Samir Ahmed Shawky and Habiba El-fendy

CHAPTER

2

Introduction, 10 The cell membrane, 10 Biochemical structure, 10 Receptors proteins, 10 Attachment proteins, 10 Transfer of molecules, 11 Cell junctions, 12 The cytoplasm, 12 The cytoplasmic organelles, 12

Endoplasmic reticulum, 12 Golgi complex, 12 Lysosomes, 12 Peroxisomes, 12 Mitochondria, 13 Cytoskeleton, 13 Protein transport, 13 Protein degradation, 13

The nucleus, 14 DNA structure, 14 Protein synthesis, 14 Gene regulation, 15 DNA repair and cell aging, 18 Cell division, 18 Signal transduction, 20 Extracellular microenvironment, 21

Outline

INTRODUCTION

The cell theory was launched in 1838 by two German scientists; Schwann a zoologist and Schlei-den a botanist. Accordingly, cells were considered as the units of life of plants and animals. Rudolf Vir-chow in 1858 considered the cells as also the units of disease and stated that cells can only arise from preexisting cells. The detailed ultrastructure of the cell (Fig. 2-1) was revealed by the discovery of elec-tron microscope (Knoll and Ruska, 1931) and the exact function of cell organelles was disclosed through advances made in molecular biology during the end of the 20th century.

The main function of cells is to maintain homeo-stasis, which is defined as the ability to resist changes in the microenvironment and keep the normal bal-ance. Four systems are utilized by the cells to achieve this goal. The first is to sense and react to any exter-nal or internal signals (Signal transduction). The second system is to keep cell population constant (Population stability) through controlled cell division and cell loss by apoptosis. The third system is to recognize and repair any DNA damage (Genomic stability). Finally, cells contain the genetic information and produce gametes needed for sexual reproduction, hence, main-taining the species of organisms.

The present chapter is a concise review of basic structure and function of normal cell. Such knowledge is essential to understand the pathologic cell and mechanism of disease.

THE CELL MEMBRANE

Biochemical Structure The cell membranes are formed of lipids bilayer

with integrated proteins (Fig. 2-2). Carbohydrates form a minor component of cell membrane and are predominantly glycoproteins that help in intracellular recognition. Cell membrane proteins are classified into 3 main types according to their function, namely: receptor proteins, transport proteins and attachment proteins.

Receptors Proteins

These transfer signals to the cells from the exter-nal environment and are classified into two types, namely: integral and peripheral. Integral proteins are permanently embedded into the cell membrane and include two subtypes, transmembrane receptors (most of cell receptors) or attached to inner lipid layer (e.g. RAS receptors). Peripheral receptors are tempo-rary attached to one side of cell membrane and inter-acts with neiboring transmembrane receptors. Mem-brane associated enzymes and insulin belong to this subgroup.

Attachment Proteins Cell adhesion molecules (CAMs) belong to 4 main families which are grouped into two classes according to their dependence on calcium to per-form their function. Calcium dependant CAMs include cadherins and selectins whereas calcium independent

group includes the immunoglobulin superfamily and integrins. Removal of calcium from the first groups (cadherins and selectins) will lead to cellular dissocia-tion. These families of CAMs have distinct cellular distribution and function. 1.Cadherins. The major role is intercellular attach-ment with a link to the cytoplasmic cytoskeleton(Actin) through an intermediate adaptor (Fig 2-1) In case of epithelium, the adaptor molecule is catenin. This system is important in intracellular signalling.

2.Selectins. These participate in the movment of leukocytes from blood to the tissues (extravasation) in the early stage of inflammation (Chapter 5). 3.Immunoglobulin superfamily (IgSF) This large group

includes receptors of immue cells (namely: antigen receptors of leukocytes , antigen presenting cells, co-receptors and endothelial receptors CD31, (PECAM) , a member of this family is involved in leukocyte transmigration. In addition IgSF also in-cludes the sperm membrane protein receptors for fertilization of ovum.

4.Integrins. These attach epithelial cells to the ex-tracellular matrix (ECM) and play a critical role in cell migration (Fig 2-1) and (Fig-2-16). Transfer of Molecules

1.Diffusion across the lipid bilayer. A spontaneous

process that allows molecules that are small and lipo-philic (lipid-soluble), to easily enters and exit cells. A molecule’s concentration gradient drives move-ment across the membrane until the molecule is at equilibrium (Fig 2-2).

2. Protein-mediated transport. Water-soluble mole-cules and large molecules require the action of mem-brane transport proteins. There are two classes of membrane transport proteins: carrier proteins, which carry specific molecules across cell membrane, and channel proteins, which form a narrow pore through which ions can pass. Channel proteins carry out pas-sive transport, in which ions travel spontaneously down their gradients. Some carrier proteins mediate passive transport (also called facilitated diffusion),

Cell Structure and Function 11

Fig 2-1 The ultrastructure of a typical epithelial cell. The cytosol represents the internal environment of the cell

where the lumens of endoplasmic reticulum, Golgi apparatus and cytoplasmic vacuoles communicate with external

environment. Intercellular communication is mediated through gap channels and cytoskeletal signalling. (SER:

smooth endoplasmic reticulum, RER: Rough endoplasmic reticulum).

Fig 2-2 Biochemical structure of cell membrane and transport of molecules and ions. Passive transport is always down an electrochemical gradient, whereas active transport is against an electrochemical gradiant and

hence needs energy in the form of adenosin triphosphate ( ATP ). ( MDR; Multidrug resistance and other Toxins )

whereas, others are coupled to a source of energy to carry out active transport, in which a molecule is transported against its concentration gradient (Fig. 2-2).

3. Endocytosis/exocytosis. Large macromolecules (e.g., proteins, viruses, lipoprotein particles) require more complex mechanisms to traverse membranes, and are transported into and out of cells selectively via endocytosis and exocytosis (secretion) (Fig 2-1).

CELL JUNCTIONS Cell junctions consist of multiprotein complex-es that provide contact between neighboring cells or between a cell and the extracellular matrix. Cell junc-tions are especially abundant in epithelial tissues. There are three major types of cell junctions: 1-Anchoring junctions : They include adherens junc-tions , desmosomes and hemidesmosomes, which me-chanically attach cells cytoskeleton to their neighbors or to the extracellular matrix. 2- Communicating junctions (Gap junctions): mediate the passage of chemical or electrical signals from one interacting cell to its partner. 3- Occluding junctions (Tight junctions): seal cells together in an epithelium in a way that prevents small mole-cules from leaking.

THE CYTOPLASM

The main function of the cytoplasm is protein syn-thesis and transport . Structurally it is composed of 7 main organelles and the cytoskeleton (Fig 2-1). Cytoplasmic Organelles

1- Ribosomes: These are RNA-protein complex-

es, in the cytoplasm (free or ER attached), provide sites of protein synthesis.

2- Endoplasmic reticulum (ER): Formed of a net-

work of tubules (cisternae) in the cytoplasm and con-tinuous with the nuclear membrane. ER is the factory for synthesis and transportation of protein and lipids. ER is divided into rough ER (with ribosomes) and smooth ER (without ribosomes).

3- Golgi complex (GC): Formed of a network of flattened vesicles near the nucleus. GC is the site of processing and packing of protein synthesized in the ER ,as well as, formation of secretory granules.

4- Lysosomes: Sac-like structures originating from

GC and function as intracellular digestive system as it contains digestive enzymes (hydrolases).

5- Peroxisomes: It is similar to lysosomes but larger

in size and oval or irregular in shape. Contain en-zymes that use or produce H2O2. Participate in met-abolic oxidations.

12 El-Bolkainy Surgical Pathology

6. Proteasomes: These are protein complexes lo-

cated in the nucleus and the cytoplasm. The main function of the proteasome is to degrade unneeded or damaged proteins by proteolysis, a chemical reac-tion that breaks peptide bonds. Enzymes that help such reactions are called proteases. Proteasomes are part of a major mechanism by which cells regulate the concentration of particular proteins and de-grade misfolded proteins (Fig 2-3). The degradation process yields peptides of about seven to eight amino acids long, which can then be further degraded into shorter amino acid sequences (peptides) which are used to synthesiz new proteins. Proteins are tagged for degradation with a small protein called ubiquitin.

7. Mitochondria: These are double membrane

bounded organelles. The outer membrane is smooth. The inner membrane forms cisternae and contains enzymes essential for the oxidative phosphorylation process which produce most of the cellular ATP. The inner membrane also contains calcium ions trans-porter which helps to maintain the cellular calcium concentration. The mitochondria also help in build-ing certain parts of blood and hormones like testos-terone and estrogen. The liver cells mitochondria have enzymes that detoxify ammonia. The mitochon-dria also play important role in the process of apop-tosis through the release of mitochondrial pro-apoptotic proteins into the cytosol. This process is regulated by Bcl-2-like proteins and several ion chan-nels, in particular the permeability transition pore (PTP) that is activated by almost all pro-apoptotic stimuli. The mitochondria have a small amount of DNA of their own. Human mitochondrial DNA spans about 16,500 DNA base pairs, it represents a small fraction of the total DNA in cells. The mtDNA is maternally inherited and contains 37 genes. All the-se genes are essential for normal function of the mi-tochondria.

The Cytoskeleton This maintains the cell shape and internal organi-

zation, permits movement of substances within the cells and allow movement of the external projection (cilia or microvilli). The cytoskeleton is composed of network of protein filaments; namely microtubules and microfilaments (actin filaments).

1- Microtubules: Functions includes: A)

Strengthen the cell's structures. (B) Support and move organelle within the cell. (C) Transport impuls-es along nerve fibers. (D) Have roles in the inflamma-tory and immune response as well as hormone secre-tion. (E) Involved in the motility of some cells e.g. sperm. (F) Involved in cilia movement. (G) Form centrioles during mitosis.

2- Microfilaments (Actin filaments): Like micro-

tubules, it is important for cellular locomotion and maintenance of cellular shape.

Other intermediate filaments include: vimentin, des-

min, cytokeratin, neurofilaments and glial fibrillary acid protein. These structures are useful for diagnos-tic immunophenotyping of cells. Protein Transportation

Proteins produced in the ribosomes may be di-rected as exports (secretions) or as imports (to be used intracellularly). Proteins have signal sequence in their molecule to help direct it to its proper destina-tion. Proteins in the cytosol are transported inside microvesicles which proceed to Golgi apparatus where they are segregated and sorted. Secretory pro-teins are expelled through the cell membrane through a process of exocytosis (Fig 2-1).

Protien Degradation Tens of thousands of proteins are present in each cell, where they perform essential tasks for the organism. Once they have fulfilled their function, they must be degraded to avoid causing damage.

Cell Structure and Function 13

Fig 2-3 Ubiquitin-proteasome pathway (UPP) of protein degradation. (A) Misfolded or damaged protein is carried by chaperone to ubiquitin. (B) Ubiquitation of protein is accomplished through the action of enzymes ligases and adenosine triphosphate (ATP). (C) In the proteasome, ubiquitylated protein is enzymatically degraded to ubiquitin and peptides which are recycled. The alternative protein degradation pathway in the autophagy– lysosomal pathway.

The life-span of proteins varies according to its func-tion from few minutes to several months and ulti-mately degraded by one of two pathways, namely: the ubiquitin-proteasome pathway (UPP) (Fig 2-3) and the autophagy-lysosome pathway (ALP).

THE NUCLEUS

The nucleus is the largest membrane-bounded organelle. It contains most of the cellular DNA, his-tones and nucleolus. The main functions of the nu-cleus are cell division (DNA replication) and control of genetic information needed for protein synthesis (transcription) and sexual reproduction. The main function of the nucleolus is to produce ribosomal RNA (rRNA) needed for protein synthesis.

DNA STRUCTURE

DNA is a double-helix and has two strands run-ning in opposite directions (Fig 2-4, 2-5). Each chain is a polymer of subunits called nucleotides (hence the name polynucleotide). The functions of DNA are vital for inheritance, coding for proteins and the ge-netic blueprint of life. The genome of a cell contains in its DNA sequence, the information to make many thousands of different proteins and RNA molecules. A cell typically expresses only a fraction of its genes, and the different types of cells arise because different sets of genes are expressed. Moreover, cells can change the pattern of genes they express in response to changes in their environment, and signals from other cells.

PROTEIN SYNTHESIS The Transcription-Translation Process Gene expression includdes two successive pro-cesses, namely: transcription and translation.

1-Transcription. In this initial step a segment of DNA (gene) is copied into a complementary ribonu-cleic acid (RNA) strand by the enzyme RNA poly-merase, resulting into a primary RNA transcript (Fig 2-6). This primary RNA undergoes further processing or splicing by removing noncoding regions of RNA (introns) and joining the ends of coding regions (exons). This is done in the nucleus through the ac-tion of small nuclear ribonucleoproteins (SN-RNPs). The finally modified RNA will exit to the cytoplasm through nuclear pores to ribosomes.

2-Translation. This occurs on ribosomes, where m-RNA serves as a template for polypeptide chain syn-thesis with participation of other RNAs (transfer RNAs and ribosomal RNAs). The formed proteins undergo post-translation modifications (PTM) to ma-ture functioning proteins. PTM includes several bio-logical changes, including: protein folding, phosphor-ylation, cleavage or the addition of other functional groups.

The estimated one million different proteins in the human body do not match with the 25,000 genes in human genome. This discrepancy is explained by two facts. First, some genes can code for several proteins through a process of gene rearrangement, examples include immunoglobulin gene of plasma cells which produce thousands of different antibodies, as well as, surface receptors of T-lymphocytes which contribute to the production of numerous cytokines. Second, many proteins undergo post-translation modification with change of their function, and hence increase in their numbers.

14

Fig 2-4 The double helical structure of DNA. A backbone of phosphate and sugar (deoxyribose) held together by a pair of bases. According to the complementarily rule of base pairing, adenine (A) always binds to thymine (T), and guanine (G) binds to cytosine (C). A gene is the DNA sequence which codes for a protein. Of the total 25.000 genes in the human genome, only 1.5 % codes for protein and 98.5 % are non-coding and play a regulatory function.

Fig 2-5 Organization package of DNA. (A) The nucleo-somes are the structural units of chromatin formed of 140 base pairs wrapped around a histone protein with spacer connecting strands of 60 base pairs (B) chromosomes are the structural units of DNA only seen at metaphase during cell division. Chromosomal arms are further divided (Locus).

El-Bolkainy Surgical Pathology

GENE REGULATION

Gene expression is controlled by two main mechanisms, namely: genetic and epigenetic.

The Genetic Gene Control In the genetic model, the gene product (m-RNA)

is increased or decreased at gene level through acti-vating or repressor regulator genes. These act on RNA polymerase enzyme through transcription fac-tors (Fig 2-7), whereas in the Epigenetic Model, gene control is achieved by either through chromatin modification (concealing promoters) or through m-RNA interference. Epigenetics is currently gaining great importance in oncology research and clinical application, hence, this will be discussed in more details.

Epigenetic Gene Control Epigenetics describes the modification of gene

expression which does not involve change in

DNA sequence. Thus, in contrast to mutation, epigenetic changes are reversible. Three types of epigenetic mechanisms are cur-

rently recognized, namely: DNA methylation, his-tone modification and m-RNA interference, with chromatin remodeling and noncoding RNA-mediated transcription repression. The first two mechanisms occur at gene transcription level, whereas, RNA interference occurs at translation level. These mechanisms do not operate separately, but are rather functionally interdependent.

1-DNA Methylation. In humans, DNA methyla-tion occurs only in cytosines that precede guanines (called dinucleotide) in the genome, instead, there are CPG –rich regions known as CPG islands (Fig. 2-8). These islands are usually not methylated in normal cells. Hypermethylation of cytosine will in-hibit gene transcription by one of two mechanisms, namely: (a) blocking the binding of transcription factors to DNA, or (b) causing histone modification with resulting arrest in gene expression. 2-Histone modification (Chromatin Remodeling). The promoter is the region of DNA where the en-zyme DNA polymerase with its associated tran-scription factor, will bind and initiate transcription of m-RNA. In the silent deacetylated state of chro-matin, the nucleosomes are closely-packed or con-desnsed, hiding DNA promoter from DNA poly-merase and transcription factors, hence, the gene is inactive or silent (Fig 2-9). These condensed areas of chromatin appear overstained and are known as heterochromatin. Acetylation refers to the addition of acetyl group (CH3CO)to lysine residues in his-tone molecules and is mediated by the enzyme his-tone acetyl transferase (HAT). This modification reduces the binding affinity of histone to DNA leading to nucleosomal dissociation (open chroma-tin) with exposure of promotor regions of DNA to transcription factors (Fig 2-9). These acetylated are-as of loose chromatin are understained and are re-ferred to as euchromatin.

Cell Structure and Function 15

Fig 2-6 The expression of DNA to form proteins. The flow of information is from DNA to RNA to protein.

Transcription and splicing occur in the nucleus, whereas, the translation process occurs on ribosomes in cytoplasm.

Different RNA types mediate these processes. Proteins also develop posttransitional molecular modification.

Abbreviations: (m-RNA: messenger RNA, s-RNA: Splicing RNA, t-RNA; transfer RNA, r-RNA:

ribosomal RNA).

(m-RNA)

(s-RNA)

t-RNA and r-RNA

Fig 2-7 Control of gene expression by regulator genes mediated by transcription factors (TF) needed by RNA polymerase (Pol) to start transcription. The transcription factors may induce a positive or negative regulatory effect.

It is noteworthy that histone is also modified by methylation. This affects its amino acids namely ly-sine or argenin. The resulting chromatin modification depends on the amino acid affected with subsequent gene activation or repression. 3-Massenger RNA interference (silencing). This mechanism is operable through the action of mi-croRNA (mi-RNA). There are functional RNA mol-ecules of small size, that are transcribed by noncod-ing DNA

and serve important regulatory function. Micro-RNA binds and inhibits messenger RNA (mRNA) at translation level, leading to arrest of protein synthe-sis (Fig 2-110). Epigenetic mechanisms play an important role in carcinogenesis by acting as complementary to gene mutation (Chapter 13 and Chapter 17). In addi-tion, recent reports suggest valuable clinical appli-cations in the diagnosis, treatment and prognosis of cancer (Table 2-1).

Fig. 2-9 Histone modifications (Chromatin remodelling).

This epigenetic phenomena is controlled by acetylation and methylation of histones and DNA . A. In the resting state. the chromatin is packed together (closed chromatin) and genes are inactive. B. In the active state, chromatin is dispersed (opened chromatin) allowing gene expression.

Fig 2-10 Messenger RNA (mRNA) interference (Silencing). Micro-RNA (mi-RNA) binds and inactivates m-RNA, hence arrest protein synthesis. Over-expression of mi-RNA inhibits tumor suppressor genes, whereas, underex-pression will increase oncoprotein product.

Fig. 2-8 Methylation of cytosine. It occurs at carbon atom

number 5 of the ring (Arrow). Hypermethylation of cytosine in DNA results in inhibition of gene expression.

16 El-Bolkainy Surgical Pathology

Epigenetic Biomarker

Mechanism Site of Cancer

References

Prognostic miR-4656 low-expression Pancreas (Tan et al, 2016)

miR-486-5p low-expression NSCLC (Shao et al, 2016) miR-155 low-expression Stomach (Sun et al, 2014)

MiR-203 Over-expression Prostate (Huang et al, 2016) HOXD13 DNA hypermethylation Breast cancer (Zhong et al, 2015)

PCDH17 DNA hypermethylation ccRCC (lin et al, 2015) DAB2IP DNA hypermethylation ccRCC (Wang et al, 2016)

H3K4me2 and H3K18Ac

Histone modification RCC Goldberg et al, 2007

CDH1, CDH13, APC

DNA hypermethylation Bladder Can-cer

Yates et al, 2007

RSPH9 DNA hypermethylation Bladder Can-cer

Yoon et al, 2016

H3K23ac Histone modification Breast cancer Ma et al, 2016

Diagnostic

MiRNA Altered expression NSCLC & HCC

Hou et al, 2016 & Shen et al, 2016

miRNA-146a-5p Over-expression Bladder cancer Sasaki et al, 2016

miR‑598‑3p & miR‑184

Under-expression Breast cancer Fu et al, 2016

TWIST1 & NID2 DNA hypermethylation Bladder cancer Yegin et al, 2013 VHL, RASSF1A DNA hypermethylation RCC Martino et al 2012

PDE11A, SPRY4, PDE11A

DNA hypermethylation Testicular germ cell tu-

mors (TGCT)

Mirabello et al, 2012

KITLG DNA hypormethylation

TET2 Hypomethylation (identify cur-able tumors)

AML Yamazaki et al, 2012

AGTR1, FOXI2, PENK

Promotor methylation predicts invasion from

premalignant

Oral SCC Foy et al, 2015

Predictive

KLF4 deacetylated (oncogene)& acetylation (TSG)

Bladder Can-

cer

Jia et al, 2016

miR-221 Oncogenic miRNA (effective target)

HCC Liu et al, 2016

miR-124 Oncosuppressor miRNA à anti-cancer therapy.

HCC Xu et al, 2016

CRABP1 Methylation state predict Ret-inoid acid response.

Cervical can-cer

Arellano-Ortiz et al, 2016

HOXA9 Methylation state à predict re-sponse to cisplatin-based

chemotherapy

Bladder Can-cer

Xylinase et al, 2016

Table 2-1 Applications of Epigenetics in Cancer Management

Cell Structure and Function 17

DNA REPAIR AND CELL AGING

Cellular repair of a macromolecule is known to oc-cur only for DNA, a vital molecule for cell survival. DNA damage repair occurs during cell-cycle check-points with tumor suppressor gene TP53 (the guardian of the genome) playing a key role. TP53 detects DNA damage through the mediation of ataxia telangiectasia protein (pAT), then TP53 causes arrest of all cycle to give time for DNA repair enzymes. If the DNA le-sions are severe and irreparable, the mutant cells are eliminated by apoptosis.

Normal adult cells have only a limited capacity of cell division. The ends of chromosomes (telomeres) contain many copies of guanine (G)- rich repeats repeats (TTAGGG)N. The enzyme DNA polymer-ase is unable to replicate telomeres, which shorten by about 100 base pairs with each division. This is referred to as the end replication problem (Fig 2-11).

When DNA shortening reaches a critical level after 50 divisions, it is sensed by TP53 as a DNA break, resulting in expression of p21 and Bax with perma-nent arrest of cell cycle (cell senescence) and subsequent apoptosis. In this way, telomeres may be considered as death timers. The enzyme telomerase can correct telomere shortening . Germ cells, stem cells and ma-lignant cells are rich in telomerase, hence they are immortal. Conversely, normal somatic cells lacking this enzyme are short-lived.

CELL DIVISION The cell cycle (Fig 2-12).describes the four main se-quential events of cell division, namely: G1 phase (gap 1 presynthetic growth), S phase (DNA synthe-sis), G2 phase (premitotic growth) and M (mitotic phase). G0 cells are quiescent noncycling cells .The cell cycle contains two cheek points (R1 and R2) to monitor the fidelity of DNA duplication and accuracy of chromosomal segregation. The phases of cycle are

Fig 2-11 Telomere shortening due to failure of replication at chromosomal ends. It is corrected by the telomerase

enzyme equipped with its own RNA molecule.

Fig 2-12 Phases of normal cell cycle and their regulation by cyclins and cyclin-dependent kinases. Also shown are cell cycle check points (R1, R2 and spindle integrity). Cells generated from cell cycle are either terminally differentiated cells or reversible quiescent cells in G0 phase. In the S phase, DNA strands are doubled through the replication in which original strands act as a template for the two new similar ones. The duration of the cell cycle stages in houers, as well as sites of inhibition by INK4 and KIP are indicated.

18 El-Bolkainy Surgical Pathology

driven by 5 classes of proteins (cyclins A to E) to-gether with their corresponding enzymes (cyclin-department kinases, Cdks).Progression through the cycle is under control of various positive and negative growth signals (Fig 2-13). A mitogenic signal leads to the phosphorylation of retinoblastoma protein (pRb) with loss of its growth suppressive function and re-lease of bound (E2F). The latter is a transcription factors for genes whose products are required in the S phase, hence, the cell enters the S-phase of cycle. There are two types of cell division namely: mitosis and meiosis which are different both in function and mechanism (Fig 2-14). Mitosis occurs in somatic cells to assure cell re-newal and growth of the organism. Conversely, meio-sis occurs in germ cells (gonads) and functions to produce gametes needed for sexual reproduction. Mitosis involves only a single cell division producing two daughter diploid cells, with chromosomal num-ber and genetic structure exactly similar to the moth-er cell. Whereas, in meiosis two cell divisions occur producing four gametes with only half the number of chromosomes (haploid). Moreover, crossing over of genetic material only occurs in meiosis before the first cell division resulting in genetic and phenotypic variability.

Fig 2-14 Comparison of mitosis and meiosis. Mitosis occurs in somatic cells and produc-

es only two diploid cells, but meiosis occurs in germ cells of gonads, produces four haploid

gametes. Crossing over of chromosomes occurs only in meiosis resulting in some genetic het-

erogenicity in offsprings.

Fig 2-13 Regulation of cell cycle through control of G1-S transition. Cyclin/cdk-Retinoblastoma pathway is a key

regulatory mechanism, but , p53, INK4 and KIP proteins are the main inhibitors of cell cycle.

Cell Structure and Function 19

SIGNAL TRANSDUCTION It is defined as the molecular mechanisms of

transmitting signals from the external microenviron-ment to the interior of the cell, that eventually elicits a response (Fig 2-15 and Table 2-2). Classically, the three steps include: (1) a signal, also called a ligand, which may be a hormone or a growth factor (Table 2-2), binds and activates a protein receptor in cells membrane or cytoplasm, (2) this leads to the activa-tion of secondary messengers (kinase enzymes) and the production of transcription factors (3) these fac-tors enter the nucleus and bind at DNA promotor to activate gene transcription. The cellular responses to signal transduction are one of the following three reactions, namely: (1) cellular proliferation, (2) func-tional differentiation and (3) cell death by apoptosis.

Three main signal transduction pathways are rec-

ognized: (1) The MAPK/ERK pathway activated by RAS and (2) The P1-3K/AKT pathways which in-crease cellular proliferation, with cross reactivity and amplification between them. Conversely, (3) SMADS pathway is antiproliferative.

Other special signal transduction pathways are also operable, including: (1) The Notch receptor and Wnt/β-catenin pathway in the embryo, (2) the spe-cial receptors of immune cells (TCR, CD3 and CD28 in helper T-cell lymphocyte, as well as, CD21 and IgM receptors in B-lymphocytes), and (3) the cytoskeleton–mediated signal transduction) integrin/actin pathway of intercellular communication).

Fig 2-15 Signal transduction pathways. The binding of growth factors (Ligands) to cell membrane surface receptors

initiates a signal which is transmitted through cytoplasmic messengers to the nucleus where it initiates DNA transcription to produce proteins needed for cell differentiation, proliferation or apoptosis (for details refer to Tables 2-2). The main signal transduction pathways include: MAPK/ERK pathway (activated by RAS) as well as PI3K/AKT pathway which increases cellular proliferation with cross reactivity and amplification between them. Conversely, SMADS pathway is antiproliferative.

20 El-Bolkainy Surgical Pathology

THE EXTRACELLULAR MICROENVIRONMENT

The extracellular microenvironment includes both cellular and extracellular matrix (ECM) , with dynamic interaction between them (Fig 2-16). It plays important role in inflammation, immune reactions, as well as, cancer development and progression. The Cellular Components

1.Macrophage. This immune cell is activated by interferon-gamma of T-helper lymphocytes. It is phagocytic and secrete metalloproteinase enzymes causing matrix degradation. It also produce various growth factors of diverse biologic effects including: EGF, PDGF, FGF and TGF-β.

2.Mast cells. They are activated by IgE, and re-lease several vasoactive products (histamine, prosta-glandins and leukotrienes) responsible for type I immediate allergic reactions.

3.Lymphocytes. Include: Cytotoxic T-cell lympho-cyte (CD3) responsible for cellular immunity, B-cell lymphocyte (CD20) responsible for humoral im-munity and T-helper lymphocyte (CD4), the master regulatory cell of the immune system.

4.Endothelial cells and pericytes. Responsible for angiogenesis which is controlled by a balance be-tween the angiogenic agents (VEGF and FGF) and

antiangiogenic agents namely: angiostatin and en-dostatin (degeneration products of plasminogen and collagen respectively) and antiangiogenic factor Ang1 (produced by pericytes).

5.Fibroblast and myofibroblast. The latter is a more active cell rich in cytoplasmic actin, commonly seen in inflammation and malignant tumors (cancer associ-ated fibroblasts CAF). Fibroblasts make all compo-nents of the extracellular matrix including collagen seen in healed inflammation and desmoplasia associ-ated with carcinoma. Fibroblasts also produce growth factors which target epithelium (HGF), endothelium (FGF) stem cells (SCF) and multiple targets (CXCL12) which is angiogenic and chemotactic to lymphocytes and enhance cancer cell growth.

6.Stem cells. These include both mesenchymal and epithelial stem cells, the later are basal in location in surface epithelium. Normally, stem cells represent an essential source of cell renewal, but are also involved in inflammatory and neoplastic disease. The extracel-lular matrix microenvironment acts as a nest (niche) for stem cells, keeping them in a quiescent Go phase of cell cycle, to be activated when needed by growth factors or PH change (acidity) of microenvironment. The capabilities and fate of stem cells are further dis-cussed in (Chapter 12).

Cell Structure and Function 21

Target cells Mito-

genic

Anti-mitogenic

Angio-genic

Fibro-genic

Epithelium Epidermal growth factor (EGF) Transforming growth factor alpha (TGF-α) Keratinocyte growth factor (KGF) Hepatocyte growth factor (HGF)

X X X X

X

Stroma Vascular endothelial growth factor (VEGF) Fibroblast growth factor (FGF) Stem cell factor (SCF) Platelet derived growth factor (PDGF)

X X X

X X X

X X

Both epithelium and stroma Insulin growth factor (IGF) Transforming growth factor beta (TGF-β)

X

X

X

X

Table 2-2 Growth Factors which Affect Epithelial and Stromal Cells

The Extracellular Matrix (ECM)

The stroma or extracellular matrix (ECM) consist-ents of three main components:

1.Fibrous proteins, including, collagens (types I to IX) and elastins,

2. adhesion glycocoproteins: (lamina, fibrnectin, throm-bospondin and tenasin). 3. Proteoglycans (glycosaminoglycans and hyaluronic

acid). Epithelial and endothelial cells are separated from the extracellular matrix (ECM) by the base-ment membrane, which is composed of a network of typr IV collagen rich in laminin adhesive mole-cules. (Fig 2-16).

Cell adhesive molecules or receptors are also pre-sent on surface of epithelial cells and help to adhere the cells to each other (E-cadherins), to vascular endothelium (integrin VLA-4 and glycoprotein CD44), and to extracellular materic (laminin and fibronectin).

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Fig 2-16 The extracellular microenvironment includes cellular components and extracel-lular matrix (ECM). A dynamic interaction, mediated by growth factors occurs between the cellular elements, stromal cells and epithelium. Moreover cytoskeleton mediated cell sig-nalling also occurs through integrins and E-cadherins adhesion molecules. The cell microen-vironment plays important roles in inflammation , immune reactions and cancer develop-ment and progression.

El-Bolkainy Surgical Pathology