Ultrastructural evidence for the existence of two types of neurosecretory cells in the abdominal ganglia of the chelicerate arthropod,Limulus polyphemus

Substance P-like immunoreactivity is present in the central nervous system of Limulus polyphemus

The distribution of substance P-like immunoreactivity (substance P-li) in the central nervous system of Limulus polyphemus was studied by using indirect immunocytochemical techniques. Six bilaterally symmetrical pairs of cell clusters in the circumesophageal connectives and the subesophageal mass contain substance P-li. Two of those pairs are the source of a system of efferent fibers that is involved in the expression of circadian rhythms of photosensitivity by the lateral eye. Substance P-li-containing cells were also observed scattered along the length of the circumesophageal connectives, which contain abundant stained fibers and some terminals. Substance P-li fibers leave through the ventral and dorsal nerves of the posterior circumesophageal ring. The neuropil of the subesophageal mass contains an abundance of stained terminals. Immunoreactive fibers can be seen throughout the length of the two longitudinal connectives of the ventral cord, in discrete fiber tracts in the lateral edges of the interganglionic connectives, and in the dorsal and ventral nerves of abdominal ganglia 1-4. Each of these ganglia contains three pairs of substance P-immunoreactive cell body clusters: an anterolateral, a medial longitudinal, and a medial posterior cluster. Substance P-li fibers entering through the ventral (posterior) nerves form very distinctive fascicles in each side of the ganglia, giving off fibers throughout their length. The neuropil is filled with immunoreactive terminals distributed homogeneously. The anterolateral clusters of the abdominal ganglia may be involved in cardioregulation. The six pairs of clusters in the posterior circumesophageal ring, and perhaps some of those in the abdominal ganglia, are believed to constitute a neurosecretory system, projecting to multiple targets throughout the organism. This system is postulated to modulate various sensory inputs and motor activity, and could be driven by a circadian clock, as well as by other systems responsible for integrated organismic responses.

Neurosecretory system of the American Dog Tick, Dermacentor variabilis (Acari: Ixodidae). ii. Distribution of secretory cell types, axonal pathways and putative neurohemal-neuroendocrine associations; comparative histological and anatomical implications

Histological observations using specialized techniques reveal neurosecretory cells in 18 centers throughout the rind (cortex) of the central nerve mass or synganglion of Dermacentor variabilis. Many cells contribute to complicated networks of neurosecretory pathways and tracts in pre- and post-esophageal portions of the synganglion. The four types of neurohemal-neuroendocrine associations found in Dermacentor resemble structures found in soft ticks (Argasidae) and in other Arachnida, but are more diverse than those described from any other single species. Neurosecretory terminals are distributed diffusely and in two concentrated associations within the perineurium of the synganglion and major peripheral nerves. Terminals are also distributed in the perineurial layers of lateral segmental organs which lie in the general hemocoel at the level of the pedal nerves. A retrocerebral organ complex surrounds the esophagus at its junction with the midgut. The complex includes dorsal and ventro-lateral lobes (containing neurosecretory terminals and intrinsic secretory cells1 and the proventricular (neurohemal) plexus. This plexus seems to be a modified (concentrated) cardioglial association. Cardioglial associations are also formed by the neurosecretory innervation of vascular walls of the dorsal aorta and circulatory sinuses which envelope the synganglion and major peripheral nerves. Inferential considerations of neurosecretory and endocrine interactions in the Acari are based on these anatomical and histological data which also provide the basis for evolutionary considerations of anatomical relationships and specializations in the neurosecretory systems of other Arachnida.