DPQZ

Apoptosis, also called programmed cell death, is generally characterized by distinct morphological characteristics.

TUNEL staining detects DNA strand breaks by using terminal deoxynucleotidyl transferase to add labeled nucleotides to exposed 3′-OH DNA termini, generating either microscopic staining in fixed cells or tissue sections, or fluorescence/cytometric signal in cell suspensions. TUNEL positivity reflects DNA fragmentation but should not be interpreted alone as definitive apoptosis, because TUNEL can also label necrotic, autolytic, mechanically damaged, or DNA-repair-associated DNA breaks.

Immunoaffinity-based positive/negative selection without magnetic or flow cytometric separation is implemented as immunopanning, in which dissociated cells bind to antibody-coated plastic surfaces through specific cell-surface antigens; negative-selection plates remove unwanted antigen-positive cells, and positive-selection plates retain the desired antigen-positive population for recovery and downstream culture or analysis. The readout is the recovered cell fraction after sequential plate binding and washing: depleted non-adherent cells represent the negative-selection output, while cells retained on the final antibody-coated surface represent the positive-selection output; published examples include T-cell subpopulation purification, mouse and rat oligodendrocyte-lineage cell isolation, and mouse marrow progenitor enrichment.

Flow cytometric DNA-content cell-cycle staining measures the fluorescence intensity of DNA-bound fluorochromes in single cells or nuclei to estimate DNA content distributions, allowing assignment of populations to G0/G1, S, and G2/M phases by DNA histogram deconvolution. Propidium iodide (PI) intercalates into DNA, and PI fluorescence is proportional to cellular DNA content when staining is performed under conditions that make DNA accessible and minimize non-DNA signal. Cells with G2/M DNA content are expected to show approximately twice the fluorescence intensity of G0/G1 cells, while S-phase cells occupy intermediate fluorescence values. PI-based DNA-content analysis can also detect cells with fractional DNA content, often reported as sub-G1, when DNA fragmentation and extraction during staining reduce retained DNA signal in apoptotic cells. DAPI is an alternative DNA fluorochrome for univariate DNA-content analysis, while bivariate approaches combining DNA content with proliferation

Density gradient centrifugation enriches cells by buoyant density: cells sediment during centrifugation until they reach a medium layer or interface compatible with their density, allowing mononuclear cells, granulocytes, erythrocytes, and density-defined subpopulations to be recovered from separate bands or layers. Classic blood-cell applications include Ficoll/sodium-metrizoate or Ficoll-Hypaque enrichment of peripheral blood mononuclear cells, Percoll subfractionation of PBMC and T-cell populations, and Percoll-based neutrophil isolation from whole blood or leukocyte-enriched suspensions. The readout is the physical recovery of enriched cell bands, followed by cell counting, morphology, viability, and immunophenotyping to determine yield, purity, and suitability for downstream assays.

Annexin V-based apoptosis assays rely on the detection of phosphatidylserine (PS) externalization from the inner leaflet of the plasma membrane to the outer leaflet, an early biochemical hallmark of apoptosis. Fluorescently labeled Annexin V binds PS in a calcium-dependent manner, enabling identification of early apoptotic cells by flow cytometry or fluorescence microscopy. When combined with a membrane-impermeant DNA-binding dye (e. g. , propidium iodide), this approach allows discrimination between viable (Annexin V−/dye−), early apoptotic (Annexin V+/dye−), and late apoptotic or necrotic (Annexin V+/dye+) cell populations by assessing membrane integrity and PS exposure.

Bromodeoxyuridine (BrdU) incorporation assay is based on the principle that BrdU, a thymidine analog, is incorporated into newly synthesized DNA during the S phase of the cell cycle, thereby serving as a marker of DNA replication and cellular proliferation. Incorporated BrdU can be detected using anti-BrdU antibodies following DNA denaturation, enabling visualization or quantification of proliferating cells through immunochemical detection methods such as immunofluorescence or immunohistochemistry.

Microfluidic cell sorting separates target cells in microscale channels by either intrinsic physical properties or specific molecular binding. Label-free platforms use size, deformability, hydrodynamic behavior, acoustic contrast, dielectric properties, or inertial migration to alter cell trajectories without antibody labeling, while affinity-based platforms immobilize antibodies, selectins, aptamers, or ligand-bearing nanoparticles to capture cells expressing corresponding surface markers. Classic label-free examples include deterministic lateral displacement arrays, inertial focusing systems, acoustophoresis devices, dielectrophoresis systems, and physical cluster-capture devices. Classic affinity-based examples include EpCAM-coated micropost or herringbone chips, PSMA-GEDI devices, E-selectin/anti-EpCAM biomimetic surfaces, and nanoparticle-mediated capture-and-release chips.

Apoptosis is a regulated, generally non-lytic cell-death pathway that removes unwanted, damaged, infected, or abnormal cells through coordinated morphological changes, caspase activation, DNA fragmentation, and membrane remodeling. The intrinsic apoptosis pathway is controlled mainly by mitochondrial outer membrane permeabilization, BCL-2 family proteins, cytochrome c release, apoptosome formation, caspase-9 activation, and downstream executioner caspase-3/7 activation. The extrinsic apoptosis pathway is initiated by death receptors such as Fas, TNFR, and TRAIL receptors, which recruit adaptor proteins and activate caspase-8 before engaging executioner caspases or mitochondrial amplification through BID cleavage. Apoptosis is linked to many phenotypes, including cancer cell killing, tissue homeostasis, immune regulation, neurodegeneration, infection response, and treatment-induced cytotoxicity; unresolved questions include how apoptosis interacts with necroptosis, pyroptosis, ferroptos

Dielectrophoresis-based electrical cell sorting separates suspended cells by the motion generated when polarizable cells experience a non-uniform electric field; cell trajectory depends on cell size, medium conductivity, applied AC frequency, electric-field gradient, and cell dielectric properties, so cells with different DEP responses can be routed, trapped, levitated, or released without biochemical labeling. In practical DEP sorters, the readout is the spatial redistribution of cells into different outlets, traps, or recovered fractions; reported examples include DEP field-flow fractionation of leukocytes, breast cancer cells, CD34+ cells, and blood cells, continuous-flow hMSC/osteoblast sorting, and image-based single-cell recovery after DEP manipulation.

Cell-cycle analysis by flow cytometry measures DNA content in single cells to estimate the fraction of cells in G0/G1, S, and G2/M phases. Propidium iodide intercalates into DNA, and after RNA removal with RNase, fluorescence intensity reflects cellular DNA content: 2N cells are assigned to G0/G1, cells between 2N and 4N to S phase, and 4N cells to G2/M. DNA-content analysis alone cannot reliably separate G0 from G1 or G2 from M. Ki-67 can distinguish quiescent G0 cells from cycling cells, EdU or BrdU incorporation marks active DNA synthesis in S phase, and phospho-histone H3 staining identifies mitotic cells within the 4N population.

MACS enriches or depletes cells by binding antibody-targeted magnetic particles to surface antigens; labeled cells are retained in a high-gradient magnetic column, while unlabeled cells pass through, and retained cells are eluted after removal from the magnetic field. In ovalbumin-induced allergic airway inflammation, MACS can enrich immune populations such as CD4+ T cells, CD8+ T cells, dendritic cells, eosinophils, or marker-defined leukocytes for downstream analysis of airway inflammation, antigen presentation, and type 2 immune responses.