Fabiola Müller

The Meninges in Human Development

The brain and cranial meninges were studied in 61 serially sectioned embryos of stages 8–23. Much earlier stages than those examined by previous authors provided a more comprehensive view of meningeal development. As a result, the possible and probable sources of the cranial and spinal meninges are believed to be: (a) prechordal plate, (b) unsegmented paraxial (parachordal) mesoderm, (c) segmented paraxial (somitic) mesoderm, (d) mesectoderm (neural crest), (e) neurilemmal cells (neural crest), and (f) neural tube. Some of these sources (a, b, d) pertain to the cranial meninges, others (c, d, e) to the spinal coverings. The first of the future dural processes to develop is the tentorium cerebelli, which, at the end of the embryonic period proper, differs considerably in shape and composition from the later fetal and postnatal tentorium. The embryonic dural limiting layer (Duragrenzschicht) probably corresponds to the interface layer of the adult meninges. The appropriate literature was reviewed and summarized.

The development of the human brain, the closure of the caudal neuropore, and the beginning of secondary neurulation at stage 12

SummaryTwenty-four embryos of stage 12 (26 days) were studied in detail and graphic reconstructions of five of them were prepared. The characteristic features of this stage are 21–29 pairs of somites, incipient or complete closure of the caudal neuropore, and the appearance of upper limb buds. The caudal neuropore closes during stage 12, generally when 25 somititc pairs are present. The site of final closure is at the level of future somite 31, which corresponds to the second sacral vertebral level. Non-closure of the neuropore may be important in the genesis of spina bifida aperta at low levels. The primitive streak probably persists until the caudal neuropore closes, when it is replaced by the caudal eminence or end-bud (Endwulst oder Rumpfknospe). The caudal eminence, which appears at stage 9, gives rise inter alia to hindgut, notochord, caudal somites, and the neural cord. The material for somites 30–34 (which appear in stage 13) is laid down during stage 12, and its absence would be expected to result in sacral agenesis. Aplasia of the caudal eminence results in cloacal deficiency and various degrees of symmelia.The junction of primary and secondary development (primäre und sekundäre Körperentwicklung) is probably at the site of final closure of the caudal neuropore. Secondary neurulation begins during stage 12. The cavity of the already formed spinal cord extends into the neural cord, and isolated spaces are not found within the neural cord. Primary and secondary neurulation are probably coextensive with primary and secondary development of the body, respectively. The telencephalon medium has enlarged two mesencephalic segments (M1 and M2) are distinguishable, and rhombomere 4 is reduced. The sulcus limitans is detectable in the spinal cord and hindbrain (RhD), and in the mesencephalon and diencephalon, where it extends as far rostrally as the optic sulcus in D1. A marginal layer is appearing in the rhombencephalon and mesencephalon. The first nerve fibres are differentiating, chiefly within the hindbrain (from the nucleus of the lateral longitudinal tract). Optic neural crest is at its maximum, and the otic vesicle is giving crest cells to ganglion 7/8. Neural crest continues to develop in the brain and contributes to cranial ganglia 5, 7/8, and 10/11. The spinal crest extends as far caudally as somites 18–19 but shows no subdivision into ganglia yet. Placodal contribution to the trigeminal ganglion is not certain at stage 12. Such a contribution to ganglion 7/8 is not unlikely. Involvement of neural crest in the formation of the derivatives of pharyngeal arches 1 and 2 is possible but has not yet been confirmed in the human embryo.

Developmental Stages in Human Embryos: Revised and New Measurements

The staging of human embryos, as distinct from seriation, depends on a morphological scheme devised by Streeter and completed by O’Rahilly, who proposed the term Carnegie stages. To avoid misconceptions and errors, and to place new findings in perspective, it is necessary to summarize the essentials of the Carnegie system: (1) Twenty-three stages cover the embryonic period, i. e. the first 8 postfertilizational weeks of development. (2) The system is based on internal as well as external features, and the use of only external criteria is subject to serious limitations. For example, precise delineation of stages 19–23 and of the embryonic-fetal transition depends on histological examination. (3) Prenatal measurements are not an integral component of the staging system, and hence a stage should never be assigned merely on the basis of embryonic length. A 20-mm embryo, for example, could belong to any of three stages. Measurements, however, are important for the assessment of age, and very few measurements are available for staged embryos. Presented here and based on accurate staging are the maximum diameter of the chorionic sac, the crown-heel length, the greatest length exclusive of the lower limbs, the biparietal diameter, the head circumference, the length of the hindbrain, the total length of the brain, and the lengths of the limbs as well as of their segments, including the foot length. (4) Prenatal ages are also not an integral part of the staging system and hence a stage should never be assigned merely on the basis of prenatal age. Ages, however, are of clinical importance and their estimate has been rendered more precise by accurate timing of fertilization followed by ultrasonography. Prenatal age is postfertilizational and hence some 2 weeks less than the postmenstrual interval. The term gestational age is ambiguous and should be discarded. Presented here is a new graph showing proposed estimates of age in relation to stages and based on current information.

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 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 neural crest in the human

The first systematic account of the neural crest in the human has been prepared after an investigation of 185 serially sectioned staged embryos, aided by graphic reconstructions. As many as fourteen named topographical subdivisions of the crest were identified and eight of them give origin to ganglia (Table 2). Significant findings in the human include the following. (1) An indication of mesencephalic neural crest is discernible already at stage 9, and trigeminal, facial, and postotic components can be detected at stage 10. (2) Crest was not observed at the level of diencephalon 2. Although pre‐otic crest from the neural folds is at first continuous (stage 10), crest‐free zones are soon observable (stage 11) in Rh.1, 3, and 5. (3) Emigration of cranial neural crest from the neural folds at the neurosomatic junction begins before closure of the rostral neuropore, and later crest cells do not accumulate above the neural tube. (4) The trigeminal, facial, glossopharyngeal and vagal ganglia, which develop from crest that emigrates before the neural folds have fused, continue to receive contributions from the roof plate of the neural tube after fusion of the folds. (5) The nasal crest and the terminalis‐vomeronasal complex are the last components of the cranial crest to appear (at stage 13) and they persist longer. (6) The optic, mesencephalic, isthmic, accessory, and hypoglossal crest do not form ganglia. Cervical ganglion 1 is separated early from the neural crest and is not a Froriep ganglion. (7) The cranial ganglia derived from neural crest show a specific relationship to individual neuromeres, and rhombomeres are better landmarks than the otic primordium, which descends during stages 9–14. (8) Epipharyngeal placodes of the pharyngeal arches contribute to cranial ganglia, although that of arch 1 is not typical. (9) The neural crest from rhombomeres 6 and 7 that migrates to pharyngeal arch 3 and from there rostrad to the truncus arteriosus at stage 12 is identified here, for the first time in the human, as the cardiac crest. (10) The hypoglossal crest provides cells that accompany those of myotomes 1–4 and form the hypoglossal cell cord at stages 13 and 14. (11) The occipital crest, which is related to somites 1–4 in the human, differs from the spinal mainly in that it does not develop ganglia. (12) The occipital and spinal portions of the crest migrate dorsoventrad and appear to traverse the sclerotomes before the differentiation into loose and dense zones in the latter. (13) Embryonic examples of synophthalmia and anencephaly are cited to emphasize the role of the neural crest in the development of cranial ganglia and the skull.

The human brain at stages 21–23, with particular reference to the cerebral cortical plate and to the development of the cerebellum

SummaryThe development of the human brain during the eighth embryonic week was studied in serial sections of 22 embryos, and graphic reconstructions were prepared. The cortical plate appears in stage 21 in the area of the future insula and is an excellent feature for staging. The internal capsule contains neocortical fibres. Its three main outlets begin to be present in stage 22 and lead to epithalamus, to dorsal thalamus, and to mesencephalon. At this time a well developed lateral olfactory tract can be seen. The anterior commissure appears in stage 23. A clear developmental relationship between claustrum and olfactory area is described for the first time in human embryos. The optic tract reaches the ventral area of the lateral geniculate body. Scattered fibres of the lateral lemniscus reach at least as far as the caudal mesencephalon, in which superior and inferior colliculi can be distinguished at stage 23; two caudalBlindsäcke containing ventricular recesses form in stage 23. The cerebellum is still present as a plate, but its internal bulge is considerably enlarged. It possesses radially- and tangentially-arranged cells; the latter form the external germinal layer. The dentate nucleus, as well as the inferior and superior cerebellar peduncles and some of the cerebellar commissures, are present. Compared with the highly developed and probably already functional remainder of the hindbrain, the cerebellar plate shows far less differentiation. Two caudal migratory streams (marginal and submarginal) are present and represent the corpus pontobulbare. The decussation of the pyramids appears in stage 23.This article concludes the study of the developing human brain during the embryonic period, from stage 8 to stage 23. The series was based on 340 serially-sectioned embryos and graphic reconstructions from 89 brains. No comparable investigation of the fetal brain is available.

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

SummaryThirteen 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 invagionation 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, spmal part, and mesencephalon.

Olfactory structures in staged human embryos.

The olfactory region was investigated in 303 serially sectioned human embryos, 23 of which were controlled by precise graphic reconstructions. The following findings in the embryonic period are new for the human. (1) The nasal plates arise at the neurosomatic junction, as do also the otic placodes. (2) Crest comes from the nasal plates later (stage 13) than the maximum production in the neural folds (stage 10). (3) The crest arises and migrates during a much longer time (at least until the end of the embryonic period) than the neural crest of the head, where origin and migration end at stage 12. (4) Olfactory nerve fibres enter the brain at stage 17, the vomeronasal fibres and those of the nervus terminalis at stages 17 and 18. (5) Fibre connections between the olfactory tubercle and the olfactory bulb, as well as those to the amygdaloid nuclei, forebrain septum, and hippocampus, develop during and after stage 17. (6) Mitral cells appear late in the embryonic period. (7) Localized, although incomplete, lamination of the olfactory bulb is detectable at the embryonic/fetal transition. (8) Tangential migratory streams of neurons, from stage 22 to the early fetal period, proceed from the subventricular zone of the olfactory bulb towards the future claustrum; they remain within the insular region but are separated from the cortical plate. (9) In future cebocephaly morphological indications may be visible as early as stage 13. The various findings are integrated by means of staging, and current information for the fetal period is tabulated from the literature.

Segmentation in staged human embryos: the occipitocervical region revisited

The first seven somites, the rhombomeres, and the pharyngeal arches were reassessed in 145 serially sectioned human embryos of stages 9–23, 22 of which were controlled by precise graphic reconstructions. Segmentation begins in the neuromeres, somites and aortic arches at stage 9. The following new observations are presented. (1) The first somite in the human, unlike that of the chick, is neither reduced in size nor different in structure, and it possesses sclerotome, somitocoel and dermatomyotome. (2) Somites 1–4, unlike those of the chick, are related to rhombomere 8 (rather than 7 and 8) and are caudal to pharyngeal arch 4 (rather than in line with 3 and 4). (3) Occipital segment 4 resembles a developing vertebra more than do segments 1–3. (4) The development of the basioccipital resembles that of the first two cervical vertebrae in that medial and lateral components arise in a manner that differs from that in the rest of the vertebral column. (5) The two groups of somites, occipital 1–4 and cervical 5–7, each form a median skeletal mass. (6) An ‘S‐shaped head/trunk interface’, described for the chick and unjustifiably for the mouse, was not found because it is not compatible with the topographical development of the otic primordium and somite 1, between which neural crest migrates without hindrance in mammals. (7) Occipital segmentation and related features are documented by photomicrographs and graphic interpretations for the first time in the human. It is confirmed that the first somite, unlike that of the chick, is separated from the otic primordium by a distance, although the otic anlage undergoes a relative shift caudally. The important, although frequently neglected, distinction between lateral and medial components is emphasized. Laterally, sclerotomes 3 and 4 delineate the hypoglossal foramen, 4 gives rise to the exoccipital and participates in the occipital condyle, 5 forms the posterior arch of the atlas and 6 provides the neural arch of the axis, which is greater in height than the arches of the other cervical vertebrae. Medially, the perinotochord and migrated sclerotomic cells give rise to the basioccipital as well as to the vertebral centra, including the tripartite column of the axis. Registration between (1) the somites and (2) the occipital and cervical medial segments becomes interrupted by the special development of the axis, the three components of which come to occupy the height of only 2½ segments.

The first appearance of the major divisions of the human brain at stage 9

SummaryFive embryos of stage 9 (20 days) were studied in detail and graphic reconstructions were prepared. This is the first report based on more than one specimen of this rarely seen stage. Detailed measurements of the embryos are provided. The characteristic features of this stage are 1 to 3 pairs of somites, the head fold and foregut, the otic discs, and the pericardial cavity. The primitive streak is not decreasing as rapidly as in stage 8 but the caudal eminence is proliferating. The notochordal plate is not increasing as rapidly as in stage 8 and a notochord is not yet present. It is to be stressed that the 3 major dividions of the brain (prosencephalic, mesencephalic, and rhombencephalic) can be identified in the open neural groove. The rhombencephalon is the dominant feature of the brain and comprises 4 subdivisions, the last of which, previously unrecognized, is related to the (occipital) somites and represents the hypoglossal region. Features that may not be visible include the caudal fold and hindgut, endocardial tubes or plexus, neural crest, and neurenteric canal. In 2 specimens, atria, left ventricle, right ventricle, conotruncus, and first (or first and second) aortic arches are distinguishable. Two of the embryos studied are the earliest examples in which neural crest has been identified.