Looking at a simpler model of something as complex as wound healing usually provides interesting insights. Sponges are animals with a very simple structure and are known for their regenerating capacity. The reason they are interesting are:
- obvious similarities between sponge and human response to injury.
- not so obvious similarities between sponge and human response to injury.
- clues what to look for in human response to injury.
This tells us something about the basic events in wound healing. Translating these findings to human wound healing is tempting, but should be considered with caution.
Sponges are animals with a very simple structure and are known for their regenerating capacity.
The sponge consists of layers of pinacocytes and choanocytes which resemble the epithelium of more complex animals. This layer covers a gel type body wherein the cells move freely. They do not have a nervous system but are nevertheless able to respond to be environment by means of neurotransmitters which guides movement. (https://en.wikipedia.org/wiki/Sponge) A sponge actually has two types of cells which play a role in regeneration. Cells which provide matrix (collencytes) and cells which proliferate into other cells (archeocytes).
Structurally a sponge has a primitive version of an epidermis, a dermis and a skeleton and its regeneration system runs roughly on two types of cells. The similarities are striking1,2,3,4.
The obvious similarity between sponge and human response to injury.
- Age matters, tissue age limits regeneration. Young tissue regenerates faster.
- Genotype matters, regeneration varies among genetically distinct sponges.
- Wound size matters, larger wounds heal slower than small wounds.
- Wound depth matters, as more deep tissue and skeletal elements are involved wounds heal slower.
- Wound location matters, distal wounds heal faster than wounds on the base of a sponge. In general, if the transport of material for regeneration is easy, wounds heal faster.
- Water temperature matters, in warmer water the metabolism runs faster, and thus regeneration is faster.
- Food availability, if more nutrients are available, there are also more nutrients available for regeneration.
- Sedimentation matters, if more sedimentation falls into the wound, more energy is needed to remove it and this energy is not available for regeneration.
- Disturbance history, previously injured sponges are more likely to be damaged.
The not so obvious similarity between sponge and human response to injury.
- Post regeneration strategies are aimed at closing the “skin” as fast as possible to prevent fouling, the marine version of a biofilm.
- Wound perimeter matters, a circular wound heals faster than a linear wound of the same surface area.
- Energy balance is important, regeneration draws energy from the organism. Actually, there is a competition for resources between regeneration and other functions of life.
- Size matters, larger sponges generate better, simply because they have a larger metabolic capacity.
Clues what to look for in human response to injury.
- The G1 checkpoint; reduced growth of cells adjacent to the wound be due to cells being halted in their progression through the cell cycle as a result of insufficient energy to progress past the G1 checkpoint in eukaryotic cell division.
- Distance; cell proliferation is lower at 1 cm from the wound compared to cell proliferation 3 cm from the wound.
And the sponge is only one of several model organisms for wound care.
Refs: these articles are cited “loosely”.
- Henry, L. A. & Hart, M. Regeneration from injury and resource allocation in sponges and corals – A review. Int. Rev. Hydrobiol. 90, 125–158 (2005).
- Alexander, B. E. et al. Cell kinetics during regeneration in the sponge Halisarca caerulea : how local is the response to tissue damage? PeerJ 3, e820 (2015).
- Wulff, J. Regeneration of sponges in ecological context: Is regeneration an integral part of life history and morphological strategies? Integr. Comp. Biol. 50, 494–505 (2010).
- Hoppe, W. F. Reproductive patterns in three species of large coral reef sponges. Coral Reefs 7, 45–50 (1988).