Follicle Cell Processes Research Articles

Tube-like structures within the ovarian follicle of Hydrolagus colliei indicate the early origins of follicle cell processes.

Follicle cell processes (FCP) are actin-based, tube-like structures that connect the developing oocyte to the follicle cells throughout oogenesis. They were first described in Selachians (sharks) where their suggested roles were facilitating the transport of metabolites to the developing oocyte and providing structural support to the large egg cells of sharks, an early stage in the evolution of viviparity. Subsequent studies found that FCP are absent in Rajiformes (skates), suggesting that FCP may have been novel structures specific to the sharks. Here, FCP in Hydrolagus colliei, a Chimaeriform, were described. The FCP of H. colliei differ morphologically from those previously described in sharks, but as they also contain actin, they presumably play similar roles provisioning the developing oocyte and providing structural support. The presence of FCP in the order Chimaeriformes suggests that their origin predates the split of the elasmobranchs and the holocephalans.

Observations of follicle cell processes in a holocephalan.

The presence of follicular cellular processes (FCP) that cross the zona pellucida, has been recorded in the ovarian follicles of Callorhinchus callorhynchus. This constitutes the first report describing the presence of these structures in a species of the Holocephali. Considering that FCPs have only previously been reported in the Selachii, these findings suggest that FCPs could have been lost by the Batoidea after their divergence, around 280 M B.P.

Follicle cell processes: a shark thing?

Follicle cell processes (FCP) are identified in two species of carcharhinid shark (Selachii) but are absent in the little skate Leucoraja erinacea (Batoidea). This suggests that FCPs are either a unique structure that evolved in selachians or were lost by the batoids after their divergence, some 280 mya. The presence of FCPs in the selachians would be consistent with the evolution of large oocytes in this group of animals.

A novel set of structures within the elasmobranch, ovarian follicle

Elasmobranch fishes produce some of the largest oocytes known, exceeding 10 cm in diameter. Using various microscopy techniques we investigated the structural adaptations which facilitate the production of these large egg cells in three species of shark: the Atlantic sharpnose shark, Rhizoprionodon terraenovae, dusky smoothound, Mustelus canis and the little gulper shark, Centrophorus uyato. The ovarian follicle of elasmobranchs follows the typical vertebrate pattern, with one notable exception; the zona pellucida reaches extreme widths, over 70 μm, during early oogenesis. Contact between the follicle cells and the oocyte across the zona pellucida is necessary for oogenesis. We describe here a novel set of large, tube-like structures, which we named follicle cell processes that bridge this gap. The follicle cell processes are more robust than the microvilli associated with the follicle cells and the oocyte plasma membrane and much longer. During early oogenesis the follicle increases in size relatively quickly resulting in a wide zona pellucida. At this stage the follicle cell processes appear taut, uniform and radially oriented. As oogenesis continues the zona pellucida narrows and the follicle cell processes change their orientation, appearing to wrap around the oocyte. The presence of the contractile protein actin within the follicle cell processes and their change in orientation may well be an adaptation for maintaining the integrity of these large oocytes. The follicle cell processes also contain electron dense material, identical to material found within the follicle cells, suggesting a role in the transport of metabolites to the developing oocyte.

From oogonia to mature oocytes: Inactivation of the maternal centrosome in humans

The fine structure of human oogonia and growing oocytes has been reviewed in fetal and adult ovaries. Preovulatory maturation and the ultrastructure of stimulated oocytes from the germinal vesicle (GV) stage to metaphase II (MII) stage are also documented. Oogonia have large nuclei, scanty cytoplasm with complex mitochondria. During folliculogenesis, follicle cell processes establish desmosomes and deep gap junctions at the surface of growing oocytes, which are retracted during the final stages of maturation. The zona pellucida is secreted in secondary follicles. Growing oocytes have mitochondria, Golgi, rough endoplasmic reticulum (RER), ribosomes, lysosomes, and lipofuscin bodies, often associated with Balbiani bodies and have nuclei with reticulated nucleoli. Oocytes from antral follicles show numerous surface microvilli and cortical granules (CGs) separated from the oolemma by a band of microfilaments. The CGs are evidently secreted by Golgi membranes. The GV oocytes have peripheral Golgi complexes associated with a single layer of CGs close to the oolemma. They have many lysosomes, and nuclei with dense compact nucleoli. GV breakdown occurs by disorganization of the nuclear envelope and the oocyte enters a transient metaphase I followed by MII, when it is arrested and ovulated. Maturation of oocytes in vitro follows the same pattern of meiosis seen in preovulatory oocytes. The general organization of the human oocyte conforms to that of most other mammals but has some unique features. The MII oocyte has the basic cellular organelles such as mitochondria, smooth endoplasmic reticulum, microfilaments, and microtubules, while Golgi, RER, lysosomes, multivesicular, residual and lipofuscin bodies are very rare. It neither has yolk nor lipid inclusions. Its surface has few microvilli, and 1-3 layers of CGs, aligned beneath the oolemma. Special reference has been made to the reduction and inactivation of the maternal centrosome during oogenesis. The MII spindle, often oriented perpendicular to the oocyte surface, is barrel-shaped, anastral and lacks centrioles. Osmiophilic centrosomes are not demonstrable in human eggs, since the maternal centrosome is nonfunctional. However, oogonia and growing oocytes have typical centrioles, similar to those of somatic cells. The sperm centrosome activates the egg and organizes the sperm aster and mitotic spindles of the embryo, after fertilization.

Effects of cycloheximide, aminoglutethimide, indomethacin, cytochalasin B and colchicine on ovulation and the ultrastructure of the ovarian follicle wall in the stellate sturgeon Acipenser stellatus Pall

The inhibitor of protein synthesis cycloheximide, inhibitor of steroidogenesis aminoglutethimide, and inhibitor of prostaglandin synthesis indomethacin, as well as the drugs affecting the cell cytoskeleton, such as cytochalasin B and colchicine, were used for studying the mechanisms of ovulation in the stellate sturgeon Acipenser stellatus Pall. Follicles were isolated from the body cavity within certain time intervals after the injection of pituitary suspension to a female and cultivated in media with the inhibitors. In the case of follicles isolated in the middle of the period from hormonal injection until ovulation, cycloheximide, cytochalasin B, and aminoglutethimide suppressed ovulation most effectively, while in the case of oocytes isolated during the last quarter of this period, aminoglutethimide and cytochalasin B were the most effective. It was shown using TEM and SEM that cycloheximide suppressed all processes related to the preparation for ovulation, except the initial ones: contraction of follicle cells and their processes and secondary flattening of these cells. In the presence of aminoglutethimide, the follicle cells underwent pathological changes. Incubation in the media containing indomethacin and colchicine prevented degradation of the outer theca layer at the follicle apex. In the presence of cytochalasin B affecting the cytoskeleton, the drawing of follicle cell processes from the jelly coat channels was blocked, the outer theca cells were strongly contracted, but the cell layer integrity was affected and it was divided in separate fragments. A relationship is discussed between the metabolic processes and morphological changes that lead to ovulation. It was proposed on the basis of the present and previous data that the preovulatory preparation of the follicle tissues comprises two contractile and two apoptotic processes distinctly coordinated in time and space.

Ultrastructural modifications of the follicular epithelium accompanying the onset of vitellogenesis in the whirligig beetle, Gyrinus natator

The ultrastructure of the follicle cells during previtellogenesis and early vitellogenesis have been studied. In previtellogenesis follicle cells are columnar with numerous bundles of microtubules located along the lateral plasma membranes. Oocyte-follicle cell gap junctions are not found in this stage. At the onset of vitellogenesis, the bundles of microtubules disappear and are replaced by an apically located ring of microtubules. The modification of microtubular cytoskeleton is not followed by the development of intercellular spaces between the follicle cells. Concurrently, numerous gap junctions are formed between specialized follicle cell processes and oocyte microvilli, which are arranged in characteristic cone-shaped aggregations. It is suggested that cytoskeletal changes and formation of heterologous gap junctions, occurring at the onset of vitellogenesis, are induced by juvenile hormone.

Intake of phosphatase-active plasma membrane into the cytoplasm of oocytes from sexually immature hamsters.

When thiamine pyrophosphate is used as substrate, heavily stained complexly branching cytoplasmic structures are seen in developing oocytes of the hamster between 8 and 12 days after birth. This coincides with the onset of the dictyate stage of first meiotic prophase. The structures appear to arise from the oocyte plasma membrane in two ways: (1) by proliferation and infolding of the membrane, and (2) by membrane being pushed into the cytoplasm by invaginating follicle cell processes. This appears to be followed by proliferation of the invaginated plasma membrane within the oocyte cytoplasm. Incubations with ATP or beta-glycerophosphate, with and without the inhibitor L-p-bromotetramisole, indicate that the staining is not due to the action of either ATPase or alkaline phosphatases. The origin and possible developmental significance of the reactive structures are discussed.

Oocyte—follicle cell interaction during normal oogenesis and atresia in an insect

Analysis of the oocyte-follicle interface during an oogenesis cycle revealed the development of a complex cell-to-cell interaction. A variety of ultrastructural methods revealed the presence of numerous gap junctions between oocyte microvilli and follicle cell processes during previtellogenesis and vitellogenesis. Junctions were not present during atresia of chorion formation. Fluorescent dyes, fluorescein, Procion yellow, and Lucifer yellow, passed from injected oocytes into follicle cells at all stages except chorion formation and during atresia. The numerous gap junctions presumably play important roles in coordination of oocyte-follicle cell differentiation.

Passage of mengovirus through the zona pellucida of the mouse morula.

Mengo encephalitis virus, a member of the encephalomyocarditis group of picornaviruses, has been shown to pass through the zona pellucida of the 2-cell mouse embryo (Gwatkin, 1963; Gwatkin & Auerbach, 1966). Penetration presumably occurs along channels left when the follicle cell processes are with¬ drawn. Recently, Austin (1965) has observed by electron microscopy that the zona substance tends to close in and obliterate these channels after ovulation, so the question has arisen whether the zona becomes less permeable as the embryo develops (McLaren, 1965). I have explored this question by deter¬ mining whether Mengovirus is capable of penetrating the pre-implantation

The fine structure of the developing follicle of the rhesus ovary

In young follicles of the rhesus monkey ovary the oocyte is in direct contact with a single layer of follicle cells, which may be as little as 0.05 μ in thickness, and which in turn is enclosed by a basement membrane. As the follicle develops, the follicle cells proliferate and the zona pellucida forms between them and the oocyte. The zona pellucida has a dual structure, being homogeneous adjacent to the oocyte, but containing a flocculent, electron dense material adjacent to the follicle cells. Microvilli project from the surface of the oocyte, and cytoplasmic processes from the follicle cells penetrate the zona pellucida to terminate at desmosomes on the surface of the oocyte. Occasionally follicle cell processes penetrate deeply into the cortex of the oocyte. The mitochondria of the oocyte are found to exist in two interconnected forms—a filamentous type with numerous transverse cristae, and a larger round type with few or no cristae. Numerous Golgi bodies are found arranged peripherally in the oocyte just below the cell membrane. The endoplasmic reticulum consists of scattered smooth vesicles. After the formation of the zona pellucida, structures designated as cortical granules appear beneath the oolemma. The cytoplasm of the follicle cells contains centrioles and well developed Golgi bodies. These observations are discussed in relation to the ovaries of other mammals.

An unusual organelle in avian ovaries

Ovarian follicles ranging in size from 1 to 5 mm in diameter, from coturnix quail and Leghorn chickens, were examined. Two principal cell types may be distinguished in the follicular epithelium, one of which contains a relatively low concentration of ribosomes in its cytoplasm, and is more abundant in follicles of this size range than the relatively ribosome-rich cell type. Boundaries between the vitelline membrane and the latter cell type are less well defined than those between this membrane and the more common type of follicle cell. Processes of follicular epithelial cells that extend through the zona radiata and terminate within the oocyte are lined by the cell membranes of the follicle cells from which they originate and are covered by the invaginated cell membrane of the oocyte. Unusual organelles found particularly at the termini of these processes, but also present at other sites in follicle cells, differ from other cytoplasmic components in that they consist of at least two parallel curved unit membranes, a row of granules being attached to the innermost membrane. The organelles are here named transosomes . The termini of processes of follicle cells are in close contact with the endoplasmic reticulum of the oocyte, and at some sites the membranes delimiting both structures appear continuous from one structure to the other. Fine fibrils in a structurally homogeneous matrix occupy the vitelline membrane, which is interrupted by follicle cell processes and whose inner border follows the irregular contour of the oocyte. The cell membrane of the oocyte is marked by pinocytotic channels that course from the vitelline membrane into the oocyte. The zona radiata of the oocyte displays pinocytotic profiles and follicle cell processes and is interiorly delimited by a terminal web. Mitochondria, endoplasmic reticulum, and Golgi bodies are not present in the zona radiata. The possible functions of transosomes are discussed: It is suggested that transosomes direct growth of membranes of follicular epithelial cells, forming their processes that extend into the oocyte, and also direct the growth of endoplasmic reticular membranes of the oocyte. The function of the ribosome-rich cell is proposed to be that of replacement of those substances of the vitelline membrane that are subject to erosion by oocyte pinocytotic activity.

Elektronenmikroskopische und histotopochemische Untersuchungen �ber Struktur und Bildung der Zona pellucida menschlicher Eizellen

Examined with the electron microscope the zona pellucida of human oocytes represents an extracellular, amorphous substance with slight differences in density. There is a greater consolidation in the inner parts and a drecrease of density towards the periphery. The plasmalemma of the oocyte forms a large number of slender projections (microvilli) penetrating the homogeneous groundsubstance of the zona pellucida. Plasmatic elongations of the follicle cells extending towards the oocyte traverse the zona in oblique or tangential directions and end at the oocytes surface forming a contact relationship (partially characteristic desmosomes). No syncytial communication between ooplasma and follicle cell cytoplasm can be demonstrated. The follicle cell processes contain finely granular material. The cytoplasm of numerous follicle cells facing the oocyte containes branched deposits of compact and dense substances with a granular ground-structure. Histochemically these substances react like anionic polysaccharides. Alternating zones of the follicle cell membranes show vaguely outlined lesser density. Intra- and extraplasmatic granular concentrations in these areas probably represent secretion processes. Acid mucopolysaccharides are the primary substrat of the zona pellucida whose deposition begins almost in the state of a bilaminar secondary follicle. Possibly this material is incorporated in the definite complex of glycoproteins by means of loss or binding of the acid groups (uronic acids, ester sulphates). PAS-positive inclusions about 1,0 to 1,5 μ in diameter lying in the cytoplasm of the follicle cells immediately adjacent to the plasmalemma or in open communication with the perivitelline space, represent probably paraplasmatic components for the building of the zona. Studied with the electron microscope these inclusions resemble strikingly the amorphous and cloudy ground substance of the zona pellucida. Stratified structures which would embody an appositional growth of the zona have not been demonstrated.