Medical terms, defined.

Arginase is a urea cycle enzyme that hydrolyzes arginine to ornithine and urea (the final step of the cycle, in the cytosol of hepatocytes). Deficiency (ARG1) causes hyperargininemia, a rare autosomal recessive disorder that presents wit...

Dipeptidyl peptidase-4 (DPP-4, CD26) is a serine protease that rapidly inactivates incretin hormones GLP-1 and GIP by cleaving N-terminal dipeptides. DPP-4 inhibitors (gliptins: sitagliptin, linagliptin, saxagliptin, alogliptin) prolong...

Cystathionine beta-synthase (CBS) is a vitamin B6 (pyridoxal phosphate)-dependent enzyme that condenses homocysteine + serine → cystathionine in the transsulfuration pathway. Deficiency causes classic homocystinuria (autosomal recessive)...

A silencer is a cis-acting DNA regulatory element that binds repressor proteins (trans-acting factors) to decrease or shut off transcription of a gene. It is the functional opposite of an enhancer and can act at a distance and independen...

Argininosuccinate lyase (ASL) is a urea cycle enzyme that cleaves argininosuccinate into arginine and fumarate in the cytosol. Deficiency causes argininosuccinic aciduria: hyperammonemia, elevated citrulline and argininosuccinate, and of...

An enhancer is a short, cis-acting DNA regulatory sequence that increases transcription of a target gene when bound by activator proteins (transcription factors). It can lie far upstream, downstream, or within introns of the gene, works...

An exon is a segment of a gene that is retained in mature mRNA after splicing and is typically expressed (coding sequence plus UTRs). Introns are removed by the spliceosome, and exons are joined together. Mutations affecting exons or spl...

The poly-A tail is a stretch of ~200 adenine nucleotides added to the 3' end of eukaryotic pre-mRNA after transcription. It protects mRNA from degradation, aids nuclear export, and enhances translation efficiency. It is added by poly-A p...

The smooth endoplasmic reticulum (SER) is a ribosome-free network of membranous tubules responsible for steroid hormone synthesis, lipid/phospholipid metabolism, detoxification of drugs and toxins (cytochrome P450 system), and calcium st...

The anticodon is a 3-nucleotide sequence on the anticodon loop of tRNA that base-pairs (antiparallel) with the complementary mRNA codon during translation at the ribosome. It ensures the correct amino acid is added to the growing polypep...

Chaperone has two high-yield meanings. (1) Molecular chaperones (e.g., heat shock proteins such as Hsp60/Hsp70, and calnexin/calreticulin in the ER) assist protein folding, prevent aggregation, and target misfolded proteins for refolding...

p53 (encoded by TP53 on chromosome 17p) is a tumor suppressor transcription factor called the "guardian of the genome." It induces G1/S cell cycle arrest (via p21), DNA repair, or apoptosis in response to DNA damage. Loss-of-function mut...

Rb most commonly refers to the retinoblastoma gene (RB1), a tumor suppressor on chromosome 13q14 encoding the pRb protein, which restricts the G1→S transition by binding E2F. Loss of both alleles (Knudson two-hit hypothesis) causes retin...

Chaperones are proteins (molecular chaperones) that assist the folding, refolding, and assembly of other proteins and prevent aggregation, especially under stress such as heat shock. In clinical ethics, a chaperone is a trained third par...

Heat shock proteins (HSPs) are a family of molecular chaperones (e.g., HSP70, HSP90, HSP60, small HSPs like αB-crystallin) upregulated in response to heat, hypoxia, oxidative stress, and other cellular stressors. They assist in folding o...

Coronal (frontal) refers to an anatomic plane that divides the body into anterior (ventral) and posterior (dorsal) portions, lying perpendicular to the sagittal plane and parallel to the forehead/face. The term also describes the coronal...

Src is the prototypical non-receptor (cytoplasmic) tyrosine kinase and the first proto-oncogene discovered (v-src from Rous sarcoma virus, a retrovirus causing sarcoma in chickens). It phosphorylates tyrosine residues on target proteins...

Ras is a small membrane-bound GTPase (G protein) that acts as a molecular switch in the receptor tyrosine kinase signaling pathway (RTK → Ras → Raf → MEK → ERK/MAPK), driving cell proliferation and survival. It is active when bound to GT...

MYC is a proto-oncogene encoding a nuclear transcription factor (basic helix-loop-helix leucine zipper) that drives cell-cycle progression, growth, and metabolism. Its classic association is the t(8;14) translocation in Burkitt lymphoma,...

Cyclin-dependent kinases (CDKs) are serine/threonine kinases that drive the cell cycle and are active only when bound to their regulatory cyclin partner. Active cyclin-CDK complexes phosphorylate target proteins (e.g., Rb) to promote cel...

Heat shock proteins (HSPs) are a family of molecular chaperones upregulated in response to cellular stress (heat, hypoxia, toxins, infection, ischemia). They assist protein folding, prevent aggregation of denatured proteins, and target i...

The lysosome is a membrane-bound organelle containing ~50 acid hydrolases (optimal pH ~4.5–5) that degrade macromolecules, damaged organelles, and phagocytosed material. Enzymes are tagged with mannose-6-phosphate in the Golgi for target...

Caudal is an anatomical directional term meaning toward the tail (inferior end) of the body. In humans, it is largely synonymous with "inferior" for the trunk, and it is the opposite of "cranial" (or "rostral" in the head/brain context).

The membranous urethra is the shortest (~1–2 cm) and least distensible part of the male urethra. It passes through the deep perineal pouch, surrounded by the external urethral sphincter (skeletal muscle, voluntary control, pudendal nerve...

The spongy (penile/cavernous) urethra is the longest part of the male urethra (~15 cm). It runs through the corpus spongiosum of the penis from the bulb to the external urethral meatus. It is the segment most often injured in straddle in...

The "urogenital diaphragm" is the traditional term for the deep perineal space structure in the anterior perineum (urogenital triangle), described as a triangular musculofascial sheet spanning the pubic arch that contains the deep transv...

Labioscrotal folds are paired embryonic swellings lateral to the urogenital folds that form the scrotum in males (under DHT influence) or the labia majora in females (absent DHT). They are part of the bipotential external genitalia, alon...

The tunica dartos is a thin layer of involuntary (smooth) muscle in the subcutaneous tissue of the scrotum, directly beneath the skin. It contracts in cold to wrinkle the scrotal skin and bring the testes closer to the body, and it relax...

The ductus (vas) deferens is a thick-walled muscular tube that transports sperm from the epididymis to the ejaculatory duct, derived from the mesonephric (Wolffian) duct. It travels in the spermatic cord, through the inguinal canal, and...

The ovarian ligament (utero-ovarian ligament) is a fibromuscular cord that connects the medial pole of the ovary to the lateral uterus, just inferior to the uterotubal junction. It is a remnant of the superior gubernaculum and lies withi...

The tunica albuginea is a dense, white, fibrous connective tissue capsule surrounding certain organs, most notably the testis, the corpora cavernosa of the penis, and the ovary. In the testis it forms the thick capsule that sends septa i...

Cowper glands (bulbourethral glands) are paired, pea-sized, mucus-secreting accessory glands of the male reproductive tract located in the deep perineal pouch. They secrete a clear, alkaline pre-ejaculate that lubricates and neutralizes...

The perimetrium is the outermost serosal layer of the uterus, a visceral peritoneum covering the fundus and posterior body (and part of the anterior body). It is continuous with the broad ligament laterally and overlies the myometrium. I...

Bartholin glands (greater vestibular glands) are paired mucus-secreting glands located at the 4 and 8 o'clock positions of the posterolateral vaginal introitus; they lubricate the vestibule during arousal. Duct obstruction causes a Barth...

The mons pubis is the rounded, fatty prominence overlying the pubic symphysis in females, covered by pubic hair after puberty. It is the anterior-most part of the vulva and is continuous with the labia majora posteriorly and the anterior...

The ejaculatory duct is a paired ~2 cm tube formed by the union of the vas deferens and the seminal vesicle duct. It passes through the prostate and empties into the prostatic urethra at the verumontanum (adjacent to the prostatic utricl...

The seminal vesicles are paired accessory glands posterior to the bladder that produce ~60-70% of seminal fluid volume, which is alkaline, fructose-rich, and contains prostaglandins and fibrinogen-like semenogelins. Each duct joins the i...

The vas deferens (ductus deferens) is a thick-walled muscular tube that carries sperm from the epididymis to the ejaculatory duct. It derives from the mesonephric (Wolffian) duct under testosterone influence and is the structure cut/liga...

The bulbourethral (Cowper) glands are paired, pea-sized accessory glands of the male reproductive tract located in the deep perineal pouch (within the urogenital diaphragm/external urethral sphincter region). They secrete a clear, alkali...

The uterosacral ligaments are thickened condensations of endopelvic fascia (part of the cardinal–uterosacral complex) that run from the posterior cervix/upper vagina (supravaginal cervix) to the sacrum (S2–S4). They provide Level I (apic...

The Skene glands (paraurethral glands, lesser vestibular glands) are paired glands flanking the distal female urethra that open near the external urethral orifice. They are the homologue of the male prostate, secrete mucus-like fluid for...

The zona pellucida is a glycoprotein shell surrounding the oocyte (and early embryo) that mediates species-specific sperm binding, triggers the acrosome reaction, and blocks polyspermy after fertilization (zona reaction). It is shed ("ha...

Decidua parietalis (decidua vera) is the portion of the pregnant endometrium lining the uterine cavity that is NOT in contact with the implantation site or the conceptus directly. It fuses with the decidua capsularis (and chorion laeve)...

The decidua capsularis is the portion of the endometrial decidua that overlies the implanted blastocyst, separating it from the uterine cavity. It is one of three decidual regions (basalis, capsularis, parietalis) and fuses with the deci...

The corona radiata is a fan-shaped sheet of white matter composed of ascending (thalamocortical) and descending (corticospinal, corticobulbar, corticopontine) projection fibers that radiate from/converge toward the internal capsule. Lacu...

The allantois is a small endoderm-lined diverticulum arising from the caudal wall of the yolk sac that extends into the connecting stalk during week 3 of development. Its proximal part becomes the urachus (median umbilical ligament) afte...

The blastocyst is the stage of preimplantation embryo (days ~5-6 post-fertilization) consisting of an outer trophoblast layer, an inner cell mass (embryoblast), and a fluid-filled blastocoel. It is the stage that implants into the endome...

The trophoblast is the outer cell layer of the blastocyst that forms the embryonic portion of the placenta and mediates implantation. It differentiates into the inner cytotrophoblast (mitotically active, stem-cell layer) and the outer sy...

The subscapularis is the largest and strongest rotator cuff muscle, originating on the subscapular fossa and inserting on the lesser tubercle of the humerus. It internally (medially) rotates and adducts the arm, and is innervated by the...

The rotator cuff is a group of four muscles (supraspinatus, infraspinatus, teres minor, subscapularis — "SITS") and their tendons that stabilize the glenohumeral joint by holding the humeral head in the glenoid fossa. Supraspinatus is th...