Enamel

Stria of Retzius

• During development of enamel, variations in the metabolism of the organism cause variations in the amount of organic material deposited in the enamel. This causes changes in the coloration of the enamel that is layed down at that time so that alternating dark (higher organic material) and light (less organic material) banding occurs. These bands are called Stria of Retzius. The striaof retzius usually intercept the dentino-enamel junction.

Legend: A, Stria of Retzius; B, DEJ

Enamel - transverse ground section

Legend: A, Stria of Retzius; B, Enamel tuft; C, Enamel lamella; D, DEJ

• In a transverse section of tooth, the stria of Retzius appear as concentric bands parallel to the dentino-enamel junction (DEJ). In addition to the "hypo-mineralized" dark stria of Retzius, there also exist hypo-mineralized areas perpendicular to the DEJ. These are enamel lamellae (that traverse the entire thickness of enamel) and enamel tufts (that traverse the inner third of enamel adjacent to the DEJ.

Neonatal line

• The neonatal line is a dark striaof Retzius that occurs at the time of birth. It is due to the stress of birth. The neonatal line is usually the darkest and thickest stria of Retzius. The enamel at the cusp of the tooth generally exhibits a wavy pattern. This enamel is called gnarled enamel. This is NOT hypo-mineralized. The enamel rods are layed down in this pattern by ameloblasts to make the enamel strong in this region.

Legend: A, Gnarled enamel; B, Neonatal line; C, Dentin; D, DEJ

Straight enamel rods - longitudinal labiolingual section

• The enamel rods project in the direction of the arrow.

• Can you see the stria of Retzius?

Gnarled enamel

• Enamel rods are general not straight throughout their length.

• In the cuspal region, the rods are very wavy.

• This is referred to as gnarled enamel.

• In this section, you can see the end of an odontoblasticprocess penetrating the enamel just past the DEJ.

• This structure is called an enamel spindle.

Legend: A, Gnarled enamel; B, Enamel spindle

Cross-striations

• Each enamel rod demonstates closely positioned striations along its length known as cross-striations or incremental lines.

• These are thought to be formed by the daily rhythm of the ameloblast laying down more and less mineralized enamel.

• The striations are approximately 5 µm apart. This distance represents one day of enamel deposition. In this micrograph rods project in the direction of the arrow.

• Can you see the striations on each rod?They are oriented perpendicular to the length of the rod.

• Note: Not the large dark bands projecting diagonally; these are stria of Retzius.

Enamel cut

• In enamel cut in perfect cross-section the shape of the enamel rod exhibits a "keyhole"-shaped pattern. However, in a normal cross-section of enamel, as seen here, most rods are cut obliquely.

• This is because they do not travel in a straight line.

• The micrograph on the left is produced by differential interference microscopy while the micrograph on the right is from transmitted light microscopy.

Enamel rods sectioned longitudinally

• In this electron micrograph enamel rods are cut longitudinally (in parallel with their long axis). The ligher band represents the inner part of the rod or rod core, while the darker part represents the outer covering or rod sheath. This difference in appearance is caused by the packing density of the rod hydroxyapatite crystals. In the core the crystals are packed in parallel with one another and tightly together (very mineralized), whereas in the rod sheath, the crystals are losslypacked and are oriented at variable angles to one another with organic material interspersed (less mineralized). In this particular section, the cores are narrow and the sheaths wide, signifying that this area is taken from a dark cross-striation or stria of Retzius.

• Note that the rod sheaths are shared by two adjacent rods.

Legend: A, Rod sheath; B, Rod core

Enamel rods sectioned in cross-section

• In this electron micrograph enamel rods are cut perpendicular to their long axis. The ligher areas are the rod cores in which hydroxyapatite crystals are tightly packed in alignment with each other. The darker areas surrounding the rod cores are the rod sheaths in which the crystals are loosely packed at various angles. There are two main parts to a rod: the rod head and rod tail. The head has the central core (light area), and is sometimes referred to as the "rod". The tail is made of the rod sheath (less mineralized enamel). During development, one ameloblast (in position 1 in the inset diagram) makes the rod core for the rod at position 1, while three other ameloblasts (in positions 2, 3 and 4) produce the rod tail of rod 1. The tail is located between 2 and 3 and above 4.

Legend: A, Rod core; B, Rod sheath; C, Rod tail; D, Rod head

Box diagram of human enamel

• This diagram represents a 25 X 25 X 25 µm of enamel.

• It demonstrates the arrangements of hydroxyapatite crystals in the enamel rods in three planes of section.

• One rod is highlighted in blue to demonstrate the typical human rod shape.

• In the rod core, hydroxyapatitecrystals are aligned with the long axis of the rod.

• In the tail the crystals are aligned diagonally or perpendicularly to the long axis of the rod.

Alternating rod directionality

• Hunter Schreger bands are alternating light and dark bands seen in a section of enamel when cut longitudinally and illuminated in a special way.

• The bands are produced by the orientation of groups of rods.

• If the light passes through rods cut in cross-section, the band appears light.

• If the light passes through rods cut in longitudinally, the band appears dark.

Legend: A, Rods cut longitudinally; B, Rods cut cross-sectionally

Hunter-Schreger bands

• Hunter Schreger bands are seen here with special illumination in longitudinal ground sections of enamel as light and dark bands.

• The red arrows indicate three light bands.

Enamel tufts

• Enamel tufts are less mineralized areas of enamel in the inner third of enamel adjacent to the DEJ. They resemble tufts of grass.

• They are wavy due to the waviness of the adjacent rods.

• Structures rich in organic matter (i.e. less mineralized) that project to the surface of the enamel are enamel lamellae.

Legend: A, Enamel tufts; B, Enamel lamella

Enamel tufts - two planes of focus

• Enamel tufts consist of several unconnected "leaves" of hypo-calcified enamel.

• They display a wavy twisted appearance.

• Enamel spindles are the processes of odontoblasts projecting into the enamel. Legend: A, Enamel spindle;

B, Enamel tuft

Enamel tufts aligned in rows

• Enamel tufts are aligned in rows.

• They may represent planes of tension during development.

Legend: A, Enamel tufts; B, DEJ; C, Dentin

Enamel lamellae

• In this ground cross-section of tooth, you can see enamel lamellae and enamel tufts You can also see the neonatal line.

• What do all three of these structures have in common?

Answer: They are all hypocalcified. Legend: A, Enamel lamella; B, Enamel tuft;

C, Neonatal line

Decalcified tooth

• In a decalcified section of tooth, only the organic material is left behind.

• In this micrograph you can see an enamel lamella and enamel tufts.

Legend: A, Enamel lamella; B, Enamel tuft

Odontoblast process

• Odontoblast processes usually end at the DEJ. However, sometimes the ends of the process become embedded in the enamel as it forms.

• These very small, usually straight structures that you can see adjacent to the DEJ are enamel spindles.

• They are only about one tenth the length of an enamel tuft.

Legend: A, Enamel spindle; B, Odontoblastprocesses in dentin

Enamel spindles

• In this high magnification of the DEJ you can clearly see the bifurcation of the ends of some of the odontoblastprocesses as well as enamel spindles.

Legend: A, Enamel spindle; B, Odontoblast process; C, Enamel rod

This slide from:Sharifa Abdulla Alhaj

Good Luck All

Done by: Bedour Al-Arfaj.

...دعيه قبل المذاكره وبعدهاأ

قبل المذاكرة

اللهم أنً اسألك فهم النبٌٌن و حفظ المرسلٌن و المالئكة المقربٌن ، اللهم أجعل ألسنتنا عامرة بذكرك و قلوبنا بخشٌتك و أسرارنا حسبنا هللا و نعم الوكٌل .. بطاعتك أنك على كل شًء قدٌر

بعد المذاكرة

اللهم أنً استودعتك ما قرأت و ما حفظت و ما تعلمت فرده عند حاجتً الٌه انك على كل شًء قدٌر ، حسبنا هللا و نعم الوكٌل

يوم اإلمتحان

اللهم أنً توكلت علٌك و سلمت امري الٌك ال ملجأ و منجا منك إال الٌك

دخول القاعة

رب أدخلنً مدخل صدق و أخرجنً مخرج صدق و أجعل لً من لدنك سلطانا نصٌرا

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أثناء األمتحان

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عند النسيان

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بعد األنتهاء

الحمد هلل الذي هدانا لهذا و ما كنا لنهتدي لوال أن هدانا هللا