Abstract
This study elucidated the fine structure of neurosecretory cells and sinus gland in the optic ganglia of the freshwater crab Travancoriana schirnerae Bott, 1969 (Decapoda: Gecarcinucidae). The eyestalk ganglion showed the presence of four well defined ganglia arranged below the ommatidium: lamina ganglionaris, medulla externa, medulla interna and medulla terminalis of which the lamina ganglionaris, was devoid of neurosecretory cells. Groups of neurosecretory cells seen distributed along the medulla externa, interna and terminalis regions constitute the X-organs. Electron microscopic observations of the eyestalk ganglia revealed ten types of neurosecretory cells, mostly apolar with a few unipolar and bipolar cells classified according to the size, shape and density of the cell and nucleus, cell organelles/inclusions, together with the arrangement and properties of chromatin. These cells were characterized by the presence of large nuclei with unusually condensed chromatin, inclusions like vacuoles and vesicles of varying size, shape and density and organelles like Golgi, endoplasmic reticulum, ribosomes and mitochondria and neurosecretory material. The sinus gland of T. schirnerae was positioned laterally between the externa and interna regions, composed of axonal endings of the neurosecretory cells of the optic ganglia with interspersed glial cells. The axon terminals were enclosed with several small to large membrane bound homogenously dense neurosecretory granules which also occur in the preterminal areas of the axons. Based on size, shape and density of granules and axoplasmic matrix, seven terminal types could be distinguished in the sinus gland of T. schirnerae. Mostly, the granules contained in a terminal were of the same type; rarely, the same terminal enclosed granules of varying size, shape and density. The neurosecretory cell types and axon terminal types represent the types of neurohormones they contained. A precise knowledge of the morphology and cytology of neurosecretory cells in the XO-SG complex of the eyestalk that secrete neurohormones controlling major physiological processes such as growth and reproduction is imperative for successful captive breeding of a species of aquaculture potential.
Highlights
The optic ganglion in the eyestalk of decapod crustaceans is an important source of hormones that control major physiological processes such as growth and reproduction (Bliss and Welsh, 1952; Knowles, 1955; Knowles and Carlisle, 1956; Matsumoto, 1958). Upadhyaya (2000) divided the optic ganglia into four parts: lamina ganglionaris, medulla externa, interna and terminalis, of which the lamina ganglionaris region is devoid of neurosecretory cells (NSC)
The NSCs observed in groups on medulla externa, interna and terminalis regions are termed as medulla externa X organ (ME-XO), medulla interna X organ (MI-XO) and medulla terminalis X organ (MT-XO)
Light microscopy The longitudinal section of eyestalk ganglion of T. schirnerae showed the presence of four well defined ganglia arranged below the ommatidium: lamina ganglionaris (LG), medulla externa (ME), medulla interna (MI) and medulla terminalis (MT), of which the topmost or distal portion is the LG, devoid of neurosecretory cells (NSCs) (Figure 1A)
Summary
The optic ganglion in the eyestalk of decapod crustaceans is an important source of hormones that control major physiological processes such as growth and reproduction (Bliss and Welsh, 1952; Knowles, 1955; Knowles and Carlisle, 1956; Matsumoto, 1958). Upadhyaya (2000) divided the optic ganglia into four parts: lamina ganglionaris, medulla externa, interna and terminalis, of which the lamina ganglionaris region is devoid of neurosecretory cells (NSC). The axonal endings from NSCs of the optic ganglia, brain and thoracic ganglia together with interspersed glial cells constitute the sinus gland (SG), a neurohemal organ concerned with the storage and release of a number of neurohormones like the moult inhibiting hormone, gonad inhibiting hormone, crustacean hyperglycemic hormone, retinal pigment concentrating hormone and pigment dispersing hormone that regulate important physiological processes like moult and growth, gonad development, blood sugar level, colour change, tegumentary and retinal pigment position, secondary sexual characterristics, locomotion and neuronal activity (Knowles and Carlisle, 1956; Carlisle and Knowles, 1959; Naylor et al, 1973; Beltz, 1988; Mohamed and Diwan, 1991; Fingerman et al, 1993; Charmantier et al, 1997). Hisano (1974) detailed the light and electron microscopic studies of the NSCs types in the optic ganglia of the prawn Palaemon paucidens. Though sufficient information is available on morphology, cytology and distribution of the NSC types in the optic
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