Note on Anatomy and Microstructure of Periodontium

Anatomy and microstructure of periodontium

This includes a brief description of the characteristics of  the normal periodontium. It  is assumed that the reader has prior knowledge of oral embryology  and  histology.

The  periodontium  (pert = around, odontos = tooth) comprises the following tissues

(1) Gingiva (G)

(2) Periodontal  ligament (PL)

(3) Root cementum (RC)

(4) Alveolar bone(AB)- The  alveolar  bone  consists  of  two  components

                 Alveolar  bone  proper     

                 Alveolar process

The  main  function  of  the  periodontium  is  to  attach the tooth to the bone tissue of the jaws and to maintain the integrity of the surface of the masticatory mucosa of  the  oral  cavity.

The  development  of  the  periodontal  tissues  occurs during  the  development  and  formation  of  teeth.  This process starts early in the embryonic phase when cells from  the  neural  crest  (from  the  neural  tube  of  the embryo)  migrate  into  the  first  branchial  arch. During  the  cap  stage, condensation  of ectomesenchymal  cells  appears  in  relation  to  the dental  epithelium  (the  dental  organ  (DO)),  forming  the dental  papilla  (DP) that gives rise to the dentin and the pulp,  and  the  dental  follicle  (DF)  that  gives  rise  to  the periodontal supporting tissues.

The  development  of  the  root  and  the  periodontal supporting  tissues  follows  that  of  the  crown.  Epithelial cells of the external and internal dental epithelium  (the  dental  organ)  proliferate  in  apical direction  forming  a  double  layer  of  cells  named Hertwig's  epithelial  root  sheath  (RS).  The  odontoblasts  (OB)  forming  the  dentin  of  the  root  differentiate  from ectomesenchymal cells in the dental papilla under inductive influence of the inner epithelial cells.

GINGIVA

Macroscopic anatomy

The  gingiva  is  that  part  of  the  masticatory mucosa  which  covers  the  alveolar  process  and  sur-rounds the cervical portion of the teeth. It consists of an  epithelial  layer  and  an  underlying  connective  tis-sue layer called the lamina  propria.  The gingiva obtains its  final  shape  and  texture  in  conjunction  with  eruption of the teeth.

In  the  coronal  direction  the  coral  pink  gingiva  terminates  in  the  free  gingival  margin,  which  has  a  scalloped  outline.  In  the  apical  direction  the  gingiva  is continuous with the loose, darker red alveolar mucosa (lining  mucosa)  from  which  the  gingiva  is  separated by a, usually, easily recognizable borderline called either the mucogingival junction or the mucogingival line. There  is  no  mucogingival  line  present  in  the palate  since  the  hard  palate  and  the  maxillary  alveolar process  are  covered  by  the  same  type  of  masticatory mucosa.

Two parts of the gingiva can be differentiated:

1. the free gingiva (FG)

2. the attached gingiva (AG)

On  the vestibular  and  lingual  side  of  the  teeth,  the  free gingiva  extends  from  the  gingival  margin  in  apical direction  to  the  free  gingival  groove  which  is  positioned at  a  level  corresponding  to  the  level  of  the  cemento-enamel  junction  (CEJ). The attached gingiva is in apical direction  demarcated  by  the  mucogingival  junction  (MGJ).

The attached gingiva extends in the apical direction to  the  mucogingival  junction  (arrows),  where  it  be-comes continuous with the alveolar (lining) mucosa (AM).  It  is  of  firm  texture,  coral  pink  in  color,  and often  shows  small  depressions  on  the  surface.  The depressions, named "stippling", give the appearance of orange peel.

Microscopic anatomy

The free gingiva comprises all epithelial and connective tissue structures (CT) located coronal to a horizontal  line  placed  at  the  level  of  the  cemento-enamel  junction  (CEJ).  The  epithelium  covering  the free gingiva may be differentiated as follows:

 Oral epithelium (OE),  which faces the oral cavity

 Oral sulcular epithelium (OSE), which faces the tooth without being in contact with the tooth surface

 Junctional epithelium (JE), which provides the contact between the gingiva and the tooth.

The boundary between the oral epithelium (OE)  and  the  underlying  connective  tissue  (CT)  has  a wavy  course.  The  connective  tissue  portions  which project  into  the  epithelium  are  called  connective  tissue  papillae (CTP) and are separated from each other by epithelial ridges — so-called rete pegs (ER). In normal non-inflamed gingiva, rete pegs and connective tissue papillae are lacking at the boundary between the junctional  epithelium  and  its  underlying  connective  tissue.

Portion  of  the  oral  epithelium  covering the free gingival is  a  keratinized,  stratified,  squamous epithelium  which, on the basis of the degree to which the  keratin-producing  cells  are  differentiated,  can  be divided into the following cell layers:

1. basal layer (stratum basale or stratum germinativum)

2. prickle cell layer (stratum spinosum)

3. granular cell layer (stratum granulosum)

4. keratinized cell layer (stratum corneum)

PERIODONTAL LIGAMENT

The  periodontal  ligament  is  the  soft,  richly  vascular and  cellular  connective  tissue  which  surrounds  the roots  of  the  teeth  and  joins  the  root  cementum  with socket  wall.  In  the  coronal  direction,  the  periodontal ligament is continuous with the lamina propria of the gingiva  and  is  demarcated  from  the  gingiva  by  the collagen  fiber  bundles  which  connect  the  alveolar bone crest with the root (the alveolar crest fibers).

In  radiographs,  two  types  of  alveolarbone can be distinguished:

1. The part of the alveolar bone which covers the alveolus, called "lamina dura" (arrows).

2.  The  portion  of  the  alveolar  process  which,  in  the radiograph,  has  the  appearance  of  a  meshwork. This is called the "spongy bone".

The periodontal ligament is situated in the space be-tween the roots (R) of the teeth and the lamina dura or the alveolar bone proper (arrows). The alveolar bone (AB)  surrounds  the  tooth  to  a  level  approximately  1mm apical to the cemento-enamel junction (CEJ). The coronal border of the bone is called the alveolar crest (arrows).

The width of the periodontal ligament is approximately 0.25  mm  (range  0.2-0.4  mm).  The  presence  of  a  peri-odontal ligament permits forces, elicited during mas-ticatory  function  and  other  tooth  contacts,  to  be  dis-tributed  to  and  resorbed  by  the  alveolar  process  via the  alveolar  bone  proper.  The  periodontal  ligament  is also essential for the mobility of the teeth. Tooth mo-bility  is  to  a  large  extent  determined  by  the  width, height  and  quality  of  the  periodontal  ligament.

The tooth is joined to the bone by bundles of collagen fibers which  can  be  divided  into  the  following  main  groups according to their arrangement:

1. alveolar crest fibers (ACF)

2. horizontal fibers (HF)

3. oblique fibers (OF)

4. apical fibers (APF)

ROOT CEMENTUM

The cementum is a specialized mineralized tissue covering  the  root  surfaces  and,  occasionally,  small  portions of the crown of the teeth. It has many features in common  with  bone  tissue.  However,  the  cementum contains  no  blood  or  lymph  vessels,  has  no  innervation,  does  not  undergo  physiologic  resorption  or  remodeling,  but  is  characterized  by  continuing  deposition  throughout  life.  Like  other  mineralized  tissues,  it contains  collagen  fibers  embedded  in  an  organic  matrix.  Its  mineral  content,  which  is  mainly  hydroxyapatite, is about 65% by weight; a little more than that of bone  (i.e.  60%).  Cementum  serves  different  functions. It  attaches  the  periodontal  ligament  fibers  to  the  root and  contributes  to  the  process  of  repair  after  damage to the root surface.

Different forms of cementum have been described:

Acellular, extrinsic fiber cementum (AEFC)- is found in the  coronal  and  middle  portions  of  the  root  and contains  mainly  bundles  of  Sharpey's  fibers.  This type  of  cementum  is  an  important  part  of  the  attachment apparatus and connects the tooth with the alveolar bone proper.

Cellular,  mixed  stratified  cementum (CMSC)- occurs in the apical third of the roots and in the furcations. It contains  both  extrinsic  and  intrinsic  fibers  as  well as cementocytes.

Cellular,  intrinsic  fiber  cementum  (CIFC)-  is  found mainly  in  resorption  lacunae  and  it  contains  intrinsic fibers and cementocytes.

ALVEOLAR BONE

The alveolar process is defined as the parts of the maxilla and the mandible that form and support the sockets of the teeth. The alveolar process develops in conjunction  with  the  development  and  eruption  of  the teeth.  The  alveolar  process  consists  of  bone  which  is formed  both  by  cells  from  the  dental  follicle  (alveolar bone  proper)  and  cells  which  are  independent  of  tooth development.  Together  with  the  root  cementum  and the  periodontal  membrane,  the  alveolar  bone  consti-tutes  the  attachment  apparatus  of  the  teeth,  the  main function  of  which  is  to  distribute  and  resorb forces generated by, for example, mastication and other tooth contacts.

The  walls  of  the sockets are lined by cortical  bone  (arrows), and the area between  the  sockets  and  between  the  compact  jaw bone walls is occupied by cancellous  bone.  The cancel-lous  bone  occupies  most  of  the  interdental  septa  but only a relatively small portion of the buccal and pala-tal  bone  plates.  The  cancellous  bone  contains  bone trabeculae,  the architecture and size of which are partly genetically  determined  and  partly  the  result  of  the forces  to  which  the  teeth  are  exposed  during  function. Note  how  the  bone  on  the  buccal  and  palatal  aspects of  the  alveolar  process  varies  in  thickness  from  one region to another. The bone plate is thick at the palatal aspect  and  on  the  buccal  aspect  of  the  molars  but  thin in the buccal anterior region.

BLOOD SUPPLY OF THE PERIODONTIUM

fig

The dental  artery  (a.d.),  which  is  a  branch  of  the  superior  or inferior  alveolar  artery  (a.a.i.),  dismisses  the  intraseptal artery  (a.i.)  before it enters the tooth socket. The terminal branches of the intraseptal artery (rami perforantes, rr.p.)  penetrate  the  alveolar  bone  proper  in  canals  at  all levels of the socket (see Fig. 1-76). They anastomose in the  periodontal  ligament  space,  together  with  blood vessels  originating  from  the  apical  portion  of  the  periodontal  ligament  and  with  other  terminal  branches, from  the  intraseptal  artery  (a.i.).  Before  the  dental artery  (a.d.)  enters  the  root  canal  it  puts  out  branches which  supply  the  apical  portion  of  the  periodontal ligament.

The gingiva receives its blood supply mainlythrough  supraperiosteal  blood  vessels  which  are  terminal  branches  of  the  sublingual  artery  (a.s.),  the  mental artery (a.m.), the buccal artery (a.b.), the facial artery (a.f.), the greater palatine artery (a.p.), the infra orbital artery (a.i.) and the posterior superior dental artery(a.ap.).

LYMPHATIC SYSTEM OF THE PERIODONTIUM

The  smallest  lymph  vessels,  the  lymph  capillaries,  form  an  extensive  network  in  the  connective tissue. The  wall  of  the  lymph  capillary  consists  of  a single  layer  of  endothelial  cells.  For  this  reason  such capillaries  are  difficult  to  identify  in  an  ordinary  histologic section. The lymph is absorbed from the tissue fluid  through  the  thin  walls  into  the  lymph  capillaries. From  the  capillaries,  the  lymph  passes  into  larger lymph vessels which are often in the vicinity of corresponding  blood  vessels.  Before  the  lymph  enters  the blood  stream  it  passes  through  one  or  more  lymph nodes  in  which  the  lymph  becomes  filtered  and  supplied  with  lymphocytes.  The  lymph  vessels  are  like veins provided with valves. The lymph from the periodontal  tissues  is  drained  to  the  lymph  nodes  of  the head  and  the  neck.  The  labial  and  lingual  gingiva  of the  mandibular  incisor  region  is  drained  to  the  submental  lymph  nodes  (sme).  The  palatal  gingiva  of  the maxilla  is  drained  to  the  deep  cervical  lymph  nodes  (cp). The buccal gingiva of the maxilla and the buccal and lingual  gingiva  in  the  mandibular  premolar-molar region  are  drained  to  submandibular  lymph  nodes  (sma).

Except  for  the  third  molars  and  mandibular  incisors, all  teeth  with  their  adjacent  periodontal  tissues  are drained to the submandibular lymph nodes (sma). The  third  molars  are  drained  to  the  jugulodigastric lymph  node  (jd)  and  the  mandibular  incisors  to  the submental lymph nodes (sme).

NERVES OF THE PERIODONTIUM

Like other tissues in the body, the periodontium contains receptors which record pain, touch and pressure (nociceptors  and  mechanoreceptors).  In  addition  to  the different  types  of  sensory  receptors,  nerve  compo-nents  are  found  innervating  the  blood  vessels  of  the periodontium.  Nerves  recording  pain,  touch,  and pressure  have  their  trophic  center  in  the  semilunar ganglion  and  are  brought  to  the  periodontium  via  the trigeminal  nerve  and  its  end  branches.  Owing  to  the presence  of  receptors  in  the  periodontal  ligament, small  forces  applied  on  the  teeth  may  be  identified. For  example,  the  presence  of  a  very  thin  (10-30  µm) metal  foil  (strip)  placed  between  the  teeth  during occlusion  can  readily  be  identified.  It  is  also  well known  that  a  movement  which  brings  the  teeth  of  the mandible in contact with the occlusal surfaces of the maxillary  teeth  is  arrested  reflexively  and  altered  into an  opening  movement  if  a  hard  object  is  detected  in the chew. Thus, the receptors in the periodontal liga-ment, together with the proprioceptors in muscles and tendons,  play  an  essential  role  in  the  regulation  of chewing movements and chewing forces.