William A Beresford MA, D Phil ©
Professor of Anatomy
Anatomy Department, West Virginia University, Morgantown, USA
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
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
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
| 2 Cell membranes 'feel' that tissue is missing, and are
| prompted to migrate? and proliferate?
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
3 Inducer substances/factors
4 Interactions with extracellular matrix
5 Cell contacts & gap junctions
6 Mechanically and electrically
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
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
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
.. (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
3 Fibroblasts in CT become active, proliferate, migrate into the clot,
and lay down new collagen fibres, glycoproteins, and proteoglycans, in the construction of
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
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
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
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
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
l Long-bone fracture (involving the shaft)
l Initial phase
2 Early repair
- (a) Bony margins and marrow along the fracture line die, because blood
vessels are torn, causing some haemorrhage into the fracture gap.
- (b) The periosteum tears away from the bone's surface.
- (c) Young bones may break without interrupting the periosteum - so-called
'greenstick' fracture, but inflammation still occurs.
3 Later repair
- (a) Leucocytes and blood capillaries grow in, removing the
fibrin clot and necrotic soft tissues.
- (b) Osteoclasts erode the dead bone margin in places.
- (c) Fibroblasts, from the fibrous periosteum and adjacent CT, try to
grow into the gap and form collagen.
- (d) Further back from the gap, the surviving periosteal and
endosteal cells become active and lay down new bone and cartilage.
- (e) The proliferating osteoblasts, chondroblasts and fibroblasts comprise
4 Union and non-union
- (a) It is seen that new bone can be laid down upon living bone, dead bone,
calcified cartilage, and as free-standing, independent trabeculae.
- (b) The distribution of the new firm tissues, called the callus is:
- (i) On the endosteal bone, and in the marrow cavity, as a bony layer
and as trabeculae, together termed the internal/endosteal callus.
- (ii) On the living bone outside the shaft, as a bony layer and
free-standing trabeculae and, nearer to the broken margin, as a rapidly
formed, large mass of hyaline cartilage. The new bone extends
progressively into the cartilage by the process of endochondral
ossification, as seen in long-bone development.
These bulky, outside tissues constitute the external/periosteal
- (a) If the broken ends are separated by only a narrow fracture gap, the
periosteal and endosteal calluses growing from each side may meet and fuse,
resulting in union.
- (b) However, in a wider gap fibroblasts may grow in and fill it with dense
- (c) The fibrous union (non-union) reduces further formation of new
bone, and leaves the two pieces of the skeletal bone free to move,
i.e., a pseudarthrosis forms, and may even acquire cartilage and
2 Some terms used clinically
- (a) When union by bony trabeculae and hyaline cartilage takes place, the
cartilage is rapidly replaced by endochondral bone.
- (b) The bony trabeculae (of woven bone) at first fill the marrow cavity,
the space between the bone-ends, and stand proud to the outer surface of
the bone. This callus bone will be remodelled:
.. (i) to restore the marrow cavity internally,
.. (ii) to reduce the high contour of the external bone,
.. (iii) at the same time, to have its density increased by the replacement
of some woven bone by lamellar bone.
Remembering that the diaphysis of a long bone has a long axis:
3 Skull vault (compared with long bone's shaft)
- (a) One broken bone piece may be displaced off the axis of the
other, or be misplaced anatomically.
- (b) The axes of the resulting two pieces may not be aligned or parallel:
the pieces are angulated.
- (c) At the fracture site, bone may split into many small fragments -
comminution of the fracture.
- (d) These bone fragments usually die, but some of their surface cells
and neighbouring cells form new bone, which holds the dead bone in
place, where its rigid nature is of use as an accidental bone graft.
Dead or living bone may be placed surgically as an intentional graft in a
fracture gap. Any bone graft is used as a temporary hard tissue, and
will eventually be resorbed and replaced by new bone.
- (e) The ideal is not to leave a significant fracture gap, but to
intervene early to bring and hold the bone ends in close apposition,
and correctly located. This procedure is called reduction of the
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.
l As on bone, restitution of tissue is performed by the surface covering -
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