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.

Telencephalon

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

Transitional zones of the hemispheric wall, histogenesis of the neocortex

The period of cortpcal plate formation continued during the 11^th PCW. In the areas of neocortical formation, the ventricular zone (VZ), subventricular zone (SVZ), intermediate zone (IZ), corical plate (CP), and marginal zone (MZ) were revealed both in the lateral and medial walls of the hemispheres. The cortical plate thickness was significantly larger than that of ventricular zone. Compared with the fetuses at Carnegie stage 21, the thickness of the walls was increased by 1.5-2 fold.

In the ventricular zone of the fetus on the 13th gw stage, proliferative cells are densely spaced (section 132). Between the ventricular zone (matrix) and cortical plate, zones of intensive proliferation and migration lay: light zone in histological preparations consists of the radial glia (or tanicyte) processes, along which neuroblasts migrate from the ventricular zone to the cortical plate. At this period, the delineation of the subventricular zone into two subzones was visible in the lateral wall: the less tightly packed outer and more tightly packed inner parts (SVZo and SVZi, respectively). Above the subventricular zone, a wide few-cell intermediatr zone was located.

Starting from the 11^th PCW, a primary differentiation of cortical plate was detected in the human neocortex. The cortical plate mainly consists of the bipolar neuroblasts, which outgrowths are usually not longer then 4-5 of the cell diameters, thus the cortical plate anlage looks rather uniform throughout the hemispheric wall. In the ventrolateral part of cortical plate, the division into two layers started, with a denser arrangement of cells in the upper layer and a more widespread arrangement in the lower layer. The upper layer was narrower than the lower one. However, such stratification has not been detected in the dorsolateral and dorsomedial hemispheric walls.
In early nomenclatures, the neural compartment between cortical plate and intermediate zone was not distinguished (despite the fact that it was described earlier, including in 1937 by Polyakov). However, this zone was reopened first in humans and then in monkeys. This area was named subplate. The subplate plays a key role in neurodevelopment as a "waiting compartment" for ingrowing cortical afferents. However, in the early development of the human forebrain, it is impossible to separate the subplate from the intermediate zone in histological preparation. Therefore, in this study, the subplate was defined as the "decondensation" of the cortical plte lower layer according to Polyakov. The first signs of the subplate appearance on histological preparations are revealed at beginning of 11^th PCW, as a rare faction of cortical plate cells in the ventrolateral region.
The dorsomedial wall of the hemisphere was much narrower than the lateral one, while the thicknesses of the ventricular zone in the medial and lateral walls did not significantly differ.
At this time, the appearance of a granular layer composed of small cells was observed in the marginal zone. First of all, it appears on the ventrolateral surface of the fetal brain, in the area of the Sylvian fossa (the future insular cortex).

Cortical plate differentiation is heterogeneous (sections 25, 235). Medial neocortical CP anlage is thinner in comparison with lateral CP (fig., sections 25, 132, 235). (for more information see Proshchina et al., 2024).

Dorsomedial hemispheric wall

Dorsolateral hemispheric wall

The thickness of the cortical plate inside the calcarine and parietooccipital sulci and in straight parts of the cortical plate did not differ visually (Godovalova et al., 2024)

Ganglionic eminence (GE, eminentia ventricularis lateralis)

Due to the growth of the cerebral hemispheres, the ganglionic eminence (GE) acquires a C-shaped morphology (Fig., GE in blue). The GE consists of three parts: medial, lateral, and caudal. Each part contains ventricular and subventricular zones, which are populated by neuroblasts. Average GE of fetus (CRL 60 mm) width - 1,19 mm, volume - 19,98 mm3 (Proshchina et al., 2024).

Fig. 3-D reconstruction of the ganglionic eminence at gestational week 13

Histogenesis of the subpallial structure

Neostriatum
Total volume of neostriatum 11,73 mm³
Volume of caudate nucleus 3,25 mm³
Volume of putamen 8,48 mm³

Allocortex histogenesis ...