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How are cysts formed

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ADPKD is characterised by the growth of numerous cysts in the kidneys, and sometimes in other organs.

What is a cyst?

adpkd-cystFigure 1 Plastic-embedded 1micron section of a pre-dialysis ADPKD kidney cyst.

A cyst is a fluid-filled spherical structure lined by a single layer of tubular (epithelial) cells.

Where do cysts occur in the kidney?

Each normal kidney is composed of about a million functional units called nephrons. Each one of these segmented filtering unit structures has a glomerulus at one end attached to a long, convoluted, tubule. This tubule is lined by a single layer of specialised cells of 15 different types that carry out the complex fluid and ion reabsorption functions of the kidney (see figure 2).

Normal Kidney NephronFigure 2 Diagram of a normal nephron (left) comprised of a glomerulus (to filter the blood) attached to a long nephron with specialised regions.
(Wilson, P., New Engl. J. Med. 350: 151-164, 2004.)

In ADPKD, every segment of the nephron can develop cysts. These arise as out-pushings of the tube so that instead of a regular narrow tube, an expanded cystic portion is formed and soon pinches off from the nephron of origin (see figure 3).

ADPKD NephronFigure 3 During development of ADPKD nephrons, cystic outpushings can arise from every segment of the nephron. These close off from the nephron of origin early in the developmental process, rendering that nephron non-functional.
(Wilson, P., New Engl. J. Med. 350: 151-164, 2004.)

How do cysts enlarge?

Once cysts close off from the nephron and become separated, they continue to enlarge by increased cell proliferation, fluid secretion and changes to the extracellular matrix (tissue which supports the cells) surrounding the nephron.

How does cyst growth affect kidney function?

As more and more cysts are formed, fewer and fewer nephrons can function properly. Fortunately the kidney has a large functional reserve capacity and does not need to use all nephrons at once. Therefore, it is not until about 60% of nephrons have stopped functioning (due to cyst growth) that the kidneys begin to fail).

Where else can cysts form?

The most common site, other than the kidney, in which cysts form is the liver, due to excessive growth (proliferation) of the cells lining bile ducts. This causes the liver to enlarge. Although fortunately liver functionality is not usually impaired, this can lead to pain, breathlessness and reduced mobility.

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Disclaimer: This information is primarily for people in the UK. We have made every effort to ensure that the information we provide is correct and up to date. However, it is not a substitute for professional medical advice or a medical examination. We do not promote or recommend any treatment. We do not accept liability for any errors or omissions. Medical information, the law and government regulations change rapidly, so always consult your GP, pharmacist or other medical professional if you have any concerns or before starting any new treatment.

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