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M C Dean

Publications and source records attributed to M C Dean.

34 records · Page 2Linked to original sources
PubMed1987

Crown-formation time of a fossil hominid premolar tooth.

Studies using surface or internal enamel growth indicators in hominids have suggested that crown-formation times were shorter than those in modern man. The crown-formation time in a robust australopithecine premolar tooth was calculated by counting enamel cross-striations, which correspond to daily increments of formation, on a replica of the fractured internal enamel surface of cuspal enamel using scanning electron microscopy. Cervical enamel completion time was estimated using other growth indicators including striae, and using measured and calculated cross-striation repeat intervals, giving a completion time of approx. 2.4 yr. This is much shorter than reported premolar crown formation times in modern man. These findings support the concept of an abbreviated period of dental development, with implications on the duration of the growth period in early hominids.

Animals↗
PubMed1986

Age at death of the Neanderthal child from Devil's Tower, Gibraltar and the implications for studies of general growth and development in Neanderthals.

This study combines traditional methods of assessing dental developmental status based upon modern human standards with new techniques based upon histological observations in order to reassess the age at death of the Gibraltar child from Devil's Tower. The results indicate that the most likely age of this individual at death was 3 years of age. This result is in agreement with an independent assessment of the age of the temporal bone of this specimen (Tillier, AM [1982] Z. Morphol. Anthropol. 73:125-148) and is concordant with dental developmental ages given for modern humans. Moreover, the fact that this specimen appears at the low end of the age scale for calcification stages in modern humans is also supportive of the findings of Legoux (Legoux, P [1970] Arch. Inst. Paleontol. Hum. Mem. 33:53-87) and Wolpoff (Wolpoff, MH [1979] Am. J. Phys. Anthropol. 50:67-114) that dental eruption schedules in Neanderthals were also accelerated. If the cranial bones from Devil's Tower are associated with the dental material, as we believe, they indicate a remarkably precocious brain growth in this individual, which is consistent with what is known about general growth and development in Neanderthals.

Age Determination by Teeth↗
PubMed1985

The eruption pattern of the permanent incisors and first permanent molars in Australopithecus (Paranthropus) robustus.

This study aims to reassess the claim that the eruption sequence of the permanent incisor and first permanent molar teeth of Australopithecus (Paranthropus) robustus is identical with that in modern Homo sapiens. Eight fossil hominid mandibles of equivalent dental developmental age were chosen for comparative study. Emphasis has been placed upon the comparative timing of events within the growth period rather than eruption sequence alone. The results of this study indicate that Homo sapiens and Australopithecus (Paranthropus) robustus share the same pattern of permanent molar and incisor eruption and that this is significantly different from the pattern of eruption shared by the great apes, Australopithecus africanus and Australopithecus afarensis.

Haplorhini↗
PubMed1985

Variation in the developing root cone angle of the permanent mandibular teeth of modern man and certain fossil hominids.

The radiographic appearance of the developing root apex of 461 mandibular teeth was studied using ellipsopantomographs of 77 individuals. The angle formed by the mesial and distal aspects of the developing root cone was measured for each quarter of root development in the permanent mandibular teeth of each subject. Large values for this angle in the more coronal part of the root gradually decreased towards the root apex where values were small in all teeth studied. This gradual decrease in the angle of the developing root cone is concordant with the faster increase in root length but slower rates of dentinogenesis reported for the apical region of human teeth in the literature, as the decrease is likely to reflect the increasing numbers of odontoblasts that become involved in root formation as it proceeds apically. The developing root cone angles of one of the five fossil hominid specimens studied (KMN-ER 820) fall outside the range for the developing root cone angle of the developing premolar and permanent second molar roots in modern Homo sapiens and is evidence of a faster rate of root elongation than is typical for modern man.

Animals↗
PubMed1985

Comparative myology of the hominoid cranial base. II. The muscles of the prevertebral and upper pharyngeal region.

Detailed accounts of the muscular relations and attachments in the prevertebral and upper pharyngeal region of the cranial base of extant hominoids and fossil hominids are described in this paper. Marked differences exist in the cranial attachments of the longus capitis, superior constrictor, tensor palati and levator palati muscles between the three great apes and Homo sapiens. Many of the bony ridges and processes characteristic of the three great apes are also present in the 'gracile' australopithecines. Some of the differences between the muscle markings in this region of the cranial base in certain fossil hominids are discussed in the light of the findings of the dissection study of the three great apes and previous studies of the hominoid cranial base.

Animals↗
PubMed1984

Phylogeny, neoteny and growth of the cranial base in hominoids.

This study tests the hypothesis that there is a general pattern in the growth of the cranial base of Homo sapiens that is 'essentially neotenous' [Gould, 1977]. Juvenile and adult crania of Homo sapiens, Gorilla gorilla, Pan troglodytes and Pongo pygmaeus were studied and the cross-sectional growth curves for 10 measurements made on the cranial base (as viewed in norma basilaris) were compared. The results of this study suggest that relatively simple modifications to the timing or pattern of growth are insufficient to explain the observed morphological differences between the cranial base of modern Homo sapiens and the great apes.

Aging↗
PubMed1984

Comparative myology of the hominoid cranial base. I. The muscular relationships and bony attachments of the digastric muscle.

This paper aims to document accurately the soft tissue anatomy and bony attachments of the posterior belly of the digastric muscle and other closely related muscles in the mastoid region of extant hominoids and fossil hominids. Five wet specimens including individuals of Pan, Gorilla and Pongo were dissected and described. Eight casts of fossil hominid cranial bases were also studied along with measurements and notes made from the same original fossil hominid specimens to assess their soft tissue markings in the light of the findings for the three great apes. The results indicate that whereas the attachment of the posterior belly of the digastric muscle in Homo sapiens is associated with a deep groove or fossa, it originates from a widened area and leaves no bony markings on the cranial base of the three great apes. Following a change in the position of the foramen magnum and the occipital condyles in hominids and H. sapiens the insertion of the posterior belly of the digastric has remained posteriorly positioned but has become compressed into a deep groove. It is likely that this has come about by the displacement of the more medial soft tissue structures which have been moved laterally away from the occipital condyles.

Animals↗
PubMed1982

Basicranial anatomy of Plio-Pleistocene hominids from East and South Africa.

The results of a metrical analysis of the basicranium of 19 Plio-Pleistocene fossil hominid crania are presented. The sample includes crania attributed to Australopithecus africanus, Australopithecus boisei, and robustus, and Homo erectus as well as crania whose attribution is still under discussion. These results confirm significant differences between the cranial base patterns of the "gracile" and "robust" australopithecines and the three crania attributed to Homo erectus have a pattern which resembles that of modern humans. None of the crania examined from East Africa sites have base patterns which resemble that of the "gracile" australopithecines. The crania KNM-ER 407 and 732 have patterns which are compatible with them being smaller-bodied females of Australopithecus boisei; KNM-ER 1470 and 1813 have base patterns which most closely resemble that of Homo erectus. The cranial base pattern of KNM-ER 1805 is compatible with its inclusion in either Australopithecus boisei or Homo. When account is taken of the immaturity of Taung, the evidence of its cranial base pattern suggests that if it had reached adulthood it would have resembled the "gracile" australopithecine crania from Sterkfontein and Makapansgat.

Africa, Eastern↗
PubMed1981

Developing pongid dentition and its use for ageing individual crania in comparative cross-sectional growth studies.

This study of the developing pongid dentition is based on cross-sectional radiographic data of juvenile Pan troglodytes, Gorilla gorilla, and Pongo pygmaeus skulls. Comparisons with developmental features of the human dentition are made, and possible explanations for the formation of larger teeth within the reduced pongid growth period are discussed. The data presented in this study provide an alternative method for ageing individual pongid crania in comparative cross-sectional growth studies. The advantages of this method are demonstrated by ageing individual Gorilla crania form radiographs and plotting relative dental age against length of the jaw.

Age Determination by Teeth↗
PubMed1980

The suppression of cellular proliferation in SV40-transformed 3T3 cells by glucocorticoids.

Glucocorticosteroids, when added two hours after cell plating to SV40-transformed, 3T3 mouse fibroblasts in low serum (0.3% v/v), biotin-supplemented medium, suppress cellular proliferation by 24 hours. While some cell death probably occurs, the growth inhibition is not primarily due to cytotoxicity and cytolysis. This conclusion is supported by the following: 1) both dead and viable cell numbers are suppressed, 2) little cell debris is evident in the medium, and 3) very high concentrations of glucocorticoids do not cause an increase in the dead cell count. Furthermore, this growth suppression, which is specific for glucocorticoids since several non-glucocorticoid steroids have no inhibitory effect, is not permanent nor irreversible. Removal of the glucocorticoid and replacement with 10% serum restore rapid proliferation. Although higher concentrations (1% and 10%) of serum afford some protection against glucocorticoid inhibition, this protection is not simply a consequence of faster growth rates. SV3T3 cells can be grown in serum-free medium supplemented with biotin, transferrin, insulin, and epidermal growth factor (EGF). Under these conditions growth rates are comparable to high serum media, yet glucocorticoids are still powerful inhibitors. However, the omission of insulin from serum-free, glucocorticoid cultures does result in observable cell death and lysis. Flow microfluorometry and autoradiographic studies have determined that glucocorticoid-inhibited cells are partially blocked in G1. The proportions of S phase and G2 + M cells are greatly reduced with an accompanying accumulation of G1 cells. These results suggest that glucocorticoids regulate a biochemical step(s) in G1 which is critical for DNA initiation.

Animals↗
PubMed1980

The serum growth and survival requirements of SV40-transformed 3T3 cells.

Simian virus 40-transformed 3T3 cells are dependent on serum for survival and growth. This growth activity can be separated on a pH 2 Sephadex G100 column into two fractions: a high molecular weight activity and a low molecular weight substance that has recently been characterized as containing as its major agent, biotin. To replace the remainder of the serum requirement, hormones and other growth factors were tested. Both insulin at high, nonphysiological concentrations (200 to 500 ng/ml) and transferrin (5 X 10(-8) M) stimulate the growth rate in low serum medium (0.3% v/v bovine calf serum DME) individually and, when added together, are nearly as growth enhancing as 10% serum. The need for the residual serum in this medium can be eliminated by the use of crystalline trypsin during trypsinization. Under these serum-free conditions, biotin and transferrin supplementation provide for moderately good growth (20 to 30 hr population doubling time, 1 X 10(6) cells/3.2-cm dish final cell density). Insulin addition further stimulate the growth rate (16 to 20 hr) and the final density (1.5 X 10(6) cells). Although the protein growth factors, EGF (0.5 to 1.0 ng/ml) and FGF (4 to 10 ng/ml), also appear to enhance growth individually and additively, their effects are slight and very variable. Nevertheless, the complete serum-free medium (DME supplemented with biotin, transferrin, insulin, EGF, and FGF) yields growth comparable but still inferior to 10% serum supplementation (14-versus 12-hr population doubling time, 1 to 2 X 10(6) versus 2 to 3 X 10(6) cells final cell density).

Animals↗
PubMed1976

Does the rat with hereditary hypothalamic diabetes insipidus have impaired avoidance learning and/or performance?

Avoidance learning and extinction of rats with hereditary hypothalamic diabetes insipidus (Brattleboro strain) were studied in 2 experiments that differed only in shock intensity. In both experiments rats homozygous for diabetes insipidus were more deficient in both escape and avoidance responding than were their heterozygous or normal controls. Although the hterozygous animals showed improved escape performance at the higher shock intensity, their avoidance behavior was not improved. The superiority of normal and heterozygous animals in extinction performance, relative to the homozygous animals, was eliminated or reversed when the differences in terminal acquisition performance were taken into account by analyses of covariance. Deficiency of ADH, therefore, may not result in faster extinction of avoidance behavior.

Animals↗
PubMedDate not supplied

A comparative study of cross striation spacings in cuspal enamel and of four methods of estimating the time taken to grow molar cuspal enamel in Pan, Pongo and Homo.

Cusps of three second permanent molar teeth belonging to Pan, Pongo and Homo respectively, were chosen where enamel cross striations were easily observed and measured using polarizing light microscopy. Prisms were tracked outwards on photomontages from the dentine horn to the surface of the tooth just lateral to the central gnarled enamel over the cusp tip. Approximately monthly zones of enamel formation were identified and mean cross striation spacings calculated for each zone within each of the three cusps and for the whole of each cusp in total. Enamel secretion rates ranged from 2.5 to 6.5 microns per day with an overall mean value in Pan and Homo of 4 microns per day and of 4.4 microns per day in Pongo for the cuspal enamel. Two of these cusps (of Pan and Pongo) and another of a third permanent molar of Homo were selected for further analysis. Four methods were employed to estimate cuspal enamel formation time. (i) Total counts of enamel cross striations were made through the lateral cuspal enamel on photomontages. The points in the dentine and at the enamel dentine junction (EDJ) corresponding to the end of cuspal enamel formation were defined using incremental and accentuated markings. (ii) Measurements of the average daily rates of dentine formation in each cusp were divided by the length of the axial cuspal dentine formed to give the time of cuspal dentine formation. (iii) A cumulative prism length was calculated close to the EDJ to the end of cuspal enamel formation and divided by the mean cross striation repeat interval along the EDJ to give an estimate of cuspal enamel formation time. (iv) A cumulative time for cuspal enamel formation along the EDJ was calculated by summing successive extension rates for known lengths of the EDJ. This was computed using the formula derived by Shellis (Archs. oral Biol. 29: 697-705, 1984) based on the average daily rate of enamel secretion, the angle of the prisms to the EDJ and the angle of the incremental lines to the EDJ. Each of these methods gave results to within 5% or 10% of a average value for cuspal enamel formation time derived for each cusp. There was no clear indication of additional enamel decussation in any of the cusps studied beyond that recorded in the total counts of cross striations as the prisms were tracked in two dimensions through the lateral cuspal enamel. The results of this study suggest that any one of the methods outlined here may give equivalent estimates of cuspal enamel formation if suitable incremental markings exist in the region being studied.

Animals↗
PubMedDate not supplied

Observations on stria morphology in the lateral enamel of Pongo, Hylobates and Proconsul teeth.

The enamel of certain primates (orang-utans, siamangs and the early Miocene fossil hominoid, Proconsul) occasionally contains striae of Retzius that appear distinct from those more typical of the lateral and cervical enamel of other anthropoids. These striae can be described as "S-shaped" since their contour is markedly sinuous as it passes from the enamel dentine junction (EDJ) to the tooth surface. "S-shaped" striae have never been described in the comparative literature on primate enamel and yet they may be of some phylogenetic significance. This study explores the interrelationship between four variables in the enamel of an orang-utan, a siamang and a specimen of Proconsul heseloni (from Rusinga Island, Kenya) all of which contain "S-shaped" striae. The morphological components of this form of stria have been quantified here. The four variables measured were (1) prism width; (2) the angle the prisms make to the EDJ; (3) the angle the striae of Retzius make with respect to the EDJ, and (4) the daily rate of enamel formation. Each of these four variables was measured in inner enamel close to the EDJ, in enamel mid-way between the EDJ and the enamel surface, and in outer enamel. Our aim was both to quantify the components of "S-shaped" striae and to attempt to offer a developmental explanation for the appearance of these striae in fully formed enamel. We conclude that "S-shaped" striae may be associated with regions of lateral enamel where prism width either remains constant or even reduces as the enamel surface is approached. This, together with an increase in the linear daily rate of enamel secretion towards the outer enamel, a cervical inclination in the angulation of the prisms with respect to the EDJ, and an increased angulation of the striae of Retzius to the EDJ are all features that characterize "S-shaped" striae. This description will facilitate identification and quantification of any similar striae in the enamel of other primates. It will also allow careful comparison of each of the four variables one with another which may in turn help in establishing "S-shaped" striae as developmentally homologous between species.

Animals↗