Ventrodorsal Direction Research Articles

ACCURACY OF POSITIONING THE CERVICAL SPINE FOR RADIATION THERAPY AND THE RELATIONSHIP TO GTV, CTV AND PTV

The purpose of the study was to evaluate the accuracy and precision of a rigid positioning device for repositioning the cervical spine accurately and precisely during conformal radiation therapy of dogs. Fifteen purpose bred research dogs in a radiation therapy study were included. The dogs were positioned using a head holder and a deflatable pillow attached to the treatment table. Port films were reviewed retrospectively, and repositioning precision was recorded by measurements in three orthogonal planes of the head, 2nd cervical vertebra and 1st thoracic spinous process. Mean treatment position was compared to the planning position for a measurement of systematic set-up error. Mean interfraction position variation of the 2nd cervical vertebra was 0.2, 0.1 and 0.2 cm for the ventrodorsal, caudocranial and laterolateral directions respectively, and the average systematic set up error was 0.2, 0.1 and 0.2 cm for the ventrodorsal, caudocranial and laterolateral directions respectively. Knowledge of the magnitude of reposition errors should be included when determining the margins around the tumor.

Multistep role for actin in initial closure of the mesencephalic neural groove in the chick embryo.

In a previous study, we have demonstrated that initial closure of the mesencephalic neural groove in the chick embryo is different from neurulation elsewhere. The neural groove invaginates, the walls appose and make contact in a ventrodorsal direction, and subsequently separate ventrally, forming an incipient neural tube lumen, which finally widens into a definitive lumen. In this study, a role for actin in the processes of this initial mesencephalic closure is studied. Based on rhodamine-phalloidin-stained sections, three distinct actin distribution patterns emerged, and time-lapse video microscopy revealed cytochalasin-D-reversible neurulation movements. We propose that actin is involved in formation and stabilization of the neural groove hinge point, in invagination of dorsal neuroepithelial cells into the neural groove, in the origin of the incipient lumen and the reinforcement of adhesion of the dorsal neural folds, and finally in the development of a wide lumen. Such a multifunctional effect of actin microfilaments within a narrow time window and at specific sites has not been reported yet.

Definition of a moving gross target volume for stereotactic radiation therapy of stented coronary arteries

Purpose : To measure the effect of cardiac motion on coronary artery stent position during the cardiac cycle as a first step toward exploring the feasibility of stereotactic external beam radiation therapy targeted at restenotic stented coronary arteries. Methods and Materials : The three-dimensional (3D) position of eight coronary artery stents in 8 patients immobilized in a stereotactic body frame was studied noninvasively by single-breathhold ECG-gated multislice spiral computed tomography (MSCT) during 10 retrospectively selected phases, equally distributed throughout the R-R interval of consecutive cardiac cycles. The volume encompassing all measured 3D positions of the stent was measured. Results : Stent volumes measured by MSCT closely agreed with measurements by quantitative coronary angiography ( r > 0.99). The mean of the maximum 3D stent center of mass displacement between any two phases during the cardiac cycle for all eight coronary arteries was 7.5 mm (range 3.3–20.5 mm) in the lateral direction, 8.6 mm (range 2.7–21.6 mm) in the ventrodorsal direction, and 8.2 mm (range 2.5–19.7 mm) in the craniocaudal direction. As was anticipated, the volume encompassing all measured 3D positions of the stent represented only a fraction of the whole heart volume in all patients, i.e., less than 0.6%. Conclusions : ECG-gated MSCT allowed the measurement of the volume encompassing multiphase 3D positions of coronary artery stents during the cardiac cycle. This volume, a measure of the cardiac motion effect on coronary artery stent position during the cardiac cycle, represents a moving gross target for stereotactic external beam radiation therapy.

Detection of lung tumor movement in real-time tumor-tracking radiotherapy

Purpose: External radiotherapy for lung tumors requires reducing the uncertainty due to setup error and organ motion. We investigated the three-dimensional movement of lung tumors through an inserted internal marker using a real-time tumor-tracking system and evaluated the efficacy of this system at reducing the internal margin.Methods and Materials: Four patients with lung cancer were analyzed. A 2.0-mm gold marker was inserted into the tumor. The real-time tumor-tracking system calculates and stores three-dimensional coordinates of the marker 30 times/s. The system can trigger the linear accelerator to irradiate the tumor only when the marker is located within the predetermined “permitted dislocation.” The value was set at ±1 to ±3 mm according to the patient’s characteristics. We analyzed 10,413–14,893 data sets for each of the 4 patients. The range of marker movement during normal breathing (beam-off period) was compared with that during gated irradiation (beam-on period) by Student’s t test.Results: The range of marker movement during the beam-off period was 5.5–10.0 mm in the lateral direction (x), 6.8–15.9 mm in the craniocaudal direction (y) and 8.1–14.6 mm in the ventrodorsal direction (z). The range during the beam-on period was reduced to within 5.3 mm in all directions in all 4 patients. A significant difference was found between the mean of the range during the beam-off period and the mean of the range during the beam-on period in the x (p = 0.007), y (p = 0.025), and z (p = 0.002) coordinates, respectively.Conclusion: The real-time tumor-tracking radiotherapy system was useful to analyze the movement of an internal marker. Treatment with megavoltage X-rays was properly given when the tumor marker moved into the “permitted dislocation” zone from the planned position.

Effectiveness of couch height–based patient set-up and an off-line correction protocol in prostate cancer radiotherapy

Purpose: To investigate set-up improvement caused by applying a couch height–based patient set-up method in combination with a technologist-driven off-line correction protocol in nonimmobilized radiotherapy of prostate patients. Methods and Materials: A three-dimensional shrinking action level correction protocol is applied in two consecutive patient cohorts with different set-up methods: the traditional “laser set-up” group ( n = 43) and the “couch height set-up” group ( n = 112). For all directions, left-right, ventro-dorsal, and cranio-caudal, random and systematic set-up deviations were measured. Results: The couch height set-up method improves the patient positioning compared to the laser set-up method. Without application of the correction protocol, both systematic and random errors reduced to 2.2–2.4 mm (1 SD) and 1.7–2.2 mm (1 SD), respectively. By using the correction protocol, systematic errors reduced further to 1.3–1.6 mm (1 SD). One-dimensional deviations were within 5 mm for >90% of the measured fractions. The required number of corrections per patient in the off-line correction protocol was reduced significantly during the course of treatment from 1.1 to 0.6 by the couch height set-up method. The treatment time was not prolonged by application of the correction protocol. Conclusions: The couch height set-up method improves the set-up significantly, especially in the ventro-dorsal direction. Combination of this set-up method with an off-line correction strategy, executed by technologists, reduces the number of set-up corrections required.

Reduction of irradiated small bowel volume and accurate patient positioning by use of a bellyboard device in pelvic radiotherapy of gynecological cancer patients

Purpose: To reduce the volume of small bowel within pelvic treatment fields for gynecological cancer using a bellyboard device and to determine the accuracy of the prone treatment position. Materials and methods: Fifteen consecutive patients with a gynecologic malignancy who were treated with postoperative pelvic radiotherapy were selected for this study. The volume of small bowel within the treatment fields was calculated for both the supine and prone treatment positions. The patients were treated in the prone position in a so-called bellyboard device. During treatment sessions electronic portal images were obtained. An off-line setup verification and correction protocol was used and the setup accuracy of the positioning in the bellyboard was determined. Results: The average volume of small bowel within the treatment fields was 229 cm 3 and 66 cm 3 in the supine and prone treatment, respectively, which means an average volume reduction in the prone position of 64% (95% CI 56–72%), as compared with the supine position. For the position of the patient in the field, the systematic error defined by the standard deviation (SD) of the mean difference per patient between simulation and treatment images was 1.7 mm in the lateral direction, 2.1 mm in the craniocaudal direction and 1.7 mm in the ventrodorsal direction. On average, only 0.4 setup correction per patient was required to achieve this accuracy. The random day-to-day variations were 1.9 (1SD), 2.6 and 2.3 mm, respectively. Standard deviations of the systematic differences between patient positioning relative to the bellyboard were 6.2 mm in lateral direction and 9.1 mm in craniocaudal direction. Conclusions: Treatment of gynecological cancer patients in the prone position using a bellyboard reduces the volume of irradiated small bowel. An off-line verification and correction protocol ensures accurate patient positioning. Daily setup variations using the bellyboard were small (1 SD<3 mm). Therefore for pelvic radiotherapy in patients with a gynecological malignancy, the use of a bellyboard is recommended.

High-speed magnetic resonance imaging for four-dimensional treatment planning of conformal radiotherapy of moving body tumors

Purpose: High-speed magnetic resonance imaging (MRI) was applied to the determination of the planning target volume (PTV) of moving hepatobiliary tumors. Methods and Materials: Three moving tumors, including two metastatic hepatic tumors and one bile duct tumor, were examined using high-speed MRI and reference fiducial markers before external radiotherapy. Patients were examined for 30 seconds under conditions of normal breathing during the examination. The coordinates of the center of the tumor contours were shown on sagittal and coronal images displayed on the monitor. Results: The maximum length of movement was 10.6 ± 7.0 mm in a craniocaudal direction; 5.2 ± 1.8 mm in a lateral direction; and 4.6 ± 1.6 mm in a ventrodorsal direction. When the PTV was determined using MRI at exhalation phase with a 10-mm safety margin, clinical target volume (CTV) was not covered in 19% of all images in the 3 patients. With MRI at inhalation phase with a 10-mm safety margin, CTV was not covered in 36% of all images. Conclusion: Four-dimensional treatment planning using high speed MRI, and integrating time and spatial information, has the potential to determine the planning target volume of moving body tumors more precisely than does conventional CT planning.

The superficial layer of the Musculus masseter and the well-developed process of the maxilla in the tiger Panthera tigris.

The musculature and functional anatomy of the head and skull of a tiger Panthera tigris were, examined and compared with those of various other felid species. The Musculus masseter is divided into three layers. The superficial layer of which is well-developed and further sub-divided into lateral and medial components. The ventro-caudal fleshy portion of the lateral component originates behind the last upper molar, where it is attached by a strong tendon to the tubercle tuber maxillae. The medial component, which runs dorso-ventrally, has a particularly well-developed fleshy portion. The present anatomical study suggests that in the tiger, large masseteric forces are produced in both ventro-rostral and ventro-dorsal directions. In the tiger, a process is present on the tubercle of the maxilla from which the strong tendon originates. This process is also apparent in the leopard Panthera pardus, the jaguar P. onca and the puma Felis concolor. It is not found, however, in either the domestic cat F. catus or the Iriomote wild cat F. iriomotensis. Comparisons between the skulls of various species of felidae indicate that whereas the puma, the leopard and the jaguar each possess the strong tendon, which bundles the ventro-caudal fleshy portion of the lateral component, the domestic cat and the Iriomote wild cat only possess weak tendons attaching to this component.

Regional VA/Q ratios in man using 133Xe and single photon emission computed tomography (SPECT) corrected for attenuation.

We describe a technique to obtain non-invasively regional pulmonary ventilation-perfusion ratios (VA/Q) using single photon emission computed tomography (SPECT) and continuous infusion of 133Xe. Single photon transmission tomography was used for attenuation correction, for delineation of the lungs and for VA/Q calculations. Data are presented for six normal subjects and compared to those for two patients with moderate chronic obstructive pulmonary disease (COPD). The mean VA/Q for the whole lung of the normal subjects ranged from 0.49 to 0.65, group mean 0.56 +/- 0.07 (1 SD), and there was no significant difference between the right and left lung. The consistently too low VA/Q values are related to the inability to measure regional blood volume and the low resolution of the scintillation camera, giving an under-estimation of tracer input. For the normal subjects, the dispersion of VA/Q, as defined by the standard deviation of the individual distribution functions, ranged from 0.12 to 0.19. One of the patients was characterized by a low mean VA/Q of 0.35, and the other patient had a wide dispersion (SD) of VA/Q of 0.37. In the normal subjects, a consistent VA/Q gradient was found only in the ventrodorsal direction. 133Xe and SPECT can be used to obtain meaningful biological information regarding ventilation/perfusion relationships of potential clinical value.

Three-dimensional movement of a liver tumor detected by high-speed magnetic resonance imaging

Objective: Three-dimensional (3D) movement of a spherical liver tumor during respiration was investigated with magnetic resonance imaging (MRI) using a high-speed sequence. Methods: A marker was placed on the surface of the patient as a reference of distance. Repetition time (TR) was 7.7 ms, echo time (TE) was 4.2 ms, flip angle was 20°, section thickness was 8 mm, and a 256×128 matrix was used. The acquisition time was 1.0 s followed by an interval of 0.5 s. The 20 tumor contours extracted during 30 s were superimposed on sagittal and coronal MR images. Results: The maximum value of tumor edge location was 3.9 cm in the cranio–caudal direction, 2.3 cm in the ventro–dorsal direction, and 3.1 cm in the lateral direction. The mean length of tumor displacement observed was 2.1 cm in the cranio–caudal direction, 0.8 cm in the ventro–dorsal and 0.9 cm in the left–right direction, respectively. The locus of the center of the tumor contour in the sagittal cross section was inclined at 23° and in the coronal cross section was inclined at 18° to the cranio–caudal axis of body. Conclusion: In conclusion, 3D movement of a spherical liver tumor was detected using rapid MRI sequential examinations. Magnetic resonance imaging has a potential to improve the accuracy of the planning target volume of a liver tumor.

The cortico-related zones of the rabbit claustrum—study of the claustrocortical connections based on the retrograde axonal transport of fluorescent tracers

The claustrocortical connections in the rabbit were assessed for the first time by the method of axonal retrograde transport of two fluorescent tracers (Fast Blue and Diamidino Yellow). The material consisted of 23 adult New Zealand rabbits. Projection zones of spindle-like form, connected with the precentral, postcentral, temporal and occipital cortices have been delineated. They are organized topographically both in the anteroposterior and ventrodorsal direction. The precentral (motor) projection zone is localized in the anterodorsal part of the claustrum. It may be divided into two separate parts that project to the medial and lateral part of the precentral cortex. The large postcentral (somatosensory) zone occupies mainly the central part, whereas the temporal (auditory) and occipital (visual) zones are situated in the posteroventral part of the claustrum. The overlap of various claustral projection zones is differentiated, the largest being that of the somatosensory zones. In comparison to the results of study of claustral projection zones performed on other species, presumably on the rat and cat, its seems plausible to conclude that the extension of claustral projection zones and degree of their overlap in the rabbit represent an intermediate character.

Variation in prostate position relative to adjacent bony anatomy

Purpose : In prostatic cancer, the prostate cannot be discerned from a portal image. Setup information is, therefore, obtained from bony anatomy. To perform high-precision conformal therapy, knowledge about the variation in distance between the prostate and adjacent bony anatomy during an external radiotherapy treatment is mandatory. This report is concerned with that variation. Methods and Materials : Nine patients previously treated interstitially for prostatic cancer with implantation of 125I seeds, agreed to cooperate in a study. They underwent a number of simulations of external radiotherapy treatment. After the first patient setup, this setup was repeated five times for each patient as if the patient was treated by external radiotherapy. Simulator radiographs were made from each setup in the anterior-posterior and left-right lateral directions. The seeds were clearly visible in the simulator film images and reflect the position of the prostate. No bladder instructions were given and the filling of the rectum was not quantified. Results : Variation in distance between the prostate and the bony anatomy was measured and is presented in one standard deviation of the normal distribution function: 0.8 mm lateral, 1.5 mm ventrodorsal, and 1.7 mm craniocaudal. Conclusion : The bone structure of the pelvis reflects fairly well the position of the prostate. The prostate is least mobile in a lateral direction and most mobile in the craniocaudal and ventrodorsal directions.

A quality control study of the accuracy of patient positioning in irradiation of pelvic fields

Purpose : Determining and improving the accuracy of patient positioning in pelvic fields. Methods and Materials : Small pelvic fields were studied in 16 patients treated for urological cancers using a three-field isocentric technique. Large pelvic fields were studied in 17 gynecological cancer patients treated with anterior and posterior (AP-PA) parallel opposed fields. Quantitative analysis of 645 megavolt images and comparison to 82 simulation images were carried out. Results : Small pelvic fields: for the position of the patient in the field, standard deviations of the difference between simulation (SIM) and treatment (MV) images were 3.4 mm in the lateral direction, 5.3 mm in the cranio-caudal direction, and 4.8 mm in the ventro-dorsal direction. Alterations in the positioning technique were made and tested. Large pelvic fields: differences between simulation and treatment images for the position of the patient in the field were 4 mm [1 standard deviation (SD)] in the lateral direction and 6.5 mm in the cranio-caudal direction. A systematic shift of the treatment field in the cranial direction had occurred in the majority of patients. A positioning technique using laser lines and marking of the caudal field border was shown to be more accurate. Conclusion : Studies of positioning accuracy in routine irradiation techniques are needed to obtain data for definition of the margins for each treatment site at each institution. Random variations shouls be kept at a minimum by monitoring and improving positioning techniques. Treatment verification by megavolt imaging or film should be used to detect and correct systematic variations early in the treatment series.

Topographical mapping of the thalamocortical projections in rodents and comparison with that in primates

The general topographical organization of the thalamo-cortical projection of two rodents, the Siberian hamster (Phodopus sungorus) and the Guinea pig (Cavia aperta) was investigated with the HRP-method and compared with that of the new world primate marmoset (Cal-lithrix jacchus) as shown in a companion study by Brysch et al. (1990). HRP was injected into various regions of the cortex in different animals and hemispheres, and plots were made of the retrogradely stained thalamic projection neurons. The thalamocortical projection is virtually identical in both rodent species. It is topological throughout in that nearby cortical injections label nearby, though overlapping cell groups in the thalamus. Cortical injections in a rostro-caudal progression labelled thalamic projection zones on top of each other, layered like tiles on a roof or fish scales, beginning in the rostromedial and ending in the caudo-dorsal thalamus. The progression vector of thalamic zones projecting successively from more rostral to more caudal cortical zones is twisted and turns from a predominantly mediolateral direction in the anterior thalamus to an essentially ventro-dorsal direction in the posterior thalamus In the marmoset, the thalamo-cortical topography follows the same topological rule, with the exception of the lateral geniculate body which is translocated latero-ventrally and separated from the rest of the thalamus as in all primates. This suggests a general thalamo-cortical mapping rule common to all mammals which can be related to gradients and timing of cell birth in the thalamus. It is proposed that this mapping rule is the consequence of successive appositions of neurons in the medio-ventral thalamus during ontogenetic development.

Effect of oestradiol benzoate and tamoxifen on the growth of and induction of progesterone receptors in the uterus and mammary gland of immature pigs

Intramuscular injection of oestradiol benzoate (0.1, 1 or 10 micrograms/kg per day) and tamoxifen (0.1 or 1 mg/kg per day) to 6-week-old immature pigs for 7 days induced a dose-dependent increase in the wet weight of the uterus and in the total content of uterine DNA, RNA and protein. Both compounds also stimulated a dose-dependent increase in the concentration of progesterone receptors in uterine cytosolic extracts (in terms of either fmol/mg DNA or fmol/g uterus). The concurrent administration of tamoxifen with oestradiol benzoate provoked significant (P less than 0.05) increases in total uterine protein and in the concentration of progesterone receptors (P less than 0.01) compared with treatment with oestradiol benzoate alone. Hence tamoxifen is an oestrogen agonist in the uterus of immature pigs. The effects of oestradiol benzoate and tamoxifen on mammary growth in immature pigs were assessed by image analysis of mammary sections across the gland (in a ventro-dorsal direction through the teat). Oestradiol benzoate at 10 micrograms/kg per day stimulated a fourfold increase in mammary duct area (P less than 0.01), and tamoxifen, at doses of 0.1 or 1 mg/kg per day, stimulated a threefold increase (P less than 0.05). Tamoxifen partially inhibited (P less than 0.05) the effect of oestradiol benzoate. The concentration of progesterone receptors was found to be very heterogeneous in cytosol extracts of mammary tissue of immature pigs and independent of treatment with oestradiol benzoate and/or tamoxifen.

Tonotopic organization of the cochlear nucleus of the mustache bat, Pteronotus parnellii

The tonotopic organization of the cochlear nucleus (CN) of the mustache bat, Pteronotus parnellii was studied by injecting horseradish peroxidase (HRP) in physiologically characterized CN-sites known to respond to a certain frequency. The tracer was transported by the branched fibers of the auditory nerve and bands of labeled terminals were found in each of the CN-subdivisions. Low to high frequencies are orderly represented in rostrocaudal direction in the anteroventral CN (AV) and in ventrodorsal direction in the posteroventral (PV) and dorsal CN (DCN). In all 3 subnuclei a vast overpresentation of the frequency band between 54–66 kHz, which includes the dominant second harmonic of the echolocation calls, is superimposed on this basic mammalian pattern of frequency representation. A deviation from the standard mammalian scheme of tonotopic arrangement is found in the cytoarchitecturally distinct ‘marginal cell group’ (MA). This cell group extends in rostrocaudal direction along the medial margin of the AV, and the frequency representation in its most rostral 2 thirds is biased towards the low frequency range of the faint first harmonic of the echolocation call (24–32 kHz). Consequently the low frequencies lie adjacent to the regular slab organization of the range of higher frequencies represented in the AV. The temporal response patterns of MA neurons to tone stimuli are predominantly phasic or onset-types, in contrast to the ‘primary like’ response type which prevails in the AV. The frequency representation and the physiological characteristics of the MA group can be relevant in the context of target range determination by echolocation.

Extracellular Electrical Currents in the Chick Blastoderm.

Extracellular electrical currents were recorded using a computer-controlled scanning vibrating electrode from gastrulating chick blastoderms cultured in vitro. On the ventral surface, currents of 10-20 µA/cm2 diverged from the area opaca and turned around the margin of blastoderm across the vitelline membrane. Weaker currents (1-10 µA/cm2) converged to the anterolateral area pellucida and penetrated it in the ventrodorsal direction. On the dorsal surface, currents returned to the area opaca from the margin and from the center of area pellucida. During development, the pattern of currents remained similar on the dorsal surface while the leaky area on the ventral surface progressively extended to all the periphery of area pellucida. The intensity of currents in the blastoderm oscillated with a period of 5 to 6 min. This spatio-temporal organization of extracellular currents is compared to physiological activities of embryonic cells in different regions of the blastoderm.

Quantification of regional V/Q ratios in humans by use of PET. II. Procedure and normal values.

Regional measurements of tissue isotope concentration, made using positron emission tomography (PET), allow tracer models to be used in a quantitative manner to provide topographic distributions of many structural and functional parameters, each derived for the same well-defined lung element. In this paper we describe a technique to measure regional ventilation-perfusion ratios (V/Q), in absolute units, by use of PET and the continuous intravenous infusion of an inert gas isotope, 13N, and report on measurements made in 12 normal subjects (4 smokers). Data were obtained from a single lung section (slice thickness, 1.7 cm full width at half-maximum response to a line source) at the level of the right ventricle in the supine posture during quiet breathing. For the 12 subjects, volume-weighted mean values of V/Q, averaged over individual right and left lung fields, ranged from 0.50 to 1.29. Analysis of these means showed no difference between lungs: right, 0.80 +/- 0.23 SD; left, 0.76 +/- 0.20 SD. Topographically, a systematic fall of V/Q in the ventrodorsal direction was observed in eight of the subjects (mean ventrodorsal difference 0.39, range 0.19-0.90), whereas two showed a clear increase toward dependent lung regions (range 0.16-0.26). Seven of the subjects with a falling ventrodorsal V/Q gradient also exhibited discrete regions of low V/Q at the dorsal lung border. We conclude that, in normal subjects, ventilation and perfusion are generally well matched in the supine posture, but isolated mismatching often occurs in dependent lung regions.

Ontogenesis of the Three Parts of the Fetal Rat Adenohypophysis

The sequential changes in the histological pattern of anterior pituitary cytodifferentiation of the rat are described. The first labeled cells were ACTH positive and were detected in the pars tuberalis on postconceptual day 13. On day 14 ACTH cells also appeared in the ventral periphery of the pars distalis. On fetal day 15 the pars tuberalis anlage was characterized by numerous well-stained ACTH cells and by some weakly labeled FSH-beta, LH-beta, TSH-beta, GH and PRL cells while the pars distalis showed only ACTH positivity. On day 16 of gestation the ACTH cells were equally distributed throughout the whole pars distalis, while LH-beta, FSH-beta, TSH-beta, PRL and GH immunoreactive cells were localized either in the ventral region of the pars distalis only or were evenly distributed throughout the pars distalis. The present immunocytochemical data suggest that in the pars distalis the hypophyseal cell differentiation follows a clear rostrocaudal, ventrodorsal direction and that the time sequence of the functional differentiation of the adenohypophysis is pars tuberalis, pars distalis and pars intermedia.

Segment-related, mosaic neurogenetic pattern in the forebrain and mesencephalon of early chick embryos: I. Topography of AChE-positive neuroblasts up to stage HH18.

Histochemical mapping of AChE-positive neuroblasts in sectioned and whole-mounted preparations of the chick embryo mesencephalon and prosencephalon allows a correlation of early neural tube morphogenesis (segmentation, longitudinal compartmentation) with the heterochronic pattern of neurogenesis. One significant finding is that the initial appearance of neuroblasts in the forebrain does not follow neuromeric segmentation, but evolves in parallel with it. Early neuroblasts appear as separate, distinct groups within specific matrix territories at the center of the transverse neuromeric segments. Neighbouring segments display different spatiotemporal patterns of neurogenesis. Overall gradients of differentiation in the rostrocaudal and ventrodorsal directions are absent, whereas a clear-cut segment-related, mosaic pattern becomes evident. Notwithstanding this, gross regularities of heterochrony in the neurogenetic behavior of the different segments lead to a definition of elemental longitudinal compartments of the forebrain and mesencephalon (floor, paramedian, basal, and alar regions) on the basis of precocious differentiation of the basal region and retarded differentiation of the paramedian and alar regions.