Necropsy Tissue Collection and Histopathology Submission

By Dr. Zubair Khalid, DVM, MS, PhD · 2026-08-11

Key Takeaways

• Fixation Quality is Paramount: 10% neutral buffered formalin is the standard fixative, requiring a minimum 10:1 fixative-to-tissue volume ratio and tissue blocks no thicker than 1.5 cm to ensure adequate penetration and prevent autolysis.
• Organ-Specific Techniques Enhance Diagnostics: Intratracheal infusion of formalin is critical for preserving lung alveolar architecture, while gentle extraction and appropriate fixation protocols are essential for minimizing artifactual damage in brain tissue.
• Fresh Tissue is Crucial for Ancillary Testing: Samples for microbiology, PCR, toxicology, and immunohistochemistry must be collected aseptically before fixation, as fixatives can inactivate agents or degrade nucleic acids and antigens.
• Systematic Sampling Maximizes Diagnostic Yield: Representative samples from all major organs, including lymph nodes from multiple drainage basins and specific sites like kidney and liver for certain infections (e.g., leptospirosis), are vital even in the absence of gross lesions.
• Accurate Labeling and Comprehensive Documentation are Non-Negotiable: Durable, permanent labels with animal ID, tissue site, and date, coupled with detailed clinical history and gross findings on submission forms, are essential to prevent misattribution and ensure accurate histopathological interpretation.
• Transport Conditions Dictate Sample Viability: Fixed tissues require secure, leak-proof packaging at ambient temperature, while fresh tissues must be transported on cold packs (not frozen) to maintain viability for culture and molecular analyses.

This article provides a procedural framework for collecting and submitting tissues for histopathologic evaluation during necropsy. It is written for veterinary students and practicing clinicians who perform necropsies in clinical, diagnostic, or research settings. The content covers tissue selection, fixation, labeling, and submission logistics, with emphasis on how collection technique directly influences diagnostic yield. Interpretation of histologic findings is outside the scope of this article.

The quality of a histopathology report depends as much on what happens before the slide is made as on the skill of the pathologist. Poorly fixed, incorrectly sampled, or mislabeled tissues can render an otherwise complete necropsy diagnostically useless. Understanding the principles that govern tissue preservation and sample selection allows the clinician to anticipate the pathologist's needs and avoid common submission errors.

At a Glance

Purpose and Value of Systematic Tissue Collection

Necropsy serves two complementary functions: determining the cause of death and characterizing the extent and distribution of disease. Histopathology extends the gross examination beyond the limits of visual inspection, revealing cellular changes, inflammatory patterns, and infectious agents that cannot be appreciated macroscopically. A systematic approach to tissue collection ensures that representative samples are obtained from all major organs regardless of whether gross lesions are apparent.

The diagnostic value of histopathology depends on sample quality. As described in guidance on diagnostic necropsy and tissue collection in rats and mice, proper collection and handling of tissues directly impacts the quality of testing results. Artifacts introduced during collection, such as crush injury from forceps, desiccation from delayed fixation, or autolysis from prolonged postmortem intervals, can obscure or mimic pathologic changes.

Principles of Tissue Fixation

Mechanism of Formalin Fixation

Formalin cross-links proteins, stabilizing cellular structures and preventing autolysis and bacterial overgrowth. Ten percent neutral buffered formalin is the standard fixative for routine histopathology because it preserves morphology while remaining compatible with most downstream staining techniques. The buffer maintains physiologic pH and prevents the formation of formalin pigment, which can complicate interpretation.

Fixative Volume and Tissue Size

The ratio of fixative to tissue is critical. A minimum of 10 parts fixative to 1 part tissue by volume is required for adequate penetration. Formalin diffuses slowly, approximately 1 mm per hour under ideal conditions. Tissues thicker than 1.5 cm will not fix adequately in the center, leaving a core of autolyzed tissue surrounded by preserved parenchyma. Trim organs into blocks of 1 to 1.5 cm thickness before immersion, and ensure that containers are large enough to allow free circulation of fixative around each specimen.

Fixation Duration

Tissues should remain in formalin for at least 24 hours before processing to allow complete cross-linking. Prolonged fixation beyond 7 days can degrade antigenicity and nucleic acids, complicating immunohistochemistry and molecular testing. If extended storage is anticipated, transfer tissues to fresh formalin or process them for paraffin embedding earlier. For lung tissue specifically, proper inflation with fixative allows the interior of the tissue to fix within 24 hours, as demonstrated in step-by-step techniques for lung collection and perfusion.

Organ-Specific Collection Considerations

Lung

The lung presents unique challenges because alveolar architecture collapses when the organ is removed and placed directly into fixative. The resulting specimen consists largely of collapsed parenchyma with thickened-appearing septa, making evaluation of interstitial disease difficult. Intratracheal infusion of formalin at a pressure of approximately 25 cm of water re-expands the lung and preserves alveolar spaces. The trachea is cannulated after removal of the lung block, and fixative is allowed to flow until the pleural surfaces are smooth and the lung fills the thoracic cavity dimensions. The trachea is then ligated and the entire specimen immersed in formalin.

Brain

Brain tissue is soft and easily damaged during extraction. Skull reflection requires careful technique to avoid crushing the cerebral cortex or tearing the brainstem. After removing the calvarium, the cranial nerves and spinal cord are transected with a scalpel or scissors, and the brain is gently lifted from the cranial vault. The brain should be allowed to fix in situ for several hours before removal in some protocols, or removed immediately and placed in a large volume of fixative. The technique for exposing and removing the brain with minimal damage is described in diagnostic necropsy protocols for rodents, and the same principles apply to larger species with appropriate modifications.

Lymph Nodes and Other Sampling Sites

Mesenteric lymph nodes are particularly valuable for detecting enteric pathogens because viruses and bacteria may persist in lymphoid tissue after they are no longer shed in feces. This principle, noted in rodent necropsy guidance, extends to other species. When infectious disease is suspected, collect lymph nodes from multiple drainage basins, including mesenteric, tracheobronchial, and peripheral nodes. Similarly, kidney and liver samples are essential for detecting leptospiral infection, as demonstrated in multi-sample approaches for leptospire detection in mammals, where renal tissue was among the most informative samples.

Fresh Tissue Collection for Ancillary Testing

Histopathology is rarely the only diagnostic test performed on necropsy tissues. Microbiology, PCR, toxicology, and specialized staining techniques require fresh, unfixed samples collected aseptically before tissues are placed in formalin. The order of collection matters: obtain samples for culture and molecular testing first, then collect tissues for histopathology. This sequence prevents contamination of sterile sites and ensures that infectious agents are not inactivated by fixative before they can be cultured or amplified.

For suspected infectious disease, collect fresh samples from multiple sites, including liver, spleen, kidney, lung, and affected lymph nodes. Place each sample in a separate sterile container labeled with the animal identification, tissue site, and date. Refrigerate samples if processing is delayed, but do not freeze tissues intended for culture. For toxicology, specific containers and handling procedures may be required, and consultation with the testing laboratory before necropsy is advisable.

Labeling and Submission Documentation

Accurate labeling is the final safeguard against diagnostic error. Each tissue cassette or container must bear a unique identifier that links it to the animal, the tissue site, and the date of collection. Use permanent markers or printed labels that resist formalin and alcohol. When multiple animals are submitted together, ensure that labels cannot be confused and that each animal's tissues are processed in separate cassettes or containers.

The submission form should include the complete clinical history, gross necropsy findings, a list of tissues submitted, and any specific questions for the pathologist. This information allows the pathologist to tailor the examination and report to the clinical context. The Davis-Thompson Foundation provides educational resources on pathology case submission and reporting standards, and the MSD Veterinary Manual offers species-specific guidance on necropsy procedures and sample handling.

Packaging and Transport of Fixed Tissues

Once fixation is complete, tissues must be packaged to prevent desiccation, physical damage, and leakage during transport. Each specimen container should be sealed with paraffin film or a screw cap with an intact gasket, then placed in a secondary leak-proof container such as a sealed plastic bag or rigid transport box. Absorbent material should line the outer packaging to capture any fixative that might escape. The outer container must carry the submitter's contact information and a biohazard label when appropriate.

Transport duration should be minimized. Formalin-fixed tissues are stable for weeks at ambient temperature, but prolonged shipping times increase the risk of formalin degradation, particularly in warm climates. If shipment is delayed beyond several days, consider refrigerating the sealed containers, though formalin precipitation can occur at low temperatures and may require warming before processing. Never freeze fixed tissues. Ice crystal formation destroys cellular architecture and renders specimens uninterpretable.

For fresh tissues destined for culture, PCR, or toxicology, transport conditions differ fundamentally. Fresh tissues must be kept moist with saline-soaked gauze, placed in sterile containers, and shipped on cold packs, not frozen unless the laboratory specifically requests frozen samples. Each fresh tissue type should be in a separate sterile container to prevent cross-contamination. The laboratory should be contacted in advance when fresh tissues are being shipped, particularly for suspected foreign animal diseases or when the sample is time-sensitive.

Submission Forms and Clinical History

The quality of the histopathology report depends directly on the quality of the clinical history provided. A submission form should include the species, breed, age, sex, reproductive status, and body weight of the animal. The gross necropsy findings should be summarized, including organ weights if measured, lesion distribution, and any photographs taken. The clinician's differential diagnoses and the specific questions the pathologist should address must be stated explicitly.

The clinical history should include the duration and progression of clinical signs, relevant laboratory results, treatments administered with dates, and vaccination or deworming history. For production animals, include herd-level information such as morbidity and mortality rates, feed changes, and management practices. For wildlife or zoo animals, include the source of the animal, any quarantine history, and known exposures to other species. This information allows the pathologist to tailor the examination and select appropriate ancillary tests.

A complete tissue list should accompany the submission. Each tissue listed must correspond to a labelled cassette or container. The pathologist should be able to reconcile the submission form with the physical specimens without ambiguity. If tissues were collected from multiple animals, each animal's tissues must be clearly separated and identified, either by separate containers or by distinct cassette labeling.

Choosing a Diagnostic Laboratory

Laboratory selection affects turnaround time, cost, and the range of ancillary tests available. Academic veterinary pathology laboratories typically offer full necropsy services, specialised immunohistochemistry, electron microscopy, and consultation with boarded pathologists. Private diagnostic laboratories may offer faster turnaround and more convenient submission logistics but may have more limited specialised testing. The Davis-Thompson Foundation maintains educational resources and case material that can help clinicians understand the range of diagnostic approaches available in veterinary pathology.

Before submitting, contact the laboratory to confirm their specific requirements for specimen handling, acceptable fixatives, and required paperwork. Some laboratories provide submission kits with pre-printed forms and pre-labelled containers. Others require online submission portals. Confirm whether the laboratory accepts specimens from your species, particularly for exotic animals, and whether they have the specialised expertise needed for your case.

Turnaround time for routine histopathology is typically 3 to 10 working days after receipt. Urgent cases may be expedited at additional cost. If the case involves a suspected notifiable disease, contact the appropriate regulatory authority before submitting samples, as submission protocols may be mandated by law. The World Organization for Animal Health terrestrial animal health standards provide international guidance on reportable diseases and sample submission for trade-related conditions.

Common Submission Errors and Their Consequences

Several recurring errors compromise the diagnostic value of histopathology submissions. Oversized tissue sections are the most common problem. A section thicker than 5 mm will not fix adequately, and the center of the tissue will autolyse even as the periphery fixes. The pathologist may be forced to interpret partially autolysed tissue or request a second submission, which is impossible once the animal is buried or cremated.

Inadequate fixative volume produces the same result. Tissues packed tightly into a small container with minimal formalin will fix slowly and unevenly. The recommended ratio of 10 parts fixative to 1 part tissue must be maintained, and this requires using a container substantially larger than the tissue itself.

Incorrect labeling causes diagnostic delays and, in the worst case, misattribution of lesions to the wrong animal. Cassettes should be labelled with a graphite pencil or solvent-resistant marker, never with standard ballpoint pen, which dissolves in formalin and alcohol. Container labels should be written before the container is filled, since formalin fumes can smear fresh ink.

Placing multiple organs in a single container without separation makes it impossible for the pathologist to identify which tissue is which after fixation, particularly for similar parenchymal organs. Each organ should be in its own container or separated by labelled cassettes within a larger container.

Decision Framework for Sample Collection

The following table summarizes the key decisions in tissue collection and submission, with the factors that should guide each choice.

For suspected leptospirosis, the sampling strategy differs from routine collection. Kidney and urine are the primary samples, and the diagnostic yield improves when multiple sample types are tested in parallel. A multi-sample approach using culture, PCR, and serology increases the chance of detection compared with any single test, as demonstrated in field studies of wild mammals Maximizing the chances of detecting pathogenic leptospires in mammals. The pathologist should be informed when leptospirosis is suspected so that silver staining or immunohistochemistry can be performed on kidney sections.

For suspected viral infections, fresh frozen tissue is often more valuable than fixed tissue. PCR can be performed on formalin-fixed tissue, but nucleic acid degradation reduces sensitivity. Collect fresh tissue into sterile cryovials and freeze at -80 degrees Celsius if viral testing is anticipated. The mesenteric lymph node is a particularly useful sample for detecting enteric viruses, since viral nucleic acid persists in lymphoid tissue longer than it is shed in feces Diagnostic necropsy and selected tissue and sample collection in rats and mice.

Documentation of Gross Findings

The gross necropsy record is the foundation upon which histopathology interpretation rests. Each organ should be described systematically: size, color, consistency, and any lesions. Measurements should be recorded in centimetres. Photographs should be taken of all significant lesions, with a scale bar or ruler included in the frame. The location of each tissue sample should be noted on the necropsy record so that the pathologist can correlate histologic findings with gross observations.

The gross description should be written in a standard format that separates objective findings from interpretation. For example, "The liver is enlarged, pale, and friable, with multifocal 1 to 2 mm white foci scattered throughout all lobes" is an objective description. "The liver lesions are consistent with bacterial hepatitis" is an interpretation that belongs in the assessment section of the record.

Photographs serve multiple purposes. They document the gross lesions for the medical record, they guide the pathologist during slide review, and they provide material for client communication and teaching. Photograph each organ before and after sectioning. Include a label in each photograph identifying the animal and the organ. The MSD Veterinary Manual provides species-specific guidance on normal organ appearance and common gross lesions that can help the clinician recognize abnormalities during necropsy.

The gross necropsy record, the submission form, and the tissue labels must all use the same animal identification. Discrepancies between these documents create confusion and may invalidate the diagnostic work. Establish a standardized necropsy form for your practice or institution and use it consistently. The form should include a checklist of organs to be examined and sampled, ensuring that no organ is overlooked during the procedure.

Complications and Failure Modes in Tissue Collection

Several complications can compromise diagnostic yield during necropsy tissue collection. Autolysis begins immediately after death and progresses rapidly at ambient temperature. Tissues collected more than a few hours postmortem, particularly from small animals with high surface area to volume ratios, may show diffuse cellular swelling, loss of nuclear detail, and mucosal sloughing that obscure or mimic pathologic change. Refrigerating the carcass promptly slows this process, but freezing causes ice crystal artifact that distorts architecture and should be avoided when histopathology is planned.

Incomplete fixation is the most common preventable failure. Formalin penetrates tissue at roughly 1 mm per hour, so a 1 cm thick block requires approximately 24 hours of adequate immersion before trimming. A lung that has collapsed during removal will fix only on its outer surface, leaving the parenchyma unfixed and prone to autolysis. Perfusion fixation of the lung through the trachea or bronchi, as described in the diagnostic necropsy and tissue collection guidance for rats and mice, ensures that alveolar septa are preserved in an inflated state suitable for evaluation. The same principle applies across species: any hollow organ or air-filled structure must be opened or inflated before immersion.

Incorrect tissue orientation at trimming produces tangential sections that can mimic fibrosis or obscure lesions. The brain is especially vulnerable because of its soft consistency and regional heterogeneity. Removing the brain with minimal traction and allowing adequate fixation before cutting, as demonstrated in the rodent necropsy collection protocol, reduces distortion. For larger species, suspending the brain in fixative by the basilar artery or using a sling prevents flattening of the ventral surface.

Common Errors by Less Experienced Collectors

Students and early-career clinicians frequently under-sample. A single section of liver or kidney may miss a focal lesion, and the absence of gross lesions does not preclude microscopic disease. The multi-sample approach used in leptospire surveillance demonstrates that sampling multiple tissues from each animal increases detection sensitivity for pathogens that localize unpredictably. When the clinical question is infectious disease, collect kidney, liver, spleen, lung, and mesenteric lymph node at minimum, and add brain and heart if neurologic or cardiac signs were present.

Mislabeling is another recurring error. Cassettes written in pencil can smudge, labels placed on the outside of a jar instead of on a paper tag inside can separate during transport. Write the animal identifier, tissue name, and date directly on the cassette with an alcohol-resistant marker, and place a duplicate paper label inside the container. Cross-check labels against the submission form before sealing.

Sampling the wrong site within an organ is common. For the kidney, collect a wedge that includes cortex, medulla, and pelvis. For the lung, sample the cranioventral and caudodorsal regions separately, because many diseases have a lobar predilection. For the gastrointestinal tract, take full-thickness sections at multiple levels and identify each site on the form. The red panda amdoparvovirus investigation illustrates how tissue distribution of an agent can vary, with viral nucleic acid detected in lymph nodes and epithelium but not in all organs, reinforcing the value of systematic multi-site collection.

Limitations of Current Evidence and Areas of Expert Disagreement

The evidence base for optimal tissue collection protocols is strongest for laboratory rodents and production species, with comparatively fewer standardized studies in companion animals, wildlife, and exotic species. Fixation times, trimming protocols, and sampling strategies are often extrapolated across species, and expert opinion differs on several points.

The optimal fixation interval before trimming is one such area. Most authorities recommend 24 to 72 hours for routine specimens, but some pathologists prefer longer fixation for brain and eye to reduce artifact, while others advocate shorter times to preserve antigenicity for immunohistochemistry. Where molecular testing is anticipated, fresh frozen tissue should be collected at necropsy regardless of the fixation schedule.

The value of routine histopathology on grossly normal tissues is also debated. Some clinicians submit only visibly abnormal tissue to control cost, while pathologists generally recommend sampling a standard panel of organs even when gross lesions are absent, because microscopic disease can precede macroscopic change. The toxicity study of cyanobacterial extracts in mice found histologic lesions such as hepatic hemorrhage and disorganization in animals that showed only transient clinical signs, supporting the practice of systematic sampling.

Referral, Consultation, and Regulatory Reporting

Referral to a veterinary pathologist is warranted when gross findings are ambiguous, when the case has medicolegal implications, when a novel or emerging disease is suspected, or when the submitting clinician lacks confidence in tissue selection. Diagnostic laboratories with boarded pathologists can advise on sampling strategy before necropsy, and many accept phone consultations for difficult cases. The Davis-Thompson Foundation provides educational case material and can direct clinicians to training opportunities in diagnostic pathology.

Regulatory reporting obligations vary by jurisdiction and species. Reportable diseases, including certain zoonoses and transboundary animal diseases, must be notified to the appropriate authority. The WOAH terrestrial animal health standards define international notification requirements for listed diseases, and national veterinary authorities publish local reporting lists. When a reportable disease is suspected, contact the relevant authority before completing the necropsy, because additional sampling or fresh tissue submission may be required. The AVMA practice resources and the MSD Veterinary Manual provide species-specific guidance on zoonotic disease handling and biosafety precautions.

Frequently Asked Questions

How much tissue should I collect when the clinical budget or owner resources are limited?

Prioritize organs with gross lesions and collect paired samples from the same site, one fixed and one fresh frozen. If only one sample is possible, submit the fixed sample for histopathology, since formalin-fixed tissue can later be used for PCR in many laboratories, though sensitivity may be reduced. For suspected infectious disease, fresh frozen tissue is often more valuable, particularly for viral detection. Communicate clearly with the laboratory about financial constraints before submission. Many laboratories offer tiered pricing or will hold fixed tissues while you decide which blocks to process. The Davis-Thompson Foundation pathology resources provide guidance on diagnostic approaches that balance thoroughness with practical limits.

What should I do when the ideal necropsy equipment is unavailable?

A scalpel blade, a clean cutting board, and a sharp knife can replace most specialized instruments. Use a new scalpel blade for each case to reduce cross-contamination. If a perfusion pump is unavailable for lung fixation, instill fixative gently through a syringe and catheter placed in the trachea, using just enough pressure to expand the lung to its normal inspiratory volume. For brain removal without proper bone cutters, use a small saw or heavy scissors, working carefully to avoid crushing the tissue. Fixation quality matters more than instrument sophistication. The rodent necropsy and tissue collection protocol demonstrates that careful technique with basic tools yields diagnostic samples.

How does tissue collection differ for wildlife, exotic, or zoo species?

Wildlife cases often require working with partially autolyzed tissues, so collect samples as soon after death as possible and prioritize organs with the most diagnostic value. For protected species, check local regulations before retaining tissues or shipping samples across borders. The WOAH terrestrial animal health standards address international movement of diagnostic specimens. In zoo species, consider zoonotic risk and consult the institution's veterinary team about prior testing or known colony pathogens. A case series in red pandas illustrates how necropsy tissue distribution studies can identify novel pathogens, so collect a broad set of tissues including lymph nodes and intestinal epithelium even when gross lesions are absent.

How should I document sample collection for legal or regulatory cases?

Photograph each organ before and after collection, and photograph the labeled container next to the organ to create a visual chain of custody. Record the time of death, time of necropsy, and time of fixation for each sample. Use tamper-evident seals on containers and maintain a written log of everyone who handles the samples. For suspected foreign animal disease or food safety issues, contact the relevant authority before shipping, as they may specify particular collection protocols. The AVMA practice resources include guidance on medical records and evidence handling. Keep duplicate fixed samples in a separate location when litigation is anticipated.

How do I explain the value of necropsy tissue collection to a client who declines the procedure?

Frame the necropsy as a diagnostic test, not a disposal service. Explain that gross findings alone often cannot distinguish between similar diseases, and that histopathology provides a definitive answer that can inform the health of remaining animals in the household or herd. For production animals, emphasize that the cost of one necropsy may prevent losses across the group. Offer a limited necropsy option, such as collecting only the affected organ system, to reduce cost while preserving diagnostic value. The MSD Veterinary Manual provides species-specific disease information that can help you explain what conditions are being ruled out.

What are the consequences of poor labeling or incomplete submission forms?

Mislabeled samples can lead to incorrect diagnoses, inappropriate treatment decisions, and legal liability. A sample with no clinical history may be processed with the wrong stains or interpreted without relevant context, leading to a nondiagnostic report. Laboratories may reject samples with illegible labels or mismatched paperwork. When samples from multiple animals are submitted together, labeling errors can result in results being assigned to the wrong patient. Always include the animal identification, tissue type, fixation date, and a brief gross description on both the container and the submission form. The leptospirosis multi-sample study demonstrates that sample quality and handling directly affect diagnostic test performance, so treat labeling as part of the diagnostic chain.

Related Clinical & Scientific Guides

• Hypersensitivity Reactions: Types and Mechanisms
• Therapeutic Decision-Making for Respiratory Infections in Cattle
• Monitoring Fluid Therapy in Critically Ill Veterinary Patients

References and Further Reading

• Diagnostic necropsy and selected tissue and sample collection in rats and mice.. 2011.
• Amdoparvovirus Infection in Red Pandas ( Ailurus fulgens).. 2018.
• Analysis of the toxicity and histopathology induced by the oral administration of Pseudanabaena galeata and Geitlerinema splendidum (cyanobacteria) extracts to mice.. 2014.
• Effects of doxycycline on early infections of Dirofilaria immitis in dogs.. 2011.
• Maximizing the chances of detecting pathogenic leptospires in mammals: the evaluation of field samples and a multi-sample-per-mammal, multi-test approach.. 2011.
• Effects on animal wellbeing and sample quality of 2 techniques for collecting blood from the facial vein of mice.. 2015.
• Davis-Thompson Foundation Veterinary Pathology Resources. Davis-Thompson Foundation.
• MSD Veterinary Manual, Professional Edition. MSD Veterinary Manual.
• American Veterinary Medical Association Practice Resources. American Veterinary Medical Association.

Related Articles

• Inflammation and Tissue Healing Process
• Necropsy Equipment and Setup: Instruments and Safety
• Necropsy Fundamentals: Systematic Approach and Techniques
• Interpretation of Histopathology Reports: A Guide for Clinicians
• Diagnostic Cytology: Sample Collection and Interpretation

This article is educational professional reference material for veterinary audiences. It is not a substitute for veterinary diagnosis, individual clinical judgment, current product labeling, or applicable regulatory requirements.