Hunter-Schreger bands, commonly abbreviated as HSB, are features of the
enamel of the
teeth
A tooth ( : teeth) is a hard, calcified structure found in the jaws (or mouths) of many vertebrates and used to break down food. Some animals, particularly carnivores and omnivores, also use teeth to help with capturing or wounding prey, t ...
in
mammals, mostly
placental
Placental mammals (infraclass Placentalia ) are one of the three extant subdivisions of the class Mammalia, the other two being Monotremata and Marsupialia. Placentalia contains the vast majority of extant mammals, which are partly distinguishe ...
s.
[Line and Bergqvist, 2005, p. 924] In HSB,
enamel prism
An enamel prism, or enamel rod, is the basic unit of tooth enamel. Measuring 3-6 μm in diameter, enamel prism are tightly packed hydroxyapatite crystals structures. The hydroxyapatite crystals are hexagonal in shape, providing rigidity to the ...
s are arranged in layers of varying thickness at about right angles to each other. HSB strengthen the enamel and prevent cracks from propagating through the tooth.
[Martin, 1994, p. 121]
HSB are first observed in early
Paleocene
The Paleocene, ( ) or Palaeocene, is a geological epoch that lasted from about 66 to 56 million years ago (mya). It is the first epoch of the Paleogene Period in the modern Cenozoic Era. The name is a combination of the Ancient Greek ''pal ...
mammals, but at this time the HSB occupy only a small portion of the incisor and the angle between the bands is low. By the late Paleocene, HSB is seen to extend throughout the enamel and the bands are located at nearly right angles to each other.
Under oblique reflected light HSB can be seen as dark and light strips of variable width.
Among Glires
Glires (, Latin ''glīrēs'' 'dormice') is a clade (sometimes ranked as a grandorder) consisting of rodents and lagomorphs ( rabbits, hares, and pikas). The hypothesis that these form a monophyletic group has been long debated based on morp ...
, the group containing rodent
Rodents (from Latin , 'to gnaw') are mammals of the order Rodentia (), which are characterized by a single pair of continuously growing incisors in each of the upper and lower jaws. About 40% of all mammal species are rodents. They are n ...
s, lagomorphs, and their primitive relatives, the absence of HSB from the incisor
Incisors (from Latin ''incidere'', "to cut") are the front teeth present in most mammals. They are located in the premaxilla above and on the mandible below. Humans have a total of eight (two on each side, top and bottom). Opossums have 18, wher ...
s has been considered primitive.[Martin, 2004, p. 411] Some early representatives, including '' Eurymylus'', lack HSB, but others, including '' Matutinia''[Martin, 2004, p. 417] and some mimotonids, have double-layered incisor enamel with HSB in the inner portion (portio interna, PI). Other mimotonids have single-layered enamel with HSB. All leporids studied also exhibit this pattern, except for an early Eocene
The Eocene ( ) Epoch is a geological epoch that lasted from about 56 to 33.9 million years ago (mya). It is the second epoch of the Paleogene Period in the modern Cenozoic Era. The name ''Eocene'' comes from the Ancient Greek (''ēṓs'', " ...
, indeterminate leporid with HSB only in the PI.[Martin, 2004, p. 418] Ochotonids also have HSB in the PI only.[Martin, 2004, p. 419]
In rodents, HSB are usually present in the PI. Three types of HSB can be separated—pauciserial, uniserial, and multiserial. Pauciserial HSB, present in some primitive Paleogene rodents, are usually three to six prisms thick and have inter-prismatic matrix (IPM) surrounding the prisms, which have irregular cross-sections. Uniserial HSB, present in most living rodents, consist of a single layer of prisms. Multiserial HSB, three to seven prisms thick, characterize the living Hystricognathi
The Hystricognathi are an infraorder of rodents, distinguished from other rodents by the bone structure of their skull
The skull is a bone protective cavity for the brain. The skull is composed of four types of bone i.e., cranial bones, fa ...
and the gundis (Ctenodactylidae) and springhare
''Pedetes'' is a genus of rodent, the springhares, in the family Pedetidae. Members of the genus are distributed across southern and Eastern Africa.
Species
A number of species both extant and extinct are classified in the genus ''Pedetes''. ...
s (''Pedetes'').[ There are three subtypes on the basis of the orientation of the IPM. In the first, the IPM runs mostly parallel to the HSB, but does not surround it as in pauciserial enamel; in the second, the IPM makes an angle of about 45° with the HSB; and in the third, the two are located at right angles to each other. The last type serves to further strengthen the enamel and is characteristic of the Octodontoidea.][Martin, 1994, pp. 121–122]
Among carnivorans, the hyenas exhibit specialized HSB, which form a complex, three-dimensional zigzag pattern, a specialization for bone-eating.[Barycka, 2007, p. 276]
In humans, their average width is 50 micrometers.
References
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Literature cited
*Barycka, E. 2007
Evolution and systematics of the feliform Carnivora
(subscription required). Mammalian Biology 72(5):257–282.
*Line, S.R.P. & Bergqvist, L.P. 2005
Enamel structure of Paleocene mammals of the São José de Itaboraí Basin, Brazil. 'Condylarthra', Litopterna, Notoungulata, Xenungulata, and Astrapotheria
(subscription required). Journal of Vertebrate Paleontology 25(4):924–928.
*Martin, T. 1994
On the systematic position of ''Chaetomys subspinosus'' (Rodentia: Caviomorpha) based on evidence from the incisor enamel microstructure
(subscription required). Journal of Mammalian Evolution 2(2):117–131.
*Martin, T. 2004
Evolution of incisor enamel microstructure in Lagomorpha
(subscription required). Journal of Vertebrate Paleontology 24(2):411–426.
Mammal anatomy