is a term for biological studies involving tissues, organs, or cells maintained outside their native organism under controlled laboratory conditions. By carefully managing factors such as temperature, oxygenation, humidity, and nutrient delivery, as well as by perfusing a nutrient solution through the vasculature, researchers sustain the extracted materials' viability, structural integrity, and physiological function temporarily to conduct experiments that would be difficult or unethical in a living body. Exvivo models are a middle ground between in vitro () models, which typically use isolated cells, and in vivo () studies conducted inside living organisms.
Ex vivo models are used for pharmacological screening, toxicology testing, organ transplant evaluation, developmental biology, as well as studies of disease mechanisms across fields ranging from cardiology to neuroscience, dermatology, and orthopedics. Since the models often use human tissues obtained from clinical procedures or biobanks, they can reduce reliance on live-animal experimentation; their utility, however, is limited by finite viability, incomplete systemic integration, and post-mortem () biochemical changes that accumulate over time. The earliest perfusion studies were conducted in the mid-19thcentury, and subsequent advances in sterilization, imaging, and microfluidics enabled wider adoption into the 20th and 21stcenturies. Regulatory oversight depends on specimen origin: human exvivo research is subject to informed consent, whereas animal-derived specimens fall under animal care guidelines.
Principles and definition
Ex vivo, literally 'out of the living' in Latin, refers to biological studies involving tissues, organs, or cells maintained outside their native organism under tightly controlled laboratory conditions. These studies preserve the extracted materials' viability, physiological function, and structural integrity temporarily by managing conditions such as oxygenation, temperature, nutrient delivery, and humidity, depending on the specific requirements of the model. These conditions are often facilitated through cell culture media (nutrient-rich liquid formulations) or perfusion chambers that continuously pump fluid through tissue to mimic blood flow.
Physiological relevance is the degree to which an experimental system reproduces the structural, mechanical, biochemical, and functional characteristics of a living organism. In vivo models preserve the full complexity of physiology by conducting experiments within living organisms, therefore rank highest in terms of physiological relevance. As an intermediate approach between in vivo models and in vitro ones (the latter of which typically use isolated cells in artificial environments), exvivo models preserve more of the native tissue architecture than traditional cell cultures, while allowing greater experimental control than whole-organism studies. In doing so, exvivo models address some of the limitations of invitro work, such as oversimplified cellular interactions (how cells signal to or influence one another), and can help mitigate the systemic variability inherent to invivo models, as when fluctuations in an organism's hormones, immune status, and overall health obscure the effect being studied.
In the preclinical development of therapies for bone diseases, for example, in vitro cell studies are typically performed prior to in vivo testing in animals, as the latter approach is more costly, time-intensive, and complex, requiring large sample sizes to yield statistically meaningful results. However, in vitro findings do not always predict in vivo responses due to the absence of native tissue architecture, including the extracellular matrix (ECM), and the lack of cell–cell/cell–matrix interactions. Ex vivo bone explant cultures preserve many aspects of native tissue architecture by maintaining tissue integrity outside the organism for a limited period, and reduce the complexity of in vivo testing by excluding systemic variables and enabling controlled investigation of specific biological or mechanical factors. Another example is the use of ex vivo models during the preclinical evaluation of intestinal drug transport. Unlike in vivo studies, which rely on animal models and are therefore affected by interspecies differences, ex vivo approaches can utilize resected human intestinal segments, more accurately representing human physiological conditions.
The boundary between exvivo and invitro models remains contested, particularly in the fields of regenerative medicine and tissue engineering, where the terms have been used interchangeably in many studies. Invitro systems have progressed from simple two-dimensional cell cultures to advanced three-dimensional constructs, such as organoids and organ-on-a-chip devices, that replicate aspects of tissue architecture, further complicating the distinction. Klein and Hutmacher (2024) propose that a model may be classified as exvivo if it meets one or more of the following criteria:
It preserves the native structure and composition of a cell, tissue, or organ without disrupting its cellular or extracellular components. It is used in therapeutic contexts where cells, organs, or tissues are removed and then reimplanted. It links explanted organs or tissues to artificial circulation via perfusion.
According to these criteria, systems involving extensive reorganization or manipulation—including organoids, organ-on-a-chip, and organotypic cultures—are considered invitro, even when they replicate certain organ-level functions.
In general, invitro models are more flexible and relatively inexpensive, allowing researchers to test new treatments quickly and adjust experimental parameters as needed. Exvivo systems are less adaptable but often provide a more reliable indication of how a treatment will work in the human body and what side effects it might cause. They are, however, subject to inherent limitations, including post-mortem () alterations in biophysical properties, progressive tissue degradation, limited viability duration, and typically the absence or only artificial replication of circulation and innervation. These factors can restrict the models' capacity to reproduce long-term or systemic physiological effects. Some of these limitations also complicate direct comparisons with invivo systems; for example, studies measuring how electric fields behave in primate brain tissue during stimulation have found that results differ markedly between in vivo and ex vivo conditions—and the longer the tissue has been removed from the body, the greater the discrepancy, partly due to cooling and loss of normal biological function. In addition, microglia undergo substantial alterations under ex vivo conditions.
Techniques and applications Organ perfusion
Organ perfusion involves circulating oxygenated solutions through isolated organs to sustain their viability. For example, ex vivo lung perfusion (EVLP) is used to evaluate donor lungs prior to transplantation. An system includes a ventilator connected to the lung via an endotracheal tube to simulate natural respiration and enable alveolar gas exchange. The typical perfusion circuit consists of a centrifugal pump to circulate the perfusate, a reservoir to collect effluent, a heat exchanger for temperature regulation, a leukocyte filter to remove white blood cells, a flow probe to measure perfusate flow, and a membrane gas exchanger to adjust oxygen and carbon dioxide concentrations in the circulating fluid. Cannulas link the circuit to the lung's pulmonary artery and left atrial cuff.
In cardiovascular research, a Langendorff heart preparation removes a heart and perfuses it with a nutrient solution, preserving structure and conduction pathways for investigation of arrhythmias or drug effects without the complexity of an in vivo model. Several ex vivo perfusion systems have been developed to reduce ischemic injury during the organ preservation phase. One of them is the Organ Care System (OCS), which maintains the heart in a non-beating, but metabolically active, state by circulating donor blood; the blood is treated with heparin to prevent clotting and supplemented with a proprietary perfusate formulation. In translational pharmacology, perfusion platforms restore pulsatile blood flow in isolated human organs, enabling direct measurement of absorption, metabolism, and toxicity prior to first-in-human trials. By supplying pharmacokinetic data on viable human tissue rather than relying on animal models or cell assays, the platforms inform clinical trial decisions and may reduce animal testing.
Not all forms of organ perfusion are ex vivo; in situ perfusion techniques are employed during organ retrieval to restore blood flow to organs while they remain within the body, minimizing ischemic injury and preserving viability for transplantation. A related example is selective insitu perfusion during surgery, such as isolated hepatic perfusion (IHP), which is used for targeted chemotherapy.
Organ culture Organ culture typically involves maintaining organ sections or small fragments in static or semi-static conditions without active perfusion. The use of culture media to sustain excised tissues or organs does not alter their classification as ex vivo models, provided that the native tissue architecture remains preserved, in accordance with the criteria proposed by Klein and Hutmacher (2024). In dermatological research, human skin organ culture (HSOC) is a technique in which excised human skin is maintained in an artificial medium that preserves its native architecture. models are employed to study wound healing, drug penetration, and toxicological responses. By retaining the structural complexity of human skin, these models facilitate the investigation of conditions that are not reproducible in animal models, such as keloid formation.
Cell culture involves isolating individual cells from tissue and growing them in a medium enriched with nutrients and growth factors. While these cultures retain some functional characteristics of their tissue of origin, they often exhibit changes in phenotype and gene expression over time. Primary cell cultures, taken directly from tissue, retain physiological characteristics that immortalized cell lines lose over time, which is useful for studying cellular behavior, disease mechanisms, and drug effects in conditions closer to those found in vivo.
Microscopy and imaging Ex vivo microscopy (EVM) uses advanced digital microscopes (such as confocal or optical-coherence devices) to produce microscopic images of fresh tissue, without mounting thin sections on glass slides. Because the tissue stays intact, surgeons can assess tumor margins or examine biopsy samples during surgery.
Computed tomography (CT) is used in ex vivo research to produce non-destructive, high-resolution images of internal structures.
Energy–tissue interaction studies
Human skin explants from surgical procedures allow researchers to observe early-stage physiological responses to laser treatments in ways that closely resemble invivo conditions, though processes like re-epithelialization occur more slowly than in living tissue. In intervertebral disc research, ex vivo models that retain vertebral bone allow for testing potential drugs and investigating loading effects on disc degeneration and repair.