Structural and functional development in airways throughout childhood: Children are not small adults.

Children are not small adults and this fact is particularly true when we consider the respiratory tract. The anatomic peculiarities of the upper airway make infants preferential nasal breathers between 2 and 6 months of life. The pediatric larynx has a more complex shape than previously believed, with the narrowest point located anatomically at the subglottic level and functionally at the cricoid cartilage. Alveolarization of the distal airways starts conventionally at 36-37 weeks of gestation, but occurs mainly after birth, continuing until adolescence. The pediatric chest wall has unique features that are particularly pronounced in infants. Neonates, infants, and toddlers have a higher metabolic rate, and consequently, their oxygen consumption at rest is more than double that of adults. The main anatomical and functional differences between pediatric and adult airways contribute to the understanding of various respiratory symptoms and disease conditions in childhood. Knowing the peculiarities of pediatric airways is helpful in the prevention, management, and treatment of acute and chronic diseases of the respiratory tract. Developmental modifications in the structure of the respiratory tract, in addition to immunological and neurological maturation, should be taken into consideration during childhood.

The development of the cricoid cartilage and its implications for the use of endotracheal tubes in the pediatric population.

BACKGROUND: The rigid cricoid cartilage is functionally the narrowest portion of the larynx. There is some controversy over the shape of the pediatric cricoid cartilage in the transverse plane. It is important to understand the development of the cricoid cartilage so that endo-traceheal tubes can be used more safely. AIM: To determine changes in the internal diameter and shape of the cricoid cartilage during development and explore the implications of those changes for the selection of ETT type and size for children. METHODS: The cervical computed tomography scans were reviewed in patients aged 1-20 years. After performing the multiplanar reconstruction and correcting the slant, the transverse and anteroposterior internal diameters of the inlet and outlet of the cricoid cartilage were measured, respectively. The angle between the arch and the lamina of the cricoid cartilage in the middle sagittal plane was measured. The ratios of transverse to anteroposterior diameter for the inlet and outlet of the cricoid cartilage were calculated, respectively. RESULTS: In females, the internal diameters of the cricoid cartilage increased linearly with age. In males, the internal diameters of the cricoid cartilage exhibited a growth spurt during adolescence. The transverse diameter of the inlet was the smallest diameter of the cricoid cartilage, and the predicting formula of the transverse diameter of the inlet for children aged 1-12 was 0.4 × age (year) + 5.1, R2  = .758. The angle between the arch and lamina of the cricoid cartilage and the ratios of transverse to anteroposterior diameter correlated weakly with age. CONCLUSION: The transverse inner diameter of the inlet is the smallest diameter of the cricoid cartilage. The "funnel shape" of the cricoid cartilage remains unchanged during development. The outer diameter should be considered when selecting an endotracheal tube.

The anatomy of the pediatric airway: Has our knowledge changed in 120 years? A review of historic and recent investigations of the anatomy of the pediatric larynx.

BACKGROUND: There is disagreement regarding the anatomy of the pediatric airway, particularly regarding the shape of the cricoid cartilage and the location of the narrowest portion of the larynx. AIMS: The aim of this review is to clarify the origin and the science behind these differing views. METHODS: We undertook a review of published literature, University Libraries, and authoritative textbooks with key search words and phrases. RESULTS: In vivo observations suggest that the narrowest portion of the airway is more proximal than the cricoid cartilage. However, in vitro studies of autopsy specimens measured with rods or calipers, confirm that the nondistensible and circular or near circular cricoid outlet is the narrowest level. These anatomic studies confirmed the classic "funnel" shape of the pediatric larynx. In vivo studies are potentially misleading as the aryepiglottic, vestibular, and true vocal folds are in constant motion with respiration. These studies also do not consider the effects of normal sleep, inhalation agents, and comorbidities such as adenoid or tonsil hypertrophy that cause some degree of pharyngeal collapse and alter the normal movement of the laryngeal tissues. Thus, the radiologic studies suggesting that the narrowest portion of the airway is not the cricoid cartilage may be the result of an artifact depending upon which phase of respiration was imaged. CONCLUSION: In vivo studies do not take into account the motion of the highly pliable laryngeal upper airway structures (aryepiglottic, vestibular, and vocal folds). Maximal abduction of these structures with tracheal tubes or bronchoscopes always demonstrates a larger opening of the glottis compared to the outlet of the cricoid ring. Injury to the larynx depends upon ease of tracheal tube or endoscope passage past the cricoid cartilage and not passage through the readily distensible more proximal structures. The infant larynx is funnel shaped with the narrowest portion the circular or near circular cricoid cartilage confirmed by multiple in vitro autopsy specimens carried out over the past century.

Pediatric upper airway dimensions using three-dimensional computed tomography imaging.

INTRODUCTION: Computed tomography- (CT) and magnetic resonance imaging (MRI)-based measurements have recently suggested that the narrowest dimension of the pediatric airway is the subglottic region. These data are contrary to the previously held tenets of a funnel- or conical-shaped airway. The current study evaluates airway volumes and shapes using three-dimensional CT images of the air way column in spontaneously breathing children. METHODS: The study included CT-based radiological images of the neck in children who required imaging unrelated to airway symptomatology. The children were evaluated during spontaneous ventilation during natural sleep or with sedation without airway devices in place. The three-dimensional images of the airway column were evaluated, volumes calculated, and comparisons made between the subglottic, cricoid, and tracheal volumes and shapes. RESULTS: The study cohort included 54 children, ranging in age from 2 months to 8 years. An increase in the airway volumes was observed from the subglottic (0.17 ± 0.06 mm3 ) to the cricoid (0.19 ± 0.06 mm3 ) to the tracheal regions (0.22 ± 0.07 mm3 ). The volumes of the subglottic, cricoid, and tracheal regions demonstrated a linear relationship with age. CONCLUSION: This study confirms recent studies demonstrating that the subglottic region not the cricoid is the narrowest part of the airway.

Age-dependent structural and radiological changes in the larynx.

AIM: To define radiological physiological changes in the larynx by establishing an age-related scale. MATERIALS AND METHODS: The present retrospective study used radiological records of patients that had undergone lateral cervical imaging. Three hundred patients were included. Thyroid cartilage was divided into anatomical regions. The hyoid bone was evaluated as the body and greater horns. Cases were compared by grouping by age and gender. RESULTS: Thyroid tissue and cricoid cartilage only became visible after the second decade. Ossification in the thyroid cartilage began in the posterior inferior horn and progressed to the superior horn and central lamina. It also began in the posterior part of the cricoid cartilage and moved forward with age. In the first decade, the body and greater horn parts of the hyoid bone could be seen more distinctly, and after the third decade the hyoid bone appeared as a single bone. The hyoid bone was the only structure ossified in the laryngeal region below the age of 20 and formed an image on direct imaging. CONCLUSIONS: Age-related changes to the laryngeal tissues are evident on radiographs. Clinicians should bear this in mind when evaluating neck radiographs.

Alignment of cricoid cartilage and esophagus and its potential influence on the effectiveness of Sellick maneuver in children.

OBJECTIVE: The effectiveness of cricoid pressure in preventing aspiration of gastric contents during rapid sequence intubation may be limited if the esophagus is laterally displaced from the trachea at the level of the cricoid cartilage. Esophageal lateral displacement has been reported to occur in 50% to 90% of adults. Children 8 years and older assume the anatomic airway characteristics of adults, and therefore, we hypothesized that esophageal displacement would be significantly more common in older versus younger children. The purposes of this study were to determine the alignment of the trachea to the esophagus at the level of the cricoid cartilage on cervical spine or neck computed tomographic (CT) scans and to compare the frequency and quantity of esophageal displacement between children younger than 8 years and children 8 years and older. METHODS: This is a retrospective cross-sectional study of children (aged 0-17 years) who had cervical spine/neck CT scans performed at a 110-bed urban children's hospital. Two pediatric radiologists blinded to the patients' clinical symptoms and signs and final diagnosis independently determined the alignment of the airway at the level of the cricoid cartilage with the esophagus from cervical spine/neck CT scans. Lateral displacement of the esophagus from the airway was determined by measuring the distance from the ipsilateral outer wall edges of the esophagus and trachea. RESULTS: There were 172 cervical spine/neck CT scans reviewed. Of 87 children younger than 8 years, 27 were excluded, and of 85 children 8 to 17 years, 25 were excluded. The remaining 120 patients were eligible for the study, 60 patients were younger than 8 years and 60 patients were aged 8 to 17 years. For children younger than 8 years, their mean age was 3.58 years. There were 34 (57%) males. The most common indication for CT scan of the cervical spine/neck was motor vehicle crash 26 (46%). For children aged 8 to 17 years, their mean age was 13.3 years. There were 30 (50%) males. The most common indication for CT scan of the cervical spine/neck was motor vehicle crash 34 (57%). Alignment of the airway with the esophagus showed esophageal displacement in 36 (30%) of the patients with displacement in 27 (45%) of the younger children compared with 9 (15%) of the older children. The rate of displacement was significantly greater in the younger children (difference in rates was 30% and 95% confidence interval was 14%-46%). All displacements were to the left. The mean distance of esophageal displacement was significantly greater in the older children (2.42 vs 1.81 mm). The difference in the means was 0.61 mm, and the 95% confidence interval was 1.2 to 0.02 mm. CONCLUSIONS: This is the first pediatric study on the rate and degree of esophageal displacement from the airway at the level of the cricoid cartilage. Lateral displacement of the esophagus occurred at a significantly greater rate in the younger (45%) compared with the older (15%) children, which was directly opposite of our hypothesis. Of the 36 children (30%) with esophageal displacement, all had displacement to the left of the cricoid cartilage.

Matrix metalloproteinase inhibition causes luminal narrowing and ring thickening in the cricoid cartilage.

OBJECTIVE: Luminal expansion of the cricoid cartilage appears to be stunted by loss of luminal epithelium (LE) and can be enhanced by transforming growth factor-beta3 (TGF-beta3). When both the LE and perichondrium are disrupted, matrix metalloproteinase (MMP) levels within adjacent chondrocytes are diminished but can be restored by exogenous TGF-beta3. Cricoid growth stunting and luminal expansion that occur in the absence and presence of MMP activity, respectively, suggest that MMPs play an important role in normal subglottal development. The study objective was to determine if MMP inhibition affects cricoid expansion and by what mechanism, which will in turn help to define the mechanism of action of TGF-beta3-induced luminal expansion. STUDY DESIGN: Ex vivo, in vitro whole organ culture of subglottises grown with and without the presence of an MMP inhibitor. SETTING: Tertiary care facility. SUBJECTS AND METHODS: Subglottises from 20 neonatal mice were divided into 10 grown with an MMP inhibitor, GM6001, and 10 grown in basic medium alone. The luminal cross-sectional area, apoptosis levels, cell proliferation rates, and presence or absence of cleaved aggrecan fragments were determined. RESULTS: Subglottises that were exposed to the MMP inhibitor displayed statistically significant luminal narrowing, accompanied by apparent circumferential thickening of the cricoid ring, relatively decreased apoptosis, increased chondrocyte proliferation, and decreased amounts of aggrecan cleavage fragments in the extracellular matrix. CONCLUSION: Matrix metalloproteinases likely play a significant role in growth of the cricoid cartilage such that their inhibition leads to marked changes in the shape of the ring.

Pediatric transtracheal and cricothyrotomy airway devices for emergency use: which are appropriate for infants and children?

Cricothyrotomy or insertion of a transtracheal device is a life-saving maneuver that may be performed on an emergent or semi-elective basis as a means of bypassing an obstructed upper airway. A surgeon is trained to perform this life-saving procedure whereas most anesthesiologists are not facile with the scalpel. It is for this reason that many percutaneous devices have been developed for use by surgeons and nonsurgeons alike. Unfortunately, the majority of such devices are designed for use in adults and/or teenagers but are not appropriate for neonates and infants. The unique anatomy of the infant larynx, the small size of the cricothyroid membrane, and the technical difficulty of locating the correct anatomical structures make the use of most of these devices impractical if not outright dangerous in neonates and infants. This paper will review many (but not all) of the available devices, associated literature, pitfalls and dangers. It is emphasized that each clinician should become familiar with the advantages and disadvantages of these devices and obtain training with simulators or animal models. A strategy for management of the 'cannot ventilate, cannot oxygenate, cannot intubate' situation should be developed with age and size appropriate equipment.

Pediatric laryngeal dimensions: an age-based analysis.

BACKGROUND: In children, the cricoid is considered the narrowest portion of the "funnel-shaped" airway. Growth and development lead to a transition to the more cylindrical adult airway. A number of airway decisions in pediatric airway practice are based on this transition from the pediatric to the adult airway. Our primary aim in this study was to measure airway dimensions in children of various ages. The measures of the glottis and cricoid regions were used to determine whether a transition from the funnel-shaped pediatric airway to the cylindrical adult airway could be identified based on images obtained from video bronchoscopy. METHODS: One hundred thirty-five children (ASA physical status 1 or 2) aged 6 mo to 13 yr were enrolled for measurement of laryngeal dimensions, including cross-sectional area (G-CSA), anteroposterior and transverse diameters at the level of the glottis and the cricoid (C-CSA), using the video bronchoscopic technique under general anesthesia. RESULTS: Of the 135 children enrolled in the study, seven patients were excluded from the analysis mainly because of poor image quality. Of the 128 children studied (79 boys and 49 girls), mean values (+/-standard deviation) for the demographic data were age 5.9 (+/-3.3) yr, height 113.5 (+/-22.2) cm and weight 23.5 (+/-13) kg. Overall, the mean C-CSA was larger than the G-CSA (48.9 +/- 15.5 mm(2) vs 30 +/- 16.5 mm(2), respectively). This relationship was maintained throughout the study population starting from 6 mo of age (P < 0.001, r = 0.45, power = 1). The mean ratio for C-CSA: G-CSA was 2.1 +/- 1.2. There was a positive correlation between G- and the C-CSA versus age (r = 0.36, P < 0.001; r = 0.27, P = 0.001, respectively), height (r = 0.34, P < 0.001; r = 0.29, P < 0.001, respectively), and weight (r = 0.35, P < 0.001; r = 0.25, P = 0.003, respectively). No significant gender differences in the mean values of the studied variables were observed. CONCLUSION: In this study of infants and children, the glottis rather than cricoid was the narrowest portion of the pediatric airway. Similar to adults, the pediatric airway is more cylindrical than funnel shaped based on these video bronchoscopic images. Further studies are needed to determine whether these static airway measurements in anesthetized and paralyzed children reflect the dynamic characteristics of the glottis and cricoid in children.

Morphometric study on the anatomy of the fetal cricoid cartilage and comparison between its inner diameter and endotracheal tube sizes.

OBJECTIVES: The high incidence of respiratory disorders is one of the main problems in perinatal medical care. With the increased use of intubation, the incidence of laryngeal injury causing stenosis has also increased. The principal constriction point in the infant's larynx is the midcricoid area. We sought to provide detailed morphometric data on the anatomy of the cricoid cartilage and its relationship with growth and body characteristics of fetuses at 5 to 9 months of gestational age. METHODS: Nineteen larynges obtained from 17 stillborn infants and 2 newborn infants ranging in gestational age from 5 to 9 months were studied. Measurements of the cricoid cartilage were made with a millimeter-graded caliper. RESULTS: Weight was the variable most correlated with cricoid measurements. The cricoid lumen configuration showed an almost elliptic shape and did not change with gestational age. The mean inner subglottic cricoid area was 19.27 +/- 9.62 mm2 and was related to weight and body surface area. Cricoid growth was more pronounced at the outer portion of the cartilage. CONCLUSIONS: The cricoid lumen configuration was elliptic, and its mean area was smaller than that of available endotracheal tubes. This lumen area was most influenced by weight and height.

Transforming growth factor beta3 increases chondrocyte proliferation and decreases apoptosis in murine cricoid cartilage in vitro.

OBJECTIVE: To determine if the luminal epithelium and/or exogenous transforming growth factor beta (TGFbeta) affects growth of the cricoid. DESIGN: Subglottises from 20 neonatal mice were subdivided into four groups: A, five subglottises with luminal epithelium grown in basic medium; B, five epithelium-free subglottises in basic medium; C, five epithelium-free subglottises in basic medium with supplemental TGFbeta1, and D, five epithelium-free subglottises in basic medium with supplemental TGFbeta3. RESULTS: Groups A and D demonstrated the greatest luminal area expansion. Group A rings demonstrated statistically higher chondrocyte proliferation than Groups B and C and lesser amounts of luminal apoptosis. Groups B and C rings demonstrated the least amount of cell proliferation, and greater luminal apoptosis relative to Group A. Groups A and D rings had similar apoptotic and proliferative results. CONCLUSIONS: Luminal epithelium exerts influence over the cricoid by increasing chondrocyte proliferation and decreasing the relative proportion of luminal chondrocytes that undergo apoptosis. Exogenous TGFbeta3, not TGFbeta1, also increases chondrocyte proliferation within the cricoid and appears to influence apoptosis as well.

[Effect of anteroposterior cricoid split on cartilage growth in rabbits].

OBJECTIVE: To investigate the effect of the anteroposterior cricoid split on cartilage growth. METHOD: The rabbits were killed 8 months after the surgery of anteroposterior cricoid split. The larynxes were harvested and the cross-sectional area of the cricoid cartilage were calculated. RESULT: There was no significant difference in the cross-sectional area of the cricoid cartilage of these three groups. CONCLUSION: The anteroposterior cricoid split might have no adverse effect on subsequent growth of cricoid cartilage.

[Quarter section cricoid split: its effect on cartilage growth in rabbits].

OBJECTIVE: To investigate the effect of the quarter-section cricoid split on cartilage growth. METHOD: The rabbits were killed 8 months after surgery of anterior, anteroposterior and quarter section cricoid split. The larynxes were harvested and the cross sectional areas of the cricoid cartilage were calculated. RESULT: There was no significant in the cross sectional area of the cricoid cartilage of any of the four groups. CONCLUSION: The quarter section cricoid split has no adverse effect on subsequent growth of cricoid cartilage and animals.

Local control of murine subglottis development.

OBJECTIVE: To determine if subglottic development is at least partially under local control and to determine which tissue layer(s) is predominantly responsible. DESIGN: The suglottises of 12 day-3 CD1 mice were grown in whole organ culture. The 12 subglottises were divided into 3 individual groups: +++, -++, and ---. Group+++ had all tissue layers of the subglottis intact: luminal epithelium, cricoid cartilage, inner and outer perichondrium. Group-++ had all layers intact with the exception of luminal epithelium. Group--- had all layers removed (luminal epithelium, inner and outer perichondrium) resulting in cricoid cartilage-only rings. All rings were grown in basic medium without the use of growth factors or serum for 15 days. Measurements of the rings were taken before and after organ culture growth. RESULTS: Group+++ was the only group that experienced growth. Only luminal growth was statistically significant although all rings experienced growth in both the luminal and external diameter. Group-++ did not experience any growth. Group--- lost structural integrity with collapse of the ring and did not experience growth of any dimension of the cartilage. CONCLUSIONS: Growth of the subglottis is under local control but may have additional influences from the outside that were not investigated here. Removal of just the epithelium stunts growth of the entire ring, but preferentially the lumen more so than the external diameter. Removal of all tissue layers around the cricoid cartilage results in a structural collapse of the ring, suggesting that the cartilage in this age group is dependent on surrounding tissues for structural integrity.

Laryngotracheal growth following cricotracheal resection combined with laminotomy. An experimental study on rabbits.

Cricotracheal resection (CTR) has been shown to be successful in the treatment of pediatric subglottic stenosis. Whether CTR combined with posterior midline division or partial resection of the cricoid plate affects the stability and growth of the larynx is still questionable. The study included 24 infant female New Zealand White rabbits that were divided into 4 groups of 6 animals each. Group 1 was the unoperated control group. Group 2 underwent removal of three quarters of the anterior cricoid circumference, followed by a primary anastomosis (CTR). In addition to CTR, the rabbits in group 3 underwent a laminotomy by splitting of the cricoid plate midline. Group 4 underwent both CTR and partial resection of the lamina. No stents were used in this study. After 6 months, the experiment was terminated, and the following observations were made. Groups 2 and 3 presented a large and stable subglottic lumen, comparable to that of the control group. After partial cricoid plate resection, 4 of the rabbits in group 4 suffered from a subglottic collapse that caused an airway obstruction and resulted in death. These results support the conclusion that CTR alone, as well as combined with laminotomy, can be performed in a relatively safe and effective manner without interfering with the development of the larynx in young rabbits. Cricotracheal resection together with partial resection of the cricoid plate, however, leads to an unstable situation with an impaired airway.

Developmental changes of laryngeal dimensions in unparalyzed, sedated children.

BACKGROUND: Knowledge of the influence of age on laryngeal dimensions is essential for all practitioners whose interest is the pediatric airway. Early cadaver studies documented that the larynx is conically shaped, with the apex of the cone caudally positioned at the nondistensible cricoid cartilage. These dimensions change during childhood, as the larynx assumes a more cylindrical shape. The authors analyzed laryngeal dimensions during development to determine if this relationship continues in unparalyzed children in whom laryngeal muscles are tonically active. The authors determined the relationships between the vocal cord, sub-vocal cord, and cricoid ring dimensions and the influence of age on these relationships. METHODS: Infants and children undergoing magnetic resonance imaging with propofol sedation had determinations of the transverse and anterior-posterior (AP) dimensions of the larynx at the most cephalad level of the larynx (vocal cords) and the most caudad level (cricoid). Most patients had an additional measurement (sub-vocal cord) at a level between the vocal cords and the cricoid ring. Relationships were obtained by plotting age against laryngeal dimensions and the ratio of laryngeal dimensions at different levels within the larynx. RESULTS: The authors measured transverse and AP laryngeal dimensions in 99 children, aged 2 months-13 yr. The relationship between the transverse and AP dimensions at all levels of the larynx did not change during development. Transverse and AP dimensions increased linearly with age at all levels of the larynx. In all children studied, the narrowest portion of the larynx was the transverse dimension at the level of the vocal cords. Transverse dimensions increased linearly in a caudad direction through the larynx ( P< 0.001), while AP dimensions did not change relative to laryngeal level. The shape of the cricoid ring did not change throughout childhood. CONCLUSIONS: In sedated, unparalyzed children, the narrowest portions of the larynx are the glottic opening (vocal cord level) and the immediate sub-vocal cord level, and there is no change in the relationships of these dimensions relative to cricoid dimensions throughout childhood.

Gross and histologic changes in the developing rabbit subglottis in response to a controlled depth of injury.

OBJECTIVE: Our goal was to determine the effects of both perichondrial and intracartilaginous injury in the developing rabbit subglottis versus normal development. DESIGN: We conducted a descriptive, pilot study of changes in the shape and histology of the subglottis after a controlled depth of injury in 27 New Zealand White rabbits, ages 4 weeks, 8 weeks, and 1(1/2) years. INTERVENTION: Within each age group, 3 animals underwent no surgery, 3 underwent perichondrial injury, and 3 underwent intracartilaginous injury. RESULTS: Perichondrially injured animals in the 4-week age group developed a marked abnormality in the shape of the cricoid cartilage in the injured region. Cartilage of the perichondriallly injured animals in the 8-week and 1(1/2)-year groups became histologically consistent with fibrous tissue. The cartilage of all animals that underwent intracartilaginous injury was replaced with fibrous tissue. CONCLUSION: In this observational study, we identified 3 relevant findings. First, the responses of the cartilage to a perichondrial injury suggest that the luminal soft tissues may exert some morphologic control in developmentally young animals. Second, only the 4-week-old group's cartilage was tolerant of a perichondrial injury with continued growth of the ring. Third, no animal's cartilage could withstand an intracartilaginous injury regardless of age.

Apoptosis in the developing human cricoid cartilage: a pilot study.

Apoptosis is widely recognized as a major phenomenon in normal development. Deficiencies in this process may lead to developmental abnormalities such as congenital subglottic stenosis. We studied apoptosis using in situ end labeling of the 3'-OH ends of fragmented DNA in 5 progressively older, normal, human cricoid cartilage specimens. Results show that apoptosis is a very active process in fetal and neonatal tissue. The process gradually slows with advancing age. In the 4- and 13-year-old specimens, minimal to no apoptosis was seen. We conclude that apoptosis plays a critical role in the intraluminal and extraluminal expansion of the cricoid cartilage.

Growth and development of the human cricoid cartilage: an immunohistochemical analysis of the maturation sequence of the chondrocytes and surrounding cartilage matrix.

OBJECTIVES: The goal was to determine maturational changes in the human cricoid cartilage. STUDY DESIGN: The study involved immunohistochemical staining of collagen II (a marker of proliferating chondrocytes), matrilin-1 (a marker of post-proliferative chondrocytes), and collagen X (a marker of hypertrophic chondrocytes). Specimens included uninjured human cricoid cartilages at 18 and 41 weeks' gestation and 1, 4, and 13 years postpartum. RESULTS: This study demonstrated that type II collagen peaks in concentration at approximately 41 weeks' gestation. Matrilin-1 is present in progressively lower concentration in the central core of the cricoid ring, but the peripheries of the ring contain the protein in relatively high concentration. Type X collagen is not expressed in the age groups tested. CONCLUSIONS: These biochemical markers lend further support to a chondrocyte proliferative phase that slows between 1 and 4 years of age. Chondrocytes then enter a phase histologically similar to the hypertrophic phase but are biochemically different than hypertrophic chondrocytes destined for endochondral ossification.