Lecture 8: PMI estimation in the late Postmortem period and influencing factors Flashcards
Stages of decomposition
-Decomposition is a continuous process
-Starts from the moment of clinical death
-Dependent on may variables
-Traditionally divided into 5 categories:
>Fresh
>Bloat
>Active decay
>Advanced decay
>Skeletonisation
Mechanisms of decomposition
-Catalysis: enzymatic and chemical reactions transforming complex compounds into simple molecules
>Bacteria
>Autolysis followed by putrefaction
-Communication: physical/mechanical breakdown of matter
>Insects, wind, sun, tides, scavengers
-Leaching: soluble material removed by water
Bloat stage
> Uncontrolled activity of internal bacteria
Build-up of gas and pressure in intestines and abdomen
Greenish discolouration, caused by sulfhaemoglobin
Marbling
Skin sloughing
Blisters in skin
Bacterial putrefaction is main driver of decomposition
Active decay
> Maggots hatch and destroy integrity of the body
Bacterial action continues
Gases released- rupturing
Temperature rises due to maggot mass-accelerates bacterial and enzyme action
Bacteria and insects are main drivers of decomposition
Advanced decay
> Gases still releases
Decomposition fluids leach into surroundings
Drying out of remains
Maggots retreat, beetles more active
Bacterial action reduced
Insects and mechanical forces are main drives of decomposition
Skeletonisation
> Decomposition slows down
Gradual exposure of the bones
Some tendons and ligaments still visible
Disarticulation
Scavengers can take away small bones
Main drivers of decomposition are more mechanical
-Wind, rain, soil, abrasion, etc
Further implications for forensic investigation I
-Difficult to perform post-mortem examination and therefore establish COD and reconstruct the events surrounding the death
>external examination e.g.
wounds
>CT scan/ MRI
>Autopsy can be inconclusive
>Histological examination
>Position of the body
>Additional materials i.e
weapons or restraints
Further implications for forensic investigation II
-Difficult to establish identity
>Visual ID- Facial features,
tattoos, skin colour, clothing
>Fragmentation
>DVI- resources are limited
>Fingerprints- desiccation, skin
sloughing
>Dental- robust
>DNA- skeletal, dental
>Anthropological analysis
Influencing factors
> The rate of decomposition= the speed at which a body transfers from a fresh whole body to complete skeletonization
Combination of:
-Extrinsic factors
(external/environmental)
-Intrinsic factors
(internal/individual)
Extrinsic factors: temperature
> Bodies decay best in warm damp environments
Hot/warm weather:
-Maggots and flies thrive in
warm weather
-A body can becomes
skeletonised in 2-4 weeks
Cold weather:
-Maggot action slowed
outside the body, but fie in
body cavities (generate
their own heat)
-Mould can develop
Not necessarily dependent
on the weather- hot/cold
microclimates can exist
Extrinsic factors: access by insects
> Insects are very important agents of decomposition
Insects are attracted by the odour/VOCs
Blowflies colonise during fresh and bloat stages, dermestid beetles in the advanced decay stage
Maggots consume the tissues of the body
Indoor Vs outdoor environments
Extrinsic factors: Moisture/ humidity/rainfall
> Decomposition requires moisture/humidity to process
Dry environments retard decomp
Water and rain can help to liquefy the body- speeds up
Can rehydrate mummified remains- kick start bacterial decomposition again
However, water can physically was away maggots
Extrinsic factors: sun/shade
> Exposure to sunlight- can warm remains and promote bacterial decay
Insect lay eggs just under skin or covering
Exposure to sun can cause bleaching, cracking, weathering of bone
Extrinsic factors: burial environment
-Composition
>Moisture content/drainage
>pH
>Oxygen
>VOCs release
>Temperature
>Microbial profile
-Particle size
-depth
-coverings, wrappings, coffins
Extrinsic factors: burial environment –> promote decomposition
> well aerated soils
Coarse textured
Free draining soils
Acidic sands and gravel; poor survival of bone in sandy, acidic soils
Sites featuring concentrations of ‘sarcosaprophagous’ microorganisms:
-Graveyards
-Manure dumps
-Cess pits
Extrinsic factors: burial environment –> retard decomposition
> Poorly drained soils- moisture retention
Clay-based soils- small participial size
Poor aeration- anaerobic bacteria
Adiopocere formation and preservation of soft tissue
Extrinsic factors: burial depth
- Decomposition is up to 4x slower at greater depth
- Temperature is lower
- Access by insects/scavengers is reduced
- Coffins retard decomposition
- Exposure of remains I shallow burials is common
- Limits release of gases/VOCs
Extrinsic factors: coffins
- Can act as physical barriers to insects
- Except Megaselia Scalaris (‘coffin flies’) that burrow down to >0.5m depth and get through cracks in coffin
- Cast iron coffins:
-Delays onset of
putrefaction by decades if
sealed correctly
-Soft tissues and hair
present after >100 years
-Adiopocere present
-Clothing preserved in very
good condition
-Preservation of internal
organs including heart,
kidneys, liver
Extrinsic factors: submersion in water
> Temperature is usually more constant, but cooler than air
Insects can only access exposed body parts, once bloat has occurred
Oxygen is depleted- aerobic bacteria need oxygen
-Can’t respire underwater
Decomposition rate is slower by half in water
Chemical composition important: pH, salinity etc
Mechanical destruction due to tides, currents, scavengers and debris
Transportation of bodies and bones
Postmortem submersion interval (PMSI)
Extrinsic factors: hanging above ground
> A body left hanging will decompose more slowly than one lying on the round
No dark place underneath the body which is favoured by insects
Maggots may fall off or be washed off by rain
More difficult for some scavengers to access the remains
Body fluids drain away- gravity
Differential decomposition
Extrinsic factors: scavengers
> Scavengers break up the body pulling apart ligaments, muscles, damaging bone to get to the bone marrow
Different populations based on location
Aquatic scavengers
Domestic scavengers
Size, shape, and position of scavenging marks can also help with identification of scavengers
Extrinsic factors: dismemberment/ fragmentation
> Started destructive process already
Increase surface area to volume ratio than whole body
More exposed flesh for oviposition
But –> some parts may not have gut bacterial populations
Limbs may mummify
Extrinsic factors: desiccation–> ‘drying’
> Browning of the skin/membranes due to drying/exposure to heat or air
Skin retreats
Desiccation of the extremities can happen first
-Differential
decomposition
Body composition and surface area to volume ratio
Drying of the eyes- Toche noir
Intrinsic factors: Body size
> Large surface area to volume ratio (small body) –> mummified
Small surface area to volume ratio (larger body) –> faster decomposition
Fat can act as waterproofing- prevent moisture loss
Very thin people decompose slower- more likely to desiccate/mummify
Infant remains decompose slower and may mummify –> due to still breastfeeding so don’t have the same intestinal bacteria
