Читать книгу Invertebrate Histology - Группа авторов - Страница 24

Asteroids and echinoids are dioecious and each has gonads suspended by mesenteries either as five paired structures within the ray or as five individual gonads suspended from the interradius. The gonad is connected by a short gonoduct to a gonopore opening at the base of the arms in asteroids or in the genital plates on the aboral surface of echinoids. Gonads have similar structures, whether ovary or testicle. They consist of an outer genital sac which has a thin connective tissue wall, an outer coelomic epithelial lining and an internal lining of germinal epithelium. Muscle fibers may be sparsely present within the connective tissue. Germ cells develop peripherally and mature centrally. Ovaries contain oogonia progressing to large well‐developed vitellogenic oocytes centrally. Testicles contain spermatogonia progressing to small round spermatozoa centrally (Figure 1.25). Sex may be histologically indiscernible in reproductively inactive or immature individuals. Somatic cells (nutritive phagocytes) are present in both sexes of echinoids and dominate during periods between and leading up to gonadogenesis (Figure 1.26) (Walker et al. 2007).

Holothuroids are dioecious but gonochoric and have an ovotestis rather than a separate ovary and testicle. The gonad is composed of a large tuft of finely branched tubules covered by thin layers of coelomic epithelium and muscle. It is lined by germinal epithelium that shows differentiation toward both ova and sperm. It is connected by a gonadal duct to a gonopore located immediately behind the mouth at the base of the buccal podia. This gonopore is lined by a simple columnar epithelium.

Figure 1.24 Histology of the ventral nerve cord (N) in a mottled star. 100×, LMB.

Figure 1.25 Histology of the ovary (a) in a Caribbean thorny star, and testicle (b) in a mottled star. 200×, HE.

Source: (a) Image courtesy of Elise LaDouceur.

Figure 1.26 Nutritive support cells in the gonad of a sand dollar. 100×, HE.