STRUCTURE AND FUNCTION OF CELL PART 2.pptx

biology of cell and structure of cell along with cells in periodontology

1. CELL- Part 2 Dr. Pranav Kulkarni Dept of Periodontology

2. CONTENTS PART - I HISTORY INTRODUCTION OF CELL CELL DIVERSITY COMPONENTS OF CELL CHARACTERISTICS OF CELL CLASSIFICATION PART - II CELL GROWTH CELL JUNCTIONS CELL REPRODUCTION CELL METABOLISM CELLULAR ADAPTATIONS CELL INJURY. APOPTOSIS CELLS OF PERIODONTIUM

3. CELL GROWTH A) Cell size : Cell size is highly variable among organisms, with some algae such as  Caulerpa taxifolia  being a single cell several meters in length. Plant cells are much larger than animal cells, and protists such as Paramecium can be 330 μm long, while a typical human cell might be 10 μm . In some cell, cell division is not initiated until cell reaches a certain size. The term cell growth can be studied into two different ways.

4. B) Cell population : Growth of cell population is by means cell reproduction . Cell populations go through a particular type of  exponential growth . Thus, each  generation  of cells should be twice as numerous as the previous generation. However, the number of generations only gives a maximum figure as not all cells survive in each generation. Cells can reproduce in the stage of Mitosis , where they double and split into two genetically equal cells .

5. CELL JUNCTIONS Cell junctions consist of  multiprotein complexes  that provide contact between neighboring cells  or between a cell and the  extracellular matrix. Cell junctions are especially abundant in epithelial tissues. Cell junctions are especially important in enabling communication between neighboring cells via specialized proteins called  communicating junctions . Cell junctions are also important in reducing stress placed upon cells.

6. CELL JUNCTIONS

7. Combined with  cell adhesion molecules and  extracellular matrix , cell junctions help hold animal cells  together . There are three major types of cell junction: Anchoring junctions D esmosomes  H emidesmosomes   Adherens junctions Gap junctions  ( communicating junction ) Tight junctions ( occluding junctions )

8. Cells within tissues and organs must be anchored to one another and attached to components of the extracellular matrix. Anchoring-type junctions not only hold cells together but provide tissues with structural cohesion . Three types of anchoring junctions are observed, - Desmosomes - Hemidesmosomes - Adherens junctions

9. A) DESMOSOMES A cell structure specialized for cell-to-cell adhesion.   Intermediate filaments composed of  keratin  or  desmin  are attached to membrane-associated attachment proteins that form a dense plaque on the cytoplasmic face of the membrane. The importance of desmosome junctions is demonstrated by some forms of the potentially fatal skin disease  pemphigus . Affected individuals make antibodies against one of their own desmosomal  cadherin proteins.

10. B) HEMIDESMOSOMES Hemidesmosomes , or half-desmosomes, resemble desmosomes morphologically and in connecting to intermediate filaments , and, like desmosomes, they act as rivets to distribute tensile or shearing forces through an epithelium. They connect the basal surface of an epithelial cell to the underlying basal lamina. Genetic or acquired diseases that cause disruption of hemidesmosome components can lead to skin blistering disorders between different layers of the skin. These are collectively coined  epidermolysis bullosa , or EB.

11. C) ADHERENS JUNCTION Adherens junction take the form of small punctate or streaklike attachments that indirectly connect the cortical actin filaments beneath the plasma membranes of two interacting cells.  Eg – Adhesion belt Adherens junctions are composed of the following proteins - cadherins - gamma-catenin - delta catenin - alpha-catenin

12. 2. GAP JUNCTION With the exception of a few terminally differentiated cells such as skeletal muscle cells and blood cells, most cells in animal tissues are in communication with their neighbors via gap junctions. Gap junctions allow for direct chemical communication between adjacent cellular cytoplasm through diffusion without contact with the extracellular fluid.

13. 3. TIGHT JUNCTIONS Junctions or  zonulae occludentes  (singular,  zonula occludens ) are multiprotein  junctional complexes  whose general function is to prevent leakage of transported solutes and water and seals the paracellular pathway. There are at least 40 different proteins composing the tight junctions. All tight junctions are impermeable to macromolecules.  They function as barriers to the diffusion of some membrane proteins (and lipids) between apical and basolateral domains of the plasma membrane. 

14. CELL REPRODUCTION Prokaryotic cells such as bacterial cells reproduce by “ Binary Fission ”. This form of  asexual reproduction  and  cell division  is also used by some organelles within eukaryotic organisms (e.g., mitochondria). Binary fission results in the reproduction of a living prokaryotic cell (or organelle) by dividing the cell into two parts , each with the potential to grow to the size of the original. Eukaryotic cell reproduction involves either- a) Mitosis (Somatic cell cycle). b) Meiosis (Reproductive cell cycle).

15. SOMATIC CELL CYCLE The series of events that take place in a cell that cause it to divide into two daughter cells Resting (G0) - A phase where the cell has left the cycle and has stopped dividing . Interphase : Period between cell division. Consists of : a) G1 phase - Cells increase in size b) S phase - DNA replication c) G2 phase - the cell will continue to grow Cell Division (Mitosis-M) - Cell growth stops at this stage and cellular energy is focused on the orderly division into two daughter cells.

16. MITOSIS The period during which the cell is actively dividing is the phase of mitosis. The daughter cells must have chromosomes identical in number (and in genetic content) to those in the mother cell. This type of cell division is called mitosis The relatively brief  M phase  consists of nuclear division ( karyokinesis ). It is a relatively short period of the cell cycle. M phase is complex and highly regulated. The sequence of events is divided into phases, corresponding to the completion of one set of activities and the start of the next.

17. MITOSIS

18. MEIOSIS A special type of cell division that reduces the  chromosome number by half , creating four haploid cells , each genetically distinct from the parent cell that gave rise to them. This process occurs in all sexually reproducing single-celled and multicellular eukaryotes, including animals, plants, and fungi.  Meiotic cell divisions are an essential process during  oogenesis  and  spermatogenesis . Meiosis is divided into meiosis I and meiosis II which are further divided into karyokinesis I and cytokinesis I and karyokinesis II and cytokinesis II respectively. The preparatory steps that lead up to meiosis are identical in pattern and name to interphase of the mitotic cell cycle. Significance of Meiosis In this kind of cell division, there is reduction of the number of chromosomes from diploid to haploid . At the time of fertilization, the diploid number (46) is restored. This provides consistency of chromosome number from generation to generation.

19. MEIOSIS

20. CELL METABOLISM Cellular metabolism is the set of chemical reactions that occur in living organisms in order to maintain life . Cellular metabolism involves complex sequences of controlled biochemical reactions , better known as metabolic pathways . These processes allow organisms to grow and reproduce, maintain their structures, and respond to environmental changes.

21. Enzymes are crucial to metabolism and allow the fine regulation of metabolic pathways to maintain a constant set of conditions in response to changes in the cell's environment, a process known as homeostasis . Cellular metabolism has two distinct divisions: - Anabolism - Catabolism

22. CATABOLIC REACTIONS Catabolism is the metabolic process by which the cell breaks down complex molecules. The purpose of catabolic reactions is to provide the energy and components needed by anabolic reactions. Catabolic reactions are normally exothermic and are further subdivided according to their substrate into carbohydrate, fat, and protein catabolism.

23. ANABOLIC REACTIONS Anabolism is a constructive metabolic process in which a cell uses energy to construct molecules such as enzymes and nucleic acids and perform other essential life functions. Anabolism involves three basic stages: The production of precursors such as amino acids, monosaccharides , isoprenoids and nucleotide. Their activation into reactive forms. The assembly of these precursors into complex molecules.

24. Macro molecules that cannot enter the cell by active or passive transport, such molecules are taken into the cell by a process called “Endocytosis”. The material to be internalized is surrounded by an area of cell membrane, which then buds off inside the cell to form a vesicle containing ingested material. Endocytosis includes pinocytosis and phagocytosis . ENDOCYTOSIS

25. Also called as “ Cell drinking ”. Process is similar to endocytosis, where the vesicles formed are used for absorption of fluid inside the cell. In cellular biology, pinocytosis, otherwise known as  fluid endocytosis  and  bulk-phase pinocytosis , is a mode of endocytosis in which small particles suspended in extracellular fluid are brought into the cell through an  invagination  of the cell membrane, resulting in a suspension of the particles within a small vesicle  inside the cell. These pinocytotic vesicles subsequently fuse with  endosomes  to hydrolyze (break down) the particles. PINOCYTOSIS

26. PHAGOCYTOSIS The process of engulfment of solid particulate material by the cells. Cells performing this function are called as Phagocytes . Two main types of cells- a) Polymorphonuclear neutrophils (PMNs). Also called as microphages. b) Circulating monocytes and fixed tissue mononuclear phagocytes called as macrophages. In a multicellular organism's immune system, phagocytosis is a major mechanism used to remove pathogens and cell debris . The ingested material is then digested in the phagosome . Bacteria, dead tissue cells, and small mineral particles are all examples of objects that may be phagocytized.

27. CELLULAR ADAPTATIONS In cell biology and pathophysiology, cellular adaptation refers to changes made by a cell in response to adverse or varying environmental changes .  The adaptation may be physiologic (normal) or pathologic (abnormal). Five types of morphological adaptations include  Atrophy Hypertrophy Hyperplasia Dysplasia Metaplasia

28. 1. ATROPHY Atrophy is the decrease in the mass of a tissue or organ due to decreased size and/or number of cells after it has reached its normal size. The shrinkage of atrophied tissue is caused by decreased size or loss of its principal cells. The causes of cellular or tissue atrophy include nutrient deprivation or loss of hormonal stimulation, decreased workload (disuse atrophy), denervation (especially in skeletal muscles), and compression (e.g., adjacent to neoplasms, other masses, or distended body cavities). If enough cells in an organ undergo atrophy the entire organ will decrease in size.  Tissue and organs especially susceptible to atrophy include skeletal muscle, cardiac muscle, secondary sex organs, and the brain.

29. 2. HYPERTROPHY Hypertrophy refers to an increase in size and volume of a tissue or organ due to increase in cell size   Hypertrophy may be caused by  mechanical signals (e.g., stretch) or  trophic signals (e.g., growth factors). Hypertrophy may involve an increase in intracellular protein  as well as  cytosol  (intracellular fluid) and other cytoplasmic components . For example, adipocytes (fat cells) may expand in size by depositing more lipid within cytoplasmic  vesciles .

30. 3. HYPERPLASIA Hyperplasia implies an increase in number of the principal cells of a tissue or organ. Any epithelial cells (e.g., hepatocytes and epithelia of the epidermis and intestinal mucosae) are quick to undergo hyperplasia in response to hormonal stimulation, inflammation, or physical trauma . The two types of physiologic hyperplasia are  compensatory  and  hormonal . Compensatory hyperplasia permits tissue and organ regeneration. It is common in epithelial cells of the epidermis and intestine, liver hepatocytes, bone marrow cells, and fibroblasts.

31. 4. METAPLASIA Metaplasia is a change from one differentiated (mature) cell type to another differentiated cell type of the same germline .  When cells are faced with physiological or pathological stresses , they respond by adapting in any of several ways, one of which is metaplasia . Typically, squamous metaplasia is a reparative response to chronic inflammation (e.g., in mammary ducts in chronic mastitis), hormonal imbalance (e.g., estrogen -induced squamous metaplasia in the prostate gland), vitamin A deficiency, or trauma.  If stress persists, metaplasia can progress to dysplasia and eventually carcinoma.

32. 5. DYSPLASIA Dysplasia implies an abnormality in formation of a tissue. Dysplasia implies an increase in the number of poorly differentiated or immature cells and can be a precursor to neoplasia  Microscopically, dysplastic epithelial cells have atypical features, such as abnormal variation in size ( anisocytosis ) and shape ( poikilocytosis ), hyperchromatic nuclei, increased nuclear size ( karyomegaly ), and increased number of mitotic figures. 

33. CELL INJURY Cell injury is a variety of changes of stress that a cell suffers due to external as well as internal envi r onmental changes . Amongst other causes, this can be due to physical, chemical, infectious, biological, nutritional or immunological factors. Cell damage can be reversible or irreversible . Depending on the extent of injury, the cellular response may be adaptive and where possible, homeostasis is restored.  Cell death occurs when the severity of the injury exceeds the cell’s ability to repair itself. Cell death may occur by necrosis or apoptosis .

34. REVERSIBLE CELL INJURY Cell injury is classified as reversible if the injured cell can regain homeostasis and return to a morphologically (and functionally) normal state. The initial response of the cell to perturbation of homeostasis is acute cell swelling . If the injury is not too severe or too prolonged, the cell can recover and return to normal structure and function. Therefore acute cell swelling is, up to a point, a reversible change. It is typical early change of irreversible cell injury.

35. The ultrastructural changes of reversible cell injury include: Blebbing Blunting Distortion of microvilli Loosening of intercellular attachments Mitochondrial changes Dilation of the endoplasmic reticulum.

36. IRREVERSIBLE CELL INJURY Necrosis- necrosis is characterized by cytoplasmic swelling, irreversible damage to the plasma membrane, and organelle breakdown leading to cell death. The stages of cellular necrosis include: - pyknosis : clumping of chromosomes and shrinking of the nucleus of the cell . karyorrhexis : fragmentation of the nucleus and break up of the chromatin into unstructured granules.   karyolitic : dissolution of the cell nucleus.

37. APOPTOSIS Apoptosis is the programmed cell death of superfluous or potentially harmful cells in the body. It is an energy dependent process mediated by proteolytic enzymes called caspases , which trigger cell death through the cleaving of specific proteins in the cytoplasm and nucleus. The dying cells shrink and condense into apoptotic bodies. The cell surface is altered so as to display properties which lead to rapid phagocytosis by macrophages or neighbouring cells

38. CELLS IN PERIODONTIUM The periodontium (peri = around, odontos = tooth) comprises the following tissues: Gingiva Periodontal ligament Root cementum Alveolar bone

39. Cells present in the gingiva Cells in gingival epithelium: Cells in gingival connective tissue Cells in Periodontal ligament Cells in Alveolar bone

40. Cells in gingival epithelium: Keratinocyte, Non Keratinocyte Langerhans cells, Merkel cells, Melanocytes.

41. KERATINOCYTE The principal cell type of gingival epithelium –is the keratinocyte. Keratinocyte make up 90% of the gingival cell population. Keratinocytes are the only cells which contains melanosomes in periodontium. Origin Keratinocytes origin from ectodermal germ layer. FUNCTION Wounds to the skin will be repaired in part by the migration of keratinocytes to fill in the gap created by wound. This cell is a barrier against environmental damage. This can produce pro inflammatory mediators such as chemokines

42. NONKERATINOCYTES These are the cells do not contain the large number of tonofilaments and desmosomes seen in epithelial keratinocytes, and none participates in the process of maturation seen in oral epithelia, there fore, they often called collectively non keratinocytes. Types of non keratinocytes - Langerhans cells, Merkel cells, Melanocytes

43. LANGERHANS CELLS These are the dendritic cells located among non keratinocytes at all supra basal levels. ORIGIN They belong to mononuclear phagocyte system as modified monocytes derived from bone FUNCTION Evidence suggest that Langerhans cells have an immunological function, recognizing and processing antigenic material that enters the epithelium from the external environment and presenting it to T lymphocytes.

44. MERKEL CELL Merkel cells also known as Merkel Ranvier cells are oval shaped cells located in the deeper layers of the epithelium ORIGIN They are derived from neural crest FUNCTION Merkel cell contain dense core granules, thus they have neuroendocrine function. This cell act as a mechanoreceptor.

45. MELANOCYTE Melanocytes are dendritic cells located in the basal and spinouts layers of gingival epithelium. Origin Melanocytes arise embryologically from the neural crest ectoderm and enter the epithelium at about 11 weeks of gestation. FUNCTION Produce melanin pigmentation in gingiva. Protect gingiva from environmental factors such as UV rays.

46. Cells in gingival connective tissue Fibroblast, Mast cells, Inflammatory cells

47. Fibroblasts They are widely distributed between the gingival fibers and is found in perivascular locations. The fibroblasts are typically stellate shaped and have extensive processes that are joined by intercellular junctions to the processes of the other fibroblasts. Origin: Gingival connective tissue fibroblasts develop from perifollicular mesenchyme, a derivative of the stomodeal mesoderm

48. FUNCTIONS of Fibroblasts They secrete collagenase and are active in matrix degradation. During normal development of the periodontium, gingival fibroblasts do not come into contact with the tooth surface. In general, gingival connective tissues have a high potential for regeneration. The collagen of gingival connective tissue turns over more rapidly than that of skin and bone but more slowly than that of the periodontal ligament.

49. DIFERENCE BETWEEN GINGIVAL AND PERIODONTAL FIBROBLAST GINGIVAL FIBROBLAST PERIODONTAL FIBROBLAST Gingival connective tissue fibroblasts develop from perifollicular mesenchyme , a derivative of the stomodeal mesoderm. Fibroblasts of the periodontal ligament originate from the dental follicle (sac), a derivative of the neural crest ecto mesenchyme. During normal development of the periodontium, gingival fibroblasts do not come into contact with the tooth surface. The fibroblasts of the periodontal ligament become juxtaposed to the tooth surface soon after the disruption of the root sheath. They have the capacity to fabricate an attachment matrix (acellular cementum) gingival connective tissues FIBROBLAST have a high potential for regeneration Pdl fibroblast have less potential for regeneration compare to gingival fibroblast Gingival fibroblast have less proliferative rate Pdl fibroblast have greater proliferative rate .

50. MAST CELLS Mast cell (also known as a mastocyte or a labrocyte is a type of white blood cell ORIGIN : Specifically, it is a type of granulocyte derived from the myeloid stem cell that is a part of the immune and neuro immune systems and contains many granules rich in histamine and heparin. Function : The cytoplasmic granules contains histamine which mediates the early stages of inflammation. They also produce heparin which may modulate the rate of bone resorption. Mast cells prevent parasitic infections through IgE signaling

51. INFLAMMATORY CELLS In clinically normal gingiva, small foci of plasma cells and lymphocytes are found in the connective tissue near the base of the sulcus. Neutrophils can be seen in relatively high numbers in both the gingival connective tissue and the sulcus. These inflammatory cells usually are present in small amounts in clinically normal gingiva The infiltrate in the area below the junctional epithelium of healthy gingiva in recently erupted teeth in children is mainly composed of T-lymphocytes (helper, cytotoxic, suppressor and natural killer) As time elapses, B-lymphocytes and plasma cells appear in greater proportions to elaborate specific antibodies against already recognized antigens that are always present in the sulcus of clinically normal gingiva.

52. Cells in Periodontal ligament Fibroblasts Monocytes and Macrophages The epithelial rests of Malassez Cementoblasts

53. Monocytes and macrophages Cells of the monocyte and macrophage lineage are normal inhabitants of the PDL. These cells are typically found in the perivascular and peri neural cuffs of loose connective tissue and are rarely observed to reside in the dense connective tissue of the principal fiber bundles. ORIGIN Monocytes are derived from monoblasts and macrophage derived from multipotent cells of bone marrow known as hematopietic stem cells. Function : Numerous substances, including endotoxin, immune complexes, and Iymphokines , can activate monocytes. Activated monocytes secrete collagenase, elastase, plasminogen activator (PA), and lysosomal hydrolases and are thus highly capable of degrading extracellular matrices. Macrophage plays an important role in wound healing. Macrophage play a key role in removing dead cells and cellular debris.This cells have a crucial role in regulating immune response.

54. The epithelial rests of Malassez Epithelial cell rests of Malassez (ERM) are integrated in the periodontal ligament Epithelial cell rests of Malassez were first described by Augustin Serres . ORIGIN originate from the degeneration of Hertwig’s epithelial root sheath to form quiescent cell rests that persist as the sole epithelial cells in the periodontium. FUNCTION Play a role in maintaining constant the width of the periodontal ligament and their additional Play a role in the regeneration of periodontal tissues. Characterization of the cytokine profile of ERM have brought to light important bone resorbing factors, growth factors, chemokines and related proteins which are directly implicated in the process of bone remodeling..

55. CEMENTOBLAST A cementoblast is a biological cell that forms from the follicular cells around the root of a tooth, and whose biological function is cementogenesis , which is the formation of cementum (hard tissue that covers the tooth root). Cementum covers the roots of teeth and serves to anchor gingival and periodontal fibers of the periodontal ligament by way of Sharpey fibers to the alveolar bone (some types of cementum may also form on the surface of the enamel of the crown at the cementoenamel junction {CEJ}). FUNCTION Cementoblasts lay down the organic matrix of cementum called cementoid which later gets mineralized by minerals from oral fluids. Cementoblasts lay down collagen and secrete osteocalcin and sialoprotein.

56. Cells in alveolar bone Osteoblasts, Osteocytes, Bone-lining cells. Osteoclast

57. OSTEOBLAST This are the cells with a single nucleus that synthesize bone. ORIGIN Osteoblasts arise from mesenchymal stem cells. FUNCTION Osteoblasts are specialized, terminally differentiated products of mesenchymal stem cells. They synthesize dense, crosslinked collagen and specialized proteins in much smaller quantities, including osteocalcin and osteopontin , which compose the organic matrix of bone

58. OSTEOCYTE An osteocytes, a star-shaped type of bone cell, is the most commonly found cell in mature bone tissue, and can live as long as the organism itself. HALF LIFE: Osteocytes have an average half life of 25 years, ORIGIN : osteocytes develop in mesenchymal. They are derived from osteoprogenitor cells. FUNCTION Although osteocytes are relatively inert cells, they are capable of molecular synthesis and modification, as well as transmission of signals over long distances, in a way similar to the nervous system Osteocytes are thought to be mechanosensor cells that control the activity of osteoblasts and osteoclasts within a basic multicellular unit (BMU), a temporary anatomic structure where bone remodeling occurs. Osteocytes generate an inhibitory signal that is passed through their cell processes to osteoblasts for recruitment to enable bone formation. The osteocyte is an important regulator of bone mass and a key endocrine regulator of phosphate metabolism..

59. Bone-lining cells Bone-lining cells extend flat cytoplasmic sheets over the bone surface. It is estimated that 80% of the total bone surface is covered by bone-lining cells. Approximately 20 bone-lining cells line every linear millimeter of resting bone surface. Beneath the bone-lining cell, the osteoid is replaced by a narrow zone of unmineralized connective tissue matrix ORIGIN This cell derived from embryo mesenchyme . FUNCTION Bone-lining cells act as gatekeepers, protecting the bone surface from osteoclasts, regulating the ionic composition of bone fluid, and regulating the initiation of new bone formation. Bone-lining cells can be stimulated to incorporate thymidine, divide and rise to osteoblasts. The osteoprogenitor capacity of bone-lining cells is important in responding to increased strain and in forming a fracture callus during bone repair.

60. Osteoclast An osteoclast is a type of bone cell that breaks down bone tissue. This function is critical in the maintenance, repair, and remodelling of bones of the vertebral skeleton. The osteoclast disassembles and digests the composite of hydrated protein and mineral at a molecular level by secreting acid and a collagenase, a process known as bone resorption. This process also helps regulate the level of blood calcium. FUNCTION Once activated, osteoclasts move to areas of microfracture in the bone by chemotaxis. Osteoclasts lie in a small cavity called Howship's lacunae, formed from the digestion of the underlying bone..

61. REFERENCES Essentials of Biochemistry Pankaja Naik Textbook of Microbiology 5 th edition Dr. C.P. Baveja Textbook of Human Physiology for Dental Students Indu Khurana 2nd Molecular Biology of the Cell. 4th edition . Textbook of Medical Physiology, Guyton. CELLS OF PERIODONTIUM A.H.MELCHER.PERIODONTAL 2000,1993 OCT 39-38 TISSUES AND CELLS OF THE PERIODONTIUM, THOMAS M.HASSELL, OCT 1993; 9-38, PERIODONTAL 2000