Smooth endoplasmic reticulum
Reticulum endoplasmicum nongranulosum
Also known as: SER, smooth ER, agranular ER, sarcoplasmic reticulum
A network of ribosome-free, anastomosing membrane tubules, continuous with the RER, specialised for lipid and steroid synthesis, drug detoxification, glycogen metabolism and Ca²⁺ storage.
Overview
The SER lacks ribosomes and is usually tubular rather than sheet-like. Its enzymes and functions differ greatly between cell types. In all cells it synthesises phospholipids and cholesterol for membranes. In steroid-secreting cells (adrenal cortex, Leydig cells, ovarian theca and corpus luteum) it carries several enzymes of steroid hormone synthesis, working together with mitochondria.
In hepatocytes the SER holds the cytochrome P450 monooxygenases and UDP-glucuronyltransferase that make drugs, toxins, bilirubin and alcohol more water-soluble for excretion (biotransformation). Repeated exposure (phenobarbital, alcohol, rifampicin) causes SER proliferation and enzyme induction — the cellular basis of drug tolerance and many drug interactions. Hepatocyte SER also contains glucose-6-phosphatase, which releases free glucose from glycogenolysis and gluconeogenesis into the blood.
In muscle the SER is specialised as the sarcoplasmic reticulum, a Ca²⁺ store that wraps each myofibril: Ca²⁺ is released through ryanodine receptors (RyR1) when the T-tubule depolarises and pumped back by SERCA ATPases, triggering contraction and relaxation. In most cells the ER is the main intracellular Ca²⁺ store, releasing Ca²⁺ through IP₃ receptors during signalling.
- Location
Cytoplasm; continuous with RER at transitional elements (ER exit sites). Abundant in hepatocytes, steroidogenic cells, muscle (sarcoplasmic reticulum) and intestinal absorptive cells (chylomicron lipid assembly).
Parts & subdivisions
- Specialised SER of muscle fibres; terminal cisternae flank T-tubules (forming triads in skeletal muscle).
- Transitional ER (ER exit sites)Ribosome-free regions of RER from which COPII vesicles bud toward the Golgi.
Anatomy
- Histology
Steroid-secreting cells have pale, eosinophilic, finely vacuolated ("foamy") cytoplasm owing to SER, lipid droplets and mitochondria with tubular cristae. The SER itself is identified only by EM.
Function
- Synthesis of phospholipids, cholesterol and steroid hormones.
- Detoxification and drug metabolism (cytochrome P450, glucuronidation).
- Glycogen metabolism — glucose-6-phosphatase (liver).
- Storage and regulated release of Ca²⁺.
- Re-esterification of absorbed lipids for chylomicron assembly (enterocytes).
Clinical significance
Malignant hyperthermia
Autosomal dominant RYR1 (or CACNA1S) mutations make the sarcoplasmic reticulum release Ca²⁺ uncontrollably on exposure to volatile anaesthetics or succinylcholine → rigidity, hypercapnia, hyperthermia, rhabdomyolysis. Treated with dantrolene, which blocks RyR1.
Von Gierke disease (GSD type Ia)
Deficiency of glucose-6-phosphatase in hepatocyte ER: severe fasting hypoglycaemia, lactic acidosis, hyperuricaemia, hyperlipidaemia and hepatomegaly from stored glycogen.
Enzyme induction
P450 inducers (rifampicin, carbamazepine, phenytoin, St John's wort, chronic alcohol) lower levels of drugs such as warfarin and oral contraceptives; inhibitors (macrolides, azoles, grapefruit juice) raise them.
Test yourself
Which organelle is most abundant in the cells of the adrenal cortex, Leydig cells and hepatocytes exposed to barbiturates?
- Rough endoplasmic reticulum
- Smooth endoplasmic reticulum
- Lysosomes
- Golgi apparatus
Answer: Smooth endoplasmic reticulum
SER synthesises steroid hormones and lipids and houses cytochrome P450 detoxification enzymes; drug exposure induces SER proliferation (and drug tolerance). Plasma cells and pancreatic acinar cells are instead rich in RER.
Compiled from standard anatomy and physiology references — see the Cells & Tissues sources. For education only; not medical advice.