Medical terms, defined.

A drug-drug interaction (DDI) occurs when one drug alters the pharmacokinetics (absorption, distribution, metabolism, excretion) or pharmacodynamics (additive, synergistic, or antagonistic effects) of another, changing its efficacy or to...

Osteocytes are the most abundant cells in mature bone (~90-95%), derived from osteoblasts that become entombed in mineralized matrix within lacunae. They act as mechanosensors and orchestrators of bone remodeling, secreting sclerostin (i...

Adipocytes are the lipid-storing cells of adipose tissue, storing energy as triglycerides and acting as an endocrine organ (leptin, adiponectin). White adipocytes are unilocular and store energy. Brown adipocytes are multilocular, mitoch...

Kupffer cells are the specialized resident macrophages lining the hepatic sinusoids. They phagocytose gut-derived bacteria, endotoxin (LPS), senescent RBCs, and immune complexes, and they are part of the mononuclear phagocyte (reticuloen...

Chondrocytes are the only cell type in mature cartilage. They sit in lacunae and produce and maintain the extracellular matrix (type II collagen, aggrecan/proteoglycans). They derive from mesenchymal stem cells via chondroblasts. Cartila...

The dorsal root ganglion (DRG) is a cluster of pseudounipolar sensory neuron cell bodies located in the dorsal (posterior) root of each spinal nerve, within the intervertebral foramen. It relays peripheral sensory information (touch, pai...

The lumbosacral plexus is the network of nerves formed by the ventral rami of L1–S4 (often divided into the lumbar plexus, L1–L4, and the sacral plexus, L4–S4) that innervates the lower limb, pelvis, and perineum. Key branches include th...

Mesangial cells are specialized pericyte-like smooth muscle–type cells located between glomerular capillary loops that provide structural support, regulate glomerular filtration (via contractility), phagocytose immune complexes/debris, a...

Langerhans cells are dendritic antigen-presenting cells residing in the stratum spinosum of the epidermis. They derive from bone marrow (monocyte/yolk sac–fetal liver precursors), express CD1a, langerin (CD207), and MHC II, and contain B...

Paneth cells are secretory epithelial cells at the base of the small intestinal crypts of Lieberkühn (most abundant in the ileum). They contain eosinophilic granules with antimicrobial products (lysozyme, α-defensins/cryptdins, TNF-α) an...

Dendritic cells (DCs) are the most potent professional antigen-presenting cells (APCs) and the key link between innate and adaptive immunity. They capture antigen in peripheral tissues, migrate via lymphatics to regional lymph nodes, and...

Paneth cells are secretory epithelial cells at the base of the small intestinal crypts of Lieberkühn (most abundant in the ileum). They secrete antimicrobial peptides (defensins, lysozyme) and support the intestinal stem cell niche, cont...

Chief cells are pepsinogen-secreting (and gastric lipase-secreting) cells located in the base of gastric glands in the fundus and body of the stomach. They are basophilic due to abundant rough ER and have zymogen granules; secretion is s...

Goblet cells are unicellular, mucin-secreting columnar epithelial cells found interspersed in the respiratory tract and intestinal epithelium (increasing in number from duodenum to colon). They secrete mucus (MUC2 in the gut, MUC5AC/MUC5...

G cells are enteroendocrine cells located mainly in the gastric antrum (also duodenum) that secrete gastrin. Gastrin stimulates parietal cells to secrete HCl and ECL cells to release histamine, and it has trophic effects on gastric mucos...

Theca cells are ovarian follicular cells surrounding the granulosa cells; under LH stimulation they convert cholesterol to androgens (androstenedione, testosterone). Granulosa cells then use FSH-induced aromatase to convert these androge...

Alpha cells are glucagon-secreting endocrine cells located in the periphery of the pancreatic islets of Langerhans (~15-20% of islet cells). Glucagon raises blood glucose during fasting/hypoglycemia by stimulating hepatic glycogenolysis,...

Pinocytosis ("cell drinking") is a form of endocytosis in which the cell non-selectively engulfs extracellular fluid and dissolved solutes via small invaginations of the plasma membrane, forming small vesicles. It occurs constitutively i...

Beta cells are insulin-secreting endocrine cells located in the pancreatic islets of Langerhans (~60-70% of islet cells, concentrated in the core). They also co-secrete C-peptide and amylin (IAPP). Autoimmune destruction causes type 1 di...

A missense mutation is a point mutation (single nucleotide substitution) that changes a codon so it codes for a different amino acid. Effects range from silent-like (conservative substitution) to severe loss or gain of function, dependin...

A silent mutation is a point mutation (usually a base substitution in the third "wobble" position of a codon) that changes the codon but still codes for the same amino acid, so the protein sequence is unchanged. It results from the degen...

A nonsense mutation is a point mutation that changes a codon specifying an amino acid into a premature stop codon (UAA, UAG, or UGA). This produces a truncated, usually nonfunctional protein, and the mRNA is often degraded by nonsense-me...

A frameshift mutation is an insertion or deletion of nucleotides in a number not divisible by 3, which shifts the mRNA reading frame downstream of the mutation. It usually produces a completely altered amino acid sequence and often a pre...

The clotting (coagulation) cascade is a series of proteolytic activations of serine protease zymogens that ends in thrombin converting fibrinogen to fibrin, forming a stable clot after primary hemostasis (platelet plug). The intrinsic pa...

A dominant negative mutation produces an abnormal protein that not only is nonfunctional but also interferes with the function of the normal (wild-type) protein produced from the other allele. It is classically seen in multimeric protein...

The common pathway is the part of the coagulation cascade where the intrinsic (aPTT) and extrinsic (PT) pathways converge, starting with activation of Factor X and ending with thrombin-mediated conversion of fibrinogen to cross-linked fi...

An insertion mutation is the addition of one or more nucleotides into a DNA sequence. If the number inserted is not a multiple of 3, it causes a frameshift mutation, which alters every downstream codon and often creates a premature stop...

A splice site mutation alters the conserved sequences at intron-exon boundaries (5' GU donor, 3' AG acceptor) or branch point, disrupting normal pre-mRNA splicing. Results include exon skipping, intron retention, or use of cryptic splice...

A transversion is a point mutation in which a purine (A, G) is replaced by a pyrimidine (C, T), or vice versa. It is less common than a transition (purine↔purine or pyrimidine↔pyrimidine) but is more likely to change the amino acid, so i...

A deletion mutation is the loss of one or more nucleotides (or a larger chromosomal segment) from DNA. Deletions of a number of bases not divisible by 3 cause a frameshift mutation, usually producing a premature stop codon and a truncate...

A transition mutation is a point mutation in which a purine is replaced by another purine (A↔G) or a pyrimidine by another pyrimidine (C↔T). Transitions are more common than transversions (purine↔pyrimidine), often arise from spontaneous...

A chromosomal inversion is a structural rearrangement in which a chromosome segment breaks, flips 180°, and reattaches in reverse orientation. Paracentric inversions exclude the centromere; pericentric inversions include it. Carriers are...

A chromosomal translocation is the exchange or transfer of genetic material between nonhomologous chromosomes. It can be balanced (no net loss of material, usually phenotypically normal carrier) or unbalanced (gain or loss of material, c...

47,XXX (Trisomy X, "Triple X syndrome") is a sex chromosome aneuploidy in phenotypic females caused by an extra X chromosome, usually from maternal meiotic nondisjunction (risk rises with maternal age). Most are asymptomatic or mildly af...

Monosomy X (45,X) is the karyotype of Turner syndrome: complete or partial absence of one X chromosome in a phenotypic female. Classic features are short stature, streak ovaries with primary amenorrhea, webbed neck, shield chest, and lef...

Polyploidy is a chromosomal abnormality in which a cell or organism has more than two complete haploid sets of chromosomes (a multiple of n greater than 2n, e.g., triploidy 69,XXX/XXY/XYY; tetraploidy 92,XXXX/XXYY). It is usually lethal...

45,X (monosomy X) is the karyotype of Turner syndrome, the most common sex chromosome abnormality in phenotypic females. It causes short stature, streak ovaries with primary ovarian insufficiency, and a webbed neck. It is also associated...

Chromosomal duplication is a structural chromosomal abnormality in which a segment of a chromosome is present in extra copies, producing partial trisomy for that region. It arises from unequal crossing over, non-allelic homologous recomb...

47,XXY is the karyotype of Klinefelter syndrome, the most common sex chromosome disorder in males (~1/600 male births). It results from meiotic nondisjunction and presents with tall stature, small firm testes, hypogonadism, infertility (...

A chromosomal deletion is loss of a segment of a chromosome, producing partial monosomy for the genes in that region. It can be terminal or interstitial, arises from unequal crossover or breakage errors, and causes classic syndromes such...

A germline mutation is a heritable DNA alteration present in gametes (sperm or egg), so it is passed to offspring and found in every cell of the offspring's body (constitutional). It underlies inherited disorders and hereditary cancer sy...

A somatic mutation is an acquired change in the DNA of a non-germline (body) cell. It is not transmitted to offspring, but it can be passed to daughter cells, so it drives clonal expansion, cancer, and mosaicism. Most cancers arise from...

A primary bone tumor arises de novo from bone or bone-associated tissues (osteoid, cartilage, fibrous tissue, marrow, notochord), unlike metastatic tumors, which are far more common in adults. Major malignant types are osteosarcoma, chon...

Metastatic bone tumors (secondary bone cancers) are malignancies that spread to bone from a distant primary site and are far more common than primary bone tumors in adults. The most common primaries are prostate, breast, lung, kidney, an...

Primary (spontaneous) peritonitis is infection of the peritoneal cavity without an identifiable intra-abdominal source (no perforation or abscess). The classic form is spontaneous bacterial peritonitis (SBP) in cirrhotic patients with as...

Secondary peritonitis is peritoneal inflammation/infection resulting from a identifiable intra-abdominal source, typically perforation, ischemia/necrosis, or leakage of a hollow viscus (e.g., perforated ulcer, appendicitis, diverticuliti...

Latent syphilis is the stage of Treponema pallidum infection with positive serology (nontreponemal and treponemal tests) but no clinical signs or symptoms, and a normal CSF exam if checked. It is divided into early latent (acquired withi...

Primary herpes is the first clinical infection with HSV-1 or HSV-2 in a previously seronegative (HSV-naive) person. It is typically more severe and longer than recurrences, with painful vesicles/ulcers (gingivostomatitis or genital lesio...

Recurrent herpes is reactivation of latent HSV-1 or HSV-2 (or VZV as zoster) from sensory/autonomic ganglia, causing recurrent mucocutaneous vesicular lesions, usually milder and shorter than primary infection. Triggers include stress, f...

Bile salts are amphipathic, conjugated derivatives of cholesterol (cholate and chenodeoxycholate conjugated to glycine or taurine) synthesized in hepatocytes via the rate-limiting enzyme cholesterol 7α-hydroxylase (CYP7A1). They emulsify...