Synthesis of pyridine derivatives of L-phenylalanine as antisickling reagents

Several bicyclic agents composed of L-phenylalanine coupled to various pyridines were synthesized: 2-, 3-, and 4-(L- phenylalanylamino )pyridine. All three compounds at 3 mM gave positive morphological antisickling effects on homozygous SS cells under reduced O2 tension. Studies on two of these compounds, 2- and 3-(L- phenylalanylamino )pyridine, showed that these agents increase the deoxy-HbS solubility ratio, Cs/ Cs0 , by 14% at 20 mM. Observed changes in the mean corpuscular hemoglobin concentration (MCHC) values of treated cells ranged from 4% at 1.3 mM to 15% at 5.6 mM in compound concentration. Very minor lytic activity was found for treated cells, indicating water uptake is responsible for changes in the MCHC. Further, exposure of sickle cells to a 3 mM concentration of these agents also increased by 6- to 7-fold cellular deformability of a treated erythrocyte population as compared to an untreated one at the same total O2 saturation of 47%. These agents demonstrate the potential of bicyclic compounds composed of a common constituent, L-Phe, in the development toward a viable therapeutic agent.

Genetic control of murine antibody-dependent cell-mediated cytotoxicity. Partial identity with the genetic control of NK activity

We have observed that the intensity of the direct antibody-dependent cell-mediated cytotoxicity (ADCC) response after an inoculation of foreign tumour cells varies with the strain of mice studied. The inoculation of a human lymphoblastoid cell-line into CBA/J, BALB/c, or DBA/2 mice gives rise to a good cytotoxic response by the host K cells armed with specific antibodies. In contrast, A/J, B10.A, C57BL/6 and B10.S mice respond poorly under the same conditions. The high response is dominant in F1 hybrids between high and low responders and is also expressed among F2 backcrosses with the H-2 phenotype of low responders, suggesting that non-H-2 genes are also implicated in the regulation of ADCC. The genetic control is not exerted at the level of antibody secretion but at that of K-cell activity, since sera from high or low responders are equally effective in arming an ADCC reaction, whereas K cells from low-responder strains are less efficient than those from high-responder strains. The natural killer (NK) activity of the same strains has been screened. The results show a good correlation with some high- and low-responder strains, such as CBA and DBA/2 or A/J and SJL, respectively, but not with C57BL/6, B10.S or B10.A strains. Thus, in addition to common genes controlling both lytic functions, there are specific genetic factors influencing the balance between NK and K cells. These findings confirm the general view that NK and K cells represent only partially identical subsets.

Sclerosteosis: neurogenetic and pathophysiologic analysis of an American kinship

We studied an American kinship with sclerosteosis, an autosomal-recessive disorder of bone remodeling and bone overgrowth of the calvaria, skull base, and tubular bones. Unlike osteopetrosis, which is attributed to abnormal immune and osteoclast function as well as bone resorption, sclerosteosis appears to be primarily a disorder of osteoblast (bone formation) hyperactivity. Related to cranial vascular and neural foraminal narrowing and reduced intracranial volume, affected patients with sclerosteosis demonstrate frequent seventh nerve palsy, progressive optic and cranial neuropathies, mixed hearing loss, brainstem compression, intracranial hypertension with increased elastance, and sudden, premature death. Management should involve early childhood identification of homozygotes, monitoring and aggressive treatment of intracranial hypertension, and extensive bone removal from skull, posterior fossa, and cervical spine.

The contribution of late-generated neurons to the callosal projection in rat: a study with prenatal x-irradiation

Studies utilizing horseradish peroxidase tracing methods have suggested that there are species differences in the relative contribution of the different neocortical layers to the callosal projection. The present investigation utilized x-irradiation at different gestational ages to eliminate the late-generated neurons in the rat neocortex. The caudorostral gradient of reduction in the neuronal population of the supragranular layers is closely correlated with the gradient of reduction in the size of the corpus callosum. Furthermore, the callosal projection is absent in anteroposterior cortical segments in which the development of the supragranular layers was prevented without a reduction of the number of neurons in the infragranular layers of the neocortex. These results indicate that late-generated neurons residing primarily in the supragranular layers are essential for the formation of the corpus callosum.

Lumbosacral skin lesions as markers of occult spinal dysraphism

Early treatment of occult spinal dysraphism may prevent progressive neurological deficits. However, diagnosis is often delayed until the onset of irreversible neurological damage. A review of data from the literature and patients at Johns Hopkins Hospital suggests that lumbosacral skin abnormalities such as tufts of hair, hemangiomas, lipomas, skin tags, or pigmented nevi should alert the physician to search for occult spinal dysraphism. In the asymptomatic patient with a skin lesion, roentgenography of the lumbosacral spine is a useful screening procedure for identifying treatable underlying problems.

The National Association of Insurance Commissioners (NAIC) Medical Malpractice Closed Claim Study 1975-1978. A review of dermatologic claims

The National Association of Insurance Commissioners (NAIC) Medical Malpractice Closed Claim Study 1975-1978 is the first major uniform nationwide survey of its kind. A total of 71,782 closed claims were reviewed during a 3 1/2-year period. Approximately one third of these claims involved indemnity payments. A review of the dermatologic closed claims with indemnity payments included in the study revealed that dermatologists accounted for 127 claims representing 0.7% of total paid claims, with payments totaling +2,549,125 representing 0.6% of total indemnity dollars paid. Since our specialty represents 1.4% of all practicing physicians, it is clear that the NAIC study reaffirms dermatology as being among the lowest malpractice risk specialties.

Development of the brain stem in the rat. V. Thymidine-radiographic study of the time of origin of neurons in the midbrain tegmentum

Groups of pregnant rats were injected with two successive daily doses of 3H-thymidine from gestational day E12 and 13 (E12 j3) until the day before parturition (E21 k2) in order to label in their embryos the proliferating precursors of neurons. At 60 days of age the proportion of neurons generated (no longer labeled) on specific embryonic days was determined quantitatively in 18 regions of the midbrain tegmentum. The neurons of the oculomotor and trochlear nuclei are generated concurrently on days E12 and 13. There was a mirror image cytogenetic gradient in these nuclei and this was interpreted as the dispersal of neurons derived from a common neuroepithelial source to the medial longitudinal fasciculus. Neurons in three other components of the tegmental visual system are produced in rapid succession after the motor nuclei. In the nucleus of Darkschewitsch peak production time was on day E12 and 13, extending to day E15; in the Edinger-Westphal nucleus the time span was the same but with a pronounced between days E13; finally, the neurons of the parabigeminal nucleus were produced between days E13 and E15 with a peak on day E14. The neurons of the periaqueductal gray were generated between days E13 and 17 with a pronounced ventral-to-lateral and lateral-to-dorsal gradient. In the red nucleus the neurons were produced on days E13 and E14 with a caudal-to-rostral gradient: the cells of the magnocellular division preceding slightly but significantly the cells of the parvocellular division. The neurons of the interpeduncular nucleus originated between days E13 and E15; the peak in its ventral portion was on day E13, in its dorsal portion on days E14 and E15. A ventral-to-dorsal gradient was seen also in both the dorsal and the median raphe nuclei in which neuron production occurred between days E13 and E15. The neurons of the pars compacta and pars reticulate of the substantia nigra were both produced between days E13 and E15 with a modified lateral-to-medial gradient. This gradient extended to the ventral tegmental area where neurons of the pars medialis were produced between days E14 and E16. With the exception of the central gray, neuron production was rapid and relatively early in the structures situated ventral to the midbrain tectum. A comparison of the cytogenetic gradients in the raphe nuclei of the lower and upper medulla, the pontine region, and the midbrain suggests that they originate from at least three separate neuroepithelial sources.

Early postnatal x-irradiation of the hippocampus and discrimination learning in adult rats

Rats with X-irradiation-produced degranulation of the hippocampal dentate gyrus were trained in the acquisition and reversal of simultaneous visual and tactile discriminations in a T-maze. These experiments employed the same treatment, apparatus, and procedure but varied in task difficulty. In the brightness and roughness discriminations, the irradiated rats were not handicapped in acquiring or reversing discriminations of low or low-moderate task difficulty. However, these rats were handicapped in acquiring and reversing discriminations of moderate and high task difficulty. In a Black/White discrimination, in which the stimuli were restricted to the goal-arm walls, the irradiated rats were handicapped in the acquisition (low task difficulty) and reversal (moderate task difficulty) phases of the task. These results suggest that the irradiated rats were not handicapped when the noticeability of the stimuli was high, irrespective of modality used, but were handicapped when the noticeability of the stimuli was low. In addition, these results are consistent with the hypothesis that rats with hippocampal damage are inattentive due to hyperactivity.

Time of origin of neurons of the rat inferior colliculus and the relations between cytogenesis and tonotopic order in the auditory pathway

Groups of pregnant rats were injected with two successive daily doses of 3H-thymidine from gestational day 12 and 13 (E12+13) until the day before parturition (E21+22) in order to label in their embryos the proliferating precursors of neurons. At 60 days of age the proportion of neurons generated (or no longer labelled) on specific embryonic days was determined quantitatively in six vertical strips of the inferior colliculus. It was established that the neurons of the inferior colliculus are produced between days E14 and the perinatal period in an orderly sequence: the earliest generated cells are situated rostrally, laterally and ventrally in the principal nucleus, the latest generated cells are situated caudally, medially and dorsally in the pericentral nucleus. This cytogenetic gradient suggested that the cells are produced dorsally in the caudal recess of the embryonic aqueduct and are deployed in an "outside-in" pattern. This study has brought to a conclusion our datings of neuron production in the central auditory pathway of the rat. The results revealed that in those structures in which a cytogenetic gradient could be recognized, the orientation of this gradient and the regional tonotopic order (demonstrated mostly in species other than the rat) tended to be aligned. Moreover, with the exception of the medial trapezoid nucleus and the dorsal nucleus of the lateral lemniscus (which receive contralateral input from the cochlear nuclei), sites with early-produced neurons correlated with units responding preferentially to high frequency tones and vice versa. This suggested that the orderly production of neurons within different components of the auditory system is a factor in their subsequent topographic organization. A comparison of the temporal order of neuron production in different components of the auditory pathway suggested that the establishment of orderly topographic relations between some of the structures (e.g., the medial geniculate body and the primary auditory cortex) takes place before this spatial relationship could be specified as a cochleotopic order.

Time of origin of neurons of the rat superior colliculus in relation to other components of the visual and visuomotor pathways

Groups of pregnant rats were injected with two successive daily doses of 3H-thymidine from gestational day 12 and 13 (E12 + E13) until the day before parturition (E21 + 22) in order to label all the multiplying precursors of neurons. At 60 days of age the proportion of neurons generated (or no longer labelled) on specific days was determined in the separate layers of the superiorr colliculus. Neurogenesis begins with layers V and IV on day E12; the bulk (87%) of these cells are generated on day E13. This early-produced band of large neurons, the intermediate magnocellular zone, divides the superior colliculus into two cytogenetically distinct regions. In both the deep and the superficial superior colliculus neuron production is relatively protracted. In the deep superior colliculus neuron production peaks on day E15 in lay VII, on day E15 and E16 in lay VI, and on day E16 (the large neurons excluded) in layer V, indicating an inside-out sequence. In the superficial superio coliculus peak production time of both layer I and II is on day E16 but in the latter region neuron production is more prolonged and ends on day E18. One interpretation of these results is that the two pairs of superficial layers are produced in an outside-in sequence. These three cytogenetic subdivisions of the superior colliculus may be correlated with its structural-functional parcellation into an efferent spinotectal, a deep somatomotor and a superficial visual component. A comparison of neurogenesis in different components of the visuomotor and visual pathways of the rat indicates that the motor neurons of the extraocular muscles, the abducens, trochlear and oculomotor nuclei, and neurons of the nucleus of Darkschewitsch are produced first. Next in line are source neurons of efferents to the bulb and the spinal cord: those of the Edinger-Westphal nucleus and the intermediate magnocellular zone of the superior colliculus. These are followed by the relay neurons of the dorsal nucleus of the lateral geniculate body. The neurons of the superficial superior colliculus and of the visual cortex implicated in visual sensori-motor integrations are produced last.

The cell populations mediating natural killing-(NK) and antibody-dependent cell-mediated cytotoxicity are only partially identical

A cold target inhibition assay has been used to investigate the relationship between the effector cells responsible for NK and ADCC activity. Two models of ADCC have been studied in parallel with NK, one involving in vivo armed effector cells and the other involving normal effector spleen cells tested on antibody-precoated target cells. Bivalent effector cells active in ADCC and NK as well as monovalent NK cells and monovalent FcR-positive ADCC killer cells appear to exist concomitantly. However, the variability from experiment to experiment of the blocking efficiency displayed by the heterologous cold targets, suggests that the proportions of monovalent to bivalent cells may vary between batches of mice depending on their immunological status at the time of the test. This should be taken into careful consideration when interpreting the results of this type of investigation.

Development of the brain stem in the rat. III. Thymidine-radiographic study of the time of origin of neurons of the vestibular and auditory nuclei of the upper medulla

Groups of pregnant rats were injected with two successive daily doses of 3H-thymidine from gestational days 12 and 13 (E12 + 13) until the day before parturition (E21 + 22). In adult progeny of the injected rats the proportion of neurons generated on specific embryonic days was determined quantitatively in the vestibular and auditory nuclei of the upper medulla. In the vestibular nuclei, neurons are generated between days E11 and E15 in an overlapping sequential order, yielding a lateral-to-medial and a rostral-to-caudal internuclear gradient. In the lateral vestibular nucleus peak production time is day E12; in the superior nucleus, E13; in the inferior nucleus, E13 and E14; and in the medial nucleus, E14. The early generation of neurons of the lateral vestibular nucleus may reflect the early differentiation of the circuit from the gravity receptors (utricle) to neurons of the spinal cord controlling postural balance. The later production of neurons of the superior vestibular nucleus may reflect the subsequent differentiation of the circuit from the rotational receptors (semicircular canals) to the neurons of the brain stem controlling eye movements. The generation time of neurons of the nucleus prepositus hypoglossi overlaps with that of the medial vestibular nucleus. The neurons of the anteroventral and posteroventral cochlear nuclei are produced from days E13 to E17, with no temporal differences between the two nuclei. The neurons of the dorsal cochlear nucleus are generated over a very long time span, beginning on day E12 and extending into the postnatal period. There is a sequence in the production of neurons forming the different layers of the dorsal cochlear nucleus in the following order: pyramidal cells, cells of the inner layer, cells of the outer layer and, finally, cells of the granular layer. There is also a sequential production of neurons in four nuclei of the superior olivary complex. In the lateral trapezoid nucleus peak production time is day E12; in the medial superior olivary nucleus, day E13; in the medial trapezoid nucleus, day E15; and in the lateral superior olivary nucleus, day E16. This order yields a medial-to-lateral gradient in the dorsal aspect of the superior olivary complex, and a lateral-to-medial gradient ventrally. These mirror-image gradients were also seen intranuclearly in the lateral superior olivary nucleus and the medial trapezoid nucleus. The cytogenetic gradients could not be related to tonotopic representation; however, they could be related to the lateral location of ipsilateral cochlear nucleus input to the lateral superior olivary nucleus and the medial location of the contralateral cochlear nucleus input to the medial trapezoid nucleus.

Development of the brain stem in the rat. IV. Thymidine-radiographic study of the time of origin of neurons in the pontine region

Groups of pregnant rats were injected with two successive daily doses of 3H-thymidine from gestational day 12 and 13 (E12 + 13) until the day before parturition (E21 + 22) in order to label in their embryos the proliferating precursors of neurons. At 60 days of age the proportion of neurons generated (or no longer labeled) on specific embryonic days was determined quantitatively in 14 nuclei of the pontine region. Peak production time of neurons of the trigeminal mesencephalic nucleus was on day E11 or earlier, with a small proportion generated on day E12. Peak production time of the trigeminal motor neurons was on day E12, with a small proportion produced earlier. Neurons of the principal sensory nucleus were generated between days E13 and E16, with a peak on day E14; the late-produced neurons tended to belong to a class of intermediate and large cells. The bulk of the neurons of the supratrigeminal and infratrigeminal nuclei arose on days E15 and E16. Neurons of the locus coeruleus are produced mostly on day E12, with about 20% of the cells arising on day E13. The bulk of the neurons of the dorsal tegmental nucleus (Gudden's) are produced between days E13 and E15, whereas most of the neurons of the deep (ventral) tegmental nucleus are produced on day E15. A dorsal-to-caudal gradient was also obtained between the dorsal and ventral nuclei of the lateral lemniscus, the neurons of the former being generated between days E12 and E15; the latter, between days E13 and E17. The neurons of both the pars lateralis and the pars medialis of the parabrachial nucleus were produced simultaneously between days E13 and E15, with a peak on day E13. The heterogeneous collection of neurons of the pontine paramedial reticular formation was produced for day E11 (or earlier) until day E15. Finally, the neurons of the raphe pontis parvicellularis were generated at an even rate between days E13 and E15, whereas the bulk of the neurons of the raphe pontis magnocellularis were produced on days E15 and E16. On the basis of datings obtained for 9 subdivisions of the entire brain stem trigeminal complex, hypotheses were offered of the cytogenetic components of the system. The sequence of neuron production in the dorsal and deep tegmental nuclei was related to their connections with divisions of the mammillary and habenular nuclei on a "first come-first serve" basis.

Development of the brain stem in the rat. I. Thymidine-radiographic study of the time of origin of neurons of the lower medulla

Groups of pregnant rats were injected with two successive daily doses of 3H-thymidine from gestational days 12 and 13 (E12 + 13) until the day before birth (E21 + 22). In adult progeny of the injected rats the proportion of neurons generated on specific days was determined quantitatively in the major nuclei of the lower medulla. The earliest generated cells form two motor nuclei: the hypoglossal and dorsal vagal nuclei. The bulk of hypoglossal neurons are produced on day E12, with a small proportion earlier; the bulk of dorsal vagal neurons are produced, likewise, on day E12, with a small proportion on day E13. The neurons of the third motor nucleus of the region, the ambiguous, are generated later, with a peak on day E15. Neurons of the sensory relay nuclei, the gracilis, cuneatus, and solitarius are produced over a more extended period, with peaks on day E13; the exception was the external cuneate nucleus in which peak generation time was on day E15. In the caudal nucleus of the trigeminal complex, neurons of the subnucleus magnocellularis arise earliest, with a peak on day E14, and those of the subnucleus marginalis last, with a peak on day E15, and extending into day E16. The neurons of the nuclei raphe pallidus and obscurus, and of the dorsal and ventral portions of the caudal medullary reticular formation, are produced between days E12 and E15, without any obvious peaks. The neurons of the nucleus parasolitarius and the nucleus of Roller are produced relatively late, and the area postrema contains a germinal cell population throughout the embryonic period, presumably supplying cells to the choroid plexus of the fourth ventricle. On the basis of absolute datings, duration of neuron production, intranuclear and internuclear gradients, and other criteria, it is postulated that the neurons of the lower medulla are derived from at least eight different cytogenetic zones.

Development of the brain stem in the rat. II. Thymidine-radiographic study of the time of origin of neurons of the upper medulla, excluding the vestibular and auditory nuclei

Groups of pregnant rats were injected with two successive daily doses of 3H-thymidine from gestational days 12 and 13 (E12 + 13) until the day before birth (E21 + 22). In radiographs from adult progeny of these rats the proportion of neurons generated on specific days was determined in the major nuclei of the upper medulla, with the exception of the vestibular and auditory nuclei. The neurons of the motor nuclei are generated over a brief period. Neurons of the retrofacial nucleus are produced first, with more than 60% of the cells arising on day E11 or earlier. Peak generation time of abducens neurons is day E12 and of the neurons of the facial nucleus is day E13. In contrast, the neurons of the superior salivatory nucleus are produced late, predominantly on day E15 and some on day E16. The generation of the (sensory relay) neurons of the nucleus oralis of the trigeminal complex takes place over an extended period between days E12 and E15; the last generated cells include the largest neurons of this nucleus. Neurons of the raphe magnus are produced between days E11 and E14, the neurons of the rostral medullary reticular formation between days E12 and E15. The latest generated neurons of the upper medulla (excluding the cochlear nuclei) belong to a structure identified as the granular layer of the raphe. Combining these results with those of the preceding paper (Altman and Bayer, '80a) and with additional data, it is postulated that the laterally and ventrally situated motor nucleus of the trigeminal, the facial nucleus, and the nucleus ambiguous form a single longitudinal zone of branchial motor neurons with a rostral-to-caudal cytogenetic gradient. In contrast, the medially and dorsally situated (juxtaventricular) hypoglossal nucleus and abducens nucleus (together with the other nuclei of the ocular muscles) form a longitudinal somatic motor zone with a caudal-to-rostral gradient. The dorsal nucleus of the vagus and the superior salivatory nucleus may constitute a preganglionic motor zone, also with a caudal-to-rostral cytogenetic gradient.

Effects of arrested cerebellar development on locomotion in the rat. Cinematographic and electromyographic analysis

The activity of the rat hindlimb during treadmill stepping was studied in normal adult rats and in rats in which cerebellar development was interfered with by early-postnatal focal X-irradiation. Based on cinematographic and electromyographic data from over 100 step cycles in 15 normal rats, correlations were made between joint angles and muscle activity to obtain a detailed picture of the locomotor pattern of the rat hindlimb. It was possible to relate most of the features of limb movement to activity in one or more of six primary flexors and extensors of the hindlimb. Compared with available data in the cat or dog, the joint angle curves were similar in shape except that the knee joint angle was usually greater at foot contact than at lift-off, while in cats and dogs the reverse is usually the case. This difference is due to a more crouched stepping posture in the rat in which the leg is not extended as far backward as in the cat or dog. It was also noticed that there was more side-to-side bowing of the spine in the rat during stepping. Finally, in rats there was no correlate to the stance phase burst seen in the semitendinosus in cats. In rats with cerebellar X-irradiation there was little effect on the stepping cycle if the animal's equilibrium was maintained externally, either by a supporting harness or by immersion in water (swimming). However, when stepping without external support, animals were unable to adequately compensate for perturbations in equilibrium, resulting in an ataxic gait. This problem was compound by the presence of high-frequency (18 Hz) tremor which, by producing hyper- or hypotonia during critical periods of stepping such as foot placement or during corrective reflex movements, was a major disturbing force to the animal's equilibrium.

Mechanisms of cell-mediated cytotoxicity in mice rejecting xenogeneic human lymphoblastoid cells

The in vivo immunization of mice with human lymphoblastoid cell line LNH13 generates direct cell-mediated cytotoxicity by spleen cells. The lytic activity appears as early as day 3 after the intraperitoneal inoculation of 7.5 x 10(6) cells and persists at least until day 11. The killer cells do not adhere to plastic and are not retained on nylon wool columns or on Degalan beads coated with mouse Ig plus rabbit-anti-mouse Ig. The effector cells are partly inhibited by treatment with anti-Thy-1.2 serum plus complement, but this inhibition appears to be non-specific since anti-serum alone or normal serum plus complement have the same effects. Heat-aggregated IgG strongly inhibits cytotoxicity, indicating that the effector cells are Fc-positive and that such receptors are implicated in lysis. Altogether, these features strongly argue for an ADCC phenomenon. The involvement of antibodies is demonstrated by the fact that eluates (56 degrees C, 30 min) from immune cells alone induce lysis in the presence of normal spleen cells as effectors. The lytic activity of these eluates can be removed by specific adsorption on protein A coupled to Sepharose beads and on the human lymphoid target cells. Positive complementation between immune and non-immune spleen cells suggest that the arming process may occur in vitro during the assay, when antibodies are released by plasmacytes.

Development of the diencephalon in the rat. V. Thymidine-radiographic observations on internuclear and intranuclear gradients in the thalamus

Groups of pregnant rats were injected with two successive daily doses of 3H-thymidine from gestational days 13 and 14 (E13 + 14) until the day before birth (E21 + 22). Internuclear and intranuclear cytogenetic gradients were examined in radiograms of the thalamus sectioned in the coronal, sagittal and horizontal planes. There was a precise and segregated lateral-to-medial gradient between and within the habenular nuclei. In the ventral thalamus the reticular nucleus had a lateral-to-medial gradient, the subthalamic nucleus a laterodorsal-to-medioventral gradient. There was a caudal-to-rostral gradient between the medial geniculate and dorsal lateral geniculate nuclei, and between the pars posterior and pars anterior of the lateral nucleus. A clear intranuclear gradient could not be detected in the sensory relay nuclei with the exception of the medial geniculate nucleus. A lateral-to-medial internuclear gradient was seen between the relay nuclei and the intralaminar nuclei, and between the latter and some of the midline nuclei. On the basis of a consideration of the time of origin and time span of production of neurons of various thalamic nuclei, and taking into account some of the recognizable internuclear and intranuclear gradients, the thalamus was divided into five principal cytogenetic components; the epithelamus, the ventral thalamus, the dorsal thalamus, the medial thalamus, and the posterior thalamus. The epithalamic nuclei form over a protracted period resembling the nuclei of the hypothalamus. The nuclei of the ventral thalamus are generated early and over a relatively long period. The dorsal thalamus consists of the relay nuclei and the intralaminar nuclei; they form rapidly and ahead of the medial thalamus. The medial thalamus was subdivided into the earlier-forming anteromedial nuclei and the latest-forming midline nuclei. The posterior thalamus was not examined in detail.

Development of the diencephalon in the rat. VI. Re-evaluation of the embryonic development of the thalamus on the basis of thymidine-radiographic datings

The development of the thalamus was examined in normal and X-irradiated embryos from day 13 (E13) to the day before birth (E22). The differentiating, radioresistant neurons of the lateral habenular nucleus, derived from a portion of the superior neuroepithelial lobule (SL1), were settling by day E15 and by this time the habenulopeduncular tract was forming. The neurons of the reticular nucleus, derived from the middle neuroepithelial lobe, began to settle on day E15 but a massive migration was still evident on day E16. Adjacent to the reticular nucleus the internal capsule appeared on day E16; this fiber bundle seemed to be continuous with fibers embedded in the first transitory zone of cells issuing from the dorsal neuroepithelial lobe. Because of the immaturity of the neocortex at this time, it was postulated that thalamocortical fibers of the dorsal thalamus are the earliest components of the internal capsule. By day E17 all the sensory relay nuclei of the thalamus were recognizable and it was assumed that the second transitory zone issuing from the receding dorsal neuroepithelial lobe contained the neurons of the later forming intralaminar nuclei. Suggestive evidence was obtained that the late arising neurons of the medial thalamus (the anterior nuclei, the mediodorsal nucleus, and some or all of the midline nuclei) originate in a portion of the superior neuroepithelial lobule designated as SL2. Our present and previous studies showed that the major divisions of the hypothalamus and thalamus are derived embryonically from distinguishable parts of the third ventricle neuroepithelium. This implies the te third ventricle neuroepithelium has a "mosaic" organization and suggests that the fate of hypothalamic and thalamic neurons may be determined to some extent while their precursors are still proliferating.