Silk gland morphology
Silk gland general morphology ( Fig. 5A )next section
Silk glands have been detected in pereopods 3-4 of adult females, males and juveniles (including the youngest ones), and the secretory units (D1, D2) are situated in pereopod segments 2-5 (basis to carpus). The ducts are grouped into a single bundle (DB) leading to the dactylus (segment 7), where this bundle connect to a common chamber (Ch) opening at the dactylus tip (arrow, Fig. 5A). The ducts are lined with epicuticle (2, Fig. 6E, G, I), whereas the chamber has a normal cuticle comprising endo-, exo- and epiuticle.
There are two distinct gland groups in each pereopod 3 or 4: proximal (D1) and distal (D2) (Fig. 5A). The D1 secretory granules stain more intensively with the mix of toluidine and methylene blue and are electron-dense (sg, Fig. 6, 7) compared with the D2 granules (sg, Fig. 8).
Each female pereopod contains approximately 70 secretory cells.
D1 gland location
Female D1 secretory units are only located in the pereopod basis (segment 2), and in males, these structures were also found in the merus (segment 4). The D1 secretory ducts lead to the dactylus (segment 7) (Fig. 5A).
D1 gland structural plan
D1 secretory units comprise approximately 60 secretory cells (SC) lying along 5-6 main ducts (MD, Fig. 5B, Fig. 6A-C), and short (5-8 µm) lateral canals (LD) individually branch away from the main duct to each secretory cell (Fig. 6A-C, E). Each lateral duct terminates into an intracellular globular accumulation site (as) that is 7-9 µm in diameter (Fig. 6A-C, E) and contains numerous cavities. TEM revealed that the accumulation site is composed of ramifying ductules (d) radiating from the end of the lateral duct (Fig. 6E, G-H).
D1 gland duct structure
Ductules within the accumulation site are situated directly in the secretory cell, and the lumens are separated from the cytoplasm by a thin (approximately 0.001 µm) electron-dense layer (Fig. 6E, G-H). It is not clear whether this layer comprises only membrane or if it also contains epicuticle; microvilli are lacking.
The main and lateral ducts are surrounded by a nonstaining sheath (duct wall, dw), which has a variable thickness of up to 3 µm (Fig. 6A-C). TEM (Fig 6E, G, I-J) revealed that the duct wall of each D1 gland comprises two cells: “internal” and “external”. The internal cell (which is really a duct cell, DC, Fig. 6E) contacts the duct lumen and the external cell and adjoins the secretory cell near the distal end of the lateral duct. The “external cell” (lining cell, LC) completely covers the “internal cell” and contacts secretory cells and the haemocoel; it is included in the gland lining (Fig. 6E, J). The cytoplasm of the duct (“internal”) and lining (“external”) cell is separated by two membranes (6, Fig. 6E, I), but the longitudinal sections of the lateral duct showed only one membrane between these structures (5, Fig. 6E, G). Thus, the structures what we call “duct” and “lining cells” are possibly components of a single complicated cell.
Duct cells (inner part of the duct wall) comprise the next layers (Fig. 6E, I): (1) the innermost thin electron-dense (0.0015-0.003 µm) layer, which likely includes the membrane and epicuticle; (2) a grainy electron-dense layer (0.1-0.3 µm), likely comprising actin filaments; (3) a wide electron-lucent layer (3-4 µm) containing microtubules, parallel to the duct axis, and occasionally with membrane structures; and (4) an outer cellular membrane. These layers are continuous and closed, and the radial connection between the inner and outer cellular membranes (mesaxon) is absent. The cell cytoplasm contains microtubules and different membrane structures (Fig. 6E, I, J).
The intracellular location of the main ducts remains in the subsequent appendage segments.
Secretory product was observed in the lumens of the intracellular ductules, lateral and main ducts (s, Fig. 6B-C, E, G-I). In each lumen, this product is aggregated into compact bodies, and its electron density and structure are similar to those of the secretory granules in secretory cells (sg, Fig. 6D, H and Fig. 7C-D).
D1 secretory cell structure
Secretory cells (Fig. 6A-C) vary in form and size (cell diameter of 5-35 µm) and are mononuclear. The semilunar nucleus (nu) is located near the centre of the cell, opposite the lateral duct (Fig. 6A-C). Secretory cells contain numerous secretory granules (sg, Fig. 6B, E, H) situated densely in the area adjacent to the accumulation site (cz) and more sparsely at the periphery (pz, Fig. 6B). Mitochondria (mt) and granular endoplasmic reticulum (er) are situated in the spaces between the granules (Fig. 6D, J and Fig. 7C).
Secretory granules (sg, Fig. 6B, D, H, and Fig. 7C) have permanent circular forms (approximately 0.6-1.0 µm in diameter) and stain intensely with a mix of toluidine and methylene blue. The granular material in the thin sections is electron dense and can be heterogeneous to varying degrees (sg-1, sg-2 in Fig. 6D, and sg in Fig. 7C).
The secretory cell membrane forms invaginations (i) filled with extensions of lining cell cytoplasm (Fig. 7A-D), and these invaginations can be significantly deep, reaching the accumulation site. The invaginations are narrow (0.05-0.15 µm) but widen at some sites (Fig. 7C, D), particularly at the end/tip of the invagination (Fig. 7D). Additionally, the invaginations occasionally branch (Fig. 7B). The pattern of invagination distribution was observed in two consecutive serial sections (with a 3-5-µm interval) perpendicular to the lateral duct planes (Fig. 7A-B). Therefore, we propose that these invaginations have a flattened form and are located in the plane of the lateral duct.
D1 lining structure
The lining cells (LC), located between the secretory cells and the haemocoel (h) (Fig. 6C, E, J, and Fig. 7), are strongly flattened (approximately 0.2-0.4 µm in thickness) but expand to surround the gland ducts (Fig. 6C, E) or to hold the nucleus or mitochondria. These cells form extensions into (see above) and between the secretory cells.
In some sections, the lining appears as a continuous cytoplasmic layer (Fig. 7F), while in other sections, the lining comprises multiple overlapping cytoplasmic extensions (arrowheads, Fig. 7E). The lining cytoplasm contains microtubule bundles, membrane vesicles or elongate structures (Fig. 7C, E-F). Near the accumulation site between the secretory and duct cells, we detected a structure (Fig. 6F) containing numerous microtubules (black arrowheads) and electron-dense round structures 0.006-0.1 µm in diameter (white arrowheads) that is likely axon terminal.
Distal glands (D2) ( Fig. 8 )
D2 gland location
The D2 secretory cells in adult females are located in the 2-5 appendage segments (basis-carpus) (Fig. 5A, and Fig. 8A) in three distinct cell groups along the bundle of D1 and D2 ducts. The largest group contains 10-11 cells and begins in the basis (segment 2) and ends at the merus (segment 4). Two other groups, comprising 1-3 cells, are situated in the carpus (segment 5) (Fig. 5A and Fig. 8A).
D2 glands structure
Inside each D2 secretory cell, there is an accumulation site (as) with a duct (D, resembling a D1 gland lateral duct) leading away (Fig. 8A). This duct is lost in the duct bundle (DB), and it is unclear whether it falls into any other ducts or remains separate. The accumulation site, as in the D1 glands, comprises numerous radiating ductules (d, Fig. 8E). The gland duct has a separate wall, likely containing a duct cell cytoplasm, but the wall ultrastructure has not been investigated.
The secretory cells (SC) are 25-35 µm in size, uninuclear, and filled with secretory granules (sg), and in the cell periphery, the granules are situated less densely than near the accumulation site. The space between the granules was observed to be heterogeneous, suggesting that this space is filled with poorly preserved endoplasmic reticulum (er, Fig. 8C-D). Secretory granules are occasionally round, but when densely situated, the shape changes (Fig. 8D). These cells lightly stain with toluidine blue and contain loose electron-lucent material. Within one cell, the secretory granules vary in their electron density and the regularity in which the material is arranged (sg1-3, Fig. 8D). We did not observe any regularity in the different granules distributed throughout the cell. Cell membrane invaginations were not observed in D2 secretory cells.