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).
IIa: in lateral portions of the EC, an outer layer IIa and a deeper layer IIb can be seen, separated by a narrow cell-sparse layer Insausti et al, 1997. Depending on the parcellation scheme, layer II = Layer IIa. In that case, layer II is narrower and layer IIb is considered to be superficial layer III.
IIb: in lateral portions of the EC, an outer layer IIa and a deeper layer IIb can be seen, separated by a narrow cell-sparse layer Insausti et al, 1997. Depending on the parcellation scheme, superficial layer III = Layer IIb. If layer IIb is differentiated, then layer III is narrower.
III: Cell layer; contains predominantly pyramidal cells, but also other cells of various sizes and shapes (e.g. multipolar, fusiform, horizontal and bipolar cells).
IV: Plexiform layer; contains scattered cell bodies of various sizes and shapes (e.g. pyramidal, fusiform and bipolar cells) Lingenhohl and Finch, 1991). Here we use layer IV as a synonym for the 'lamina dissecans'. However, some authors describe the 'lamina dissecans' as a separate entity (See also: Haug, 1976 (p.11), Stephan, 1975 (pp. 206-211, 650-653, 661, 665, 694). In the nomenclature by Lorente de Nó, 1933, the 'lamina dissecans' is considered to be the deep part of layer III (see also layer Va).
V: Cell layer; contains pyramidal, horizontal and polymorphic neurons.
Vb: a few rows of cells wide in diameter; contains smaller neurons compared to layer Va and are more densely packed. Insausti et al, 1997
VI: Cell layer; contains pyramidal(-like) and multipolar cells
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:
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:
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
© Copyright 2007 - 2022 N.L.M. Cappaert & N.M. van Strien. All rights reserved. Content is available under Attribution-Noncommercial-No Derivative Works 3.0 Unported. Currently this work is financially supported by the Dutch Research Council. Project DOI: 10.21942/uva.19786213.v1