Fig 1.
A male G. caespitosa extracted from its calcareous tube.
G. caespitosa consisted of a conspicuous head bearing a radiolar crown (RC) and an operculum (Op), a thorax (Th) and a segmented abdomen (Ab). The main sperm duct (or vas deferens) lay along the mid-ventral region of the abdomen with an opening (*) located at the junction of the abdomen and thorax.
Fig 2.
A longitudinal section through the abdomen of G. caespitosa.
Internally, a coiled intestine (I) ran the whole length of the abdomen. The abdomen was divided into over 80 trunk segments by intersegmental septa (IS). Each segment externally bore a pair of parapodia (Pp). Ramified blood vessels (BV) were located within the intersegmental septa. The abdominal segments served as germinal chambers (GC) where the spermatogenesis was initiated. These germinal chambers were filled up with spermatogenic cells (SC) at various stages of spermatogenesis.
Fig 3.
A schematic diagram showing a germinal chamber of the male reproductive system in G. caespitosa.
The C-shaped germinal chamber was divided into three distinct compartments: one seminiferous epithelium compartment, two nurse cell compartments and two efferent duct compartments. Spermatogenesis occurred in the seminiferous epithelium compartment and spermatogonia (Sg) initially appeared on the basement membrane (BM) of the germinal chamber. The spermatogonia underwent a series of mitotic divisions and subsequently developed into primary spermatocytes (PSc), which further produced secondary spermatocytes (SSc) through the first meiotic division. Each secondary spermatocyte subsequently developed into a pair of young spermatids (YSt) through the second meiotic division. The paired young spermatids were released from the seminiferous epithelium and migrated into the nurse cell compartments where they underwent spermiogenesis and differentiated into older spermatids (OSt) and spermatozoa (Sp). The spermatozoa then migrated towards the efferent duct compartments and were finally released into the seawater through the vas deferens (VD). CC, ciliated cells; EC, squamous epithelial cells; NC, nurse cells.
Fig 4.
A longitudinal section of two neighbouring seminiferous epithelium compartments.
The floor of the compartment was layered with seminiferous epithelium (SE), whereas the ceiling was covered by a monolayer of ciliated cells (CC). The seminiferous epithelium contained layers of spermatogonia (Sg) and spermatocytes (Sc). Spermatogonia occupied the initial layers near the basement membrane (BM), while spermatocytes with a larger size and paler colour were located in the upper layers close to the lumen. Paired spermatids (St) detached from the seminiferous epithelium and completed the remainder of spermatogenesis in the lumen of the germinal chamber.
Fig 5.
A longitudinal section of the connecting area between the seminiferous epithelium compartment (SE, on the left) and a nurse cell compartment (NC, on the right) in a germinal chamber.
Young (YSt) and older spermatids (OSt) were observed floating freely in the lumen and mixed with numerous spermatozoa (Sp) and granules secreted by the nurse cells. Noticeably, neither nurse cells nor blood vessels (BV) exhibited any physical connection with germ cells.
Fig 6.
(A) A light micrograph showing nurse cells (NC) lining the border (arrowheads) of two nurse cell compartments. The nurse cells were filled with spherical granules which were released to the lumen to support the differentiation of spermatids. Sp, spermatozoa. (B) A scanning electron micrograph showing a portion of a nurse cell compartment filled with spermatozoa (Sp). The nurse cells (NC) were densely distributed along the wall of the compartment (arrowheads), with epithelial cells (EC) of digestive tract on the other side.
Fig 7.
A light micrograph showing four neighbouring efferent duct compartments containing large numbers of spermatozoa (Sp) and spherical granules secreted by nurse cells.
Each side of the intersegmental septa (arrowheads) in this compartment was covered by a thin layer of epithelial cells.
Fig 8.
A light micrograph showing efferent duct compartments connecting to the vas deferens (*) at the terminus of each C-shaped germinal chamber.
The wall of the efferent duct in this region (arrowheads) was comparatively thicker than other regions of this compartment. It consisted of muscle cells that facilitated migration of spermatozoa from the efferent ducts to the vas deferens.
Fig 9.
A diagram showing the pattern of spermatogenesis in G. caespitosa.
Spermatogonia were in clusters and in close apposition to the basement membrane of the SE compartment. They underwent mitotic divisions and developed into primary spermatocytes while migrating away from the basement membrane. A primary spermatocyte divided into a pair of secondary spermatocytes through the first meiotic division and each secondary spermatocyte further divided into a pair of spermatids after the second meiotic division. The paired spermatids subsequently detached from the seminiferous epithelium and completed spermiogenesis while floating freely in the lumen of the germinal chamber. No direct contact between germ cells and nurse cells was observed at any spermatogenic stage.
Fig 10.
(A) A transmission electron micrograph showing a layer of type A spermatogonia packed along the basement membrane (arrowheads) of the SE compartment. The type A spermatogonia had no physical contact with the blood vessel (BV). Numerous mitochondria (M) tended to cluster in the basal cytoplasm while the nucleus (N) contained irregularly scattered clumps of chromatin blocks and an eccentrically located nucleolus (Nu). RER, rough endoplasmic reticulum. (B) A transmission electron micrograph showing two type A spermatogonia (SgA, the lower row) and four type B spermatogonia (SgB, the upper row). Type B spermatogonia were usually found one or two layers away from the basement membrane. Irregularly accumulated chromatin could be observed lining the inner nuclear membrane and in the centre of the nucleus. A conspicuous nucleolus (Nu) was frequently be observed in the nucleus adhering to the nuclear membrane of type B spermatogonia. M, mitochondria; RER, rough endoplasmic reticulum.
Fig 11.
A transmission electron micrograph showing a primary spermatocyte at late zygotene/early pachytene stage of the first meiotic division.
The most distinctive feature of the primary spermatocyte at this stage was the presence of synaptonemal complexes (*) in the nucleus. A Golgi complex with secreted irregular-sized vesicles and multivesicular bodies (MB) could be observed. Mitochondria (M) and glycogen (Gl) were scattered throughout the cytoplasm. RER, rough endoplasmic reticulum.
Fig 12.
(A) A light micrograph of a pair of young spermatids connected by a cytoplasmic bridge (CB). A proacrosomal vacuole (PV), which was the precursor of acrosome, could be seen adjacent to the oval- to spherical-shaped nucleus (N). A distinct nucleolus (Nu) was eccentrically situated in the nucleus. No flagellum had formed at this stage. (B) A light micrograph of a pair of older spermatids connected by a cytoplasmic bridge (CB). At this stage, a forming sperm flagellum (F) was observed protruding from the posterior cytoplasm of the older spermatids. The older spermatids contained comparatively less cytoplasm and a spherical-shaped nucleus (N).
Fig 13.
(A) A light micrograph of a spermatozoon. They consisted of a cap-like acrosome (A), an oval-shaped nucleus (N), a short midpiece (M) containing four round mitochondria and a fully elongated flagellum (F). (B) A scanning electron micrograph showing bundles of spermatozoa stored in the lumen of the ED compartment. A, acrosome; M, midpiece; N, nucleus; F, flagellum.