Radiographic determination of prenatal basicranial ossification

In a previous investigation on prenatal development of the human cranial base, the sequence in which the bones develop in the midsagittal region was elucidated. The purpose of the present study was to identify fetal ossification on horizontal plane roentgenograms of the occipital and sphenoid bones in the central part of the cranial base, and establish stages in bone appearance related to general fetal developmental parameters. This study is based upon roentgenograms of the cranial base of 145 human fetuses from the first half of the prenatal period. Two different maturation patterns of the sphenoid bone were observed. The first, most common pattern is characterized by a midsagittal centre of ossification and the second by bilateral centres of ossification in the corpus of the sphenoid bone. These bilateral centres might in some cases be connected by a slight bony bridge. It appears that these different maturation patterns are maintained throughout the period investigated. The material was divided into five well-defined developmental stages for both maturation patterns and general parameters of fetal development. Mapping different aspects of ossification in normal cranial development is necessary for understanding deviations of cranial maturation and growth.     

Skeletal maturation of the human fetus assessed radiographically on the basis of ossification sequences in the hand and foot

In studies of postnatal human development the skeletal maturation of the hand has been found to be a better indicator of general physical maturation than attained body height. For assessment of prenatal human development the Crown-rump length (CRL) has so far been the most commonly used measure. The object of the present study is to examine the possibility of also using the skeletal maturation of the hand as a maturity indicator in fetal development. The study is based upon a radiographic and histochemical investigation of 169 human fetuses. On the basis of counting silver-impregnated diaphyses on radiographs of the hand and foot a maturity indicator (CNO = Composite Number of Ossified bones in hand and foot) was established. Owing to the marked regularity of the recorded ossification pattern, the CNO parameter can be used for evaluating fetal maturation during the early half of the prenatal period. To supplement the assessment of skeletal maturation during the later stages of development, a classification based on the shape of some bones was included in the study. In many cases fetuses of the same size (CRL) exhibited different stages of skeletal maturation (CNO). In accordance with findings from assessment of postnatal development, a more accurate evaluation of fetal development is obtained by combining the size parameter CRL with an assessment of fetal skeletal maturation, CNO.     

Cranial base angulation and prognathism related to cranial and general skeletal maturation in human fetuses.

The purpose of the present study was to describe normal midsagittal craniofacial morphology in second trimester human fetuses. Measurements of the cranial base angle and the prognathism of the maxilla and the mandible were performed on radiographs of cranial midsagittal tissue blocks of 52 fetuses with a gestational age from 13 to 27 weeks. Special procedures were developed for the definitions of the nasion and sella reference points on the radiographs in the early stages of fetal development. Mean data were reported for stages of crown rump length (CRL) and maturation of the fetal cranial base (MSS), usable as reference in assessment of pathological fetal crania in reports and autopsy procedures. Regression equations were determined for the regression of the angular values on CRL, MSS, and general skeletal maturation (TNO). The cranial base angle was found to decrease significantly, and the angles of prognathism to increase significantly with increasing CRL, TNO, and MSS values. It was suggested that these simultaneous and similar changes in the three angles could be accounted for by the upwards movement of the sella point produced by a cranial displacement of the pituitary fossa caused by local cartilagenous growth and bony remodelling during the period of study. The study thus reflects the influence of cranial skeletal maturation on the early development in shape of the craniofacial complex.     

Prenatal human cranial development evaluated on coronal plane radiographs

The purpose of the present investigation has been to analyse prenatal cranial base development in the coronal plane and to combine the findings with results in former reports on cranial base maturation estimated in the horizontal and sagittal planes. The study is based upon cranial bases of 26 human fetuses from the first half of the prenatal period. Fetal coronal plane cranial base tissue blocks were dissected, radiographed, and sectioned for microscopic examination. Five stages in cranial base development are defined and related to general parameters for fetal size and fetal maturation. Two different maturation patterns were recognized in the sphenoid corpus. Canal structures, remnants of the craniopharyngeal canal, were observed in specimens showing bilateral centers of ossification in the sphenoid corpus. The radiographic method used is easy to record and recommended for diagnosing prenatal and neonatal cranial malformations.     

Human prenatal palatal closure related to skeletal maturity of the jaws

The purpose of the present study was to elucidate the stages of skeletal maturation of the maxilla and the mandible at the time of soft tissue palatal closure. Similar studies were not found in the literature. This investigation was based on maxillae and mandibles from 19 human embryos/fetuses selected by visual inspection of palatal structures, eight fetuses "just before palatal closure," and 11 fetuses "just after palatal closure." The findings are related to formerly described skeletal developmental stages in the maxilla, to stages in the symphysis menti region, and to development in the mandibular condylar region. The present study revealed that elevation of the palatal shelves takes place at a specific stage of maxillary skeletal maturity (stage Max III), at a time of mandibular development characterized by absence of condylar cartilage and by constancy in symphysis menti maturity (stage SM I). Knowledge of the normal sequence of prenatal skeletal development is considered essential for understanding the abnormal sequence of skeletal development.     

Craniofacial growth of fetal Macaca nemestrina: A cephalometric roentgenographic study

The prenatal growth of the macaque craniofacial skeleton is described using lateral radiographs of 82 fetal and 25 neonatal Macaca nemestrina whose known gestational ages range from 50 to 186 days. The ossification sequence of the craniofacial bones resembles that in the human fetus. During gestation, the macaque neurocranium loses its round, globular shape, becoming flattened and elongated in an anteroposterior direction. In contrast, the morphologic pattern of the face is established early in fetal life, and little change takes place during the remaining prenatal period. The macaque craniofacial dimensions develop along the general skeletal growth pattern, unlike the human craniofacial dimensions, which follow an intermediate pattern between the neural and general skeletal patterns. However, despite minor differences, the macaque and human fetal faces follow the same basic patterns of growth.     

Fetal growth and development of the coypu (Myocastor coypus): Prenatal growth,tooth eruption,and cranial ossification

We examined the fetal growth and development of the coypu (Myocastor coypus), a member of the Caviomorpha that produces extremely precocial young. Analyses of 69 fetuses derived from the latter half of the prenatal period (60–125 days of gestation) focused on external feature growth and development, tooth eruption, and cranial ossification. There were four developmental stages based on morphological characteristics; major external changes predominated over somatic growth in the early stages by 100–105 days of gestation, whereas the last stage was a time of rapid somatic growth. Growth rate was greater in hind foot length (4.3) than in fore foot length (3.4). Soft X-ray photos from 120 to 125 days of gestation show that the incisors, premolars, and first molars were completely calcified, and the second molars were present in the alveolus but not completely calcified. The occlusal surfaces of these teeth were subjected to wear. We analyzed the bone and cartilage of the coypu fetal cranium using a double-staining method. Early ossification of the jugular processes of the occipital bone was a prominent feature of coypu development. The digastric muscle originates on the jugular process, and early ossification should be linked to an adaptation to the herbivorous habit of weaned young coypu. Additionally, the sizes and closure times of six fontanelles are correlated with gestational age and are suggested as a comparative parameter for fetal maturity within and between mammalian species.     

Investigating histomorphological variations in human cranial bones through ontogeny

Through ontogeny, human cranial vault bones undergo differentiation in terms of their shape, size and tissue maturation. This differentiation is visible at both the macroscopic and microscopic levels. Preliminary data from a histological and compartmentalisation exploratory analysis of individuals with different ages suggest differences in the modelling and remodelling patterns through ontogeny. Child vault bones are primarily composed of avascular lamellar bone (largely vascularised), late juvenile or adolescent bones present the largest extension of mineralised areas (highly remodelled) and the lowest vascularisation (diploe is highly reduced), and the adult present highly vascularised bone in which the diploe is again largely extended. During childhood, the existence of an avascular lamellar bone promotes the sealing of the cranium bones surfaces whereas adult vault bones seem to become opened ectocranially due to the remodelling. We discuss the possibility that both effects could be related with the head thermoregulation.     

Long bone development in extrinsic fetal akinesia: an experimental study in rat fetuses subjected to oligohydramnios.

The transverse growth of long bones during intrauterine development was studied in rat fetuses subjected to experimental oligohydramnios in order to determine whether the skeletal changes, if any, in extrinsic fetal akinesia were similar to those observed in curarized rat fetuses with the fetal akinesia deformation sequence. Oligohydramnios was induced by daily extraction of amniotic fluid from day 17 of gestation until term. Experimental fetuses were compared with a sham-operated control group. The total area and perimeter, the absolute and relative amount of periosteum and bone trabeculae, the major and minor axes, and the elongation factor were measured in histological cross sections of the femoral metaphysis and diaphysis with an IBAS 1 image analysis system. Rat fetuses in the experimental group showed multiple articular contractures, redundant skin, and lung hypoplasia, a phenotype consistent with the oligohydramnios sequence. No alterations in femoral shape and transverse growth of the metaphysis and diaphysis were noted in these fetuses. These results suggest that the main mechanical factor related to fetal bone modeling is muscular strength, while motion would be mainly involved in fetal joint development.     

The craniofacial skeleton in anencephalic human fetuses. II. Calvarium.

A detailed study of the calvarium of twelve anencephalic and four normal human fetuses 26 to 40 weeks gestational age using gross dissection, alizarin red S staining, silver nitrate radiography and histology revealed dramatic alterations in the presence, form, location and relationship of the individual bones. In the larger dorsal cranial defects the interparietal portions of the occipital bone were relocated anteriorly to approximate the frontal bone. The occipital components were rotated anterolaterally and inferiorly with lack of fusion of the chondrocranium posterior to the foramen magnum. The squamae of the frontal bone were collapsed horizontally and reduced in size to lie peripheral to the anterior cranial fossa forming most of the orbital roofs. In anencephaly the bones derived from the chondrocranium were not as severely affected morphologically as those derived from the neurocranium. The sutures were narrow and smooth instead of wide and serrated as in the normally developing calvarium. In general the degree of maldevelopment was proportional to the extent of the dorsal cranial defect in anencephaly.     

An experimental study of intraspecific variation, developmental timing, and heterochrony in fishes

Heterochrony is widely regarded as an important evolutionary mechanism, one that may underlie most, if not all, morphological evolution, yet relatively few studies have examined variation in the sequence of development. Even fewer studies have been designed so that intraspecific variation in the relative sequence of developmental events can be assessed, although this variation must be the basis for evolutionary change. Intraspecific variation in developmental ossification sequences was documented from the zebrafish (Danio rerio) by Cubbage and Mabee (1996) and from the Siamese fighting fish (Betta splendens) by Mabee and Trendler (1996), but a quantitative analysis of the patterns within this variation was not made. Here, we quantify the effect of rearing temperature on the sequence of ossification and characterize the levels and patterns of intraspecific variation in these fishes. For Danio, there were no temperature effects on the sequence of bone development across the cranium, cranial region development, cartilage versus dermal bones, or lateral line bone versus nonassociated bones. Likewise the level of variation in relative sequence (position) of ossification was low, about two ranks, across temperatures. At higher temperatures, we found higher levels of variation in iterated cranial bones and less in bones forming early in the sequence. No temperature effects on variation were found among regions, between lateral line-associated bones and nonassociated bones, between median and paired bones, or across the entire sequence, indicating concordant variability among the three temperatures. Individual bones with the highest levels of variability were not consistent among temperatures. Baseline patterns of intraspecific variation in Danio were compared to those of Betta. For both species, the level of intraspecific variation in sequence position was low and the variability of cranial bones was concordant. Individual bones with the highest levels of variability were not consistent between species. In both species, variation was widespread (distributed evenly across the sequence). We used comparisons (among regions, between dermal and cartilage bones, between lateral line-associated and other bones, between median and paired bones, between iterated and noniterated bones, between feeding-associated bones and others) to see which subsets were most variable and thus potentially useful in predicting high levels of evolutionary change. The only subset of bones that was significantly more variable than others was cartilage bones. If interspecific patterns are parallel to these intraspecific differences, cartilage bones would be expected to show higher levels of heterochrony. Although concordance across the cranial ossification sequence and among regions in Danio, Betta, and two other teleosts, Oryzias and Barbus, suggests an evolutionarily conserved pattern of ossification, identity in sequence position across taxa was not observed for any bone. Thus, variation existed in sequence position across temperatures and species. Intraspecific variation of this sort may influence the morphological outcome and evolutionary trajectories of species.     

Studies on the growth of the fetal guinea pig. The effects of ligation of the uterine artery on organ growth and development

The effects of reduced maternal placental blood flow on the growth and development of the fetal guinea pig have been studied by unilateral ligation of the uterine artery at day 30 of pregnancy. Fetal guinea pigs were investigated about 20 or 30 days later. In about one-third of cases fetal death occurred, in another third fetuses less than 60% of normal weight were observed and in the remainder all fetuses were in the normal weight range. In the growth retarded fetuses prenatal growth occurred at about 50% of the rate in control. There was no postnatal 'catch up' as growth still remained lower than in controls. Restricted fetal growth affected particularly development of the visceral tissues in which case size declined in proportion to body weight. Brain and adrenal by comparison were less affected as their contribution to total body weight increased, but even so in the severely retarded fetuses the mass of both fell. The responses of the liver were in general consistent with a delay in the pattern of development. Thus DNA, RNA, protein and haematopoietic cell content changes occurred later than normal. In contrast an enhanced deposition of glycogen was apparent in the liver of the growth-retarded fetus. The results indicate some of the ways in which nutritional deprivation of the fetuses leads to reprogramming of growth and maturation of selected fetal tissues to allow non-essential changes to await more favourable times.     

Development of the mouse mandibles and clavicles in the absence of skeletal myogenesis

In this report we employed double-knock-out mouse embryos and fetuses (designated as Myf5-/-: MyoD-/- that completely lacked striated musculature to study bone development in the absence of mechanical stimuli from the musculature and to distinguish between the effects that static loading and weight-bearing exhibit on embryonic development of skeletal system. We concentrated on development of the mandibles (= dentary) and clavicles because their formation is characterized by intramembranous and endochondral ossification via formation of secondary cartilage that is dependent on mechanical stimuli from the adjacent musculature. We employed morphometry and morphology at different embryonic stages and compared bone development in double-mutant and control embryos and fetuses. Our findings can be summarized as follows: a) the examined mutant bones had significantly altered shape and size that we described morphometrically, b) the effects of muscle absence varied depending on the bone (clavicles being more dependent than mandibles) and even within the same bone (e.g., the mandible), and c) we further supported the notion that, from the evolutionary point of view, mammalian clavicles arise under different influences from those that initiate the furcula (wishbone) in birds. Together, our data show that the development of secondary cartilage, and in turn the development of the final shape and size of the bones, is strongly influenced by mechanical cues from the skeletal musculature.     

Problems in combining skeletal age for an individual

Maturity imbalance between bones from different areas of the body, or between different bones in the same area or even between different centers within the same bones, are not an infrequent experience in doing radiographic assessment of skeletal maturation. Data from previous works have demonstrated tremendous variation concerning the pattern, degree, and causes of the imbalance. In view of the incomplete and contradictory knowledge concerning the mechanism producing such imbalance, it was only recommended to obtain the unweighted arithmetic mean of all the separately assessed bones as the overall skeletal age for an individual. It was hoped, however, that enthusiastic collection of new data on multiple bone assessments would soon take place which would facilitate a further recognition of the significance in skeletal maturity imbalance.