PubMed Health⌕ Search

Biomedical subjects

R O'Rahilly

Publications and source records attributed to R O'Rahilly.

At least 19 recordsLinked to original sources
PubMed2004

The primitive streak, the caudal eminence and related structures in staged human embryos.

The caudal region of the trunk was reassessed in 52 serially sectioned human embryos of stages 8-23, 42 of which were controlled by precise graphic reconstructions. The following observations, new for the human, are presented. (1) The neurenteric canal is an important landmark because rostral to it the neural plate of stages 8, 9, and the main part of the notochord develop, whereas caudal to it the neural plate of stages 10-12 and the caudal portion of the notochord are formed. All somites at stages 9-11 and probably also at stage 12 arise rostral to the site of the neurenteric canal. (2) A 'chordoneural hinge' can be detected in stages 10 and 11, where the caudal part of the neural plate gives off cells that probably participate in the production of mesenchyme. (3) When apparent disappearance of the epiblast is used as a criterion, then the primitive streak seems to end during stage 9. (4) The caudal eminence, derived from the primitive streak and covered by ectoderm, forms at stage 10 caudal to the site of the former neurenteric canal and persists as a terminal cap to at least stage 14, although formation of mesenchyme continues in stages 15 to 17 or 18. (5) As the region rostral to the site of the neurenteric canal grows because of the development of somites, the caudal eminence is shifted caudally. (6) The caudal eminence is most active developmentally during stage 13, when most of the required (ca 6 out of 9) pairs of somites appear. (7) The eminence produces the caudal part of the notochord and, after closure of the caudal neuropore, all caudal structures, but it does not produce even a temporary 'tail' in the human. (8) A temporal overlap results between primary and secondary development in the caudal part of the notochord. (9) Primary development begins very early with the formation of the inner cell mass at stage 3, and includes the development of the somites rostral to the neurenteric canal, whereas secondary development, with the exception of the notochord caudally, commences at stage 12. (10) Primary neurulation lasts from stage 8 to stage 12, secondary from stage 12 to stages 17 or 18. (11) Secondary development and secondary neurulation are characterized morphologically by direct formation of structures (notochord, postcloacal gut, neural cord/neural tube) from mesenchyme.

Ectoderm↗
PubMed2004

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.

Embryonic Development↗
PubMed2003

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.

Animals↗
PubMed2003

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.

Embryonic and Fetal Development↗
PubMed2000

Prenatal ages and stages-measures and errors.

BACKGROUND: The confusing term "gestational age" is generally either not defined or is used for menstrual "age," postovulatory age, or postfertilizational age. The designation (post)menstrual weeks and/or days is very useful in obstetrics but, because prenatal age extends from fertilization to birth, menstrual "age" is a misnomer. The term "stage" has a specific morphological meaning in embryology and should not be used either for ages or for lengths. METHODS: Embryonic age is best assessed, when possible, from the 23 internationally recognized morphological stages. A morphological staging system is not available for the fetal period, and fetal age is assessed mainly from measurements. RESULTS: Among these, the unsatisfactory designation crown-rump length (CRL) should be replaced, in ultrasonic biometry as well as in embryology, by the greatest length (GL), exclusive of the lower limbs. These points are discussed in detail, and justification for the views expressed is provided. CONCLUSIONS: The continuing confusion concerning prenatal age is shown to be unnecessary once the ambiguous and superfluous term "gestational age" is abandoned.

Crown-Rump Length↗
PubMed1997

Making planes plain.

The major anatomical planes (horizontal, coronal, and sagittal, including the median plane) are discussed from a historical perspective, and their correct usage is clarified. Unofficial and unnecessary terms to be avoided (for reasons explained) include midsagittal, parasagittal, and midline.

Anatomy↗
PubMed1997

Philatelic introduction to the history of anatomy.

Representative postage stamps reproduced here provide an idea of the scope available for introducing the history of human anatomy. Included are ancient Greek pioneers, the reformer of human anatomy (Vesalius), the discoverer of the circulation of the blood (Harvey), the compound microscope, and well-known names in histology, neurohistology, and topographical anatomy.

Anatomy↗
PubMed1997

"Gestational age"?

Explore the source record for details and available documents.

Embryonic and Fetal Development↗
PubMed1997

The timing and sequence of appearance of neuromeres and their derivatives in staged human embryos.

Serial sections of 215 human embryos from Carnegie stages 6-17 were investigated, and 85 graphic reconstructions were prepared. It is proposed that neuromeres be defined as morphologically identifiable transverse subdivisions perpendicular to the longitudinal axis of the embryonic brain and extending onto both sides of the body. It is proposed further that primary neuromeres be redefined as the early-appearing larger divisions of the open neural folds, and secondary neuromeres as the smaller subdivisions that are found both before and after closure of the neural tube. In the light of these definitions, 6 primary neuromeres can be detected in the human brain at stage 9, and a maximum of 16 secondary neuromeres at stage 14. The relationships of the 8 rhombomeres to the associated neural crest, as well as to the pharyngeal arches and the exits of the cranial nerves, are tabulated. Rhombomere 8 (Rh. 8) is intermediate between the more rostral neuromeres and the spinal cord, and its neural relationships indicate that the four occipital somitic pairs do not impress a strictly repetitive pattern as in the spinal cord. Hence, it is suggested that Rh. 8 depends on both intrinsic and extrinsic factors. The synencephalon, parencephalon, and isthmic neuromere can be distinguished in stage 13. In stage 14, rostral and caudal portions of the parencephalon are recognizable, and the full complement of 16 neuromeres is now present. The medial ventricular eminence appears in the diencephalon (D1). A longitudinal organisation begins to be superimposed on the neuromeres, as now indicated by the appearance of the hypothalamic cell cord. This continues in stage 15, when the hypothalamic sulcus develops. That groove, however, is not continuous with the sulcus limitans. In the diencephalon, five longitudinal zones can be discerned. In stage 16, fibre tracts, such as the habenulo-interpeduncular (fasciculus retroflexus) and the tract of the posterior commissure, outline the boundaries of the synencephalon. In stage 17, the tract of the zona limitans intrathalamica (along the marginal ridge in the parencephalon) is an important landmark. This is the last stage in which all the neuromeres can be distinguished. The supramamillary recess becomes defined and is the termination of the sulcus limitans: the alar/basal distinction is inappropriate in the human forebrain. The number and identity of the neuromeres in the human brain, their precise sequence of appearance, and the stages at which they appear are here clarified for the first time. The results of various studies of domains of gene expression indicate that, although in some instances such territories follow the morphological neuromeres, in others they may cross interneuromeric boundaries. It is concluded that the precise morphological study of neuromeres in any given species is necessary for correlative investigations of gene expression.

Anatomy, Comparative↗
PubMed1994

Neurulation in the normal human embryo.

The neural groove and folds are first seen during stage 8 (about 18 postovulatory days). Two days later (stage 9) the three main divisions of the brain, which are not cerebral vesicles, can be distinguished while the neural groove is still completely open. Two days later (stage 10) the neural folds begin to fuse near the junction between brain and spinal cord, when neural crest cells are arising mainly from the neural ectoderm. The rostral (or cephalic) neuropore closes within a few hours during stage 11 (about 24 days). The closure is bidirectional; it takes place from the dorsal and terminal lips and may occur in several areas simultaneously. The two lips, however, behave differently. The caudal neuropore takes a day to close during stage 12 (about 26 days) and the level of final closure is approximately at future somitic pair 31, which corresponds to the level of sacral vertebra 2. At stage 13 (4 weeks) the neural tube is normally completely closed. Secondary neurulation, which begins at stage 12, is the differentiation of the caudal part of the neural tube from the caudal eminence (or end-bud) without the intermediate phase of a neural plate.

Embryonic and Fetal Development↗
PubMed1994

Occipitocervical segmentation in staged human embryos.

Serial sections of 108 human embryos from stage 11 to stage 23 were investigated, and 33 reconstructions were prepared. The existence of 4 occipital somites was confirmed. The important developmental distinction between axial (central) and lateral components obtains in the occipital as well as in the vertebral region. The lateral occipital components begin to show dense areas as the cervical region is approached. The lateral occipital and vertebral components arise in registration with the initial sclerotomes. In both the occipital and the vertebral region the related nerves and intersegmental arteries traverse the loose areas of the sclerotomes. The axial occipital region is not segmented, whereas the cervical components develop from perinotochordal loose areas. Three complete centra (known as XYZ) develop in the atlanto-axial region, although they are related to only 2 1/2 sclerotomes and only 2 neural arches. The height of the XYZ complex equals that of 3 centra elsewhere, and not 2 1/2, as previously maintained. The experimental findings in the occipitocervical region of the chick embryo show both similarities to, as well as differences from, the data for the human embryo. A scheme showing the early development of the entire vertebral column is included.

Atlanto-Occipital Joint↗
PubMed1993

Commemorating the Fabrica of Vesalius.

It is an appropriate time to commemorate the Fabrica of Andreas Vesalius, which was published in Basel 450 years ago. In addition, a dozen key references in English (books and articles) are listed for those who wish to learn more about Vesalius and particularly concerning the wonderful woodcuts in the Fabrica.

Anatomy↗
PubMed1991

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.

Abnormalities, Multiple↗