Fine structures of the cerebrospinal fluid-contacting neurons in the hypothalamus of the lamprey, Lampetra japonica.

In the preoptic, infundibular and posterior recesses of the lamprey, Lampetra japonica cerebrospinal fluid (CSF)-contacting neurons are distributed much more abundantly than in higher vertebrates. They are classified into three different types on the basis of their granules size or electron density: type 1 containing round dense granules of large size (180--230 nm in diameter), type 2 containing variously shaped dense granules of medium size (150--200 nm) and type 3 containing variously shaped granules of small size (100--150 nm). The neurons of the third type may be further classified into two subtypes: type 3a possessing dense granules and type 3b showing less dense granules. The intraventricular process of CSF-contacting neurons contains Golgi cisternae that are forming new secretory granules and well developed granular endoplasmic reticulum with dilated cisternae. It is suggested that secretory granules might be formed in the intraventricular process in addition to the perikaryon. Besides the small cytoplasmic protrusions presumably involved in microapocrine secretion, intraventricular processes occupied with many empty granules are detected which might represent a process of diacrine secretion. The intraventricular processes further contain multivesicular bodies which incorporate old or overproduced secretory granules, suggesting a process (crinophagy) and large lipid droplets likely corresponding to the final digestive product. The CSF-contacting neurons of all types are usually supplied with axo-somatic synapses on the perikaryon and subsurface cisternae are sometimes observed beneath the postsynaptic membrane. Axon terminals are also found forming synapses on the intraventricular process of type 2 and 3 neurons.