HISTOLOGY FULL-TEXT

William A Beresford MA, D Phil ©
Professor of Anatomy
Anatomy Department, West Virginia University, Morgantown, USA

Chapter 3l REGENERATION

Regeneration is the regrowth of a tissue or part of an organ after its destruction or loss. The ability to restore structural and functional integrity after injury is essential for survival, and genetic selection has left man with much of the regenerative ability of lower vertebrates.
In medicine, many of the injuries seen are complicated by such factors as infection, chemical, heat and radiation damage, extensive haemorrhage, delayed treatment, shock, multiple injury, old age, malnutrition and metabolic disease.
Nevertheless, it is helpful to know how well individual tissues and organs can mend after injury, under optimal conditions of diet, age, treatment and its timing. In man, such conditions may prevail in elective surgery. The discussion of regeneration below applies to aseptic, experimental injury in mammals.

A GENERAL CONCEPTS

l Regrowth of a tissue and its organization for function in many ways recapitulate the initial embryonic formation of the tissue. The formation of new tissue needs the development of new cells as shown below.


SOURCE CELLS     (a) Surviving differentiated cells (may de-differentiate)
     |           (b) Surviving undifferentiated stem cells
     |           (c) Circulating cells in the blood
     |
proliferation   1 Stimulated by: reduced density of cell packing?
     |             Physiological overload? Growth factors? Loss of growth
     |             inhibitors?
     |          2 Cell membranes 'feel' that tissue is missing, and are
     |             prompted to migrate? and proliferate?
     |
     |
     V                      Specialisation
GROWING CELL POPULATION -------------------------> DIFFERENTIATED CELLS
                            brought about by
                                                      
                       1 Maintained regulatory programmes
                       2 Continuity of regenerating part with mature,
                          specialized part, e.g., in skeletal muscle
                          fibre
                       3 Inducer substances/factors
                       4 Interactions with extracellular matrix
                       5 Cell contacts & gap junctions
                       6 Mechanically and electrically
                          polarized fields

2 New tissue requires new cells, derived by cell proliferation
The extent to which cell division is taking place can be determined by radioautographic study using tritiated thymidine, or BrdU-based methods. Radioautography has shown, for example, that smooth muscle has more regenerative ability than the none credited to it previously. Tissue with cells incapable of division, e.g., neurons, is unable to restore the lost cells, although individual neurons can repair some damage to their processes.

3 Organization of the differentiating cells to repair an organ
l Coordination of regenerations of more than one tissue is needed, e.g., (a) glandular cells, blood vessels, and stromal cells, ECM and later reticular elements, to build new lobules in the liver; (b) skeletal muscle fibres, connective tissues, and nerve fibres, to regenerate muscle and restore musculo-skeletal function. Tissues interact inductively and trophically.
2 Regenerations of tissues may compete, with a functionally unfavourable outcome, e.g.,
.. (a) collagenous connective tissue may outgrow the regenerating cartilage and bone in a skeletal fracture, and fill the fracture gap with a fibrous tissue not rigid enough for support;
.. (b) in the injured brain, glial cells multiply, and actively inhibit the growth of axons.

4 Requirements for regeneration
l Space: the growth of tissue requires space and may, in itself, be a response acknowledging that a spatial defect exists. Where tissue has been damaged, phagocytosis and lysis of the necrotic tissue make room for the new cells. When the defect has been filled, the cell proliferation is reduced or stopped. This inhibition happens even when the 'wrong' tissue, e.g., fibrous CT, fills the gap.
2 Communication between the participating cells by means of cytokine and other agents.
3 Adequate hormone levels, e.g., thyroid.
4 Adequate stores or intake of amino acids, vitamins, etc., for the synthesis of new protein and other materials.
5 Freedom from infection.
6 An intact blood supply and drainage for the area. Sometimes, too, a continuing innervation is a necessary coordinating and trophic factor.

5 Hyperplasia and hypertrophy
l Hyperplasia is a response by the tissue involving mitosis and the formation of new cells, increasing cell number to meet a demand for greater output, e.g., of glandular secretion.
2 Hypertrophy again tries to meet a requirement for increased effort or output, not by cell proliferation, but rather by the cells in their original number increasing their size and hence content of productive organelles and materials, e.g., uterine smooth muscle cells in pregnancy.

6 Regeneration and physiological regeneration
l Physiological regeneration is a normal process going on continuously, e.g., in gut epithelium, or continually, e.g., in hair follicle epithelium, involving cell division for the replacement of cells lost naturally.
Blood cells, epithelial, bone and connective tissue cells show the phenomenon. Muscle, cartilage and nerve cells, on the other hand, are stable and static cells, in maturity.
2 Tissues regenerating physiologically without any injury are better able to regenerate to repair damage than those with stable, long-living cells.

E EPITHELIAL AND CONNECTIVE TISSUES

l Initial events
A sterile cut into epithelium that involves its lamina propria will:
.. (a) kill some epithelial cells,
.. (b) cut CT fibres allowing the wound to gape open;
.. (c) sever small blood vessels which
.. (d) will spill blood into the gap.

2 Below the epithelium
l Blood forms a clot with fibrin fibres, platelets, and fibronectin.
2 Leucocytes migrate through intact vessel walls; polymorphs attacking the few bacteria and monocytes removing cell debris and fibrin. (The inflammatory response and agents involved in it, e.g., affecting capillary permeability and the migration of leucocytes, will be studied in pathology and pharmacology.)
3 Fibroblasts in CT become active, proliferate, migrate into the clot, and lay down new collagen fibres, glycoproteins, and proteoglycans, in the construction of granulation tissue.
4 Endothelial cells of cut capillaries concurrently proliferate and move into the clot, rebuilding the capillary network.
5 Continued activity along these several lines results in the rapid formation of a new lamina propria. Meanwhile,

3 In the epithelium
l Epithelial cells at the cut margins migrate out as a thin layer, penetrating the fibrin, and replacing the temporary substrate of fibronectin and tenascin with basal lamina as a robust and permanent support.
2 While the epithelial surface is being restored as a thin sheet, its cells start to multiply and differentiate, to restore the original thickness and variety of specialized cells of the epithelium.
3 Since the new epithelium has to grow in from the edges, the degree of gaping of the cut determines the distance over which the cells must migrate, and hence the time needed for healing. The cut margins of the wound should therefore be drawn into apposition by sutures. The sooner the epithelium is restored, the earlier is the underlying tissue protected from invasion by pathogenic organisms.
The above describes the smooth, progressive sequence of healing by first intention.
4 The surviving deep part of exocrine glands can provide a source of regenerative epithelial cells to replace the surface epithelium, in addition to the lost part of the gland. Significant examples are:
(i) in the physiological restitution of the uterine lining from the basal layer of the endometrium after menstruation;
(ii) the replacement of epidermis from the sweat glands (and hair follicles), after a second-degree burn has killed the more superficial epithelial cells.

Although the regenerations of epithelium and CT have been treated separately, in life these processes, and, indeed, their normal day-to-day working, are tightly coordinated by cytokines and cell-matrix interactions.

In general, repair can restore both epithelium and its lamina propria of connective tissue to almost as good a condition as before. When there is pathological delay, cytokines are being tried in order to speed up epithelial events and to boost construction of the lamina propria.

C GLANDULAR REPAIR

Because they are composed of epithelial cells, glands can show considerable regenerative ability. For example:
l Liver, after aseptic, surgical removal of half of its substance, can make good the deficit with organized hyperplasia and some hypertrophy of the remainder.
2 Pancreas. Ligaturing the pancreatic duct causes the enzymatic destruction of most of the acinar tissue. If the ligature is released before the duct epithelium is killed, regrowth of glandular acini and islets can take place from the duct epithelium.
3 Kidney can replace tubular epithelium injured, for example, by toxins, but lost or damaged glomeruli are not restored.

D MUSCULAR REGENERATION

l Skeletal muscle
l Some regeneration occurs at the cut ends of fibres. (A cut is insufficient injury to kill the cell throughout its length.)
2 The end-piece reverts to the narrow myotube stage, seen in embryonic growth.
3 Just outside the sarcolemma of intact muscle fibres lie satellite cells that act as residual, peripheral myoblasts, able to respond to injury by becoming active myoblasts.
4 The end grows out a little way into the defect, then increases in thickness. If the cut is not wide, myotubes regenerating from each side may fuse and restore the fibres.
5 A deep cut may sever nerves disturbing regeneration in two ways:
.. (a) Denervation of muscle fibres reduces their regenerative response.
.. (b) Dense fibrous CT then fills the gap and obstructs reinervation of the muscle.

2 Smooth and cardiac muscle
l Radioautographic studies indicate that smooth muscle cells, e.g., in the gut, are capable of some proliferation to replace damaged cells and partially restore continuity in a muscular tunic.
2 Cardiac muscle is at a disadvantage, because it cannot relax and rest for a period to permit cell division and muscle reorganization; and there may be an early and unfavourable response from its CT.
Lung, likewise, is prevented by its elasticity and motion, and other factors, from effective regeneration, despite its epithelial content.
3 Cuts into cardiac muscle fill quickly with collagenous CT, but muscle fibres injured by infections can regenerate.
In general then, a large lesion in muscle will be filled with C.T. Only a little new muscle tissue forms to replace that lost or to fill gaps. Surviving muscle fibres may hypertrophy in an attempt to restore the power of the muscle as a whole.

E BONE AND CARTILAGE

If a long bone of an extremity breaks, the animal's ability to forage, and to escape from predators is seriously curtailed. Most animals, including man, are able to repair such broken bones, and use them again for locomotion and other tasks. .

l Long-bone fracture (involving the shaft)
l Initial phase

2 Early repair 3 Later repair 4 Union and non-union 5 Consolidation 2 Some terms used clinically
Remembering that the diaphysis of a long bone has a long axis: 3 Skull vault (compared with long bone's shaft)
l Initial phase is essentially similar.
2 Early repair. The difference is that the surviving bony surfaces (periosteal/pericranial, outside; dural, inside the vault) produce only a little new bone, and only very rarely any cartilage. The fibroblasts meet little to obstruct them from filling the gap with CT, which is too soft to protect the brain.

4 Cartilage
l As on bone, restitution of tissue is performed by the surface covering - the perichondrium.
2 In youth, when it is still active in appositional growth, the perichondrium can restore significant defects.
3 In mature cartilage, defects are likely to be filled with fibrous CT, or the lesion may precipitate a degeneration of adjacent cartilage.
Lacking a perichondrium, articular surfaces are especially unable to repair damage. In end-stage osteoarthrosis, the cartilage is completely worn away, leaving painful, grinding bone ends.

F ASSORTED TISSUES

l Tooth. The enamel, deprived of its forming cells at eruption, is incapable of repair. Additional dentine can be laid down by the odontoblast layer on the pulp-chamber surface of the dentine - again an example of restitution from the surface.
2 Tendon. Fibroblasts of the cut tendon's sheath and other sources proliferate, become active, and lay down orderly collagen fibres, which can restore most of the original strength of the tendon.
3 Myeloid and lymphoid tissues
l The phagocytic filtering action can be performed in other organs, if only one member of the system is removed. Thus, splenectomy leaves the bone marrow and liver's macrophage cells with the task of treating blood, e.g., removing old RBCs, but creates a vulnerability to certain pathogens. Therefore, surgeons try to repair ruptured spleens.
2 Removal of myeloid tissues from the sternum and calvarium leaves much marrow in other bones. Surviving marrow becomes more active, and can repopulate (by passage through the bloodstream) sites denuded of haemopoietic tissue.
A drastic demand for new RBCs and granulocytes may be met by the resumption of myelopoiesis in such ectopic sites as the liver.
3 After the killing of all the blood cell-producing elements by whole-body X-irradiation, activity can be restored by the injection into the circulation of isogenous bone marrow cells.

G NERVOUS TISSUE

See Chapter
11.F.2
William A Beresford, Anatomy Department, School of Medicine, West Virginia University, Morgantown, WV 26506-9128, USA - - e-mail: -- wberesfo@wvu.edu -- wberesfo@hotmail.com -- beresfo@wvnvm.wvnet.edu -- fax: 304-293-8159