Chapter 154. The Telencephalon (first Vesicle): Phylogenesis - Review of Medical Embryology Book

Development of the telencephalon becomes more and more extensive as one goes up the evolutionary scale of vertebrates

IN CYCLOSTOMES AND FISH the telencephalic vesicle remains simple, and its cavity consists of a single ventricle. Only the olfactory bulbs are differentiated
IN AMPHIBIANS the telencephalon gives rise to 2 evaginations or hemispheres, both of which enclose a lateral ventricle
UP TO THE REPTILES development is slightly modified, and one sees 5 vesicles
IN MAMMALS the hemispheres grow extensively, both laterally and caudally, and gradually engulf the diencephalon and surround the dorsal structures of the mesencephalon and part of the metencephalon

Centers which were once dominant lose their autonomy and come under control of the telencephalon
Special areas related to the hemispheres are differentiated, such as the neocerebellum, neorubrum, et
With growth of the telencephalon, the olfactory bulbs slowly regress and essentially become appendages (in contrast to development in lower vertebrates)
The hemispheres extend progressively backward in the human fetus and increase in volume and in cortical surface, related to the association areas (?70% in man; ?20% of cortical surface in rabbits)

THERE IS AN INCREASED POTENTIAL FOR ASSOCIATION CENTERS, corresponding to the extension of the alar plates

In cyclostomes the pallium* is seen as a thin cellular layer without special structure or importanc The cells are paraventricular. The telencephalon is essentially olfactory and regulates rudimentary behavior
In fish the pallium is thicker, but the cells are still paraventricular and lack a clearcut organization. The telencephalon is not exclusively olfactory since it receives nonolfactory afferent fibers via the striatum and the septum
In amphibians the cells of the pallium migrate toward the surface and differentiate into 2 cell types: small granular receptor cells and large effector cells. Together they form the archipallium

A poorly differentiated paleopallial layer is seen on the outer surface of the ventricle which connects the olfactory afferent fibers and the archipallium

In reptiles the 2 cellular types, described above, are stratified and form the archeocortex. The paleopallial zones migrate toward the surface and show a stratification to form the paleocortex, rich in receptor and association cells

The rhinencephalon now consists of archeocortex, paleocortex, striatum, and septum and becomes the center of behavior, since it connects many afferent as well as olfactory and visual areas

In mammals the major development is a 6-layer neocortex (from neopallium), accompanied by extensive cellular migration to establish an outer gray matter and an inner white matter, reversing the primitive medullary structur With evolution, the neocortex is extended and compresses the archeocortex on the inside and the paleocortex below
In primates (man) the neocortex is so extensive that it invades most of the former cortices. The human rhinencephalon is composed of neocortical zones integrated into a new system which is no longer the "principal" brain but is superceded by the neoencephalon, which consists of neocortex plus the neostriated structures. Here all sensory afferent fibers converg Olfactory functions of the rhinencephalon regress

The neoencephalon receives much information, and as a result of its being formed predominantly of association areas, its potential for integration of afferent impulses is immense, leading to thought processes and complex associations

*Pallium [Latin: cloak]. Mantle; brain mantle; the cerebral cortex with the subjacent white substance.