In this description of the stratification/lamination of the entorhinal cortex, the most superficial layer, close to the pial surface of the brain comes first, followed by the layers below. The EC consists of six layers, four cell layers (II, III, V and VI) and two plexiform layers (I, IV or lamina dissecans). Layers II and V are sometimes split into a superficial part 'a' and deep part 'b' (Haug, 1976 (p. 17)). The Swanson atlas (2018), plates 28 - 48, show the location of layers IIa / IIb, but does not divide layer V. There are approximately 675.000 principle neurons in the EC (Schmidt, 2012). Several accounts of the cell counts in the EC and its layers/divisions have been published (e.g. Mulders, 1997; Merrill, 2001; Rapp, 2002; Gatome, 2010). For computer modelers, a book dedicated to hippocampal microcircuits is available (see Witter, 2018). In the list below, each layer is briefly described (See: Cappaert, 2015 for more details).

Several parcellation schemes exist for the entorhinal cortex. Brodmann (1909) was the first who parceled the EC into two fields: A lateral area 28a (or LEC) and a medial area 28b (or MEC). Subsequent descriptions were made by (a.o.) Stephan, 1975, Witter (2002, Insausti et al, 1997, Witter, 2017). Here we use the 2 subfield division, but several authors subdivided the EC into more than 2 subfields. These subdivisions delineate CE, ME, DIE, VIE, VLE and AE. AE was later found not to be part of the EC (see: Kemppainen et al., 2002; Majak and Pitkänen, 2003). We use the term LEC to include DLE, DIE, and VIE, whereas MEC includes CE and ME.

See the 6 subdivisions of the entorhinal cortex in the Hippocampus Atlas

Some of the characteristic cytoarchitectonic differences between LEC and MEC are:

• LEC Layer II^* is clearly demarcated; its cells are very densely packed and tend to be clustered in islands. The cells in MEC layer II are somewhat larger and show less of a distinct clustering into islands when viewed with a neuronal stain. However, the distribution of different celltypes in MEC do show a marked clustering in several species, including rat (Naumann et al, 2015, 2018).
• he border between layers II^* and III in MEC is not as sharp as in the LEC, although in both entorhinal areas the overall difference in cell size between layers II and III facilitate the delineation of the two layers.
• The lamina dissecans of the MEC is sharply delineated but is less clear in the LEC. The other cell layers, in particular layers IV–VI, can be better differentiated from each other in the MEC than in the LEC, and cells in the deep layers of the MEC generally show a more radial or columnar arrangement.

In addition to cytoarchitecture as a criterion to subdivide the EC, one can look at its connectivity to define the LEC/MEC subfields. In the rat, LEC and MEC can also be distinguished by looking at their differential connectivity 1) to the molecular layer of the Dentate Gyrus (DG) and 2) to the differential connectivity along the proximodistal axis of hippocampal subfield CA1. Specifically:

• The LEC-> DG projection terminates in the outer one third (close to the pial surface) of the dentate molecular layer, whereas the MEC->DG projection terminates in the middle one-third of the dentate molecular layer (Hjorth-Simonsen and Jeune, 1972; Hjorth-Simonsen, 1972).
• The MEC-> CA1 projection terminates in the proximal part of CA1 (closer to the DG), whereas the LEC-> CA1 projection terminates in the distal part of CA1 (see also: van Strien et al., 2009).

Canto, C.B. & Witter, M.P. (2012). Cellular properties of principal neurons in the rat entorhinal cortex. II. The medial entorhinal cortex. Hippocampus, 22(6), 1277-1299. Resolve DOI

Brodmann, Korbinian. (1909) Vergleichende Lokalisationslehre der Großhirnrinde in ihren Prinzipien dargestellt auf Grund des Zellenbaues Read original full text (German) | € Buy English copy