The fine structure of neuroglia in the lamina ganglionaris of the housefly, Musca domestica L.

Six morphologically distinct glial cell layers are described in the housefly lamina ganglionaris, a region previously thought to be composed of only three. 1. The external glial layer abuts the basement membrane of the retina. The cells of this layer have a highly involuted surface membrane and an abundance of ribosomes and rough endoplasmic reticulum (ER) throughout their cytoplasm. They envelop the traversing photoreceptor and mechanoreceptor axons as well as the large tracheoblast cells of the fenestrated layer. They are referred to as the fenestrated layer glia. 2. The second glial layer is composed of large, horizontally elongated cells with large elongate nuclei. They contain large membrane-bounded vacuoles and extensive arrays of parallel-running microtubules and smooth ER. These glia invest the photoreceptor axons through much of the multiple chiasmatic (pseudocartridge) region and are thus designated as the pseudocartridge glia. 3-4. Satellite glia comprise the third and fourth glial layers. Thin cytoplasmic processes of these multipolar glia intervene between the tightly packed monopolar neuron somata and the photoreceptor axons of the nuclear layer. The satellite glia are distinguished into two sub-groups: distal and proximal. The distal satellite glia are exclusively responsible for the large glial invaginations of the type I monopolar cell bodies. Multilaminated processes of the proximal layer of satellite glia surround the photoreceptor axons and the neurite neck of the monopolar neurons prior to their entry into the plexiform layer. The proximal satellite glia also contain prominent lipid deposits. 5. Epithelial glia are columnar cells that occupy the plexiform layer. They envelop the optic cartridges of the neuropil and are the substrate for two characteristic glial-neuronal invaginations; i.e. the capitate projection and the 'gnarl'. The cytoplasm of the epithelial glia is electron dense and contains numerous stacked arrays of infolded membrane. 6. Marginal glia form the proximal boundary of the optic neuropil. They invest the axons entering or leaving through the base of the lamina ganglionaris. Marginal glia contain large numbers of parallel microtubules and numerous polyribosomes. Fine structural evidence is presented relevant to the role of these six glial layers in the maintenance of ionic and metabolic homeostasis across the retina-lamina barrier.