Modelado de fuentes

To understand the makeup, function and location of tooth cementum, a brief explanation is necessary. Cementum is one of four tissues that support the tooth in the jaw (periodontium), the others being alveolar bone, the periodontal ligament and gingivae (see Fig. 5.1). Its primary function is to provide an attachment point for the collagen fibers of the periodontal ligament, known as Sharpey’s fibers. The periodontal ligament or membrane, which is composed of dense collagenous tissue, forms a thin fibrous attachment between the tooth root and the alveolar bone and acts as a sling for the tooth within the socket, permitting slight movements which cushion the impact of masticatory forces (Burkitt et al. 1993). Sharpey’s fibers run obliquely downwards from their attachment in the alveolar bone to their anchorage

in the cementum at a more apical position on the root surface. The points of attachment of these collagen fibers in both cementum and bone are in a constant state of reorganization to accommodate changing functional stresses upon the teeth. The alveolar bone itself is stimulated by the mechanical tugging movement of the Sharpey’s fibers exerted upon it during occlusal loading and results in osteoclastic resorption and deposition at different areas of the socket.

Figure 5.1. Longitudinal section of an incisor tooth and its basic anatomical structure (Hildebrand 1995).

This bone stimulation is somewhat akin to the way in which bones of the extremities, especially the points of tendon attachment, react to skeletal muscles during physical work or exercise strain.

Some aspects of tooth cementum are well documented yet, little is known about its origin, differentiation and the cell dynamics of the cementum-forming cell (cementoblast). It is very resistant to resorption, is formed continually throughout a lifetime, a process which allows for the perpetual reattachment of the periodontal ligament, and its heavy deposition at the apex can be viewed as a compensatory physiological response to occlusal wear (Berkovitz 2002). In addition, it is a highly responsive mineralized tissue responsible for maintaining the

integrity of the root, helping to maintain the tooth in its functional position in the mouth, and is involved in tooth repair and regeneration.

According to Burkitt et al. (1993), cementum consists of a dense, calcified organic material similar to the matrix of bone and has no osteons. Berkovitz (2002) indicates further, that cementum is classified according to the nature and origin of the fibrous matrix. If it originates from the periodontal ligament, then it contains extrinsic fibers and if it originates from cementoblasts then it contains intrinsic fibers. In humans, two basic cementum types exist, an acellular and a cellular. However, among these two types a number of varieties have been identified. The acellular form as the name suggests, contains no cells and covers the root adjacent to the dentine. Acellular extrinsic fiber cementum (AEFC) is the first cement formed and found on the cervical two thirds of the root (Fig. 5.2). AEFC layers form slowly, are approximately 15µm thick and well mineralized. Their collagen comes from Sharpey’s fibers. Cellular intrinsic fiber cementum (CIFC) is generally cellular, forms more rapidly than AEFC, has no Sharpey’s fibers, plays no role in the attachment of the root and is found on the apical third of the tooth root. When CIFC forms at a slow rate and cementocytes do not become entrapped then it is called acellular intrinsic fiber cementum (AIFC). The different cementum types AEFC and CIFC can also occur together and be observed in alternating layers. This happens primarily at the root apex and the tissue is referred to as cellular mixed stratified cementum (CMSC). When CMSC is slowly formed it produces a well mineralized acellular variety and is called acellular mixed fiber cementum (AMFC). A more rapidly formed, less mineralized cellular variety is known as cellular mixed fiber cementum (CMFC). Yet another type called afibrillar cementum, a thin acellular form with a well mineralized ground substance is found extending onto the tooth enamel near the cemento-enamel junction. The cellular cementum containing cementocytes, which is found in the apical areas, overlays the acellular cementum to some extent. Histometrically, cementum is thinnest at the cemento- enamel junction (10-15µm), becoming progressively thicker and irregular towards the tooth root apex (50-200µm, although it may exceed 600µm) where cementocytes are often entrapped in spaces called lacunae.

Figure 5.2. Volders burial 57. A photomicrograph showing the histological cross section of a tooth root illustrating the appearance of AEFC. The bright white line running through the center of the image is the cementum-dentin-junction (CDJ), directly below it is the granular layer of Tomes, which in turn is adjacent to the root dentine (premolar, 70µm, 20x).

Cementum contains on a wet-weight basis 65% inorganic material, 23% organic material and 12% water. Thin plate-like hydroxyapatite crystals averaging 55nm wide and 8nm thick compose the main portion of inorganic components. Collagen, virtually all of which is Type I, a type found mainly in fibrous supporting tissue with a biochemical composition that confer it with great tensile strength, makes up the organic matrix (Berkovitz 2002).

Cementum is deposited in an irregular rhythm resulting in the unevenly spaced incremental lines of Salter (Berkovitz 2002). In acellular cementum, these lines appear thin and evenly spaced. In the more rapidly forming cellular cementum, the incremental lines are irregular, thicker and farther apart. When viewed under a light microscope, a cross section of these incremental lines surrounding a tooth root appear as layers of alternating dark and light bands (see Fig. 5.3). Differences in the degree to which each of the two layers are mineralized results in this observable structural characteristic. Environmental and physiological factors apparently both influence cementogenesis and an interplay between parathyroid hormone, calcitonin and vitamin D is hypothesized to be the reason for the differences in mineralization. Parathyroid hormone, also called parathormone, is synthesized and released by the parathyroid gland and regulates the distribution of calcium and phosphate in the body (Müller 2004). Vitamin D, a sterol with hormone-like functions, maintains adequate plasma levels of calcium by enhancing the absorption of calcium and phosphorus from the intestine,

minimizing the loss of calcium by the kidney, and stimulating their resorption from bone or promoting their deposition in bone. The endogenous precursor of vitamin D, cholecalciferol (vitamin D3), is converted from 7-dehydrocholesterol in the dermis and epidermis through the

action of ultraviolet light during sunlight exposure (Champe et al. 1994). The intensity of UVB rays corresponds to a seasonal cycle, which in turn is thought to be reflected in the degree of mineralization seen in the incremental lines. To reiterate, a singular incremental line, consisting of one light and one dark band, are thought to represent one calendar year. Other microscopic structures adjacent to the cementum are also visible in the cross section. The cementum-dentin-junction (CDJ) also called the hyaline layer of Hopewell-Smith or intermediate cementum, is observable between the cementum and the granular layer of Tomes (see Fig. 5.2), and is the point of attachment between cementum and dentin (Ho et al. 2004). The granular layer of Tomes is an imperfectly calcified tissue made of small interglobular spaces that give it a granular appearance and is found at the peripheral border of the mantle dentin (Ten Cate 1972).

Figure 5.3. Burial 144. Photomicrograph of a tooth root cross section. Root dentin at the lower portion of the image and the incremental lines composing the cementum adhering to it. The dark black line above the granular layer of Tomes (arrow) is probably afibrillar cementum.

The incremental lines exhibited in the acellular extrinsic fiber cementum (AEFC), the layer on the cervical two thirds of the tooth root whose deposition is presumed to follow a seasonal rhythm, are the focus of interest here and the object of scrutiny in this TCA analysis.