BONE differs from reproductive tissues in that many aspects of skeletal growth, turnover, and function occur in the absence of gonadal hormones. Nevertheless, sex steroids have an important impact on bone physiology: they participate in the sexual dimorphism of the skeleton, play a role in maintenance of mineral homeostasis during reproduction, and are essential to maintaining bone balance in adults. Insufficient levels of certain sex steroids predispose the human skeleton to bone loss and to osteoporotic fractures. Estrogen appears to be the most important sex steroid in preventing osteoporosis in women and is the subject of this review. Serious difficulties must be overcome to understand the action of estrogen on mineralized tissues. The skeleton is heterogeneous and contains many cell types. The cells that synthesize (osteoblasts), lie within (osteocytes), and degrade (osteoclasts) bone matrix are present in small numbers, are difficult to isolate as pure populations, and probably do not proliferate.

Skeletal Effects of Estrogen

The tempo of tooth mineralization is under significant genetic control, and the orderly progression of morphological changes-in concert with the long span during growth in which teeth form-makes "dental age" a useful measure of a person's degree of biological maturity. The third molar is of particular interest because (1) it is the last and most variable tooth to form and (2) it is the only tooth to complete formation after puberty, which has made it attractive in forensic and legal circles as an estimator of adulthood. Age standards are described here for mandibular third molar formation stages in a cross-sectional sample of 4,010 persons (age range: 3-25 years), with proportionate sample sizes of American blacks and whites and males and females. Formation was scored against the 15-grade ordinal scheme of Moorrees, and descriptive statistics were computed using proportional hazards survival analysis. Blacks achieved each formation stage significantly ahead of whites, but not in a uniform manner. Instead, there was an enhanced advancement in blacks during crown formation and during late stages of root formation. In both races formation proceeded faster in males, which is unique for the third molar, as prior studies suggest. Sample variance increases with the stage of formation, such that 95% confidence limits span 8 or more years for root formation stages. Consequently, the third molar provides a rough gauge of an individual's chronological age, but the considerable variability precludes any precise estimate, particularly in late adolescence where most forensic interest has focused.

Mineralization of the mandibular third molar: a study of American blacks and whites.

Ovariectomy results in cancellous osteopenia in rat long bones, a condition that is prevented by treatment with estrogens. The purpose of these studies was to clarify the effects of estrogen on cancellous bone turnover using dynamic bone histomorphometry. Treatment of ovariectomized rats with diethylstilbestrol (DES) reduced the mineral apposition rate, double-label perimeter, osteoblast number, and osteoclast number, suggesting that the hormone had inhibitory effects on bone formation as well as bone resorption. However, we could not estimate the bone formation rate because of rapid resorption of tetracycline-labeled bone in the ovariectomized rat. The magnitude of loss was documented by a time course study: 58% of the tetracycline initially incorporated into the secondary spongiosa of the tibial metaphysis was resorbed after 11 days and 89% was resorbed after 22 days. Similarly, cancellous bone area was decreased by 67% after 11 days and by 88% after 22 days. Administration of either DES or tamoxifen (TAM) dramatically reduced resorption of tetracycline as well as the decrease in cancellous bone area. These results demonstrate that (1) estrogen prevents osteopenia in ovariectomized (OVX) rats, in part by inhibiting bone turnover, (2) TAM is an estrogen agonist on bone resorption, and (3) resorption of tetracycline-labeled bone leads to serious underestimation of the bone formation rate in OVX rats.

Mechanism of action of estrogen on cancellous bone balance in tibiae of ovariectomized growing rats: Inhibition of indices of formation and resorption

The evolution of male life history traits was central to the emergence of the genus Homo. Compared with earlier hominins, changes in the behavioral and physiological mechanics of growth, survivorship, reproductive effort, and senescence all likely contributed to shifts in how males contributed to the evolution of our genus. For example, the range of paternal investment in modern Homo sapiens is unusual compared with most mammals and primates, all but certainly contributing to the evolution of the suite of life history traits that define Homo, including high fertility, large bodies, altricial offspring, large brains, and long lives. Moreover, the extensive range of phenotypic and behavioral variation in somatic and behavioral reflections of male reproductive effort across modern H. sapiens is especially noteworthy. We propose that selection for a broad range of variation in traits reflective of male reproductive effort was important to the evolution of Homo. We examine factors that contribute to this varia...

https://www.jstor.org/stable/10.1086/667538

Male Life History, Reproductive Effort, and the Evolution of the Genus Homo

Currently, one of the major debates about the American peopling focuses on the number of populations that originated the biological diversity found in the continent during the Holocene. The studies of craniometric variation in American human remains dating from that period have shown morphological differences between the earliest settlers of the continent and some of the later Amerindian populations. This led some investigators to suggest that these groups—known as Paleomericans and Amerindians respectively—may have arisen from two biologically different populations. On the other hand, most DNA studies performed over extant and ancient populations suggest a single migration of a population from Northeast Asia. Comparing craniometric and mtDNA data of diachronic samples from East Central Argentina dated from 8,000 to 400 years BP, we show here that even when the oldest individuals display traits attributable to Paleoamerican crania, they present the same mtDNA haplogroups as later populations with Amerindian morphology. A possible explanation for these results could be that the craniofacial differentiation was a local phenomenon resulting from random (i.e. genetic drift) and non-random factors (e.g. selection and plasticity). Local processes of morphological differentiation in America are a probable scenario if we take into consideration the rapid peopling and the great ecological diversity of this continent; nevertheless we will discuss alternative explanations as well.

/pdf/discrepancy-between-cranial-and-dna-data-of-early-americans-3nrxe6om91.pdf

Discrepancy between cranial and DNA data of early Americans: implications for American peopling.

Two groups of weanling rats were subjected to malnutrition, one with periodic injections of testosterone (males) and the other with estradiol (females). Two other groups (castrated males or castrated females) received normal feedings. In control animals, the relative weights (mg/gm body weight) of testes, seminal vesicles, and ovaries were greater than in malnourished rats. However, relative weights of those organs in hormone-treated, malnourished animals were greater than in those subjected to malnutrition alone and still greater than in controls. Normal sexual cranial dimorphism (SCD) was decreased 16% by male castration, 23% by malnutrition, and 83% by estradiol treatment in malnourished females. On the other hand, normal SCD was increased 20% by female castration and more than 200% by testosterone treatment in malnourished males. All monosexual comparisons corroborated the bisexual range of distances found. Testicular but not ovarian secretions seemed to influence sexual cranial dimorphism. Malnutrition delayed SCD because of a deficiency of testosterone level in stressed males. It is suggested that estradiol in females may counteract sexual cranial development and that its inhibitory effect may be additive to the testosterone deficit evoked by malnutrition.

Variations in sexual dimorphism in the skulls of rats subjected to malnutrition, castration, and treatment with gonadal hormones.

The influences of protein-calorie malnutrition (PCM), and sex during lactation and post-lactation on the frequencies of 25 discontinuous cranial traits (DCT), were investigated in Holtzman rats. Significant differences were observed in about 20% of the traits. Those traits were: the interfrontal fusion, the posterior incurvation of the palatine border, the double maxillary foramen, the double posterior palatine foramen, and the double frontal foramen. Total PCM was the nutritional factor which showed the greatest influence on the variability of the DCTs. It was followed, in decreasing order, by the PCM imposed during post-lactation and lactation. Sex had more influence than early PCM but less than late PCM. It is concluded that despite their apparent stability, a substantial number of DCTs were altered by both biological (like sex) and environmental factors (like nutritional deficiencies) imposed at different stages of postnatal development.

Effects of protein‐calorie malnutrition during suckling and post‐weaning periods on discontinuous cranial traits in rats

Weanling rats of both sexes were submitted to either castration, or castration together with periodic injections of testicular extract to males or of ovarian extract to females. Control and experimental animals were sampled at 63 days of age. Cranial differentiation between sexes was estimated by Mahalanobis D2 distances. The controls showed a significant sexual cranial difference. Orchidectomy decreased cranial differences and this effect was compensated by injections of testicular extract. On the other hand, oophorectomy increased cranial differences, which were diminished by injections of ovarian extract. Sexual cranial dimorphism in the normal rat seems to be the result of a counteracting effect between testicular and ovarian hormonal secretions.

Effect of gonadal activity on the cranial dimorphism of the rat.

/pdf/influencia-del-nivel-socioeconomico-sobre-el-dimorfismo-agia4ro9sm.pdf

Influencia del nivel socioeconómico sobre el dimorfismo sexual de escolares patagónicos

/pdf/transicion-nutricional-en-patagonia-2j213ggyfo.pdf

Silvia Lucrecia Dahinten

Papers

Variations in sexual dimorphism in the skulls of rats subjected to malnutrition, castration, and treatment with gonadal hormones.

Effects of protein‐calorie malnutrition during suckling and post‐weaning periods on discontinuous cranial traits in rats

Effect of gonadal activity on the cranial dimorphism of the rat.

Influencia del nivel socioeconómico sobre el dimorfismo sexual de escolares patagónicos

Transición nutricional en Patagonia