Synaptic development in long‐term organized cultures of murine hypothalamus

Abstract

Abstract Synapse formation and development were studied in organized cultures of mouse hypothalamic tissues from perinatal stages through a high degree of maturation during several months in vitro . Original explants from fetal and newborn animals were found to contain few definitive primordial synapses (ca. 4 per 10 grid‐squares); newly‐formed synaptic configurations began to appear with regularity in explanted tissues after the third day in vitro . These “immature” synapses contain small clusters of ∼ 300–600 Å agranular vesicles adjoining the presynaptic membrane, are usually devoid of mitochondria, and display diminutive junctional areas as yet little specialized. They are principally axodendritic, and may occur in the company of short, symmetrical, avesicular membrane‐junctions which link neuritic extensions. By about one month in vitro a substantial increase in number of synapses (up to 25/grid square) as well as advances in degree of maturity are evident in the outgrown neuropil; axosomatic synapses are relatively less numerous, as in vivo . Synaptic junction complexes henceforth appear well developed, and the presynaptic portions of boutons terminaux and boutons en passant now contain mitochondria and numerous agranular vesicles (sometimes packed in crystalloid arrays). Glomerulus‐like profiles, synaptic spines displaying typical spine apparatuses, and multifarious synaptic complexes containing a variety of ∼ 500–1,400 Å granular vesicles are also in evidence. The impressive amount of synapse formation and degree of specialization observed here suggests the utility of such cultures as a model system for further hypothalamic investigations.