Fact-checked by Grok 2 weeks ago

Permanent cell

A permanent cell, also referred to as a post-mitotic or terminally differentiated cell, is a mature cell type in multicellular organisms that has permanently exited the cell cycle and lost the capacity for division or proliferation, even in response to injury or stress.^[1] These cells are characterized by their inability to undergo mitosis, distinguishing them from other cell populations that can regenerate through division.^[2] In the classification of tissues based on regenerative potential, permanent cells form a key category alongside labile cells (which continuously proliferate, such as skin epithelial cells) and stable cells (which divide only when needed, such as hepatocytes in the liver).^[1] Permanent cells include neurons in the central nervous system, cardiomyocytes in the heart, and skeletal muscle fibers, which maintain long-term function but cannot replace themselves if damaged.^[2] This limited regenerative ability has significant implications for tissue repair: damage to permanent cell-containing tissues, such as in myocardial infarction or neuronal injury, typically results in fibrosis and scar formation rather than full restoration of original structure and function, potentially leading to chronic conditions like heart failure or neurodegeneration.^[1] Research into stem cell therapies and regenerative medicine continues to explore ways to overcome this barrier, though permanent cells remain inherently non-proliferative by design to support specialized roles in homeostasis.^[3]

Definition and Characteristics

Definition

Permanent cells are terminally differentiated cells that are incapable of undergoing mitosis or proliferation in postnatal life.^[4] These cells arise from progenitor or stem cells during development, which initially possess the ability to divide and differentiate, but upon achieving maturity, they irreversibly lose their replicative capacity.^[5] In contrast to embryonic stages, where precursor cells may perform a limited number of divisions before attaining terminal differentiation, permanent cells in adults exhibit a stable, non-dividing state postnatally.^[6] This distinction underscores the developmental transition from proliferative potential to specialized function without renewal. The core mechanism underlying this non-proliferative state involves a permanent exit from the cell cycle, often enforced by the upregulation of inhibitors such as the cyclin-dependent kinase inhibitor p21 and the retinoblastoma protein (Rb).^[7] These proteins suppress DNA replication and cell division, ensuring the maintenance of cellular specialization.^[8]

Key Characteristics

Permanent cells, also known as post-mitotic or terminally differentiated cells, exhibit highly specialized morphological features that prioritize long-term functional stability over proliferative capacity. These cells develop intricate structural adaptations, such as extensive dendritic arborizations in neurons for signal integration or intercalated discs in cardiac muscle cells for synchronized contraction, which enhance their efficiency in specialized roles while rendering them incapable of division. These morphological traits arise during terminal differentiation, where cells invest resources in structural complexity rather than maintaining a proliferative state, as seen in the formation of multinucleated myotubes in skeletal muscle.^[9] At the molecular level, permanent cells are characterized by the upregulation of differentiation-specific genes and the downregulation of cell cycle regulators, ensuring irreversible exit from the mitotic cycle. For instance, transcription factors like NeuroD promote neuronal differentiation by activating genes for structural proteins, while cyclin-dependent kinase inhibitors (CKIs) such as p21 and p27 accumulate to suppress cyclin D1 and cyclin E activity, blocking the G1/S transition through Rb-family-mediated E2F repression and chromatin remodeling.^[10] This molecular profile, including persistent heterochromatin formation via HP1α and the DREAM complex, maintains quiescence and prevents aberrant re-entry into the cell cycle, distinguishing permanent cells from labile or stable proliferative types.^[11]^[12] Permanent cells achieve longevity spanning decades through robust maintenance mechanisms, including enhanced autophagy for protein and organelle turnover, and efficient DNA repair pathways to counteract accumulated damage in non-dividing states. Autophagy, involving proteins like Atg7 and LC3, clears dysfunctional components to sustain viability, while DNA repair systems—such as non-homologous end joining predominant in post-mitotic contexts—address oxidative and replication-independent lesions, though inefficiencies can lead to senescence.^[13]^[14] However, these cells remain vulnerable to apoptosis if repair fails, as unrepaired DNA damage triggers p53-mediated pathways without the option for dilution through division.^[15] To meet their elevated energy demands without proliferative turnover, permanent cells rely on a high density of mitochondria optimized for oxidative phosphorylation, supporting intense metabolic activity such as synaptic transmission in neurons or continuous contraction in muscle. This mitochondrial abundance, accounting for up to 20% of the body's resting metabolic rate in neural tissues, ensures ATP production but heightens sensitivity to dysfunction, underscoring the trade-off for functional specialization.^[16]^[17]

Cell Proliferation Types

Labile Cells

Labile cells represent a category of continuously dividing cell populations that maintain tissue integrity by perpetually replacing cells lost through normal wear, injury, or apoptosis. These cells exhibit a high capacity for mitosis throughout an organism's life, driven by stem cell activity that replenishes differentiated progeny. Prominent examples include the epithelial cells of the skin, the mucosal lining of the gastrointestinal tract, and hematopoietic stem cells in the bone marrow, which generate blood cell lineages to sustain systemic functions.^[18]^[19]^[20] The proliferative capacity of labile cells is characterized by a high mitotic index, reflecting frequent entry into the cell cycle and progression through its phases—G1, S, G2, and M—under the influence of mitogenic signals. Growth factors such as epidermal growth factor (EGF) play a central role in stimulating this process by binding to cell surface receptors, thereby activating intracellular pathways that promote DNA synthesis and division. This ongoing proliferation ensures rapid replacement of short-lived cells, with lifespans typically ranging from days to weeks, contrasting sharply with the post-mitotic nature of permanent cells.^[21]^[22] Tissue turnover rates among labile cell populations underscore their role in homeostasis and repair. In the bone marrow, hematopoietic stem cells produce approximately 500 billion new blood cells daily to maintain circulating levels. Skin epithelial cells renew every 40–56 days, allowing the epidermis to shed and regenerate its outer layers continuously. Similarly, intestinal mucosal epithelial cells turn over every 4–5 days, facilitating barrier function amid constant exposure to luminal contents.^[23]^[24]^[25] Regulation of labile cell proliferation involves intricate checkpoints and signaling mechanisms to balance renewal with prevention of uncontrolled growth, such as in oncogenesis. The restriction point in G1 phase commits cells to division only upon sufficient growth factor stimulation, while intra-S and G2/M checkpoints monitor DNA integrity to halt progression if damage is detected. These controls, including tumor suppressor pathways, mitigate risks of overproliferation in high-turnover tissues like the gut mucosa and hematopoietic system.^[26]^[27]^[28]

Stable Cells

Stable cells, also known as quiescent or conditionally renewing cells, are those that normally reside in a non-proliferative state within the G0 phase of the cell cycle but retain the capacity to re-enter the cell cycle and proliferate in response to specific stimuli such as injury or increased physiological demand.^[29] Unlike permanent cells, which enter an irreversible G0 state and lose proliferative potential entirely, stable cells maintain latent mitotic competence.^[30] Prominent examples include hepatocytes in the liver, proximal renal tubular epithelial cells, and endothelial cells lining blood vessels, which typically remain dormant under homeostatic conditions but can activate to support tissue maintenance or repair.^[30] Activation of stable cells occurs through extracellular signals, including growth factors and hormones, that promote progression from G0 to the G1 phase and subsequent DNA synthesis. For instance, hepatocyte growth factor (HGF), produced by non-parenchymal liver cells and mesenchymal tissues, binds to the c-MET receptor on hepatocytes, initiating signaling cascades such as PI3K/AKT and MAPK/ERK pathways that drive limited rounds of division, often 1-2 cycles, after which the cells return to quiescence.^[31] Similarly, renal tubular cells and endothelial cells respond to injury-induced factors like vascular endothelial growth factor (VEGF), which enhances their survival and proliferative response while mitigating apoptosis.^[32] These cells demonstrate significant regeneration potential through hyperplasia, enabling tissue restoration without relying on stem cell differentiation. A classic example is liver regeneration in mammals following partial hepatectomy, where the remnant liver lobes undergo compensatory growth to restore 70-80% of the original mass within 7-10 days, primarily via hepatocyte proliferation rather than precursor activation.^[33] This process highlights the adaptive hyperplasia of stable cells, allowing organs like the liver and kidney to recover function after acute insults such as toxin exposure or ischemia. However, the proliferative capacity of stable cells is constrained by a finite replicative lifespan, primarily due to progressive telomere shortening with each division, which eventually triggers cellular senescence and limits long-term renewal in contrast to the continuous turnover of labile cells.^[34] This telomere attrition imposes a biological barrier, ensuring that while stable cells can mount robust short-term responses, they cannot sustain indefinite proliferation without external interventions like telomerase activation.^[35]

Comparison to Permanent Cells

Permanent cells represent the endpoint of the proliferation spectrum among tissue cell types, exhibiting no capacity for division in adulthood, in contrast to labile cells, which continuously proliferate to maintain tissues like the skin and gastrointestinal epithelium, and stable cells, which remain quiescent but can re-enter the cell cycle in response to stimuli, as seen in the liver and kidneys.^[1] This classification, based on regenerative potential, underscores the post-mitotic commitment of permanent cells, such as neurons and cardiac myocytes, which exit the cell cycle permanently after differentiation.^[36] The non-proliferative state of permanent cells is maintained through epigenetic mechanisms that silence proliferation-associated genes, preventing re-entry into the cell cycle. For instance, in cardiac myocytes, the retinoblastoma proteins Rb and p130 recruit heterochromatin complexes to repress E2F-dependent cell cycle genes, ensuring stable silencing via histone modifications and DNA methylation.^[37] Similarly, in neurons, epigenetic repression of cyclin-dependent kinases and other mitotic regulators locks cells in a differentiated, non-dividing configuration, prioritizing long-term functional stability over renewal potential.^[38] This commitment to a post-mitotic state imposes functional trade-offs, where permanent cells achieve high specialization for roles like rapid signal transmission in the nervous system but become vulnerable to irreversible loss upon injury, as replacement relies solely on surviving cells or supportive glia rather than self-renewal.^[39] In evolutionary terms, the emergence of such non-regenerative cells in mammals may reflect a trade-off favoring enhanced tumor suppression and metabolic efficiency in complex tissues, at the expense of regenerative capacity, allowing for the development of intricate structures like the central nervous system while reducing cancer risk from unchecked division.^[40] Histologically, these differences manifest quantitatively: permanent cell tissues display zero mitotic figures under normal conditions, reflecting absent proliferation, whereas labile tissues exhibit frequent mitoses (often >1 per high-power field in active areas), and stable tissues show rare or stimulus-induced figures (typically <0.1 per field).^[36] This absence of mitotic activity in permanent cells highlights their terminal differentiation and informs pathological assessments of tissue repair limitations.^[41]

Examples in Human Physiology

Neurons

Neurons serve as the quintessential example of permanent cells within the nervous system, characterized by their terminally differentiated, post-mitotic state that precludes cell division throughout adulthood.^[42] These cells are essential for information processing and transmission, forming the structural and functional backbone of the central and peripheral nervous systems. Unlike proliferative glial cells, neurons exit the cell cycle permanently after differentiation, relying on their longevity to maintain neural circuits.^[43] The structure of neurons reflects their specialization for signal propagation, typically exhibiting a multipolar morphology with a central cell body (soma), multiple branching dendrites, a single elongated axon, and synaptic terminals.^[44] Dendrites receive incoming signals, the axon conducts outgoing impulses over long distances, and synapses facilitate communication with other cells via neurotransmitter release.^[45] Mature neurons lack centrioles, a key feature contributing to their inability to undergo mitosis, as centrioles are required for spindle formation during cell division.^[46] Functionally, neurons are optimized for electrochemical signaling, generating and propagating action potentials to enable rapid communication across neural networks.^[44] In the adult human brain, approximately 86 billion neurons form intricate connections, with the vast majority generated prenatally during embryonic and early postnatal development.^[47] Neurons originate from neural progenitor cells in the developing neuroepithelium, undergoing proliferation before differentiating into post-mitotic states by birth.^[48] While most neurons are produced during this period and do not divide thereafter, limited adult neurogenesis occurs in restricted regions such as the hippocampal dentate gyrus, generating new granule cells that integrate into existing circuits but insufficient to offset widespread losses.^[49] This non-replicative nature renders neurons particularly vulnerable to neurodegenerative diseases, such as Alzheimer's and Parkinson's, where irreversible loss of specific neuronal populations leads to progressive dysfunction without effective replacement.^[50] The inability to regenerate amplifies damage from stressors like protein aggregation and oxidative injury, underscoring the fragility of these long-lived cells.^[51]

Cardiac Muscle Cells

Cardiac muscle cells, or cardiomyocytes, exhibit a distinctive branched and striated morphology, featuring sarcomeres as the fundamental contractile units composed of overlapping actin and myosin filaments that enable forceful contractions. These cells are interconnected by specialized intercalated discs, which include gap junctions for rapid electrical impulse propagation and desmosomes for mechanical stability, thereby forming a functional syncytium that ensures coordinated, wave-like contractions across the myocardium. Approximately 25% of human cardiomyocytes are binucleated, a feature that emerges during development and persists throughout life without altering the post-mitotic state of the cells.^[52]^[53] Functionally, cardiomyocytes drive the involuntary, rhythmic contractions essential for systemic circulation, with the adult human heart comprising roughly 2 billion such cells that occupy 75-80% of the myocardial volume despite representing only 20-40% of total cardiac cells by number. Postnatally, cardiomyocyte addition is minimal, with annual turnover rates below 1% in adulthood, reinforcing their classification as permanent cells incapable of significant self-renewal. This fixed population supports efficient ATP-dependent contractions fueled primarily by aerobic metabolism of fatty acids and glucose, allowing the heart to pump approximately 7,000 liters of blood daily under autonomic control.^[54]^[52]00576-0) During development, cardiomyocytes arise from mesodermal progenitor cells, particularly those in the first and second heart fields, which differentiate into contractile myocardium starting around the third week of human gestation through regulated signaling pathways involving BMP, FGF, and WNT. Terminal differentiation occurs predominantly within the first week after birth, after which cells exit the cell cycle and cease proliferation, shifting reliance for cardiac growth to hypertrophy of existing cardiomyocytes rather than hyperplasia. This developmental trajectory establishes the heart's mature syncytial architecture by the late fetal to early postnatal period.^[55]^[55] Physiologically, cardiomyocytes integrate seamlessly with pacemaker cells in the sinoatrial and atrioventricular nodes to sustain the heart's intrinsic rhythmic beating at 60-100 beats per minute at rest, propagating action potentials via the syncytial network for efficient ventricular systole and diastole. However, their permanent, non-regenerative nature means that injury, such as ischemia, results in cell death followed by fibrotic replacement by non-contractile scar tissue from activated fibroblasts, impairing long-term cardiac function and contributing to conditions like heart failure.^[52]^[56]

Skeletal Muscle Cells

Skeletal muscle cells, also known as muscle fibers or myofibers, are elongated, cylindrical cells that form multinucleated syncytia through the fusion of multiple myoblasts during development.^[57] These syncytia contain hundreds to thousands of myofibrils, which are organized bundles of contractile proteins including actin and myosin filaments arranged into repeating sarcomeres, enabling the striated appearance and precise contraction characteristic of skeletal muscle.^[58] The sarcolemma, or cell membrane, surrounds each fiber, with nuclei positioned peripherally, and the fibers are bundled into fascicles encased by connective tissues such as endomysium, perimysium, and epimysium for structural support.^[57] These cells are primarily responsible for voluntary movements, posture maintenance, and force generation in the body, distinguishing them from the involuntary contractions of cardiac muscle cells.^[58] Comprising approximately 40% of total body weight in humans, skeletal muscle fibers collectively enable locomotion and manipulation of the environment through neural control from the somatic nervous system.^[59] Postnatally, these fibers do not proliferate but are supported by satellite cells, which contribute to repair and adaptation without forming new fibers.^[60] Skeletal muscle development, or myogenesis, begins in embryogenesis with progenitor cells differentiating into myoblasts that fuse to form primary and secondary myofibers, a process mediated by fusogenic proteins such as myomaker and myomerger.^[60] By birth, myogenesis is largely complete, with fibers specializing into slow-twitch (type I) or fast-twitch (type II) variants based on contractile properties.^[60] Postnatal growth occurs through hypertrophy, where existing fibers increase in size via the addition of myofibrils and myonuclei from satellite cell fusion, rather than cell division.^[58] Maintenance of skeletal muscle fibers relies on continuous protein synthesis to preserve myofibril integrity and contractile function, as these post-mitotic cells cannot regenerate by dividing.^[58] The core structure of each fiber remains irreplaceable throughout life, with adaptations to stress or injury achieved through hypertrophy or limited contributions from satellite cells that add nuclei but do not replace lost fibers.^[60]

Physiological Implications

Regeneration Limitations

Permanent cells, such as neurons and cardiac muscle cells, exhibit profound limitations in regeneration due to intrinsic and extrinsic biological barriers that prevent cell division and tissue repair. Intrinsically, these cells enter a post-mitotic state early in development, characterized by the absence of functional mitotic machinery and a stable nuclear architecture, including the nuclear lamina, which inhibits re-entry into the cell cycle; attempts to induce proliferation often lead to cell death rather than division.^[42]^[61] In cardiac muscle cells, additional barriers include cell cycle arrest and polyploidization, which further restrict proliferative potential.^[62] Telomere maintenance issues exacerbate these limitations, as progressive telomere shortening in somatic cells induces senescence in post-mitotic cells, impairing cellular function and any potential regenerative response.^[63] Extrinsically, the microenvironment post-injury becomes inhibitory; in the brain, glial scar tissue forms a physical and chemical barrier that suppresses axonal regrowth and neuronal replacement, while in the heart, fibrotic scarring replaces lost myocardium and hinders functional recovery.^[64]^[56] At the tissue level, the inability of permanent cells to regenerate results in lasting functional deficits. Loss of neurons following ischemic stroke causes irreversible brain damage and permanent neurological impairments, such as motor and sensory deficits, due to the failure of surviving neurons to compensate for the lost population.^[65]^[66] Similarly, myocardial infarction in cardiac muscle leads to extensive cardiomyocyte death, followed by scar formation that weakens the ventricular wall and predisposes to complications like left ventricular aneurysms, where thinned scar tissue bulges under pressure, impairing contractility and increasing heart failure risk.^[67]^[68] In contrast to labile tissues, such as the skin epidermis, where continuous cell division enables full regeneration and restoration of function after injury, damage to permanent cell-containing tissues results in incomplete repair, chronic scarring, and progressive functional decline without cellular replacement.^[2] These limitations manifest in quantifiable age-related impacts, particularly in sensory systems; for instance, human olfactory bulb neurons exhibit minimal turnover, with an estimated annual replacement rate of only about 0.008%, leading to uncompensated neuron loss over time that contributes to the widespread olfactory decline observed in over 50% of individuals aged 65 and older.^[69]^[70]

Compensatory Mechanisms

When permanent cells are lost, the body employs hypertrophy of surviving cells as a primary compensatory mechanism to maintain tissue function. In the heart, following myocardial infarction, the remaining cardiomyocytes undergo adaptive hypertrophy, enlarging to increase contractile force and normalize ventricular wall stress, thereby preserving cardiac output in the short term. This process involves molecular regulators such as the lncRNA Sweetheart, which promotes interventricular septum thickening and enhances cell size in response to hypoxic stress.^[71] Similarly, in skeletal muscle, surviving myofibers can hypertrophy to compensate for fiber loss, supported by auxiliary cells that bolster structural integrity. Auxiliary cells play a crucial role in providing indirect support to permanent cells after damage. In the central nervous system, glial cells such as astrocytes and microglia extend processes to neighboring territories, clearing neuronal debris through phagocytosis and offering metabolic and synaptic support to mitigate the impact of neuron loss.^[72] This glial multitasking maintains blood-brain barrier integrity and regulates synaptic activity, compensating for dysfunctional glia or neuronal injury without replacing lost neurons. In skeletal muscle, fibroblasts produce extracellular matrix components like collagens I, III, and IV, which provide mechanical stability and facilitate force transmission from remaining myofibers to tendons, aiding structural compensation post-injury.^[73] Neuroplasticity enables functional redistribution in the brain following permanent neuron loss, primarily through synaptic rewiring and circuit reorganization. After stroke or neuronal damage, surviving neurons form new connections via axonal sprouting and enhanced synaptic strength in peri-infarct regions, such as increased dendritic spine density in cortical layer V, allowing contralesional areas to assume lost functions.^[74] This adaptive plasticity occurs in phases, with rapid early improvements followed by slower consolidation, often enhanced by rehabilitative training to redistribute motor or cognitive tasks.^[74] These compensatory mechanisms offer only temporary efficacy, as chronic or repeated cell loss overwhelms adaptations, leading to organ dysfunction. In the heart, sustained hypertrophy alters calcium handling—prioritizing diastolic control at the expense of systolic dynamics—eventually contributing to decompensation and heart failure after multiple infarcts.^[75] Similarly, prolonged neuronal loss diminishes neuroplasticity's capacity for rewiring, resulting in progressive neurodegeneration, while excessive fibroblast activity in muscle can lead to fibrosis and impaired contractility.^[75]

Research and Clinical Relevance

Historical Context

In the mid-19th century, histologists began using light microscopy to examine tissue structures, leading to early observations of non-dividing cells in the nervous system. Rudolf Virchow, a pioneer in cellular pathology, distinguished neurons from glial cells through his studies of brain tissue, noting their morphological differences and contributing to the view of specialized, stable cell types in multicellular organisms.^[76] These findings, published in his 1858 work Die Cellularpathologie, aligned with the emerging cell theory.^[77] By the 1890s, embryological research further clarified the developmental origins of neurons. Wilhelm His, through serial sectioning of human embryos, demonstrated that neurons originate from cells lining the neural tube and migrate to form the nervous system, becoming differentiated units.^[78] Concurrently, Santiago Ramón y Cajal's application of Camillo Golgi's silver staining technique provided histological evidence for the neuron doctrine, portraying neurons as discrete, independent cells that do not regenerate through division after development. Cajal's 1890s publications argued that this fixed state underscores the nervous system's reliance on precise embryonic patterning rather than ongoing cellular proliferation.^[79] These observations aligned with broader theoretical shifts from vitalism—which attributed life's processes to an immaterial force without cellular specificity—to the cell theory's mechanistic framework. Formulated by Theodor Schwann and Matthias Schleiden in 1838–1839 and extended by Virchow's 1855 dictum omnis cellula e cellula, the theory established cells as the fundamental units of life arising from preexisting cells, positioning differentiated cells like neurons as endpoints of developmental lineages.^[80] This integration reframed biological organization around cellular autonomy and specialization, diminishing vitalistic explanations for tissue stability. A key milestone in confirming the post-mitotic state came in the 1950s with electron microscopy advancements. Studies by Sidney Palay and colleagues revealed the ultrastructure of neurons, providing insights into their mature, stable morphology that supported the concept of terminally differentiated entities.^[81] This structural validation bridged early histological insights with modern cytology.

Modern Therapeutic Approaches

Modern therapeutic approaches to address the limitations of permanent cells, such as neurons and cardiac muscle cells, primarily involve stem cell-based regeneration and gene editing technologies to restore function or induce proliferation where natural repair is absent. Induced pluripotent stem cells (iPSCs) have emerged as a key tool for neuron replacement, particularly in neurodegenerative diseases like Parkinson's, where dopaminergic precursors derived from iPSCs are transplanted to replenish lost cells. In the 2020s, multiple clinical trials have advanced this strategy; for instance, a Phase I/II trial initiated in Japan in 2025 (jRCT2090220384) demonstrated that allogeneic iPSC-derived dopaminergic progenitors survived implantation, produced dopamine, and showed no tumor formation in Parkinson's patients after one year.^[82] Similarly, two Phase I and II trials reported in 2025 evaluated the safety and efficacy of transplanting early-stage dopamine-producing cells, yielding preliminary improvements in motor symptoms without severe adverse events.^[83] Another ongoing trial, NCT06482268, assesses the safety of iPSC-derived dopaminergic progenitors in advanced Parkinson's cases, focusing on incidence and severity of treatment-emergent adverse events.^[84] Gene editing techniques, notably CRISPR activation (CRISPRa), target genes to promote cardiovascular progenitor formation or correct mutations in post-mitotic cardiac cells for myocardial repair. In the 2010s, foundational studies in mice used CRISPR/Cas9 to edit genes like PRKAG2, correcting cardiomyopathy mutations in postnatal hearts and restoring normal cardiac function without off-target effects. More recent applications of CRISPRa have reprogrammed fibroblasts into cardiovascular progenitors by activating endogenous genes such as Mef2c and Gata4, promoting improved heart function in mouse models of infarction.^[85] These approaches build on studies from the late 2010s, including 2017 experiments demonstrating in vivo CRISPR editing in cardiomyocytes, offering proof-of-concept for genetic interventions in non-regenerative mammalian hearts.^[86] For skeletal muscle fibers, another type of permanent cell, research as of 2025 explores gene editing and cell therapies to address damage in conditions like muscular dystrophy. CRISPR-based approaches have targeted dystrophin mutations in preclinical models, with early-phase trials evaluating safety of edited myoblasts for transplantation.^[87] Despite these advances, challenges persist, including immune rejection of transplanted cells, which can trigger innate and adaptive responses leading to graft failure in allogeneic therapies. Strategies to mitigate rejection, such as hypoimmunogenic iPSC derivatives, have shown promise in preclinical models by evading host immune detection. Insights from zebrafish models, which achieve partial heart regeneration through epicardial activation and cardiomyocyte dedifferentiation post-injury, have informed human trial designs by identifying pathways like NF-κB signaling essential for tissue repair. These models highlight conserved mechanisms that could enhance mammalian regeneration, guiding ongoing efforts to translate findings into clinical applications. As of 2025, clinical status includes FDA-approved neural prosthetics serving as interim bridges for permanent cell damage, such as deep brain stimulation devices for Parkinson's that modulate neural circuits to alleviate symptoms. For spinal cord injury, several Phase II trials are underway, including a 2025 multicenter study evaluating intrathecal anti-Nogo-A antibodies, which improved upper limb function in tetraplegic patients compared to placebo.^[88] Another Phase II trial assesses combined mesenchymal stem cells and Schwann cells for neuropathic pain relief in complete spinal cord injuries, reporting enhanced sensory recovery without major safety concerns.^[89]

References

[1]
Tissue Homeostasis, Inflammation, and Repair - NCBI
Tissues can be described as labile, stable, or permanent based on the type and degree of cell turnover, Medzhitov explained. Labile tissues have stem cells like ...Tissue Homeostasis · Cellular Division Of Labor · Minimal Tissue Units And...Missing: biology | Show results with:biology
[2]
Wound healing – regeneration and scarring - Pathologia
Labile cells – Keratinocytes, intestinal epithelial cells; Stable cells – Hepatocytes, renal tubular epithelial cells; Permanent cells – Neurons, cardiac ...Labile cells · Stable cells - renal tubular... · Stable cells - hepatocytes and...
[3]
Permanent Cell - an overview | ScienceDirect Topics
Permanent cells are defined as cells that are unable to replicate in postnatal life. Nervous cells, also termed neurons, together with skeletal muscle and ...
[4]
https://www.sciencedirect.com/topics/agricultural-and-biological-sciences/permanent-cell
[5]
Coordinating cell proliferation and differentiation - PubMed Central
Precursor cells continue division before acquiring a fully differentiated state, while terminal differentiation usually coincides with proliferation arrest and ...
[6]
P21 and Retinoblastoma Protein Control the Absence of DNA ...
We show that the inactivation of Rb and p21 through the binding of the adenovirus E1A protein leads to the induction of DNA replication in differentiated muscle ...Results · Inactive Cyclin E--Cdk2... · E1a And P21 Can Physically...
[7]
Cell cycle regulation: p53-p21-RB signaling - Nature
Mar 31, 2022 · The p53 protein is likely also the best studied tumor suppressor. Its main functions are the induction of apoptosis and cell cycle arrest.
[8]
Mechanisms controlling cell cycle exit upon terminal differentiation
This review focuses on describing recent advances in deciphering how terminal differentiation and exit from the cell cycle are coordinated.
[9]
https://doi.org/10.1016/j.devcel.2007.02.020
[10]
https://pmc.ncbi.nlm.nih.gov/articles/PMC2700000/
[11]
https://doi.org/10.1038/sj.emboj.7601789
[12]
https://pmc.ncbi.nlm.nih.gov/articles/PMC3986830/
[13]
The Complex Mechanisms by Which Neurons Die Following DNA ...
Furthermore, neurons are also particularly vulnerable to DNA damage, given their post-mitotic nature. However, in contrast to proliferating cells, in ...
[14]
Mitochondria, energy, and metabolism in neuronal health and disease
Jan 28, 2022 · The brain has a high energy demand, which makes it particularly sensitive to mitochondrial dysfunction. Distinct cellular events causing ...
[15]
Focusing on mitochondria in the brain: from biology to therapeutics
Apr 17, 2024 · The brain demands 20% of the resting metabolic rate and holds highly active mitochondrial activities.
[16]
Labile Cell - an overview | ScienceDirect Topics
Labile cells are continuously dividing cells that replace lost cells, and include surface epithelial cells like skin and gastrointestinal cells.
[17]
Cellular Adaptations - Cell Populations - TeachMePhysiology
Apr 9, 2024 · Tissue type, Stem cell activity, Examples ; Labile, Stem cells divide repeatedly to replenish losses. Surface epithelia, e.g. gut mucosa; Bone ...Missing: turnover | Show results with:turnover
[18]
Labile cells – Knowledge and References - Taylor & Francis
A labile cell is a type of cell that is capable of regeneration, in contrast to permanent cells that cannot divide or regenerate.<|separator|>
[19]
https://teachmephysiology.com/histology/tissue-structure/cellular-adaptations/
[20]
Regulation of Cell Cycle Progression by Growth Factor-Induced Cell ...
Nov 26, 2021 · While EGF promotes cell survival in mitosis, it does not alter mitosis progression significantly. The only effect observed is the longer ...Missing: labile capacity index
[21]
Bone marrow: the workhorse organ - PubMed
In a healthy adult, bone marrow produces approximately 500 billion new blood cells daily to maintain steady-state levels in the peripheral circulation.
[22]
Making an epidermis - PMC - NIH
In humans, it is estimated that the epidermis turns over every 40–56 days,, whereas in mice the estimated epidermal turnover time is 8–10 days. This constant ...
[23]
Stem cells, self-renewal, and differentiation in the intestinal epithelium
The mammalian intestine is covered by a single layer of epithelial cells that is renewed every 4-5 days. This high cell turnover makes it a very attractive ...
[24]
The Restriction Point of the Cell Cycle - NCBI - NIH
The point at G1 at which commitment occurs and the cell no longer requires growth factors to complete the cell cycle has been termed the restriction (R) point.
[25]
Cell cycle checkpoints (article) | Khan Academy
This article gives a high-level overview of cell cycle control, outlining the factors that influence a cell's decision to pause or progress at each checkpoint.
[26]
Cell cycle checkpoint in cancer: a therapeutically targetable double ...
Sep 27, 2016 · In this review we summarize developing concepts on how targeting cell cycle checkpoints may provide substantial improvement to cancer therapy.
[27]
Cellular Mechanisms and Regulation of Quiescence - PMC - NIH
For most quiescent cells, this arrest takes place in G0, a resting phase outside of the cell cycle that occurs prior to S phase, but is distinct from the G1 ...
[28]
Stable Cell - an overview | ScienceDirect Topics
The parenchymal cells of the most solid glandular organs such as liver and kidney are stable cells. They are capable of undergoing rapid division upon injury to ...
[29]
The discovery of Hepatocyte Growth Factor (HGF) and its ...
The fact that blood-born HGF triggers liver regeneration was demonstrated in 1992: when recombinant human HGF (rh-HGF) was intravenously administrated into ...
[30]
Vascular endothelial growth factor is a survival factor for renal ...
Because VEGF induced a proliferative and an antiapoptotic response in renal tubular epithelial cells, these data suggest that VEGF may act as a survival factor ...<|control11|><|separator|>
[31]
Liver Regeneration after Partial Hepatectomy - NIH
Liver regeneration after partial hepatectomy is one of the most studied models of cell, organ, and tissue regeneration.
[32]
Roles of Telomere Biology in Cell Senescence, Replicative ... - NIH
Jan 15, 2019 · Reintroduction of telomerase to the cells null of telomerase increases the replicative lifespan, indicating a pivotal role of telomere length ...
[33]
Telomere dysfunction in ageing and age-related diseases - Nature
Feb 14, 2022 · As replicative cellular senescence is caused by telomere shortening ... Naturally occurring p16Ink4a-positive cells shorten healthy lifespan.
[34]
Labile Cell - an overview | ScienceDirect Topics
Examples. 1. LABILE CELLS, rapid proliferation and cell turnover, gut-lining epithelial cells. 2. STABLE CELLS, slow proliferation and cell turnover ...
[35]
Rb and p130 control cell cycle gene silencing to maintain the ...
Aug 8, 2011 · Both Rb and p130 are required for the recruitment of heterochromatin proteins that mediate silencing of proliferation genes in adult cardiac ...
[36]
Epigenetic regulation of cardiac myocyte differentiation - PMC - NIH
Recent studies suggest the upregulation of adult cardiac-specific genes together with the silencing of cell cycle genes may be mediated by epigenetic mechanisms ...
[37]
Review Central nervous system regeneration - ScienceDirect.com
Jan 6, 2022 · Neurons of the mammalian central nervous system fail to regenerate. Substantial progress has been made toward identifying the cellular and molecular mechanisms ...Missing: permanent | Show results with:permanent
[38]
Re“evolutionary” Regenerative Medicine - PMC - NIH
Dec 21, 2010 · It has been postulated that during evolution mammals lost regenerative potential as a trade-off for cancer protection. The tumor suppressor ...
[39]
Tissue Remodeling - Lab
Tissues are classified as labile, stable, and permanent, according to the proliferative capacity of their cells. · Labile and stable tissues contain stem cells ...
[40]
The post-mitotic state in neurons correlates with a stable nuclear ...
Neurons become terminally differentiated (TD) post-mitotic cells very early during development yet they may remain alive and functional for decades.
[41]
Cell cycle regulators in neural stem cells and postmitotic neurons
Even while postmitotic neurons remain in permanent mitotic quiescence, they express a number of cell cycle regulators required for cell cycle progression.
[42]
Neuroanatomy, Neurons - StatPearls - NCBI Bookshelf
A. Two connected neurons. Neurons have a soma that contains a nucleus, an axon, and a dendritic tree. A single synapse (red circle) is formed at the point ...Missing: morphology centrioles
[43]
Types of neurons - Queensland Brain Institute
Motor neurons have the most common type of 'body plan' for a nerve cell - they are multipolar, each with one axon and several dendrites. Interneurons. As the ...Missing: absence centrioles
[44]
Microtubules are organized independently of the centrosome in ...
Dec 6, 2011 · In developing and mature neurons, centrioles were not surrounded by the core nucleation protein γ-tubulin. This suggests that the centrioles do ...Centrioles Have Variable... · The Core Microtubule... · The Centriole Does Not Seem...<|control11|><|separator|>
[45]
The remarkable, yet not extraordinary, human brain as a scaled-up ...
Jun 22, 2012 · Remarkably, at an average of 86 billion neurons and 85 billion nonneuronal cells (25), the human brain has just as many neurons as would be ...
[46]
The Basics of Brain Development | Neuropsychology Review
Nov 3, 2010 · Neurons are post-mitotic cells; once formed they are no longer capable of dividing and producing new cells. From the end of gastrulation ...
[47]
Adult Neurogenesis in the Hippocampus: From Stem Cells to Behavior
Nov 3, 2016 · In this review, we summarize the current knowledge about DG neurogenesis, its origins, regulation, and relevance to disease.
[48]
Selective Neuronal Vulnerability in Neurodegenerative Diseases
Jul 14, 2011 · Neurodegenerative diseases selectively target subpopulations of neurons, leading to the progressive failure of defined brain systems.
[49]
Neuronal cell death in neurodegenerative diseases - NIH
This review describes the main mechanisms of neuronal cell death, particularly apoptosis, necrosis, excitotoxicity and autophagic cell death.
[50]
Physiology, Cardiac Muscle - StatPearls - NCBI Bookshelf
Cardiac muscle cells (cardiomyocytes) are striated, branched, contain many mitochondria, and are under involuntary control. Each myocyte contains a single, ...Missing: mitotic | Show results with:mitotic
[51]
Evidence That Human Cardiac Myocytes Divide after Myocardial ...
Jun 7, 2001 · ... myocytes constituted 75 percent and binucleated cells 25 percent of the cell population. This proportion was not affected by disease, age ...
[52]
Cardiomyocyte proliferation vs progenitor cells in myocardial ... - NIH
They found that while normal hearts contained ∼2 billion cardiomyocytes, hearts with pathologic hypertrophy contained up to ∼4.8 billion cardiomyocytes. These ...
[53]
The molecular mechanisms of cardiac development and related ...
Dec 23, 2024 · Many molecular signals regulate cardiac development, including various growth and transcription factors and signaling pathways.
[54]
Mechanisms of Cardiac Regeneration - PMC - PubMed Central
In response to cardiac injury, adult mammals—including humans—fail to regenerate the majority of the lost cardiomyocytes and instead replace necrotic muscle ...
[55]
Anatomy, Skeletal Muscle - StatPearls - NCBI Bookshelf - NIH
Each skeletal muscle consists of thousands of muscle fibers wrapped together by connective tissue sheaths. The individual bundles of muscle fibers in a skeletal ...Missing: maintenance | Show results with:maintenance
[56]
Physiology, Skeletal Muscle - StatPearls - NCBI Bookshelf
Jul 30, 2023 · The main functions of skeletal muscle are to contract to produce movement, sustain body posture and position, maintain body temperature, store ...
[57]
Skeletal muscle: a brief review of structure and function - PubMed
In humans, skeletal muscle comprises approximately 40% of total body weight and contains 50-75% of all body proteins.
[58]
Skeletal muscle: molecular structure, myogenesis, biological ...
Jul 10, 2024 · Myofibers are formed after undergoing cell differentiation, cell–cell fusion, myonuclei migration, and myofibril crosslinking among other ...
[59]
Biomaterial-engineering and neurobiological approaches for ...
The loss of Rb in post-mitotic neurons induces cell-cycle reentry and causes cell death, probably because of the tight coupling of proliferation and neuronal ...2.3. Cell Cycle · 3.1. Biomaterials Without... · 3.2. Biomaterials With Cell...
[60]
Unlocking Cardiomyocyte Renewal Potential for Myocardial ... - NIH
Adult mammalian cardiomyocyte cell cycle arrest and polyploidization have been proposed as major barriers to heart regeneration.
[61]
Telomere shortening impairs organ regeneration by inhibiting cell ...
Telomere shortening limits the regenerative capacity of primary cells in vitro by inducing cellular senescence characterized by a permanent growth arrest of ...
[62]
Neurorepair and Regeneration of the Brain - PubMed Central - NIH
Apr 7, 2021 · The glial scar tissue is well known for its inhibitory effect on axonal growth. One of the main studied glial scar growth-inhibitory ...
[63]
Neuronal injuries in cerebral infarction and ischemic stroke - PMC
Ischemic stroke leads to severe outcomes, including cerebral infarcts, permanent brain damage and neural functional deficits. Therefore, decreasing and ...
[64]
Selective neuronal loss in ischemic stroke and cerebrovascular ...
The aim of this review is to address selective neuronal loss (SNL) in ischemic cerebrovascular disease, with a focus on anterior circulation stroke and large ...
[65]
Myocardial Infarction With Ventricular Wall Aneurysm: A Case Report
Sep 11, 2022 · True aneurysms are complications seen in transmural infarcts. Thinned-out scar tissue paradoxically bulged during systole, and toughened ...
[66]
Two Rare Complications of Post Myocardial Infarction: A Case Report
The injured myocardium is gradually replaced with scar tissue formation. The "border zone" of the aneurysm (ie, between the aneurysm and the normal myocardium) ...
[67]
The Age of Olfactory Bulb Neurons in Humans
May 24, 2012 · We cannot exclude that there may be low-grade turnover of neurons, but at a constant rate, the annual turnover would be 0.008% ±. 0.08% (mean ...
[68]
The influences of age on olfaction: a review - Frontiers
Decreased olfactory function is very common in the older population, being present in over half of those between the ages of 65 and 80 years.
[69]
The lncRNA Sweetheart regulates compensatory cardiac ... - Nature
Nov 2, 2023 · After myocardial infarction in the adult heart the remaining, non-infarcted tissue adapts to compensate the loss of functional tissue.
[70]
Glia multitask to compensate for neighboring glial cell dysfunction
Sep 10, 2024 · It is therefore possible that the neighboring glial cells infiltrate the cortex to clear neuronal debris in a compensatory fashion. Such ...
[71]
Skeletal Muscle Fibroblasts in Health and Disease - PMC - NIH
As the primary producer of extracellular matrix (ECM) proteins in skeletal muscle, fibroblasts play an important role providing structural support to muscle.Missing: compensation | Show results with:compensation
[72]
Compensatory Relearning Following Stroke: Cellular and Plasticity ...
Compensatory relearning is likely to be accompanied by gradual shaping of these regions and pathways, with participating neurons progressively adapting cortico- ...
[73]
Compensatory and decompensatory alterations in cardiomyocyte ...
May 21, 2017 · Elevated left ventricular afterload leads to myocardial hypertrophy, diastolic dysfunction, cellular remodelling and compromised calcium ...
[74]
Rudolf Virchow and the discovery of the Müller cell - PMC
Sep 5, 2024 · Virchow recognized there are two major cell types in the brain, neurons and glial cells. Neurons were first depicted by Christian Ehrenberg ( ...
[75]
Rudolf Carl Virchow (1821-1902) | Embryo Project Encyclopedia
Mar 17, 2012 · Virchow's research at Würzburg helped to establish the concept of cellular pathology, the idea that all diseases are caused by changes in normal ...
[76]
Historical perspective on neuroembryology: Wilhelm His ... - PubMed
The article places His' work in the scientific context of 19th century embryology, concerned with reconciling the embryonic layers theory, the cell theory and ...
[77]
Cajal, the neuronal theory and the idea of brain plasticity - PMC
Cajal also discovered that the extensions of the nerve cells terminate freely and communicate with each other by contact, not by continuity (Cajal, 1889), term ...Missing: permanent | Show results with:permanent
[78]
Cell theory | Definition, History, Importance, Scientists ... - Britannica
Sep 25, 2025 · Cell theory, fundamental scientific theory of biology according to which cells are held to be the basic units of all living tissues.Missing: shifts vitalism<|control11|><|separator|>
[79]
THE FINE STRUCTURE OF NEURONS - PMC - PubMed Central - NIH
Thin sections of representative neurons from intramural, sympathetic and dorsal root ganglia, medulla oblongata, and cerebellar cortex were studied