Lysosomes: Cell Digestion and Recycling Centers
Lysosomes: Cell Digestion and Recycling
Lysosomes are a set of structures whose function is to digest and lyse materials captured by the cell, including waste substances. These structures could be considered the digestive system of the cell. They are cytoplasmic vesicles, offering great polymorphism, surrounded by a unit membrane and a granular matrix that is dense and heterogeneous. Lysosomes act as digestive vacuoles and are present in all eukaryotic cells. The contents of the lysosome have an acidic pH. The set of enzymes present is variable and depends on the functional specialization of each cell. Identification based on morphological criteria is not enough; membrane markers (Lamp-1 and Lamp-2) or contained lysosomal markers (cathepsin L) must be included.
Composition
The lipid fraction consists mainly of phospholipids. The internal protein layer has about thirty different proteins with molecular weights ranging from 20 to 200 kD; most are glycoproteins. Transmembrane glycoprotein enzymes, such as acid phosphatase, are present. The membrane of lysosomes contains Lamp-1 and Lamp-2, which are lysosomal markers.
Function
- Heterophagy: This consists of the degradation of products captured from outside the cell, either by phagocytosis or endocytosis.
- Phagosomes: These vacuoles are formed as a result of phagocytosis, by which macrophages or leukocytes introduce extraneous fragments of necrotic cells. Depending on the cells involved, the following functions are performed:
- Defense
- Cell turnover
- Protein absorption
- Glycoprotein catabolism
- Catabolism of cholesterol
- Secretion of thyroid hormones
- Hormone processing
- Optical disc renewal
- Autophagy: This is a mechanism by which the cell is released from its damaged or non-useful organelles, or those that are in high proportions. Depending on the cells involved, the following functions are carried out:
- Renewal of organelles
- Differentiation
- Reversible cell damage
- Protein turnover
- Cell fasting
- Crinophagy: A special mechanism of autophagy that occurs in secretory endocrine cells, such as the pituitary gland, when there is an excess of secretion granules. It is done by autophagy, and other granules fuse directly with lysosomes.
Discharge of Contents Outside the Cell
Some cell types are able to secrete the lysosomal contents into the extracellular environment where hydrolytic enzymes exert their function, e.g., osteoclasts (bone tissue).
- Defense: White blood cells have the ability to secrete lysosomal enzymes.
- Fertilization: The acrosome of sperm is a specialized lysosome, and its contents are poured into the surrounding medium when it comes into contact with the egg, favoring its passage through the ovular membranes because it promotes the degradation of this membrane. The egg, once fertilized, releases its enzymes and destroys the receptors, preventing more sperm from entering.
- Storage: Lysosomal organelles may also store materials. This occurs in certain vegetables, forming structures called aleurone bodies.
Congenital Pathobiology
The membrane, content, or function of lysosomes can be altered. Examples include:
- Chédiak-Higashi disease
- Type I cell disease
- Accumulation or Thesaurismosis Disease: Clinical manifestations include tachypnea and cough associated with heart problems. Death occurs late in the first year of life. Anatomically, cardiomegaly, sometimes hepatomegaly, muscular hypotonia, hypertrophy, and macroglossia are noted.
Acquired Pathobiology
- Pneumoconiosis
- Gout
- Altered Function: A group of microorganisms that enter the cell can alter the normal functioning of the lysosomal complex.