Читать книгу Invertebrate Histology - Группа авторов - Страница 20

In Asteroidea, hemal sinuses at the margins of the gut drain to the hemal ring that surrounds the base of the esophagus. The axial duct arises from the hemal ring, courses with the stone canal to the dorsal/aboral body, and enters the axial complex beneath the madreporite. The axial organ is adjoined by the axial duct, forming a junction between the coloemic cavity, water vascular system, and hemal system. The exact role of the axial complex is currently undetermined. Hypotheses include roles in respiration, excretion, and waste disposal, an immune organ, a gland of unknown purpose, a coelomocyte‐producing organ, a site of cell degradation, or a heart (Ziegler et al. 2009).

Histologically, hemal sinuses (or lacunae) have a wall of connective tissue that is lined exteriorly by coelomic epithelium. Muscle fibers in circular or longitudinal profile are scant throughout the wall. There is no inner lining or endothelium. Pigmented cells presumed to be phagocytes laden with melanin are often within vessels of the hemal system, and these may increase with age. The axial gland (or axial organ) is associated with the stone canal and consists of meshwork of connective tissue populated by coelomocytes (Figure 1.21) (Ziegler et al. 2009). Invaginations of the coelomic lining and lacunae penetrate the hemal sinuses. Cells containing melanin pigment are often within the stroma (Bachmann and Goldschmid 1978). The external surface of the axial gland is lined by coelomic epithelium.

Figure 1.21 Axial gland in a white urchin. 400×, HE.

Five pairs of Tiedemann's bodies adorn the hemal ring at the interradial areas in Asteroidea and the dorsal lantern in Echinoidea. In echinoidea they are formed where the coelomic lining of the dorsal lantern engages with evaginations from the radial canals (Cavey and Märkel 1994). Histologically, these are similar to the axial organ (Figure 1.22). A meshwork of connective tissue is permeated by canaliculi lined by coelomic epithelium. Coelomocytes and pigmented cells are also similarly frequent.