Early Stages of Development
By the third trimester of intrauterine life, the human fetus weighs approximately 1000 gm and though far from ready for life outside the protective intrauterine environment, can often survive premature birth. During the last 3 months of intrauterine life, continued rapid growth results in a tripling of body mass to about 3000 gm. Dental development, which begins in the third month, proceeds rapidly thereafter (Table 3-1). Development of all primary teeth and the permanent first molars starts well before birth.
Although the proportion of the total body mass represented by the head decreases from the fourth month of intrauterine life onward because of the cephalocaudal gradient of growth discussed earlier, at birth the head is still nearly half the total body mass and represents the largest impediment to passage of the infant through the birth canal. Making the head longer and narrower obviously would facilitate birth, and this is accomplished by a literal distortion of its shape (Figure 3-1). The change of shape is possible because at birth, relatively large uncalcified fontanelles persist between the flat bones of the brain case. As the head is compressed within the birth canal, the brain case (calvarium) can increase in length and decrease in width, assuming the desired tubular form and easing passage through the birth canal.
FIGURE 3-1 This photograph of a newborn infant clearly shows the head distortion that accompanies (and facilitates) passage through the birth canal. Note that the head has been squeezed into a more elliptical or tubular “cone-head” shape, a distortion made possible by the presence of the relatively large fontanelles.
The relative lack of growth of the lower jaw prenatally also makes birth easier, since a prominent bony chin at the time of birth would be a considerable problem in passage through the birth canal. Many a young dentist, acutely aware of the orthodontic problems that can arise later because of skeletal mandibular deficiency, has been shocked to discover how incredibly mandibular deficient his or her own newborn is and has required reassurance that this is a perfectly normal and indeed desirable phenomenon. Postnatally, the mandible grows more than the other facial structures and gradually catches up, producing the eventual adult proportions.
Despite the physical adaptations that facilitate it, birth is a traumatic process. In the best of circumstances, being thrust into the world requires a dramatic set of physiologic adaptations. For a short period, growth ceases and often there is a small decrease in weight during the first 7 to 10 days of life. Such an interruption in growth produces a physical effect in skeletal tissues that are forming at the time because the orderly sequence of calcification is disturbed. The result is a noticeable line across both bones and teeth that are forming at the time. However, bones are not visible and are remodeled to such an extent that any lines caused by the growth arrest at birth would soon be covered over at any rate.
Teeth, on the other hand, are quite visible, and the extent of any growth disturbance related to birth can be seen in the enamel, which is not remodeled. Almost every child has a “neonatal line” across the surface of the primary teeth, its location varying from tooth to tooth depending on the stage of development at birth (Figure 3-2). Under normal circumstances, the line is so slight that it can be seen only if the tooth surface is magnified, but if the neonatal period was stormy, a prominent area of stained, distorted, or poorly calcified enamel can be the result.1
FIGURE 3-2 Primary teeth shown on a developmental scale that indicates the expected location of the neonatal line. From a chart of this type, the timing of illness or traumatic events that led to disturbances of enamel formation can be deduced from the location of enamel lines on various teeth.
Birth is not the only circumstance that can have this effect. As a general rule, growth disturbances lasting 1 to 2 weeks or more, such as the one that accompanies birth or one caused by a febrile illness later, will leave a visible record in the enamel of teeth forming at the time. Permanent as well as primary teeth can be affected by illnesses during infancy and early childhood.
The general pattern of physical development after birth is a continuation of the pattern of the late fetal period: rapid growth continues, with a relatively steady increase in height and weight, although the rate of growth declines as a percentage of the previous body size (Figure 3-3).
FIGURE 3-3 Graphs of growth in length and weight in infancy for boys (the curves for girls are almost identical at these ages). Note the extremely rapid growth in early infancy, with a progressive slowing after the first 6 months. (Based on data from the National Center for Health Statistics, Washington, DC.)
1. Premature Birth (Low Birth Weight). Infants weighing less than 2500 gm at birth are at greater risk of problems in the immediate postnatal period. Since low birth weight is a reflection of premature birth, it is reasonable to establish the prognosis in terms of birth weight rather than estimated gestational age. Until recent years, children with birth weights below 1500 gm often did not survive. Even with the best current specialized neonatal services, the chances of survival for extremely low birth weight infants (less than 1000 gm) are not good, though some now are saved.
If a premature infant survives the neonatal period, however, there is every reason to expect that growth will follow the normal pattern and that the child will gradually overcome the initial handicap (Figure 3-4). Premature infants can be expected to be small throughout the first and into the second years of life. In many instances, by the third year of life premature and normal-term infants are indistinguishable in attainment of developmental milestones.2
FIGURE 3-4 Growth curves for two at-risk groups of infants: small-for-gestational age (SGA) twins and twins of less than 1750 gm birth weight (premature birth). In this graph, 100 is the expected height and weight for normal, full-term infants. Note the recovery of the low birth weight infants over time. (Redrawn from Lowery GH. Growth and Development of Children. 8th ed. Chicago: Year Book Medical Publishers; 1986.)
2. Chronic Illness. Skeletal growth is a process that can occur only when the other requirements of the individual have been met. A certain amount of energy is necessary to maintain life. An additional amount is needed for activity, and a further increment is necessary for growth. For a normal child, perhaps 90% of the available energy must be “taken off the top” to meet the requirements for survival and activity, leaving 10% for growth.
Chronic illness alters this balance, leaving relatively less of the total energy available to support growth. Chronically ill children typically fall behind their healthier peers, and if the illness persists, the growth deficit is cumulative. An episode of acute illness leads to a temporary cessation of growth, but if the growth interruption is relatively brief, there will be no long-term effect. The more chronic the illness, the greater the cumulative impact. Obviously, the more severe the illness, the greater the impact at any given time. Children with congenital hormone deficiencies provide an excellent example. If the hormone is replaced, a dramatic improvement in growth and recovery toward normal height and weight often occurs (Figure 3-5). A congenital heart defect can have a similar effect on growth, and similarly dramatic effects on growth can accompany repair of the defect.3 In extreme cases, psychologic and emotional stress affect physical growth in somewhat the same way as chronic illness (Figure 3-6).
FIGURE 3-5 The curve for growth in height for a boy with isolated growth hormone deficiency. No treatment was possible until he was 6.2 years of age. At that point, human growth hormone (HGH) became available, and it was administered regularly from then until age 19, except for 6 months between 12.5 and 13 years. The beginning and end of HGH administration are indicated by the arrows. The open circles represent height plotted against bone age, thus delay in bone age is represented by the length of each horizontal dashed line. It is 3.5 years at the beginning of treatment and 0.8 years at 11 to 12 years, when catch-up was essentially complete. Note the very high growth rate immediately after treatment started, equal to the average rate of a 1-year-old infant. (Redrawn from Tanner JM, Whitehouse RH. Atlas of Children’s Growth. London: Academic Press; 1982.)
FIGURE 3-6 The effect of a change in social environment on growth of two children who had an obviously disturbed home environment, but no identifiable organic cause for the growth problem. When both children were placed in a special boarding school where presumably their psychosocial stress was lessened, both responded with above-average growth, though the more severely affected child was still outside the normal range 4 years later. The mechanism by which psychosocial stress can affect growth so markedly is thought to be induction of a reversible growth hormone deficiency, accompanied by disturbance of the nearby appetite center. (Redrawn from Tanner JM, Whitehouse RH. Atlas of Children’s Growth. London: Academic Press; 1982.)
3. Nutritional Status. For growth to occur, there must be a nutritional supply in excess of the amount necessary for mere survival. Chronically inadequate nutrition therefore has an effect similar to chronic illness. On the other hand, once a level of nutritional adequacy has been achieved, additional nutritional intake is not a stimulus to more rapid growth. Adequate nutrition, like reasonable overall health, is a necessary condition for normal growth but is not a stimulus to it.
An interesting phenomenon of the last 300 or 400 years, particularly the twentieth century, has been a generalized increase in size of most individuals. There has also been a lowering in the age of sexual maturation, so that children recently have grown faster and matured earlier than they did previously. Since 1900, in the United States the average height has increased 2 to 3 inches, and the average age of girls at first menstruation, the most reliable sign of sexual maturity, has decreased by more than 1 year (Figure 3-7). This “secular trend” toward more rapid growth and earlier maturation has continued until very recently and may still be occurring,4 although there is some evidence that this trend is leveling off.5 Signs of sexual maturation now appear in many otherwise-normal girls much earlier than the previously accepted standard dates, which have not been updated to match the secular change.
FIGURE 3-7 Age at menarche declined in both the United States and northern European countries in the first half of the twentieth century. On average, children are now larger at any given age than in the early 1900s, and they also mature more quickly. This secular trend seems to have leveled off in the early part of the twenty-first century. (Redrawn from Tanner JM. Foetus into Man. Cambridge, Mass: Harvard University Press; 1978; 1995 U.S. data from Herman-Giddens ME, et al. Pediatrics 99:505-512, 1997; 1995 British data from Cooper C, et al. Br J Obstet Gynaecol 103:814-817, 1996; Russian data from Dubrova YE, et al. Hum Biol 67:755-767, 1995.)
The trend undoubtedly is related to better nutrition, which allows the faster weight gain that by itself can trigger earlier maturation. Physical growth requires the formation of new protein, and it is likely that the amount of protein may have been a limiting factor for many populations in the past. A generally adequate diet that was low in trace minerals, vitamins, or other minor but important components also may have limited the rate of growth in the past, so even a small change to supply previously deficient items may in some instances have allowed a considerable increase in growth. Because a secular trend toward earlier maturity also has been observed in populations whose nutritional status does not seem to have improved significantly, nutrition may not be the entire explanation. Exposure to environmental chemicals that have estrogenic effects (like some pesticides, for instance) may be contributing to earlier sexual maturation.
Secular changes in body proportions, which presumably reflect environmental influences, also have been observed. It is interesting that skull proportions changed during the last century, with the head and face becoming taller and narrower.6 Some anthropologists feel that such changes are related to the trend toward a softer diet and less functional loading of the facial skeleton (see Chapter 5), but firm evidence does not exist.
The principal physiologic functions of the oral cavity are respiration, swallowing, mastication, and speech. Although it may seem odd to list respiration as an oral function, since the major portal for respiration is the nose, respiratory needs are a primary determinant of posture of the mandible and tongue.
At birth, if the newborn infant is to survive, an airway must be established within a few minutes and must be maintained thereafter. As Bosma7 demonstrated with a classic radiographic study of newborn infants, to open the airway, the mandible must be positioned downward and the tongue moved downward and forward away from the posterior pharyngeal wall. This allows air to be moved through the nose and across the pharynx into the lungs. Newborn infants are obligatory nasal breathers and do not survive without immediate medical support if the nasal passage is blocked at birth. Later, breathing through the mouth becomes physiologically possible. At all times during life, respiratory needs can alter the postural basis from which oral activities begin.
Respiratory movements are “practiced” in utero, although the lungs do not inflate at that time. Swallowing also occurs during the last months of fetal life, and it appears that swallowed amniotic fluid may be an important stimulus to activation of the infant’s immune system.
Once an airway has been established, the newborn infant’s next physiologic priority is to obtain milk and transfer it into the gastrointestinal system. This is accomplished by two maneuvers: suckling (not sucking, with which it is frequently confused) and swallowing.
The milk ducts of lactating mammals are surrounded by smooth muscle, which contracts to force out the milk. To obtain milk, the infant does not have to suck it from the mother’s breast and probably could not do so. Instead, the infant’s role is to stimulate the smooth muscle to contract and squirt milk into his mouth. This is done by suckling, consisting of small nibbling movements of the lips, a reflex action in infants. When the milk is squirted into the mouth, it is only necessary for the infant to groove the tongue and allow the milk to flow posteriorly into the pharynx and esophagus. The tongue, however, must be placed anteriorly in contact with the lower lip so that milk is in fact deposited on the tongue.
This sequence of events defines an infantile swallow, which is characterized by active contractions of the musculature of the lips, a tongue tip brought forward into contact with the lower lip, and little activity of the posterior tongue or pharyngeal musculature. Tongue-to-lower lip apposition is so common in infants that this posture is usually adopted at rest, and it is frequently possible to gently move the infant’s lip and note that the tongue tip moves with it, almost as if the two were glued together (Figure 3-8). The suckling reflex and the infantile swallow normally disappear during the first year of life.
As the infant matures, there is increasing activation of the elevator muscles of the mandible as the child swallows. As semisolid and eventually solid foods are added to the diet, it is necessary for the child to use the tongue in a more complex way to gather up a bolus, position it along the middle of the tongue, and transport it posteriorly. The chewing movements of a young child typically involve moving the mandible laterally as it opens, then bringing it back toward the midline and closing to bring the teeth into contact with the food. By the time the primary molars begin to erupt, this sort of juvenile chewing pattern is well established. Also, by this time, the more complex movements of the posterior part of the tongue have produced a definite transition beyond the infantile swallow.
Maturation of oral function can be characterized in general as following a gradient from anterior to posterior. At birth, the lips are relatively mature and capable of vigorous suckling activity, whereas more posterior structures are quite immature. As time passes, greater activity by the posterior parts of the tongue and more complex motions of the pharyngeal structures are acquired.
This principle of front-to-back maturation is particularly well illustrated by the acquisition of speech. The first speech sounds are the bilabial sounds /m/, /p/, and /b/, which is why an infant’s first word is likely to be “mama” or “papa.” Somewhat later, the tongue tip consonants like /t/ and /d/ appear. The sibilant /s/ and /z/ sounds, which require that the tongue tip be placed close to but not against the palate, come later still. The last speech sound, /r/, which requires precise positioning of the posterior tongue, often is not acquired until age 4 or 5.
Nearly all modern infants engage in some sort of habitual non-nutritive sucking—sucking a thumb, finger, or a similarly shaped object. Some fetuses have been reported to suck their thumbs in utero, and the vast majority of infants do so during the period from 6 months to 2 years or later. This is culturally determined to some extent, since children in primitive groups who are allowed ready access to the mother’s breast for a long period rarely suck any other object.8
After the primary molars erupt during the second year of life, drinking from a cup replaces drinking from a bottle or continued nursing at the mother’s breast, and the number of children who engage in non-nutritive sucking diminishes. When sucking activity stops, a continued transition in the pattern of swallow leads to the acquisition of an adult pattern. This type of swallow is characterized by a cessation of lip activity (i.e., lips relaxed, the placement of the tongue tip against the alveolar process behind the upper incisors, and the posterior teeth brought into occlusion during swallowing). As long as sucking habits persist, however, there will not be a total transition to the adult swallow.
Surveys of American children indicate that at age 8, about 60% have achieved an adult swallow, while the remaining 40% are still somewhere in the transition.9 After sucking habits are extinguished, a complete transition to the adult swallow may require some months. This is complicated, however, by the fact that an anterior open bite, which may well be present if a sucking habit has persisted for a long time, can delay the transition even further because of the physiologic need to seal the anterior space. The relationship of tongue position and the pattern of swallowing to malocclusion is discussed further in Chapter 5.
The chewing pattern of the adult is quite different from that of a typical child: an adult typically opens straight down, then moves the jaw laterally and brings the teeth into contact, whereas a child moves the jaw laterally on opening (Figure 3-9). The transition from the juvenile to the adult chewing pattern develops in conjunction with eruption of the permanent canines, at about age 12. Interestingly, adults who do not achieve normal function of the canine teeth because of a severe anterior open bite retain the juvenile chewing pattern.
At birth, neither the maxillary nor the mandibular alveolar process is well developed. Occasionally, a “natal tooth” is present, although the first primary teeth normally do not erupt until approximately 6 months of age. The natal tooth may be a supernumerary one, formed by an aberration in the development of the dental lamina, but usually is merely a very early but otherwise normal central incisor. Because of the possibility that it is perfectly normal, such a natal tooth should not be extracted casually.
The timing and sequence of eruption of the primary teeth are shown in Table 3-1. The dates of eruption are relatively variable; up to 6 months of acceleration or delay is within the normal range. The eruption sequence, however, is usually preserved. One can expect that the mandibular central incisors will erupt first, closely followed by the other incisors. After a 3- to 4-month interval, the mandibular and maxillary first molars erupt, followed in another 3 or 4 months by the maxillary and mandibular canines, which nearly fill the space between the lateral incisor and first molar. The primary dentition is usually completed at 24 to 30 months as the mandibular, then the maxillary, second molars erupt.
Spacing is normal throughout the anterior part of the primary dentition but is most noticeable in two locations, called the primate spaces. (Most subhuman primates have these spaces throughout life, thus the name.) In the maxillary arch, the primate space is located between the lateral incisors and canines, whereas in the mandibular arch, the space is between the canines and first molars (Figure 3-10). The primate spaces are normally present from the time the teeth erupt. Developmental spaces between the incisors are often present from the beginning but become somewhat larger as the child grows and the alveolar processes expand. Generalized spacing of the primary teeth is a requirement for proper alignment of the permanent incisors.
Late childhood, from age 5 or 6 to the onset of puberty, is characterized by important social and behavioral changes (see Chapter 2), but the physical development pattern of the previous period continues. The normally different rates of growth for different tissue systems, however, must be kept in mind. The maximum disparity in the development of different tissue systems occurs in late childhood (see Figure 2-2).
By age 7, a child has essentially completed his or her neural growth. The brain and the brain case are as large as they will ever be, and it is never necessary to buy the child a larger cap because of growth (unless, of course, the growth is of uncut hair). Lymphoid tissue throughout the body has proliferated beyond the usual adult levels, and large tonsils and adenoids are common. In contrast, growth of the sex organs has hardly begun and general body growth is only modestly advanced. During early childhood, the rate of general body growth declines from the rapid pace of infancy, then stabilizes at a moderate lower level during late childhood. Both nutrition and general health can affect the level at which stabilization occurs.
In planning orthodontic treatment, it can be important to know how much skeletal growth remains, so an evaluation of skeletal age is frequently needed. A reliable assessment of skeletal age must be based on the maturational status of markers within the skeletal system. The ossification of the bones of the hand and the wrist was for many years the standard for skeletal development (Figure 3-11). A radiograph of the hand and wrist provides a view of some 30 small bones, all of which have a predictable sequence of ossification. Although a view of no single bone is diagnostic, an assessment of the level of development of the bones in the wrist, hand, and fingers can give an accurate picture of a child’s skeletal development status. To do this, a hand–wrist radiograph of the patient is simply compared with standard radiographic images in an atlas of the de/>