Transitory macrophages in the white matter of the developing visual cortex. II. Development and relations with axonal pathways.

Clusters of 'gitter cells' develop in the white matter of the occipital cortex of the cat at the end of the first postnatal week. These clusters, and others already present at birth, disappear by the end of the first postnatal month. The life span of the clusters in the occipital white matter corresponds to the period when transitory callosal axons are eliminated. The clusters have close contact with callosal axons and can be labeled by HRP injected in the contralateral hemisphere and transported through the corpus callosum. One of the clusters clearly forms in a part of the white matter crossed by transitory callosal axons. The 'gitter cells' might be involved in the elimination of these axons. Consistent with this hypothesis, ultrastructural observations show groups of axons completely surrounded by 'gitter cell' cytoplasm as if they were being phagocytosed.