The timing and sequence of events in the development of the human respiratory system during the embryonic period proper

Development of the foregut and the formation of the trachea and esophagus in rat embryos. A symphony of confusion

Introduction: During embryonic development, the trachea emerges from an area of the foregut, which is often referred to as "anterior" or "common" foregut tube or simply foregut. To explain this process of differentiation, four competing models exist to date. The outgrowth and watershed models propose a foregut that remains constant in length. In the outgrowth model, the trachea buds off and elongates from the foregut, while in the watershed model, a mesenchymal wedge splits the growing foregut into the trachea and esophagus. In contrast, the septation model proposes a cranial splitting and thus a shortening of the "common" foregut tube into the trachea and esophagus by an emerging septum. Finally, the splitting and extension model describes an interaction of cranial splitting of the foregut and simultaneous caudal tracheal and esophageal growth. Methods: Here we examine the development of the undifferentiated foregut by micro computed tomography, which allows precise measurements. Results: Our results show that this area of the foregut transforms into the larynx, a process, which is independent from tracheal and esophageal development. Discussion: These observations are only consistent with the outgrowth model.

The bronchial tree of the human embryo: an analysis of variations in the bronchial segments

A classical study has revealed the general growth of the bronchial tree and its variations up to Carnegie stage (CS) 19. In the present study, we extended the morphological analysis CS by CS until the end of the embryonic period (CS23). A total of 48 samples between CS15 and CS23 belonging to the Kyoto Collection were used to acquire imaging data by performing phase-contrast X-ray computed tomography. Three-dimensionally reconstructed bronchial trees revealed the timeline of morphogenesis during the embryonic period. Structures of the trachea and lobar bronchus showed no individual difference during the analyzed stages. The right superior lobar bronchus was formed after the generation of both the right middle lobar bronchus and the left superior lobar bronchus. The speed of formation of the segmental bronchi, sub-segmental bronchi, and further generation seemed to vary among individual samples. The distribution of the end-branch generation among five lobes was significantly different. The median branching generation value in the right middle lobe was significantly low compared with that of the other four lobes, whereas that of the right inferior lobe was significantly larger than that of both the right and left superior lobes. Variations found between CS20 and CS23 were all described in the human adult lung, indicating that variation in the bronchial tree may well arise during the embryonic period and continue throughout life. The data provided may contribute to a better understanding of bronchial tree formation during the human embryonic period.

Spatial relationships of the bronchial arteries to the left recurrent laryngeal nerve in the sub-aortic arch area

PURPOSE

To safely perform lymphadenectomy in the sub-aortic arch area during esophagectomy for esophageal cancer, we investigated the spatial relationships between the bronchial arteries (BAs) and the left recurrent laryngeal nerve (LRLN).

METHODS

For this macro-anatomical study, 72 cadavers were used.

RESULTS

Of the 195 dissected BAs, 15 (7.7%) arteries ran dorsally across the LRLN. Such a running pattern of the BA was found in 15 (20.8%) of the 72 cadavers. Fourteen (93.3%) of the 15 arteries ran anteriorly along the left side of the esophagus, and 13 (86.7%) passed further to the lateral side of the left main bronchus to reach the ventral surface of the tracheobronchus; we named this running pattern "Type III". Of the 51 arteries with the Type III pattern, 25.5% ran across the dorsal side of the LRLN.

CONCLUSION

Approximately 20% of the cadavers had BAs running dorsally to the LRLN in the sub-aortic arch area. Most of these arteries had the Type III pattern. One-quarter of the BAs with the Type III pattern showed this running pattern. Care must be practiced to safely perform lymphadenectomy for esophageal cancer in patients with Type III BAs.

The Early Development of the Larynx in Staged Human Embryos

More than 15 serially sectioned human embryos from stage 8 to stage 15 were examined, together with relevant reconstructions and photographs. Their lengths ranged from 1 to 7 mm, and their ages from 18 to 33 postovulatory days. The necessity of employing a recognized staging system is stressed. The foregut appears either late in stage 8 or during stage 9. The median pharyngeal groove that appears during stage 9 presages the first indication of the respiratory system and includes the future larynx. The laryngotracheal sulcus begins to be circumscribed at stage 10 and a caudal expansion represents the pulmonary primordium. The tracheoesophageal septum appears at stage 12. The right and left lung buds become definite by stage 13. The hypopharyngeal eminence, arytenoid swellings, and epithelial lamina of the larynx are detectable at stage 14. Vestibulotracheal and pharyngotracheal canals are distinguishable at stage 15. Hence, from the first appearance of the foregut at about 19 days, the larynx has developed into a recognizable organ two weeks later, namely, by 33 days.

A landmark-free morphometric staging system for the mouse limb bud

We have created a 2D morphometric analysis of the developing mouse hindlimb bud. This analysis has provided two useful resources for the study of limb development. First, a temporally accurate numerical description of shape changes during normal mouse limb development. Second, a web-based morphometric staging system, which has the advantage of being easy to use, and with a reproducibility of about ±2 hours. It allows users to upload a dorsal-view photo of a limb bud, draw a spline curve and thereby stage the bud within a couple of minutes. We describe how the system is constructed, its robustness to user variation and illustrate one application: the accurate tracking of spatiotemporal dynamics of gene expression patterns.

The association between autism and errors in early embryogenesis: what is the causal mechanism?

The association between embryonic errors and the development of autism has been recognized in the literature, but the mechanism underlying this association remains unknown. We propose that pleiotropic effects during a very early and specific stage of embryonic development-early organogenesis-can explain this association. In humans early organogenesis is an embryonic stage, spanning Day 20 to Day 40 after fertilization, which is characterized by intense interactivity among body parts of the embryo. This implies that a single mutation or environmental disturbance affecting development at this stage can have several phenotypic effects (i.e., pleiotropic effects). Disturbances during early organogenesis can lead to many different anomalies, including limb deformities, craniofacial malformations, brain pathology, and anomalies in other organs. We reviewed the literature and found ample evidence for the association between autism and different kinds of physical anomalies, which agrees with the hypothesis that pleiotropic effects are involved in the development of autism. The proposed mechanism integrates findings from a variety of studies on autism, including neurobiological studies and studies on physical anomalies and prenatal influences on neurodevelopmental outcomes. The implication is that the origin of autism can be much earlier in embryologic development than has been frequently reported.

Tracheobronchial Stenting in an Infant with an Anomalous Right Main Bronchus

Congenital anomalies of the right main bronchi have not been fully described. Bronchial anomalies are usually asymptomatic unless they coexist with other abnormalities. We describe a rare bronchial anomaly in a 2-month-old girl in which the right upper lobe, middle lobe, and lower lobe bronchi all originated at the same point as a result of a developmental teratogenic long right main bronchus and an absent bronchus intermedius. Furthermore, the left main bronchus contained a proximal segment of stenosis. This combination of anomalies, along with right upper lobe atelectasis and left-sided hyperinflation, resulted in a severe right mediastinal shift and respiratory distress. The mediastinum was returned to midline with endoscopic placement of an in-dwelling tracheobronchial stent into the left main bronchus. To the best of our knowledge, the specific developmental anomaly in our patient has not been previously described.

Analysis of the conducting airway system in the lung: a new method combining morphometry with mathematical modeling for airway classification

Although the lung is structurally complex, it is suitable for morphometric analysis of the structural determinants of lung function in health and disease. Analysis of the organized branching airways has been problematic because of the need to identify and classify airways before structural characteristics of different-order branches can be determined. Airway casts have been used to identify relationships between branches, measure some structural features, and develop mathematical models that describe simply the relationships between generations. However, cast preparation destroys surrounding tissue, including the airway wall, thus precluding analysis of these structural elements. We describe a new approach using tissue sections which combines the classification of airways into Strahler order (SO) with tissue structural analysis. Lung-tissue sections are prepared, and outer (OD) and inner (ID) diameters are determined over a wide range of airways. The line equation relating log OD vs. SO is determined using measured values for SO1 (terminal bronchioles) and SO8 (first branch bronchi). Mean ODs can then be calculated for each of the other SO groups, and measurements can be classified. Calculations can be made for the mean number of branches and airway lengths (given the log linear relationship of these factors with SO and morphometrically determined volume densities for airway lumen), and for individual airway resistance and total resistances for each SO. For an example, mean data are presented for airways in the adult sheep (n = 13). The methodology presented allows identification of subtle alterations in airway structures which may be affecting selected orders of airways, with specific implications for changes in lung function.

Separation between the digestive and the respiratory lumina during the human embryonic period: morphometric study along the tracheo-oesophageal septum

An isolated tracheo-oesophageal fistula could be caused by close proximity of the epithelia of both organs (O'Rahilly & Müller, 1984; Kluth et al. 1987) at certain embryonic stages, the most frequent location being the tracheal bifurcation. Thus the relative position and degree of separation between the digestive and the respiratory tubes throughout their development may be relevant to the origin of this anomaly. The aim of this study was to analyse along the different segments of the tracheo-oesophageal septum (TES) where the closest relationship between both lumina occurred and what degree of separation was present at each segment. Computer imaging techniques were applied on cross sections of a graded series of normal human embryos (Carnegie stages (CS) 13-23). In addition, the differentiation of the primitive TES was also studied (from CS 12) by light microscopy. Between CS 13 and 16 both tubes tended to separate (phase of separation), principally at the proximal segments of the laryngopharyngeal and the tracheo-oesophageal portions of the TES. During this phase the separation between the trachea and oesophagus was wider than between the larynx and pharynx. From CS 17 to CS 23 the digestive and respiratory lumina reached their widest separation at different levels of the laryngopharyngeal portion. Below these levels they tended to come closer together, principally at the proximal segment of the tracheo-oesophageal portion, but also at the distal part of the laryngopharyngeal portion. During this phase of approximation they reached their closest relationship at the proximal (CS 17) and the distal (from CS 18) segments of the tracheo-oesophageal portion. When finally the distal segment of the trachea (which includes the bifurcation) comes closest to the oesophagus, the coats of both organs have already undergone an appreciable differentiation. According to these observations, the origin of the most frequent isolated tracheo-oesophageal fistula at the bifurcation region could not be explained from the normal development of the TES.

Effect of immediate versus slow intrauterine reduction of congenital diaphragmatic hernia on lung development in the sheep: a morphometric analysis of term pulmonary structure and maturity

The incidence of congenital diaphragmatic hernia (CDH) is 1:1,200-5, 000, and the condition is associated with high mortality and morbidity attributed principally to associated pulmonary hypoplasia. One treatment approach has been for intrauterine intervention to induce lung growth to a sufficient level to allow survival at birth. Repair of the hernia in utero has been attempted, using a method of immediate reduction and repair of the hernia (patch) compared to a slow reduction method using a silastic "silo" sewn over the diaphragm defect to contain the hernial contents. In animal studies, this second method has been associated with lower fetal morbidity and mortality. This study, utilizing the sheep model of CDH, focuses on analysis of lung structural development and maturation, comparing the efficacy of the immediate vs. slow methods of hernial repair in preventing/reversing pulmonary hypoplasia. We hypothesized that: a) Both the immediate (patch) and slow (silo) methods of hernia repair performed in the lamb model of CDH will stimulate lung growth and structural development and restore lung structure and maturity towards normal levels by term gestation; b) Effects will be detectable by morphometric measurement of the following parameters: lung volume; parenchyma to nonparenchyma tissue ratio; volume density of connective tissue in nonparenchyma; gas exchange tissue to airspace ratio; gas exchange surface area; capillary loading; alveolar/airspace density; and alveolar perimeter; c) Effects will be seen in all lobes of the lung; and d) There will be no significant difference in lung size or structural parameters between the two groups. Forty-four pregnant ewes were allocated randomly to one of four groups. Fetal lambs in three groups (n = 36) underwent CDH creation at days 72-74 of gestation. Of surviving lambs showing an adequate hernia, 9 were not operated on further, 11 underwent "repair" using a silastic chimney around the hernial contents (slow reduction), and 11 underwent "repair" by a silastic patch over the diaphragmatic defect (immediate reduction). The fourth group were normal controls. All surviving lambs (n = 8 in each group) were delivered by Cesarian section at 141-143 days (term = 145-149 days). Lungs were obtained at autopsy, inflation-fixed, divided into lobes, and sampled, and morphometric analysis was performed. Comparisons were made between these groups and with matched normal controls and CDH untreated animals prepared in conjunction and previously reported. The lungs from the CDH animals treated by both methods of fetal hernia repair showed significant lung growth and structural development and maturation, although they remained significantly hypoplastic compared to normal. There were minor differences in the lung parameters between these two groups, with a tendency for the slow method to provide more normal parameter values. An exception was the increase in lung volume that was greater for the immediate (patch) method, particularly in the left lower lobe. In conclusion, intrauterine hernia repair by both methods is capable of partially reversing total lung and lobar structural hypoplasia and immaturity. The slow reduction method, with reduced potential for mortality and morbidity, is at least as good at reversing pulmonary hypoplasia as the immediate method. Alternative intrauterine interventions to prevent or reverse pulmonary hypoplasia are discussed and compared with the hernia repair methods used in this study.

Morphology of the human larynx during the first five years of life studied on whole organ serial sections

The morphologic development of the human larynx during the first years of life is poorly understood to date. This study used plastinated whole organ serial sections to determine the growth and structure of the infant larynx. The larynges of 43 children 1 to 60 months old were plastinated. Whole organ serial sections were obtained by cutting the resulting specimen with a diamond band saw. The slices were then submitted to computer-assisted morphometric investigation. We found that the subglottic airway rapidly increases in size during the first 2 years of life. Further growth follows a linear mode. The relative proportion of the mucosal lining decreases likewise. In contrast to that in adults, and comparable to that in most mammals, the cartilaginous glottis accounts for 60% to 75% of the vocal folds' length at <2 years. No sexual dimorphism of the larynx exists during childhood. This study supplies detailed morphometric data on the growth and structure of the human larynx during the first years of life. It is the first to use plastinated whole organ serial sections for morphology of the pediatric larynx. Therefore, this study provides quantitative anatomic data of clinical interest that have not been available to date.

Effect of antenatal tracheal occlusion on lung development in the sheep model of congenital diaphragmatic hernia: a morphometric analysis of pulmonary structure and maturity

The incidence of congenital diaphragmatic hernia (CDH) is 1:1,207-5,000, and the condition is associated with high mortality and morbidity, attributed principally to associated pulmonary hypoplasia. Repairing the diaphragmatic defect by antenatal surgery has high mortality, mainly due to premature labor. Antenatal tracheal occlusion, which is achievable by less invasive methods, stimulates lung growth (weight and DNA). However, its effectiveness in reversing structural and maturational abnormalities and its optimal timing requires further investigation. We hypothesized that (1) antenatal tracheal occlusion performed in the lamb model of congenital diaphragmatic hernia will stimulate lung growth and structural development and restore lung structure and maturity toward normal levels by term gestation; (2) effects will be detectable by morphometric measurements of the following parameters: lung volume, ratio of parenchyma to nonparenchyma, volume density of connective tissue within nonparenchyma, ratio of gas exchange tissue to airspace in parenchyma, gas exchange surface area, capillary loading, alveolar/airspace density and alveolar perimeter; (3) effects will be seen in all lobes of the lung; and (4) a greater effect will be observed when tracheal occlusion is performed early rather than late in gestation. Fourteen lambs underwent CDH creation at gestation day 72-74 followed by tracheal occlusion at day 101 (n = 7) or 129 (n = 7). They were delivered by Cesarean section at 143 days (term = 145-149). Lungs were obtained at autopsy, inflation fixed, divided into lobes, and sampled; morphometric analysis was performed. Comparisons were made with previously reported results from control lungs of normal lambs and lambs with untreated CDH. In comparison with untreated lungs, antenatal tracheal occlusion at both times resulted in increased volumes for total lung and lobes, increased volume density of parenchyma and of airspace within parenchyma, and increased gas exchange surface areas. Normal values for gas exchange surface area density, and alveolar density and perimeter were attained and the lungs appeared more mature than non-occluded lungs. Tracheal occlusion earlier in gestation produced a greater effect, achieving greater than normal values for lung volumes and volume densities, whereas the capillary loading value was similar to normal lung. Later occlusion achieved less than normal values for lung volumes and volume densities, with a reduced capillary loading value. We conclude that antenatal tracheal occlusion is capable of reversing structural total lung and lobar hypoplasia and immaturity caused by CDH as determined by morphometrically determined parameters. The effect is greater when tracheal occlusion is performed early rather than late in gestation. The results are encouraging for development of treatment methods for humans with antenatally diagnosed CDH.

Unusual association of two unilateral anomalies present in adulthood: pulmonary hypoplasia and renal agenesis. Embryology and clinical expression

We report a case of pulmonary hypoplasia associated with renal agenesia diagnosed in a 46-year-old woman admitted to hospital because of a viral pleuro-pericarditis. The chest x-ray raised the suspicion of left pulmonary hypoplasia. The CT scan and the Magnetic Resonance Image confirmed the presence of the congenital lung malformation and detected the absence of the left kidney. The clinical presentation, radiological diagnosis and embryological basis are described.

Septation of the respiratory and digestive tracts in human embryos: crucial role of the tracheoesophageal sulcus

Esophageal atresia and tracheoesophageal fistula, common malformations of the respiratory and digestive tracts, are of unsettled pathogenesis. Part of the difficulty in understanding these abnormalities arises from the uncertainties about the normal developmental processes in the region. This study examined the development and fate of the tracheoesophageal septum. Normal human embryos from the Carnegie Embryological Collection and fetuses from the Hopkins Pathology Collection were examined, and reconstructions of selected specimens were made from photomicrographs of serial histologic sections. The results show that the lung bud appears in Carnegie stage 12, rapidly enlarges, and bends caudally, thereby producing a sulcus between the foregut and the respiratory system on its caudal aspect. The cranial aspect of this tracheoesophageal sulcus remains fixed at the levels of the first cervical vertebra throughout subsequent embryonic and fetal development. At the same time the trachea and esophagus elongate to bring those part of the respiratory and digestive systems into their definitive anatomic positions. Examination of the tracheoesophageal sulcus shows that its growth-limiting properties may be explained by its catenoidal configuration. Catenoidal, or saddle-shape, sulci have been shown to have similar regional growth-limiting properties in the embryonic heart. These regions contrast with outwardly convex regions in both the developing heart and lung where growth of the tissues occurs. The observations made here suggest that the origin of the tracheoesophageal malformations must be sought in a configurational abnormality in the area of the developing lung bud in Carnegie stage 12.

Recurrent tetraamelia and pulmonary hypoplasia with multiple malformations in sibs

A term amelic female infant was born to an apparently nonconsanguineous Arab Moslem couple. This was followed by the birth of 4 normal children. Afterwards, in 2 subsequent pregnancies, 2 amelic fetuses were diagnosed by transabdominal ultrasonography in the 18th and 12th week of gestation. Pregnancies were terminated and on autopsy both amelic fetuses had severe lung hypoplasia and aplasia of the peripheral pulmonary vessels. The first fetus also had apparently low-set ears and micrognathia, whereas the last had hydrocephaly and left cleft lip beside the lung hypoplasia and aberrant pulmonary artery. This appears to be a new autosomal recessive malformation syndrome.

Laryngeal dysgenesis

A case of a previously undescribed anomaly of the larynx is presented and offers insight and support of current concepts of fetal laryngeal development. The patient possessed unilateral absence of true and false vocal cords, laryngeal ventricle, and saccule. Current concepts of embryological development would place the development of this anomaly and most of the patient's other multiple anomalies in the period of the 6th to 9th weeks of fetal life. This report lends substantiation to the stages of laryngeal development by demonstrating an aberration in the normal sequential development. Additionally, the time period of this maldevelopment is suggested by the concurrence of multiple other anomalies. Aspects of laryngeal embryology relevant to laryngeal maldevelopment are reviewed.

The first appearance of the neural tube and optic primordium in the human embryo at stage 10

Thirteen embryos of stage 10 (22 days) were studied in detail and graphic reconstructions of most of them were prepared. The characteristic feature of this stage is 4-12 pairs of somites. Constantly present are the prechordal and notochordal plates (the notochord sensu stricto is not yet apparent), the neurenteric canal or at least its site, the thyroid primordium, probably the mesencephalic and rhombencephalic neural crest and the adenohypophysial primordium. During this stage, the following features appear: terminal notch, optic sulcus, initial formation of neural tube, oropharyngeal membrane, pulmonary primordium, cardiac loop, aortic arches 1-3, intersegmental arteries, and laryngotracheal groove. The primitive streak is still an important feature. Graphic reconstructions have permitted the detection of the telencephalic portion of the forebrain, for the first time at such an early stage. It is proposed that the remainder of the forebrain comprises two subdivisions: D1, which becomes largely the optic primordium during stage 10, and D2, which is the future thalamic region. The optic sulcus is found in D1 but does not extent into D2, as has been claimed in the literature. An indication of invagination of the otic disc appears towards the end of the stage. As compared with the previous stage, the prosencephalon has increased in length, the mesencephalon has remained the same, the rhombencephalon has decreased, and the spinal part of the neural plate has increased fivefold in length. The site of the initial closure of the neural groove is rhombencephalic, upper cervical, or both. The neural plate extends caudally beyond the site of the neurenteric canal. Cytoplasmic inclusions believed to indicate locations of great activity were always detected in the forebrain (especially in the optic primordium), and also in the rhombencephalon, spinal part, and mesencephalon.

Bronchial atresia associated with a bronchogenic cyst. Evidence of early appearance of atretic segments

We report a case of congenital lobar emphysema in an adult due to bronchial atresia and presenting characteristically with a solitary pulmonary nodule due to a mucous plug. In the same patient the presence of a bronchogenic cyst leads to a hypothesis that the atretic segment is the result of an insult occurring during the 5th or 6th week of intrauterine life rather than one occurring after airway development is complete.

Chevalier Jackson lecture. Respiratory and alimentary relations in staged human embryos. New embryological data and congenital anomalies

The early development of the digestive and respiratory systems is summarized in relation to embryonic staging. It has frequently been emphasized that the digestive and respiratory tubes do not arise from a common chamber, that they pursue separate courses as soon as the lung bud appears, that a mesenchymal septum comes to intervene between them, and that the two tubes rapidly acquire independent outer coats. Some commonly held views such as the supposed caudorostral separation of the trachea from the esophagus, have been shown to be incorrect. These ideas often arose from the use of unstable landmarks during development. It is pointed out that, for 3 weeks after its appearance, the tracheoesophageal separation point remains at a constant level, whereas the tracheal bifurcation descends. The application of valid embryological data to the interpretation of several congenital anomalies has been shown to be of value. Although the modes of origin in some instances are still obscure, considerable advance has been made in understanding the timing of the relevant events.

The development of the lung in mammals: an analysis of concepts and findings

To evaluate one model of mammalian-lung development, i.e., division into periods, pre- and postnatal lung development in the CPB-S mouse strain was divided into the currently distinguished periods: the pseudoglandular period, covering establishment of the air-conducting portion; and the canalicular, terminal-sac, and alveolar or postnatal periods, in which the respiratory portion develops. The last three periods would each cover the formation of a different component of the respiratory unit or pulmonary acinus (acinus pulmonaris) (nonalveolated respiratory bronchiole, nonalveolated duct and sac, and alveolar pouch). However, determination of the nature of the relevant structures on the basis of recent findings concerning the epithelia showed that these hypotheses are not tenable. Since the tubule with cuboidal epithelium (appearing in the pseudoglandular and following periods) is the basic structure in the genesis of the pulmonary acinus, the development of the respiratory portion must start in the pseudoglandular period. Likewise, since the definitive components of the acinus are derived from this acinar tubule, their establishment may not be restricted to one of the other periods. Because other postulated divisions of mammalian-lung development were based on similar histological interpretations, they cannot reflect the course of mouse-lung development either. Therefore, a developmental scheme based on the recent findings concerning the epithelia is given as well as a tentative scheme for the human lung. The respiratory portion proved to develop by budding of acinar tubules, the mode of budding being not restricted to any particular pattern.

The human larynx at the end of the embryonic period proper. I. The laryngeal and infrahyoid muscles and their innervation

The larynx at stage 23 (about 27-32 mm C-R, 8 postovulatory weeks) is being studied in detail, and findings on the musculature and its innervation are presented here. Serial sections of six embryos all belonging to the one developmental stage were investigated and compared, and graphic reconstructions were made from five of the specimens. All the infrahyoid and most of the major laryngeal muscles are present, and their innervation follows closely the adult pattern. The ary-epiglottic and thyro-epiglottic apparently may or may not be identifiable. The vocalis is beginning to differentiate and is noted here in the embryonic period proper for the first time. The histological degree of development of the intrinsic laryngeal muscles is not as advanced as that of adjacent muscles in the neck, including the infrahyoid group. Variation within this one stage of development are small but definite. They include differences in the position, level, and shape of the skeletal components, and the precise position and extent of development of some of the muscles.

Laryngeal development and congenital lesions

In considering laryngeal development, functional development and the comparative anatomical approach of Sir Victor Negus must be included in addition to classical morphological development; postnatal questions are considered in addition to embryological and fetal development.

Clinical correlation of anomalies of the supraglottic larynx with the staged sequence of normal human laryngeal development

The clinical correlation of the authors' observations on human laryngeal development with existing literature supports the suggestion that earlier concepts, specifically by Frazer, may not be valid. It is further proposed that development of the human larynx may similarly correspond in man to that of the rat. Congenital laryngeal malformations are fundamentally associated with a disturbance of epithelial mesenchymal interactions, particularly related to a premature arrest of the normally vigorous epithelial activity.

The timing and sequence of events in the development of the human digestive system and associated structures during the embryonic period proper

A documented scheme of the early development of the human digestive system is presented. It is based on (1) reports of workers who personally studied staged embryos, and (2) personal observations and confirmations. The necessity of studying staged embryos in order to determine the precise sequence of developmental events is stressed.

The timing and sequence of events in the development of the limbs in the human embryo

A documented scheme of the early development of the human limbs is presented. It is based on (1) published and unpublished personal observations, and (2) reports of other workers who personally studied staged human embryos. The necessity of studying staged embryos in determining the precise sequence of developmental events is stressed.

Some aspects of fetal laryngeal development

Some aspects of morphological and physiological development of the fetal larynx are presented. Histochemical confirmation and determinations of fetal glandular activity are needed. Anomalous development of the larynx is still conjecture, but congenital cystic formation with glandular development appears possible after the fifth month of human fetal life.

The early development of the larynx in staged human embryos. I. Embryos of the first five weeks (to stage 15)

More than 15 serially sectioned human embryos from stage 8 to stage 15 were examined, together with relevant reconstructions and photographs. Their lengths ranged from 1 to 7 mm, and their ages from 18 to 33 postovulatory days. The necessity of employing a recognized staging system is stressed. The foregut appears either late in stage 8 or during stage 9. The median pharyngeal groove that appears during stage 9 presages the first indication of the respiratory system and includes the future larynx. The laryngotracheal sulcus begins to be circumscribed at stage 10 and a caudal expansion represents the pulmonary primordium. The tracheoesophageal septum appears at stage 12. The right and left lung buds become definite by stage 13. The hypopharyngeal eminence, arytenoid swellings, and epithelial lamina of the larynx are detectable at stage 14. Vestibulotracheal and pharyngotracheal canals are distinguishable at stage 15. Hence, from the first appearance of the foregut at about 19 days, the larynx has developed into a recognizable organ two weeks later, namely, by 33 days.