Ultrastructural Evidence of Oogenesis of the Rusty Parrotfish, Scarus ferrugineus (Teleostei: Scaridae)
Ultrastructural study of oogenesis of the Scarus ferrugineus was carried out during the period of oocyte growth, particularly the changes in nuclear morphology, cytoplasmic organelles, and wall formation. Materials produced in the nucleus and transported to the cytoplasm known as nuage were observed in the oogonia and primary oocytes at premeiotic and perinucleolar stages. This material gives rise to germ plasma in the growing oocytes. In the perinucleolar oocyte, the cytoplasm appears loaded with free ribosomes and dense bodies. Short strands of endoplasmic reticulum (ER), perinuclear mitochondrial ring, and a simple layer of flattened granulosa cells were also detected. Protrusion of short buds from oocyte and granulose surfaces to the perioocyte space was clearly detected in the oocyte. From the early vitellogenic stage (yolk vesicle oocyte) the vitelline envelope was observed as a single electron-dense mesh pattern layer, becoming thicker during the vitellogenic stage. The granulosa cells (GCs) become organelle-rich with elongate mitochondria, free ribosomes, dilated tubular RER, and a Golgi system. Both GCs and thecal cells (TCs) show ultrastructural steroidogenic features, suggesting their primary roles in the production and secretion of steroid hormones at this stage of ovarian development. Remarkable ultrastructural changes were shown in vitellogenic oocytes including increase in RER, proliferation of mitochondria, and protrusion of long microvilli from oocyte and granulosa cells into pore canals of the vitelline membrane. The microvilli withdraw by the end of yolk deposition and the granulosa cells dissociate in fully grown oocytes. In atretic oocytes, the vitelline membrane homogenized and fragmented. Also, GCs, and maybe TCs, proliferated, hypertrophied, and phagocytosed the oocyte remnants forming temporary structure. Degeneration of previtellogenic oocytes prior to sex change was described and discussed.
Light micrographs (LM) of portions of ovaries of Scarus ferrugineus showing: (A) Previtellogenic oocytes in chromatin nucleolar (1), perinucleolar (2), and cortical alveolar (3) stages. Note oogonial nest (Og) adjacent to ovarian lamellae (Ol), nucleolus (Nu), micronuclei (mn), thin chorion (c), follicular layer (Fl) (TB, bar = 50 µm); (B) Vitellogenic oocytes contain mature yolk (my), cortical alveoli (arrows) with cortical granules, thick chorion (C), and follicular wall (fw). Note yolk vesicle oocytes with cortical granules, thin chorion (c), and previtellogenic oocyte (Pr) (TB, bar = 25 µm).
Transmission electron micrographs (TM) of portions of ovaries of Scarus ferrugineus showing: (A) Group of primordial germ cells containing nuclei (N) with marginal nucleoli (Nu) and thin cytoplasm contains free ribosomes and scarce membranous organelles and pseudopodia like extensions (unlabeled arrows). Note mitochondria (m), endoplasmic reticulum strands (ER), granulose wall (g), phagosomes (Ph) (bar = 1 µm); (B) Portion of an oogonium. Note dense electron bodies close to the nuclear membrane (nuage, ng) and also associated with mitochondrial groups. Note also, mitochondria (M) with light matrix and scarce cristae, voluminous nucleus with prominent granular nucleolus (Nu) and numerous nuclear pores. Note also, nucleolar extrusion (arrow), endoplasmic reticulum (ER) (bar = 2 µm); (C) Chromatin nucleolar oocyte contains nucleus with smooth chorion (c) and one voluminous nucleolus (Nu) with electron-dense granular chromatin. Note electron-dense cytoplasm with inconspicuous organelles (bar = 1 µm); (D) Portion of chromatin nucleolar oocyte. Note irregular nuclear membrane (NM) surrounded by nuage (ng) material and clumped membranous organelles penetrate the electron-dense cytoplasm (arrows). Nucleolus (Nu), micronucleoli (mn) (bar = 5 µm).
Transmission electron micrographs (TM) in perinucleolar oocyte of Scarus ferrugineus showing: (A) Follicular wall consisting of squamous granulosa cell (GC) and squamous thecal cell (TC) with elongated nucleus and scarce organelles separated by basal membrane (bm). Note marginal mitochondria with light matrix and few cristae and microvilli (mv) projecting from oocyte surface (bar = 20 µm); (B) Outer part of perinucleolar oocyte surrounded by granulosa cell containing ovoid nucleus (b) with marginal heterochromatin blocks and dark cytoplasm with marginal vesicles (v) and scattered mitochondria (M) (bar = 1 µm); (C) Nuclear area of the same oocyte above showing undulated nuclear membrane (arrow) surrounded by nuage materials. Note granular micronucleoli (mn), ribosomes (r), vesicles (v), fibrous materials (F) (bar = 1 µm); (D) Portion of perinucleolar oocyte showing micronucleolus (mn) with fibrous and filamentous components, mitochondrial groups (m), and nuage (ng) around the outer nuclear (Nu) surface, nuage (ng) (bar = 20 µm).
Transmission electron micrographs (TM) in portion of alveolar stage oocyte of Scarus ferrugineus showing: (A) Start of deposition of vitelline envelope or chorion (c) perforated by pore canals and granulosa cell with large nucleus (N) with marginal heterochromatin blocks and cytoplasm containing smooth endoplasmic reticulum (SER), coated vesicles (arrow), and free ribosomes. (TC) thecal cell, (CA) cortical alveolus, (M) mitochondria; (B) Marginal portion of cortical alveolar oocyte showing cortical granule (arrow), mitochondria (M) with tubular cristae and dense bodies, rough endoplasmic reticulum (RER), lysosomes (ly), and plenty of free ribosomes; (C) Portion of cytoplasm of alveolar stage oocyte showing smoothing of the nuclear membrane (arrow) of the nucleus (N) and cytoplasm with plenty of organelles (smooth endoplasmic reticulum [SER], rough endoplasmic reticulum [RER], mitochondria [M]) with dispersed cristae and glycogen particles (g); (D) Granulosa cell with rough endoplasmic reticulum and smooth endoplasmic reticulum, free ribosomes (R) and mitochondria (M) with tubulovesicular cristae. Note also, intercellular spaces without cellular junctions between granulosa cells. Note thecal cell (TC), basal lamella (bl), nucleus (N), transport vesicles (arrows). All scale bars = 1 μm.
Transmission electron micrographs (TM) of vitellogenic stage oocyte of Scarus ferrugineus showing: (A) Outer portion of vitellogenic oocyte showing thick striated (lamellated) chorion (C), yolk granules (YG), and cortical alveoli (arrow) with cortical granules. Note thecal cells (TC) with irregular nuclei (N), previtellogenic oocyte (PV) (bar = 5 µm); (B) Higher magnification showing the electron light zona externa (ze) and electron-dense zona interna (zi) of the chorion crossed by pore canals (arrows) and transport vesicles (v) in previtelline space. Note thecal cell (TC) with elongated nucleus and cytoplasm with SER. Granulosa cell (GC), mitochondria (M), basal lamina (bl) (bar = 5 µm); (C) Portion of vitellogenic oocyte cytoplasm contains pleomorphic mitochondria (M) loaded with tubular cristae, rough endoplasmic reticulum (arrow), and free ribosomes. Note oil droplets (O), electron-dense masses (e) (bar = 1 µm); (D) Retraction and vacuolation (v) of granulosa cells layer (arrows) from oocyte surface (bar = 5 µm).
Transmission electron micrographs (TM) of atretic vitellogenic oocyte from post spawning ovary of Scarus ferrugineus showing: (A) Proliferation and rounding of granulosa cell (GC) and undulation of the chorion (c) with disappearance of the pore canal (bar = 5 µm); (B) Fragmentation of the chorion and invasion of the atretic oocyte (bar = 2 µm); (C) Portion of the proliferated granulosa cells of atretic oocyte. Note nucleus (N) with dilated nuclear membrane (NM) and marginal heterochromatin blocks. Also, accumulation of phagolysosomes (arrow), vesiculated rough endoplasmic reticulum (RER), mitochondria with scarce cristae, and deposits and myeline figures in GC cytoplasm. Vacuolation (v), oil droplets (O) (bar = 1 µm); (D) Portion of atretic oocyte cytoplasm showing vacuolation (v) and deformation of yolk granules and appearance of lysosomes. Mitochondria (M) (bar = 1 µm).
Transmission electron micrographs (TM) of the post spawning gonads of female Scarus ferrugineus at the beginning of natural sex reversal showing: (A) Degenerated previtellogenic oocyte with clumping and lysed mitochondria. Inset, pyknosis and vacuolation of oocyte; (B) Vacuolation (v) of cytoplasm and clumping of mitochondria (M), disappearance of nucleoli and dilated nuclear membrane; (C) Necrotic previtellogenic oocyte. Note autophagolysosomes (Ph), myeline figure (my), pyknotic nucleoli (N), fragmented cytoplasm (arrows) with abnormal cristae; (D) Degenerated oogonium. Note pyknosis of nucleoli and clumping of mitochondria. All scale bars = 1 µm.
Contributor Notes
Associate Editor: T. Grande.