(named after its resemblance to the seahorse, from the Greek hippos meaning “horse” and kampos meaning “sea monster”) is a major component of the brain. It belongs to the limbic system.

Humans have two hippocampi, one in each side of the brain. The hippocampus is located under the cerebral cortex.

It contains two main interlocking parts: Ammon's horn and the dentate gyrus.

The internal structure of the human hippocampus is challenging to map using histology or neuroimaging due to its complex archicortical folding.

DeKraker et al. aimed to overcome this challenge using a unique combination of three methods. First, they leveraged a histological dataset with unprecedented 3D coverage, BigBrain. Second, they imposed a computational unfolding framework that respects the topological continuity of hippocampal subfields, which are traditionally defined by laminar composition. Third, they adapted neocortical parcellation techniques to map the hippocampus with respect to not only laminar but also morphological features. Unsupervised clustering of these features revealed subdivisions that closely resemble gold standard manual subfield segmentations. Critically, they also show that morphological features alone are sufficient to derive most hippocampal subfield boundaries. Moreover, some features showed differences within subfields along the hippocampal longitudinal axis. Our findings highlight new characteristics of the internal hippocampal structure and offer new avenues for its characterization with in-vivo neuroimaging 1).

In Alzheimer's disease, the hippocampus is one of the first regions of the brain to suffer damage; memory loss and disorientation are included among the early symptoms. Damage to the hippocampus can also result from oxygen starvation (hypoxia), encephalitis, or medial temporal lobe epilepsy. People with extensive, bilateral hippocampal damage may experience anterograde amnesia—the inability to form or retain new memories.

Since different neuronal cell types are neatly organized into layers in the hippocampus, it has frequently been used as a model system for studying neurophysiology. The form of Neuroplasticity known as long-term potentiation (LTP) was first discovered to occur in the hippocampus and has often been studied in this structure. LTP is widely believed to be one of the main neural mechanisms by which memory is stored in the brain.

Resection of the hippocampus can cause verbal memory decline, especially in the pediatric population. Thus, preservation of the hippocampus can be crucial for the quality of life of children with intractable temporal lobe epilepsy (TLE) who are candidates for epilepsy surgery. We investigated techniques that determine whether the hippocampus is part of the epileptogenic zone and the outcomes of pediatric surgery aimed to spare the hippocampus.

We accessed data of children with normal hippocampus on MRI, who underwent surgery for medically refractory TLE. To identify epileptogenic areas, electrocorticography was performed in patients with space occupying lesions adjacent to the hippocampus, and long term invasive monitoring in patients with nonlesional TLE. Postoperative seizure control was classified according to Engel I-IV; Class I indicates seizure-free.

Eleven females and 11 males met study inclusion criteria; the mean age at surgery was 11.3 years. Cortical and hippocampal electrocorticography was performed in 15 patients and long term invasive hippocampal monitoring in seven. The hippocampus was preserved in 16 patients (73%) while hippocampectomy was performed in 6 (27%). At the end of a mean follow-up of 3.5 years, 94% (15/16) of the patients who did not undergo hippocampectomy were classified as Engel I, compared to 50% (3/6) who underwent hippocampectomy.

Sparing the hippocampus in temporal lobe epilepsy surgery is possible with excellent seizure outcome, while using the proper intraoperative technique 2).

Duvernoy HM: The Human Hippocampus: Functional Anatomy, Vascularization and Serial Sections With MRI,ed 3. Berlin: Springer-Verlag, 2005.

DeKraker J, Lau JC, Ferko KM, Khan AR, Köhler S. Hippocampal subfields revealed through unfolding and unsupervised clustering of laminar and morphological features in 3D BigBrain. Neuroimage. 2019 Nov 1:116328. doi: 10.1016/j.neuroimage.2019.116328. [Epub ahead of print] PubMed PMID: 31682982.
Benifla M, Bennet-Back O, Shorer Z, Noyman I, Bar-Yosef R, Ekstein D. Temporal lobe surgery for intractable epilepsy in children: What to do with the hippocampus? Seizure. 2017 Oct 2;52:81-88. doi: 10.1016/j.seizure.2017.09.020. [Epub ahead of print] PubMed PMID: 29017082.
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