the eyes are open. The lids meet at the medial canthus and lateral canthus respectively. The inner medial canthus is ?xed; the lateral canthus more mobile. An epicanthus is a fold of skin which covers the medial canthus in oriental races.

Each lid consists of several layers. From front to back they are: very thin skin; a sheet of muscle (orbicularis oculi, whose ?bres are concentric around the palpebral ?ssure and which produce closure of the eyelids); the orbital septum (modi?ed near the lid margin to form the tarsal plates); and ?nally, lining the back surface of the lid, the conjunctiva (known here as tarsal conjunctiva). At the free margin of each lid are the eyelashes, the openings of tear glands which lie within the lid, and the lacrimal punctum. Toward the medial edge of each lid is an elevation known as the papilla: the lacrimal punctum opens into this papilla. The punctum forms the open end of the cannaliculus, part of the tear-drainage mechanism.

Orbit The bony cavity within which the eye is held. The orbits lie one on either side of the nose, on the front of the skull. They a?ord considerable protection for the eye. Each is roughly pyramidal in shape, with the apex pointing backwards and the base forming the open anterior part of the orbit. The bone of the anterior orbital margin is thickened to protect the eye from injury. There are various openings into the posterior part of the orbit – namely the optic canal, which allows the optic nerve to leave the orbit en route for the brain, and the superior orbital and inferior orbital ?ssures, which allow passage of nerves and blood vessels to and from the orbit. The most important structures holding the eye within the orbit are the extra-ocular muscles, a suspensory ligament of connective tissue that forms a hammock on which the eye rests and which is slung between the medial and lateral walls of the orbit. Finally, the orbital septum, a sheet of connective tissue extending from the anterior margin of the orbit into the lids, helps keep the eye in place. A pad of fat ?lls in the orbit behind the eye and acts as a cushion for the eye.

Conjunctiva A transparent mucous membrane that extends from the limbus over the anterior sclera or ‘white of the eye’. This is the bulbar conjunctiva. The conjunctiva does not cover the cornea. Conjunctiva passes from the eye on to the inner surface of the eyelid at the fornices and is continuous with the tarsal conjunctiva. The semilunar fold is the vertical crescent of conjunctiva at the medial aspect of the palpebral ?ssure. The caruncle is a piece of modi?ed skin just within the inner canthus.

Eye muscles The extra-ocular muscles. There are six in all, the four rectus muscles (superior, inferior, medial and lateral rectus muscles) and two oblique muscles (superior and inferior oblique muscles). The muscles are attached at various points between the bony orbit and the eyeball. By their combined action they move the eye in horizontal and vertical gaze. They also produce torsional movement of the eye (i.e. clockwise or anticlockwise movements when viewed from the front).

Lacrimal apparatus There are two components: a tear-production system, namely the lacrimal gland and accessory lacrimal glands; and a drainage system.

Tears keep the front of the eye moist; they also contain nutrients and various components to protect the eye from infection. Crying results from excess tear production. The drainage system cannot cope with the excess and therefore tears over?ow on to the face. Newborn babies do not produce tears for the ?rst three months of life. LACRIMAL GLAND Located below a small depression in the bony roof of the orbit. Numerous tear ducts open from it into predominantly the upper lid. Accessory lacrimal glands are found in the conjunctiva and within the eyelids: the former open directly on to the surface of the conjunctiva; the latter on to the eyelid margin. LACRIMAL DRAINAGE SYSTEM This consists of: PUNCTUM An elevated opening toward the medial aspect of each lid. Each punctum opens into a canaliculus. CANALICULUS A ?ne tube-like structure run-ning within the lid, parallel to the lid margin. The canaliculi from upper and lower lid join to form a common canaliculus which opens into the lacrimal sac. LACRIMAL SAC A small sac on the side of the nose which opens into the nasolacrimal duct. During blinking, the sac sucks tears into itself from the canaliculus. Tears then drain by gravity down the nasolacrimal duct. NASOLACRIMAL DUCT A tubular structure which runs down through the wall of the nose and opens into the nasal cavity.

Visual pathway Light stimulates the rods and cones of the retina. Electrochemical messages are then passed to nerve ?bres in the retina and then via the optic nerve to the optic chiasm. Here information from the temporal (outer) half of each retina continues to the same side of the brain. Information from the nasal (inner) half of each retina crosses to the other side within the optic chiasm. The rearranged nerve ?bres then pass through the optic tract to the lateral geniculate body, then the optic radiation to reach the visual cortex in the occipital lobe of the brain.... eye

This may occur in adults as ACUTE RESPIRATORY DISTRESS SYNDROME (ARDS), or in newborn children, when it is also known as HYALINE MEMBRANE DISEASE. The adult syndrome consists of PULMONARY OEDEMA of non-cardiac origin. The process begins when tissue damage stimulates the autonomic nervous system, releases vasoactive substances, precipitates complement activation, and produces abnormalities of the clotting cascade – the serial process that leads to clotting of the blood (see COAGULATION). The activation of complement causes white cells to lodge in the pulmonary capillaries where they release substances which damage the pulmonary endothelium.

Respiratory distress syndrome is a complication of SHOCK, systemic SEPSIS and viral respiratory infections. It was ?rst described in 1967, and – despite advances with assisted ventilation

– remains a serious disease with a mortality of more than 50 per cent. The maintenance of adequate circulating blood volume, peripheral PERFUSION, acid-base balance and arterial oxygenation is important, and assisted ventilation should be instituted early.

In newborns the mechanism is diferent, being provoked by an inability of the lungs to manufacture SURFACTANT.... respiratory distress syndrome

a pathological condition of the cornea of the eye due to failure in the functioning of its endothelium. It results in corneal oedema, seen as small blisters in the cornea that cause blurring of vision. See Fuchs’ endothelial dystrophy.... bullous keratopathy

n. the transparent circular part of the front of the eyeball. It refracts the light entering the eye onto the lens, which then focuses it onto the retina. The cornea contains no blood vessels and it is extremely sensitive to pain. The innermost layer (corneal endothelium) consists of a single layer of cells that cannot regenerate. —corneal adj.... cornea

the membrane that forms the deepest layer of the *stroma of the cornea of the eye. The endothelium lies between it and the aqueous humour. [J. Descemet (1732–1810), French anatomist]... descemet’s membrane

n. any tumour arising from or resembling endothelium. It may arise from the linings of blood or lymph vessels (haemangioendothelioma and lymphangioendothelioma respectively); from the linings of the pleural cavity or the peritoneal cavity (see mesothelioma); or from the meninges (see meningioma).... endothelioma

n. the tissue that covers the external surface of the body and lines hollow structures (except blood and lymphatic vessels). It is derived from embryonic ectoderm and endoderm. Epithelial cells may be flat and scalelike (squamous), cuboidal, or columnar. The latter may bear cilia or brush borders or secrete mucus or other substances (see goblet cell). The cells rest on a common basement membrane, which separates epithelium from underlying *connective tissue. Epithelium may be either simple, consisting of a single layer of cells; stratified, consisting of several layers; or pseudostratified, in which the cells appear to be arranged in layers but in fact share a common basement membrane (see illustration). See also endothelium; mesothelium. —epithelial adj.... epithelium

a hereditary condition in which the corneal endothelium loses its functional ability, usually with age. It may result in thickening and swelling of the cornea (*bullous keratopathy) and thus affect vision. Cornea guttata, small whitish deposits of hyalin, are seen on the inner surface of the cornea and signify a reduced number of endothelial cells. A corneal transplantation (see keratoplasty) may become necessary in certain cases. [E. Fuchs (1851–1913), German ophthalmologist]... fuchs’ endothelial dystrophy

(corneal graft) n. an eye operation in which any diseased part of the cornea is replaced by clear corneal tissue from a donor. In penetrating keratoplasty all layers of the cornea are replaced, in lamellar keratoplasty only the superficial layers are replaced, and in Descemet’s stripping endothelial keratoplasty (DSEK) the endothelium only is replaced.... keratoplasty

a vertical linear deposit of brown pigment on the inner surface of the cornea (corneal endothelium), appearing in cases of pigment dispersion syndrome. [F. E. Krukenberg (1871–1946), German pathologist]... krukenberg’s spindle

a pathological process involved in *atherosclerosis of arteries and vein grafts, and in *restenosis, that may be in response to angioplasty and stent placement. Damage to the endothelium of the artery exposes the underlying smooth muscle cells in the *media to cytokines, growth factors, and other plasma components in the circulation, which results in loss of their contractile characteristics. These abnormal muscle cells migrate to the *intima, where they proliferate and eventually form a thick layer of tissue (neointima), which occludes the artery.... neointimal hyperplasia

n. a derivative of *prostaglandin H produced by the endothelium lining blood vessel walls. It inhibits blood clotting by preventing platelet aggregation and causes vasodilatation (compare thromboxane A2). *Epoprostenol is a pharmaceutical preparation of prostacyclin.... prostacyclin

A parasitic disease caused by four species of PLASMODIUM: P. falciparum, P. vivax, P. ovale, and P. malariae. Clinically, malaria is characterised by recurrent episodes of high fever, sometimes associated with RIGOR; enlargement of the SPLEEN is common. P. falciparum infection can also be associated with several serious – often fatal – complications (see below): although other species cause chronic disease, death is unusual.

During a bite by the female mosquito, one or more sporozoites – a stage in the life-cycle of the parasite – are injected into the human circulation; these are taken up by the hepatocytes (liver cells). Following division, merozoites (minute particles resulting from the division) are liberated into the bloodstream where they invade red blood cells. These in turn divide, releasing further merozoites. As merozoites are periodically liberated into the bloodstream, they cause the characteristic fevers, rigors, etc.

Malaria occurs in many tropical and subtropical countries; P. falciparum is, however, con?ned very largely to Africa, Asia and South America. Malaria is present in increasingly large areas; in addition, the parasites are developing resistance to various preventative and treatment drugs. The disease constitutes a signi?cant problem for travellers, who must obtain sound advice on chemoprophylaxis before embarking on tropical trips – especially to a rural area where intense transmission can occur. Transmission has also been recorded at airports, and following blood transfusion.

The World Health Organisation (WHO) has listed malaria as one of Europe’s top ten infectious diseases. In 1992, 20,000 cases were reported: this had risen to more than 200,000 by the late 1990s. The resurgence of malaria has been worldwide, in part the result of the development of resistant strains of the disease, and in part because many countries have failed (or been unable) to implement environmental measures to eliminate mosquitoes. Nearly 40 years ago the WHO forecast that by 1980 only four million people would be affected worldwide; now, at the beginning of the 21st century, around 500 million people a year are contracting malaria with about 3,000 people a day dying from the infection – as many as 70 per cent of them children under the age of ?ve, according to WHO ?gures. The apparently steady advance of global warming means that countries with temperate climates may well warm up su?ciently to enable malaria to become established as an ENDEMIC disease. In any case, the great increase in international air travel has exposed many more people to the risk of malaria, and infected individuals may not exhibit symptoms until they are back home. Doctors seeing a recent traveller with unexplained pyrexia and illness should consider the possibility of malarial infection.

Diagnosis is by demonstration of trophozoites – a stage in the parasite’s life-cycle that takes place in red blood cells – in thick/thin blood-?lms of peripheral blood. Serological tests are of value in deciding whether an individual has had a past infection, but are of no value in acute disease.

P. vivax and P. ovale infections cause less severe disease than P. falciparum (see below), although overall there are many clinical similarities; acute complications are unusual, but chronic ANAEMIA is often present. Primaquine is necessary to eliminate the exoerythrocytic cycle in the hepatocyte (liver cell).

P. falciparum Complications of P. falciparum infection include cerebral involvement (see BRAIN – Cerebrum), due to adhesion of immature trophozoites on to the cerebral vascular endothelium; these lead to a high death rate when inadequately treated. Renal involvement (frequently resulting from HAEMOGLOBINURIA), PULMONARY OEDEMA, HYPOTENSION, HYPOGLYCAEMIA, and complications in pregnancy are also important. In complicated disease, HAEMODIALYSIS and exchange TRANSFUSION have been used. No adequate controlled trial using the latter regimen has been carried out, however, and possible bene?ts must be weighed against numerous potential side-effects – for instance, the introduction of a wide range of infections, overload of the circulatory system with infused ?uids, and other complications.

P. malariae usually produces a chronic infection, and chronic renal disease (nephrotic syndrome) is an occasional sequel, especially in tropical Africa.

Gross SPLENOMEGALY (hyper-reactive malarious splenomegaly, or tropical splenomegaly syndrome) can complicate all four human Plasmodium spp. infections. The syndrome responds to long-term malarial chemoprophylaxis. BURKITT’S LYMPHOMA is found in geographical areas where malaria infection is endemic; the EPSTEIN BARR VIRUS is aetiologically involved.

Prophylaxis Malaria specialists in the United Kingdom have produced guidance for residents travelling to endemic areas for short stays. Drug choice takes account of:

risk of exposure to malaria;

extent of drug resistance;

e?cacy of recommended drugs and their side-effects;

criteria relevant to the individual (e.g. age, pregnancy, kidney or liver impairment). Personal protection against being bitten by

mosquitoes is essential. Permethrinimpregnated nets are an e?ective barrier, while skin barrier protection and vaporised insecticides are helpful. Lotions, sprays or roll-on applicators all containing diethyltoluamide (DEET) are safe and work when put on the skin. Their e?ect, however, lasts only for a few hours. Long sleeves and trousers should be worn after dark.

Drug prophylaxis should be started at least a week before travelling into countries where malaria is endemic (two or three weeks in the case of me?oquine). Drug treatment should be continued for at least four weeks after leaving endemic areas. Even if all recommended antimalarial programmes are followed, it is possible that malaria may occur any time up to three months afterwards. Medical advice should be sought if any illness develops. Chloroquine can be used as a prophylactic drug where the risk of resistant falciparum malaria is low; otherwise, me?oquine or proguanil hydrochloride should be used. Travellers to malaria-infested areas should seek expert advice on appropriate prophylactic treatment well before departing.

Treatment Various chemoprophylactic regimes are widely used. Those commmonly prescribed include: chloroquine + paludrine, me?oquine, and Maloprim (trimethoprim + dapsone); Fansidar (trimethoprim + sulphamethoxazole) has been shown to have signi?cant side-effects, especially when used in conjunction with chloroquine, and is now rarely used. No chemotherapeutic regimen is totally e?ective, so other preventive measures are again being used. These include people avoiding mosquito bites, covering exposed areas of the body between dusk and dawn, and using mosquito repellents.

Chemotherapy was for many years dominated by the synthetic agent chloroquine. However, with the widespread emergence of chloroquine-resistance, quinine is again being widely used. It is given intravenously in severe infections; the oral route is used subsequently and in minor cases. Other agents currently in use include me?oquine, halofantrine, doxycycline, and the artemesinin alkaloids (‘qinghaosu’).

Researchers are working on vaccines against malaria.... malaria

n. a large glycoprotein that acts as a host defence mechanism. In the plasma it induces phagocytosis and on the cell surface it induces protein linkage which is important in the formation of new epithelium in wound healing. It is also involved in platelet aggregation. It is concentrated in connective tissue and the endothelium of the capillaries and is a component of the extracellular matrix. In pregnancy, fetal fibronectin (fFN) is found in high concentrations in secretions from the cervix and vagina before fusion of the membranes occurs at around 21 weeks of gestation. Inflammation or trauma to the fetal–maternal surface after then causes secretion of fFN into the cervix and vagina. Vaginal swabs that detect fFN can be used to predict preterm birth between 22 and 34 weeks gestation.... fibronectin

n. (pl. synechiae) 1. an adhesion between the iris and another part of the eye. An anterior synechia is between the iris and the endothelium of the cornea or the part of the sclera that normally hides the extreme outer edge of the iris from view. A posterior synechia is between the iris and the lens. 2. a small adhesion that forms in the nose after surgery.... synechia