The Body Wall and Musculature of the Marine Triclad Palombiella stephensoni (Palombi, 1938): Part One: General Tissue Structure as seen with the Light Microscope
The Epidermis (Plate 1). The epidermis is a single, cellular layer. The cells are cuboidal to columnar, being more cuboidal on the ventral surface, and columnar on the dorsal. The entire epidermis is ciliated, but the ciliation is heavier on the ventral surface. The nuclei are spherical, drop-like, or oval structures. Unicellular basiphil glands occur scattered throughout the epidermis but especially on the dorsal and ventral surfaces at the head and tail ends of the body. These gland cells do not differ in appearance from other epidermal cells, except for the secretion which they contain.
When seen in plan view, the cells of the epidermis fit together closely to give an approximately hexagonal arrangement. In sections they have a distinctive appearance which leads to the conclusion that the outer ⅓ region of the cells possesses a system of internal "channels" (Plate 1, Fig. 1). These "channels" are regularly spaced, are parallel to each other, and are perpendicular to the base of the cells. They are most prominent in the dorsal epidermis and are well demonstrated by sections tangential to the surface of the animal (Pl. 1, Fig. 2, 3).
The Basement Membrane (Pl. 2, Figs. 1, 2). Immediately below the epidermis and in contact with it is a thick (up to 2µ) non-cellular layer. It stains red with Van Gieson's picrofuchsin stain, blue with Mallory's triple stain and green with the triple stain of Wineera (1968). No reticular or elastin fibres could be demonstrated in this layer by the methods used. Sections of mouse and rabbit skin which were stained at the same time as those of P. stephensoni gave a positive result for both elastin and reticulin fibres. The sulphuric acid-haematoxylin method for basement membranes failed to stain this layer. The basement membrane has the same appearance when cut in transverse or sagittal section. At a magnification of x1250, fibres are recognizable in some areas, otherwise it is homogenous and non-cellular throughout. It is intruded between the epidermal cells at their bases (Pl. 2, Fig. 1).
Muscles (Pl. 2, figs. 1, 2). Immediately below the basement membrane of the epidermis is a thin layer of circular muscle. The fibres in this layer are roundish in transverse section and are approximately 1µ in diameter. Beneath the circular muscles are ribbon like longitudinal muscle fibres which are approximately 2.5µ wide and 1µ thick. Muscle fibres are separated by thin connective tissue fibres which are continuous with the basement membrane below the epidermis (Pl. 2, Fig. 2). At the anterior end of the animal the longitudinal muscles are often bipinnate in arrangement and very similar, when seen in transverse section, to the longitudinal muscles of the earthworm. Dorso-ventral muscle fibres of similar size to the longitudinal muscle fibres are abundant throughout the body, especially at the sides of the animal (pl. 2, Fig. 1, 2). No striations of any sort have been seen on the muscle fibres. The nuclei of muscle cells are smaller than those of the epidermal cells. They are ovoid to elongate, and are orientated with their long axis parallel to the muscle fibre.page 4
Parenchyma (Pl. 2, Fig. 1, 2, 3, Pl. 3, Fig. 1, 2). Between the organs the body is filled with parenchymatous tissue. Numerous nuclei can be seen, but cell boundaries are indistinct except in the case of the neoblast cells. Hyman (1951, p. 78) defines this tissue as a syncytium.
The neoblast cells (Pedersen, 1959) are free cells with a characteristic morphology (Text Fig. 1). In sections fixed in Lillie's AAF they are seen as small to large cells with a round or oval nucleus invested with a thin layer of basiphil cytoplasm. The nucleus measures up to 5µ in diameter, and usually contains one or two large nucleoli. The whole cell measures approximately 7µ in diameter, with the cytoplasm being fairly evenly distributed around the nucleus. Sections treated with Mallory's Triple Stain show the nucleus as blue grey, with coarsely granular nucleoplasm. The nucleoli stain a vivid red-orange, and the cytoplasm, which is finely granular, colours a darker blue-grey. Neoblasts occur throughout the parenchyma, but they tend to be most numerous on the ventral side, and are scarce in the cephalic and caudal regions.
Text Fig. 1 cy., cytoplasm; n., nucleus; nuc., nucleolus.
Typical neoblast cells. For description see text.
Connective tissue fibres and muscle fibres are abundant in the parenchyma, and various vacuoles are also present. At least two types of subepidermal gland cell also occur in the parenchyma. One type of gland occurs as scattered single cells mainly in the ventral part of the animal, but it may be found anywhere in the parenchyma. In the head and tail regions these cells are aggregated into clusters. They are long, slender and are basiphil, staining with aniline blue in the Mallory technique and with haematoxylin. They open to the surface through long necks which penetrate between epidermal cells, mostly at the ventral surface, but also on the dorsal side. In the cephalic region of the worms these glands pour a profuse secretion to the ventral surface through large ducts.page 5
A second type of gland cell is restricted mostly to the parenchyma of the lateral body margins. These cells are smaller than the basiphil gland cells but are also elongate. They open by way of long slender necks which usually penetrate epidermal cells at the ventral surface of the marginal area of the animal. However, they also open at other places on the ventral surface, and often extend from the lateral margins towards the dorsal midline. These cells are eosinophil and are shown very well by the Falg method of staining (Pl. 3, Figs. 2, 3, 4). With the Mallory technique they stain a distinct purple colour. When the necks of these cells penetrate epidermal cells they branch to form a fan-shaped system of ducts (Pl. 3, Fig. 2).
Pigment (Pl. 1, Fig. 1). Pigment, which is sub-epidermal, is confined mostly to the dorsal side of the parenchyma. On this side the pigment may be so dense as to completely obliterate other features of the anatomy (Pl. 1, Fig. 1). On the ventral side, however, pigment is present only as scattered granules. The pigment is laid down at the site of connective tissue fibres, particularly where these fibres course between the circular and longitudinal muscle fibres of the dorsal body wall. Other regions of the body are free of pigment. The pigment granules are very small, and are irregularly shaped. They can be bleached by hydrogen peroxide and by potassium permanganate solution.