Rat inferior colliculi were fixed by intracardiac perfusion of 4% methanol-free formalin in an isotonic phosphate-sucrose buffer, dehydrated in ethanol and studied during the first two weeks of postnatal life. The extracellular space during the first week was made up of intercellular gaps about 100-150 A wide and large intercellular lakes (1-3 p long) which diminished in size and number as maturation proceeded. Similar intercellular lakes were found in tissue fixed by immersion in glutaraldehyde containing ruthenium red. In these tissues an intense ruthenium red reaction product, distributed extracellularly, suggested an intercellular substance comparable to that described in adult brain. Intercellular lakes were also found in tissue dehydrated in acetone. Morphometric estimations indicated the extracellular space of neuropil during the first week to be about twice that of the adult, which was approached progressively.The development of the rat central nervous system during the first few weeks after birth is characterized by rapid changes in physiological, biochemical and morphological parameters. An adult-type spontaneous EEG can be recorded by the tenth day (Crain, '52). KC1-induced spreading depression by the thirteenth day (Deza and Eidelberg, '67) and cortical evoked potentials to sound between the tenth and fifteenth days (Bures, '57). A "blood-brain barrier" to ferrocyanide, which does not exist at birth, appears 12-14 days later (Stern and Peyrot, '27). Incorporation of C14 glucose into brain lipid becomes maximal between the eighth and eleventh days (Maker and Hauser, '67) and total gangliosides increase most rapidly between the sixth and sixteenth days (Mokrasch and Manner, '63). In cerebral cortex between the first and tenth days, "neuronal packing density" decreases maximally while nuclear, perikaryonal and total cell volume increase most rapidly (Brizzee and Jacobs, '59).Several lines of evidence indicate that the extracellular space of developing rat brain is larger than that of the adult. Na+ and C1-contents are highest between the fourth and seventh postnatal days (Millichap et al., '58) and on the basis of inulin and C1-analyses it has been estimated that the extracellular space at eight days is 1.5 times larger than that in the adult (Vernadakis and Woodbury, '65). Additionally, an electron micrograph of fetal rat cerebral cortex showing large intercellular spaces has been published (Gershenfeld and De Robertis, '67). The relationship of this relatively large extracellular space to other maturational events is unclear, in part, because the morphology of the developing extracellular space during the immediate postnatal period has not been defined and the nature of the intercellular substance in immature brain is not known.The present electron microscope study was designed to examine the extracellular space of chemically fixed rat brain during the first two weeks after birth. The inferior colliculus was chosen, in part, for its large size and relatively homogeneous cell population, which...
