Medical terms, defined.

The second pharyngeal pouch (endoderm-lined) gives rise to the epithelial lining of the palatine tonsil crypts and the tonsillar fossa/supratonsillar fossa. Its mesenchymal/lymphoid component is later populated by B and T cells. It lies...

The second pharyngeal (hyoid) arch gives rise to the muscles of facial expression, stapedius, stylohyoid, posterior belly of digastric, and the stapes, styloid process, lesser horn and upper body of the hyoid, and is innervated by the fa...

The foramen primum is the temporary opening at the inferior edge of the septum primum during fetal heart development, between the free edge of the septum primum and the endocardial cushions. It allows right-to-left shunting of blood betw...

The fourth pharyngeal arch gives rise to most of the laryngeal cartilages (thyroid, cricoid, arytenoid, corniculate, cuneiform), the pharyngeal constrictors, the cricothyroid muscle, and the levator veli palatini. Its nerve is the superi...

The sixth pharyngeal arch gives rise to most intrinsic laryngeal muscles (all except the cricothyroid), the laryngeal cartilages (cricoid, arytenoid, corniculate, cuneiform), and the ductus arteriosus/proximal pulmonary arteries (arch ar...

The foramen secundum is a perforation that forms in the septum primum during fetal heart development, preserving right-to-left shunting across the atria while the septum secundum grows. Not to be confused with an ASD secundum, the most c...

The third pharyngeal pouch (endoderm) gives rise to the inferior parathyroid glands (dorsal wings) and the thymus (ventral wings). Failure of development/migration causes DiGeorge (22q11.2 deletion) syndrome with hypocalcemia (absent par...

Endocardial cushions are mesenchymal tissue swellings in the atrioventricular (AV) canal of the developing heart (weeks 4–5) that fuse to divide the common AV canal into the right and left AV canals (future tricuspid and mitral orifices)...

The fourth pharyngeal pouch gives rise to the superior parathyroid glands (parathyroid IV) and contributes to the parafollicular C cells of the thyroid via the ultimobranchial body (with the 5th pouch, when described). Mnemonic: "3 down,...

Endocardial cushion defect (atrioventricular septal defect, AVSD) is a congenital heart defect caused by failed fusion of the superior and inferior endocardial cushions, producing defects of the atrial septum (primum ASD), ventricular in...

The cloaca is the common endodermal chamber at the caudal end of the hindgut in the early embryo, receiving the hindgut and allantois and bounded by the cloacal membrane. In weeks 4–7 the urorectal septum divides it into the anterior uro...

Rathke pouch is an embryologic invagination of oral ectoderm (stomodeum roof) that ascends toward the brain and forms the anterior pituitary (adenohypophysis). It contrasts with the posterior pituitary, which derives from neuroectoderm (...

The buccopharyngeal (oropharyngeal) membrane is a bilaminar embryonic structure of ectoderm directly apposed to endoderm, with no intervening mesoderm, at the cranial end of the embryonic disc. It ruptures around week 4 to connect the st...

The cloacal membrane is a bilaminar embryonic structure (ectoderm externally, endoderm internally, with no intervening mesoderm) at the caudal end of the embryonic disc. It marks the future site of the anal and urogenital openings. It ru...

The proctodeum is the ectodermal invagination (pit) at the caudal end of the embryo that forms the anal canal below the pectinate (dentate) line. It meets the endoderm-derived hindgut at the cloacal (anal) membrane, which ruptures around...

The stomodeum (primitive mouth, oral pit) is an ectoderm-lined depression on the ventral face of the embryo, separated from the foregut by the buccopharyngeal (oropharyngeal) membrane. Rupture of this membrane around week 4 connects the...

The urogenital membrane (perineal membrane) is a thin, tough, triangular fibromuscular sheet spanning the anterior pelvic outlet (urogenital triangle) between the ischiopubic rami. It lies inferior to the deep perineal pouch and is pierc...

SRY (Sex-determining Region Y) is a gene on the short arm of the Y chromosome (Yp11.3) that encodes the testis-determining factor (TDF), a transcription factor with an HMG (high-mobility group) DNA-binding domain. Its expression in the b...

Müllerian inhibiting factor (MIF), also called anti-Müllerian hormone (AMH), is a glycoprotein of the TGF-β family secreted by fetal Sertoli cells. It causes ipsilateral regression of the paramesonephric (Müllerian) ducts, which prevents...

The genital (gonadal) ridge is a paired mesodermal thickening on the medial aspect of the urogenital ridge (intermediate mesoderm plus overlying coelomic epithelium) that forms around week 5 of development. It is the precursor of the bip...

The gonadal (genital) ridge is a paired mesodermal thickening on the medial aspect of the urogenital ridge (intermediate mesoderm) that forms around week 5 of development. It is the primordium of the bipotential gonad, which is populated...

Physiologic umbilical herniation is the normal protrusion of the midgut loop through the primitive umbilical ring into the extraembryonic coelom (umbilical cord) from ~weeks 6–10 of development, because the abdominal cavity is too small...

The vitellointestinal (vitelline/omphalomesenteric) duct connects the midgut to the yolk sac in the embryo and normally obliterates by weeks 5-9. Persistence causes Meckel diverticulum (the most common GI congenital anomaly, "rule of 2s"...

Midgut rotation is the normal embryologic process in which the midgut loop herniates into the umbilical cord (week 6), rotates 270° counterclockwise around the superior mesenteric artery (SMA) axis (viewed from the front), and returns to...

A pelvic kidney is a congenital renal ectopia in which the kidney fails to ascend from the pelvis to the normal retroperitoneal lumbar position (L1-L3) during embryogenesis. It is usually asymptomatic and found incidentally, but it predi...

Simple cuboidal epithelium is a single layer of cube-shaped cells with a round central nucleus, specialized for secretion and absorption. It lines renal tubules (especially the proximal convoluted tubule, which has a brush border), thyro...

Epispadias is a congenital penile malformation in which the urethral meatus opens on the DORSAL surface of the penis (vs. hypospadias = ventral). It results from faulty migration of the genital tubercle mesenchymal cells, is much rarer t...

Pseudostratified columnar epithelium is a single layer of columnar cells that all contact the basement membrane but have nuclei at different heights, falsely appearing stratified. It is typically ciliated with goblet cells (respiratory e...

Stratified cuboidal epithelium is a rare epithelium made of two (sometimes more) layers of cube-shaped cells. It lines the larger ducts of exocrine glands (sweat glands, salivary glands, mammary glands) and functions mainly in protection...

Spongy (cancellous/trabecular) bone is the porous, lattice-like inner bone tissue made of trabeculae with marrow-filled spaces. It makes up ~20% of skeletal mass but has high surface area and metabolic turnover, so it is found in vertebr...

Type II collagen is the main collagen of hyaline cartilage, the nucleus pulposus, and the vitreous humor. It is encoded by COL2A1. Mutations cause chondrodysplasias such as achondrogenesis type II and Stickler syndrome, and it is an auto...

Ground substance is the amorphous, gel-like, highly hydrated component of the extracellular matrix (ECM) that fills the space between cells and fibers in connective tissue. It is composed mainly of glycosaminoglycans (GAGs), proteoglycan...

Enterochromaffin (EC) cells are neuroendocrine cells (Kulchitsky-type) in the GI mucosa that produce ~90% of the body's serotonin (5-HT) from tryptophan. They regulate gut motility and secretion, and are the cell of origin of carcinoid (...

White pulp is the lymphoid (immune) component of the spleen, consisting of the periarteriolar lymphoid sheath (PALS, T cells), lymphoid follicles (B cells), and the marginal zone. It surrounds the central arteries and mediates immune res...

Compact (cortical) bone is the dense, outer shell of bones, making up ~80% of skeletal mass. It is organized into osteons (Haversian systems) and provides mechanical strength and protection, while trabecular (cancellous) bone has higher...

The paracortex is the T-cell zone of a lymph node, located between the superficial cortex (B-cell follicles) and the medulla. It contains naive T cells (CD4+ and CD8+), dendritic cells, and high endothelial venules (HEVs). It is underdev...

Mucous cells are epithelial cells that synthesize and secrete mucin glycoproteins, forming protective mucus. They include surface mucous (foveolar) cells and mucous neck cells in the stomach, goblet cells in the intestines and airways, a...

Elastic fibers are extracellular matrix components made of an elastin core surrounded by fibrillin-1 microfibrils. They give tissues the ability to stretch and recoil (lungs, large arteries, skin, ligaments). Defects cause Marfan syndrom...

Reticular fibers are thin, branching networks of type III collagen that form the delicate supportive stroma (scaffold) of soft organs such as the liver, spleen, lymph nodes, bone marrow, and adipose tissue. They stain black with silver s...

High endothelial venules (HEVs) are specialized postcapillary venules with plump, cuboidal endothelial cells found in lymph node paracortex, Peyer patches, and tonsils (but NOT spleen). They are the main site where naive and central memo...

Hassall corpuscles (thymic corpuscles) are whorled clusters of concentrically arranged, keratinized epithelial reticular cells found in the thymic medulla. They are a histologic hallmark of the thymic medulla and are involved in T-cell r...

Intercalated cells are specialized cells in the late distal tubule and collecting duct that regulate acid-base balance. Type A (alpha) cells secrete H+ via apical H+-ATPase and H+/K+-ATPase and reabsorb HCO3- via basolateral Cl-/HCO3- ex...

Macula densa cells are specialized epithelial cells in the wall of the distal thick ascending limb/early distal convoluted tubule, located where the tubule contacts its own glomerulus between the afferent and efferent arterioles. They se...

A germinal center is a site within the follicle (cortex) of secondary lymphoid organs (lymph nodes, spleen, Peyer patches, tonsils) where activated B cells proliferate, undergo somatic hypermutation, class-switching, and affinity maturat...

Juxtaglomerular (JG) cells are modified smooth muscle cells in the wall of the afferent arteriole that synthesize, store, and secrete renin. They act as baroreceptors (sensing low renal perfusion pressure) and are stimulated by β1-sympat...

Principal cells are the major cell type of the late distal tubule and collecting duct in the kidney. They reabsorb Na+ (via ENaC) and secrete K+ (via ROMK) under aldosterone control, and they mediate water reabsorption via ADH (vasopress...

Beta intercalated cells are a subtype of intercalated cells in the late distal tubule and collecting duct that secrete bicarbonate (HCO3-) into the lumen and reabsorb H+ (via basolateral H+-ATPase), via the apical Cl-/HCO3- exchanger pen...

The myogenic response is the intrinsic ability of vascular smooth muscle (especially afferent arterioles) to contract when stretched by increased intravascular pressure and relax when pressure falls. It maintains constant blood flow/GFR...

The Bainbridge reflex (atrial reflex) is an increase in heart rate in response to increased venous return/right atrial stretch. Stretch of atrial receptors triggers afferent signals via the vagus to the medulla, causing reduced vagal ton...

Fenestrated capillaries have endothelial cells with small pores (fenestrae, ~60–80 nm) that are usually covered by a thin diaphragm, allowing rapid exchange of water and small solutes. They are found where high filtration or absorption o...