Cisternae are flattened, disc-like compartments found inside eukaryotic cells, serving as integral components of the internal membrane network. They are part of the endomembrane system, which manages the production, modification, and transport of cellular materials. Cisternae create distinct internal environments necessary for the sequential modification and sorting of proteins and lipids.
Defining the Structure of Cisternae
A cisterna is a sac bounded by a continuous membrane composed of a phospholipid bilayer. This membrane completely encloses a distinct internal volume known as the lumen, or cisternal space, which maintains a biochemical composition separate from the surrounding cytoplasm. The characteristic flattened, disc-like shape is a consistent physical trait across the various organelles where cisternae are found. This specific morphology is a structural advantage for promoting efficient cellular processes.
The large surface area generated by these flattened compartments maximizes the space available for embedding specific membrane-bound enzymes and transport proteins. The shape of the cisterna is not static but is actively maintained and influenced by the cell’s internal scaffolding, the cytoskeleton, and specific integral membrane proteins. Certain proteins can stabilize the high curvature found at the edges of these membrane sheets.
The Role of Cisternae in the Endoplasmic Reticulum
The endoplasmic reticulum (ER) contains an extensive, interconnected network of cisternae and tubules spread throughout the cytoplasm, often continuous with the outer nuclear membrane. In the rough ER (RER), cisternae appear as flattened membrane sheets clustered near the nucleus. The outer surface of RER cisternae features attached ribosomes, which synthesize proteins destined for secretion or membrane insertion.
As new polypeptide chains are synthesized, they are threaded directly into the cisternal lumen for modification. The lumen contains specialized chaperone proteins that assist with proper folding. RER cisternae also facilitate post-translational modifications, such as the initial addition of carbohydrate groups to form glycoproteins, preparing them for further processing.
The smooth ER (SER), in contrast, is primarily composed of fine, tubular membrane vesicles. SER cisternae are specialized for diverse metabolic activities, including the synthesis of lipids, cholesterol, and steroid hormones. In the liver, the SER plays a prominent role in detoxification, utilizing enzymes to convert lipid-soluble compounds, such as drugs and metabolic wastes, into water-soluble forms for easier excretion.
A specialized type of SER, known as the sarcoplasmic reticulum (SR), is adapted for its unique function within muscle cells. The SR cisternae are responsible for the storage and rapid, regulated release of calcium ions into the cytoplasm. This precise control over calcium concentration within the cisternal space is the mechanism that directly initiates and regulates muscle contraction.
The Functional Stack of Golgi Cisternae
The Golgi apparatus cisternae are organized into a polarized stack, known as a dictyosome. A typical Golgi stack may contain between four and eight separate cisternae, each exhibiting structural and functional differences. This structure is defined by three functionally distinct regions: the cis face, the medial cisternae, and the trans face.
The cis face is the entry point positioned closest to the ER, where proteins and lipids arrive via small transport vesicles. These vesicles fuse with the cis cisterna, delivering contents into the lumen to begin their journey. Materials then sequentially progress through the intervening medial cisternae, where biochemical modifications occur.
Each cisterna within the stack contains a unique set of enzymes, enabling a stepwise refinement of the cargo as it moves through the stack. This processing includes the modification of carbohydrate regions on glycoproteins. This distinct enzyme localization allows for the highly regulated maturation of molecules as they move from one cisterna to the next.
The trans face marks the exit point, often forming the trans Golgi network (TGN), which is primarily responsible for sorting the processed molecules. Modified proteins and lipids are packaged into new vesicles that bud off from the cisterna’s margins at this stage. This final sorting step directs the molecules to their final cellular destination, such as the cell membrane, specialized organelles like lysosomes, or for secretion outside the cell. The movement of cargo is often explained by the cisternal maturation model, which posits that cis cisternae physically progress and mature into medial and then trans cisternae over time.