Fig 2: YKL-40 immunoreactivity in a coronal section of visual cortex from a 21st wpc fetus (CRL: 200 mm). A low power overview is shown in (A) where the calcarine sulcus (CS) is indicated with an arrow. The boxed area is shown in higher magnification in (B) which provides an overview of the distribution of the YKL-40 positive cells in the inner and outer subventricular zones separated by the inner fibrous layer with migrating positive cells. The boxed area in (B) is shown in higher magnification in (C) where it is obvious that the YKL-40-immunoreactive population is not present in the ventricular zone whereas immunostained cells occupy a substantial part of the inner subventricular zone of the visual cortex. The many unstained cells might belong to groups of other progenitor cells, interneurons or microglia. Scale bars: A: 2,000 µm; B: 500 µm; C: 100 µm. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

Fig 3: The inner subventricular zone (ISVZ) of occipital cortex from a 21st wpc human fetus (CRL: 200 mm). Double immunolabeling with antibodies against SSEA-4 (red) and YKL-40 (green), with nuclei labeled with DAPI (blue). (A) The SSEA-4 and YKL-40 positive cells are oriented in the same direction on a migratory trajectory. Merged images of double-immunolabeled cells are boxed in (A) and shown at higher magnification in (D) and (G), with individual channels in (B), (C) and (E), (F), respectively. The double-labeled cells show immunoreactivity within the same cell compartments (ER/Golgi apparatus) in close apposition to the nucleus [(D), (G)]. Abbreviations: ISVZ: inner subventricular zone. Scale bar: 10 µm. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

Fig 4: Glia limitans in adult human brain stained for GFAP, MAP2, and YKL-40 (A–C). Adult human cerebral cortex immunostained for GFAP in (A) depicts the glia limitans (GL) as a dense multilayered network of GFAP positive astrocytic processes and few small fibrous astrocytes (arrowheads). Note the larger protoplasmic astrocytes (arrows) deeper in the molecular layer. Protrusions of astrocytic processes, which look like remnants of end feet (RE) are seen as patches facing the subarachnoid space (SAS). Immunostaining for MAP2 (B) defines the inner border of glia limitans (GL) toward the rest of the molecular layer and the unstained outer border with the empty-looking protrusions/remnants of end feet (RE) toward the subarachnoid space (SAS) is clearly depicted. Immunoreactivity for YKL-40 of glia limitans (GL) is seen in (C). Many protrusions exhibit distinct apical membrane reactivity for YKL-40, and YKL-40 positive spheroid bodies corresponding to corpora amylacea are seen within the glia limitans but also in the border zone toward the MAP2 positive part of the molecular layer (arrowheads). (A–C) same magnification. (A) Scale bar: (A) 20 µm.

Fig 5: A different area of occipital cortex from a 21st wpc human fetus (CRL: 200 mm) outside the calcarine sulcus. The entire section stained for YKL-40 is shown in (A) at low magnification. The subventricular zone contains a large pool of YKL-40- immunoreactive cells. Reactivity is also observed in the subpial layer, as described previously. The boxed area corresponds to a similar area of a neighboring section used for double immunolabeling. (B) Examination of YKL-40 immunoreactivity with RGCs labeled with antibodies against GFAP shows a GFAP-lined ventricular zone and an YKL-40 positive population in the SVZ. The boxed area, seen in (C) at higher magnification, shows a differential pattern of GFAP positive cells compared to YKL-40 positive cells. This is highlighted in three compartments, marked I–III. The ventricular zone (I) contains GFAP positive radial fibers anchoring at the ventricular surface. No YKL-40 reactivity is observed in this compartment. The basal part of the VZ (II) is a GFAP/YKL-40 positive zone with hardly any double-labeled cells, while the deeper layer of the inner SVZ only contains YKL-40 positive and GFAP negative cells (III). Abbreviations: ISVZ: inner subventricular zone; VZ: ventricular zone. Scale bars: A: 2,000 µm; B: 100 µm; C: 20 µm. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]