Cell biology

Pillar cell

Pillar cells (rods of Corti) are inner and outer supporting cells of the mammalian organ of Corti that form the tunnel of Corti; FGF8-FGFR3 and β-Catenin specify them, and inner pillars survive hearing loss best in human cochleas.

Pinocytosis

Pinocytosis, or cell drinking, is fluid-phase endocytosis in which cells ingest extracellular fluid and solutes into small vesicles. Discovered and named by Warren Lewis in 1931; includes macropinocytosis, clathrin-mediated, and caveolae-mediated uptake.

Pituicyte

A pituicyte is a specialized glial cell of the neurohypophysis, the posterior pituitary, where it regulates the release of oxytocin and vasopressin into the bloodstream.

PK-1

PK-1 (PK1, CVCL_4717) is a human pancreatic ductal adenocarcinoma cell line established in 1984 from a liver metastasis; male donor, Japanese origin, KRAS G12D, TP53 mutant, held as RCB1972/TKG 0239. Distinct from KP-1N and LLC-PK1.

PK-15

PK-15 (CVCL_2160, ATCC CCL-33) is a spontaneously immortalized porcine kidney epithelial cell line cloned in 1957 from PK-2a, used widely for swine virus research, vaccine production, and PERV-inactivation studies; it is aneuploid and carries PCV1 and endogenous C-type retrovirus.

PL-21

PL-21 (PL21, CVCL_2161) is a human acute promyelocytic leukemia cell line established in 1981 from peripheral blood, peroxidase-positive, lacking t(15;17), with FLT3-ITD, KRAS and TP53 mutations.

Plakin

Plakins: family of seven giant cytolinker proteins (BPAG1, MACF1, plectin, desmoplakin, envoplakin, periplakin, epiplakin) crosslinking actin, microtubules, and intermediate filaments to cell junctions; mutations cause skin and muscle diseases.

Plant cell

A plant cell is a eukaryotic cell that builds plant tissues, distinguished from animal cells by a cellulose cell wall, chloroplasts that carry out photosynthesis, and a large central vacuole.

Plastid

Plastid (chloroplast family): semi-autonomous organelles of plants and algae, originating from a single primary cyanobacterial endosymbiosis about 1.5 billion years ago, then spread by secondary and tertiary endosymbioses; types include chloroplasts, chromoplasts, and leucoplasts.

Plastid differentiation and interconversion

Plastid differentiation and interconversion is the process by which a single proplastid develops into chloroplasts, etioplasts, amyloplasts, and chromoplasts, all of which can convert between forms in response to light and tissue context.

Plastoquinone

Plastoquinone (PQ, PQ-9), a 2,3-dimethyl-nonaprenyl benzoquinone in chloroplast thylakoid membranes, carries two electrons and two protons from photosystem II to cytochrome b6f and signals redox state; discovered by Kofler, rediscovered by Crane.

Platelet

Platelet (thrombocyte): anucleate 2–4 µm blood cell fragment from megakaryocytes, 150–450 × 10⁹/L, 7–10 day lifespan, ~10¹¹ made daily; drives primary hemostasis and arterial thrombosis, transfused for thrombocytopenia.

Platelet degranulation

Platelet degranulation: calcium-regulated exocytosis of platelet alpha granules, dense granules, and lysosomes via SNARE-mediated membrane fusion, releasing ADP, ATP, serotonin, fibrinogen, vWF, and growth factors at vascular injury sites.

PML nuclear bodies

PML nuclear bodies (ND10) are membrane-less nuclear organelles built by the PML tumor suppressor protein, 0.2–1.0 µm foci with a PML shell and protein core, disrupted in acute promyelocytic leukemia and responsive to interferon, oxidative stress, and arsenic trioxide.

Podocyte

Podocyte (glomerular visceral epithelial cell): kidney epithelial cell whose interdigitating foot processes and slit diaphragms form the outer layer of the glomerular filtration barrier; injury causes foot process effacement and proteinuria.

POLG-related mitochondrial disease

POLG-related mitochondrial disease is a group of inherited disorders caused by pathogenic variants in the POLG gene, which encodes DNA polymerase gamma; they range from fatal infantile epilepsy and liver failure to late-onset progressive external ophthalmoplegia.

Polyploid cell

Polyploid cell: a cell with more than two complete chromosome sets, formed by endoreplication, cytokinesis failure, cell fusion, or mitotic slippage; common in liver, heart, and placenta, linked to larger cell size and to cancer risk.

Pore cell

Pore cell: a unicellular tube cell in the Caenorhabditis elegans excretory system, connecting the excretory duct to the exterior. A seamed tube sealed by an autocellular junction; the G1 pore is replaced by G2.p during larval development.

Potassium transporter

Potassium transporter: membrane protein moving K+ across membranes. Two families, Trk/Ktr/HKT and HAK/Kup/KT, occur in bacteria, fungi, and plants, absent from animals, mediating K+ uptake, osmotic adjustment, and salt tolerance.

PPIF

PPIF (CYP3, CypD) is a human chromosome 10 gene encoding cyclophilin D, the mitochondrial matrix peptidyl-prolyl isomerase that positively regulates the mitochondrial permeability transition pore and is a drug target in ischemia-reperfusion and neurodegeneration.

Preadipocyte

Preadipocyte (CL:0002334), adipocyte precursor or adipose progenitor cell: an undifferentiated fibroblast in the adipose stromal vascular fraction that differentiates into fat cells; studied via 3T3-L1, OP9, SGBS models; impaired in obesity.

Precursor cell

Precursor cell (CL:0011115) is a Cell Ontology term for any cell that, by division or terminal differentiation, gives rise to other cell types; broader than progenitor cell, distinct from self-renewing stem cells, and used inconsistently across fields.

Primary neuron (sensu Teleostei)

Primary neuron (sensu Teleostei), Cell Ontology CL:0000530, denotes early-born teleost fish neurons with large bodies and pioneer axons, including Rohon-Beard, CaP/MiP/RoP motor, and Mauthner cells, distinct from later secondary neurons.

Primitive reticulocyte

Primitive reticulocyte: the reticulocyte formed when a primitive (yolk-sac, EryP) embryonic erythroblast enucleates in the embryonic bloodstream; large, embryonic-globin (Gower) producing first red cell, distinct from definitive reticulocytes.

Profilin

Profilin: a small (12-15 kDa), ubiquitous actin-monomer-binding protein that accelerates nucleotide exchange and delivers ATP-actin to formins and Ena/VASP; humans have four isoforms (PFN1-PFN4) with distinct tissue roles and disease links including ALS.