Page 207 - Veterinary Histology of Domestic Mammals and Birds, 5th Edition

Digestive system (apparatus digestorius) 189

dense and is rich in collagen fibres. The outermost portion Other cells present in the periodontal ligament include
VetBooks.ir of dentin, known as mantle dentin, is separated from the cementoblasts, cementocytes, osteoblasts and osteoclasts.
overlying enamel by a basal lamina (formerly membrana The periodontal ligament is vascularised and contains free
nerve endings that serve as pressure and pain sensors.
praeformativa).
Dental pulp (pulpa coronalis) ALVEOLAR BONE
The dental pulp lies within the pulp cavity. Its original Alveolar bone, to which the tooth is connected by the peri-
mesenchymal characteristics are retained into adulthood odontal ligament (Figure 10.12), is composed of lamellar
(Figure 10.12). The constituents of dental pulp include bone that is perforated to facilitate the passage of blood
loose connective tissue, blood vessels and nerve fibre vessels and nerves to and from the dental pulp.
bundles. Lymphatic vessels are absent. Myelinated and
unmyelinated nerve fibres extend along the processes of Salivary glands (glandulae oris)
odontoblasts into the dentinal tubules. The structural core The salivary glands develop from cordlike down-growths
of the dental pulp comprises a delicate network of type of the epithelium of the oral mucosa into the underlying
III collagen fibres in which fibroblasts and reticular cells tela submucosa. Their ends branch to form compound
are embedded. Odontoblasts form a layer at the periphery. tubulo-acinar secretory organs. Secondary lumen forma-
Abundant amorphous ground substance is also present. tion gives rise to the excretory system of ducts through
The odontoblasts in the outer dental pulp produce den- which the secretions of the salivary glands are conveyed
tin throughout the life of the organism. The pulp cavity to the mucosal surface.
thus becomes smaller with age, with concomitant reduc- The secretory product, saliva, performs various func-
tion in metabolic exchange. tions including lubrication of the non-glandular oral
Towards its proximal end, the pulp cavity narrows to mucosa and foodstuffs during mastication, thus aiding the
form the root canal (canalis radicis dentis), which ends swallowing of solids. Dissolution of water-soluble nutri-
in the apical foramen (foramen apicale dentis) (Figure ents in saliva facilitates gustation and sensory investigation
10.12). Nerves and blood vessels enter and leave the pulp of food. The large volumes of alkaline saliva produced by
cavity through this opening. ruminants (approximately 90–180 l per day in cattle) assist
in neutralising short-chain fatty acids produced by micro-
Attachment apparatus of the teeth bial fermentation in the rumen. In the pig, the saliva is
The tissues involved in the attachment of the teeth are the: rich in amylase, an enzyme involved in the breakdown of
carbohydrate.
· cementum, Saliva contains immunoglobulin A and lactoperoxidase,
· periodontal ligament and and thus also has a role in immune defence.
· alveolar bone. Secretion of saliva is under neural (sympathetic,
parasympathetic) and hormonal control. Sympathetic
CEMENTUM stimulation produces saliva rich in organic compounds,
Cementum (Figure 10.12) forms by appositional growth on while the saliva secreted under parasympathetic stimula-
the outer surface of the root of brachydont teeth, and over tion (cranial nerves VII, IX and X) is watery and thin.
the entire length of hypsodont teeth. Resembling bone in In addition, production of saliva is influenced by the
its structure, cementum is composed of cementocytes smell, taste and composition of food, and by the process
(cementocyti) and mineralised organic matrix. Near of swallowing. In the dog, dry food induces the synthe-
the enamel margin, the cementum is acellular. Collagen sis of watery saliva whereas moist food promotes a more
fibres extend from the alveolar bone into the cementum viscous secretion.
(Sharpey’s fibres, see below).
Salivary gland structure
PERIODONTAL LIGAMENT Based on their size and location, the salivary glands are
The periodontal ligament consists predominantly of col- divided into the minor salivary glands (glandulae sali-
lagen fibres (Sharpey’s fibres) that traverse the space variae minores) and major salivary glands (glandulae
between the alveolar bone and the cementum (Figure salivariae majores). Located within the tela submucosa
10.12). The orientation of fibre bundles varies in different of the oral cavity, the minor salivary glands comprise the
portions of the ligament. The fibres are firmly bound to glands of the lips, cheeks, tongue and palate. These are
the tooth, anchoring it to the alveolus and aiding in the included in the description of the individual components
absorption of compressive forces. of the oral cavity.
In addition to collagen fibres, the periodontal ligament The major salivary glands are connected to the oral
contains fibroblasts that replenish its collagen content. cavity by a system of excretory ducts. They include the:

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