and Hillson Text Ch 8
Hillson pp 198-20 Dental Cement
Cementum in humans forms in layers; however, those layers are very difficult to see in prepared ground sections. Due to uneven production and resorption, the layering is difficult to see beyond more than just a small area. Its use in humans is extremely limited. In some other animals (such as marine animals), cementum layering can be used as an ageing method.
Some teeth in some individuals experience a massive overproduction of cementum which is called hypercementosis. It even occurs in unerupted teeth.
Cementum preserves poorly in archaeological specimens.
Growth and Development I
(Note to the reader: The organization and general content in this section and the remaining three units are based on the previous textbook used in this course, Harrison et al Human Biology 3rd ed. and the classic texts in the field by Baer, Bogin, Krogman, Sinclair, and Tanner.
A comprehensive new publication, Ulijasjek, S., Johnson, F., and Preece, M. eds The Cambridge Encyclopedia of Human Growth and Development New York: Cambridge University Press, 1998 is a superb compilation of current literature in this field. It has been consulted throughout in the present revision of these notes. Citations within the text will be abbreviated as: CEDG page -.)
THE HUMAN GROWTH CURVE
I. AUXOLOGY: THE STUDY OF GROWTH (Harrison et al p 339)
The term auxology (Gr. auxein=to grow, to increase) describes the study of biological growth.
The first known instrument for measuring humans in the anthropometria, described in Italy nearly three centuries ago. It was the artists of the Renaissance who preceded science in an understanding of changing proportions in growth and development. The first book about changing proportions was by a professor of art; is also entitled Anthropometria.
A new tradition in growth study appeared in the 19th century as a reaction to the appalling conditions of the poor and their children in the Industrial Revolution. In the 1870s, Roberts and Galton contrasted the growth achieved by working class children and the privately educated upper class. Social class differences have been much investigated since. There is a close historical link between American and European longitudinal growth studies. They are a direct result of the child welfare movements in the 1920s. (See CEGD pp54-56; 5-7)
A fundamental observation was the human growth curve. A famous example is illustrated below. It was soon realized that individuals varied in two important parameters-size and rate of maturation.
The oldest published record of child growth is from 1759-1777, when Count Philibert Gueneau de Montbeillard measured the growth of his son. He plotted out the growth that single boy at 6 month intervals from six months to 18 years. It was the first longitudinal study of human growth.
Buffon, another Frenchman, wrote on the adolescent growth spurt and the more rapid maturation of girls. He also noted the seasonal variation of growth and the daily variation in stature--a boy was tallest arising from sleep in the morning than after a day of work and play. Others of the time observed that the youth of upper classes grew faster than those who were disadvantaged
During the Industrial Revolution in England, reformers prohibited employment by children under nine. Age was defined by dental maturation; the 2nd permanent molar became known as 'the factory tooth.'
(from history of growth in Spencer, F. History of Physical Anthropology: An Encyclopedia. 2 vols. New York: Garland Publishing Inc., 1997. see also Bogin, B. Also see CEGH pp 3-12)
Primates (and humans) have a slow reproductive turnover and drawn out life histories. 30% of our lifespan is devoted to growing (but less with longer lives).
Primates in general, and humans in particular have an extended time period between birth and sexual maturity. Humans and some primates make up for some of the early slowness ingrowth with accelerated growth at puberty.
Humans have a distinctly large brain and rapid post-natal brain growth is made necessary by the size limitations of the pelvic inlet. Some have said that humans have evolved 'post-birth fetal brain growth' to get around the pelvic opening limitation
Much of our lives are devoted to growing. Also, much of our lives are devoted to learning. We are dependent upon culture for survival.
(For a creative illustration of environment-brain wiring, see Burke, J. and Ornstein, R. The Axemaker's Gift. New York: Putnam, 1997. See CEGD pp 96-107.)
A Basic Understanding of Growth
If you think of growth as motion you can plot it graphically as distance traveled or as velocity.
Stature varies in the 24-hour day; therefore, in a precise longitudonal study it should be measured at the same time each day. We are tallest in the morning. Astronauts in weightlessness come home taller than they were before leaving the ground. This seems due to vertebral disc compression. Children are measurably shorter after vigorous play. (see Krogman p 3, Baer p 30)
II. PRENATAL GROWTH (Harrison et al p 340)
The velocity curve of growth in height begins long before birth. It reaches its peak at 18 weeks post menstrual. Growth in weight at 34th week post menstrual in utero and then slows down due to the influence of the maternal uterus. This response to the in utero is adaptive. It allows for a genetically large baby--and multiple births to be successfully complete the birthing process.
Growth retardation in uteri relates to maternal constitution, health, and the external environment. The implications for health are enormous. Persons having suffered growth retardation before births have more physical and mental disabilities plus certain chronic diseases later in life. (See CED pp 147-175 for a wealth of information on this subject.)
Birth weight and birth size reflect maternal environment more than the genotype of the child. Did you read that? Disadvantaged, malnourished young women having babies too soon give birth to low weight babies. Public health officials observe globally that birth weight and the height of children are leading indicators of a population's well being. (See Bogin p20)
There is a 'slowdown' before birth and a 'catch up' after birth. Thus, fetal growth slows, but rebounds following parturition so that the child 'catches up' to the overall growth pattern. A similar growth pattern is documented in chickens, pre- and post-hatching. (See Bogin p 44).
Postnatal growth is, for at least some tissues, a period of development and enlargement of existing cells rather than the formation of new ones. Fat cells are an exception--they increase until puberty.
III. FITTING GROWTH CURVES (Harrison et al p 343
Growth is an exceedingly regular process. Growth, development and maturation are consistent for all children. It follows a predictable pattern. In contrast, there is no biological or genetic plan to aging and senescence. (Growth curves can be expressed mathematically; more than 200 now exist. See Harrison et al pp 344-346 and Bogin, ch 7.)
Many children show a regular seasonal variation with a 6-month rhythm--an increase in the spring and a decrease rate in the fall. A child grows 3x faster in his/her annual time of greatest growth than in his/her slowest time of the year.
Growth Cyclicities and Pulsatilities
Don't be put off by this formal title. In the popular press, you'd call this mini topic 'biorhythms.'
Well-nourished children growth more quickly in height during spring and summer than they do during fall and winter. Could it be due to sunlight? A Swedish study exposed a group of boys to winter time 'sun lamp' treatments. During the treatment period, they grew 1.5 cm more than the control group. (Our neuro-endocrine system that regulates growth seems influenced by light in two ways, by the eyes and by the skin.)
But, this story has a twist: The control group made it up later in the year on their own. There was ultimately no size difference between the groups. Implication: Children seem to have a built-in one year endogenous growth cycle.
The lowest rate of weight gain correlates with the highest rate of height gain, and vice versa.
Another finding is this: careful study has shown that on a daily or weekly basis, periods of little or no growth are followed by bursts of rapid growth. How these patterns are controlled in unknown. (CEGD pp 220-221)
Trivia Did you know that normal adults have a marked daily rhythm of deep body temperature? It falls to 36.2 degrees Celsius in early morning and rises to 37.2 degrees Celsius in the afternoon. The rhythm is absent in newborns; the adult pattern is achieved by age six months. It seems to be associated with heart rates. (CEGD pp 191-192.
These are some factors that influence growth (see Sinclair Ch 7, Bogin Ch 5)
(1) Genetic control We inherit our genome.
(2) Neural control Some suspect that the brains's hypothalamus exerts control over some aspects of growth.
(3) Hormonal control The role of hormones is well verified.
(5) Secular (generational) growth trend
(6) Socioeconomic class Well-off children grow taller than those who are disadvantaged.
(7) Season, climate, and altitude Growth in height is faster in the spring than the fall; weight is the opposite-more weight goes on in the fall--a possible relic of evolution. Daylight may be a factor. The rhythm is random in totally blind children.
(9) Emotional deprivation This was well verified in a World War II internment camp for children run by nasty people.
(10) Migration and Urbanization This topic is complex. The best way to summarize this is to cite from medical anthropology: when people of any ethnic background move and take up a new culture, they tend to acquire the growth characteristics (and diseases) of that culture.
There are two general ways to study growth
Both studies have their uses but they do not give the same information and cannot be handled in the same way.
IV. GROWTH CURVES OF DIFFERENT TISSUES AND DIFFERENT PARTS OF THE BODY (Harrison et al p 348)
A better descriptive title for this unit would be patterns of systemic growth. (see Krogman p 5, Baer p 7, Tanner p 11) In the 1920s, Richard Scammon produced his famous chart. It has decorated the slide shows of orthodontists for decades. (See CEGD pp 212-213 for a current article.)
The chart is a consolidation of many specific curves. You should understand that this chart illustrates key differences between non primate and primate growth patterns. Primates--especially humans--delay body growth and reproductive development, but do not delay brain growth. In one obvious way, this pattern of growth relates to the fact that primates are learning creatures par excellence.
As a rule, the dimensions of most bones, muscles, and organs such as the liver and kidney follow the same growth curve as does stature. Height and most body measurements follow the 'body' curve. (Bunney p 99)
Keep in mind that the Scammon illustration is a DISTANCE chart. It assumes 0% at birth, 100% at adulthood. These values are for attained growth at a given age, NOT velocity!
Nearly every textbook illustrates the change in body proportion from birth to adulthood in a similar fashion as illustrated here.
(1) Neural Growth.
This includes the brain, spinal cord, optic apparatus, cranium, and the vertebral column. The brain curve is for total weight.
At birth, the brain is 25% of its adult weight; by age five it is 90%. Thus, there is little if any pubertal spurt evident in its growth pattern.
By the age of seven or eight, the brain has achieved 95% of its adult size, shape, and its nerve pathway architecture.
The curve of course represents the growth of the cranium, the braincase. This growth curve has biological, cultural, and evolutionary significance. As the brain (and head) get bigger, there is a problem--how to deliver it through the bony ring of the pelvis. The human pelvis is a compromise between efficient upright posture, bipedalism, and the size of the birth canal in relation to childbirth.
In sexual dimorphism, the pelvis achieves a compromise between upright walking and size of the birth canal. This is reflected in the difference between male and female Olympic running and jumping records.
(My note: male and female performance in sports has been studied. The energy cost of physical activity is the same in men and women. There are no differences in technical skill. Women have some advantages in swimming. See Astrand pp 247-256 and CEGD pp 216-219.)
Female women give birth to a fetus with a large head--this is partially accommodated by the requirement for a 90% rotation in transit thru the pelvis
There is a curious cultural implication: Humans are born face backwards unlike other hominoids. Somewhere along the way hominids acquired birth attendants. Incidentally, in the majority of cultures, birth attendants are female. Male doctors are an aberration. The trend back to midwives is a return to the traditional pattern.
(My note: Before the rise of the European universities and men in medicine, the herbal knowledge of birth control was passed down through midwives. Did you know, for example, that the seed of Queen Anne's lace, a member of the carrot family is a potent anti fertility agent? The seeds need to be chewed to be effective. For a fascinating review of this subject, see Riddle, J. Eve's Herbs.) Cambridge: Harvard University Press, 1997.)
The popular notion is that our evolutionary trend is for larger and larger brains & heads: it could have some bizarre implications. (See Baer)
Human babies are born after a long gestation period. They are relatively large compared to the mother's size. Unlike many mammals, eyes and ears are open. Like most mammals, there is a coat of hair (lanugo Latin=soft down) that is shed just before birth. The brain is delivered immaturely; a large proportion of the brain's growth takes place after birth. Rapid brain growth continues for a year after birth--a specialization unique to humans.
Some have said that 'human gestation is 21 months long with the majority of it (12 months) in the mother's care post-partum.' (Bunney p 86)
We complete this brief sketch of the biological, cultural, and evolution significance of brain and head growth with this: we are dependent on culture for child rearing, enculturation, learning, language development, emotional maturation and all the rest that makes us human. Our brain immaturity 'sets us up' for these events.
The head is more advanced at birth than the rest of the body; also the cranium is more advanced than the face. Don't you recall the old rule from developmental biology that in embryonic growth that growth proceeds in a cephalic-caudad direction?
(2) General Growth
This includes the external dimension of the body as a whole (except the brain and eye). More specifically, the curve represents growth in stature or body weight (CEGD p 92). The General Growth curve resembles the distance curve for Montebeillard's son.
This also includes the respiratory, vascular, and respiratory systems, skeleton, musculature, kidney and the bladder. The general curve is mathematically a sigmoid, an S curve on its side. It is like the distance curve for human growth.
This curve might also be called the 'body' curve. Krogman calls it the 'bone, brawn, blood. breathing, bowel and bladder curve.' (Krogman p 7)
(3) Lymphoid Tissue
The lymphoid tissue--tonsils, adenoids, appendix, intestinal, lymphatic masses have a very different curve. It rises to reach 2x adult size (between ages ten and 15) and then recedes. It is probably under the influence of the sex hormones as it declines to its adult value. (Bunney p 99)
In traditional physiology, the lymphatic system was described as returning blood from tissue spaces, and redistributing fluid in the body, and removing particular matter such as bacteria from the body.
More recently, especially because of AIDS research, it is known that the lymphatic system has a fundamental role in the functioning of the immune system. (See Sinclair p 94) Lymphocytes, part of the immune system are widely distributed in the body-inthe thymus, llymph nodes, other tissues and in the circulation. (See CEGD p 213)
The thymus is a prominent feature in the chest of an infant. In time, 'daughter' colonies form elsewhere in the body which become self supporting. The thymus is difficult to find in the adult; in old age it regresses to just a remnant of scar tissue.
This curve is similar to the general (body) curve, but is different in scale. It is based on the weight of the gonads. The growth of the external and internal reproductive organs before puberty is slow, but its growth at adolescence is very rapid. (Bunney p 99, Krogman p 7)
Why is the pubertal growth spurt separated from the rapid growth experienced after birth. One possible evolutionary reason is that delayed pregnancy/child rearing responsibilities are postponed to allow for an extended period of learning. Another reason is suggested by the allocation of labor in pre industrial societies. In herding and subsistence farming societies, child labor is very important. It contributes to survival of the society as a whole. Thus, delayed sexual maturation has survival value. (see Bogin p 61)
We grow up in various proportions. The proportions of growth can be illustrated
by the "2,3,4,5 rule."
2x head and neck
Body Composition and Body Fat (See CEGD pp 212-215)
Body composition changes throughout growth and development. For understanding, it is convenient to classify the body anatomically into two components: lean body mass and body fat.
(1) Lean Body Mass
The growths of many organs mirror the growth in height; others like the brain, reproductive organs, and the thymus follow their own trajectory.
Before adolescence, boys have only a slightly greater mean body mass than girls; however, with the changes wrought by adolescence, young adult men have 50% more lean body mass than do young adult women.
(2) Body Fat (See also CEGD pp 230-232)
Fat is one of the most labile tissues in the body and is subject to both genetic and environmental factors.
Human newborns have 15% body fat; this rises to 30% at age one year. By young adulthood, women have 50% more body fat that young men. To put this in perspective, however, post adolescent males have about 12% body fat whilst females have about 20% body fat.
There is a redistribution of body fat at adolescence, creating the body contours of manhood and womanhood.
The distribution of body fat with more of it accumulating on the trunk rather than the extremities has important health implications. Trunk fat deposits are a risk factor for type II diabetes and cardiovascular disease for persons living in industrialized societies. 'Modernization' has brought with it increases in body weight and an increasing incidence of obesity. This is confirmed by many studies of immigrant groups in the United States, Australia, and elsewhere. Some groups are more susceptible than others.
V. Organization and Disorganization of the Growth Process (Harrison et al p 352)
Growth in a single direction is a very regular process, yet there is differential growth of tissues and portions of the body. Differential growth makes us different from non primate species.
The process of growth is self-stabilizing, or better said-- is 'target seeking'. Canalization means a sort of moving homeostasis--the tendency to return to the trajectory after 'being pushed off'. The catch up is called 'compensatory growth' by nutritionists. If the insult is unusually prolonged and severe then the catch up is incomplete. Girls are less likely to be thrown off their growth curve by insult than our boys. (See Park, M Biological Anthropology. Mountain View: Mayfield Publishing Company, 1995 p 292.)
The mechanism for canalization is obscure.
(B) Growth gradients
Think of this as differential growth for different parts of the body. The head is at all ages in advance of the trunk.
The head is in advance of the trunk which is in advance of the limbs. But, within the trunk it doesn't quite fit the 'cephalic first' rule.
The regulative forces do not always succeed. Downs trisomy 21 is an example.
It is suspected that there are genes, or groups of genes that determine amounts of growth and rates of development. Specific genes for growth have not yet been identified. Studies of chromosomal anomalies suggest that genetic factors on the Y chromosome produce the male pattern of growth in body proportion and skeletal development. (See Bogin pp 169-173)
(C) Sensitive periods
This recognizes that some tissues are sensitive for brief time periods. The induction of tooth germ tissues are an example. They are time sensitive.
Tanner cites the kitten's eye example in the text; we mentioned the critical earning hypothesis for language last time.
VI. POST-ADOLESCENT GROWTH (Harrison et al p 357)
Growth of the skeleton does not entirely cease at the end of the adolescent period. In humans, the epiphysises of the long bones close completely and cannot afterwards be stimulated to grow again.
The vertebral column continues to grow from age 20 to 30 years by apposition to the tops and bottoms of the vertebral bodies. Height increases by 3-5 mm (3/16th") during these years.
From 30-50 years height is constant. Thereafter, height begins to decline.
Most head and face measurements continue to increase after adolescence steadily, though very slowly to at least age 60. The increase from ages 20-60 is 2-4% of the age 20 values.
For practical purposes, adult height is achieved at 17.5 yrs for boys, 16.0 yrs for girls, + or - two years.
Another hallmark of adulthood is reproductive maturity. Adolescent girls who become pregnant early after menses tend to have a high percentage of spontaneous abortions. Teenage girls often have low birth weight infants. Experts agree that full female productive maturity is not achieved until the end of adolescent life.
On average, boys start producing sperm at age 14 1/2 years. The ability to successfully father children is usually achieved late in adolescence, although numerous exceptions make it to the supermarket tabloids.
The course of growth and development during adulthood is not easily described. There is a lack of precisely timed or sequenced physiological events.
Exercise training increases muscle size and caloric intake excess certainly can add pounds of adipose tissue.
The most striking feature of the adult stage of life is its stability (homeostasis) and its resistance to pathological influences by parasites or psychological stress. (Bogin pp 38-40)
VII THE HUMAN GROWTH CURVE AS A PRIMATE CHARACTERISTIC (See Harrison et al p 358)
We share our type of growth curve with apes & monkeys; it is not seen in rodents or cattle. The acquisition of the adolescent growth spurt is seen as an evolutionary advance by primates. As noted above, it is advantageous for learning and child labor.
The prolonged delay in human growth due to the evolution of a childhood period of growth distinguishes it from all others.
THE ADOLESCENT GROWTH SPURT AND DEVELOPMENTAL AGE
The adolescent growth spurt is a constant phenomenon and occurs (with variable intensity) in all children. It occurs about two years earlier in girls. It is not easily discernable in cross-section curvers of growth; it is most dramatic in a distance curve for a single individual.
The conclusion of the spurt is followed by a rapid slowing of growth. Girls reach 98 % of their final height by age 16 1/2 years; for boys it is achieved at about 17 3/4 years. There is wide individual variation. (Sinclair p 31)
The later occurence in males allows for the greater increase and the dimorphic difference in height between the sexes.
The facial profile becomes straighter and the incisors more upright.
Chronic and severe illness, malnutrition, and psychological stress can obliterate the growth spurt. Quechua Indian youth in the Peruvian Andes have a late and poorly defined growth spurt. Their adolescent growth period is prolonged, lasting to 22 years and beyond. (Bogin p 62)
I. SEX DIFFERENCES (Harrison et al p 362)
Hormonal changes account for changes in body proportion at puberty.
The most striking difference between male and female growth is the earlier maturation of the female. This was noticed by Buffon and others over two centuries ago.
II. DEVELOPMENT OF THE REPRODUCTIVE SYSTEM (Harrison et al p 363)
This section in the Harrison text is especially well written. These general remarks express the essence of this unit:
(1) The sequence of events is less variable than the age at which they take place.
(2) For boys, the first signs are the accelerated growth of the testis, scrotum, and pubic hair.
(3) For girls, appearance of the budding breast and (usually) pubic hairs are significant first signs.
Krogman speaks of the "events of puberty" and says that the sequence of these events can vary. (See Tanner p 77, Krogman pp 103-104 and p 108). Here is a somewhat eclectic summary of the events at puberty as summarized by Krogman:
Important: secondary sex characteristics and skeletal age are closely related; they seem to share hormonal control.
There is much less correlation between dental development and skeletal age than there is between skeletal age and secondary sex characteristics.
Dental development seems unaffected by male and female sex hormones or the adrenal hormones. Dental development is affected by hypothyroidism.
Menarche occurs just after the peak in the growth spurt.
III. CHANGES IN PHYSIOLOGICAL FUNCTION AND MOTOR DEVELOPMENT (Harrison et al p 367)
Changes in physiological function are greater in boys than girls. The male at adolescence becomes more adapted to hunting, fighting, and handling heavy objects.
The athletic ability of boys greatly increases at adolescence.
IV. DEVELOPMENTAL AGE AND THE CONCEPT OF PHYSIOLOGICAL MATURITY (Harrison et al p 369)
Once again, the age at which the events of adolescence occur is more variable than the sequence.
What kind of 'clock' or measure of age do we use? Chronological age in our culture is dated from birth. That is ethnocentric; many cultures date age from the time of conception. (See Krogman p 5)
In our discussion forthcoming we will discuss biological age which is classified as follows:
(1) Skeletal Maturity
A time-honored way to measure this is by a hand wrist film and to use an atlas such as Greulich and Pyle. You should be aware that there are others: Todd, Tanner & Whitehouse, to name just two of the standards developed for the wrist. (see Tanner pp 56-61). The Todd standard is based on upper income American children of the 1930s. The Greulich and Pyle atlas is an updated and modified version of the Todd standards. Tanner & Whitehouse was based on British children of ordinary means in the 1950s. (See CEGD pp 42-44)
Skeletal age is a measure of how far along the bones are in their course of development as recorded by X-ray appearance.
The left hand is used-a leftover from the anthropological tradition of measuring the left side. Does right or left side matter for assessing skeletal age with an atlas? No.
There are 51 different centers available for measurement.
The sequence of events in each bone is essentially the same in all individuals, irrespective of whether the bone is advanced or retarded in relation to chronological age. Read that statement again. You should remember that there is an orderly sequence in skeletal growth. If growth and development are advanced or retarded, the sequence remains the same.
Skeletal maturity is judged both on the number of centers present and the stage of development.
Earlier maturities are seen in girls than in boys.
Skeletal maturation appears to be under tight genetic control. Tissue transplant experiments where an immature limb bone is transplanted under the skin of a mouse of the same inbred strain will grow and undergo ossification at the normal place and time. (Tanner pp 62-63)
(2) Dental Maturity (See Tanner p 67)
The staging is done on the same principle as skeletal age, using eruption or appearance of a center as a marker. This thus acknowledges the use of eruption or calcification as a marker as a criterion. Clinicians define eruption as emergence into the oral cavity through the gingival tissues. Workers with skeletal material define eruption as emergence through alveolar bone.
The onset of crown calcification, root formation, emergence from alveolar bone and appearance in the oral cavity have been used as measures of maturation. There is a wealth of dental literature on tooth eruption.
Tables in dental textbooks tend to be unisex which blurs the significant differences between males and females in dental maturity. Clinically, the sex difference is not an important issue, but it does become a significant factor in scientific study.
Tooth formation proceeds in an invariant sequence. (See CEGD "1.9 Dental Maturation" pp 45-48) Timing, however can vary from person to person. Most adverse conditions can slow down the formation and emergence of the teeth. Thus, conditions that cause somatic growth to slow down also slow the rate of tooth formation and eruption.
Further, there is evidence of a secular trend in dental maturation: teeth now form and erupt at earlier chronological ages than in the past. Europeans are rather slow maturers; sub-Saharan Africans and African-Americans develop their dentitions sooner than whites. Asians are somewhat in between (CEGD p 47)
Important Things to Remember
(a) Statistically, girls shed their deciduous teeth earlier than boys.
(b) Statistically, girls have every permanent tooth come in earlier by 2-11 months than boys. (Tanner p 70)
(c) Dental age is of LESS value than skeletal age or secondary sexual characteristics age in judging physical maturity. (Tanner pp 73 and 77)
(d) Statistically, there is no difference between right and left sides. Thus, there is no directional asymmetry in human tooth eruption.
(See CEGD pp 209-211)
Historical trivia: In the industrial revolution in Britain, a child was judged old enough to work with the emergence of the permanent second molar; it was called the 'factory tooth.' The 'wisdom' tooth is called that because supposedly a person had discretion by that age. (Sinclair p 109)
(3) Relationships between different measures of maturity
Skeletal age and menarche are closely related. Bone age is NOT related to the appearance of breasts or pubic hair. Why? An occasional girl may show the other outward signs of physical maturity, but has not yet menstruated. In this case--the skeletal age would also be behind.
These consistencies or inconsistencies reflect differences in the organization of the hypothalmus and pituitary. Children who are advanced or delayed in skeletal age tend to remain so in their development. This exemplifies 'canalization' and illustrates one of the traditional observations on growth: it is target-seeking.
Teeth and bodily development tend to be independent of one another. (My comment: this statement brings up an inconsistency in our thinking. On one hand we say that dental age correlates poorly with skeletal age. On the other hand in archaeology we consider dental age very accurate in staging skeletal material. You can resolve this one for yourself.)
The percentage of adult height attained at a given age is quite closely related to skeletal age from about age 7 onwards.
Girls are on average ahead of boys in skeletal maturity and adult dentition, but not for deciduous development.
Skeletal age difference begins in fetal life. At birth, boys are four weeks behind. The sex difference in maturity is widespread; it is seen in most mammals.
If a person develops early in physical development, then mental development is more advanced also.
V. DEVELOPMENTAL VARIATION IN CHILDHOOD (This is a collection of 'odds and ends' from Overfield pp 39-46)
(My concluding remark: if you are doing research on any aspect of biologic variation, health or illness--look at the Overfield text. It is incredibly well documented and is very eclectic in style.
.... CJ '99
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