The endocrinology of the pubertal growth spurt

Recent experimental and clinical findings in the skeleton associated with loss of estrogen hormone or estrogen receptor activity

Studies on rodent models and rare human disorders of estrogen production or response have revealed an increased complexity of the actions of estrogen on bone. ERalpha disruption in human males results in delayed epiphyseal maturation, tall stature, trabecular thinning, marked cortical thinning, genu valgum and significantly reduced cortical vBMD, but trabecular number is preserved and there is normal to increased periosteal expansion. Aromatase deficiency results overall in a similar phenotype, although less is known about skeletal architecture. Importantly, estrogen replacement in these individuals, even if provided late in the third decade, may normalize aBMD. Less certain is whether there is complete recovery of normal skeletal architecture and strength. Rodent models, in general, are consistent with the human phenotype but are confounded by inherent differences between mouse and human physiology and issues regarding the completeness of the different knock-out lines. Both human and rodent studies suggest that residual effects of estrogen through ERbeta, truncated ERalpha forms or nonclassical estrogen receptors might account for different phenotypes in the hERKO man, aromatase deficient subjects and rodents. Importantly, androgen, particularly by preserving trabecular number and augmenting both periosteal and epiphyseal growth, also has significant actions on bone.

Growth and development after hematopoietic cell transplant in children

Hematopoietic cell transplantation (HCT) following high-dose chemotherapy or chemoradiotherapy for children with malignant or nonmalignant hematologic disorders has resulted in an increasing number of long-term disease-free survivors. The preparative regimens include high doses of alkylating agents, such as CY with or without BU, and may include TBI. These agents impact the neuroendocrine system in growing children and their subsequent growth and development. Children receiving high-dose CY or BUCY have normal thyroid function, but those who receive TBI-containing regimens may develop thyroid function abnormalities. Growth is not impacted by chemotherapy-only preparative regimens, but TBI is likely to result in growth hormone deficiency and decreased growth rates that need to be treated with synthetic growth hormone therapy. Children who receive high-dose CY-only have normal development through puberty, whereas those who receive BUCY have a high incidence of delayed pubertal development. Following fractionated TBI preparative regimens, approximately half of the patients have normal pubertal development. These data demonstrate that the growth and development problems after HCT are dependent upon the preparative regimen received. All children should be followed for years after HCT for detection of growth and development abnormalities that are treatable with appropriate hormone therapy.

Pubertal development in obese children and adolescents

OBJECTIVE

To investigate clinical and laboratory markers of pubertal development in a large sample of obese children and adolescents.

METHODS

Analysis of parameters of sexual maturation in 1232 obese individuals (582 boys) aged 6-18 years (mean 13.0+/-2.42 years). Clinical evaluation of pubertal stage and determination of bone age in a subset (227 patients).

RESULTS

Mean Height--standard deviation scores (height-SDS) was positive during childhood and reached zero approximately at age 14 years followed by a turn to negative mean height-SDS in both genders. Accordingly, bone age was accelerated until age 14. No significant differences in average time points of occurrence of pubic hair stages PH 2 to PH 4 in boys and PH 2 to PH 5 in girls were observed as compared to references of the First Zurich Longitudinal Study. In girls, breast stage B 3 was reached earlier (11.6 vs 12.2 years, P=0.03). In boys, mean volume of testis revealed no significant deviation from reference. Mean dehydroepiandrosterone sulfate (DHEAS) levels were elevated in boys (within age ranges 8-10 years and 12-16 years, P<0.02) and in girls (within age ranges 6-8 years and 12-18 years, P<0.005) and mean testosterone levels in boys >12 years were lower as compared to reference ranges (all P-values <0.0001).

CONCLUSION

The study data suggest normal development of pubarche and gonadarche in obese German boys and normal timing of pubarche in girls. Breast development in obese girls seems to be slightly advanced. In obese boys, an obvious dissociation of clinical and laboratory parameters of pubertal development was observed. Despite significantly increased height-SDS and increased DHEAS levels, gonadal development was normal and testosterone levels were decreased. Elevated DHEAS levels in both genders may contribute to the acceleration of bone maturation, a lower final body height and could increase cardiovascular risk.

Biochemical markers of bone formation in Thai children and adolescents

The measurement of biochemical markers of bone turnover is essential in the study of skeletal metabolism in health and diseases. Due to variations in the rate of bone growth in different age groups and possible ethnic differences, age-specific reference ranges for biochemical markers should be established in a particular pediatric population. In this study, biochemical markers of bone formation, bone-specific alkaline phosphatase (BAP), and osteocalcin (OC) in healthy Thai children and adolescents aged 9 to 18 years were evaluated in relation to their ages and pubertal development. Serum BAP levels in boys increased with age and peaked at about 12 to 13 years. In contrast, there was a progressive decline of serum BAP levels with advancing age in girls older than 9 years. Serum OC also increased with age and reached a peak at ages 12 and 13 years in girls and boys, respectively. In addition, both serum BAP and OC levels also varied with pubertal stages. The BAP levels in boys increased sharply at pubertal stage 3 and decreased at pubertal stage 5. In girls, the BAP levels showed a fairly constant high level up to stage 3, followed by a remarkable decrease thereafter. The OC levels in boys increased sharply at pubertal stage 4 and decreased thereafter. In girls, OC started to increase at pubertal stage 3 with no subsequent changes. The levels of serum BAP and OC were higher in boys than in girls at pubertal stages 3 to 5 and at stages 2, 4, and 5, respectively. Moreover, only serum BAP level showed significant positive correlation with height velocity in both genders. In multiple regression analyses, gender, age, and pubertal stage were consistently correlated with both serum BAP and OC levels. In summary, male and female adolescents have different patterns of changes in biochemical markers of bone formation.

Role of estrogen and androgen in pubertal skeletal physiology

Since both estrogen and androgen are present in each sex, it has been difficult to discern the exact role that each sex steroid plays in skeletal physiology. However, studying clinical syndromes in which there is either only estrogen or androgen action has allowed us to gain insight into the unique role that each sex steroid plays in the growing skeleton. In complete androgen insensitivity syndrome (AIS) the only functional sex steroid receptor is that for estrogen. Effected XY females have a pubertal growth spurt that is typical of normal females, both in magnitude and timing. Individuals with AIS have a mild reduction in bone density but it is difficult to distinguish whether this is the result of androgen resistance or estrogen deficiency. These observations suggest that estrogen action only is sufficient to induce a normal pubertal growth spurt, epiphyseal maturation, and near normal bone mineral accretion in women. Until recently, the skeletal effects of estrogen were not thought to be of importance in the male. Conventional wisdom dictated that, in the male, testosterone mediated these skeletal changes. The notion that estrogen is of little importance in the male has been challenged by the recent discovery of two human syndromes in which estrogen action is lacking. In males with either estrogen resistance (inability to respond to circulating estrogen) or aromatase deficiency (inability to synthesize estradiol), as a result of the lack of estrogen action, a pubertal growth spurt does not appear to occur. Furthermore, complete epiphyseal maturation does not take place allowing for continued growth in adulthood and resultant tall stature. Finally normal bone mineral accretion does not take place resulting in severe osteoporosis. These findings indicate that estrogen plays a critical role in skeletal physiology of males as well as females.

Oestrogen receptor deficiency: consequences for growth

These oestrogen-resistant and aromatase-deficient cases now establish a more complete picture of how androgen and oestrogen combine to regulate pubertal growth (47, 48) and demonstrate that oestrogen deficiency states need not be lethal (1). Whereas original concepts suggested that skeletal maturation and pubertal growth were attributable to androgen and, therefore, were not sexually dimorphic, current evidence preserves the notion of common mechanisms in both sexes, but points to oestrogen as the principal steroid involved in the final phases of skeletal maturation. Oestrogen can be viewed as a primary determinant of the final height of a child, in the sense that oestrogen initiates and completes epiphyseal closure. The full scope of the effects of androgen on epiphyseal maturation and linear height regulation is less clear but evidence suggests that androgen has direct and indirect effects through its aromatization to oestrogen (Fig. 2). Remarkably, under unusual circumstances, a rather prolonged period of continued growth can be achieved with minimal bone-age advancement, as long as oestrogen concentrations and/or sensitivity are low. Given this more complete understanding of the role of oestrogen, and with the availability of more sensitive assays for oestradiol (49), a new era in sex-steroid physiology and pubertal growth has been inaugurated (50). However, any therapies directed towards manipulating growth with sex steroids will need to take account of the possibility of untoward effects on other processes, such as the accretion of bone mineral mass (51,52).

The role of estrogen in pubertal skeletal physiology: epiphyseal maturation and mineralization of the skeleton

The year 1994 is likely to be remembered by many endocrinologists as the year in which dramatic new light was shed on the role played by estrogen in human skeletal physiology. It was in 1994 that two new syndromes were described, each representing a human model in which estrogen action was lacking. The first case was a female with an aromatase defect and a resultant inability to synthesize estrogen, and the second case was a man with an estrogen receptor gene defect that resulted in a non-functioning estrogen receptor and complete estrogen resistance. By examining the phenotypes of these two individuals, we were able, for the first time, to see what pubertal skeletal changes occur in the absence of estrogen action and directly extrapolate the role of estrogen in skeletal physiology. What has become abundantly clear is that it is estrogen and not androgen that is responsible for pubertal epiphyseal maturation and skeletal mineralization.

The two faces of growth: benefits and risks to bone integrity

Bones grow by two processes: cortical bone is made by periosteal apposition (growth in width), and cancellous bone is made by endochondral ossification (growth in length). In both the axial and appendicular skeleton, about half of peak adult bone mass is accumulated during the adolescent growth spurt, which occurs two years earlier in girls than in boys, and is under pituitary control via interactions between growth hormone and sex hormones. Throughout growth, but particularly during adolescence, the ability of bone to adapt to mechanical loading is much greater than after maturity. This is the main reason why the effects of physical activity on bone are greater in cross-sectional studies in young athletes than in longitudinal studies in previously sedentary adults. In wild animals, by the time growth has ceased, the bones must be as strong as they will ever need to be, and attainment of further strength after cessation of growth would serve no biologic purpose. Adaptation of growing bone to mechanical loading is the purpose of the mechanostat, which enables physiologic adaptation in individuals to establish and maintain a species-specific property of the bones that is determined by evolutionary adaptation in populations. But growth confers risks as well as benefits to the skeleton. The large increase in incidence of upper extremity (particularly lower forearm) fractures, coincident with the adolescent growth spurt in both sexes, is due to an increase in cortical porosity as a consequence of an increase in intracortical bone turnover, which supplies some of the calcium needed by the growing ends of the long bones. This enables an increased demand for calcium to be spread over a longer time, analogous to the cyclic physiologic osteoporosis which occurs during the antler growth cycle in deer. The subsequent decline in cortical porosity is responsible for the continued increase in radial bone density after cessation of growth, referred to as consolidation. In the present state of knowledge, an increased incidence of fracture during the adolescent growth spurt is the inescapable consequence of an appropriate level of physical activity, and is the price that has to be paid in order to maximize bone accumulation during growth and minimize fracture risk in old age.

Biochemical markers of bone turnover in girls during puberty

OBJECTIVE

Bone turnover and the rate of bone growth increase dramatically during puberty. A number of new assays for the estimation of bone resorption and formation rates have been developed over recent years, and puberty acts as a convenient model for evaluation of these measurements. The aim of this study was to explore the interrelationships between pubertal development, biochemical markers of bone turnover, insulin-like growth factor I and oestradiol in healthy pubertal girls.

SUBJECTS

Ninety-one healthy girls (ages 11.6-15.5 years) were studied. All subjects were apparently healthy, and were not taking medications known to influence calcium homeostasis. Breast examination was performed to assess pubertal stage according to Tanner. The adult reference range for biochemical markers of bone turnover was obtained from concurrent studies on 42 healthy premenopausal women ranging in age between 20 and 45 years.

DESIGN AND MEASUREMENTS

Blood samples were obtained from subjects between 0800 and 1000 h. Urine samples were collected between 1330 and 1600 h. We measured total and bone specific alkaline phosphatase, osteocalcin, and type I procollagen carboxyterminal propeptide as markers of bone formation. Tartrate resistant acid phosphatase, carboxyterminal pyridinoline cross-linked telopeptide, creatinine corrected urinary deoxypyridinoline, immunoreactive urinary pyridinolines, and urinary galactosyl hydroxylysine were measured as markers of bone resorption.

RESULTS

Bone turnover as reflected by each of the markers was maximal in mid puberty (breast Tanner stages II and III) and decreased towards adult levels in late puberty (P < 0.001). However, the magnitude of the mid-pubertal increase differed between markers. In particular, the pubertal increase in levels of bone specific alkaline phosphatase, osteocalcin and urinary deoxypyridinoline were higher than the increase shown by the other markers. All markers were significantly lower after the menarche. Circulating insulin-like growth factor I and insulin like growth factor binding protein-3 were not important determinants of pubertal changes in bone turnover. In contrast, there was a significant negative correlation between oestradiol and all markers of bone formation and resorption during puberty.

CONCLUSIONS

The greater pubertal increase in levels of bone specific alkaline phosphatase, osteocalcin and urinary deoxypyridinoline suggests that these markers may be relatively more sensitive as indicators of skeletal health during puberty. The differences between markers may reflect differences in the bone specificity of the analytes, or differing mechanisms of production and clearance. The negative correlation between oestradiol and markers of bone resorption and formation suggests that this hormone may be responsible for the reduction in bone turnover in late puberty.

Does early physical maturity influence breast cancer risk?

Earlier onset of menarche and tallness in adult women are mainly confirmed as risk markers for breast cancer. Recent disparate case-control studies have reported abdominal-type obesity and higher circulating levels of insulin, testosterone and insulin-like growth factor 1, to be further risk markers for breast cancer. There is evidence that abdominal-type obesity is recognisable in girls even before puberty, and disparate studies have shown it to be correlated with earlier onset of menarche, insulin resistance leading to hyperinsulinaemia, and an abnormal sex steroid profile. The implications are that earlier onset of puberty in a subset of girls can lead to more prolonged exposure of developing breast tissue to an abnormal sex steroid profile and also to a higher circulating level of insulin. It is postulated that these metabolic/endocrine concomitants of abdominal-type obesity could play a role in promoting mammary carcinogenesis at a young age, particularly if genetic predisposition is present.

Growth in patients with isolated gonadotrophin deficiency

The growth pattern of 66 patients (50 males, 16 females) with isolated gonadotrophin deficiency (IGnD), who had reached their final height with epiphyseal closure, was evaluated. For the purpose of analysis the males were divided into two groups according to age at referral: group 1 less than 16 years (n = 23) and group 2 greater than or equal to 16 years (n = 27). Sex hormone treatment was initiated at a mean (SD) chronological age of 15.8 (1.3) and 18.6 (1.2) years in groups 1 and 2 in the males and at 15.3 (1.3) years in the females. The duration of treatment (until epiphyseal closure) in the males was 3.9 (1.5) years in group 1 and 2.1 (1.0) years in group 2 and 2.8 (1.3) years in the females. There was no significant difference between the mean final height in groups 1 and 2, but it was significantly higher than the mean parental height (mean height SD score (HtSDS): 0.1 (1.1) v -0.8 (0.9)) and they were significantly correlated. For females the mean HtSDS compared with parental height was 0.4 (1.5) v -0.6 (1.2). It is concluded that the timing of induction of puberty by sex hormones in males and females with IGnD has no significant effect on final height provided that moderate doses are used. Furthermore final height was significantly correlated to mid-parental height.