Читать книгу Invertebrate Histology - Группа авторов - Страница 16

The body wall of echinoderms consists of three major layers: (i) an outer monolayered epidermis, (ii) a middle connective tissue dermis containing an endoskeleton and muscle, and (iii) an internal monolayered coelomic epithelial lining (Figure 1.8a–c). There is a sensory nerve net (ectoneural nerve net or subepidermal nerve plexus) associated with the epidermis. A similar sensory and motor nerve net is associated with the coelomic epithelium (hyponeural nerve net). Nerve nets can be difficult to appreciate on hematoxylin & eosin (HE) stained histologic sections. A multilayered cuticle composed of proteoglycans and mucopolysaccharides covers the epidermal surface, but is frequently lost during fixation and processing (Holland and Nealson 1978; McKenzie and Grigolava 1996). Cuticular layers can be discerned by TEM and are summarized in Table 1.2. In Echindoidea, Asteroidea, and Ophiuroidea, there are essentially three described layers: (i) fibrous outer layer; (ii) granular middle layer; (iii) fibrous inner layer. Crinoidea lack an inner fibrous layer. Holothuroidea have a unique outer rodlet layer and fibrogranular inner layer. In some species, symbiotic bacteria occupy the space between the cuticle and the epidermis. The microvilli and cilia of the epidermal cells project into the lower two layers of the cuticle but do not extend into the outer coat (Ameye et al. 2000).

The epidermis is composed of simple cuboidal to columnar epithelium of several cell types, best differentiated by electron microscopy. These include supporting cells, secretory cells, pigmented cells (chromatophores and iridophores), sensory cells, nerve cells, and coelomocytes. Supporting cells have microvilli along their apex and may have cilia. They have basally located oval nuclei and a prominent nucleolus. Secretory cells are nonciliated with microvilli present only at the apex. Although five types of secretory cells are recognized by electron microscopy, the features discernible by light microscopy are variations in vacuolar size, shape, and staining characteristics. This discerns essentially two cell types: mucous gland cells, with finely granular contents, and muriform cells filled with coarse spherules (Hyman 1955) (Figure 1.9). In some echinoderms, especially echinoids, epithelial cell types may be difficult to differentiate histologically. In areas surrounding papulae (eversions of the coelomic cavity used for respiration in Asteroidea), the epidermis may contain multicellular glands with specialized secretions. Sensory nerve cell bodies and their axons may be visible basally within the epidermis, often referred to as the subepidermal plexus (or the ectoneural nerve net). The sensory layer is thinnest near the papulae and thickest in the oral region where it forms a circumoral nerve ring. The sensory layer often forms a ring around the base of ossified appendages. Coelomocytes may be present in the epidermis due to their role in phagocytosis and excretion of waste products to the environment. Their features are described later. The inner body wall consists of a simple layer of squamous sparsely ciliated epithelial cells that line the coelomic cavity.

Table 1.1 Organs for histologic evaluation in Echinodermata.^a

Organ system	Organs
Body wall/musculoskeletal	Cuticle, epidermis, dermis/mutable collagenous tissue, dermal ossicles, skeletal muscle, paxillaeb, pedicellariaeb
Water vascular system	Madreporite, stone canal, circumoral ring canal, radial canal, tube feet
Digestive	Alimentary canal	Mouth, esophagus, stomach, intestineb, rectumb
	Pyloric and rectal cecae	Digestive tubules, pyloric duct, rectal duct
Excretory		Heart, axial canal, axial hemal vessel, tube feet, papulae
Circulatory		Heart, axial organ, axial hemal vessel, hyponeural (oral) hemal ring, gastric hemal ring, genital hemal ring
Immune	Coelomocytes
Respiratory		Papulae (gills), tube feet
Nervous		Circumoral nerve ring, radial nerve, superficial and deep nerve nets
Reproductive	Male	Testis, sperm ducts
	Female	Ovary, oviduct
	Ovotestisb	Ovary, testis
Special senses/organs		Eyespots, sensory tube feet

^a Alternative names for organs are provided parenthetically, in italics.

^b If present in a given species.

Figure 1.8 Low‐magnification image of the histology of the body wall of an (a) ochre sea star (Pisaster ochraceus), (b) white sea urchin, and (c) California giant sea cucumber. Hematoxylin & eosin (HE), 100×, 40×, 100×, respectively. D, dermis; E, epidermis; G, gonads; O or arrows, ossicles; P, papulae; Pd, pedicellaria; T, tube feet.

Table 1.2 Cuticular layers in echinoderms (Holland).

Class	Layers present
Crinoidea	Fibrous outer layer (“fuzzy layer”) Granular inner layer
Echinoidea	Fibrous outer layer Granular middle layer Fibrous inner layer
Asteroidea	Fibrous outer layer Granular middle layer Fibrous inner layer
Ophiuroidea	Fibrous outer layer Granular middle layer Fibrous inner layer
Holothuroidea	Outer, rodlet layer Granular middle layer Fibrogranular inner layer

The dermis is composed of mutable collagenous tissue and an endoskeleton composed of interconnected plates, which may be articulated to form a rigid structure. The endoskeleton is composed of magnesium‐rich calcium carbonate, as magnesian calcite, devoid of an organic matrix (Cavey and Märkel 1994). Magnesium, substituting for calcium, is a unique feature of the echinoderm skeleton relative to other invertebrates (Raup 1966). Endoskeletal plates are of various shapes and are often called ossicles. Ossicles are separated into small interdigitating sections that are adjoined by collagenous ligaments and skeletal muscle (Figure 1.10). They are typically adorned by tubercles that articulate with movable ossified appendages, such as spines or calcareous protuberances, pedicellariae, and sphaeridia. Specialized ossicles called paxillae are present on the aboral surface of certain sea star species and facilitate burrowing. In ophiuroids, ossicles form larger plates called shields and each arm segment (article) is composed of four shields, two lateral, one aboral and one oral, with the lateral shields having large spines. Echinoids lack a muscle layer in the body wall because skeletal plates are fused and immobile, although muscle tissue is still present at the sites of articulation of the spines. In holothurorids, the ossicles are present but microscopic and are randomly distributed throughout the dermis. Some have paired specialized ossicles, the anchor and anchor plate, which assist in attaching species that lack tube feet to the substrate. A ring of well‐developed ossicles is present around the mouth and esophagus providing attachment sites for the buccal podia. Well‐developed longitudinal bands of smooth muscle are present along each ambulacrum.

Figure 1.9 Histology of the epidermis of a sunflower sea star (Pycnopodia helianthoides). Individual cell types are difficult to discern with light microscopy. The columnar epidermis (E) has occasional secretory cells (S). The subjacent dermis (D) contains many coelomocytes (C). 400×, Lee's methylene blue (LMB).

Figure 1.10 Low‐magnification image of the histology of a sunflower sea star ossicle demonstrating dermal, ligamentous, and muscular attachments. 200×, von Kossa.

Figure 1.11 Higher magnification image of the histology of an ochre sea star ossicle demonstrating the sclerocyte lattice (plastinated section). 400×, LMB.

Histologically, the endoskeleton consists of a three‐dimensional crystalline latticework, the stereom. Post decalcification, the calcite trabeculae are evident as clear spaces that may be artifactually collapsed. The fluid‐rich stroma that marginates trabeculae forms a honeycomb structure and contains sclerocytes that produce, modify, and envelop the skeleton (Figure 1.11). Sclerocytes are stellate mesenchymal cells that are typically in contact with trabeculae, and may be sparse within fully developed ossicles (Märkel and Roser 1983). In growing ossicles, sclerocytes form syncytia. Coelomocytes (discussed later) are common among the stroma, but may not necessarily be evenly distributed and can lead to a false impression of inflammation. Specialized phagocytes are capable of reabsorbing calcite from the ossicles (Ruppert et al. 2004). In echinoids, these are termed skeletoclastic cells and they are syncytial phagocytes that resemble osteoclasts (Cavey and Märkel 1994).

Figure 1.12 Histology of the base of a white sea urchin spine at the ball and socket joint. 400×, HE. M, muscle; L, ligament; T, test.

The osseous appendages have components that are similar to the body wall. All are covered in epidermis and contain an assemblage of dermal tissues described above. The echinoid spine consists of similar latticed endoskeleton with a central meshwork or hollow area surrounded by radiating longitudinal septae. The base of a spine adjoins to a tubercle of the test with ligaments of mutable collagenous tissue (i.e., the catch apparatus) encircled by bundles of smooth muscle cells (Figure 1.12). Distal spines of some urchins may be surrounded by a poison sac that has a collagenous connective tissue wall, and a lumen containing dissociated cells and debris (Cavey and Märkel 1994). Pedicellariae, present in Echinoidea and Asteroidea, clean the body surface and protect against sediment and small organisms. Microscopically, they consist of a stalk bearing a moveable head (Figure 1.13). Pedicellariae can be classified into a variety of types based on the size and shape of the head, and the number of jaws (i.e., tridentate, trifoliate, ophiocephalous, and globiferous). Most often, they have three elongate and distally narrowed jaws, each supported by a valve‐type ossicle, and supplied by adductor, abductor, and flexor muscles. The latter may be composed of smooth or striated myocytes. The stalk is supported by a rod‐shaped ossicle that may distally transition to a cavity filled with mucosubstances (Ghyoot et al. 1987). The epidermis is similar to that covering the test, but may be heavily ciliated along the stalk and inner jaws. Globiferous pedicellariae may carry venom sacs or epidermal glands on the inner jaws and these may be composed of more than one type of secretory epithelial cell (Ghyoot et al. 1994).

Figure 1.13 Histology of white sea urchin appendages including pedicellaria (P), spine (S), and tube foot (T). 100x. HE.

Dermal spaces between the endoskeleton are composed of fibrous connective tissue populated by stellate cells (Hyman 1955). A unique connective tissue termed mutable collagenous tissue is present in the body wall of all classes of echinoderms. Mutable collagenous tissue is controlled through a nonmuscular nervous system and can change its mechanical properties within one second to a few minutes from flaccid to rigid (Motokawa 1984, 2011; Wilkie 2002). The histologic features of mutable collagenous tissue (also called catch connective tissue) are not unlike dense irregular and regular connective tissues present in vertebrates. It is composed of individual collagen fibers with intervening ground substance that are arranged in perpendicular or parallel arrays depending on the species (Motokawa 1984). Interspersed among the fibers and ground substances are small numbers of immune cells (morula cells, coelomocytes). The function of this tissue varies by species and body wall structure. In holothuroids and asteroids, this tissue plays a significant role in overall body tone. In asteroids and echinoids, it plays a role in spine posture and prevents spine disarticulation. In crinoids, it controls the flexibility of the stalk (cirral) ligaments. In all species, it plays a significant role in autotomy (Motokawa 1984).