Z. Zellforsch. 101, 88--97 (1969)

The Sensory Papilla X-organ in Boreomysis arctica (Kroyer) (Crustacea Malacostraca Mysidacea) ERIK ])AHL and CLAES VO}r MECKLENBURG *

Department of Zoology, University of Lnnd, Sweden

Received May 29, 1969

Summary. The SPX-organ in Boreomysis arctica (KROYER) (Crustacea Malacostraca Mysidacea) was investigated light and electron microscopically. The organ consists of a group of cells (the SPX-cells) and a vesicle surrounded by a connective tissue sheath. It is situated near the base of the sensory papilla of the eye-stalk. Neurosecretory material is produced in the SPX-cells and transported in axon-like projections from these cells into the vesicle. These processes contain no neurotubuli. Numerous fibres from an afferent nerve emanating from the medulla terminalis also enter the vesicle, where they form a dense irregular meshwork. This nerve transports no neurosecretory material. There are numerous synaptic contacts between the afferent nerve fibres and the neurosecretory processes from the SPX-cells. The neuroseeretory material released from them accumulates in haemocoelic spaces in the vesicle. Release is most probably effected by the afferent nerve.

Key-Words: Sensory papilla--X-organ--Neurosecretion--Boreomysis arctica--Crustacea.

I n contras t to the highly complicated sensory papilla X-organ (SPX) in decapods, the corresponding organ in the mysids is simple (HANsTRSM, 1933, 1937, 1947). I n the lophogastr id genus Eucopia, HANSTRS~ showed tha t i t consists of one elliptic vesicle. The vesicle is surrounded by a layer of cells and contains " a colloid-like, sometimes s t r ingy secretory subs tance" (HANSTR(~M, 1947). Adjo in ing the vesicle lies a group of sensory cells, with fibres reaching in to the sensory papilla of the eye-stalk. The whole s t ructure is connected with the medulla terminal is by means of a simple nerve. I n other mysids invest igated by ttAN- STROM the a r rangement was largely similar. The object of the present invest igat ion was to examine more closely this apparen t ly simple SPX-organ.

Material and Methods The species was Boreomysis arctica (KROYER) obtained from Biologisk Stasjon, Espegrend,

Norway, in the deeper parts of the Korsfjord. For light microscopy, material was fixed in Bouin made with seawater, sectioned in paraffin and polyester wax. The stains used were: Heidenhains azan, ehrome-haematoxylln (Gomori), paraldehyde-fuchsin, and silver impregna- tion (Bodian). For electron microscopy the SPX-organ was removed, fixed in osmium tetra- oxide (both Palade and sea water) and embedded in Vestopal. Sections were made with an LKB-microtome Ultrotome and examined in an Akashi TRS-50 and a Hitachi HS-7 S.

Results

Light Microscopy o/ the SPX-Vesicle. I n Boreomysis arctica the eye papilla is well developed, project ing as a b lun t outgrowth from the dorsal side no t far from

* Acknowledgements. We are indebted to Prof. H. BRATTSTROM and the staff of Biologisk Stasjon, Espegrend for working facilities and material. Mrs B. MORAWETZ and Mrs L. ERIKSSON gave us skilled technical assistance. The investigation was made possible by grants from the Swedish Natural Science Research Council.

The Sensory Papilla X-organ in Boreomysi8 arctica (KROu 89

Fig. 1. Superficial section through the eye-stalk of Boreomysis arctica showing the SPX- vesicle and the SPX-cells. Chrome-haematoxylin (Gomori). 1 SPX-cells, 2 SPX-vesicle,

3 Eye muscle, 40mmatidia

the proximal edge of the compound eye. Below it, in the interior of the eye-stalk, lies the SPX-organ, consisting of a spherical vesicle, surrounded by a membrane with flat tend nuclei. A group of cells, the SPX-cells, lies below the membrane on the posterior and proximal part of the vesicle and is in direct contact with the interior of the vesicle (Fig. 1).

The nerve from the medulla terminalis on reaching the SPX-vesicle splits up into fine fibre bundles, encircling the outer wall, which they penetrate a t irregular intervals (Fig. 2). After entering the vesicle they split up still further, forming brush-like structures, each strand of which in silver impregnation appears to consist of numerous fine fibres. Together these bundles form a dense irregular meshwork which fills most of the vesicle {Fig. 3).

Staining with chrome haematoxylin (Gomori) produces a very different picture. No positive reaction can be seen in the afferent nerve bundles. On the other hand the interior of the vesicle gives a strong positive reaction. Strands and blotches of blue material appear to fill it more or less completely (Fig. 4). The chrome haematoxylin staining gives no definite information concerning the site of production of the positively staining substance within the SPX-cells. But there are very strong indications tha t it is formed in the SPX-cells, for although no blue-staining material can be discerned in the cytoplasm of the cell body, axon- like structures with blue material in their more distal par t radiate from the cells and enter the vesicle to merge with the large masses of stained material described

90 E. DAIIL and C. VON M]~CKLENBURG:

Fig. 2. Tangential section of SPX-vesicle in B. arctica showing bundles of afferent nerve fibres entering the vesicle at irregular intervals. Silver impregnation (Bodian). 1 Nerve bundle,

2 Nucleus of SPX-cell, 3 Nucleus of connective tissue cell of vesicle wall

above (Fig. 4). On the whole the SPX-cells give the impression of being active, with large vesicular nuclei.

SPX-Cells

Electron Microscopy. In the electron microscope the nuclei of the SPX-cells are spherical to ovoid, sometimes indented. The area adjoining the nucleus is occupied by a dense, smooth endoplasmic reticulum. Scattered throughout this area are Golgi membranes. Numerous large mitochondria are particularly to be found in parts of the cell periphery (Figs. 5--6).

In some of the specimens examined there were large accumulations of bodies with a diameter of the order of magnitude of 500--1,000 nm. These accumulations were scattered throughout the cell and with a bounding membrane.

The Vesicle

The interior of the vesicle contains two types of fibre profiles and large open spaces which do not give the impression of being artefacts (cf. below and Figs. 7, 8).

The one type of fibre contains neurotubuli. They are of varying diameter and give the impression of dividing into finer and finer branches which often form bundles (Figs. 7, 8). They never contain granules. Their bounding membranes are thin. These are nerve fibres from the afferent nerve.

The other type contains a varying number of electron dense granules of uniform size (50 nm) and with no distinct bounding membrane. The surrounding

The Sensory Papilla X-organ in Boreomysis arctica (KRoYER) 91

Fig. 3. Transversal section of SPX-vesicle in B. arctica showing meshwork of nerve fibres after silver impregnation (Bodian). 1 Nerve fibres in interior of SPX-vesicle, 2 Afferent nerve

bundle, 3 Vesicle wall

Fig. 4. Section through the SPX-cell group and part of the SPX-vesicle after staining with chrome-haematoxylin (Gomori). 1 Material stained with chrome haematoxylin, 2 SPX-cells, 3 Axon-like structures emanating from the SPX-eells and containing Gomori-positive material

in their more distal parts

92 E. DAn~ and C. vo~ MECKLENBURG:

Fig. 5. Electron micrograph of SPX-cell. 1 Nucleus, 2 Golgi membranes, 3 Mitochondria, 4 Electron-dense bodies, probably representing early stages of neuroseeretory material

m e m b r a n e of th is second t ype appears more electron dense t han in the f irst type . I n th is t y p e no neuro tubu l i are found.

I n the open spaces there are often ve ry large and dense accumula t ions of the same t y p e of granules as descr ibed in the fibres (Figs. 7, 8).

I n m a n y places there are indica t ions t h a t the f irst t ype of fibres, which are obvious ly nervous are in synap t i c contac ts wi th cell processes of the second t y p e (Fig. 8).

The Sensory Papilla X-organ in Boreomysis arctica (KBoY~R) 93

Fig. 6. Electron micrograph of border area between SPX-cells and SPX-vesicle. 1 Accumula- tion of mitochondria, 2 Afferent nerve bundles, 3 Neuroseeretory granules

Comparison between Light and Electron Microscopy Genera l ly i t can be s t a t ed t h a t l ight and e lect ron microscopic f indings cor-

robora te and supp lemen t each other.

94 E. DAttL and C. vo~ MECKLENBURG:

:Fig. 7. Electron micrograph from the interior of the SPX-vesicle in B. arctica. 1 Nerve fibre from the afferent nerve, 2 Cross-sectioned fibre probably derived from SPX-cell and containing

neurosecretory granules, 3 Haemocoelie space with neurosecretory granules

The afferent nerve bundles are easily impregnated with silver and their course can be t raced for considerable distances. Eteetron microscopically they have a characterist ic and uniform appearance with central neurotubul i . They conta in no

The Sensory Papilla X-organ in Boreomysis arctica (KR~Y]~R) 95

Fig. 8. Interior of SPX-vesicle in higher magnification than in Fig. 7, showing the close connections {including numerous synapses) between the afferent nerve fibres and the neuro- secretory processes. 1 Nerve fibre from the afferent nerve, 2 Neuroseeretory fibre, 3 Haemo-

coelic space. Arrows indicate synaptie contacts

neurosecretory granules and give no positive staining with Gomori haematoxylin. They can be seen to enter the vesicle, and neither light nor electron microscopy gives any indications of synaptie connections with the SPX-cells. On the other hand, as already mentioned, there are numerous synapses with the other kind of fibre in the vesicle.

These other tubules contain granules and obviously represent the elements, which stain positively with chrome haematoxylin (Gomori). Both with the light and the electron microscope such fibres have been seen to emanate from the SPX-eells.

96 E. ]~)AHL and C. vo~ MECKLENBURG:

. . . . .

Fig. 9. Diagram of the SPX-organ in B. arctica illustrating the probable relationships between the various elements. 1 Neurons and fibres of afferent nerve. 2 Connective tissue cells of the vesicle sheath. 3 SPX-cells, sites of production of neuresecretory material which is trans- ported through. 4 The axon-like neurosecretory eellprocesses. Release of neurosecretory material is probably effeeted by the afferent nerve by means of numerous synapses (indicated by arrows) upon the neurosecretory fibres. 5 Accumulations of neurosecretory material occur

in haemocoelic spaces throughout the vesicle. 6 Medulla terminalis

The high degree of act ivi ty which obviously characterizes the SPX-cells, and the accumulations of granular material within some of them, point to these cells as the site of formation of the neurosecretory material. The t ransport of this mater ial from the site of formation in the SPX-cell body to the site of release in the vesicle is accompanied by a transformation with respect to the size of the granules. In the cell they have a diameter of 500--1,000 am. There are indications tha t these comparat ively large electron dense bodies are gradually subdivided during the t ransport towards the axon-like processes, and tha t in the la t ter they have a t ta ined a much smaller and much more uniform size of about 50 am. This transformation appears also to be combined with some biochemical change, for positive staining of the granules with chrome-haematoxylin (Gomori) does not take place until just before the processes enter the vesicle.

The material is obviously transported into the vesicle within the projections from the SPX-cells.

The release mechanism is probably directed by the afferent nerves. This assumption is supported by the numerous synapses between these and the other fibres. This release leads to the accumulation of large quantities of neurosecretory material in the haemocoelic spaces of the vesicle.

I t is not known whether the picture described here is subject to seasonal variation. Our material was collected during the autumn.

The Sensory Papilla X-organ in Boreomysis attica (KR0r~.R) 97

I n a n y case our observa t ions show t h a t the S P X - o r g a n in Boreomysis arctica is a neu rosec re to ry organ in close con tac t wi th a neurohemal release site.

D i s c u s s i o n

The p ic ture ob ta ined of the S P X - o r g a n of Boreomysis arctica differs m a t e r i a l l y f rom t h a t descr ibed in the Decapoda (CARLISLE, 1953; KNOWLES a n d CARLISLE, 1956; CAI~LISLE and K~OWLES, 1959). The ma in difference is the t o t a l absence in B. arctica of a n y t h i n g resembl ing the so-called " o n i o n bod ie s " , i n t e rp re t ed b y CARLISLE as d i l a t ed nerve endings emana t ing f rom neurosecre to ry neurons in the medul la t e rmina l i s gangl ion layer .

I n B. arctica we have found no ind ica t ion wha t soever of a n y in t r aneu rona l t r a n s p o r t of neurosec re to ry ma te r i a l in e i ther di rect ion be tween the medul la t e rmina l i s and the SPX-organ . The s t ruc ture of the S P X - o r g a n in B. arctica can be s u m m a r i z e d as follows (Fig. 9):

1. A connect ive t issue shea th wi th cells wi th f l a t t ened nuclei sur rounding the S P X - o r g a n ;

2. The S P X - o r g a n itself consist ing of a) a t y p e of h igh ly ac t ive sec re to ry cells, the SPX-ce l l s ; b) a vesicle f i l led wi th cell processes of two types , one wi th neuro- tubu l i obv ious ly represent ing the t e rmina l pa r t s of the afferent neurons f rom the medul la terminal is , and one wi thou t neuro tubu l i bu t wi th neurosecre to ry granules de r ived f rom the SPX-cel ls . Outs ide these s t ruc tures lie haemocoel ie spaces p a r t l y f i l led wi th neurosecre to ry mater ia l .

The S P X - o r g a n of B. arctica is an organ wi th s t r i c t ly localized product ion , s to rage and release of neurosecre to ry mater ia l . The S P X - o r g a n a p p a r e n t l y re- p resen t s the only neurosecre to ry sys tem in the species s tudied, whereas condi t ions descr ibed b y CARLISLE in Lysmata seem to ind ica te the exis tence of a t leas t two sys tems. I t is also obvious t h a t the S P X - o r g a n in mys ids should be looked upon as no t on ly a neurosecre to ry organ bu t also as a neurohaemal organ (cf. GERSCH, 1964).

R e f e r e n c e s

CArLISLe, D. B. : Studies on Lysmata seticaudata Risso (Crustaeea Decapoda). VI. Notes on the structure of the neurosecretory system of the eye-stalk. Pubbl. Staz. Zool. Napoli 24, 434 ~46 (1953).

- - , and F. G. K~OWLES: Endocrine control in crustaceans. Cambridge: Cambridge University Press 1959.

G~.~seH, M.: Vergleichende Endokrinologie der wirbellosen Tiere. Leipzig: Geest & Portig 1964.

HX~STRSM, B. : Neue Untersuchungen tiber Sinnesorgane und Nervensystem der Crustaceen. II . Zool. Jb., Abt. Anat. u. Ontog. 56, 387--520 (1933).

- - Inkretorische Organe und Hormonfunktionen bei den Wirbellosen. Ergebn. Biol. 14, 143--224 (1937).

- - The brain, the sense organs, and the incretory organs of the head in the Crustacea Mala- costraca. Lunds Univ. Arsskr. N.F. Avd. 2, 43, 1--44 (1947).

K~ow~Es, F. G., and D. B. CARLmL~: Endocrine control in the Crustacea. Biol. Rev. 31, 396--473 (1956).

Dr. ERIK DAHL Dr. CLtES VON ~V[ECKLENBURG Department of Zoology University of Lund Lund, Schweden

7 Z. Zellforsch., Bd. 101