Cortical granule exocytosis is coupled with membrane retrieval in the egg of Brachydanio

Zebrafish prss59.1 is involved in chorion development

The prss59.1 gene was identified as one of 11 genes that were highly upregulated during the induction of ovulation in zebrafish by using an in vivo ovulation assay. Previously, we conducted biochemical characterization of Prss59.1 and revealed it to be a trypsin-like proteolytic enzyme. In this study, we established a prss59.1 gene knockout strain using the CRISPR/Cas9 system. Phenotypic analysis of prss59.1 knockout fish showed that prss59.1 is associated with chorion elevation, a prominent event in egg activation during fertilization. The chorions of heterozygous and homozygous prss59.1 mutant zebrafish were smaller than those of the wild type. The results suggested that Prss59.1 is necessary for chorion expansion. The homozygous prss59.1 mutant strain, with a small chorion, showed an extremely low survival rate. Fiber-supported knob-like structures (KS) on the chorion showed an abnormal structure in prss59.1 mutants. Prss59.1 was detected in the KS on the chorion. The pores on the chorion were smaller in the prss59.1 mutants than in the wild type. Transmission electron microscopy (TEM) observations of the cross sections of the chorions showed abnormalities in the chorion structure in prss59.1 mutants. These results demonstrated that Prss59.1 is involved in chorion elevation and in proper formation of the chorion, which is necessary for embryo development.

Alternative signal pathways underly fertilization and egg activation in a fish with contrasting modes of spawning

BACKGROUND

The processes of fertilization and egg activation are vital for early embryogenesis. However, while the mechanisms associated with key events during these processes differ among species and modes of spawning, the signal pathways underlying these processes are opaque for many fishes, including economically important species.

RESULTS

We investigated phenotypic traits, ultrastructure and protein expression levels in the eggs of the topmouth culter (Culter alburnus), a protected and economically important freshwater fish that exhibits two spawning modes, producing semi-buoyant eggs and adhesive eggs. Unfertilized eggs of C. alburnus were examined, as well as eggs at fertilization and 30 min post fertilization. Our results showed that in semi-buoyant eggs, energy metabolism was activated at fertilization, followed by elevated protein expression of cytoskeleton and extracellular matrix (ECM)-receptor interactions that resulted in rapid egg swelling; a recognized adaptation for lotic habitats. In contrast, in adhesive eggs fertilization initiated the process of sperm-egg fusion and blocking of polyspermy, followed by enhanced protein expression of lipid metabolism and the formation of egg envelope adhesion and hardening, which are adaptive in lentic habitats.

CONCLUSION

Our findings indicate that alternative signal pathways differ between modes of spawning and timing during the key processes of fertilization and egg activation, providing new insights into the molecular mechanisms involved in adaptive early embryonic development in teleost fishes.

Zebrafish stm is involved in the development of otoliths and of the fertilization envelope

Using an in vivo assay, we selected 11 genes that were highly upregulated during the induction of ovulation in zebrafish using microarray analysis and RNA sequencing. The starmaker gene (stm) was one of these genes. Although stm has been previously reported to be involved in otolith formation during the early development of zebrafish, we detected its expression in eggs and showed that stm was related to fertilization by establishing an stm gene knockout strain using the CRISPR/Cas9 system. Further phenotypic analysis of stm knockout fish was conducted in this study. With a higher nonfertilization rate, the stm mutant strain showed an extremely low survival rate. Otoliths of stm homozygous mutant zebrafish showed abnormal morphology in embryos and adult fish. However, fish did not show any abnormalities in swimming behaviour in either embryos or adults. Stm proteins were detected on the chorion of ovulated eggs before spawning. Fibre-supported knob-like structures on the fertilization envelope (FE) also showed abnormal structures in stm mutants. The Stm protein is necessary for otolith formation, and a lack of Stm causes abnormal otolith formation. The partial defect of otolith formation does not cause defects in swimming behaviour. The Stm protein is expressed in the chorion and is responsible for the formation of fibre-supported knob-like structures on the FE. It was suggested that a lack of Stm caused a lower fertilization rate due to inadequate formation of the FE.

Lay summary

In zebrafish, the protein Starmaker (Stm) was identified as having a role in ovulation. Stm is also known to be required for the formation of ear stones (otoliths) which are needed to keep the body in balance. Zebrafish lacking Stm were produced by genome editing. As expected, Stm-deficient fish formed abnormal otoliths. To investigate the role of Stm in ovulation, fertilization and early development, we tried mating of Stm mutants and observed their juveniles. Although no problem found in ovulation, we found low fertilization rate and abnormal structure of knob-like structure (small pit) on the egg membrane. Survival rate of embryos with abnormal egg membrane was extremely low. It was demonstrated that Stm protein is necessary to form the functional egg membrane to protect embryos from the outside environment.

Comparative ultrastructure of the fertilized egg envelope in Corydoras adolfoi and Corydoras sterbai, Callichthyidae, Teleostei

In teleost, the structural characteristics of fertilized egg and egg envelope are very important for classification of genus or species. The structures of fertilized egg and egg envelope from Corydoras adolfoi and Corydoras sterbai, Callichthyidae, Siluriformes in teleost were examined by scanning and transmission electron microscopes to confirm whether these morphological structures have specificities of species and family or not. The fertilized eggs of C. adolfoi and C. sterbai were non-transparent, spherical, demersal, and strong adhesive. There were no structural differences between two species through the light microscope. The size of the fertilized eggs of C. adolfoi was 1.95 ± 0.03 mm (n = 20), and that of C. sterbai was 1.92 ± 0.03 mm (n = 20). The perivitelline space was almost not developed in both species. In both species, the adhesive protuberances structures were on the outer surface of egg envelope. And fibrous structures were specially located at attachment part of spawning bed. And the egg envelope consisted of two layers, an inner lamellae layer and an outer strong adhesive layer with high electron dense protuberances structures in cross section. Consequentially, the fertilized eggs, outer surface on the egg envelope and cross section of egg envelope have identical structure. So, these structural characteristics of fertilized eggs and egg envelope show genus Corydoras specificity.

High-Pressure Freezing Electron Microscopy of Zebrafish Oocytes

Oogenesis is an essential cellular and developmental process to prepare the oocyte for propagation of a species after fertilization. Oocytes of oviparous animals are enormous cells endowed with many, big cellular compartments, which are interconnected through active intracellular transport. The dynamic transport pathways and the big organelles of the oocyte provide the opportunity to study cellular trafficking with outstanding resolution. Hence, oocytes were classically used to investigate cellular compartments. Though many novel regulators of vesicle trafficking have been discovered in yeast, tissue culture cells and invertebrates, recent forward genetic screens in invertebrate and vertebrate oocytes isolated novel control proteins specific to multicellular organisms. Zebrafish is a widely used vertebrate model to study cellular and developmental processes in an entire animal. The transparency of zebrafish embryos allows following cellular events during early development with in vivo imaging. Unfortunately, the active endocytosis of the oocyte also represents a drawback for imaging. The massive amounts of yolk globules prevent the penetration of light-beams and currently make in vivo microscopy a challenge. As a consequence, electron microscopy (EM) still provides the highest resolution to analyze the ultra-structural details of compartments and organelles and the mechanisms controlling many cellular pathways of the oocyte. Among different fixation approaches for EM, High Pressure Freezing (HPF) in combination with freeze substitution significantly improves the samples preservation closest to their natural status. Here, we describe the HPF with freeze substitution embedding method for analyzing cellular processes in zebrafish oocytes using electron microscopy.

Hecate/Grip2a acts to reorganize the cytoskeleton in the symmetry-breaking event of embryonic axis induction

Maternal homozygosity for three independent mutant hecate alleles results in embryos with reduced expression of dorsal organizer genes and defects in the formation of dorsoanterior structures. A positional cloning approach identified all hecate mutations as stop codons affecting the same gene, revealing that hecate encodes the Glutamate receptor interacting protein 2a (Grip2a), a protein containing multiple PDZ domains known to interact with membrane-associated factors including components of the Wnt signaling pathway. We find that grip2a mRNA is localized to the vegetal pole of the oocyte and early embryo, and that during egg activation this mRNA shifts to an off-center vegetal position corresponding to the previously proposed teleost cortical rotation. hecate mutants show defects in the alignment and bundling of microtubules at the vegetal cortex, which result in defects in the asymmetric movement of wnt8a mRNA as well as anchoring of the kinesin-associated cargo adaptor Syntabulin. We also find that, although short-range shifts in vegetal signals are affected in hecate mutant embryos, these mutants exhibit normal long-range, animally directed translocation of cortically injected dorsal beads that occurs in lateral regions of the yolk cortex. Furthermore, we show that such animally-directed movement along the lateral cortex is not restricted to a single arc corresponding to the prospective dorsal region, but occur in multiple meridional arcs even in opposite regions of the embryo. Together, our results reveal a role for Grip2a function in the reorganization and bundling of microtubules at the vegetal cortex to mediate a symmetry-breaking short-range shift corresponding to the teleost cortical rotation. The slight asymmetry achieved by this directed process is subsequently amplified by a general cortical animally-directed transport mechanism that is neither dependent on hecate function nor restricted to the prospective dorsal axis.

One of the earliest and most crucial events in animal development is the establishment of the embryonic dorsal axis. In amphibians and fish, this event depends on the transport of so-called “dorsal determinants” from one region of the egg, at the pole opposite from the site where the oocyte nucleus lies, towards the site of axis induction. There, the dorsal determinant activates the Wnt signaling pathway, which in turn triggers dorsal gene expression. Dorsal determinant transport is mediated by the reorganization of a cellular network composed of microtubules. We determine that hecate, a zebrafish gene active during egg formation that is essential for embryonic axis induction, is required for an early step in this microtubule reorganization. We find that hecate corresponds to glutamate receptor interacting protein 2a, which participates in other animal systems in Wnt-based pathways. We also show that the microtubule reorganization dependent on hecate results in a subtle symmetry-breaking event that subsequently becomes amplified by a more general transport process independent of hecate function. Our data reveal new links between glutamate receptor interacting protein 2a, Wnt signaling and axis induction, and highlights basic mechanisms by which small changes early in development translate into global changes in the embryo.

hnRNP I is required to generate the Ca2+ signal that causes egg activation in zebrafish

Egg activation is an important cellular event required to prevent polyspermy and initiate development of the zygote. Egg activation in all animals examined is elicited by a rise in free Ca(2+) in the egg cytosol at fertilization. This Ca(2+) rise is crucial for all subsequent egg activation steps, such as cortical granule exocytosis, which modifies the vitelline membrane to prevent polyspermy. The cytosolic Ca(2+) rise is primarily initiated by inositol 1,4,5-trisphosphate (IP(3))-mediated Ca(2+) release from the endoplasmic reticulum. The genes involved in regulating the IP(3)-mediated Ca(2+) release during egg activation remain largely unknown. Here we report on a zebrafish maternal-effect mutant, brom bones, which is defective in the cytosolic Ca(2+) rise and subsequent egg activation events, including cortical granule exocytosis and cytoplasmic segregation. We show that the egg activation defects in brom bones can be rescued by providing Ca(2+) or the Ca(2+)-release messenger IP(3), suggesting that brom bones is a regulator of IP(3)-mediated Ca(2+) release at fertilization. Interestingly, brom bones mutant embryos also display defects in dorsoventral axis formation accompanied by a disorganized cortical microtubule network, which is known to be crucial for dorsal axis formation. We provide evidence that the impaired microtubule organization is associated with non-exocytosed cortical granules from the earlier egg activation defect. Positional cloning of the brom bones gene reveals that a premature stop codon in the gene encoding hnRNP I (referred to here as hnrnp I) underlies the abnormalities. Our studies therefore reveal an important new role of hnrnp I in regulating the fundamental process of IP(3)-mediated Ca(2+) release at egg activation.

Adenosine triphosphate levels in steelhead (Oncorhynchus mykiss) eggs: an examination of turnover, localization and role

The dynamics of energy production and utilization in fish eggs before and shortly after fertilization may be critical for embryo survival. Therefore, the current study examined the turnover of adenosine triphosphate (ATP) as well as examined the possible role and localization of ATP in unfertilized steelhead (Oncorhynchus mykiss) eggs and early embryos. The mean ATP level in unfertilized steelhead eggs was 1.92+/-0.10 (mean+/-S.E.M., n=17) nmol ATP per egg. Exposure of the unfertilized egg to 10 degrees C water (water activation) and fertilization resulted in comparable and substantial decreases (approx. 20-50%) in egg ATP levels within 3 min. This suggests that the energy expended at fertilization is used in response to water activation rather than fertilization per se. Unfertilized eggs maintained in ovarian fluid for 9 days at 10 degrees C under air showed a progressive decline of fertility that reached zero after 6 days. In contrast, no significant changes were seen in ATP levels throughout this 9 days period. Thus, fertility does not positively correlate with egg ATP levels in stored eggs. In the unfertilized egg, the ATP stored in the yolk accounted for approximately 1.5% of the total egg ATP. After fertilization, the concentration of ATP in the yolk increased approximately seven-fold, with the yolk and blastoderm each now accounting for approximately 20% of the total remaining ATP. Finally, to estimate the changes in oxidative metabolism following fertilization, the cyanide (KCN)-sensitive decline in total ATP was determined for unfertilized eggs and 1 day embryos. In the presence of KCN, ATP levels declined to approximately 50% within 24 h in both unfertilized eggs as well as embryos; the rates of ATP decline were not different. Therefore, there was not a discernible increase in ATP generation by oxidative phosphorylation at the time of fertilization.

The biology of cortical granules

An egg-that took weeks to months to make in the adult-can be extraordinarily transformed within minutes during its fertilization. This review will focus on the molecular biology of the specialized secretory vesicles of fertilization, the cortical granules. We will discuss their role in the fertilization process, their contents, how they are made, and the molecular mechanisms that regulate their secretion at fertilization. This population of secretory vesicles has inherent interest for our understanding of the fertilization process. In addition, they have import because they enhance our understanding of the basic processes of secretory vesicle construction and regulation, since oocytes across species utilize this vesicle type. Here, we examine diverse animals in a comparative approach to help us understand how these vesicles function throughout phylogeny and to establish conserved themes of function.

Reorganization of filamentous actin and myosin-II in zebrafish eggs correlates temporally and spatially with cortical granule exocytosis

The zebrafish egg provides a useful experimental system to study events of fertilization, including exocytosis. We show by differential interference contrast videomicroscopy that cortical granules are: (1) released nonsynchronously over the egg surface and (2) mobilized to the plasma membrane in two phases, depending upon vesicle size and location. Turbidometric assay measurements of the timing and extent of exocytosis revealed a steady release of small granules during the first 30 seconds of egg activation. This was followed by an explosive discharge of large granules, beginning at 30 seconds and continuing for 1–2 minutes. Stages of single granule exocytosis and subsequent remodeling of the egg surface were imaged by either real-time or time-lapse videomicroscopy as well as scanning electron microscopy. Cortical granule translocation and fusion with the plasma membrane were followed by the concurrent expansion of a fusion pore and release of granule contents. A dramatic rearrangement of the egg surface followed exocytosis. Cortical crypts (sites of evacuated granules) displayed a purse-string-like contraction, resulting in their gradual flattening and disappearance from the egg surface. We tested the hypothesis that subplasmalemmal filamentous (F-) actin acts as a physical barrier to secretion and is locally disassembled prior to granule release. Experimental results showed a reduction of rhodamine-phalloidin and antimyosin staining at putative sites of secretion, acceleration of the timing and extent of granule release in eggs pretreated with cytochalasin D, and dose-dependent inhibition of exocytosis in permeabilized eggs preincubated with phalloidin. An increase in assembled actin was detected by fluorometric assay during the period of exocytosis. Localization studies showed that F-actin and myosin-II codistributed with an inward-moving, membrane-delimited zone of cytoplasm that circumscribed cortical crypts during their transformation. Furthermore, cortical crypts displayed a distinct delay in transformation when incubated continuously with cytochalasin D following egg activation. We propose that closure of cortical crypts is driven by a contractile ring whose forces depend upon dynamic actin filaments and perhaps actomyosin interactions.

Permeability of dechorionated one-cell and six-somite stage zebrafish (Brachydanio rerio) embryos to water and methanol

The permeability of dechorionated one-cell and six-somite stage zebrafish (Brachydanio rerio) embryos to water and the cryoprotectant methanol at 22 degreesC was studied, using real-time video microscopy to determine the volumetric changes of the embryos during cryoprotectant exposure. The equilibrium volumetric behavior of the embryos and the Boyle-van't Hoff relationships were determined using sucrose as a nonpermeating compound. These showed the embryos to behave as nearly ideal osmometers over the range of 253-1724 mOsm, with osmotically inactive volumes of 72.9 and 82.6% for one-cell and six-somite stage embryos, respectively. The Boyle-van't Hoff relationship of the ovary eggs was also determined for comparison and their osmotically inactive volume was 63.9%. The Kedem-Katchalsky parameters of water permeability (Lp), cryoprotectant permeability (Ps), and reflection coefficient (sigma) were determined using DIFFCHAM software. The parameters reported in this study are phenomenological parameters referring to the overall embryo response. The mean values of these parameters were Lp = 0.34 and 0.35 (micrometer/min*atm), Ps = 0.45 and 0.04 (micrometer/s), and sigma = 0.88 and 0.93 for one-cell and six-somite stage embryos, respectively. While the water permeability of the dechorionated zebrafish embryos at different developmental stages remained relatively stable, the permeability to the cryoprotectant methanol (Ps) appeared to decrease during embryo development. The Ps and sigma values for methanol are the first reported for dechorionated fish embryos at these stages.

Long-lasting exocytosis and massive structural reorganisation in the egg periphery during cortical reaction in Platynereis dumerilii (Annelida, Polychaeta)

The course of the cortical reaction in the Platynereis dumerilii egg is described from live observation and from sectioned fixed material and is found to differ in several aspects from the course of cortical reactions in better-known systems. Cortical granules are unusually numerous. They are discharged by exocytosis during a period of about 25 min following fertilisation (18 degrees C). Most of the surplus membrane material brought to the egg surface by exocytosis is set free into the perivitelline space. Swelling of egg jelly precursor secreted by cortical granule exocytosis may be causal for the detachment of the vitelline envelope from the egg cell surface which, however, remains attached punctately to the vitelline envelope by about 30,000 microvilli. Under the strain of the distending vitelline envelope, the bases of the microvilli move and line up, pulling the cell surface into a network of ridges. The grooves in between the ridges are the sites of exocytoses. Cytochalasin B, generally destabilising actin filaments, induces rupture of the microvilli and exaggerated distension of the vitelline envelope during the cortical reaction. In a final phase of the cortical reaction the vitelline envelope wrinkles and falls back onto the egg cell surface, the microvilli shorten and the egg cell transiently becomes deformed by local contractions. The cortical reaction in the nereid egg is discussed as a process of distortion and reorganisation of the egg cortex and plasmalemma. The abundance of cortical granules accommodating egg jelly precursor in the Platynereis oocyte is attributed to the mode of so-called diffuse oogenesis characteristic of nereids, i.e. of differentiation of oocytes freely suspended in the coelomic fluid. In nereids, egg jelly therefore forms after fertilisation as opposed to ovulation.

The sperm entry site during fertilization of the zebrafish egg: localization of actin

The sperm entry site (SES) of zebrafish (Brachydanio rerio) eggs was studied before and during fertilization by fluorescence, scanning, and transmission electron microscopy. Rhodamine phalloidin (RhPh), used to detect polymerized filamentous actin, was localized to microvilli of the SES and to cytoplasm subjacent to the plasma membrane in the unfertilized egg. The distribution of RhPh staining at the SES correlated with the ultrastructural localization of a submembranous electrondense layer of cortical cytoplasm approximately 500 nm thick and containing 5- to 6-nm filaments. Actin, therefore, was organized at the SES as a tightly knit meshwork of filaments prior to fertilization. Contact between the fertilizing sperm and the filamentous actin network was observed by 15-20 sec postinsemination or just before the onset of fertilization cone formation. Growing fertilization cones of either artificially activated or inseminated eggs exhibited intense RhPh staining and substantial increase in thickness of the actin meshwork. Collectively, TEM and RhPh fluorescence images of inseminated eggs demonstrated that the submembranous actin became rearranged in fertilization cones to form a thickened meshwork around the sperm nucleus during incorporation. The results reported here suggest that activation of the egg triggers a dramatic polymerization of actin beneath the plasma membrane of the fertilization cone. Furthermore, the actin involved in sperm incorporation is sensitive to the action of cytochalasins.

Extracellular Mg2+ induces a loss of microvilli, membrane retrieval, and the subsequent cortical reaction, in the oocyte of the prawn Palaemon serratus

Surface changes induced by sea water were analyzed in the ovulated oocyte of the prawn Palaemon serratus. They depended on the presence of external Mg2+ but not on external Ca2+ alone. Increasing external Mg2+ from 0 mM to 30 mM stimulated first a progressive disappearance of preexisting microvilli, which was over within 30 min of incubation. This is correlated with membrane removal via internalization of coated vesicles, ascertained by observations of endocytosis of an extracellular fluid-phase marker and by measurement of a diminution in membrane capacitance (Cm). Thirty-five minutes after sea water contact, the prawn oocyte underwent a cortical reaction independent of fertilization. It consists in a heavy exocytosis of ring-shaped elements, leading to the deposition of a thick capsule, and requiring a threshold Mg2+ concentration of greater than or equal to 10 mM and at least a 3-min incubation with Mg2+. Concurrently, the values of the membrane capacitance (Cm) and conductance (Gm) increased about 2 and 10 times their initial values, respectively. The calcium ionophore ionomycin, added to Mg(2+)-free artificial sea water, stimulated the cortical reaction with requirement of external Ca2+. Other divalent cations (Mn2+, Zn2+, Co2+, Ni2+, Cd2+) instead of Mg2+, induced the cortical reaction, but Ba2+, Sr2+, and La3+ did not. When eggs are fertilized, the cortical reaction takes place in two steps, the first being a discrete exocytosis of a foamy material and the second always involving ring-shaped elements.

An electron-microscope and freeze-fracture study of the egg cortex of Brachydanio rerio

We have examined the cortex of the teleost (Brachydanio rerio) egg before and during exocytosis of cortical granules by scanning, transmission, and freeze-fracture electron microscopy. In the unactivated egg, the P-face of the plasma membrane exhibits a random distribution of intramembranous particles, showing a density of 959/micron2 and an average diameter of 8 nm. Particles over P- and E-faces of the membranes of cortical granules are substantially larger and display a significantly lower density. An anastomosing cortical endoplasmic reticulum forms close associations with both the plasma membrane of the egg and the membranes of cortical granules. Exocytosis begins with cortical granules pushing up beneath the plasma membrane to form dome-shaped swellings, coupled with an apparent clearing of particles from the site of contact between the apposed membranes. A depression in the particle-free plasma membrane appears to mark sites of fusion and pore formation between cortical granules and plasma membranes. Profiles of exocytotic vesicles undergo a predictable sequence of morphological change, but maintain their identity in the egg surface during this transformation. Coated vesicles form at sites of cortical granule breakdown. Differences in particle density between cortical granules and egg plasma membranes persist during transformation of the exocytotic profiles. This suggests that constituents of the 2 membrane domains remain segregated and do not intermix rapidly, lending support to the view that the process of membrane retrieval is selective (i.e., cortical granule membrane is removed).

Cortical alveolus breakdown in the eggs of the freshwater teleost Rhodeus ocellatus ocellatus

Cortical alveoli and their breakdown in the eggs of the rose bitterling were morphologically investigated. Cortical alveoli of various sizes existed in multiple layers within the cortical cytoplasm of each egg. Two types of cortical alveoli were recognized--one possessed homogeneous contents; the other possessed heterogeneous contents with electron-less dense spots. The eggs were activated by a penetrating spermatozoon or by immersion in fresh water. The immersion of the eggs into fresh water induced a wave of cortical alveolus breakdown (CABD), which was propagated from the vegetal pole to the animal pole. When eggs were inseminated in fresh water, CABD progressed from both the vegetal and the animal poles of the egg, and was complete within about 20 min. The contents of the cortical alveoli were released into the perivitelline space through a large opening formed by the fusion of the plasma and cortical alveolar membranes. For a short time after CABD, the smooth inner surfaces of the cortical alveoli were readily discernible from the rougher original plasma membrane. About 30 min after immersion in fresh water, the inner surfaces of the cortical alveoli were indistinguishable from the original egg surfaces. Aequorin-injected eggs became luminescent immediately after their immersion in fresh water. The luminescence reached a maximum level about 15 min later and subsided after roughly 30 min. The result shows that increase in intracellular free Ca ions occurs during CABD.

Ultrastructural localization of lysosomal enzymes in the egg cortex of Brachydanio

The localization of acid phosphatase (E.C. 3.1.3.2), inorganic trimetaphosphatase (E.C. 3.6.1.2), and aryl sulfatase (E.C. 3.1.6.1) in the cortex of unactivated and activated eggs of Brachydanio was examined by ultrastructural cytochemistry. Using a lead capture method, activity for all three acid hydrolases was demonstrated in organelles of the cortex before and after egg activation. Acid phosphatase (AcPase) reaction product was consistently present in primary lysosomes, secondary lysosomes, multivesicular bodies, and yolk bodies. AcPase activity was absent from mitochondria, profiles of the endoplasmic reticulum, coated pits of exocytosed cortical granules, and coated vesicles. Although most cortical granules of the mature, unactivated egg were unreactive for this enzyme, a few showed AcPase reaction product. It is not clear whether the AcPase-positive granules might be an immature form of cortical granules or a subpopulation of these organelles with lysosomal properties. Most cisternae of the Golgi apparatus did not stain for AcPase; however, reaction product was occasionally localized in a single cisterna as well as several small vesicles at the inner face of the Golgi. The intensity of the reaction product and the pattern of distribution of trimetaphosphatase (Tm-Pase) activity was very similar to that of AcPase. However, TmPase was never observed in cortical granules. Cortices of unactivated and activated eggs showed less overall aryl sulfatase (ArSase) activity when compared with AcPase and TmPase. The presence of ArSase reaction product in lysosomes and multivesicular bodies confirmed the acid hydrolytic nature of these organelles. AcPase and TmPase, and to a lesser extent ArSase, are adequate markers of a cortical lysosomal system in the danio egg.(ABSTRACT TRUNCATED AT 250 WORDS)

Fertilization induces endocytosis in Xenopus eggs

Fertilization-induced endocytosis in Xenopus eggs was shown by direct visualization of fluorescent dye in semithin sections. The eggs were incubated in a medium containing 0.1% Lucifer yellow CH for 20 min before, during and after fertilization and then fixed at different times after fertilization. The eggs incubated during or immediately after fertilization contained fluorescent vesicles in the cortex. These vesicles were mainly distributed in the animal hemisphere.

Scanning electron microscope studies of sperm incorporation into the zebrafish (Brachydanio) egg

Morphological studies on the gametes and entry of the spermatozoan into the egg of the zebra danio, Brachydanio rerio, were conducted primarily with scanning electron microscopy. The spermatozoan showed a spherical head, which lacked an acrosome, a midpiece containing several mitochondria, and a flagellum. Observations of the unfertilized egg confirmed and extended prior studies showing a distinct cluster of microvilli on the plasma membrane, identified as the sperm entry site, beneath the inner micropylar aperture (Hart and Donovan, '83). The fertilizing spermatozoan attached to the sperm entry site within 5 seconds of the mixing of a gamete suspension. Binding to the egg microvilli appeared restricted to the equatorial surface of the spermatozoan. Fusion between the plasma membranes of the interacting gametes was followed by the formation of a distinct, nipple-shaped fertilization cone. The sperm head was partially incorporated into the fertilization cone cytoplasm by 60 seconds postinsemination. The incorporation of the entire sperm head, midpiece, and a portion of the flagellum occurred between 1 and 2 minutes. During this time, the fertilization cone shortened and was transformed into a massive, blister-like cytoplasmic swelling. Concurrently, upward movements of the ooplasm resulted in the gradual disappearance of the original depression in the egg surface containing the sperm entry site. The second polar body, fully developed by 10 minutes postinsemination, formed approximately 10-15 microns from the site of sperm penetration. Development of the fertilization cone, formation of the second polar body and exocytosis of cortical granules at the sperm entry site readily occurred in parthenogenetically activated eggs, indicating that these surface rearrangements do not require sperm binding and/or fusion.