Classification according to conformity to the lost structure
Historically, the doctrine of reparative regeneration took shape in the late 19th century, when the biologist T. Morgan identified the main pathways of this process. The basic criterion for classifying restorative reactions is how closely the newly formed structure matches the one lost to trauma.
Based on this principle, two main types are distinguished:
- Homomorphosis. An ideal recovery option in which the newly formed organ or tissue is completely similar to the lost one.
- Heteromorphosis. A process in which tissues or organs completely atypical for the given area develop instead of the lost ones. Although this pathway does not lead to a full recreation of the original, the study of heteromorphosis is of immense importance to biology. It allows scientists to determine the factors influencing the course of regeneration. In nature, heteromorphosis is most pronounced and widespread in lower animals: arthropods, annelids, as well as some chordates.
Epimorphosis: outgrowth from the wound surface
Epimorphosis is a method of restoration in which the lost organ grows directly from the wound surface. This process can be described as a gradual "buildup" of missing structures. Classic examples of this phenomenon include the regeneration of a severed lizard tail or a lost newt limb.
The mechanism of epimorphosis proceeds in a strict sequence:
- Resorption of destroyed tissues adjacent to the damage zone.
- Onset of intensive cell proliferation in the defect area.
- Formation of a specific regenerative primordium called a blastema.
- Differentiation of blastema cells with the formation of specialized tissues of the new organ.
Cicatrization (scarring) is closely related to epimorphosis. During scarring, the wound closes, but restoration of the lost organ itself does not occur. The defect is simply replaced by non-specialized tissue.
Morphallaxis: complete internal restructuring
Morphallaxis represents a fundamentally different pathway—a complete internal rearrangement of the entire organism or its remaining part. This mechanism predominates in lower-organized animals such as hydras, polyps, planarians, and is also observed during the regeneration of a cockroach leg.
The underlying mechanism is a significant growth of the body part remaining after trauma. From this available cellular material, an entire new organ or even an independent organism is formed. An important feature of morphallaxis is that the restored object initially has dimensions significantly smaller than the original and only increases in size over time.
Note: In real-world wildlife, epimorphosis and morphallaxis rarely occur in absolute isolation. Most often, they accompany each other, but one of the mechanisms clearly predominates (for example, during the regeneration of a newt leg or a lizard tail).
Endomorphosis (compensatory hypertrophy)
Endomorphosis, or compensatory hypertrophy, differs cardinally from the processes occurring on the body surface. This is restoration taking place deep within an organ. Its key feature is that during regeneration, the original mass of the organ is restored, but not its shape.
This method of restoration is characteristic primarily of internal parenchymatous organs. These include the liver, lungs, kidneys, spleen, pancreas, and reproductive organs.
Stages of endomorphosis:
- Primary wound healing at the site of traumatic injury.
- Intensive multiplication (proliferation) of cells in the remaining healthy part of the organ.
- Hypertrophy (increase in size) of cells in this surviving part.
- Gradual growth of the organ, which stops only when the restoration of the initial working mass is achieved.
Briefly: answers to topical questions
How does regenerative hypertrophy differ from compensatory hypertrophy?
The provided sources do not directly compare regenerative and compensatory hypertrophy. It is indicated that hypertrophy in diseases is a mechanism of compensation, and compensatory and regenerative types are listed among its kinds. During substitution, regenerative hypertrophy of the preserved functional tissue often develops around the scar, directly termed a compensatory process. In post-infarction cardiosclerosis, regenerative (compensatory) cardiomyocyte hypertrophy is described in the preserved myocardium. Consequently, according to the sources, the distinction between these terms is not disclosed.
From which tissue does the scar form during incomplete epimorphosis?
The sources do not use the term "incomplete epimorphosis." At the same time, it is indicated that during scarring (which is adjacent to epimorphosis), wound closure occurs without restoration of the lost organ, and during incomplete regeneration (substitution), the scar is formed from granulation tissue, which matures into a connective tissue scar.
To avoid confusing the mechanisms: EPImorphosis is growth from the "outside" (tail regrows), ENDOmorphosis is compensation from the "inside" (liver mass grows), and MORPHallaxis is a change in shape (restructuring of the entire planarian body from a small piece).
Class and exam questions
What is the fundamental difference between homomorphosis and heteromorphosis?
In homomorphosis, a structure identical to the lost one regenerates. In heteromorphosis, atypical organs or tissues not matching the original ones are restored at the injury site.
What is a blastema and at what stage does it appear?
Blastema is a regenerative primordium that forms during epimorphosis. It appears as a result of intensive cell proliferation after the resorption of damaged tissues, and subsequently its cells differentiate into a new organ.
Is the shape of the organ restored during compensatory hypertrophy?
No. During compensatory hypertrophy (endomorphosis), only the original mass of the organ is restored through cell proliferation and hypertrophy, but its initial anatomical shape is not recovered.
How do epimorphosis and morphallaxis relate to each other?
They usually accompany each other during the restoration of complex structures (such as amphibian limbs), but one of the mechanisms always assumes a leading role.
What else to study on this topic
- Features of blastema cell differentiation.
- The role of heteromorphosis in studying regeneration factors.
- Morphological differences between scarring and full epimorphosis.
- Regularities of cellular hypertrophy in endomorphosis.
- Specifics of morphallaxis in coelenterates and flatworms.
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