Fig. 3

Ultrastructure and development of secretory activity in pleuropodia of S. gregaria. (a)-(e) Main features of the cells in the fully formed pleuropodia. Pleuropodia just before dorsal closure are shown: (a-d), 8 days; (e), 8.5 days. (a) Cross section through the pleuropodium. Apical (“outer”) and basal (“inner”) side of the cells is labeled. (b) Stalk cell. The short microvilli at the apical side are associated with the deposition of fibres in the embryonic cuticle (EC1). (c)-(e) Glandular cells. Note in (e) that the secretion granule is located at the base of the microvilli (brush-border); the tips of the microvilli produce fibrous material that is a part of the embryonic cuticle (EC1). (f)-(p) Ontogenesis of the glandular cells. Note the development of the microvilli, which is for day 4–7 shown at higher magnification in insets (f)-(i) (compare with fully developed microvilli shown at high magnification in (d) and (e)), accumulation of mitochondria below, mitochondria entering the microvilli, presence of smooth and rough endoplasmic reticula and the onset of secretion (appearance of secretion granules within and above the microvilli). (a) is a toluidine blue stained semithin section, (b)-(p) TEM micrographs. Secretion granules are marked with magenta arrows. Asterisks mark mitochondria inside microvilli. Black arrowheads mark the infolding of the basal plasma membrane (basal labyrinth) shown in (c), white arrowheads mark the spaces between neighboring cells. bm, basement membrane; cch, condensed chromatin; cj, cell junction; dv, dense vesicle; EC1, the first embryonic cuticle; gly, glycogene; ld, lipid droplet; mit, mitochondria; mv, microvilli; nu, nucleus; rer, rough endoplasmic reticulum; ser, smooth endoplasmic reticulum; sp., “spot” of a different electron-density in the pleuropodial granules. Scale bars: in (b), (c), (d), (e) and (f) for (f)-(p), 2 μm; inset in (f) for inset in (f) and (g), 500 nm, inset in (h) for inset in (h) and (i), 500 nm