Table 2.

Features of outer brain barrier and meningeal development.

WPC	Feature	Species	References
6th	Appearance of a primary meninx around most parts of the brain	Human	O'Rahilly and Muller, 1986
	Amoeboid microglial cells penetrate the brain by crossing the pial basement membrane	Human	Verney et al., 2010
7th	The skeletogenous layer becomes visible	Human	O'Rahilly and Muller, 1986
	Differentiation of a leptomeningeal meshwork that is presumed to contain liquid and represent the future SAS	Human	O'Rahilly and Muller, 1986
8th	The dural limiting layer is almost complete in hindbrain and midbrain but only present in the area adjacent to the lateral fossa in the forebrain	Human	O'Rahilly and Muller, 1986
	The fenestrated sinusoids of the pia-arachnoid become non-fenestrated (E14)	Rat (E14)	Balslev et al., 1997
	Most of the cisternae of the adult is already present	Human	O'Rahilly and Muller, 1986
7–10th	Initial communication between the ventricular and subarachnoid compartments	Human and rat (E17)	Brocklehurst, 1969; Johansson et al., 2008
11th	Completion of the subpial end feet layer (E16) and claudin-11 positive arachnoid blood-CSF barrier (E18) and thereby appearance of a clearly defined subarachnoid space	Rat (E16 and E18)	Balslev et al., 1997
12–13th	Second wave of microglial cells penetrate the brain via the BBB and inner CSF-brain barrier	Human	Verney et al., 2010
12 (13–15th)	RGCs begin to transform into astrocytes	Rhesus monkey (E64) and newborn ferret	Schmechel and Rakic, 1979; Voigt, 1989
25–28th	Transition from subpial end feet layer to glia limitans	Human	Kadhim et al., 1988

Data were mainly based on human material and approximated to weeks post-conception. It is clearly stated when results from other mammals are used. The work of Clancy et al. (2001) was used for conversion to estimated human age.