Sunday, 21 July 2013

The Lysosome - A Cell Organelle in Health and in Disease

A cell organelle is a structure within a cell that carries out a specific function (or functions) to keep the cell alive. A lysosome is an organelle that consists of a membrane-bound sac containing hydrolytic enzymes. These enzymes digest cell materials such as food particles, other molecules, parts of viruses and bacteria and old, damaged or unneeded organelles. An animal cell may contain hundreds of lysosomes. They perform an important role in recycling the cell's components and in breaking down materials that enter the cell.

Parts of an animal cell
Illustration by, public domain image
The description above refers to a "conventional" lysosome.
Researchers say that there are actually two types of lysosomes: conventional lysosomes and secretory lysosomes. In this article I'll describe conventional lysosomes.

Current research indicates that lysosome activity is far more complicated than was once thought. In addition, scientists have discovered that lysosomes are so important that they can cause a group of disorders known as lysosomal storage diseases or lysosome storage disorders when they malfunction. These diseases may be very serious.

Endocytosis, Endosomes and Lysosomes

In order to do its job, a conventional lysosome fuses with another organelle called an endosome. An endosome is a vesicle (sac) formed during the process of endocytosis.

In endocytosis, part of the outer covering of an animal cell, known as the cell membrane, forms an invagination or pocket as it surrounds material from outside the cell. The invagination eventually complete surrounds the material, breaks away from the cell membrane and enters the cell. The vesicle that forms is known as an endosome. The damage to the cell membrane caused by endosome formation is repaired.

Examples of Endocytosis
Illustration by Mariana Ruiz Villarreal, public domain image

The leading theory for lysosome action says that the lysosome and the endosome join to form a "hybrid" organelle. Within this hybrid organelle digestion occurs. The lysosome then separates from the endosome and joins with another endosome to repeat the digestion process. It's thought that the lysosome also fuses with vesicles that are carrying materials originating within the cell in order to digests their contents. 

Lysosomal Storage Disorders or Diseases

There are about forty lysosomal storage diseases. They are quite rare, but that's no comfort for someone suffering from one of the diseases. They're caused by a genetic error. As a result, digestive enzymes in the lysosomes are either absent or defective. Material that should be broken down collects in the cells.

Lysosomal storage diseases affect different parts of the body, and some have a more serious effect than others. Tay-Sachs disease is one example of a serious condition. The patient's lysosomes lack the enzyme necessary to break up compounds called gangliosides. As these compounds collect in nerve cells, they destroy them. Research into the causes and treatments for lysosomal storage diseases is ongoing, as well as research into genetic counseling.

The video below is a good summary of lysosomal enzyme function. It was created by the Lysosomal Storage Research Group at The Hospital For Sick Children in Toronto, Canada.

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