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 coordinated 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 replication, growth, death and replacement 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 represent beneficial adaptations, 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.

Definitions

Growth

Growth is the process of increase in size resulting from the synthesis of specific tissue components. The term may be applied to populations, individuals, organs, cells, or even subcellular organelles such as mitochondria.

Types of growth in a tissue ( Fig. 4.1A ) are:

Multiplicative , involving an increase in numbers of cells (or nuclei and associated cytoplasm in syncytia) by mitotic cell divisions. This type of growth occurs in all tissues during embryogenesis.
Auxetic , resulting from increased size of individual cells, as seen in growing skeletal muscle.
Accretionary , an increase in intercellular tissue components, as in bone and cartilage.
Combined patterns of multiplicative, auxetic and accretionary growth as seen in embryological development, where there are differing directions and rates of growth at different sites of the developing embryo, in association with changing patterns of cellular differentiation.

Differentiation

Differentiation is the process whereby a cell develops a distinct specialised function and morphology (phenotype). There are many different cell types in the human body, but all somatic cells in an individual have identical genomes. Differentiation thus involves the coordinated and selective expression and repression of specific genes and gene products to produce a cell with a specialised function ( Fig. 4.1B ). The fertilised ovum has the ability to produce daughter cells that ultimately give rise to all of the cell types in the body, as well the extraembryonic tissues such as the placenta and membranes. As embryogenesis progresses, the differentiation potential of emerging cell populations is sequentially restricted so that although the various adult tissues ultimately formed may retain populations of cells capable of renewal, these tissue-specific stem cells are generally only capable of producing the particular cell types necessary to renew a specific tissue.

Morphogenesis

Morphogenesis is the highly complex process of development of the structural shape and form of organs, limbs, facial features and so on, from primitive cell masses during embryogenesis. For morphogenesis to occur, primitive cell masses must undergo coordinated growth and differentiation, with movement of some cell groups relative to others, and focal programmed cell death (apoptosis) to remove unwanted features. Morphogenesis remains the least well understood of the biological processes discussed here, but the consequences of disrupted morphogenesis may be striking.

Normal Growth, Differentiation and Morphogenesis

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 to changing requirements for growth. In both fetal and adult life, tissue growth depends upon the balance between the increase in cell numbers due to cell proliferation, and the decrease in cell numbers due to cell death. Nonproliferative cells are termed ‘quiescent’; such cells differentiate and adopt specific phenotypes capable of carrying out their specific function ( Fig. 4.2 ).

In fetal life, growth is rapid and all cell types proliferate, but even in the fetus there is constant cell death, some of which is an essential component of morphogenesis. In postnatal and adult life, however, the cells of many tissues lose their capacity for proliferation at the high rate of the fetus, and cellular replication rates are variably reduced. Some cells continue to divide rapidly and continuously, some divide only when stimulated by the need to replace cells lost by injury or disease, and others are unable to divide whatever the stimulus.

Regeneration and replication

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Process of replacing injured or dead cells
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Cell types vary in regenerative ability
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Labile cells : very high regenerative ability and rate of turnover (e.g. intestinal epithelium)
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Stable cells : good regenerative ability but low rate of turnover (e.g. hepatocytes)
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Permanent cells : no regenerative ability (e.g. neurones)

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-specific reservoir of ‘parent’ stem cells (discussed below). The presence of tissue stem cells with the ability to proliferate governs the regenerative potential of a specific cell type. Mammalian tissues fall into three classes according to their regenerative ability:

labile
stable
permanent.

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) that interfere with cell division. Examples of labile cells include:

haemopoietic cells of the bone marrow, and lymphoid cells
epithelial cells of the skin, mouth, pharynx, oesophagus, the gut, exocrine gland ducts, the cervix and vagina (squamous epithelium), endometrium, urinary tract (transitional epithelium), and so on.

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 skin which includes the dividing basal cells from an unburned donor site, and fixes it firmly to the burned graft site where the epithelium has been lost ( Ch. 5 ). Dividing basal cells in the graft and the donor site ensure regeneration of squamous epithelium at both sites, enabling rapid healing in a large burned area where regeneration of new epithelium from the edge of the burn would otherwise be prolonged.

Stable cells (sometimes called ‘conditional renewal cells’) divide very infrequently under normal conditions, but their 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.

Permanent cells normally divide only during fetal life. Their active stem cells do not persist long into postnatal life, and they cannot be replaced when lost. Cells in this category include neurones, retinal photoreceptors in the eye, cardiac muscle cells and skeletal muscle (although skeletal muscle cells do have a very limited capacity for regeneration). There is much research interest in developing artificial methods for regenerating tissues comprised of such cells, through the in vitro creation of stem cells which can both replicate and differentiate appropriately (see p. 63 ).

The cell cycle

Successive phases of progression of a cell through its cycle of replication are defined with reference to DNA synthesis and cellular division ( Fig. 4.3 ). 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 cell cycle — the S phase . Another distinct phase of the cycle is the cell-division stage or M phase comprising nuclear division (mitosis) and cytoplasmic division (cytokinesis). Following the M phase, the cell enters the first gap phase (G ₁ ) and, via the S phase, the second gap phase (G ₂ ) before entering the M phase again. Although initially regarded as periods of inactivity, it is now recognised that these ‘gap’ phases represent periods when critical processes occur, preparing the cells for DNA synthesis and mitosis.

After cell division (mitosis), individual daughter cells may reenter G ₁ to undergo further division if appropriate stimuli are present. Alternatively, they may leave the cycle and become quiescent or ‘resting’ cells — a state often labelled as G ₀ . Entry to G ₀ may be associated with 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 neurones. Other quiescent cells retain some ability to proliferate by reentering G ₁ if appropriate stimuli are present.

Molecular events in the cell cycle

Cell division is a highly complex process and cells possess elaborate molecular machinery to ensure its successful completion. A number of internal ‘checkpoints’ exist to ensure that one phase is complete before the next commences (see Fig. 4.3 ). This is vital to ensure, for example, that DNA replication has been performed accurately and that cells do not divide before DNA replication is complete. The various proteins and enzymes that carry out DNA replication, mitotic spindle formation, etc., are typically only present and active during the appropriate phases of the cycle. The timely production and activation of these proteins is regulated by the activity of a family of evolutionarily conserved proteins called cyclin-dependent kinases (CDKs), which activate their target proteins by phosphorylation. The activity of CDKs is, in turn, regulated by a second family of proteins, the cyclins . Transitions from one phase of the cycle to the next are initiated by rises in the levels of specific cyclins. The transition from G ₀ to G ₁ at the initiation of the cell cycle, for example, is triggered by external signals such as growth factors leading to rises in the levels of cyclin D . Problems during cell division, such as faulty DNA replication, result in rises in the levels of a third family of proteins, the CDK inhibitors (CDKIs) , which can prevent CDKs from triggering the next phase of cell division until the issue is resolved. In the face of major failures, cells will typically initiate apoptosis (see below) rather than permit the generation of improperly formed progeny. Damage to the genes that encode proteins involved in the regulation of cell-cycle progression is seen in many cancers ( Ch. 10 ).

Duration of the cell cycle

In mammals, different cell types divide at very different rates, with observed cell cycle times (also called generation times) ranging from as little as 8 hours, in the case of gut epithelial cells, to 100 days or more, exemplified by hepatocytes in the normal adult liver. However, the duration of the individual phases of the cycle is remarkably constant and independent of the rate of cell division. The principal difference between rapidly dividing cells and those that divide slowly is the time spent temporarily in G ₀ between divisions; some cells remain in the G ₀ phase for days or even years between divisions, whilst others rapidly reenter G ₁ after mitosis.

Therapeutic interruption of the cell cycle

Many of the drugs used in the treatment of cancer affect particular stages within the cell cycle ( Fig. 4.4 ). These drugs inhibit the rapid division of cancer cells, but since they are administered systemically there is often inhibition of other rapidly dividing cells, such as the cells of the bone marrow and lymphoid tissues. Thus anaemia, a bleeding tendency and suppression of immunity may be clinically important side effects of cancer chemotherapy.

Fig. 4.4, Pharmacological interruption of the cell cycle.

Apoptosis: physiological cell death in growth and morphogenesis

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Individual cell deletion in physiological growth control and in disease
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Activated or prevented by a variety of intracellular and extracellular stimuli
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Reduced apoptosis contributes to cell accumulation, for example, neoplasia
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Increased apoptosis results in excessive cell loss, for example, atrophy

Apoptosis is a physiological cellular process in which a defined and programmed sequence of intracellular events leads to the removal of a cell without the release of products harmful to surrounding cells. The coexistence of apoptosis alongside mitosis 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. It is an energy-dependent, biochemically specific mode of cell death characterised by the enzymatic digestion of nuclear and cytoplasmic contents, and the phagocytosis of the resultant breakdown products whilst still retained within the cell membrane. Apoptosis must be distinguished from necrosis ( Table 4.1 ) — the latter representing unintended cell death in response to cellular injury; indeed, the mechanisms of apoptosis act to suppress the inflammatory response triggered by necrosis. Disturbances in apoptosis play a role in a variety of diseases. Defective apoptosis is important in neoplasia ( Ch. 10 ), and autoimmune disease ( Ch. 8 ) may at least in part reflect a failure of induction of apoptosis in lymphoid cells directed against host antigens. Some viruses enhance their survival by inhibiting apoptosis of cells they infect. Diseases in which increased apoptosis is probably important include acquired immune deficiency syndrome (AIDS), neurodegenerative disorders and anaemia of chronic disorders ( Ch. 23 ). In AIDS, human immunodeficiency virus proteins may activate CD4 on uninfected T-helper lymphocytes, inducing apoptosis with resulting immunodepletion.

Table 4.1

Comparison of cell death by apoptosis and necrosis

Feature	Apoptosis	Necrosis
Induction	May be induced by physiological or pathological stimuli	Invariably due to pathological injury
Extent	Single cells	Cell groups
Biochemical events	Energy-dependent fragmentation of DNA by endogenous endonucleases	Energy failure Impairment or cessation of ion homeostasis
	Lysosomes intact	Lysosomes leak lytic enzymes
Cell membrane integrity	Maintained	Lost
Morphology	Cell shrinkage and fragmentation to form apoptotic bodies with dense chromatin	Cell swelling and lysis
Inflammatory response	None	Usual
Fate of dead cells	Ingested (phagocytosed) by neighbouring cells	Ingested (phagocytosed) by neutrophil polymorphs and macrophages
Outcome	Cell elimination	Defence, and preparation for repair

Regulation of apoptosis

Apoptosis is triggered by both extracellular and intracellular signals. External signals may include detachment from the extracellular matrix, the withdrawal of growth factors, or specific signals from other cells. Intracellular factors include DNA damage or failure to conduct cell division correctly. Factors controlling apoptosis thus include substances outside the cell and internal metabolic pathways.

Inhibitors include growth factors, extracellular cell matrix, sex steroids, some viral proteins.
Inducers include growth factor withdrawal, loss of matrix attachment, glucocorticoids, some viruses, free radicals, ionising radiation, DNA damage, ligand-binding at ‘death receptors’.

Apoptosis is initiated via two broad pathways, the extrinsic and intrinsic pathways, which converge upon a final common effector pathway characterised by the activation of proteases and DNAses ( Fig. 4.5 ).

The intrinsic pathway

The intrinsic pathway acts to integrate multiple external and internal stimuli, leading to alterations in the relative levels of pro- and antiapoptotic members of the B-cell lymphoma 2 (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 that induce apoptosis. In contrast, Bax — another member of the same family — forms Bax–Bax dimers which enhance apoptotic stimuli. Thus 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. The intrinsic pathway responds to stimuli such as growth factors (or their withdrawal) and biochemical stress. DNA damage (e.g. due to radiation or cytotoxic chemotherapy) represents a particular form of cell stress, which leads to stabilisation of the protein product of the p53 gene. p53 is a multifunctional protein which induces cell cycle arrest and initiates DNA damage repair. However, if this is unsuccessful, p53 can induce apoptosis via activation of proapoptotic members of the Bcl-2 family.

The extrinsic pathway

The extrinsic pathway is a specific mechanism for the activation of apoptosis characterised by ligand-binding at so-called death receptors on the cell surface. Receptors include members of the tumour necrosis factor receptor (TNFR) gene family, for example, TNFR1 and Fas (CD95). Ligand binding at these receptors promotes clustering of receptor molecules on the cell surface, and the initiation of a signal transduction cascade resulting in the activation of caspases. This pathway is the mechanism by which the immune system eliminates lymphocytes that would otherwise produce self-antigens.

The execution phase

Activation of apoptosis by either the intrinsic or extrinsic pathways results in a cascade of activation of caspases . Caspases are proteases, normally present as inactive procaspase molecules. Triggering of apoptosis first leads to the activation of initiator caspases such as caspase 8, which in turn cleaves other procaspases to produce active executioner caspases which cause degradation of many targets including the cytoskeletal framework and nuclear proteins. Caspase-3 activates DNAse which fragments DNA. The nucleus shrinks (pyknosis) and fragments (karyorrhexis). The cell shrinks, retaining an intact plasma membrane ( Fig. 4.6 ), but alteration of this membrane rapidly induces phagocytosis. Dead cells not phagocytosed fragment into smaller membrane-bound apoptotic bodies . There is no inflammatory reaction to apoptotic cells, probably because the cell membrane is intact. 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.

Apoptosis in development

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 the genetically programmed death of specific cells, is part of the metamorphosis of a frog. It is now clear that physiological cell death has an important role in human development and in the regulation of tissue size throughout life.

The removal of cells by apoptosis is responsible for alterations in tissue form and shape, including:

interdigital cell death responsible for separating the fingers ( Fig. 4.7 )
cell death leading to the removal of redundant epithelium following fusion of the palatine processes during development of the roof of the mouth
cell death in the dorsal part of the neural tube during closure, required to achieve continuity of the epithelium, the two sides of the neural tube and the associated mesoderm
cell death in the involuting urachus, required to remove redundant tissue between the bladder and umbilicus.

Failure of apoptosis in these four sites is a factor in the development of syndactyly (webbed fingers), cleft palate , spina bifida , and bladder diverticulum (pouch) or fistula (open connection) from the bladder to the umbilical skin, respectively.

Apoptosis is also seen, in the hormonally controlled differentiation of the accessory reproductive structures from the Müllerian and Wolffian ducts. In the male, for instance, anti-Müllerian hormone produced by the Sertoli cells of the fetal testis causes regression of the Müllerian ducts (which in females form the fallopian tubes, uterus and upper vagina) by the process of apoptosis. Finally, apoptosis is also involved in the removal of vestigial remnants from lower evolutionary levels, such as the pronephros.

Differentiation and morphogenesis

Differentiation is the process whereby a cell develops an overt specialised function that was not present in the parent cell. Embryonic development requires the establishment of correctly located populations of cells with different phenotypes. Effective morphogenesis thus requires mechanisms to signal the direction of differentiation to cells within different parts of the embryo, as well as intracellular mechanisms that yield the selective, coordinated gene expression that distinguishes one cell type from others, and from primitive, undifferentiated cells. In adult life, these distinct phenotypes must be maintained in the face of changing cellular environments, even in labile cell populations and tissues with ongoing cell turnover.

Control of normal differentiation

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Embryonic differentiation of cells is controlled by genes, systemic hormones, position within the fetus, local growth factors and matrix proteins
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Maintenance of the differentiated state is dependent upon persistence of some of these factors as well as epigenetic changes passed from cell to progeny

Individual cell types are distinct only because, in addition to the many universal proteins required by all cell types for ‘housekeeping’ functions such as cellular metabolism, each cell produces a characteristic set of specialised proteins which define that particular cell type. There are very few exceptions to the rule that differentiated cells contain an identical genome to that of the fertilised ovum (one exception, for example, would be B and T lymphocytes which have antigen receptor genes rearranged to endow them with a large repertoire of possible receptors ( Ch. 8 ). The fact that differentiated cells contain the same genome as the fertilised ovum has been demonstrated elegantly by injecting the nucleus of a differentiated tadpole gut epithelial cell into an unfertilised frog ovum, the nucleus of which was previously destroyed using ultraviolet light; the result was a normal frog with the normal variety of differentiated cell types ( Fig. 4.8 ). More recently a variety of mammalian species — most notably a sheep — have been cloned from somatic cells using an analogous approach.

Fig. 4.8, Potential of the genome of somatic cells.

The success rate of cloning using the approach presented above is in fact low — and lower in mammals than it is in amphibians or lower organisms. The ability of cells to recapitulate the generative potential of the zygote diminishes rapidly after fertilisation. At the 4-cell or 8-cell stage embryos can be artificially separated into separate cell groups, each capable of forming a complete organism ( artificial twinning ), but this ability diminishes rapidly with subsequent divisions as individual cells lose their generic developmental potential and begin to establish specific fates. By observing the effects of selective marking or obliteration of cells, a ‘fate map’ of the future development of cells in even primitive embryos can be constructed. Thus some of the cells of somites become specialised at a very early stage as precursors of muscle cells, and migrate to their positions in primitive limbs. These muscle-cell precursors resemble many other cells of the limb rudiment, and it is only after several days that they differentiate and manufacture specialised muscle proteins. Thus long before they differentiate, the developmental path of these cells is planned; such a cell which has made a developmental choice before differentiating is said to be determined . A determined cell must:

have differences that are heritable from one cell generation to another
be committed and commit its progeny to specialised development
change its internal character, not merely its environment.

Determination therefore differs from differentiation, in which there must be demonstrable tissue specialisation.

Cell position and inductive phenomena

The mechanisms responsible for anatomical development are complex but some core principles are established and are helpful in understanding disruptions of morphogenesis. It is tempting to imagine embryonic development as occurring through a series of preprogrammed steps, with each individual cell dividing, differentiating or undergoing apoptosis according to an intrinsic genetically determined programme without regard for neighbouring cells or their surroundings. A contrasting model might consider cells as purely reactive, simply responding to extracellular signals that guide development. The reality appears to be that both processes operate, with embryogenesis emerging as extracellular signals induce cells to select appropriate programmed pathways of determination and differentiation, which in turn produce extracellular signals that govern subsequent developmental steps.

As the fields of cells over which spatial chemical signals act are generally small, large-scale changes to the whole individual are the result of factors operating very early in embryonic development, whilst more specific minor features of differentiation within small areas of an organ or limb are specified later and depend on the position of the cell within the structure. Simple changes may occur in response to a diffusible substance (such as vitamin A in the developing limb bud), and serve to control local cell growth and/or differentiation according to the distance from the source. Additional differentiation changes may, however, occur as a result of more complex cellular interactions.

Many organs eventually contain multiple distinct populations of cells that originate separately but later interact. The pattern of differentiation in one cell type may be controlled by another, a phenomenon known as induction . Examples of induction include:

the action of mesoderm on ectoderm at different sites to form the various parts of the neural tube
the action of mesoderm on the skin at different sites to form epithelium of differing thickness and accessory gland content
the action of mesoderm on developing epithelial cells to form branching tubular glands
the action of the ureteric bud (from the mesonephric duct) to induce the metanephric blastema in kidney formation.

Inductive phenomena also occur in cell migrations, sometimes along pathways that are very long, controlled by generally uncertain mechanisms (although it is known, for example, that migrating cells from the neural crest migrate along pathways that are defined by the host connective tissue). Inductive phenomena control the differentiation of the migrating cell when it arrives at its destination — neural crest cells differentiate into a range of cell types, including sympathetic and parasympathetic ganglion cells.

Control of gene expression in the establishment of phenotype

As virtually all differentiated cells have an identical genome, differences between cell types cannot be due to amplification or deletion of genes. The cells of the body thus differ not in the range of genes present in each cell, but in how those genes are expressed, that is, transcribed and translated into proteins. Paradoxically, the complete sequencing of the human genome in recent years has highlighted the fact that although our biology is indeed determined by the sequence of our DNA, the controlled regulation of gene expression is an equally critical determinant of cellular form and function. The mechanisms that govern cellular differentiation are only now beginning to be understood and, although knowledge of this fundamental cellular process has advanced rapidly in recent years, much remains to be learned.

The synthesis of a gene product can, in theory, be controlled at several levels:

transcription : controlling the formation of mRNA
transport : controlling the export of mRNA from the nucleus to the ribosomes in the cytoplasm
translation : controlling the formation of gene product within the ribosomes.

In practice, regulation of transcription appears to be the main mechanism through which gene expression is controlled. There is now ample evidence that the regulation of transcription of entire groups of genes is mediated by the gene products of a small number of ‘control’ genes, the protein products of which are known as transcription factors . These genes themselves may be regulated by other transcription factors, acting as ‘master’ control genes ( Fig. 4.9 ). Much insight into possible control mechanisms behind determination, differentiation and morphogenesis has been gained from observations of the fruit fly, Drosophila . Disturbances of single ‘master’ genes in Drosophila have been shown to result in major malformations, such as the development of legs on the head in place of antennae, mediated by the response of many controlled genes to the alteration in ‘master’ gene product. Such a homeotic mutation (the transformation of one body part into another part that is usually found on a different body segment) highlights the importance of the position of a cell within an embryo at a given time and of genetically predefined programmes of development. In Drosophila , a group of genes, which individually cause a range of homeotic mutations, have been found to share a 60–amino acid sequence domain which is common to genes controlling normal larval segmentation. This sequence, named the homeobox , has also been demonstrated in vertebrates, including humans ( Ch. 3 ). Homeobox-containing genes (also known as homeobox genes) are transcription factors influencing morphogenesis. Parts of human anatomy appear to be constructed on a segmental basis, for example rows of somites, teeth and limb segments, and here it is probable that homeobox genes have an important morphogenetic role.

Epigenetic regulation of gene expression

Gene expression is not simply governed by the presence or absence of appropriate transcription factors. The term epigenetic regulation refers to alterations in the structure (not sequence) of DNA which modulate the expression of specific genes and are heritable from a cell to its progeny. These changes appear to act in concert with transcription factors in regulating gene expression. DNA methylation is the best understood epigenetic regulator of gene expression. Such methylation occurs in lengths of DNA rich in sequential adjacent cytosine and guanine bases — referred to as CpG islands — which typically occur in the promoter region upstream of the coding region of individual genes. Methylation inhibits transcription and gene expression. Methylation is stable and preserved during DNA replication, so patterns of methylation are passed from cells to their progeny, providing a heritable mechanism of gene expression regulation which appears to play a key role in cell determination and differentiation. Disturbances in the pattern of DNA methylation are thought to be important in the development of cancer. A second mechanism of epigenetic gene expression regulation may be conferred by histone proteins. Within the nucleus, DNA is usually tightly packed into chromatin. Histones are structural proteins involved in this packaging and in conferring high-order structure to chromatin. Posttranslational modification (e.g. methylation, acetylation) of these proteins appears to alter chromatin structure, potentially signalling to the transcriptional machinery whether or not a particular genomic region is active or silenced. As with DNA methylation, histone modifications can be passed from a cell to its progeny. There remains some controversy as to whether histone modifications are directly implicated in epigenetic regulation (as opposed to being proxy markers for transcriptional activity), but it is likely that they play at least some role in cell determination and differentiation.

Stem cells and transdifferentiation

As mentioned, stem cells are ‘parent’ cells that retain replicative potential, and whose progeny may differentiate into different types of ‘daughter’ cell. However, different stem cell types have varying potential for differentiation.

The fertilised human ovum (zygote) and cells from its first two divisions are totipotent —able to form all of the cells of the embryo and placenta.
Embryonic stem cells derived from the early blastocyst are pluripotent — producing almost all cells derived from the endoderm, mesoderm and ectoderm (but not cells from the placenta or its supporting tissues).
In normal circumstances, most individual tissues have either multipotent or unipotent stem cells, capable of generating only small numbers of cell types, or only one cell type, respectively.

The presence or absence of tissue stem cells within a particular tissue governs the ability of that tissue to regenerate after physiological or pathological cell loss or destruction. Thus haemopoietic stem cells in bone marrow replace the different blood cell types after haemorrhage (blood cells are ‘labile’ cells), while brain neurones (‘permanent’ cells) cannot be replaced, because there are no functioning neuronal stem cells in the adult brain.

When organs (such as the kidneys) or cells (such as brain neurones) fail because of ageing or disease, a patient may die or suffer increasing disability. In some cases, organ transplantation may be possible, although there are insufficient organ donors, and the transplanted organ may be rejected. In 1998 human embryonic stem (ES) cells were successfully extracted from blastocysts and aborted fetuses and grown in vitro. Such ES cells have been successfully artificially induced to differentiate into a variety of different individual cell types. Because of the ethical issues associated with the use of embryonic stem cells, more recent research has focused on the possibility of inducing stem cells from one organ system, such as haemopoietic stem cells (bone marrow cells differentiating into red and white blood cells and platelets), to develop into cells of other organ systems (e.g. kidney, liver or brain) by a process of ‘transdifferentiation’. Techniques have been developed to ‘reprogramme’ a variety of somatic cells to induce pluripotency alongside proliferative potential. This is achieved through the demethylation of genes associated with pluripotency and the activation of specific transcription factors. Through such ‘adult stem cell plasticity’, it is in principle possible that an adult patient's own bone marrow stem cells could be induced artificially to transdifferentiate to replace cells from organs that have been damaged by disease. This would also avoid the risk of immunological rejection of transplanted organs. For the time being the potential for artificial organogenesis remains largely unfulfilled, however, not least because of the difficulties in recapitulating the complex microanatomy of many organs, with multiple cell types and specialised stroma arranged in an intricate histological structure. Providing cells with a synthetically produced connective tissue scaffold to guide their growth is one potential way forward in this respect.

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