The diencephalon comprises three functionally distinct regions: synencephalon, dorsal thalamus, and ventral thalamus. Patterning of the diencephalon has been proposed to involve subdivision of its anteroposterior axis into segments, neuromeres or prosomeres (Bergquist and Kallen, 1954; Vaage, 1969; Figdor and Stern, 1993; Rubenstein et al., 1994; Redies et al., 2000; Yoon et al., 2000). However, the number and sequence of diencephalic neuromeres, or even their existence, are uncertain. We have examined the proposed subdivisions by morphology, gene expression, acquisition of boundary-specific phenotypes, and cell lineage restriction. We find that at stage 16 in chick the diencephalon is divided into synencephalon and parencephalon. The synencephalon exhibits neuromeric morphology, expresses Prox, and acquires neuromere boundary properties at its interface with both the midbrain and the parencephalon. Although the mesencephalic/synencephalic boundary restricts cell mixing, the synencephalic/parencephalic boundary does not. Similarly, there is no lineage restriction between the parencephalon and the more rostral forebrain (secondary prosencephalon). Subdivision of the parencephalon into ventral and dorsal thalamus involves the formation of a narrow intraparencephalic territory, the zona limitans intrathalamica (zli). This is correlated with the acquisition of cell lineage restriction at both anterior and posterior borders of the zli, the appearance of boundary-specific properties, and Gbx2 and Dlx2 expression in dorsal thalamic and ventral thalamic territories, respectively. At stage 22, the synencephalon is divided into two domains, distinguished by differential gene expression and tissue morphology, but associated with neither a boundary phenotype nor cell lineage restriction. Our results suggest that the diencephalon does not have an overt segmental pattern.

https://www.jneurosci.org/content/jneuro/21/13/4699.full.pdf

Boundary Formation and Compartition in the Avian Diencephalon

In the last 10 years our concept of the developing diencephalon has changed dramatically. This is a consequence of an increasing number of morphological, chemoarchitectural, gene expression and experimental data that resist a satisfactory interpretation within the usual morphological schema suggested by textbooks, represented by the so-called columnar view of the vertebrate forebrain. The columnar paradigm was instaurated by Herrick (1910), who divided the vertebrate diencephalon into four superposed columns separated by ventricular sulci. This schema was later supported by numerous adherents, among which Kuhlenbeck played a singular role (Kuhlenbeck 1973, and earlier work reviewed therein). The four columns were called epithalamus, thalamus dorsalis, thalamus ventralis and hypothalamus (from dorsal to ventral). They were held to be longitudinal parts of the neural tube, though this view is only possible by arbitrary disregard of the notorius axial bending of the rostral neural tube at the cephalic flexure (Keyser 1972; Puelles and Rubenstein 1993; Puelles 1995). Among other problems (see Puelles 1995), this schema typically dealt poorly with the pretectum, causing many authors to fail to distinguish it adequately from the dorsal thalamus, the epithalamus or the midbrain roof. Various descriptive embryologists noticed over the years the difficulties of the columnar approach, favoring a segmental paradigm, but did not achieve a substantial impact with their alternative interpretations (Rendahl 1924; Tello 1934; Bergquist 1954; Coggeshall 1964; Keyser 1972; Gribnau and Geijsberts 1985). These, nevertheless, finally constituted the base of the present conceptions, together with parallel work on non-mammalian vertebrates.

Neurogenetic Compartments of the Mouse Diencephalon and some Characteristic Gene Expression Patterns

Contemporary study of molecular patterning in the vertebrate midbrain is handicapped by the lack of a complete topological map of the diverse neuronal complexes differentiated in this domain. The relatively less deformed reptilian midbrain was chosen for resolving this fundamental issue in a way that can be extrapolated to other tetrapods. The organization of midbrain centers was mapped topologically in terms of longitudinal columns and cellular strata on transverse, Nissl-stained sections in the lizard Gallotia galloti. Four columns extend along the whole length of the midbrain. In dorsoventral order: 1) the dorsal band contains the optic tectum, surrounded by three ventricularly prominent subdivisions, named griseum tectale, intermediate area and torus semicircularis, in rostrocaudal order; 2) a subjacent region is named here the lateral band, which forms the ventral margin of the alar plate and also shows three rostrocaudal divisions; 3) the basal band forms the basal plate or tegmentum proper; it appears subdivided into medial and lateral parts: the medial part contains the oculomotor and accessory efferent neurons and the medial basal part of the reticular formation, which includes the red nucleus rostrally; the lateral part contains the lateral basal reticular formation, and includes the substantia nigra caudally; 4) the median band contains the ventral tegmental area, representing the mesencephalic floor plate. The alar regions (dorsal and lateral) show an overall cellular stratification into periventricular, central and superficial strata, with characteristic cytoarchitecture for each part. The lateral band contains two well developed superficial nuclei, one of which is commonly misidentified as an isthmic formation. The basal longitudinal subdivisions are simpler and basically consist of periventricular and central strata.

Cytoarchitectonic subdivisions in the subtectal midbrain of the lizard Gallotia galloti.

The inbuilt intrinsic cholinergic nervous apparatus of the gastric wall of the cat was studied by using two thiocholine methods for mapping the acetylcholinesterase-positive nerves and nerve cells. A rich distribution of acetylcholinesterase-positive nerves was observed in all layers of the gastric wall, except the superficial half of the lamina propria (with the epithelium), which was completely devoid of acetylcholinesterase activity, and the submucosa, in which a scarce distribution of large nerve fascicles and nerve trunks was observed. Acetylcholinesterase-positive ganglia were observed both in the subserous layer and in the myenteric plexus of Auerbach, whereas none were recognized in the submucous plexus of Meissner. This obviously fits well to the results of some electrophysiological experiments which indicate that the submucous plexus of Meissner includes an important intramural pathway from the extrinsic vagus nerves to the antrum region; so the submucous plexus of Meissner seems to be mainly involved in direct rapid conduction of nerve impulses without integrative activities, like a cable. Certain clear differences exist in the pattern of organization of the cholinergic intrinsic nervous apparatus within the different layers of the gastric wall in the fundic and pyloric regions. These differences seem to correspond quite logically to the different types of motor, secretory and neurohumoral activities of these main regions of the stomach. The activity of the non-specific cholinesterases was localized both in the neural elements and the smooth muscle, as well as in some epithelial cells.

Cholinergic innervation of the gastric wall of the cat.

An experimental model for the study of hyperthyroidism in embryonic development was achieved in the local anuran larvae, Bufo melanostictus , by treatment with potassium perchlorate at a critical dose and time of embryogenesis.

Phocomely-hemimely and digital deformities of fore- and hind limbs in high percentages of the population were found associated with such critically stimulated tadpoles. Similar deformities were reproduced by exogenous treatment with L-thyroxine sodium. The deformities were severely aggravated if the tadpoles had anti-thyroid treatment before critical stimulation with thyroxine in the sensitive phase of development.

Incoordinate cellular differentiation in the malformed limbs was indicated histologically by undifferentiated mesenchyme in the distal extremities although the overlying ectoderm had already formed mature epidermis with specialized skin glands. The differentiated mesodermal components (muscles, cartilages and connective tissues) more proximally were afflicted with degenerative changes. Lytic spinal motor neurones and involuted Mauthner's neurones with precipitate metamorphosis were accompanying features. Embryonic thyrotoxicosis is discussed in relation to congenital deformities of limbs.

W C Wong

Papers

Cholinesterase localization in the developing foregut of the toad (Bufo melanostictus).