Molecular basisi of growth


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Molecular basisi of growth

  1. 1. MOLECULAR BASIS OF GROWTH INDIAN DENTAL ACADEMY Leader in continuing dental education
  3. 3.
  4. 4. Craniofacial development  Facial development in the embryo involves the origin of the facial mesenchyme which arises from neural crest cells.  Unusually, they disrupt the ectodermal- mesodermal junction and migrate into the underlying tissue as ectomesenchymal cells.
  5. 5.  Among the derivatives of the cephalic neural crest cells are the maxilla, mandible, zygomatic, nasal bones, and bones of the cranial vault.  This process is presumed to be under the control of genes known as homeobox genes (1984), which endow neural crest cells (NCC) with a positional identity, which mediates aspects of craniofacial morphogenesis and patterning.
  6. 6. Role of Homeobox genes  genes which are highly conserved throughout evolution of diverse organisms  fundamental in evolution of the specialised body parts of many animal species
  7. 7.  master genes of the head and face controlling patterning, induction, programmed cell death, and epithelial mesenchymal interaction during development of the craniofacial complex.  Those of particular interest in craniofacial development include the Hox group, Msx1 and Msx2 (muscle segment), Dlx (distalless), Otx (orthodontical), Gsc (goosecoid), and Shh (sonic hedgehog).
  8. 8.  Proteins encoded by these homeobox genes are transcription factors, which can switch genes on and off by activating or repressing gene expression, and therefore control other genes producing a co-ordinated cascade of molecular events which, in turn, control patterning and morphogenesis . (Thesleff, 1995).
  9. 9. At a cellular level this control is expressed through two main groups of regulatory proteins, the growth factor family and the steroid/ thyroid/retinoic acid super family (Evans, 1988), that regulate growth (Johnston and Bronsky, 1995).  These regulatory molecules in the mesenchyme are fibroblast growth factor (FGF), epidermal growth factor (EGF), transforming growth factor alpha (TGF ), transforming growth factor beta (TGF ), and bone morphogenetic proteins (BMPs)
  10. 10. MSX genes  Homologous to drosophila msh (muscle segment homeobox)  Murine msx family – msx1, msx2, msx3  Msx1 & 2 – transcriptina; repressors in cellular differentiation.  Along with dlx genes (activator)- mutually modulate their own transcriptional activities
  11. 11. Mice studies  Msx1- targeted disruption – loss of palatal shelves & maxillary bones , slight shortening of maxilla &/or mandible, no tooth development beyond bud stage  Msx2-(less well defined role in lip & palate development) deficiency-skull ossification defects & persistent calvarial foramen; mutation- craniofacial malformation- mandibular hypoplasia, cleft palate medial facial
  12. 12. DLX genes  Homologous to drosophila distal-less (Dll gene)  At least 6 members in mice- dlx1,2, 3, 5,6 &7 – encode transcription factors involved in orofacial patterning from neural crest cells
  13. 13.  Dlx 1 & 2-development of maxi arch (development of palatine & pterygoid bones of palate)  Dlx 1, 2, 3, 5, 6 –development of mandible
  14. 14. LHX genes  (lim homeobox genes)  Tissue patterning/specification & differentiation of different cell types  Lhx 6 & 7- expressed prior to initiation of tooth formation in oral & odontogenic mesenchyme of maxi & mandi processes (mice)
  15. 15.  Lhx 8(L3)-differentially expressed in maxilla, mandible & ventral forebrain; continuous expression in mesenchyme during diff stages of palatogenesis- candidate gene for isolated nonsyndromic form of cleft palate
  16. 16. PRRX gene  Paired related homeobox  Homologous to drosophila paired & gooseberry genes & to mouse pax3, 6 & 7 genes.  2 best studied prrx genes- prrx1(mhox/phox1/prx1/K2) & 2(S8/prx2)-similar expressions in cranium, branchial arches, body wall & limbs
  17. 17.  Prrx 1- detected in most of the ectoderm including brain precursors- inactivation (mice)- microcephaly, low set ears, pointed snout, cleft palate & mild mandibular hypoplasia.  Prrx 2 – no expression in developing brain – inactivation is compensated by prrx 1 functionality
  18. 18. GOOSECOID genes  Involved in the final stages of formation of craniofacial structures like the ear, nose & mouth.  Disruption – lower mandible & associated musculature including tongue, nasal cavity, nasal pits, malleus & external auditory meatus, malformation of various bones of base of skull (e.g;palatine)
  19. 19. RYK gene  Related to tyrosine kinases (receptor proteins).  Rugulate diverse cellular functions-mitogenesis, differentiation & morphogenesis
  20. 20.  Disruption (mice study– Halford 2000) – some limb abnormalities with slightly smaller & more rounded cranial vault, shorter snout premaxillary/maxillary hypoplasia(flattened midface) & reduced mandible with clefting of palate.
  21. 21. GROWTH FACTORS - FGF  FUNCTIONS – angiogenesis, wound healing, embryonic development & malignant transformation, regulate cell proliferation, differentiation & migration in different tissues  At least 7 members are expressed in the developing face – Fgf 1,2,4,5,8,9 & 12
  22. 22.  Receptors – FGFRs – fgfrs 1,2 & 3- expressed in development of face & later associated with some regions of chondrogenesis  FGFR activating mutations – embryonic lethality & limb & craniofacial abnormalities such as CP & reduced maxillary bone (Apert, Crouzon syndromes)
  23. 23. PDGF & PDGFR  2 RECEPTORS – pdgfr-alpha & pdgfr-beta  Pdgfs & Pdgfrs –regulatory roles in development of CNS, vascular system, in maintenance of tissue homeostasis & in wound healing along with imp role in palate development  Homozygous knockout of Pdgfra – midline defects & underdevelopment of face with absence of some facial bones (Soriano 1997)
  24. 24. TGF  TGF ALPHA & BETA-contribute to facial development, especially palate formation.  Tgf A – some human studies indicate a positive association between tgfa & CL with/without CP(Ardinger 1989)  Rather than a necessary & sufficient determinant , it has been postulated that tgfa acts as a modifier gene.
  25. 25. TGF Beta superfamily  Tgf beta 1,2,3,4 & 5 and more distantly related bone morphogenetic proteins  Tgh beta 1,2, 3-expressed in early embryogenesis & are associated later with some regions of skeletal development
  26. 26.  Depletion of tgf beta 3(Brunet 1995)-prevents in vitro fusion of palatal shelves.  Critical role of tgf beta2& 3 in molecular control of orofacial clefting (Lidral, 1998, Sanford 1997)
  27. 27. EGF & EGFR  Necessary for normal craniofacial development.  Increases matrix metalloproteinases(MMPs) secretion (downstream signal transduction effector)
  28. 28.  Mice studies –(Miettinen 1999) egfr knockout-facial mediolateral defects like narrow & elongated snouts, underdeveloped lower jaws, CP & diminished secretion of MMPs in palatal shelf tissues
  29. 29. GABA  Major inhibitory neurotransmitter with many critical functions as an intercellular signaling molecule in the CNS  Receptors – gaba A,B & C. gaba A can be modulated by steroids, benzodiazepines.
  30. 30.  Gaba- capable of promoting survival, differentiation & migration of embryonic neurons  Mice studies(Miller 1975, Homanics 1997) Abberations in gaba/gaba A – induces clefting of palate
  31. 31. Growth factors  This collective term originally referred to substances that promote cell growth.
  32. 32.  The genes and mechanisms of our body are tirelessly operating day in, day out, and are part of a long interconnected chain of reactions that make our body work.  There are various factors involved that affect growth. It is the genetic coding of our bodies that determine the way we are and how we work, with the external environment either emphasizing or inhibiting the effectiveness of some of these genes.
  33. 33. Growth factors  Introduction  Growth factors are proteins that bind to receptors on the cell surface, with the primary result of activating cellular proliferation and/or differentiation. Many growth factors are quite versatile, stimulating cellular division in numerous different cell types; while others are specific to a particular cell-type.
  34. 34.  Growth factors comprises of molecules that function as growth stimulators but also as growth inhibitors (sometimes referred to as negative growth factors ), factors that stimulate cell migration or as chemotactic agents or inhibit cell migration or invasion of tumor cells, factors that modulate differentiated functions of cells, factors involved in apoptosis , factors involved in angiogenesis , or factors that promote survival of cells without influencing growth and differentiation.
  35. 35.  Growth factors are polypeptides that belongs to a number of families.  Cell surface receptors capture them.  Upon capturing receptor interacts with membrane and cytoplasmic bound components to bring about alteration in gene expression of a cell.  Thus a growth factor is an inductive agent.
  36. 36.  A growth factor produced by one cell and acting on another is described as paracrine regulation.  Whereas the process of a cell that recaptures its own product is known as autocrine regulation.  Few growth factors act during embryogenesis.
  37. 37.  By contrast, the retinoic acid family freely enters a cell to complex with intracellular receptors, which eventually affect gene expression.  Both growth factors and the retinoids regulate the expression of the homeobox genes, which, in turn, regulate the expression of growth factors(role of regulatory loops in development).
  38. 38.  Epidermal Growth Factor (EGF)  Platelet-Derived Growth Factor (PDGF)  Fibroblast Growth Factors (FGFs)  Transforming Growth Factors-b (TGFs-b)  Transforming Growth Factor-a (TGF-a)  Erythropoietin (Epo)  Insulin-Like Growth Factor-I (IGF-I)  Insulin-Like Growth Factor-II (IGF-II)  Interleukin-1 (IL-1)  Interleukin-2 (IL-2)  Interleukin-6 (IL-6)  Interleukin-8 (IL-8)  Tumor Necrosis Factor-a (TNF-a)  Tumor Necrosis Factor-b (TNF-b)  Interferon-g (INF-g)  Colony Stimulating Factors (CSFs)
  39. 39.  Osteoblasts synthesize and regulate the deposition and mineralization of the extracellular matrix of bone. Systemic and locally active hormones, growth factors, ions, lipid metabolites and steroids are regulators of osteoblastic activity and/or differentiation.
  40. 40.  Members of the transforming growth factor beta (TGF-b) family, particularly TGF-b and the bone morphogenetic proteins (BMPs) are important to bone homeostasis. These factors modulate osteoblast proliferation and differentiation .
  41. 41. Matrixmetalloproteinases  Extracellular matrix degrading metallo enzymes are known collectively as Matrixmetalloproteinases(MMPs).  Tissue inhibitors of metalloproteinases(TIMPs).  Depend on Zn²+ and Ca²+ for activity.
  42. 42. Matrixmetalloproteinases  Rather than being primarily involved in matrix degradation MMPs have equally or more important roles as efficient processing enzymes of many bioactive mediators such as cytokines, chemokines, growth factors, their receptors and specific matrix protein anchors for these molecules.
  43. 43. Cytokines  Includes a family of molecules which are small proteins with either paracrine or endocrine functions which are involved in local inflammation or immunoregulation.  Within this definition growth factors could be included.
  44. 44. Cytokines.  Cytokines are a unique family of growth factors.  Secreted primarily from leukocytes, cytokines stimulate both the humoral and cellular immune responses, as well as the activation of phagocytic cells.  Lymphokines  Monokines.
  45. 45. Cytokines  A large family of cytokines are produced by various cells of the body.  Many of the lymphokines are also known as interleukins (ILs), since they are not only secreted by leukocytes but also able to affect the cellular responses of leukocytes.
  46. 46. Cytokines  Specifically, interleukins are growth factors targeted to cells of hematopoietic origin.  The list of identified interleukins grows continuously with the total number of individual activities now at 22.
  47. 47.
  48. 48. Factor Principal Source Primary Activity Comments PDGF platelets, endothelial cells, placenta promotes proliferation of connective tissue, glial and smooth muscle cells two different protein chains form 3 distinct dimer forms; AA, AB and BB
  49. 49. Factor Principal Source Primary Activity EGF submaxillary gland, Brunners gland promotes proliferation of mesenchymal, glial and epithelial cells
  50. 50. Factor Principal Source Primary Activity Comments TGF-a common in transformed cells may be important for normal wound healing related to EGF
  51. 51. Factor Principal Source Primary Activity Comments FGF wide range of cells; protein is associated with the ECM promotes proliferation of many cells; inhibits some stem cells; induces mesoderm to form in early embryos at least 19 family members, 4 distinct receptors
  52. 52. Factor Principal Source Primary Activity Comments NGF promotes neurite (axites & dendrites) outgrowth and neural cell survival several related proteins first identified as proto- oncogenes; trkA (trackA), trkB, trkC
  53. 53. Factor Principal Source Primary Activity Comments Erythropoietin kidney promotes proliferation and differentiation of erythrocytes
  54. 54. Factor Principal Source Primary Activity Comments TGF-b activated TH1 cells (T-helper) and natural killer (NK) cells anti-inflammatory (suppresses cytokine production and class II MHC expression), promotes wound healing, inhibits macrophage and lymphocyte proliferation at least 100 different family members
  55. 55. Factor Principal Source Primary Activity Comments IGF-I primarily liver promotes proliferation of many cell types related to IGF- II and proinsulin, also called Somatomedin C
  56. 56. Factor Principal Source Primary Activity Comments IGF-II variety of cells promotes proliferation of many cell types primarily of fetal origin related to IGF-I and proinsulin
  57. 57.  Factors modulating growth, chemotactic behavior and/or functional activities of smooth muscle cells include Activin A , Adrenomedullin , aFGF, ANF , Angiogenin , Angiotensin-2 , Betacellulin , bFGF , CLAF , ECDGF (endothelial cell-derived growth factor ), ET (Endothelins ), Factor X , Factor Xa , HB-EGF , Heart derived inhibitor of vascular cell proliferation , IFN-gamma , IL1 , LDGF (Leiomyoma-derived growth factor ), MDGF (macrophage-derived growth factor , monocyte-derived growth factor ), Oncostatin M , PD-ECGF , PDGF , Prolactin , Protein S , SDGF (smooth muscle cell-derived growth factor ), SDMF (Smooth muscle cell-derived migration factor ), Tachykinins , TGF-beta , Thrombospondin .
  58. 58.  Factors modulating growth, chemotactic behavior and/or functional activities of vascular endothelial cells include AcSDKP , aFGF , ANF , Angiogenin , angiomodulin , Angiotropin , AtT20-ECGF , B61 , bFGF , bFGF inducing activity , CAM-RF , ChDI , CLAF , ECGF , ECI , EDMF , EGF , EMAP-2 , Neurothelin , Endostatin , Endothelial cell growth inhibitor , Endothelial cell-viability maintaining factor , Epo , FGF-5 , IGF-2 , HBNF , HGF , HUAF , IFN-gamma , IL1 , K-FGF , LIF , MD-ECI , MECIF , NPY , Oncostatin M , PD-ECGF , PDGF , PF4 , PlGF , Prolactin , TNF-alpha , TNF-beta , Transferrin , VEGF .
  59. 59. A mind once stretched by a new idea, never regains its original dimensions.
  60. 60. Bone – enormous reservoir 1.PDGF 2.IGF – I IGF – II 3.BMP (PART OF TGF-β FAMILY) Binding proteins to keep these factors within bone itself.
  61. 61. Transforming Growth Factors-β (TGFs- β) ALTERNATIVE NAMES  CIF-B ( cartilage inducing factor B =TGF- beta-2 )  DIF ( differentiation-inhibiting factor )  DSF ( decidual suppressor factor =TGF-beta- 2 )  EIF ( Epstein-Barr virus inducing factor )  EGI (epithelial growth inhibitor )
  62. 62.  MDGF ( milk-derived growth factor )  MGF ( milk growth factor )  MGF-a ( milk growth factor =TGF-beta-2 )  MGF-b ( milk growth factor =TGF-beta-1 )  TCGF ( transformed cell growth factor )  TGI ( tissue-derived growth inhibitor )  TIF-1 ( tumor inducing factor-1 ).
  63. 63. Sources of TGFs- β  Found predominantly in spleen and bone tissues.  Platelets - milligrams of TGF-beta/ kg.  other tissues - microgram TGF/kg.  Human milk – MGF.  Synthesized also by - macrophages(TGF-beta-1 ), lymphocytes(TGF-beta-1 ), endothelial cells(TGF-beta- 1 ), keratinocytes(TGF-beta-2 ), granulosa cells(TGF- beta-2 ), chondrocytes (TGF-beta-1 ), glioblastoma cells(TGF-beta-2 ), leukemia cells(TGF-beta-1 ).
  64. 64. Inducers of TGFs- β  secretion can be induced by a no. of different stimuli including: steroids, retinoids, EGF , NGF , activators of lymphocytes, vitamin D3 , and IL1 .
  65. 65. Inhibitors of TGFs- β  The synthesis of TGF-beta can be inhibited by: EGF , FGF , dexamethasone, calcium, retinoids and follicle stimulating hormone .  TGF-beta also influences the expression of its own gene and this may be important in Wound healing .
  66. 66. TGFs- β  With the extracellular matrix as a complex with betaglycan and decorin.  Stored in a biologically inactive form.  The exact molecular mechanisms underlying its release from these reservoirs is unknown.
  67. 67. PROTEIN CHARACTERISTICS  Five isoforms  TGF-beta-1 , TGF-beta-2 , TGF-beta-3 , TGF-beta-4 , TGF-beta-5 .  They are not related to TGF-alpha .  Their amino acid sequences display homologies on the order of 70-80 percent.  TGF-beta-1 - prevalent form.
  68. 68. PROTEIN CHARACTERISTICS  The biologically active forms of all isoforms are disulfide-linked homodimers.  Sometimes hetrodimers.  The isoforms of TGF-beta arise by proteolytic cleavage of longer precursors.
  69. 69. PROTEIN CHARACTERISTICS  Isoforms isolated from different species are evolutionarily closely conserved and have sequence identities on the order of 98 percent.  Mature human, porcine, simian and bovine TGF-beta-1 are identical and differ from murine TGF-beta-1 in a single amino acid position.  Carboxy terminal end and Amino terminal end of precursor.
  70. 70. Biosynthesis and processing of mature TGF-beta (dark blue)
  71. 71. L - TGF  Almost all forms of TGF-β are released as biologically inactive forms that are known also as L-TGF ( latent TGF ).  Latent forms are complexes of TGF-β, an aminoterminal portion of the TGF-beta precursor, designatedTGF-LAP ( TGF- latency associated peptide ), and a specific binding protein, known as LT-BP ( latent TGF binding protein).
  72. 72. L-TGF  L-TGF - localized at the cell surface by binding to the mannose-6-phosphate/IGF-2 receptor.  Biologically active TGF-beta results after dissociation from the LAP complex.  The nature of the activation mechanism of L- TGF in vivo is unclear.  Direct cell-to-cell contacts, proteases, specifically plasmin, transglutaminases Thrombospondin .
  73. 73. Alpha2M/TGF-beta complexes  The main fraction of the factor in the serum is covalently attached to one of the acute phase proteins , Alpha-2-Macroglobulin (Alpha2M).  Alpha2M/TGF-beta complexes are believed to represent TGF-beta molecules released by platelets after tissue injuries and destined to degradation.
  74. 74. Mutant TGF- β  Mutant forms of TGF- β have been created.  They form wild-type/mutant heterodimers deficient in assembly or processing.  Such mutants behave as dominant negative mutants and are useful in investigation of the role of TGF- β in normal and pathological conditions.
  75. 75. GENE STRUCTURE  The different isoforms of TGF-β are encoded by different genes.  All genes have a length of more than 100 kb and contain seven exons.  The genes map to different chromosomes.
  76. 76. GENE STRUCTURE  The TGF-beta-1 gene maps to human chromosome 19q13.  The TGF-beta-2 gene maps to human chromosome 1q41.  The TGF-beta-3 gene maps to human chromosome 14q24.  These genes are expressed differentially.  The TGF-beta-3 gene is expressed strongly in embryonic heart and lung tissues but only marginally in liver, spleen, and kidney tissues. TGF-beta-1 is expressed strongly in spleen tissues.
  77. 77. RELATED FACTORS  TGF-beta is the prototype of a protein family known as the TGF-beta superfamily.  This family includes Inhibins , Activin A , MIS (Müllerian inhibiting substance ), BMP (bone morphogenetic proteins ), dpp (decapentaplegic ) and Vg-1 .
  78. 78. RECEPTORS  An entire family of glycoprotein receptors for TGF-beta has emerged.  Some of these proteins do not bind TGFbeta- related factors belonging to the TGF-beta family.  Type-1 receptors (hematopoietic progenitor cells) and type-2 receptors.
  79. 79. RECEPTOR AFFINITY  Individual TGF-b isotypes - varying affinities.  E.g., TGF-beta-1 binds approximately tenfold better than TGF-beta-2.  Expression of the TGF-beta receptors is decreased by EGF (Receptor transmodulation).  In endothelial cells the expression of the TGF-beta receptor is decreased by bFGF .
  80. 80. RECEPTORS  Almost all types of cells express, type-3 receptor.  This receptor type is not expressed in primary epithelial, endothelial, and lymphoid cells .  The type-3 receptor is a proteoglycan (Betaglycan), binds TGF-beta-1 and TGF- beta-2 equally well.
  81. 81. BIOLOGICAL ACTIVITIES  Not species-specific.  TGF-beta-2 is the only variant that does not inhibit the growth of endothelial cells.  Most pronounced differences in the TGF-beta isoforms is their spatially and temporally distinct expression of both the mRNAs and proteins in developing tissues, regenerating tissues, and in pathologic responses.
  82. 82. BIOLOGICAL ACTIVITIES  TGF-beta is the most potent known growth inhibitor for normal and transformed epithelial cells, endothelial cells, fibroblasts, neuronal cells, lymphoid cells and other hematopoietic cell types (CFU-S ), hepatocytes, and keratinocytes.
  83. 83. BIOLOGICAL ACTIVITIES  Inhibits the proliferation of T-lymphocytes.  Inhibits the growth of natural killer cells in vivo.  Deactivates macrophages.  Blocks the antitumor activity mediated in vivo by IL2 and transferred lymphokine-activated or tumor infiltrating lymphocytes .
  84. 84. BIOLOGICAL ACTIVITIES  Inhibits the growth of immature hematopoietic progenitor cells .  In particular growth of CFU-GEMM .  Inhibits megakaryocytopoiesis.  Antagonizes the biological activities of EGF , PDGF , aFGF and bFGF .  Latent form of TGF-beta is a strong inhibitor of erythroleukemia cell lines.
  85. 85. BIOLOGICAL ACTIVITIES  The extent of growth inhibition induced by TGF-beta depends on the cell type, on the concentration of TGF-beta, and on the presence of other factors.  The growth-inhibitory activities of TGF-beta can be abolished by HGF (hepatocyte growth factor ).
  86. 86. BIOLOGICAL ACTIVITIES  At concentrations of 1-2 fg/cell - growth inhibition for smooth muscle cells, fibroblasts, and chondrocytes.  At higher concentrations - stimulation.  This bimodal activity is mediated in part by PDGF .  Low concentrations of TGF-beta - synthesis and secretion of PDGF.  Higher concentrations – lower expression of the PDGF receptors and hence diminish the biological effects of PDGF .
  87. 87. BIOLOGICAL ACTIVITIES  Overproduction of TGF-beta-1 by tumor cells - neovascularization and may help promote tumor development in vivo.  TGF-beta is an autocrine growth modulator for malignant gliomas.  It stimulates the growth of fibroblasts and osteoblasts in vivo and in vitro.  TGF-beta induces the synthesis of bone matrix proteins in osteoblasts.
  88. 88. BIOLOGICAL ACTIVITIES  Factors that promote bone resorption (IL1 , vitamin D3 , parathormone) induce the synthesis of TGF-beta in bone cells.  While calcitonin, an inhibitor of bone resorption, reduces the synthesis of TGF- beta.  It suppresses the expression of class II MHC antigens .  Microglial cells.
  89. 89. BIOLOGICAL ACTIVITIES  TGF-beta stimulates the synthesis of the major matrix proteins including collagen, proteoglycans , glycosaminoglycans, fibronectin , integrins, Thrombospondin , osteonectin, osteopontin .  It inhibits degradation mainly by inhibiting the synthesis of neutral metalloproteinases and by increasing the synthesis of proteinase inhibitors.
  90. 90. BIOLOGICAL ACTIVITIES  Involved in metastatic processes.  It is responsible for the transformation of epithelial cells into mesenchymal cells.  Suppressive effects on the immune system .  TGF-beta-1 is the most potent known chemoattractant for neutrophils .
  91. 91. CLINICAL USE AND SIGNIFICANCE  It may be a potent regulator of Wound healing and of bone fracture healing.  Local application of TGF-beta has been shown to accelerate wound repair.  In combination with bone morphogenetic protein-2 it causes development of ossification of the posterior longitudinal ligament of the cervical spine.
  92. 92. CLINICAL USE AND SIGNIFICANCE  The factor may be helpful in the treatment of traumatic tissue injuries.  Treatment of osteoporosis.  Reverses age- or glucocorticoid-impaired Wound healing even if given 24 hours before wounding.
  93. 93. Bone morphogenetic protein (BMP)  Responsible for osteoinductive activity in bone matrix.  Non-collagenous.  Water soluble.
  94. 94. Bone morphogenetic protein (BMP)  The cellular and molecular events governing bone formation in the embryo, healing of a fractured bone, and induced bone fusion follow a similar pattern.  Bone is unique of all the tissues.  When injured, it heals by formation of new bone.
  95. 95. BMP-Introduction  The molecular and cellular processes that lead to the development of the skeletal structures within the embryo are very similar to the cascades that occur in the healing process in an injured bone.  Thus, there is a common theme in the development of bone from primitive mesenchymal tissues to a well-structured, well-organized mature bone.
  96. 96. BMP-Introduction  The ongoing remodeling process in an adult organism, which is exposed to external physical and hormonal influences, is also modulated through a similar molecular mechanism.  Intracartilaginous process.  Intramembranous process.
  97. 97. BMP-Introduction  Postfracture healing - intracartilaginous ossification process.  Very high concentration of BMP - intramembranous route may be taken.  It is unclear what factor(s) direct(s) one process as opposed to the other in the embryonic phase or during fracture healing.
  98. 98. Stages of bone healing and remodeling
  99. 99. GF and cytokines involved in generation of new bone
  100. 100. History  Senn(1889)-Decalcified ox bone promotes healing of osteomyelitic defects.  Lexer(1908)-Necrotic bone tissue released stimulating factors that affect osteoblasts.  Polettini(1922)-Substance released from graft tissue resulted in differentiation of fibroblasts into bone and cartilage forming cells.
  101. 101. History  Leriche(1928)-Ca materials contained in the graft tissue were the agents inducing new bone formation.  Levander(1934)-crude alcohol extracts of bone induce bone formation in muscle.  Sharrard and Collin(1961)-EDTA decalcified allograft induced spinal fusion in children.
  102. 102. History  Urist(1965)-acid-decalcified bone induced ectopic bone in rat model. He coined the term "bone morphogenetic protein" or "osteogenic protein" .  Reddi and Sampath(1983)-crude but reproducible bioassay for BMP; bone matrix when dissociated from BMP ineffective in bone induction; reconstituted matrix effective.
  103. 103. History  Johnson(1992)-first clinical study; purified human BMP successful clinically.  Creative biomolecules and genetic institute(1990s)- simultaneous gene sequencing for various BMP’s and related patent dispute.  Stryker Corp., Medtronic Sofamor Danek(2002)-FDA approval of OP-1 (BMP-7) for long bone defects and BMP-2 in a collagen carrier within a cage for anterior lumbar interbody fusions.
  104. 104. Classification of BMP  Bone morphogenic proteins are members of TGF superfamily.  The BMP subfamily comprises more than 10 proteins, and newer ones are being discovered.  Several structural homologies between BMPs and TGF growth factors.  The amino acid sequence of BMPs is considered to be as old as 600 million years.
  105. 105. Classification  Because of this conservation, human recombinant BMPs are highly effective in lower life forms, including fruit flies.  BMPs are synthesized as precursor proteins.  The mature portion of the protein is located at the carboxy terminal of the precursor molecule.
  106. 106.
  107. 107. BMP  It is the only morphogen of all known growth factors that has the ability to transform connective tissue cell into osteoprogenitor cells.  Thus, it is not only a mitogen but can be a morphogen as well.  All other growth factors such as TGF, insulin-like growth factor, fibroblast growth factor, PDGF, and vascular endothelial growth factor all induce multiplication of cells but do not transform one cell type into the other.
  108. 108. Signaling Mechanism of BMP  BMP receptors - Type I and Type II serine/threonine kinase proteins.  The binding of the ligand to the Types I and II serine/threonine kinase transmembrane receptors results in the activation of the signaling cascade.  Type II receptor kinase phosphorylates the Type I receptor.
  109. 109. Signaling Mechanism of BMP  Type I receptor phosphorylates the intracytoplasmic signaling molecules Smads 1, 5, and 8.  Smads 1, 5, and 8 bind to Smad 4.  Translocate into the cell nucleus.  Activation of transcriptional factors for the early BMP response genes.
  110. 110. Dosage  Normal bone contains approximately 0.002 mg of BMP per kilogram of pulverized bone.  At a fracture site, presumably the BMP is released at a higher concentration.
  111. 111. Dosage  The concentration required for ideal induced bone bridging in osseous defects depends on several factors. - state of the organism in the evolutionary scale. - type of defect. Bone induced under the influence of BMP matures faster than natural healing of the bone.
  112. 112. Other uses  Brain protective agent.  kidneys are their primary source in the human adult. In chronic renal disease levels of BMP are lower. systemic administration of BMP may restore some of the renal functions.  Local application for dialysis patient in osteodystrophy.
  113. 113. BMP  Named because of their osteoinductive ability.  Role in embryonic and post embryonic development.  Signaling molecules in no. of tissues.  Implicated in mesodermal patterning, neurogenesis and organogenesis.
  114. 114. BMP  BMP signaling pathways ↔ other growth factors ↔ hormonal signaling pathways.  Cross talk between them must be evaluated to avoid side effects of BMP based therapies.
  115. 115. BMP  Mutations perturbing functions of BMP genes:  BMP-5 gene mutation – short ear – abnormal growth & patterning of skeletal structures and diminished repair of bone fracture.
  116. 116. BMP  GDF-5 gene mutation – brachypod phenotype in mice & in autosomal recessive syndromes Hunter – Thompson chondrodysplasia in humans – shortening of appendicular skeleton and loss or abnormal development of some joints.
  117. 117. BMP  BMP-2 & BMP-4 knock out mice die early in embryonic development, long before development of skeleton, because of defects in gastrulation.  BMP-7 knock out – eye and kidney defects, only mild skeletal defects.
  118. 118. BMP  Several extracellular proteins regulate activities of BMP.  Noggin (BMP-4 & BMP-2)  Follistatin (BMP-4 & BMP-7)  Protein chordin  Astacin family of metalloproteases – cleaves chordin.
  119. 119. Platelet-Derived Growth Factor (PDGF) ALTERNATIVE NAMES  MDGF ( monocyte-derived growth factor )  ODGF ( osteosarcoma-derived growth factor ) SOURCES  megakaryocytes  stored in the alpha granules of platelets(PDGF- BB/AB)  released after cell activation of platelets for example by thrombin .
  120. 120. Sources  Unstimulated cells of osteoblastic lineage – PDGF-AA.  Other cells - macrophages, endothelial cells, fibroblasts, glial cells, astrocytes, myoblasts, smooth muscle cells, and a number of tumor cell lines.  Synthesis of PDGF can be induced by IL1 , IL6 , TNF-alpha , TGF-beta and EGF .
  121. 121. Platelet-Derived Growth Factor (PDGF)  PDGF is composed of two distinct polypeptide chains, A and B, that form homodimers (AA or BB) or heterodimers (AB).  3 isoforms.  PDGF receptors have intrinsic tyrosine kinase activity.
  122. 122. PDGF  PDGF-BB – binds to receptor – activates extracellular signal regulated kinase 1&2 – cellular proliferation by accelerating cell recycle & inducing quiescent cells into the proliferation portion of the cell cycle.  This effect is mediated by protein kinase B, a serine- threonine protein kinase.  TGFb1 – inhibits receptor autophosphorylation – neutralizes mitogenic effect of PDGF.
  123. 123. PDGF  Proliferative responses to PDGF action are exerted on many mesenchymal cell types.  Two related receptors, called PDGFR alpha or PDGFR beta.  PDGF is not released into the circulation.  The biological half-life is less than two minutes after intravenous administration.
  124. 124. PDGF  In the adult organism PDGF is involved in Wound healing processes.  The dimeric form of PDGF is mainly mitogenic for cells of mesenchymal origin while monomeric forms of PDGF are mainly chemotactic.  Disruption of PDGF signaling – perinatal lethality > 50%.
  125. 125. PDGF  At low concentrations PDGF is a chemoattractant for fibroblasts.  PDGF is also chemotactic and activating for monocytes and neutrophils.  PDGF (alone and in combination) may be useful in promoting bone formation.  Promotes fracture healing.  Doesn’t provides entire osteogenic properties itself.
  126. 126. PDGF  Osteoblasts can specifically bind and proliferate in response to PDGF.  Enhanced proliferation of both osteoblasts and osteoclasts.  In tissue culture, PDGF alone has not yet been proved to be osteoinductive in vivo.  In osteosarcoma – positive feedback loop.  Platelet gel.
  127. 127. PDGF  PDGF is chemotactic for both alkaline phosphatase positive and negative cells.  It may so contribute to recruitment of bone cells during remodeling and repair.  Used in implants and periodontal therapies.  With or without IGF-I.
  128. 128. Fibroblast Growth Factors (FGFs)  19 distinct members  FGF1 (acidic-FGF, aFGF) and FGF2 (basic- FGF, bFGF).  Studies of human disorders & gene knock-out studies in mice show the prominent role for FGFs is in the development of the skeletal system in mammals.
  129. 129. aFGF SOURCE  Best sources of aFGF is brain tissue.  The mechanism underlying the release of aFGF is unknown.
  130. 130. bFGF SOURCES  Almost all tissues of mesodermal and neuroectodermal origin.  Also in tumors derived from these tissues.  Endothelial cells.
  131. 131. FGF  potent inducers of mesodermal differentiation in early embryos.  Specific cell-surface receptors.  4 distinct receptor types identified as FGFR1 - FGFR4.  Receptors has intrinsic tyrosine kinase activity .  autophosphorylation of the receptor is the immediate response to FGF binding.
  132. 132. FGF  FGFs also bind to cell-surface heparan-sulfated proteoglycans with low affinity  The FGF receptors are widely expressed in developing bone.  Mutations in the FGFR genes-autosomal dominant disorders of bone growth e.g. achondroplasia(FGFR3).  FGFR3 is predominantly expressed in quiescent chondrocytes & it restricts chondrocyte proliferation and differentiation.
  133. 133. FGF  bFGF stimulates the growth of fibroblasts, myoblasts, osteoblasts, endothelial cells, chondrocytes, and many other cell types.  bFGF is not only a mitogen for chondrocytes but also inhibits their terminal differentiation.
  134. 134. FGF  Animals experiments with bFGF - promotes endosteal, but not periosteal, bone formation.  bFGF may thus be a potential agent for treatment of osteoporosis which may increase bone mass without causing outward deformation of the skeletal bones.
  135. 135. FGF  Craniosynostosis syndromes have been shown to result from mutations in FGFR1, FGFR2 and FGFR3.  Sometimes the same mutation can cause two or more different craniosynostosis syndromes.
  136. 136. Affected Receptor Syndrome Phenotypes FGFR1 Pfeiffer broad first digits, hypertelorism FGFR2 Apert mid-face hypoplasia, fusion of digits FGFR2 Beare- Stevenson mid-face hypoplasia, corrugated skin
  137. 137. Affected Receptor Syndrome Phenotypes FGFR2 Crouzon mid-face hypoplasia FGFR2 Jackson- Weiss mid-face hypoplasia, foot anamolies FGFR2 Pfeiffer same as for FGFR1 mutations
  138. 138. Affected Receptor Syndrome Phenotypes FGFR3 Crouzon mid-face hypoplasia, acanthosis nigricans FGFR3 Non-syndromatic craniosynostosis digit defects, hearing loss.
  139. 139. Insulin-Like Growth Factor  IGFs are single chain peptides.  2 isoforms (IGF-I and IGF-II).  40-50% homology with insulin. Still all 3 have unique binding site to their receptors.  IGF also has general activity (metabolic & growth promoting) in many tissue types.
  140. 140. IGF-I SOURCE  Mainly liver.  IGF – responsible for fetal and postnatal growth and development in general.
  141. 141. IGF-I  IGF-I receptor gene – deleted mice died at birth putatively due to poor muscular development.  IGF-I: pre + postnatal development.  IGF-II: prenatal stages only.  IGF-I/ IGF-II ratio increases with age in many tissues.  Role in skeletal development and skeletal mass maintenance and development of teeth.
  142. 142. Insulin-Like Growth Factor-I (IGF-I)  Called somatomedin C(considered as circulating mediator of growth hormone).  Primary protein involved in responses of cells to growth hormone (GH)  IGF-I is produced in response to GH and then induces subsequent cellular activities, particularly on bone growth.
  143. 143. IGF-I  IGF-I has autocrine and paracrine activities in addition to endocrine activities on bone.  Family of transmembrane IGF-I(tyrosine kinase), IGF-II(mannose-6-phosphate receptor) & insulin receptor.  Receptor has intrinsic tyrosine kinase activity.  Plays role in general growth and maintenance of body skeleton.
  144. 144. Insulin-Like Growth Factor-II (IGF-II)  Exclusively expressed in embryonic and neonatal tissues.  Following birth, the level of detectable IGF-II protein falls significantly.  The IGF-II receptor is identical to the mannose-6-phosphate receptor.
  145. 145. IGF  Osteoblast aging is associated with impaired production of the stimulatory components of the IGF-system, that may contribute to age- related decline in osteoblast functions.  Of all IFG binding proteins, IGFBP-5 is abundant in bone matrix.
  146. 146. IGF  IGF-I & II are potent survival factors for fibroblasts, hematopoetic cells, cardiac muscle cells & pancreatic beta cell.  IGF-I has anti apoptotic activity in these cell types and in certain tumors.  Autocrine loop – tumor promoting effect.  IGF-I has chemotactic effect on osteoblasts in a dose dependent manner. IGF-II effects only at lowest conc.
  147. 147. IGF  It promotes expression of bone specific protein e.g. bone sialoprotein, and osteopontin.  In vivo, systemic application of IGF-I – rapidly activated bone turnover – increase in serum osteocalcin, increased collagen marker of bone formation, and an increased urinary ratio of Ca/creatinine.
  148. 148. IGF  In inflammatory tissue (e.g. fracture repair) – IL1 increases IGF-I production.  IGF-I activity can be suppressed by NSAIDs e.g. indomethacin.
  149. 149.
  150. 150.  Taken from the AJO-DO on CD-ROM (Copyright © 1997 AJO-DO), Volume 1993 Aug (121 - 131): Heritability of skeletodental relationships - King, Harris, and Tolley, Fig. 6.  --------------------------------  Fig. 6. The epigenetic landscape (redrawn from Waddington82).
  151. 151.  Taken from the AJO-DO on CD-ROM (Copyright © 1997 AJO-DO), Volume 1981 Oct (366 - 375): Genetics, epigenetics, and causation - Moss, Fig. 5.  --------------------------------  Fig. 5. This figure shows how boundaries might be analogously demonstrated to operate in the epigenetic regulation. If the ball at the top of the figure is taken to represent a cell, or a tissue, capable of rolling down the convoluted surface shown, then the developmental pathway will be regulated by the convex ridges of that inclined surface. These epigenetic boundaries will effectively prevent the ball from passing into adjacent grooves, and thus determine the developmental pathway. (Adapted from Waddington: New Patterns in Genetics and Development, New York, 1962, Columbia University Press.)
  152. 152.  The ball may be considered, for example, as a simple, totipotential cell with its complete, genomically encoded information regulatory of the full range of species-specific polypeptide synthesis. The subsequent history of this cell and its descendants is a function of which developmental pathway (or "Chreod") it moves along. Some initial epigenetic factor or process determines the initial path selected, at which time portions of the genome become, respectively, repressed and derepressed, so that initial cytodifferentiation occurs. This new state, or epigenetic environment, keeps the vital material moving along this particular pathway until another bifurcation point occurs (Fig. 5). Once again, the instantaneous epigenetic state regulates this decision, and a "catastrophic" event occurs; that is, a new structually higher-order state or path is evolved. These pathways become "deeper" (have higher "walls") as they progressively become hierarchically more complex. This represents, in such a model, the fact that there is an increasing ability to withstand lateral, homeostatic perturbations during "movement'' along the landscape. This movement is termed homeorrhesis. In this model, the selection of pathways is not genomically but epigenetically regulated. Yet the genomic information must be present to permit synthetic activity by the cell and is one type of intrinsic, necessary information needed for ontogenesis to occur. The constantly added epigenetic information is the other type of necessary causation required. There is no reason for conflict between the genomic and epigenetic hypotheses of ontogenetic regulation when it is perceived that they are interdependent, yet different, categories of necessary causes and that only their unity provides the sufficient condition for growth and development to occur.74
  153. 153.  (1) all of the extrinsic (extraorganismal) factors impinging on vital structures, including importantly mechanical loadings and electroelectric states and (2) all of the intrinsic (intraorganismal) biophysical, biomechanical, biochemical, and bioelectric microenvironmental events occurring on, in, and between individual cells, extracellular materials, and cells and extracellular substances.  As previously noted,99 epigenetic factors include (1) all of the extrinsic, extraorganismal, macroenvironmental factors impinging on vital structures (for example, food, light, temperature), including mechanical loadings and electromagnetic fields, and (2) all of the intrinsic, intraorganismal, biophysical, biomechanical, biochemical, and bioelectric microenvironmental events occuring on, in, and between individual cells, extracellular materials, and cells and extracellular substances.
  154. 154.
  155. 155.
  156. 156.
  157. 157. AJO-DO:1993 Mar - Sandy, Farndale, and Meikle  Simplified diagram depicting interactions of molecules at focal contacts. The model depicts how the cytoskeleton is linked through the membrane glycoprotein integrin to the extracellular matrix. Many of the extracellular matrix proteins which are responsible for cell adhesion contain common peptide sequences as cell recognition sites. These sites are recognized by integrins that are a family of glycoproteins, which span the cell membrane from the cytoplasm to the extracellular matrix. Integrin does not bind directly to microfilament structures, such as actin, but is dependent on associated proteins for this function. Integrin binds to fibronectin in the extracellular matrix and to talin on the cytoplasmic surface. Actin and vinculin then bind to this talin-integrin complex.
  158. 158.
  159. 159. From macro to - micro
  160. 160. From macro to - micro
  161. 161.
  162. 162. Terminology  Cell – basic living, structural & functional unit of body  Chromosome – highly coiled & folded DNA molecule combined with protein molecules, present in the nucleus of cell  Gene-The unit of heredity: one or more nucleic acid sequences incorporating information necessary for the generation of a particular peptide or RNA product (AJODO-1997 : MOSS - Meier AE, editor. A is for . . . gene. Sci Med 1996;3:72.) These are located on the chromosomes
  163. 163. Terminology  Apoptosis – physiological cell death occuring in normal tissues/in diseased organs, not associated with inflammatory reactions  Chemotaxis – process of migration of cells towards an attractant  Ontogenesis - growth and development of the cell
  164. 164. Growth 3 possibilities :  Hypertrophy – increase in the size of cells  Hyperplasia – increase in the number of cells  Secretion of extra cellular material MAINLY SEEN IN HARD TISSUES – BONE, CARTILAGE, TEETH  Atrophy – diminished size and number of cells due to extreme failure of development  Interstitial growth – occurring at all points within the tissue; hyperplasia primarily & hypertrophy secondarily, with/without secretion of ECM IN SOFT TISSUES & UNCALCIFIED CARTILAGE
  165. 165. Resolving synthesis (AJODO-1997 : MOSS)  Morphogenesis is regulated (controlled, caused) by the activity of both genomic and epigenetic processes and mechanisms. Both are necessary causes; neither alone are sufficient causes; and only their integrated activities provides the necessary and sufficient causes of growth and development. Genomic factors are considered as intrinsic and prior causes; epigenetic factors are considered as extrinsic and proximate causes.  It is probable that ontogeny involves nonlinear processes and is not fully predictable; that is, growth and development, to a significant extent, exhibit both random behaviors and frequent
  166. 166. Resolving synthesis - Complexity and self- organization  The highly ordered morphological properties of adult complex biological systems (for example, functional matrices and skeletal units) result from the operation of a series of spontaneous and self-organized ontogenetic processes and mechanisms.  Environmental factors play a decisive role in all ontogenetic processes. But it is the organism itself that, as an integrated system, dictates the nature of each and every developmental response . . . the living organism self-organizes on the basis of its own internal structuring, in continuous interaction with the environment in which it finds itself.
  167. 167. Mechanotransduction (AJODO-1997 : MOSS)  Occurs in single bone cells  All vital cells are "irritable" or perturbed by and respond to alterations in their external environment. Mechanosensing processes enable a cell to sense and to respond to extrinsic loadings by using the processes of mechanoreception and of mechanotransduction.  The former transmits an extracellular physical stimulus into a receptor cell; the latter transduces or transforms the stimulus's energetic and/or informational content into an intracellular
  168. 168.  There are several mechanotransductive processes, for example, mechanoelectrical and mechanochemical. Whichever are used, bone adaptation requires the subsequent intercellular transmission of the transduced signals.  When an appropriate stimulus parameter exceeds threshold values, the loaded tissue responds by the triad of bone cell adaptation processes…..trio of possible osteoblastic responses to loading (deposition, resorption, or maintenance of bone tissue)
  169. 169. Osseous mechanotransduction is unique in four ways:  (1) Most other mechanosensory cells are cytologically specialized, but bone cells are not;  (2) one bone-loading stimulus can evoke three adaptational responses, whereas nonosseous processes generally evoke one;  (3) osseous signal transmission is aneural, whereas all other mechanosensational signals use some afferent neural pathways and,  (4) the evoked bone adaptational responses are confined within each "bone organ" independently, e.g., within a femur, so there is no necessary "interbone" or organismal involvement.
  170. 170. 1. Electromechanical  As in most cells, the osteocytic plasma membrane contains voltage-activated ion channels, and transmembrane ion flow may be a significant osseous mechanotransductive process.  Stretch-activated channels - Several types of deformation may occur in strained bone tissue. One of these involves the plasma membrane stretch- activated (S-A) ion channels, a structure found in bone cells, in many other cell types and significantly in fibroblasts. When activated in strained osteocytes, they permit passage of a certain sized ion or set of ions, including K+, Ca2+, Na+. Such ionic flow may, in turn, initiate intracellular electrical events.
  171. 171. 2. Electrokinetic  Bound and unbound electric charges exist in bone tissue, many associated with the bone fluid(s) in the several osseous spaces or compartments. Electrical effects in fluid-filled bone are not piezoelectric, but rather of electrokinetic, that is, streaming potential (SP) origin. The SP is a measure of the strain-generated potential (SGP) of convected electric charges in the fluid flow of deformed bone. The usually observed SPG of 2 mV can initiate both osteogenesis and osteocytic action potentials.
  172. 172. 3. Electric field strength  Bone responds to exogenous electrical fields. Although the extrinsic electrical parameter is unclear, field strength may play an important role. A significant parallel exists between the parameters of these exogenous electrical fields and the endogenous fields produced by muscle activity.
  173. 173. Gap junctions & Connected cellular network (CCN)  Gap junctions are regions on the lateral surfaces of cells where the gap between the adjoining plasma membranes is reduced from 20 nm to 2nm in width. Pits/holes may be present in these regions, permeable to small tracer particles  All bone cells, except osteoclasts, are extensively interconnected by gap junctions that form an osseous CCN. In these junctions, connexin is the major protein. Each osteocyte, enclosed within its mineralized lacuna, has many (n = 80) cytoplasmic (canalicular) processes, 15 mm long and arrayed three- dimensionally, that interconnect with similar processes of up to 12 neighboring cells. These processes lie within mineralized bone matrix channels (canaliculi)
  174. 174.  Gap junctions are found where the plasma membranes of a pair of markedly overlapping canalicular processes meet. Gap junctions also connect superficial osteocytes to periosteal and endosteal osteoblasts. All osteoblasts are similarly interconnected laterally. Vertically, gap junctions connect periosteal osteoblasts with preosteoblastic cells, and these, in turn, are similarly interconnected.  In addition to permitting the intercellular transmission of ions and small molecules, gap junctions exhibit both electrical and fluorescent dye transmission. Gap junctions are electrical synapses, in contradistinction to interneuronal, chemical synapses, and, significantly, they permit bidirectional signal traffic, e.g., biochemical,
  175. 175.  Mechanotransductively activated bone cells, e.g., osteocytes, can initiate membrane action potentials capable of transmission through interconnecting gap junctions.  The CCNs show oscillation, i.e.,reciprocal signaling (feedback) between layers. This attribute enables them to adjustively self-organize.  Gap junctions, permitting bidirectional flow of information, are the cytological basis for the oscillatory behavior of a CCN.
  176. 176. Integrins  A family of membrane integral proteins that span the cell membrane from the cytoplasm to the extracellular matrix.  Many of the extracellular matrix proteins which are responsible for cell adhesion contain common peptide sequences as cell recognition sites, these sites are recognized by integrins.  Changes in cell shape produce a range of effects mediated by integrins and the cytoskeleton, which may be important in transducing mechanical deformation into a meaningful biologic
  177. 177. Integrins  A family of glycoproteins. These are connected extracellularly with the macromolecular collagen of the organic matrix and intracellularly with the cytoskekeletal actin. The molecules of the latter, in turn, are connected to the nuclear membrane,  These aid in transmitting signals from the ECM directly to the intranuclear genome. This informational transfer between cells and ECM is dynamic, reciprocal, and continuous.
  178. 178. Osteoblasts (AJO-DO:1993 Mar - Sandy, Farndale, and Meikle)  They are now recognized as the cells that control both the resorptive and the formative phases of the remodeling cycle, and receptor studies have shown them to be the target cells for resorptive agents in bone.  The osteoblast is perceived as a pivotal cell, controlling many of the responses of bone to stimulation with hormones and mechanical forces.
  179. 179. Genes  All somatic cells commonly share approximately 5000 different polypeptide chains, each specific cell type is characterized only by approximately 100 specific proteins. And it is claimed that "these quantitative (protein) differences are related to differences in cell size, shape and internal architecture”.  The encoding of the DNA exists in two families; the vastly preponderant "housekeeping" genes and the nonabundant "structural" genes. The former regulate the normal molecular synthesis of agents involved in (1) the common energetic (metabolic, respiratory) activities of all cells and, (2) the specific activities of special cell types (e.g., neurons, osteoblasts, ameloblasts etc.)
  180. 180.  Prenatal craniofacial development is controlled by two interrelated, temporally sequential, processes: (1) initial regulatory (homeobox) gene activity and (2) subsequent activity of two regulatory molecular groups: growth factor families and steroid/thyroid/retinoic acid super-family. For example, "homeobox genes coordinate the development of complex craniofacial structures" and in "both normal and abnormal development, much of the regulation of the development of virtually all of the skeletal and connective tissue of the face is dependent on a cascade of overlapping activity of homeobox genes.“
  181. 181.  It is claimed that regulatory molecules can (1) "alter the manner in which homeobox genes coordinate cell migration and subsequent cell interactions that regulate growth" and (2) be involved in the "genetic variations causing, or contributing to, the abnormal development of relatively common craniofacial malformations . . . perhaps modifying Hox gene activity."
  182. 182. Thank you For more details please visit