Chapter 157. The Telencephalon (first Vesicle): Development of The Rhinencephalon - Review of Medical Embryology Book

The rhinencephalon is derived from the archeo- and paleopallium and consists of a number of intracerebral structures, namely, the limbic lobe and attached structures, and the olfactory bulbs

THE RHINENCEPHALON is the essentially olfactory brain of the lower vertebrates without a neocortex and regulates their behavior

In mammals, growth of the neocortex brings about the relative regression of the rhinencephalon and, in humans, where regression is maximal, olfactory function is markedly reduce Nevertheless, with its neocortical parts, the rhinencephalon still regulates a great deal of fundamental behavior

The hemispheric vesicle has a simple form at about 2 1/2 months. The archipallium is located at the internal face of the hemispheres and the paleopallium at their ventral face, below and outside the striated bodies
Between months 3 and 5, the topography of the cortex is greatly modified by both the longitudinal and transverse growth of the neocortex to form the temporal lobes
As it grows, the neocortex invades most of the dorsal archeocortex to form the convolution of the corpus callosum. It also invades most of the paleocortex to form the fifth temporal convolution. Both convolutions merge posteriorly to form the limbic lobe

The structure of the limbic lobe is intermediate between that of the neocortex and that of the corresponding cortices, except anteriorly where the limbic lobe is directly related to the afferent fibers of the olfactory bulbs. In these areas, it remains an archeocortical or paleocortical organization and give rise to the olfactory areas of the cortex

Archeocortical elements which are generally spared by the neocortex and more or less are regressed in man
The dorsal hippocampus (hippocampal cortex) consists of a thin, atrophied band of gray matter on the dorsal surface of the corpus callosum (the hippocampal rudiment)
The ventral hippocampus is more developed and projects into the ventricular lumen of the inferior horn of the lateral ventricle and forms the intraventricular hippocampus and the dentate gyrus
The intra- or interhemispheric fiber systems of association

The hippocampus sends projection fibers via the fimbria, the fornix system, and its commissures to the septum pellucidum, the anterior nucleus of the thalamus, the premamillary region of the hypothalamus, the nuclei of the mamillary body, and to the hippocampus of the opposite side

The olfactory areas of the cortex connect with lower structures (hypothalamus and brainstem) and with upper structures (neocortex of the limbic lobe and frontal cortex) by way of the amygdaloid nucleus, the nuclei of the septum, and the hippocampus

DEVELOPMENT OF THE OLFACTORY BULBS is the result of a cortical formation which takes place under the inductive influence of nerve fibers from the olfactory epithelium of the nasal cavities

At first, it is a hollow evagination, which gradually fills and elongates under the frontal lobe of the cerebrum. The enlarged end of the evagination forms the olfactory bulb
The narrower stem by which the bulb is attached to the hemisphere forms the olfactory tract
The olfactory bulb connects with the entorhinal paleocortex and the archeocortex of the subcallosal convolution