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* These authors contributed equally
This article details murine congenital heart disease (CHD) diagnostic methods using fetal echocardiography, necropsy, and Episcopic fluorescence image capture (EFIC) using Episcopic confocal microscopy (ECM) followed by three-dimensional (3D) reconstruction.
Congenital heart diseases (CHDs) are major causes of infant death in the United States. In the 1980s and earlier, most patients with moderate or severe CHD died before adulthood, with the maximum mortality during the first week of life. Remarkable advances in surgical techniques, diagnostic approaches, and medical management have led to marked improvements in outcomes. To address the critical research needs of understanding congenital heart defects, murine models have provided an ideal research platform, as they have very similar heart anatomy to humans and short gestation rates. The combination of genetic engineering with high-throughput phenotyping tools has allowed for the replication and diagnosis of structural heart defects to further elucidate the molecular pathways behind CHDs. The use of noninvasive fetal echocardiography to screen the cardiac phenotypes in mouse models coupled with the high fidelity of Episcopic fluorescence image capture (EFIC) using Episcopic confocal microscopy (ECM) histopathology with three-dimensional (3D) reconstructions enables a detailed view into the anatomy of various congenital heart defects. This protocol outlines a complete workflow of these methods to obtain an accurate diagnosis of murine congenital heart defects. Applying this phenotyping protocol to model organisms will allow for accurate CHD diagnosis, yielding insights into the mechanisms of CHD. Identifying the underlying mechanisms of CHD provide opportunities for potential therapies and interventions.
Congenital heart diseases (CHDs) are the most common neonatal birth defect1,2, affecting about 0.8%-1.7% of neonates and resulting in significant neonatal mortality and morbidity3. A genetic etiology is strongly indicated with CHDs4,5. Genetically modified mouse models have been used widely to understand the complexity of CHDs and the mechanisms that cause them due to the mice having four-chamber hearts and comparable cardiac developmental DNA sequences in mouse and human fetuses6. Identifying the phenotype of the mouse mutants is the fundamental first step in characterizing the function of the targeted gene. Mouse models expressing gene dosage effects, in which a single genetic mutation can result in a spectrum of cardiac defects that mimic human CHDs, are important for understanding the complexity of CHDs and the mechanisms that cause them.
This article outlines a pipeline to characterize cardiac phenotypes in mouse models. The applied methods utilize fetal echocardiogram7, followed by necropsy and ECM histopathology7,8, which can display the detailed anatomy of developing murine cardiac phenotypes. A fetal echocardiogram is a noninvasive modality that allows direct visualization of multiple embryos with reasonable imaging resolution. In addition, a fetal echocardiogram provides a quick determination of the total number of embryos in a litter, their developing stages, and the relative orientation and location in the uterine horn. Using a spectral Doppler/color flow, abnormal embryos can be identified based on the structure, the hemodynamic disturbance, the growth restriction, or the development of hydrops. Since a fetal echocardiogram study is a noninvasive technique, it can be used to scan on multiple days and to observe the changes in hemodynamics or cardiac morphology. Obtaining high-quality imaging of fetal echocardiograms requires practice and skill, as specific heart defects may be missed due to a lack of experience and knowledge. Because of this, a more definitive analysis of cardiac morphology may be obtained through a combination of necropsy and ECM histopathology. Necropsy provides direct visualization of the arch structure, the relative relationships of the aorta and pulmonary artery, the size of the ventricles and atria, the position of the heart relative to the chest, and the bronchopulmonary structures. However, interior features such as the heart valves and wall thickness may be difficult to assess through necropsy alone. Thus, ECM histopathology is recommended for a conclusive diagnosis. ECM histopathology is a high-resolution visualization technique that allows for both 2D and 3D reconstruction of the image stack9. These images are obtained through serial Episcopic fluorescent imaging of a paraffin-embedded sample as it is thinly sectioned at a consistent interval by an automatic microtome. Unlike classical histology, images are captured as a section before it is cut from the block such that all images are captured within the same reference frame. Because of this, the 2D image stack produced by ECM histopathology may easily and reliably be reconstructed in three dimensions. This is done using a DICOM viewer, which allows 3D visualization of the images in the three anatomical planes: coronal, sagittal, and transverse. From these high-resolution 3D reconstructions, a definitive cardiac diagnosis may be made. The application of these three different visualization modalities, either individually or in combination, can provide accurate characterizations of structural heart defects in mouse embryos.
The use of mice for these studies is necessary as mice have four-chambered hearts that can mimic human CHDs. Mice were provided veterinary care and housed in the institution's Association for Assessment and Accreditation of Laboratory Animal Care (AAALAC)-accredited animal care facility. Strict protocols were followed to minimize the mice's discomfort, stress, pain, and injury. Mice were euthanized using CO2 gas, which is acceptable for small rodents according to the American Veterinary Medical Association Guidelines on Euthanasia. The studies on mice in this manuscript were carried out with an approved IACUC protocol at the University of Pittsburgh.
1. Fetal echocardiogram
NOTE: An echocardiogram is a powerful tool for identifying cardiovascular malformation and extracardiac defects in mice. Due to the small size of the mouse embryos (about 1-2 mm at midgestation, 3.5 mm at birth), ultrahigh-frequency echocardiographic equipment with ultrasound biomicroscopy (UBM) is required. UBM provides different high-frequency (30-50 MHz) probes with a small imaging window (15 mm x 14 mm) that provides the resolution (30 µm axial x 68 µm lateral) to visualize one mouse fetus at a time. A 40 MHz transducer provides high-resolution images to identify cardiovascular phenotypes7.
2. Necropsy
NOTE: Once abnormal cardiac phenotypes are suspected using fetal echocardiography, fetuses are collected and fixed via full-body submersion in the fixative solution: either 10% buffered formalin phosphate or 4% paraformaldehyde (PFA). Inspect the sample's external and internal morphology, looking for macroscopic anatomical abnormalities or malformations.
3. Embedding
4. Episcopic confocal microscopy (ECM)
NOTE: After appropriate embedding, embryos undergo image collection serially via ECM for histopathology analysis. Individual slides can be recovered from the microtome for further studies.
5. Three-dimensional (3D) reconstruction
NOTE: The purpose of 3D reconstruction is to process a 2D image stack from ECM imaging into 3D videos in the coronal, sagittal, and transverse orientations and to use the 3D videos for diagnosis of the structural and anatomical abnormalities in the samples.
The mouse embryos with significant hemodynamic defects were noted to be embryonic lethal. A wide variety of CHDs can be identified through the high output, noninvasive fetal echocardiogram using different views (Figure 1).
Septal defects: The most common CHDs are septal defects such as a ventricular septal defect (VSD), an atrioventricular septal defect (AVSD), and an atrial septal defect (ASD)1. VSD or AVSD can be easily v...
Genetically modified mice have been used to understand the pathomechanisms of congenital heart defects. The protocols we provide in this study attempt to streamline and standardize the process of assessing murine fetal heart defects. However, there are critical steps to note during the protocol. Mouse embryos grow significantly during each day of gestation, and the correct time to harvest a mouse can be determined by performing a fetal echocardiogram accurately. The fetal echocardiogram can be used to screen the fetal ca...
The authors declare no conflicts of interest in this manuscript.
None.
Name | Company | Catalog Number | Comments |
1x phosphate-buffered saline solution (PBS), PH7.4 | Sigma Aldrich | P3813 | |
1.5 mL Eppendorf tubes (or preferred vial for tissue storage) | SealRite | 1615-5599 | |
10% buffered formalin phosphate solution | Fisher Chemical | SF100-4 | |
100% Ethanol | Decon Laboratories | 2701 | |
16% paraformaldehyde (PFA) fixative | ThermoScientific | 28908 | 4% working concentration freshly prepared in 1x PBS at 4 °C |
50 mL tubes | Falcon | 352070 | |
6–12 Well plate or 20 mL vial for embryo storage | Falcon | 353046 | |
Dissecting microscope | Leica | MDG36 | |
Dissecting Pins (A1 or A2 grade) | F.S.T | 26002-15 | |
Dissecting Plate | F.S.T | FB0875713 | Petri dish with paraffin base |
Embedding molds | Sakura | 4133 | |
Extra narrow scissors (10.5 cm) | F.S.T | 14088-10 | 1–2 pairs |
Fiji application/Image J | NIH | Fiji.sc | |
Fine tip (0.05 mm x 0.01 mm) Dissecting Forceps (11 cm) | F.S.T | 11252-00 | 2 Pairs |
Hot forceps | F.S.T | 11252-00 | For orientation of embryos |
Industrial Marker for Wax Blocks | Sharpie | 2003898 | Formatted for labratory use |
Jenoptik ProgRes C14plus Microscope Camera | Jenoptik | 017953-650-26 | |
Jenoptik ProgRess CapturePro acquisition software | Jenoptik | jenoptik.com | |
Large glass beaker | Fisher Scientific | S111053 | For melting paraffin |
Leica M165 FC binocular microscope (16.5:1 zoom optics) | Leica | M165 FC | |
OsiriX MD Version 12.0 | OsiriX | osirix-viewer.com | |
Paraplast embedding paraffin wax | Millipore Sigma | 1003230215 | |
Small glass beaker | Fisher Scientific | S111045 | |
Small, perforated spoon (14.5 cm) | F.S.T | 10370-17 | |
Straight Vannas Scissors (4–8 mm) | F.S.T | 15018-10 | A pair |
Vevo2100 ultrahigh-frequency ultrasound biomicroscope | FUJIFILM VisualSonics Inc. | Vevo2100 | |
Xylene | Fisher Chemical | UN1307 |
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