Hippocampal Formation. The hippocampus, the posterior part of the dentate gyrus and the indusium griseum are sometimes grouped together as the hippocampal formation. In humans, the attenuated gray and white structures of this formation are produced by the enormous enlargement of the corpus callosum, which encroaches upon the parahippocampal and dentate gyri and the hippocampi, thus expanding them. The hippocampus is a part of the marginal cortex of the parahippocampal gyrus that has been invaginated, or rolled, into the floor of the inferior horn of the lateral ventricle by the exuberant growth of the nearby temporal cortex. The curved hippocampal eminence is composed mostly of gray matter, and its anterior end is expanded and grooved like a paw, the pes hippocampi. Axons conveying efferent impulses from the pyramidal cells of the hippocampus form a white layer on its surface, the alveus, and then converge toward its medial edge to form a white strip, the fimbria. The hippocampus is an important part of the olfactory apparatus in lower animals; in humans, few or no secondary olfactory fibers end in it. However, it possesses substantial connections with the hypothalamus, which regulates many visceral activities that influence emotional behavior and with temporal lobe areas reputedly associated with memory.
The dentate gyrus (dentate fascia) is a crenated fringe of cortex occupying the narrow furrow between the fimbria of the hippocampus and the parahippocampal gyrus. Anteriorly, this fringe fades away on the surface of the uncus, and posteriorly, it becomes continuous with the indusium griseum through the gyrus fasciolaris.
The hippocampus contains pyramidal cells in regions CA1 and CA3 that project via the efferent fornix to the septal nuclei and hypothalamus. The subiculum receives input from the hippocampal pyramidal cells and also projects via the fornix to the mammillary nuclei and anterior nucleus of the thalamus. It is connected reciprocally with the amygdala and sends axons to cortical association areas of the temporal lobe.
The dentate gyrus contains granule cells that project to the pyramidal cells of the hippocampus and subiculum and receive hippocampal input. The afferent connections to the hippocampal formation include the cerebral association cortices, prefrontal cortex, cingulate cortex, the insular cortex, amygdaloid nuclei, and olfactory bulb via projections to the entorhinal cortex. Afferent cholinergic axons from septal nuclei traverse the fornix to provide the dentate gyrus and hippocampal CA regions.
There exist several clinical conditions where damage unique to the hippocampal formation occurs. CA1 neurons are particularly susceptible to ischemic conditions as seen in cardiorespiratory arrest. Also, patients with temporal lobe epilepsy can suffer CA1 neuronal loss. The most common clinical scenario affecting the hippocampal formation is Alzheimer disease (AD). AD is pathologically associated with neuronal cell loss, neurofibrillary tangles, neuritic amyloid plaques, and granule vacuolar degeneration of the hippocampal region. AD is discussed in more detail in Plates 2-24 to 2-26.