Somites, spinal Ganglia, and centra. Enumeration and interrelationships in staged human embryos, and implications for neural tube defects.

Serial sections of 99 human embryos from Carnegie stages 8-23 were investigated and 38 graphic reconstructions were evaluated. At stage 9 somite 1 is of appreciable size and is separated from the otic disc, as also in the next several stages by rhombomeres and pharyngeal arches 3 and 4, thereby differing from the chick. At stage 10 somite 1 begins to differentiate into sclerotome and dermatomyotome. At stage 11 spinal neural crest begins to develop. At stage 12 parts of somites 1-4 are being transformed into the hypoglossal cell cord. It is stressed that the numbers of somites present at stages 9-12 are part of the definition of those stages. At stage 13 dense and loose zones begin to be detectable rostrally in the sclerotomes and also, although out of phase, in the perinotochord. Spinal ganglia begin to develop in phase with the somites. At stages 14-16 the maximum number of somites observed was 38-39 rather than 42-44, as usually given. Moreover, they did not extend to the tapered end of the trunk, which is not a (vertebrated) 'tail'. At stages 17-23 the maximum number of centra was 38-39, including coccygeal vertebrae 4-5. Although most of the somites appear during primary development, all of the spinal ganglia develop during secondary development (stages 13-18). The number of ganglia was at a maximum of 35 at stage 18, but was reduced to 32 already by stage 23. Important points confirmed in this study are that the number of occipital somites in the human is four, and that the level of final closure of the caudal neuropore is future somite 31, which represents approximately future sacral vertebra 2. The interpretation of relevant neural tube defects is discussed in the light of the findings. The ascensus of the conus medullaris during the fetal period is well established, but a concomitant ascent of the situs neuroporicus is proposed here, and has implications for defects that involve tethering of the spinal cord. The main results are integrated in comprehensive graphic representations of the levels and the interrelationships of (a) somites and centra, and (b) somites, neural crest, and spinal ganglia. These may aid in the elucidation of some frequently occurring anomalous conditions.

The prechordal plate, the rostral end of the notochord and nearby median features in staged human embryos.

The enigmatic structure known as the prechordal plate and also the precursors of the notochord were reassessed in 101 human embryos of stages 8-14; 36 were controlled by precise graphic reconstructions. Various measurements were made and the appearance of median structures was tabulated. The prechordal plate, which has been unequivocally found first at stage 7, is usually detectable at stage 8 as a highly developed mesendodermal mass in contact with the floor of the neural groove. At stages 9 and 10 the plate is related to neuromere D1. Cellular migration laterad at stages 9-11 gives rise to the bilateral premandibular condensations, which are lateral to the adenohypophysial primordium, and at stages 13 and 14 these condensations are closely related to the future tentorium cerebelli. The notochordal process is first visible at stage 7, and its dorsal part constitutes the notochordal plate at stage 8. At stages 8-10 the notochordal and prechordal plates appear continuous, but they are distinguishable histologically. The notochordal plate becomes intercalated in the endoderm of the foregut and begins to give rise to the notochord at stages 10 and 11. Bifurcation occurs rostrally at stage 12: the dorsal limb disappears, whereas the ventral limb is the definitive continuation. The topographical relationships of the prechordal and notochordal plates, the notochord, the adenohypophysis, and the oropharyngeal membrane are documented. Definitions and pertinent remarks on terminology are included, comparative data are considered, and the origin and derivatives of the prechordal plate are discussed. In addition to giving rise to external ocular muscles, the possibility of contributions to the heart and the tentorium cerebelli is raised. The importance of the plate in the development of the forebrain, as well as in the production of median anomalies such as holoprosencephaly and cyclopia, is stressed.

A laboratory exercise in the study of the gross structure of the eye.

The dissection of the bovine eye, when it is based on procedures used in ophthalmic surgery, results in an enthusiastic reception by medical students.

Development of anencephaly and its variants.

Extreme variants of anencephaly in two human embryos of the same stage, namely 22 (54 days), shed new light on problems such as craniocerebral interrelationships and the timing of developmental events. Embryo X had a chondrocranium that possessed features typical of a holoacranial anencephalic skull and an extremely well-preserved brain, in which some of the neural tracts were comparable to those in a normal control. On the other hand, embryo Y of the same stage had a completely degenerated brain, although the chondrocranium was more nearly normal and represented the precursor of a meroacranial skull. A comparison of the two cases seems to indicate a certain independence between skull and brain. Moreover, it appears possible that the disturbances are related primarily to the skeletal, and only secondarily to the nervous, component. Comparisons with experimental data allow the conclusion that the maldevelopment involves mostly paraxial mesenchyme and little or no disturbance of neural crest. The timing of the mesenchymal defect is probably as early as stages 8 and 9 (18-20 days). This is also the time at which mesenchymal defects can result in failure of the neural tube to close.

Mediobasal prosencephalic defects, including holoprosencephaly and cyclopia, in relation to the development of the human forebrain.

Four very early synophthalmic embryos were studied in serial sections and reconstructed graphically by the point-plotting method. Three belonged to stage 16 (5 weeks) and one to stages 19/20 (7 weeks). Recently completed accounts and reconstructions of the normal brains of staged human embryos served as controls for comparison with the abnormal examples. The embryos shared in common: holoprosencephaly, arhinencephaly sensu stricto (absence of olfactory nerve fibers, bulbs, and tracts), presence of a proboscis, synophthalmia with two lens vesicles, a retarded telencephalic wall, absence of the mediobasal part of the telencephalon (the future septal area and the commissural plate: future anterior commissure and corpus callosum), irregularity of the diencephalon, mensural changes in the brain, absence of the rostral part of the notochord and consequent cranial defects, and small ganglia of the cranial nerves. Where it could be determined (at least in the three less advanced specimens), the adenohypophysial primordium was either small and isolated or was absent; a tentorial condensation appeared to be missing; and disturbances of the primordia of the orbital muscles and their innervation were noted. The corpus striatum is single and corresponds to only the diencephalic part (medial eminence) of normal embryos. Interference with induction by the prechordal plate at or before stage 8 (18 days) would be expected to affect the future mediobasal part of the neural plate (median prosencephalic dysgenesis) and the future optic primordium (cyclopia sensu stricto). Insufficient formation of material from the prechordal plate would account for disorders of the orbital musculature and, possibly, for inadequacy of the tentorium cerebelli. Disturbance a couple of days later (stage 9) would result in synophthalmia. Cyclopia and synophthalmia entail arhinencephaly and holoprosencephaly, both of which may arise independently. Defective distribution of the cephalic mesenchyme points to a derangement of the mesencephalic neural crest (stages 10 and 11), causing such features as an incomplete chondrocranium and reduction in size of the ganglia of the cranial nerves. Failure of bilateral division of the telencephalon would occur at or before 4 weeks (stages 13 and 14). It is concluded that all the above conditions arise during the first 4 postovulatory weeks.

The development of the human brain from a closed neural tube at stage 13.

Twenty-five embryos of stage 13 (28 days) were studied in detail and graphic reconstructions of seven of them were prepared. Thirty or more somitic pairs are present, and the maximum is possibly 39. The notochord is almost entirely separated from the neural tube and the alimentary epithelium, and its rostral tip is closely related to the adenohypophysial pocket. Caudal to the cloacal membrane, the caudal eminence is the site of secondary neurulation. The eminence, which usually contains isolated somites, is the area where new notochord, hindgut, and neural tube are forming. The neural cord develops into neural tube without the intermediate phase of a neural plate (secondary neurulation). Canalization is regular and the lumen is continuous with the central canal. The neural tube is now a closed system, filled with what may be termed "ependymal fluid." The brain is widening in a dorsoventral direction. Neuromeres are still detectable. The following features are distinguishable: infundibular area of D2, chiasmatic plate of D1, "adult" lamina terminalis, and commissural plate (at levels of nasal plates). The beginning of the synencephalon of D2 can be discerned. The retinal and lens discs are being defined. The mesencephalic flexure continues to diminish. The midbrain possesses a sulcus limitans, and the tegmentum may show the medial longitudinal fasciculus. The isthmic segment is clearly separated from rhombomere 1. Lateral and ventral longitudinal fasciculi are usually present in the hindbrain, and the common afferent tract is beginning. Somatic and visceral efferent fibres are seen in certain nerves: 6, 12; 5, 7, 9-11. The first indication of the cerebellum may be visible in the alar lamina of rhombomere 1. The terminal-vomeronasal crest appears. Various cranial ganglia (e.g., vestibular, superior ganglia of 9, 10) are forming. The trigeminal ganglion may show its three major divisions. Epipharyngeal placodes of pharyngeal arches 2 to 5 contribute to cranial ganglia 7, 9, and 10. The spinal neural crest is becoming segregated, and the spinal ganglia are in series with the somites. Ventral spinal roots are beginning to develop.

The first appearance of the future cerebral hemispheres in the human embryo at stage 14.

Thirty-five embryos of stage 14 (32 days) were studied in detail and graphic reconstructions of four of them were prepared. Characteristic features of this stage include the beginning formation of the future cerebral hemispheres and the cerebellar plates. The ventral boundary between telencephalon medium and diencephalon is the preoptic recess. Although a velum transversum is not yet distinguishable as a dorsal boundary, its site is indicated by a change in the thickness of the roof of the forebrain. As the cerebral vesicles (future hemispheres) begin to evaginate, a di-telencephalic sulcus and a corresponding lateral ventricle and ventricular ridge (torus hemisphericus) develop. The telencephalic wall is mainly ventricular layer but three areas show advanced differentiation: olfactory area, future amygdaloid body (which lies at first mainly in the diencephalon), and primordium of the hippocampus. The telencephalon is growing in length, and the forebrain now occupies almost one quarter of the total length of the brain. The two neuromeres of the diencephalon are no longer as clearly delineated. The floor of D1 presents a thickened chiasmatic plate; that of D2 includes the infundibulum, which is closely related to the adenohypophysial pouch. The ventricular surface of D1 presents elevations for the dorsal and ventral thalami, separated by the sulcus medius. Other features of the diencephalon include the ventricular eminence (medial ventricular ridge) of the basal nuclei and the hypothalamic cell cord, from which the preopticohypothalamotegmental tract arises. The roof of D2 contains the evaginating part of the synencephalon. The mesencephalic angle continues to diminish. Two neuromeres, M1 and M2, are still distinguishable. The oculomotor nucleus emits nerve fibres, as does also the trochlear nucleus, which lies in the isthmic segment. Some extracerebral oculomotor fibres are present, but decussating and extracerebral trochlear fibres have not yet appeared. In the region of the tectum, two nuclei are discernible, and will form the medial tectobulbar tract and the mesencephalic root of the trigeminal nerve, respectively. The medial longitudinal fasciculus is present. A "median ventricular formation" is sometimes found in the mesencephalic roof. The cerebellum is the widest part of the brain. Two neuromeres (isthmic segment and Rh1) are involved in its formation. Most of the cerebellar plate has differentiated an intermediate layer, and the future rhombic lip is discernible. Indications of an efferent fibre system are present. In addition to the cerebellum, the rhombencephalon includes Rh1 to Rh7, and RhD.(ABSTRACT TRUNCATED AT 400 WORDS)

The human larynx at the end of the embryonic period proper. 2. The laryngeal cavity and the innervation of its lining.

The laryngeal cavity was studied in eight serially sectioned embryos of stage 23 and in three early fetuses, and graphic reconstructions were prepared. After the isolation of the tracheal from the pharyngeal cavity during stages 16 through 22, a communication (not necessarily the pharyngotracheal duct) appears again during stage 23. At this time (8 postovulatory weeks) the laryngeal cavity comprises 1) the coronal and parts of the sagittal clefts of the vestibule (uniting later at the laryngeal inlet); 2) the ventricles, which are not yet completely formed; and 3) the subglottic cavity, which appeared already in earlier stages. The characteristic events of stage 23 are the dissolution of the epithelial lamina and the development of the ventricles. The disruption of the epithelial lamina is an active process that comprises rearrangement and growth, but not loss of cells. The ventricles, which begin as solid outgrowths in stage 20, do not represent fifth pharyngeal pouches. They now point toward the middle of the still paired thyroid laminae and are not at the level of the future glottis, which lies more caudally. In the absence of the median part of the soft palate, the nasopharynx communicates widely with the oral cavity. The epithelium of the respiratory tube, including the larynx, resembles that of the pharynx and esophagus in being pseudostratified columnar and showing a clear basement membrane. It is ciliated over that part of the epiglottis that surmounts the arytenoid swellings, and also over the tip and back of the latter. The transitional area between the laryngopharynx and the esophagus is already innervated by the recurrent laryngeal nerve. Nerve fibers have not yet reached the epithelium of the coronal cleft and the ventricles, but fibers are present near the sagittal cleft of the vestibule. The sensory innervation of the pharynx and larynx has been followed and plotted for the first time in an embryo, and previously unrecorded silver-impregnated receptors have been observed.

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.

The Dandy-Walker and Arnold-Chiari malformations. Clinical, developmental, and teratological considerations.

Five patients with the Dandy-Walker syndrome had dysgenesis of the cerebellar vermis, cystic dilatation of the fourth ventricle, and a high position of the tentorium cerebelli. When only these features are present, the patient may lead a normal life. Additional defects usually account for the prominent clinical and pathological features of this syndrome. In this series, one patient had aqueductal stenosis, four had agenesis of the corpus callosum, two had hydrocephalus, one had cerebral abiotrophy, and one (a 72-year-old man) had no additional defects and no symptoms from his Dandy-Walker syndrome. An analysis of development and teratological considerations indicates that the Dandy-Walker and Arnold-Chiari malformations are complex disorders that have different causes and mechanisms and begin at different times in the emryonic period. The causes are still unknown.