Elephants are characterized by a highly pneumatic skull with a premaxilla that descends vertically, bearing the roots of the paired tusks, which represent enormously enlarged maxillary incisors. The growth of the tusk is continuous throughout life and its size at any age is dependent on the sex of the animal, the rate of attrition and breakage of the tooth, as well as genetic and environmental factors. The record length of a single tusk is 3.51 m and record weight 117 kg. The bulk of the tusk consists of ivory (or dentine), ensheathed by a layer of cementum.

The unique chequered pattern of polished ivory, which has not been described in great detail in the literature, has made it a sought-after product in the manufacturing of jewellery and other works of art. Our purpose now was to propose a hypothesis on the histogenesis of the unique chequered pattern of elephant ivory.

Twenty fragments of ivory and five tusks of African elephant (Loxodonta Africana) and a developing tusk of a 22-month fetus were obtained through the National Parks Board of South Africa. The ivory was harvested from animals that had died as a result of natural causes or as part of the population-control programme of the National Parks Board. The fetal tissue was obtained within 20 min of death. The biopsies were taken with a sharp scalpel from the peripulpal ivory of the proximal part of the tusk, cut in thin slices and then fixed in 10% phosphate-buffered formalin at 21°C.

The five tusks, with masses between 0.7 and 9.3 kg, were sectioned and polished in sagittal and coronal planes. The morphological characteristics of the chequered pattern were described on polished surfaces prepared in both planes. Irregularities affecting the pattern were noted.

Ground sections were prepared parallel and perpendicular to the long axis of 12 different fragments of ivory by cutting 300-?m thick slices of ivory from the respective planes with a rotating diamond saw. The thick slices were fixed to glass slides with methylmethacrylate cement and ground to a thickness of approx. 40 ?m on a rotating disc utilizing different grades of diamond pastes. The ground sections were covered with glass cover-slips after debridement with a weak acid solution (0.1 M HCl) and a soft brush, and examined by transmission light microscopy under standard illumination.

Standardized techniques for the preparation of scanning electron micrographs of hard tissues were employed to visualize the morphology and distribution of dentinal tubules of six different tusks when viewed from the pulpal surface of the ivory. The presence of light or dark bands was noted during the preparation of an additional 10 sections at different levels perpendicular to the long axis of the tubules. The levels were selected so that five sections passed through light bands and five through dark bands. All surfaces were cleaned before scanning with a 0.1 M HCl solution and a soft brush, and flushed with deionized water to remove debris. The distance between tubules was expressed as the smallest linear measurement between two adjacent tubules and the tubule density as the number of tubules per mm². At least 10³ tubules were counted for each measurement. An image analyser was used (Flexible Image Processing System, CSIR, Pretoria) and findings were subjected to the Student’s t-test for unequal variances.

The entire outer surfaces of the tusks were covered by a layer of cellular cementum. Ivory was exposed on the working surfaces of erupted tusks through cemental abrasion. The cementum–ivory junction was visible as a dark, concentric ring demarcating the peripheral border of the ivory. In cross-sections through the tusk, the cementum–ivory junction followed an undulating circular course, forming irregular excrescences alternating with shallow convexities or concavities. The excrescences were more pronounced in tusks with smaller diameters. Occasional invaginations of cementum, which were 2–3 mm deep, were present in the outer layer of the ivory. The external contour of the cementum followed the morphology of the cementum–ivory junction, resulting in longitudinal and parallel ridges and troughs had been visible upon external examination of the tusks. These ridges and troughs had been abraded away on the anterior and more exposed surface of the tusk. The pulpal surface of the ivory was smooth and the pulpal cavity conical in shape, with the tip of the tusk composed of solid ivory and the apical foramen wide open.

Fig. 1.Cross-section through the solid part of a tusk. Note the undulating course the cementum–ivory junction follows and the geometric chequered pattern (bar=4 cm).

Fig. 2.Tusk of an elephant bull (top) and cow (bottom). Note the parallel ridges and troughs on the posterior surface of the bull’s tusk (arrows). The broken line in the lower tusk represents the wall of the conical pulpal cavity (bar=10 cm).

The unique chequered pattern was evident on polished surfaces of cross-sections through the tusk. The pattern consisted of two systems of alternating light and dark lines which started adjacent to the cementum. One system swept clockwise and the other anticlockwise towards the centre of the tusk, intersecting at regular intervals. The chequered pattern corresponded to parallel alternating light and dark lines evident on polished surfaces prepared in the sagittal plane. The cementum–ivory junction was straight when viewed in this plane. Planes of fracture through ivory tended to follow the contours of the dark bands.

Fig. 3.Sagittal section through a tusk. Note the conical pulpal cavity and parallel light and dark bands in the ivory, the latter of which are evident in the outer third (bar=4 cm).

The outermost layer of ivory (mantle ivory) was between 40 and 80 ?m thick and consisted of irregularly spaced dentinal tubules which were slanted apically at a sharp angle to the cementum–ivory junction. These tubules appeared to branch extensively, a feature which was best observed when focusing at different planes in the section. Morphologically, the mantle ivory resembled the granular layer of human radicular dentine. After passing through the mantle ivory, the tubules became more regularly spaced and gradually changed their course by curving towards the tip of the tusk. This curvature was the beginning of the regular, sinusoidal course followed by the dentinal tubules in a pulpal direction and was evident only in sections prepared in the sagittal plane. The convexities and concavities of this sinusoidal pattern corresponded to the alternating light and dark bands seen macroscopically on surfaces prepared in the sagittal plane. The dark bands correlated with that part of the dentinal tubules that curved apically. On high-power magnification, many dentinal tubules appeared to end blind and scattered tubules appeared to fuse forming one tubule. The blind-ending tubules were more frequent in the dark bands in ivory (16 blind-ending tubules per 100 tubules, SD 7 tubules) than in the light bands (6 blind-ending tubules, SD 3 tubules) as measured over 2500 tubules in each of the dark and light bands, respectively (p<0.001). The process of apparent fusion involved the main dentinal tubule and was distinct from the fine lateral branches that seemed to anastomose with those of adjacent tubules. Demineralized sections of newly formed ivory in the fetal tissue showed crowding of distally slanted odontoblasts and scattered pyknotic cells, highly suggestive of individual cell death.