Disorders of growth, differentiation and morphogenesis

4 Disorders of growth, differentiation and morphogenesis



Growth, differentiation and morphogenesis are the processes by which a single cell, the fertilised ovum, develops into a large, complex, multicellular organism with co-ordinated organ systems containing a variety of cell types, each with individual specialised functions. Growth and differentiation continue throughout adult life, as many cells of the body undergo a constant cycle of death, replacement and growth in response to normal (physiological) or abnormal (pathological) stimuli.


There are many stages in human embryological development at which anomalies of growth and/or differentiation may occur, leading to major or minor abnormalities of form or function, or even death of the fetus. In postnatal and adult life, some alterations in growth or differentiation may be beneficial, as in the development of increased muscle mass in the limbs of workers engaged in heavy manual tasks. Other changes may be detrimental to health, as in cancer, where the outcome may be fatal.


This chapter explores the wide range of abnormalities of growth, differentiation and morphogenesis which may be encountered in clinical practice, relating them where possible to specific deviations from normal cellular functions or control mechanisms.



DEFINITIONS







REGENERATION


Regeneration enables cells or tissues destroyed by injury or disease to be replaced by functionally identical cells. These replaced ‘daughter’ cells are usually derived from a tissue reservoir of ‘parent’ stem cells (discussed below, page 72). The presence of tissue stem cells, with their ability to proliferate, governs the regenerative potential of a specific cell type. Mammalian cells fall into three classes according to their regenerative ability:





Labile cells proliferate continuously in postnatal life; they have a short-lifespan and a rapid ‘turnover’ time. Their high regenerative potential means that lost cells are rapidly replaced by division of stem cells. However, the high cell turnover renders these cells highly susceptible to the toxic effects of radiation or drugs (such as anticancer drugs) which interfere with cell division. Examples of labile cells include:




The high regenerative potential of the skin is exploited in the treatment of patients with skin loss due to severe burns. The surgeon removes a layer of the split skin which includes the dividing basal cells from the unburned donor site, and fixes it firmly to the burned graft site where the epithelium has been lost. Dividing basal stem cells in the graft, and dividing stem cells from residual basal and adnexal structures (such as the cells from the neck of pilosebaceous units) from the donor sites, ensure that squamous epithelium at both sites regenerates. This enables rapid healing to take place in a large burned area, when natural regeneration of new epithelium from the edge of the burn would otherwise be prolonged. Skin epithelium from a donor site can now be grown in the laboratory by tissue/organ culture for eventual grafting onto burned areas, and this is important for patients with extensive burns.


Stable cells (sometimes called ‘conditional renewal cells’) divide very infrequently under normal conditions, but stem cells are stimulated to divide rapidly when such cells are lost. This group includes cells of the liver, endocrine glands, bone, fibrous tissue and the renal tubules. Thus the liver is able to regenerate to its normal weight even after large partial resections for neoplastic disease.


Permanent cells normally divide only during fetal life, but their active stem cells do not persist long into postnatal life, and they cannot be replaced when lost. Cells in this category include neurons, retinal photoreceptors and neurons in the eye, cardiac muscle cells and skeletal muscle cells (although skeletal muscle cells do have a very limited capacity for regeneration).



CELL CYCLE


Successive phases of progression of a cell through its cycle of replication are defined with reference to DNA synthesis and cellular division. Unlike the synthesis of most cellular constituents, which occurs throughout the interphase period between cell divisions, DNA synthesis occurs only during a limited period of the interphase: this is the S phase of the cell cycle. A further distinct phase of the cycle is the cell-division stage or M phase (Fig. 4.3) comprising nuclear division (mitosis) and cytoplasmic division (cytokinesis). Following the M phase, the cell enters the first gap (G1) phase and, via the S phase, the second gap (G2) phase before entering the M phase again.



Some cells (e.g. some of the stable cells) may ‘escape’ from the G1 phase of the cell cycle by temporarily entering a G0 ‘resting’ phase: others ‘escape’ permanently to G0 by a process of terminal differentiation, with loss of potential for further division and death at the end of the lifetime of the cell: this occurs in permanent cells, such as neurons.





CELL DEATH IN GROWTH AND MORPHOGENESIS


It seems illogical to think of cell death as a component of normal growth and morphogenesis, although we recognise that the loss of a tadpole’s tail, which is mediated by genetically programmed cell death, is part of the metamorphosis of a frog. Cell death is a paradox of growth, and it is now clear that cell death has an important role in the development of an embryo, and in the regulation of tissue size throughout life. Alterations in the rate at which cell death occurs are important in situations such as hormonal growth regulation, immunity and neoplasia.



APOPTOSIS


The term ‘apoptosis’ is used to define the type of individual cell death which is related to growth and morphogenesis, but which appears to have an opposite function in regulating the size of a cell population. Apoptosis is a biochemically specific mode of cell death characterised by activation of non-lysosomal endogenous endonuclease, which digests nuclear DNA into smaller DNA fragments. Morphologically, apoptosis is recognised as death of scattered single cells which form rounded, membrane-bound bodies; these are eventually phagocytosed (ingested) and broken down by adjacent unaffected cells.


The coincidence of both mitosis and apoptosis within a cell population ensures a continuous renewal of cells, rendering a tissue more adaptable to environmental demands than one in which the cell population is static.


Apoptosis can be triggered by factors outside the cell or it can be an autonomous event (‘programmed cell death’). In embryological development, there are three categories of autonomous apoptosis:











Regulation of apoptosis


When cells within tissues are stimulated to divide by mitogens the tissues enter a high turnover state, in which mitotic activity is accompanied by some degree of coincident apoptosis (Fig. 4.6). The ultimate fate of individual cells within the tissue – whether the cell will survive or undergo apoptosis – depends upon the balance between apoptosis inducers (survival inhibitors) and apoptosis inhibitors (survival factors). Although apoptosis can be induced by diverse signals in a variety of cell types, a few genes appear to regulate a final common pathway. The most important of these are the members of the bcl-2 family (bcl-2 was originally identified at the t(14:18) chromosomal breakpoint in follicular B-cell lymphoma, and it can inhibit many factors which induce apoptosis). The bax protein (also in the bcl-2 family) forms bax-bax dimers which enhance apoptotic stimuli. The ratio of bcl-2 to bax determines the cell’s susceptibility to apoptotic stimuli, and constitutes a ‘molecular switch’ which determines whether a cell will survive (leading to tissue expansion), or undergo apoptosis (leading to tissue contraction).



The study of factors regulating apoptosis is of considerable importance in finding therapeutic agents to enhance cell death in malignant neoplasms. In retinoblastoma (a malignant neoplasm of the eye found in infants), the neoplasm has a very high mitotic rate, but also has extensive apoptosis. Occasionally the neoplasm undergoes spontaneous regression (possibly due to increased apoptosis), and agents which increase apoptosis might also induce this regression therapeutically.



NORMAL AND ABNORMAL GROWTH IN SINGLE TISSUES


Within an individual organ or tissue, increased or decreased growth takes place in a range of physiological and pathological circumstances as part of the adaptive response of cells to changing requirements for growth.



INCREASED GROWTH: HYPERTROPHY AND HYPERPLASIA


The response of an individual cell to increased functional demand is to increase tissue or organ size (Fig. 4.7) by:






The stimuli for hypertrophy and hyperplasia are very similar, and in many cases identical; indeed, hypertrophy and hyperplasia commonly coexist. In permanent cells (see pp. 13, 53) hypertrophy is the only adaptive option available under stimulatory conditions. In some circumstances, however, permanent cells may increase their DNA content (ploidy) in hypertrophy, although the cells arrest in the G2 phase of the cell cycle without undergoing mitosis; such a circumstance is present in severely hypertrophied hearts, where a large proportion of cells may be polyploid.


An important component of hyperplasia, which is often overlooked, is a decrease in cell loss by apoptosis; the mechanisms of control of this decreased apoptosis are unclear, although they are related to the factors causing increased cell production (Fig. 4.6).




REPAIR AND REGENERATION


The proliferation of vascular (capillary) endothelial cells and myofibroblasts in scar tissue, and the regeneration of specialised cells within a tissue, are the important components of the response to tissue damage.





Angiogenesis

This is the process whereby new blood vessels grow into damaged, ischaemic or necrotic tissues in order to supply oxygen and nutrients for cells involved in regeneration and repair. Briefly, vascular endothelial cells within pre-existing capillaries are activated by angiogenic growth factors such as vascular endothelial growth factor (VEGF), released by hypoxic cells or macrophages. On activation, the endothelial cells secrete plasminogen activator and other enzymes, including the matrix metalloproteinases, which selectively degrade extracellular matrix proteins to allow endothelial cell migration to occur. Tissue inhibitors of metalloproteinases exist to prevent excessive matrix breakdown. Thus, activated endothelial cells migrate (mediated by integrins, a family of cell-surface adhesion molecules) and proliferate towards the angiogenic stimulus to form a ‘sprout’. Adjacent sprouts connect to form vascular loops, which canalise and establish a blood flow. Later, mesenchymal cells, including pericytes and smooth muscle cells, are recruited to stabilise the vascular architecture, and the extracellular matrix is remodelled. Two other initiating mechanisms exist in addition to the above ‘sprouting’ form of angiogenesis: existing vascular channels may be bisected by an extracellular matrix ‘pillar’ (intussusception), and the two channels extend towards the angiogenic stimulus; and the third mechanism involves circulating primordial stem cells which are recruited at sites of hypoxia and differentiate into activated vascular endothelial cells. (Note that a similar process of angiogenesis occurs in response to tumour cells, as an essential component of the development of the blood supply of enlarging neoplasms. Such angiogenesis is an important new therapeutic target in the treatment of malignant neoplasms, although theoretically such drugs might impair angiogenesis and, therefore, delay healing of wounds.)


(Note that the term ‘vasculogenesis’ should be reserved specifically for the blood vessel proliferation which occurs in the developing embryo and fetus.)




Skin


The healing of a skin wound is a complex process involving the removal of necrotic debris from the wound and repair of the defect by hyperplasia of capillaries, myofibroblasts and epithelial cells. Fig. 4.8 illustrates some of these events, most of which are mediated by growth factors.



When tissue injury occurs, there is haemorrhage into the defect from damaged blood vessels; this is controlled by normal haemostatic mechanisms, during which platelets aggregate and thrombus forms to plug the defect in the vessel wall. Because of interactions between the coagulation and complement systems, inflammatory cells are attracted to the site of injury by chemotactic complement fractions. In addition, platelets release two potent growth factors – platelet-derived growth factor (PDGF) and transforming growth factor beta (TGFβ) – which are powerfully chemotactic for inflammatory cells, including macrophages; these migrate into the wound to remove necrotic tissue and fibrin.


In the epidermis, PDGF acts synergistically with epidermal growth factor (EGF) and the somatomedins (IGF-1 and IGF-2) to promote the progression of basal epithelial cells through the cycle of cell proliferation (p. 53). PDGF acts as a ‘competence factor’ to move cells from their ‘resting’ phase in G0 to G1. EGF and IGFs then act sequentially in cell progression from the G1 phase to that of DNA synthesis. Thereafter, the cell is independent of growth factors. In the epidermis, EGF is derived from epidermal cells (autocrine and paracrine mechanisms), and is also present in high concentrations in saliva when the wound is licked. IGF-1 and IGF-2 originate from the circulation (endocrine mechanisms) and from the proliferating cell and adjacent epidermal and dermal cells (autocrine and paracrine mechanisms).


(Note that once a specialised adnexal structure such as a pilosebaceous unit has been destroyed, new units cannot regenerate from the basal layer of the epidermis. Hairs will, therefore, not grow in areas where deep burns have destroyed adnexal tissues, even if split skin grafting is successful. Similarly, in ‘scarring alopecia’, hair loss is permanent once hair follicles have been destroyed.)


In the dermis, myofibroblasts proliferate in response to PDGF (and TGFβ); collagen and fibronectin secretion is stimulated by TGFβ, and fibronectin then aids migration of epithelial and dermal cells.


Capillary budding and proliferation are stimulated by angiogenic factors such as vascular endothelial growth factor (VEGF: see above). The capillaries ease the access of inflammatory cells and fibroblasts, particularly into large areas of necrotic tissue.


Hormones (e.g. insulin and thyroid hormones) and nutrients (e.g. glucose and amino acids) are also required. Lack of nutrients or vitamins, the presence of inhibitory factors such as corticosteroids or infection, or a locally poor circulation with low tissue oxygen concentrations, may all materially delay wound healing; these factors are very important in clinical practice.



Ulcers and erosions


An ulcer is a full-thickness defect in a surface epithelium or mucosa, which may also extend into subepithelial or submucosal tissue. An erosion is a partial-thickness defect in a surface epithelium or mucosa.


Both ulcers and erosions occur when adverse tissue circumstances (‘ulcerating factors’, such as hypoxia, factors such as gastric acid forming the local physicochemical environment, or infection) cause local death of cells which cannot be replaced by regenerative cell proliferation, leading to net loss of epithelial or mucosal tissue. The presence of one or more of these ‘ulcerating factors’, therefore, overpowers the local ‘survival factors’, such as the regenerative potential and oxygenation of the tissue, and an ulcer or erosion develops.


Once the ‘ulcerating factor or factors’ are removed, however, the residual ‘survival and healing factors’, or healing capacity of the tissue predominates, and cell proliferation exceeds cell loss, producing net tissue growth to fill the ulcer cavity. In deep ulcers (Fig. 4.9), angiogenic growth factors (produced by macrophages in the necrotic ulcer crater) stimulate growth and migration of capillaries into the base of the ulcer (producing vascular ‘granulation tissue’, seen as finely granular red tissue in the ulcer base). Myofibroblasts also migrate into the ulcer crater, where they proliferate and secrete collagen and matrix proteins, filling the ulcer crater. Once this has happened, the epithelial cells at the edge of the ulcer migrate over the new scar tissue: eventually the ulcer crater is filled, and the epithelium totally covers the former ulcer. Eventually, subepithelial scar tissue contracts (caused by myofibroblast contraction), and myofibroblasts differentiate into mature fibroblasts.


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Fig. 4.9 Healing in an ulcer. These basic mechanisms apply to all ulcers, in different tissues of the body. In this ulcer the predominance of ‘ulcerating factors’ (factors such as anoxia, gastric acid, and infection) has caused loss of both the epithelium and subepithelial tissue (top left). Once these factors have been corrected, however, ‘survival/healing factors’ predominate, and healing, repair and regeneration can take place (sequence a–f). (a) Macrophages have migrated into the necrotic tissue of the ulcer, where they ingest necrotic debris. In addition, however, they secrete angiogenic growth factors (A), which diffuse into the tissue at the base of the ulcer. (b) Angiogenic growth factors stimulate vascular (capillary) endothelial cells to proliferate and migrate into the ulcer (forming ‘sprouts’). (c) Adjacent endothelial cells sprouts join to form loops, and canalise (a lumen forms), allowing blood flow through the loop. New endothelial cell sprouts then develop. Myofibroblasts proliferate and migrate into the newly vascularised base of the ulcer. (d) More proliferation of capillaries occurs, producing granulation tissue (seen macroscopically as a red granular base to the ulcer). Myofibroblasts continue to proliferate, and produce collagen and other intracellular matrix proteins (to strengthen the developing scar). (e) Once the blood vessels and proliferating myofibroblasts fill the cavity of the ulcer, epithelial cells from the edge of the ulcer proliferate (stimulated by epithelial growth factors) and migrate over the regenerating subepithelial tissue (migration is aided by fibronectin secreted by myofibroblasts). (f) Eventually the myofibroblasts contract, with resulting contraction of the scar. The epithelium has now regenerated completely.


If ‘ulcerating factors’ persist, or if there are recurrent cycles of ulceration – healing – ulceration, an ulcer may become ‘chronic’, with a large deep crater and very extensive scar formation, perhaps leading to marked deformity of the tissue (for example, an ‘hour glass’ deformity with possible stenosis in a stomach with a large chronic gastric ulcer).


At the epithelial edge of large chronic ulcers, persistent attempts to regenerate occasionally lead to the development of a malignant neoplasm (carcinoma).


If an ulcer fails to heal after ‘ulcerating factors’ have been removed, this may indicate that there is an underlying neoplasm. Many malignant neoplasms, which arise in (or invade) epithelial or mucosal tissues, ulcerate as they outgrow their blood supply or invade local blood vessels. A classical example is basal cell carcinoma of the skin (a ‘rodent’ ulcer), but other examples include breast adenocarcinoma ulcerating overlying skin, and large ulcerated bowel adenocarcinomas.


Note that epithelial proliferation and regeneration alone are required to heal an erosion, once the causative factor has been removed.



Peritoneum


The practice of abdominal surgery requires an understanding of the mechanisms of peritoneal healing and of the development of intra-abdominal fibrous adhesions (scars). In one large study, 31% of all cases of intestinal obstruction were due to adhesions, and of these patients 79% had undergone previous abdominal surgery, whilst 18% had inflammatory adhesions and 3% had congenital bands.


The process of healing and repair of a peritoneal defect is very different to that of an ulcerated epithelial surface, as the mesothelial surface cells do not grow over the defect from its edges. If even large peritoneal defects are left open (not sutured), macrophages migrate into the area to remove necrotic debris (Fig. 4.10). This is followed by a proliferation and migration of peritoneal perivascular connective tissue cells (which resemble primitive mesenchymal cells) into the defect, which eventually fills with these cells. The connective tissue cells on the ‘new’ surface then undergo metaplasia into mesothelial cells. As a result, peritoneal defects heal very rapidly, large defects heal as rapidly as small ones, and peritoneal healing occurs without formation of adhesions.


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Fig. 4.10 Factors affecting peritoneal wound healing and adhesions. Representation of two opposing peritoneal surfaces (top and bottom), with two surgically created wounds which have removed the mesothelium and some submesothelial connective tissue. In lesion 1 (left) the surgeon has left the defect open, and has carefully removed foreign body (FB) material and fibrin from the surface. Under these circumstances: (1a) macrophages remove necrotic material from the wounded area, then (1b) subperitoneal perivascular connective tissue cells proliferate and migrate into the base of the defect, and (1c) fill the defect. Finally (1d) the surface layer of these cells undergoes metaplasia into mesothelial cells. As a result, healing takes place with no adhesions to adjacent peritoneal surfaces. In lesion 2 (right), by contrast, foreign material and fibrin have not been removed by the surgeon. In addition, the peritoneal defect has been sutured, and as a result (2a) the tissue is relatively ischaemic as a result of the tension of the suture. Under these circumstances (2b) angiogenesis occurs, and proliferating blood vessels extend into the ischaemic tissue and into the fibrin on the surface of the wound, eventually accompanied by the proliferating myofibroblasts which grow over the adjacent mesothelium, and which eventually form the adhesions to the adjacent peritoneal surface (top). Contraction of these myofibroblasts, and accompanying scar contraction, may cause intestinal obstruction if the peritoneal adhesions are extensive.


If, however, peritoneal defects are sutured, the suture compresses or tensions the mesothelium and underlying connective tissue, which tends to become relatively ischaemic as a result (Fig. 4.10). As a result, angiogenesis (new blood vessel formation) is stimulated, and capillaries (and later fibroblasts) migrate into the area. If fibrin and/or foreign material such as starch (used to lubricate the inside of surgical gloves) are on the peritoneal surface, the capillaries and fibroblasts grow into the area, and are likely to cause adhesions to adjacent peritoneal surfaces, which may ultimately cause intestinal obstruction. In abdominal and pelvic surgery, therefore, peritoneal surfaces which are left unsutured are less likely to cause adhesions, and both removal of fibrin and prevention of contamination by foreign body materials will reduce the chances of adhesion formation.


Peritoneal mesothelial cells have fibrinolytic activity, but damage to these cells at surgery reduces their ability to remove the peritoneal fibrin which promotes development of adhesions. In addition, growth factors such as epidermal growth factor (EGF) and transforming growth factor beta (TGFβ) may directly influence cell growth in peritoneal healing. However, TGFβ (released in large quantities from platelets at sites of haemorrhage) and tumour necrosis factor (TNF) both probably increase plasminogen-activator inhibitor-1 (PAI-1) activity in peritoneal mesothelial cells, blocking fibrinolytic activity (and hence fibrin removal), and thereby promoting adhesion formation. This is an important field in which further research may well influence the clinical management of patients undergoing abdominal surgery.



Bone


Cellular mechanisms involved in the healing of bone fractures are similar to those in healing in other tissues (Fig. 4.11 illustrates the events involved). Haemor-rhage at the fracture site (inside and around the bone) produces a haematoma, in which there are fragments of necrotic bone, bone marrow, and soft tissues. As is the case in other sites, these necrotic tissues are removed by macrophages. Organisation of the haematoma in bone is accomplished by ingrowth of capillaries and fibroblasts (as in other sites in the body), but is modified in bone by ingrowth of osteoblasts; the resulting proliferation of these cells produces an irregular mass of new irregularly woven bone, called ‘callus’. Internal callus forms within the medullary cavity of the bone; external callus forms in relation to the periosteum, where it acts as a splint until it is finally removed by resorption and remodelling. Eventually, woven bone of the callus is remodelled into lamellar bone, with lamellae oriented according to the direction of mechanical stress on the bone.



Occasionally bone is lost at the time of fracture (for example, the fractured end of a bone may be removed by the surgeon if heavy contamination has occurred when a compound fracture has penetrated the skin). Under such circumstances the two ends of the bone may be pinned and externally fixed and oriented on an external frame. After initial contact, the bone ends may be gradually separated by increasing traction over several weeks, allowing the bone to be drawn to its correct length whilst the healing process occurs.


Bone healing may be delayed or inhibited as a result of movement, gross misalignment, soft tissues interposed between the ends of the bone, infection, bone disease (such as osteoporosis or Paget’s disease, or primary or secondary neoplasms), severe systemic illness or malnutrition. Excessive movement and soft tissue interposition may prevent bone fusion, and fibrous union of the bone may occur (perhaps producing a ‘false joint’).


Note that multiple fractures of different ages seen on x-ray may indicate an underlying bone disease such as severe osteoporosis or congenital osteogenesis imperfecta. In infants, children and weak dependant adults, however, such fractures may be the result of non-accidental injury (physical abuse).




Dec 12, 2016 | Posted by in GENERAL & FAMILY MEDICINE | Comments Off on Disorders of growth, differentiation and morphogenesis
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