Gaussia luciferase for bioluminescence tumor monitoring in comparison with firefly luciferase

Gaussia luciferase (Gluc) is a secreted reporter, and its expression in living animals can be assessed by in vivo bioluminescence imaging (BLI) or blood assays. We characterized Gluc as an in vivo reporter in comparison with firefly luciferase (Fluc). Mice were inoculated subcutaneously with tumor cells expressing both Fluc and Gluc and underwent Fluc BLI, Gluc BLI, blood assays of Gluc activity, and caliper measurement. In Gluc BLI, the signal from the tumor peaked immediately and then decreased rapidly. In the longitudinal monitoring, all measures indicated an increase in tumor burden early after cell inoculation. However, the increase reached plateaus in Gluc BLI and Fluc BLI despite a continuous increase in the caliper measurement and Gluc blood assay. Significant correlations were found between the measures, and the correlation between the blood signal and caliper volume was especially high. Gluc allows tumor monitoring in mice and should be applicable to dual-reporter assessment in combination with Fluc. The Gluc blood assay appears to provide a reliable indicator of viable tumor burden, and the combination of a blood assay and in vivo BLI using Gluc should be promising for quantifying and localizing the tumors.     

In vivo bioluminescence imaging

In vivo bioluminescent imaging (BLI) is a versatile and sensitive tool that is based on detection of light emission from cells or tissues. Bioluminescence, the biochemical generation of light by a living organism, is a naturally occurring phenomenon. Luciferase enzymes, such as that from the North American firefly (Photinus pyralis), catalyze the oxidation of a substrate (luciferin), and photons of light are a product of the reaction. Optical imaging by bioluminescence allows a low-cost, noninvasive, and real-time analysis of disease processes at the molecular level in living organisms. Bioluminescence has been used to track tumor cells, bacterial and viral infections, gene expression, and treatment response. Bioluminescence in vivo imaging allows longitudinal monitoring of a disease course in the same animal, a desirable alternative to analyzing a number of animals at many time points during the course of the disease. We provide a brief introduction to BLI technology, specific examples of in vivo BLI studies investigating bacterial/viral pathogenesis and tumor growth in animal models, and highlight some future perspectives of BLI as a molecular imaging tool.     

Light emission requires exposure to the atmosphere in ex vivo bioluminescence imaging

The identification of organs bearing luciferase activity by in vivo bioluminescence imaging (BLI) is often difficult, and ex vivo imaging of excised organs plays a complementary role. This study investigated the importance of exposure to the atmosphere in ex vivo BLI. Mice were inoculated with murine pro-B cell line Ba/F3 transduced with firefly luciferase and p190 BCR-ABL. They were killed following in vivo BLI, and whole-body imaging was done after death and then after intraperitoneal air injection. In addition, the right knee was exposed and imaged before and after the adjacent bones were cut. Extensive light signals were seen on in vivo imaging. The luminescence disappeared after the animal was killed, and air injection restored the light emission from the abdomen only, suggesting a critical role of atmospheric oxygen in luminescence after death. Although no substantial light signal at the right knee was seen before bone cutting, light emission was evident after cutting. In conclusion, in ex vivo BLI, light emission requires exposure to the atmosphere. Bone destruction is required to demonstrate luciferase activity in the bone marrow after death.     

Dual-wavelength imaging of tumor progression by activatable and targeting near-infrared fluorescent probes in a bioluminescent breast cancer model

Bioluminescence imaging (BLI) has shown its appeal as a sensitive technique for in vivo whole body optical imaging. However, the development of injectable tumor-specific near-infrared fluorescent (NIRF) probes makes fluorescence imaging (FLI) a promising alternative to BLI in situations where BLI cannot be used or is unwanted (e.g., spontaneous transgenic tumor models, or syngeneic mice to study immune effects).In this study, we addressed the questions whether it is possible to detect tumor progression using FLI with appropriate sensitivity and how FLI correlates with BLI measurements. In addition, we explored the possibility to simultaneously detect multiple tumor characteristics by dual-wavelength FLI (~700 and ~800 nm) in combination with spectral unmixing. Using a luciferase-expressing 4T1-luc2 mouse breast cancer model and combinations of activatable and targeting NIRF probes, we showed that the activatable NIRF probes (ProSense680 and MMPSense680) and the targeting NIRF probes (IRDye 800CW 2-DG and IRDye 800CW EGF) were either activated by or bound to 4T1-luc2 cells. In vivo, we implanted 4T1-luc2 cells orthotopically in nude mice and were able to follow tumor progression longitudinally both by BLI and dual-wavelength FLI. We were able to reveal different probe signals within the tumor, which co-localized with immuno-staining. Moreover, we observed a linear correlation between the internal BLI signals and the FLI signals obtained from the NIRF probes. Finally, we could detect pulmonary metastases both by BLI and FLI and confirmed their presence histologically.Taken together, these data suggest that dual-wavelength FLI is a feasible approach to simultaneously detect different features of one tumor and to follow tumor progression with appropriate specificity and sensitivity. This study may open up new perspectives for the detection of tumors and metastases in various experimental models and could also have clinical applications, such as image-guided surgery.     

Factors influencing quantification of in vivo bioluminescence imaging: application to assessment of pancreatic islet transplants

The aim of this study is to determine and characterize factors influencing in vivo bioluminescence imaging (BLI) and apply them to the specific application of imaging transplanted pancreatic islets. Noninvasive quantitative assessment of transplanted pancreatic islets poses a formidable challenge. Murine pancreatic islets expressing firefly luciferase were transplanted under the renal capsule or into the portal vein of nonobese diabetic-severe combined immunodeficiency mice and the bioluminescence was quantified with a cooled charge coupled device camera and digital photon image analysis. The important, but often neglected, effects of wound healing, mouse positioning, and transplantation site on bioluminescence measurements were investigated by imaging a constant emission, isotropic light-emitting bead (lambda = 600) implanted at the renal or hepatic site. The renal beads emitted nearly four times more light than hepatic beads with a smaller spot size, indicating that light absorption and scatter are greatly influenced by the transplant site and must be accounted for in BLI measurements. Detected luminescence decreased with increasing angle between the mouse surface normal and optical axis. By defining imaging parameters such as postsurgical effects, animal positioning, and light attenuation as a function of transplant site, this study develops BLI as a useful imaging modality for quantitative assessment of islets post-transplantation.     

High resolution in vivo bioluminescent imaging for the study of bacterial tumour targeting

The ability to track microbes in real time in vivo is of enormous value for preclinical investigations in infectious disease or gene therapy research. Bacteria present an attractive class of vector for cancer therapy, possessing a natural ability to grow preferentially within tumours following systemic administration. Bioluminescent Imaging (BLI) represents a powerful tool for use with bacteria engineered to express reporter genes such as lux. BLI is traditionally used as a 2D modality resulting in images that are limited in their ability to anatomically locate cell populations. Use of 3D diffuse optical tomography can localize the signals but still need to be combined with an anatomical imaging modality like micro-Computed Tomography (μCT) for interpretation.In this study, the non-pathogenic commensal bacteria E. coli K-12 MG1655 and Bifidobacterium breve UCC2003, or Salmonella Typhimurium SL7207 each expressing the luxABCDE operon were intravenously (i.v.) administered to mice bearing subcutaneous (s.c) FLuc-expressing xenograft tumours. Bacterial lux signal was detected specifically in tumours of mice post i.v.-administration and bioluminescence correlated with the numbers of bacteria recovered from tissue. Through whole body imaging for both lux and FLuc, bacteria and tumour cells were co-localised. 3D BLI and μCT image analysis revealed a pattern of multiple clusters of bacteria within tumours. Investigation of spatial resolution of 3D optical imaging was supported by ex vivo histological analyses. In vivo imaging of orally-administered commensal bacteria in the gastrointestinal tract (GIT) was also achieved using 3D BLI. This study demonstrates for the first time the potential to simultaneously image multiple BLI reporter genes three dimensionally in vivo using approaches that provide unique information on spatial locations.     

Advancing molecular therapies through in vivo bioluminescent imaging

Effective development of therapeutics that target the molecular basis of disease is dependent on testing new therapeutic moieties and delivery strategies in animal models of human disease. Accelerating the analyses of these models and improving their predictive value through whole animal imaging methods, which provide data in real time and are sensitive to the subtle changes, are crucial for rapid advancement of these approaches. Modalities based on optics are rapid, sensitive, and accessible methods for in vivo analyses with relatively low instrumentation costs. In vivo bioluminescent imaging (BLI) is one of these optically based imaging methods that enable rapid in vivo analyses of a variety of cellular and molecular events with extreme sensitivity. BLi is based on the use of light-emitting enzymes as internal biological light sources that can be detected externally as biological indicators. BLI has been used to test spatio-temporal expression patterns of both target and therapeutic genes in living laboratory animals where the contextual influences of whole biological systems are preserved. BLI has also been used to analyze gene delivery, immune cell therapies, and the in vivo efficacy of inhibitory RNAs. New tools for BLI are being developed that will offer greater flexibility in detection and analyses. BLI can be used to accelerate the evaluation of experimental therapeutic strategies and whole body imaging offers the opportunity of revealing the effects of novel approaches on key steps in disease processes.     

Multi-modality imaging identifies key times for annexin V imaging as an early predictor of therapeutic outcome

Radiolabeled annexin V may provide an early indication of the success or failure of anticancer therapy on a patient-by-patient basis as an in vivo marker of tumor cell killing. An important question that remains is when, after initiation of treatment, should annexin V imaging be performed. To address this issue, we obtained simultaneous in vivo measurements of tumor burden and uptake of radiolabeled annexin V in the syngeneic orthotopic murine BCL1 lymphoma model using in vivo bioluminescence imaging (BLI) and small animal single-photon emission computed tomography (SPECT). BCL1 cells labeled for fluorescence and bioluminescence assays (BCL1-gfp/luc) were injected into mice at a dose that leads to progressive disease within two to three weeks. Tumor response was followed by BLI and SPECT before and after treatment with a single dose of 10 mg/kg doxorubicin. Biodistribution analyses revealed a biphasic increase of annexin V uptake within the tumor-bearing tissues of mice. An early peak occurring before actual tumor cells loss was observed between 1 and 5 hr after treatment, and a second longer sustained rise from 9 to 24 hr after therapy, which heralds the onset of tumor cell loss as confirmed by BLI. Multimodality imaging revealed the temporal patterns of tumor cell loss and annexin V uptake revealing a better understanding of the timing of radiolabeled annexin V uptake for its development as a marker of therapeutic efficacy.     

Bioluminescence and magnetic resonance imaging of macrophage homing to experimental abdominal aortic aneurysms

Macrophage infiltration is a prominent feature of abdominal aortic aneurysm (AAA) progression. We used a combined imaging approach with bioluminescence (BLI) and magnetic resonance imaging (MRI) to study macrophage homing and accumulation in experimental AAA disease. Murine AAAs were created via intra-aortic infusion of porcine pancreatic elastase. Mice were imaged over 14 days after injection of prepared peritoneal macrophages. For BLI, macrophages were from transgenic mice expressing luciferase. For MRI, macrophages were labeled with iron oxide particles. Macrophage accumulation during aneurysm progression was observed by in situ BLI and by in vivo 7T MRI. Mice were sacrificed after imaging for histologic analysis. In situ BLI (n = 32) demonstrated high signal in the AAA by days 7 and 14, which correlated significantly with macrophage number and aortic diameter. In vivo 7T MRI (n = 13) at day 14 demonstrated T?* signal loss in the AAA and not in sham mice. Immunohistochemistry and Prussian blue staining confirmed the presence of injected macrophages in the AAA. BLI and MRI provide complementary approaches to track macrophage homing and accumulation in experimental AAAs. Similar dual imaging strategies may aid the study of AAA biology and the evaluation of novel therapies.     

Making the Brain Glow: In Vivo Bioluminescence Imaging to Study Neurodegeneration

Bioluminescence imaging (BLI) takes advantage of the light-emitting properties of luciferase enzymes, which produce light upon oxidizing a substrate (i.e., d-luciferin) in the presence of molecular oxygen and energy. Photons emitted from living tissues can be detected and quantified by a highly sensitive charge-coupled device camera, enabling the investigator to noninvasively analyze the dynamics of biomolecular reactions in a variety of living model organisms such as transgenic mice. BLI has been used extensively in cancer research, cell transplantation, and for monitoring of infectious diseases, but only recently experimental models have been designed to study processes and pathways in neurological disorders such as Alzheimer disease, Parkinson disease, or amyotrophic lateral sclerosis. In this review, we highlight recent applications of BLI in neuroscience, including transgene expression in the brain, longitudinal studies of neuroinflammatory responses to neurodegeneration and injury, and in vivo imaging studies of neurogenesis and mitochondrial toxicity. Finally, we highlight some new developments of BLI compounds and luciferase substrates with promising potential for in vivo studies of neurological dysfunctions.     

Secreted Luciferase for In Vivo Evaluation of Systemic Protein Delivery in Mice

A naturally secreted Gaussia luciferase (Gluc) has been utilized as a reporter for bioluminescence imaging (BLI) evaluation. However, the potential application of Gluc for in vivo monitoring of systemic protein delivery, as well as its natural biodistribution, has not been studied. To examine Gluc secretion and uptake profile, we injected Gluc-encoding plasmids into mice by hydrodynamic tail-vein injection. Whole-body BLI showed that imaging quantification obtained at pawpad was directly correlated to blood Gluc activities. When gene expression was restricted to the liver by the use of a hepatic promoter, in vivo Gluc biodistribution analysis revealed the kidney/bladder, stomach/intestine, and lung as the major uptake organs. Three-dimensional BLI identified liver/stomach and lung as the main internal luminescent sources, demonstrating the feasibility of detecting major uptake organs in live animals by 3D BLI with high-background signals in circulation. Notably, Gluc levels in capillary-depleted brain samples from Gluc-injected mice were comparable to controls, suggesting that Gluc may not cross the blood?Cbrain barrier. Gluc uptake kinetics and intracellular half-life were assessed in various types of cell lines, implicating the involvement of non-specific pinocytosis. These results suggest that Gluc-based system may provide a useful tool for in vivo evaluation of protein/agent biodistribution following systemic delivery.     

Illuminating the Regenerative Properties of Stem Cells In Vivo with Bioluminescence Imaging

Preclinical animal studies are essential to the development of safe and effective stem cell therapies. Bioluminescence imaging (BLI) is a powerful tool in animal studies that enables the real-time longitudinal monitoring of stem cells in vivo to elucidate their regenerative properties. This review describes the application of BLI in preclinical stem cell research to address critical challenges in producing successful stem cell therapeutics. These challenges include stem cell survival, proliferation, homing, stress response, and differentiation. The applications presented here utilize bioluminescence to investigate a variety of stem and progenitor cells in several different in vivo models of disease and implantation. An overview of luciferase reporters is provided, along with the advantages and disadvantages of BLI. Additionally, BLI is compared to other preclinical imaging modalities and potential future applications of this technology are discussed in emerging areas of stem cell research.     

DNA nanoparticles: detection of long-term transgene activity in brain using bioluminescence imaging

In this study, we used bioluminescence imaging (BLI) to track long-term transgene activity following the transfection of brain cells using a nonviral gene therapy technique. Formulations of deoxyribonucleic acid (DNA) combined with 30-mer lysine polymers (substituted with 10 kDa polyethylene glycol) form nanoparticles that transfect brain cells in vivo and produce transgene activity. Here we show that a single intracerebral injection of these DNA nanoparticles (DNPs) into the rat cortex, striatum, or substantia nigra results in long-term and persistent luciferase transgene activity over an 8- to 11-week period as evaluated by in vivo BLI analysis, and single injections of DNPs into the mouse striatum showed stable luciferase transgene activity for 1 year. Compacted DNPs produced in vivo signals 7- to 34-fold higher than DNA alone. In contrast, ex vivo BLI analysis, which is subject to less signal quenching from surrounding tissues, demonstrated a DNP to DNA alone ratio of 76- to 280-fold. Moreover, the ex vivo BLI analysis confirmed that signals originated from the targeted brain structures. In summary, BLI permits serial analysis of luciferase transgene activity at multiple brain locations following gene transfer with DNPs. Ex vivo analysis may permit more accurate determination of relative activities of gene transfer vectors.     

Multimodality imaging of somatostatin receptor-positive tumors with nuclear and bioluminescence imaging

Multimodal bioluminescence (BLI) and single-photon emission computed tomography/computed tomography (SPECT/CT) imaging were investigated as means to monitor somatostatin receptor subtype 2 (SST2)-positive neuroendocrine tumors as both a subcutaneously implanted and a liver metastasis animal model in mice and rats. Ultimately, such a model will be of use for studying SST2-targeted peptide receptor radionuclide therapy (PRRT). CA20948 cells were transfected with a green fluorescent protein/luciferase plasmid construct. Cells were inoculated subcutaneously in the shoulder of nude mice: nontransfected cells in the left shoulder and transfected cells in the right shoulder. BLI, SPECT/CT imaging, biodistribution analysis, and ex vivo autoradiography of the tumors were performed. BLI and SPECT/CT imaging were also performed on an intrahepatic tumor model in the rat. Caliper volume measurement of transfected tumors could be correlated with BLI measurements (R2 = .76). SPECT/CT imaging showed high levels of accumulation of 111In-DTPA-octreotide in control and transfected tumors, which was confirmed by biodistribution analysis and autoradiography. Subcapsular inoculation of transfected cells in rat liver resulted in an intrahepatic tumor, which could be visualized by both SPECT/CT and BLI. Transfection of CA20948 tumor cells did not alter the growth properties of the cell line or the expression of SST2. Transfected tumors could be clearly visualized by BLI and SPECT/CT imaging. The transfected SST2-positive tumor cell line could represent a novel preclinical model for tumor monitoring in studies that aim at further optimizing PRRT for neuroendocrine tumors.     

Real-time in vivo bioluminescence imaging of lentiviral vector-mediated gene transfer in mouse testis

Although much research has focused on transferring exogenous genes into living mouse testis to investigate specific gene functions in spermatogenic, Sertoli, and Leydig cells, relatively little is known regarding real-time gene expression in vivo. In this study, we constructed a bicistronic lentiviral vector (LV) encoding firefly luciferase and enhanced green fluorescence protein (EGFP); this was a highly efficient in vivo gene transfer tool. After microinjecting LV into the seminiferous tubules the ICR mouse testis, we detected luciferase and EGFP expression in vivo and ex vivo in the injected tubules using bioluminescence imaging (BLI) with the IVIS-200 system and fibered confocal fluorescence microscopy (CellViZio), respectively. In addition, with an in vivo BLI system, luciferase expression in the testis was detected for ∼3 mo. Furthermore, EGFP expression in seminiferous tubules was confirmed in excised testes via three-dimensional fluorescent imaging with a confocal laser-scanning microscope. With immunostaining, EGFP expression was confirmed in several male germ cell types in the seminiferous tubules, as well as in Sertoli and Leydig cells. In conclusion, we demonstrated that real-time in vivo BLI analysis can be used to noninvasively (in vivo) monitor long-term luciferase expression in mouse testis, and we verified that EGFP expression is localized in seminiferous tubules after bicistronic LV-mediated gene transfer into mouse testes. Furthermore, we anticipate the future use of in vivo BLI technology for real-time study of specific genes involved in spermatogenesis.     

In vivo bioluminescence for tracking cell fate and function

Tracking the fate and function of cells in vivo is paramount for the development of rational therapies for cardiac injury. Bioluminescence imaging (BLI) provides a means for monitoring physiological processes in real time, ranging from cell survival to gene expression to complex molecular processes. In mice and rats, BLI provides unmatched sensitivity because of the absence of endogenous luciferase expression in mammalian cells and the low background luminescence emanating from animals. In the field of stem cell therapy, BLI provides an unprecedented means to monitor the biology of these cells in vivo, giving researchers a greater understanding of their survival, migration, immunogenicity, and potential tumorigenicity in a living animal. In addition to longitudinal monitoring of cell survival, BLI is a useful tool for semiquantitative measurements of gene expression in vivo, allowing a better optimization of drug and gene therapies. Overall, this technology not only enables rapid, reproducible, and quantitative monitoring of physiological processes in vivo but also can measure the influences of therapeutic interventions on the outcome of cardiac injuries.     

Switching on the lights for gene therapy

Strategies for non-invasive and quantitative imaging of gene expression in vivo have been developed over the past decade. Non-invasive assessment of the dynamics of gene regulation is of interest for the detection of endogenous disease-specific biological alterations (e.g., signal transduction) and for monitoring the induction and regulation of therapeutic genes (e.g., gene therapy). To demonstrate that non-invasive imaging of regulated expression of any type of gene after in vivo transduction by versatile vectors is feasible, we generated regulatable herpes simplex virus type 1 (HSV-1) amplicon vectors carrying hormone (mifepristone) or antibiotic (tetracycline) regulated promoters driving the proportional co-expression of two marker genes. Regulated gene expression was monitored by fluorescence microscopy in culture and by positron emission tomography (PET) or bioluminescence (BLI) in vivo. The induction levels evaluated in glioma models varied depending on the dose of inductor. With fluorescence microscopy and BLI being the tools for assessing gene expression in culture and animal models, and with PET being the technology for possible application in humans, the generated vectors may serve to non-invasively monitor the dynamics of any gene of interest which is proportionally co-expressed with the respective imaging marker gene in research applications aiming towards translation into clinical application.     

A quantitative study of factors affecting in vivo bioluminescence imaging

In vivo bioluminescence imaging (BLI) has the advantages of high sensitivity and low background. By counting the number of photons emitted from a specimen, BLI can quantify biological events such as tumour growth, gene expression and drug response. The intensities and kinetics of the BL signal are affected by many factors and may confound the quantitative results acquired from consecutive imaging sessions or different specimens. We used three different mouse models of tumours to examine whether anaesthetics, positioning and tumour growth may affect the consistency of the BL signal. The results showed that BLI signal could be affected by different anaesthetics and repetitive positioning. Using the same anaesthetics produced consistent peak times, while other factors were held constant. However, as the tumours grew the peak times shifted and the time course of BL signals had different shapes, depending on the positioning of the mice. The data indicate that a carefully designed BLI experiment is required to generate optimal and consistent results. Copyright © 2008 John Wiley & Sons, Ltd.     

Continuous delivery of D-luciferin by implanted micro-osmotic pumps enables true real-time bioluminescence imaging of luciferase activity in vivo

Bioluminescence imaging (BLI) of luciferase reporters in small animal models offers an attractive approach to monitor regulation of gene expression, signal transduction, and protein-protein interactions, as well as following tumor progression, cell engraftment, infectious pathogens, and target-specific drug action. Conventional BLI can be repeated within the same animal after bolus reinjections of a bioluminescent substrate. However, intervals between image acquisitions are governed by substrate pharmacokinetics and excretion, therefore restricting temporal resolution of reinjection protocols to the order of hours, limiting analyses of processes in vivo with short time constants. To eliminate these constraints, we examined use of implanted micro-osmotic pumps for continuous, long-term delivery of bioluminescent substrates. Pump-assisted d-luciferin delivery enabled BLI for > or = 7 days from a variety of luciferase reporters. Pumps allowed direct repetitive imaging at < 5-minute intervals of the pharmacodynamics of proteasome- and IKK-inhibiting drugs in mice bearing tumors stably expressing ubiquitin-firefly luciferase or IkappaBalpha-firefly luciferase fusion reporters. Circadian oscillations in the olfactory bulbs of transgenic rats expressing firefly luciferase under the control of the period1 promoter also were temporally resolved over the course of several days. We conclude that implanted pumps provide reliable, prolonged substrate delivery for high temporal resolution BLI, traversing complications of repetitive substrate injections.     

Bioluminescence imaging captures the expression and dynamics of endogenous p21 promoter activity in living mice and intact cells

To interrogate endogenous p21(WAF1/CIP1) (p21) promoter activity under basal conditions and in response to various forms of stress, knock-in imaging reporter mice in which expression of firefly luciferase (FLuc) was placed under the control of the endogenous p21 promoter within the Cdkn1a gene locus were generated. Bioluminescence imaging (BLI) of p21 promoter activity was performed noninvasively and repetitively in mice and in cells derived from these mice. We demonstrated that expression of FLuc accurately reported endogenous p21 expression at baseline and under conditions of genotoxic stress and that photon flux correlated with mRNA abundance and, therefore, bioluminescence provided a direct readout of p21 promoter activity in vivo. BLI confirmed that p53 was required for activation of the p21 promoter in vivo in response to ionizing radiation. Interestingly, imaging of reporter cells demonstrated that p53 prevents the extracellular signal-regulated kinase/mitogen-activated protein kinase pathway from activating p21 expression when quiescent cells are stimulated with serum to reenter the cell cycle. In addition, low-light BLI identified p21 expression in specific regions of individual organs that had not been observed previously. This inducible p21(FLuc) knock-in reporter strain will facilitate imaging studies of p53-dependent and -independent stress responses within the physiological context of the whole animal.