Description 13th gw

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The sample presented in the reference atlas refers to a part of the collection that was processed by histological methods in the late 80s - early 90s. The clinical information and protocol have not been preserved, so the basic information about the sample is taken from the label. In accordance with modern tables for determining of the fetal age by CRL (60mm CRL), the age of this fetus coincides with the specified one. Since the entire head of this fetus was processed by histological methods, the description of fissures was carried out on the serial sections.

The collection contains a complete series of cross-sections with a thickness of 20 microns. The preparations are stained with Mallory and hematoxylin and eosin.


1. Surface of the cerebral hemispheres.

The formation of primary sulci continues. On the dorsolateral surface, the interhemispheric fissure (fissura interhemispherica) is most noticeable, passing through the entire dorsal surface of the brain and separating the hemispheres (#25 - 235). On the ventro-lateral surface of the hemispheres, the separation of the temporal lobe and the formation of the islet area continue. A shallow depression or fossa is formed on the lateral surface of the hemispheres, which does not affect the deep layers of their walls (#25).

On the medial surface, the wide depression of the arcuate fissure is most noticeable. Closer to the anterior (frontal) pole on the medial surface, the primary first marginal fissure (f. marginalis prima) (#25) appears, passing as a branch of arcuate fissure (Fig.1). The second marginal fissure appears next. These primary fissures affect the entire wall of the hemisphere, including the ventricular zone. Closer to the posterior (occipital) pole, the parietooccipital and calcarine fissures are detected (sections are not presented).

The primary olfactory sulcus is well defined on the ventral surface of the brain closer to the anterior pole of the brain (#25). On the section #132, the hippocampal fissure (f. hippocampi) is shown on the ventral brain surface.

Fig.1 Main fissure of the brain medial surface at 11 weeks of gestational development

2. Transitional zones of the hemispheric wall, histogenesis of the neocortex

At the stages later 13th pcw, differentiation of the cortical plate (lamina corticalis, CP), which earlier began, continues. The cortical plate mainly consists of the bipolar neuroblasts, which outgrowths are usually not longer then 4-5 of the cell diameters, thus the CP anlage looks rather uniform throughout the hemispheric wall. Between the ventricular zone (matrix, zona ventricularis, VZ) and CP, zones of intensive proliferation and migration lay: lighter zone in histological preparations consists of the radial glia (or tanicyte) processes, along which neuroblasts migrate from the VZ to CP. These well-known maturation events accompany with differentiation. 

In VZ of the fetus on the 13th gw stage, proliferative cells are densely spaced. Subventricular zone (zona subventricularis, SVZ) lies next to VZ and consists of the large number of neuroblasts. The proper intermediate zone (zona intermedia, IZ) is a cell-poor large layer adjoining SVZ.

Cytoarchitecture of the CP at this stage possesses two distinct cell layers. IZ is bordered with undifferentiated CP, and externally light marginal zone (lamina zonalis, zona marginalis, MZ) is situated (fig.). After 13th gw subpial granular subzone (stratum granulare subpiale) could be distinguished, that consisted of small interneurons arisen mainly from the lateral ganglionic eminence (LGE) and radial orientated neurons migrated from VZ. Firstly, it has separated in the ventro-lateral part of the hemispheric wall. 

In the human neocortex at the 12 gw, further differentiation of the CP could be distinguished: CP has differentiated into upper solid and thin layer and deep extended and loose layer – subplate (zona sublaminaris, SP). CP differentiation is heterogeneous: frontal CP anlage is wider, it gets narrow caudal ward (sections #25, #235). Medial neocortical CP anlage is thinner in comparison with lateral CP (fig., sections #25, #132, #235). SVZ, as well as IS, depending on the position, can differ by a factor of two in size, which reflects the initial differentiation heterogeneity of the neocortex.

Dorsomedial hemispheric wall

Dorsolateral hemispheric wall

3. Allocortex histogenesis ...

4. Histogenesis of the subpallial structure ....