Main types of morphological restoration
In biology, there are two key directions of morphological restoration of lost structures, which depend on the accuracy of reproducing the original organ:
- Homomorphosis is a process in which the exact restoration of a lost body part occurs, and the new structure is completely identical to the original. A classic example of this type is the normal regeneration of a dropped lizard tail.
- Heteromorphosis is the development of a structure that is morphologically different from the originally lost one. An example of heteromorphosis is the formation of an abnormal tail shape in an axolotl after injury.
Regenerative potential of various vertebrate classes
The capacity for restoration naturally decreases as the level of evolutionary organization of animals increases:
- Fish: Demonstrate high capabilities for structure recreation. They are able to well regenerate damaged fins, skin integument along with scales, as well as areas of injured jaw. In addition, they can regenerate the lens, gills, and even the tail section of the spinal cord.
- Amphibians: Representatives of the order Urodela stand out particularly. They possess an extremely high regenerative capacity and can regrow entire limbs, tails, eyes, and even the tip of the snout.
- Reptiles: In this group, the potential is noticeably reduced. Although lizards regenerate their tail excellently, attempts at limb restoration proceed extremely poorly.
- Birds: Their capabilities are limited to the tissue level. They are able to regenerate muscle tissue, gonads, and the liver.
Features of tissue regeneration in mammals and humans
In higher vertebrates and humans, the process is predominantly of a tissue nature and often ends with incomplete compensation.
Skin integument Upon skin damage, both the epidermis and dermis are able to regenerate. However, the main feature is that the proliferation rate of loose connective tissue cells significantly exceeds the division rate of epithelial cells. As a result, the defect is rapidly filled with connective tissue fibers, and scar tissue is formed. This is regarded as incomplete compensation. In clinical practice, surgical skin grafting is often resorted to for the adequate closure of extensive defects.
Musculoskeletal system
- Bone tissue: Generally regenerates well, although there are pronounced species differences. For example, extensive cranial bone defects are capable of spontaneous closure in rabbits, whereas in dogs and humans such defects do not regenerate.
- Cartilage tissue: Restores very poorly. Successful regeneration is possible exclusively in small-volume defects and occurs due to the cellular elements of the perichondrium.
- Skeletal muscles: Upon trauma, restoration of the continuity of the muscle fibers themselves does not occur. The damaged area is replaced by dense connective tissue, which is also an example of incomplete compensation.
Restoration of internal organs and the nervous system
Internal organs (such as the liver, spleen, pancreas) are characterized by a special type of restoration — endomorphosis (hypertrophy). The process is often triggered by an increased functional load on the organ. Striking examples include working hypertrophy of the heart and skeletal muscles in professional athletes, as well as pathological myocardial hypertrophy in patients suffering from hypertension.
Nervous tissue
- Central nervous system (CNS): Regeneration of brain and spinal cord structures does not occur.
- Peripheral nervous system: Only the restoration of neuronal processes is possible. The mechanism is as follows: after trauma, the peripheral nerve segment degenerates. The cells of the surviving sheath begin to actively proliferate, forming a peculiar cellular "pathway". It is along this guiding pathway that the central segment of the damaged nerve begins to grow.
- Surgical tactics: A vital condition is the mandatory surgical suturing of the severed nerve ends. If this is not done, a scar forms consisting of randomly intertwined nerve processes (amputation neuroma). This leads to pathological regeneration, which does not restore the lost function, but causes pronounced painful sensitivity in the scar area.
Briefly: answers to topical questions
What mechanisms of intracellular regeneration exist?
Intracellular regeneration consists in the restoration of the structures of surviving cells.
This process is implemented through the following mechanisms:
- Renewal of ultrastructures — restoration of normal cell architecture after partial death of its elements.
- Hyperplasia — active proliferation of surviving intracellular organelles to replace lost ones.
Thanks to these mechanisms, cells are able to maintain their functioning during the restoration process.
What humoral factors and hormones stimulate the process of tissue regeneration?
The process of tissue regeneration is stimulated by a number of hormones, growth factors, cytokines, and mediators.
Stimulating factors include:
- Hormones — somatotropin, insulin, and thyroxine have a direct stimulating effect; mineralocorticoids activate regeneration.
- Growth factors — fibroblast growth factors, epidermal, nerve, insulin-like growth factor; also CTGF, FGF9 or GAF, KGF stimulate tissue restoration.
- TGF-α — ensures cellular proliferation in granulation tissue.
- TGF-β — stimulates fibroblasts for collagenogenesis.
- Cytokines — monokines, lymphokines, interleukins, including IL-1 and IL-2–7.
- Inflammatory mediators — leukotrienes, kinins, and biogenic amines stimulate cell division.
- Anti-inflammatory mediators — glucocorticoids, antioxidants, and hydrolase inhibitors are indicated as proliferation activators.
- Endogenous opiates — enhance regeneration.
Homomorphosis = Identity (normal lizard tail). Heteromorphosis = Alterity (abnormal axolotl tail). Endomorphosis = Growth from within, in volume (hypertrophy).
Class and exam questions
Why do deep skin wounds leave a scar?
This is because the proliferation rate of loose connective tissue is much higher than that of epithelium. Connective tissue rapidly fills the wound, forming a fibrous scar, which is an example of incomplete compensation.
Can nerves regenerate after rupture?
Regeneration is impossible in the CNS. In the peripheral nervous system, neuronal processes can regenerate, but this requires mandatory surgical suturing of the nerve ends so that the processes grow along the correct "pathway".
How does liver and spleen regeneration occur?
Internal organs recover via endomorphosis — hypertrophy (increase in volume) of the remaining organ tissue occurs, often in response to enhanced function.
What else to study on this topic
- Cellular mechanisms of homomorphosis and heteromorphosis
- Features of lens and tail spinal cord restoration in fish
- Causes of species differences in the healing of cranial bone defects
- The role of the perichondrium in cartilage tissue regeneration
- Pathogenesis of neuroma formation in the absence of nerve suturing
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