Inclusion (cell)
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In cellular biology, inclusions are diverse intracellular[1] non-living substances (ergastic substances)[2] that are not bound by membranes. Inclusions are stored nutrients/deutoplasmic substances, secretory products, and pigment granules. Examples of inclusions are glycogen granules in the liver and muscle cells, lipid droplets in fat cells, pigment granules in certain cells of skin and hair, and crystals of various types.[3] Cytoplasmic inclusions are an example of a biomolecular condensate arising by liquid-solid, liquid-gel or liquid-liquid phase separation. They are different from inclusion bodies.

These structures were first observed by O. F. Müller in 1786.[1]

Examples

[edit]
Glycogen granules in Spermiogenesis in Pleurogenidae (Digenea)

Glycogen: Glycogen is the most common form of glucose in animals and is especially abundant in cells of muscles, and liver. It appears in electron micrograph as clusters, or a rosette of beta particles that resemble ribosomes, located near the smooth endoplasmic reticulum.[3] Glycogen is an important energy source of the cell; therefore, it will be available on demand. The enzymes responsible for glycogenolysis degrade glycogen into individual molecules of glucose and can be utilized by multiple organs of the body.[4][2]

Lipids: Lipids, which are stored as triglycerides, are the common form of inclusions. They are stored not only in specialized cells (adipocytes) but also are located as individuals droplets in various cell types, especially hepatocytes.[3] These are fluid at body temperature and appear in living cells as refractile spherical droplets. Lipids yield more than twice as many calories per gram as do carbohydrates. On demand, they serve as a local store of energy and a potential source of short carbon chains that are used by the cell in its synthesis of membranes and other lipid containing structural components or secretory products.[3][4]

Crystals: Crystalline inclusions have long been recognized as normal constituents of certain cell types such as Sertoli cells and Leydig cells of the human testis, and are found occasionally in macrophages.[4] It is believed that these structures are crystalline forms of certain proteins, and are located everywhere in the cell, including the nucleus, mitochondria, endoplasmic reticulum, Golgi body, and free in cytoplasmic matrix.[3][4]

Pigments: The most common pigment in the body, besides hemoglobin of red blood cells, is melanin, manufactured by melanocytes of the skin and hair, pigment cells of the retina and specialized nerve cells in the substantia nigra of the brain.[3] These pigments have protective functions in the skin and aid in the sense of sight in the retina, but their function in neurons is not understood completely. Furthermore, cardiac tissue and central nervous system neurons show a yellow to brown pigment called lipofuscin, believed by some to have lysosomal activity.[4]

References

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Inclusion (cell)

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In cell biology, cellular inclusions are non-living, non-membrane-bound structures residing within the cytoplasm of eukaryotic cells, primarily functioning as storage depots for metabolic products, nutrients, and waste materials essential to cellular homeostasis and specialized functions.[1] Unlike membrane-bound organelles such as mitochondria or the endoplasmic reticulum, which exhibit active metabolic processes, inclusions lack independent enzymatic activity and are transient, appearing and disappearing based on the cell's physiological needs.[2] These structures are particularly prominent in long-lived cells like hepatocytes, neurons, and adipocytes, where they accumulate to support energy reserves, pigmentation, or detoxification.[1] Common types of cellular inclusions include glycogen granules, which store carbohydrates as a readily accessible energy source in liver and muscle cells; lipid droplets, serving as fat reserves for energy metabolism and membrane synthesis in various tissues; and pigment granules such as melanin, which provide ultraviolet radiation protection in skin cells.[2] Other notable examples encompass protein crystals in certain plant cells, such as in seeds, for storage of reserve proteins,[3] secretory products like hormones or enzymes in glandular cells, and residual bodies such as lipofuscin, which represent indigestible waste from lysosomal activity and accumulate with cellular aging.[1] In prokaryotic cells, inclusions often take the form of polyphosphate granules or gas vacuoles, aiding in nutrient storage or buoyancy, respectively, though the term is most frequently applied to eukaryotic contexts.[4] Beyond storage, cellular inclusions play diverse roles in cellular adaptation and pathology; for instance, they can form as inclusion bodies in response to viral infections or protein misfolding in neurodegenerative diseases, disrupting normal cellular architecture.[1] In plants, inclusions like calcium oxalate crystals deter herbivory, while in animals, they contribute to processes such as biomineralization or detoxification.[5] Visible under light or electron microscopy, these structures highlight the cell's capacity for dynamic material management, underscoring their importance in both normal physiology and disease states.[2]

Definition and Characteristics

Definition

Cellular inclusions are non-membrane-bound, non-living structures located within the cytoplasm of both prokaryotic and eukaryotic cells, primarily functioning to store or accumulate substances such as nutrients, pigments, or waste products.[1] These structures represent aggregates of molecules or materials that are not enclosed by lipid membranes, distinguishing them from the more organized, dynamic components of the cell.[6] Key characteristics of cellular inclusions include their variable size, which can range from tens of nanometers to several micrometers depending on the type and cellular context, and their complete lack of independent metabolic activity, as they do not possess the enzymatic machinery for self-sustained processes.[7] Additionally, inclusions exhibit a transient nature, forming and degrading in response to fluctuating cellular needs, such as nutrient availability or environmental stresses, without contributing to ongoing metabolic pathways.[8] Unlike membrane-bound organelles, which actively participate in cellular metabolism, inclusions serve as passive repositories.[9] From an evolutionary perspective, cellular inclusions are ancient adaptations that likely emerged in early prokaryotic cells to manage resources efficiently in fluctuating primordial environments, enabling survival through the sequestration of essential or excess materials long before the development of complex eukaryotic structures.[6] Their presence across diverse taxa underscores their fundamental role in cellular economy, conserved through billions of years of evolution.[10]

Distinction from Organelles

Cellular inclusions differ from organelles primarily in their structural organization, vitality, and functional roles. Organelles are specialized, often membrane-bound structures—such as mitochondria or the endoplasmic reticulum—or non-membrane-bound entities like ribosomes that exhibit defined composition and actively participate in metabolic processes, including energy production and macromolecular synthesis.[1] In contrast, inclusions are non-membrane-bound structures or aggregates of non-living materials, lacking enzymatic activity and serving passive roles like storage or accumulation of waste products.[1] This distinction underscores that while organelles are essential, dynamic components integral to cellular homeostasis, inclusions are transient byproducts of cellular metabolism without independent metabolic capabilities.[11] Boundary cases highlight the nuances in classification, particularly for non-membrane-bound structures. Ribosomes, for instance, are categorized as organelles despite their lack of a delimiting membrane, owing to their ribosomal RNA-protein complex structure and critical enzymatic function in translating mRNA into polypeptides.[12] Polyphosphate granules, however, exemplify true inclusions, comprising inorganic polyphosphate chains stored in prokaryotic and eukaryotic cells for phosphate buffering and energy reserve, without any associated catalytic machinery.[11]

Classification

By Chemical Composition

Cellular inclusions can be classified based on their predominant chemical composition, which provides insight into their structural and material characteristics. This taxonomy groups them into major categories such as carbohydrate-based, lipid-based, protein-based, mineral-based, and pigment-based, reflecting the diverse molecular substances they store or accumulate within the cytoplasm.[13] Carbohydrate-based inclusions, such as glycogen granules, consist primarily of polysaccharides that serve as energy reserves. These structures are formed by the polymerization of glucose units and appear as irregular, electron-dense particles under microscopy.[13] Lipid-based inclusions, exemplified by triglyceride droplets, are composed of neutral fats and oils, often enveloped by a phospholipid monolayer. These droplets form through lipid synthesis and accumulation, exhibiting a hydrophobic core that excludes water. Polyhydroxyalkanoates, another lipid-like polymer, represent similar storage forms in certain cells.[13][14] Protein-based inclusions, including crystalloids, are aggregates of amino acid polymers or enzymes, often arranged in ordered, crystalline lattices. These may include protein shells surrounding enzymatic contents, contributing to their compact, insoluble nature.[13][14] Mineral-based inclusions, such as calcium oxalate crystals or calcium deposits, incorporate inorganic ions like calcium, forming insoluble salts or complexes. Hemosiderin, an iron-storage complex, also falls into this category due to its mineral-like iron core derived from heme breakdown. These structures often result from precipitation of ions in supersaturated cellular environments.[13] Pigment-based inclusions, like melanin granules, contain organic pigments such as melanins or lipofuscin, which are oxidation products of biomolecules. Melanin consists of polymerized phenolic compounds, while lipofuscin arises from lipid peroxidation residues, both imparting color and insolubility to the inclusions.[13] Physical properties of inclusions vary by composition, influencing their behavior in cellular environments. Carbohydrate-based inclusions, like glycogen, are generally water-soluble with low density, allowing diffuse storage in the cytoplasm. Lipid-based droplets are hydrophobic, low-density structures that float in aqueous media and resist aqueous solvents. Protein-based crystalloids exhibit moderate density and insolubility in neutral pH, often appearing hyaline or eosinophilic. Mineral inclusions, such as calcium oxalate, are highly insoluble and dense, forming rigid crystals with high refractive indices. Pigment inclusions show variable solubility but are typically insoluble, with densities influenced by their organic-inorganic mix, and distinct coloration due to light absorption. Staining affinities further distinguish them; for instance, glycogen displays magenta coloration with periodic acid-Schiff (PAS) stain, lipids take up Oil Red O or Sudan black for red-black hues, proteins appear pink with hematoxylin and eosin (H&E), minerals like calcium basophilize with von Kossa stain, and pigments like hemosiderin turn blue with Prussian blue.[13] Detection of inclusion composition relies on targeted methods to verify molecular makeup. Biochemical assays, such as enzymatic digestion (e.g., diastase for glycogen) or extraction (e.g., chloroform-methanol for lipids), quantify specific components and confirm solubility profiles. Histochemical staining, including PAS for carbohydrates, Oil Red O for lipids, and special stains like von Kossa for minerals, provides compositional identification in tissue sections. Electron microscopy, particularly transmission electron microscopy (TEM), reveals ultrastructural details like electron-dense cores in protein crystalloids or lipid envelopes, while cryo-electron tomography aids in visualizing protein shells. These techniques collectively ensure precise chemical taxonomy without relying on functional inference.[13][14]

By Function

Cellular inclusions are classified by their physiological functions, which encompass nutrient storage, detoxification, pigmentation, and structural support, thereby enabling cells to adapt to diverse environmental challenges. This functional categorization complements compositional analyses by emphasizing the adaptive roles these structures play in maintaining cellular homeostasis and survival under stress. Inclusions often integrate multiple functions, with their prevalence and capacity varying based on external pressures such as nutrient availability or radiation exposure. Nutrient storage inclusions primarily accumulate energy reserves to sustain cellular metabolism during periods of scarcity. In prokaryotes, poly-β-hydroxybutyrate (PHB) granules, composed of lipid polymers, serve as carbon and energy depots, accumulating to up to 90% of the cell's dry mass under unbalanced growth conditions like excess carbon relative to nitrogen.[15] Similarly, glycogen granules, polysaccharide aggregates of glucose units, provide readily mobilizable energy, with granules typically ranging from 20-100 nm in diameter.[16] In eukaryotes, analogous structures include glycogen rosettes (α-particles, 100-400 nm) and lipid droplets of triglycerides, which store long-term glucose and fats, respectively, and can be visualized via PAS or Sudan staining.[17] Under nutrient stress, these inclusions expand to occupy substantial cellular volume—PHB alone can constitute a major fraction of cytoplasmic space—facilitating prolonged viability in resource-limited environments.[18] Detoxification inclusions sequester toxic compounds or byproducts to prevent cellular damage. Prokaryotic sulfur globules store elemental sulfur, enabling the oxidation of harmful hydrogen sulfide (H₂S) into less toxic sulfate, thus supporting metabolism in sulfide-rich settings.[16] Polyphosphate (volutin) granules bind heavy metals and maintain phosphate homeostasis, contributing to tolerance against metal toxicity.[16] In eukaryotes, hemosiderin aggregates iron from hemoglobin breakdown, averting oxidative stress from free ferric ions, while lipofuscin accumulates oxidized lipids and proteins in lysosomes, potentially buffering reactive oxygen species.[17] These structures enhance resilience in contaminated or metabolically active conditions, with accumulation levels scaling with exposure intensity. Pigmentation inclusions shield cells from light-induced damage through light absorption and antioxidant properties. Eukaryotic melanin granules, electron-dense organelles (0.3 × 0.7 μm), absorb UV radiation, reducing photochemical harm to DNA and proteins.[17] In prokaryotes, carotenoid granules act similarly, dissipating excess energy from UV exposure and neutralizing singlet oxygen, thereby preventing lipid peroxidation.[19] These pigments adapt cells to high-irradiance environments, with granule density increasing under chronic light stress to optimize photoprotection without impeding other functions. Structural support inclusions provide mechanical or positional advantages for cellular orientation and stability. Prokaryotic gas vacuoles, assemblies of gas-filled protein cylinders (75 nm diameter, 200-1,000 nm length), occupy 3-10% of cell volume to decrease density and enable buoyancy adjustment in aqueous media.[20] Magnetosomes, chains of magnetite (Fe₃O₄) crystals (40-100 nm), facilitate magnetotaxis, aligning cells along geomagnetic fields for efficient navigation toward optimal conditions.[16] In eukaryotes, crystalline inclusions like Reinke crystals offer rigidity, though less prevalent. These structures are crucial for habitat exploitation, with volume adjustments directly influencing adaptive positioning. The adaptive significance of these functions is evident in how inclusions respond to environmental pressures; for example, storage capacities amplify under nutrient or osmotic stress, while osmoregulatory inclusions like ion-binding granules in extremophiles maintain turgor by countering salinity gradients.[17] Overall, functional versatility allows inclusions—often referencing prior compositional details like lipid or protein matrices—to integrate into broader stress responses, prioritizing survival over growth.

Formation and Dynamics

Biosynthesis

The biosynthesis of cellular inclusions involves distinct mechanisms tailored to their chemical composition, such as carbohydrates, lipids, and minerals. For carbohydrate-based inclusions like glycogen granules, formation occurs through enzymatic polymerization initiated by glycogenin, a self-glucosylating primer protein that attaches UDP-glucose units to form a short oligosaccharide chain. Glycogen synthase then elongates this chain by catalyzing the addition of glucose monomers via α-1,4-glycosidic bonds, creating branched structures with the aid of branching enzyme. This process assembles glycogen particles visible as electron-dense inclusions in the cytoplasm, particularly in liver and muscle cells.[21] Lipid inclusions, such as lipid droplets, form via phase separation of neutral lipids within the endoplasmic reticulum (ER) bilayer. Neutral lipids like triglycerides and sterol esters are synthesized by enzymes including diacylglycerol acyltransferase (DGAT) and acyl-CoA:cholesterol acyltransferase (ACAT), leading to their accumulation and coalescence into an oil lens between the ER leaflets. This lens buds outward, acquiring a phospholipid monolayer from the ER to form mature droplets that detach into the cytosol. The process is facilitated by proteins like seipin, which stabilize the ER curvature at budding sites.[22][23] Mineral inclusions arise through crystallization under conditions of supersaturation, often mediated by biomineralization pathways in prokaryotes and eukaryotes. These mechanisms ensure ordered assembly rather than random precipitation.[24][25] Protein inclusions, such as crystals in certain plant cells, form through supersaturation of soluble proteins, leading to nucleation and growth of ordered lattices influenced by cellular factors like pH, ionic strength, and macromolecular crowding.[26] Regulatory factors, including hormonal signals and nutrient availability, tightly control inclusion biosynthesis across compositions. For instance, insulin activates glycogen synthase by promoting its dephosphorylation via protein phosphatase 1, enhancing glucose uptake and polymerization in response to high blood sugar. Nutrient levels, such as elevated fatty acids, induce DGAT expression to drive lipid droplet formation during energy surplus. Similarly, mineral crystallization is modulated by environmental ions and cellular pH, with supersaturation thresholds dictating onset.[21][27] Biosynthesis timescales vary by inclusion type, reflecting their biophysical properties. Lipid droplets can assemble rapidly, often within minutes of neutral lipid synthesis in the ER, allowing quick adaptation to nutrient influx. In contrast, mineral crystals accumulate more slowly, over hours, as supersaturation builds and nucleation propagates growth. Glycogen granules form on an intermediate timescale, with initial priming in seconds but full particle maturation requiring minutes to hours depending on glucose availability.[22][24]

Degradation

The degradation of cellular inclusions involves specific catabolic pathways that dismantle these non-membrane-bound structures to release their components for metabolic reuse, ensuring cellular homeostasis under varying physiological demands.[28] For carbohydrate-based inclusions like glycogen granules, degradation primarily occurs through enzymatic hydrolysis in the cytosol. Glycogen phosphorylase catalyzes the phosphorolytic cleavage of α-1,4-glycosidic bonds, releasing glucose-1-phosphate from the non-reducing ends of glycogen chains, while a debranching enzyme handles α-1,6 branch points to yield free glucose.[28] This process is complemented by lysosomal degradation in some contexts, where acid α-glucosidase hydrolyzes glycogen within lysosomes.[29] Lipid inclusions, such as lipid droplets, are broken down via lipolysis, a sequential enzymatic hydrolysis of triacylglycerols. Adipose triglyceride lipase (ATGL) initiates the reaction by converting triacylglycerols to diacylglycerols, followed by hormone-sensitive lipase (HSL) producing monoacylglycerols, and monoglyceride lipase (MGL) yielding free fatty acids and glycerol.[30] Additionally, lipophagy—an autophagic pathway—facilitates lysosomal fusion, engulfing lipid droplets for acid hydrolase-mediated breakdown.[30] Proteinaceous aggregates, another type of inclusion, are degraded through lysosomal fusion via macroautophagy (aggrephagy). Cytoplasmic aggregates are sequestered into autophagosomes by adaptor proteins like p62, which fuse with lysosomes where cathepsins and other hydrolases dismantle the cargo.[31] Degradation is triggered by signals such as nutrient deprivation or stress. In starvation, glucagon elevates cAMP levels, activating protein kinase A to phosphorylate and stimulate glycogen phosphorylase and HSL in liver and adipose tissue, respectively.[28][30] Cellular stress responses, including mTOR inhibition, upregulate autophagy for aggregate clearance.[31] The byproducts of inclusion degradation are efficiently recycled into central metabolism. For glycogen, approximately 90% of the polymer is recovered as glucose-1-phosphate, which is converted to glucose-6-phosphate for entry into glycolysis or gluconeogenesis, while the remaining ~10% yields free glucose.[32] Lipid breakdown provides fatty acids for β-oxidation and glycerol for gluconeogenesis, supporting energy production.[30] Protein aggregates yield amino acids that replenish the proteome or fuel biosynthesis.[31]

Biological Roles

Storage and Reserve Functions

Cellular inclusions primarily function as nutrient reservoirs, enabling cells to buffer against fluctuations in environmental nutrient availability and maintain metabolic homeostasis. These non-membrane-bound structures store essential macromolecules such as carbohydrates and lipids, which can be rapidly mobilized during periods of scarcity to support energy demands. For example, glycogen granules in liver cells serve as a key reserve of glucose, helping to stabilize blood glucose levels by releasing stored sugars as needed during fasting or increased metabolic activity.[33][17] The efficiency of inclusions as energy stores varies by their chemical composition, with lipid-based inclusions offering higher energy density than carbohydrate-based ones. Lipids stored in droplets provide approximately 9 kcal per gram, allowing for compact, high-yield energy reserves suitable for long-term storage, whereas carbohydrates like glycogen yield about 4 kcal per gram but enable quicker mobilization for immediate needs.[34][35] This differential efficiency supports cellular adaptation to diverse physiological states, prioritizing lipid reserves for sustained energy buffering in nutrient-variable conditions. From an evolutionary perspective, storage inclusions provide significant survival benefits in fluctuating environments by decoupling cellular metabolism from transient resource availability, enhancing fitness in heterogeneous habitats. In model organisms like bacteria, such as Pseudomonas and Streptomyces, accumulation of storage compounds like polyhydroxybutyrate granules improves starvation tolerance, with benefits persisting even after reserves are depleted through sustained physiological adaptations.[36][37] Similarly, microbial studies highlight how investment in these inclusions promotes diverse metabolic strategies, increasing resilience and competitive advantage in nutrient-limited settings.[38]

Structural and Protective Roles

In prokaryotes, particularly cyanobacteria, gas vacuoles serve as structural inclusions that confer buoyancy to cells, enabling vertical migration in aquatic environments to optimize light exposure for photosynthesis. These hollow, proteinaceous structures, composed of gas vesicles, reduce cell density and maintain positional stability without requiring energy input for flotation.[39] In plant cells, crystal inclusions such as silica phytoliths embedded within cell walls provide mechanical reinforcement, enhancing rigidity and resistance to physical stress by infilling lumina and strengthening the extracellular matrix. These opaline silica deposits, formed through biomineralization, contribute to overall cell wall integrity and structural support in tissues exposed to environmental pressures.[40] Protective roles of inclusions often involve shielding cells from oxidative damage and toxic substances. Pigment inclusions containing carotenoids, found in various algal and plant cells, act as antioxidants by scavenging reactive oxygen species and free radicals, thereby preventing lipid peroxidation and maintaining membrane integrity during stress conditions.[41] Similarly, detoxification granules, such as metal-accumulating structures in invertebrate cells, sequester heavy metals like cadmium and copper into insoluble forms, isolating them from sensitive cellular components to mitigate toxicity and preserve metabolic function.[42] Many inclusions interface with the cytoskeleton to ensure precise positioning and enhance cellular stability. For instance, in eukaryotic cells, protective granules and pigment bodies associate with actin filaments and microtubules via motor proteins like dynein and kinesin, facilitating directed transport and anchoring that prevents diffusion and supports localized defensive functions. This dynamic tethering not only stabilizes inclusions against mechanical perturbations but also integrates them into the broader cytoskeletal network for coordinated cellular responses.[43]

Examples in Organisms

In Prokaryotes

In prokaryotes, such as bacteria and archaea, cell inclusions primarily serve as storage depots for nutrients and other compounds, often accumulating under nutrient-rich or stress conditions to support survival and metabolic flexibility. These structures are typically non-membrane-bound or enveloped by simple protein shells, distinguishing them from the more complex organelles in eukaryotes. Common inclusions include granules of polyhydroxybutyrate (PHB), which act as carbon and energy reserves in diverse prokaryotic species.[44] PHB granules are synthesized by many bacteria, including species of Ralstonia and Azospirillum, through the polymerization of 3-hydroxybutyrate monomers via dedicated enzymes like PHB synthase, enabling cells to store excess carbon when organic substrates are abundant. These granules can constitute up to 80% of the cell's dry weight in some accumulators, providing a hydrophobic reserve that is mobilized during starvation via depolymerization to acetyl-CoA. In Caulobacter crescentus, PHB formation requires the polymerase PhaC, phasins for surface stabilization, and attachment to the nucleoid via proteins like PhaM, highlighting their role in spatial organization within the prokaryotic cytoplasm.[45][46][47] Gas vacuoles, also known as gas vesicles, are cylindrical or spindle-shaped inclusions composed of a protein shell that encloses a gas-filled interior, providing buoyancy to aquatic prokaryotes such as cyanobacteria (e.g., Anabaena species) and halophilic archaea. These structures, formed by stacking ribbed cylindrical proteins (GvpA) and stabilized by GvpC, allow cells to regulate their position in the water column for optimal light exposure during photosynthesis or to avoid UV radiation. Gas vacuoles collapse under high pressure but reform under low pressure conditions, enabling dynamic flotation control in stratified environments.[48] Magnetosomes represent a specialized inclusion unique to magnetotactic bacteria, such as Magnetospirillum species, consisting of chains of membrane-bound magnetite (Fe₃O₄) or greigite (Fe₃S₄) nanocrystals that enable magnetic navigation toward optimal microoxic environments in aquatic sediments. Biogenesis involves compartmentalized biomineralization, where dedicated genes (e.g., mam operon) control crystal size (typically 30-120 nm) and alignment, conferring a cellular magnetic moment up to 10 times stronger than abiotic analogs. This adaptation enhances chemotaxis in redox-stratified habitats, as demonstrated in Desulfovibrio magneticus, where magnetosomes align along the cell axis for directional motility.[49][50] Sulfur globules are transient inclusions in sulfur-oxidizing prokaryotes, including purple sulfur bacteria like Chromatium vinosum and colorless thiobacteria such as Thiobacillus species, where they store elemental sulfur (S⁰) as an intermediate during the oxidation of sulfide (H₂S) or thiosulfate to sulfate. These globules, often located periplasmically or intracellularly, are enveloped by proteins like sulfur globule proteins (e.g., SgpA in Thiocapsa roseopersicina), preventing toxicity and allowing gradual oxidation for energy generation under anaerobic or microaerobic conditions. In Allochromatium vinosum, globules can accumulate to 30-50% of cell volume, serving as a buffer against fluctuating sulfur inputs in chemolithotrophic niches.[51][52][53] Volutin granules, also known as metachromatic or polyphosphate granules, function as phosphate storage in prokaryotes like Corynebacterium and certain archaea, accumulating inorganic polyphosphate chains that provide phosphorus reserves and contribute to osmotic stress tolerance by maintaining turgor pressure. These granules form via polyphosphate kinases under phosphate excess and are degraded during nutrient limitation to supply phosphate for nucleic acid synthesis or energy metabolism. In osmotic stress scenarios, such as high salinity, volutin accumulation in Lactobacillus species enhances survival by sequestering ions and supporting membrane integrity.[54][55][56] Visualization of prokaryotic inclusions often relies on specific staining techniques, with the Neisser stain being particularly effective for detecting volutin and other polyphosphate-rich granules by producing a characteristic purple metachromatic color against a yellow background. This method, involving methylene blue and crystal violet, differentiates phosphate accumulators in microbial communities, as applied in studies of activated sludge bacteria where Neisser-positive filaments indicate polyphosphate storage. Transmission electron microscopy further reveals granule ultrastructure, confirming their dense, amorphous composition in species like Escherichia coli.[57][55][58]

In Eukaryotes

In eukaryotic cells, inclusions represent a diverse array of non-membrane-bound or partially enclosed structures that store vital metabolites, contrasting with the simpler, often metabolically inert inclusions found in prokaryotes. These structures enable specialized functions tailored to the complex, multicompartmental architecture of eukaryotes, such as energy reserve accumulation and pigmentation.[59] Among the most prevalent types are lipid droplets, which serve as primary fat storage sites in adipocytes and other cells. Composed of a neutral lipid core surrounded by a phospholipid monolayer and associated proteins, lipid droplets form dynamically to sequester excess lipids and prevent cellular toxicity. In muscle cells, glycogen rosettes—branched polymers of glucose appearing as rosette-like β-particles under electron microscopy—act as readily mobilizable energy reserves, supporting rapid ATP production during contraction. Hemoglobin crystals, observed in erythrocytes particularly in conditions like hemoglobin C disease, manifest as dense, rod-shaped aggregates that alter red blood cell deformability and oxygen transport efficiency.[59][60][61] Eukaryotic inclusions exhibit remarkable diversity across kingdoms, reflecting adaptations to specific physiological needs. In plants, aleurone grains in seed endosperm cells store proteins and minerals, such as phytin in globoid inclusions, facilitating nutrient mobilization during germination. Calcium oxalate crystals, common in leaves, stems, and other tissues of many plants (e.g., Dieffenbachia species), form as druse or prism shapes to regulate intracellular calcium levels, detoxify excess oxalate, and deter herbivory through their indigestible nature.[62][63] Animal-specific examples include melanin granules, aggregates of melanin pigment in skin cells that provide coloration and ultraviolet radiation protection.[17] Regarding compartmentalization, many eukaryotic inclusions associate closely with the endoplasmic reticulum (ER) or reside freely in the cytoplasm without enclosing membranes, allowing dynamic interactions with metabolic pathways. For instance, lipid droplets bud directly from the ER bilayer, integrating with cytosolic lipid synthesis enzymes, while glycogen rosettes and hemoglobin crystals distribute throughout the cytoplasm to optimize accessibility. This non-membranous or ER-proximal positioning contrasts with fully enclosed organelles, enabling rapid remodeling in response to cellular demands.[59][61]

Pathological Significance

In Disease Processes

In various neurodegenerative diseases, abnormal accumulation of cellular inclusions contributes to cellular dysfunction and toxicity. In Alzheimer's disease, intracellular aggregates of amyloid-beta protein form within neurons, disrupting normal cellular processes and promoting neurotoxicity. These inclusions, often accompanied by extracellular plaques, are hallmark features that impair synaptic function and lead to neuronal loss. Similarly, in Parkinson's disease, Lewy bodies—intracellular inclusions primarily composed of aggregated alpha-synuclein protein—accumulate in dopaminergic neurons of the substantia nigra, leading to motor deficits through mechanisms involving protein misfolding and impaired proteostasis.[64][65][66] In amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD), TAR DNA-binding protein 43 (TDP-43) forms ubiquitinated, hyperphosphorylated cytoplasmic inclusions in neurons and glia, which are present in nearly all ALS cases and a majority of FTD cases. These inclusions result from TDP-43 mislocalization from the nucleus, leading to loss of RNA processing functions and gain of toxic properties through aggregation, contributing to motor neuron degeneration and cognitive decline. Recent 2025 research highlights TDP-43's role in cellular fate decisions and prion-like propagation, advancing understanding of disease mechanisms.[67][68] Lipofuscin, an autofluorescent pigment granule, accumulates as intracellular inclusions in post-mitotic cells during aging, reflecting oxidative damage and lysosomal dysfunction. In pathological contexts, such as age-related macular degeneration or certain cardiomyopathies, excessive lipofuscin buildup exacerbates cellular stress by sequestering vital cellular components and inhibiting autophagic clearance, thereby accelerating tissue degeneration. In Huntington's disease, dysregulated biosynthesis of mutant huntingtin protein with expanded polyglutamine tracts leads to the formation of intranuclear and cytoplasmic inclusions, which exert toxicity through sequestration of transcription factors and disruption of proteasomal degradation pathways.[69][70][71] Historical observations of pathological inclusions date back to the 19th century, with early microscopic examinations revealing their presence in infectious diseases. Notably, in 1903, Adelchi Negri described Negri bodies—eosinophilic cytoplasmic inclusions in neurons infected with the rabies virus—as a diagnostic hallmark, marking a pivotal advancement in understanding viral neuropathology. These viral inclusions, composed of viral ribonucleoproteins, disrupt neuronal function and propagate infection, underscoring the long-recognized role of inclusions in disease pathogenesis.[72]

Diagnostic and Research Applications

In clinical diagnostics, immunohistochemistry (IHC) serves as a key tool for identifying cellular inclusions in tissue biopsies, enabling the characterization of pathological conditions. For instance, Russell bodies—eosinophilic inclusions within plasma cells—are detected through IHC staining for markers such as CD138 on the containing cells and immunoglobulins (e.g., kappa or lambda light chains) within the inclusions themselves, aiding in the diagnosis of plasma cell disorders like multiple myeloma or Russell body gastritis.[73][74] This approach distinguishes inclusions from mimics, such as fungal organisms or crystalline structures, by confirming their immunoglobulin composition and cellular origin.[75] In research, live-cell imaging techniques provide dynamic insights into inclusion formation and behavior, surpassing static histological methods. Fluorescence microscopy, often combined with time-lapse imaging and techniques like FRAP (fluorescence recovery after photobleaching), visualizes the intracellular trafficking and aggregation of protein inclusions, such as polyglutamine aggregates in neurodegenerative disease models.[76][77] These methods reveal divergent dynamics, like the sequestration of misfolded proteins into aggresomes, supporting models of cellular stress responses.[76] CRISPR-based models have advanced the study of inclusion body formation by enabling precise genetic perturbations in cellular systems. Genome-wide CRISPR screens, for example, have identified regulators like SRRD, which influences intermediate filament dynamics and aggresome assembly during protein inclusion formation.[78] Similarly, CRISPR/Cas9-engineered induced pluripotent stem cell (iPSC) models rapidly induce aggregation-prone proteins in neurons, facilitating high-throughput screening of inclusionopathy mechanisms without relying on transgenic animals.[79] Therapeutically, cellular inclusions such as lipid droplets are harnessed for targeted drug delivery due to their biocompatibility and natural accumulation in cells. Artificial lipid droplets, engineered as nanoparticles with a phospholipid monolayer, encapsulate hydrophobic drugs like paclitaxel, enhancing cellular uptake and sustained release while mimicking endogenous storage functions.[80][81] This approach leverages lipid droplets' role in cells for applications in cancer therapy, where they improve drug solubility and reduce systemic toxicity compared to traditional liposomes.[82]

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