The Epithalamus, Thalamus, and Hypothalamus

The term diencephalon means the “between brain.”  The two main components of the diencephalon are the thalamus and the hypothalamus.  This part of the brain links the phylogenetically older brainstem to various structures in the phylogenetically newer telencephalon.  Embryologically, the diencephalon is considered to be part of the forebrain along with the telencephalon.  We have studied all aspects of diencephalic development at the cell and tissue level by determining neurogenetic timetables in all of the adult structures in the diencephalon with long-survival 3H-thymidine autoradiography.  Using short-survival 3H-thymidine autoradiography we determined the location of source areas in the diencephalic neuroepithelium for specific nuclei.  Finally, using sequential-survival 3H-thymidine autoradiography, we followed the migratory pathways of diencephalic neurons to their settling sites in specific regions of the brain parenchyma.

Our main finding is that the diencephalic neuroepithelium is a mosaic with specific locations giving rise to specific nuclei in a spatiotemporal production sequence of neurons, migratory pathways, and settling patterns in mature nuclei.  We have published 15 papers in peer-reviewed journals from 1969 through 1990, and a monograph on hypothalamic development that appeared in  1986 (Altman&Bayer, Advances in Anatomy, Embryology, and Cell Biology, volume 100) .  The links below provide pdf copies of all the peer-reviewed papers.  Eventually, we hope to have a scanned copy of the hypothalamic monograph on this page; stay tuned.

The above series of pictures of mature rat brains at postnatal day 60 shows the clear neurogenetic gradient that existswithin the medial habenular nucleus.  The E designations indicate which embryonic days these animals were exposed to 3H-thymidine.  All of the labeled cells (those with black dots over the nuclei) originated on or after the first embryonic day of exposure.  Notice how the labeled cells “moves” from lateral (right side of the pictures) to medial (left side of the pictures) when the onset of 3H-thymidine exposures is delayed in time–that is the neurogenetic gradient.  (The flat string of labeled cells at the medial edge are the ependymal layer lining the third ventricle [III].)  Thus, not only are theregradients between nuclei throughout the diencephalon, many of these nuclei contain intrinsic gradients.  This precise developmental sequence in the thalamus (and other parts of the brain) sets the stage for proper functioning of the adult brain.

This is a schematic of the various sources of thalamic nuclei during development, modified from the latest series of 6 papers by Altman and Bayer in 1988-89 on thalamic development.

This is modified Figure 2  in the monograph on hypothalamic development that appeared in 1986.  A is a schematic illustration of the three primary divisions (tiers) of the hypothalamus in the coronal plane.  B shows the major components of the three divisions of the hypothalamus in the anteroposterior plane.  Abbreviations: AR, arcuate nucleus; c, caudal;EP, entopeduncular nucleus (also considered part of the pallidal part of the basal ganglia; l, lateral; LH, lateral hypothalamic area; m, medial; PE, periventricular nuclei; r, rostral; SU, suprachiasmatic nucleus; ZI, zona incerta (also considered part of the subthalamus).

This is modified Figure 27 in Bayer&Altman, 1987 paper on development of the preoptic area.  The preoptic area is considered an anterior extension of the hypothalamus above and in front of the optic chiasm.  This shows the sequential order of neuronal production by various zones (red to pink portions of the central curved sheet)  of the neuroepithelium (green crescent) and their successive migratory waves (arrows) to settle in older (darker red) to younger (progressively lighter shades of red) nuclear groups in the preoptic area.