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10.1: Electron carriers - Biology
If oxygen is available, aerobic respiration will go forward. In eukaryotic cells, the pyruvate molecules produced at the end of glycolysis are transported into mitochondria, which are the sites of cellular respiration. There, pyruvate will be transformed into an acetyl group that will be picked up and activated by a carrier compound called coenzyme A (CoA). The resulting compound is called acetyl CoA . CoA is made from vitamin B5, pantothenic acid. Acetyl CoA can be used in a variety of ways by the cell, but its major function is to deliver the acetyl group derived from pyruvate to the next stage of the pathway in glucose catabolism.
Regulation of photosynthetic electron transport
The photosynthetic electron transport chain consists of photosystem II, the cytochrome b(6)f complex, photosystem I, and the free electron carriers plastoquinone and plastocyanin. Light-driven charge separation events occur at the level of photosystem II and photosystem I, which are associated at one end of the chain with the oxidation of water followed by electron flow along the electron transport chain and concomitant pumping of protons into the thylakoid lumen, which is used by the ATP synthase to generate ATP. At the other end of the chain reducing power is generated, which together with ATP is used for CO(2) assimilation. A remarkable feature of the photosynthetic apparatus is its ability to adapt to changes in environmental conditions by sensing light quality and quantity, CO(2) levels, temperature, and nutrient availability. These acclimation responses involve a complex signaling network in the chloroplasts comprising the thylakoid protein kinases Stt7/STN7 and Stl1/STN7 and the phosphatase PPH1/TAP38, which play important roles in state transitions and in the regulation of electron flow as well as in thylakoid membrane folding. The activity of some of these enzymes is closely connected to the redox state of the plastoquinone pool, and they appear to be involved both in short-term and long-term acclimation. This article is part of a Special Issue entitled "Regulation of Electron Transport in Chloroplasts".
Products of Fermentation
While there are a number of products from fermentation, the most common are ethanol, lactic acid, carbon dioxide, and hydrogen gas (H2). These products are used commercially in foods, vitamins, pharmaceuticals, or as industrial chemicals. In addition, many less common products still offer commercial value. For example, the production of acetone via the acetone – butanol – ethanol fermentation was first developed by the Jewish chemist Chaim Weizmann and was important to the British war industry during Word War I.
1. What is the coenzyme regenerated by the process of fermentation?
B. NAD +
D. Lactic acid
2. Which type of fermentation occurs in muscle cells during strenuous exercise?
B. Mixed acid
C. Lactic acid
D. Butyric acid
3. Which chemist famously demonstrated the role of yeast in fermentation?
A. Chaim Weizmann
B. Louis Pasteur
C. Marie Curie
D. Antoine Lavoisier
Most eukaryotic cells have mitochondria, which produce ATP from products of the citric acid cycle, fatty acid oxidation, and amino acid oxidation. At the inner mitochondrial membrane, electrons from NADH and FADH2 pass through the electron transport chain to oxygen, which is reduced to water.  The electron transport chain comprises an enzymatic series of electron donors and acceptors. Each electron donor will pass electrons to a more electronegative acceptor, which in turn donates these electrons to another acceptor, a process that continues down the series until electrons are passed to oxygen, the most electronegative and terminal electron acceptor in the chain. Passage of electrons between donor and acceptor releases energy, which is used to generate a proton gradient across the mitochondrial membrane by "pumping" protons into the intermembrane space, producing a thermodynamic state that has the potential to do work. This entire process is called oxidative phosphorylation since ADP is phosphorylated to ATP by using the electrochemical gradient established by the redox reactions of the electron transport chain.
Mitochondrial redox carriers Edit
Energy obtained through the transfer of electrons down the electron transport chain is used to pump protons from the mitochondrial matrix into the intermembrane space, creating an electrochemical proton gradient (ΔpH) across the inner mitochondrial membrane. This proton gradient is largely but not exclusively responsible for the mitochondrial membrane potential (ΔΨM).  It allows ATP synthase to use the flow of H + through the enzyme back into the matrix to generate ATP from adenosine diphosphate (ADP) and inorganic phosphate. Complex I (NADH coenzyme Q reductase labeled I) accepts electrons from the Krebs cycle electron carrier nicotinamide adenine dinucleotide (NADH), and passes them to coenzyme Q (ubiquinone labeled Q), which also receives electrons from complex II (succinate dehydrogenase labeled II). Q passes electrons to complex III (cytochrome bc1 complex labeled III), which passes them to cytochrome c (cyt c). Cyt c passes electrons to complex IV (cytochrome c oxidase labeled IV), which uses the electrons and hydrogen ions to reduce molecular oxygen to water.
Four membrane-bound complexes have been identified in mitochondria. Each is an extremely complex transmembrane structure that is embedded in the inner membrane. Three of them are proton pumps. The structures are electrically connected by lipid-soluble electron carriers and water-soluble electron carriers. The overall electron transport chain:
Complex I Edit
In complex I (NADH ubiquinone oxireductase, Type I NADH dehydrogenase, or mitochondrial complex I EC 126.96.36.199), two electrons are removed from NADH and transferred to a lipid-soluble carrier, ubiquinone (Q). The reduced product, ubiquinol (QH2), freely diffuses within the membrane, and Complex I translocates four protons (H + ) across the membrane, thus producing a proton gradient. Complex I is one of the main sites at which premature electron leakage to oxygen occurs, thus being one of the main sites of production of superoxide. 
The pathway of electrons is as follows:
NADH is oxidized to NAD + , by reducing Flavin mononucleotide to FMNH2 in one two-electron step. FMNH2 is then oxidized in two one-electron steps, through a semiquinone intermediate. Each electron thus transfers from the FMNH2 to an Fe-S cluster, from the Fe-S cluster to ubiquinone (Q). Transfer of the first electron results in the free-radical (semiquinone) form of Q, and transfer of the second electron reduces the semiquinone form to the ubiquinol form, QH2. During this process, four protons are translocated from the mitochondrial matrix to the intermembrane space.  As the electrons become continuously oxidized and reduced throughout the complex an electron current is produced along the 180 Angstrom width of the complex within the membrane. This current powers the active transport of four protons to the intermembrane space per two electrons from NADH. 
Complex II Edit
In complex II (succinate dehydrogenase or succinate-CoQ reductase EC 188.8.131.52) additional electrons are delivered into the quinone pool (Q) originating from succinate and transferred (via flavin adenine dinucleotide (FAD)) to Q. Complex II consists of four protein subunits: succinate dehydrogenase, (SDHA) succinate dehydrogenase [ubiquinone] iron-sulfur subunit, mitochondrial, (SDHB) succinate dehydrogenase complex subunit C, (SDHC) and succinate dehydrogenase complex, subunit D, (SDHD). Other electron donors (e.g., fatty acids and glycerol 3-phosphate) also direct electrons into Q (via FAD). Complex II is a parallel electron transport pathway to complex 1, but unlike complex 1, no protons are transported to the intermembrane space in this pathway. Therefore, the pathway through complex II contributes less energy to the overall electron transport chain process.
Complex III Edit
In complex III (cytochrome bc1 complex or CoQH2-cytochrome c reductase EC 184.108.40.206), the Q-cycle contributes to the proton gradient by an asymmetric absorption/release of protons. Two electrons are removed from QH2 at the QO site and sequentially transferred to two molecules of cytochrome c, a water-soluble electron carrier located within the intermembrane space. The two other electrons sequentially pass across the protein to the Qi site where the quinone part of ubiquinone is reduced to quinol. A proton gradient is formed by one quinol ( 2 H 2 + e −
When electron transfer is reduced (by a high membrane potential or respiratory inhibitors such as antimycin A), Complex III may leak electrons to molecular oxygen, resulting in superoxide formation.
This complex is inhibited by dimercaprol (British Antilewisite, BAL), Napthoquinone and Antimycin.
Complex IV Edit
In complex IV (cytochrome c oxidase EC 220.127.116.11), sometimes called cytochrome AA3, four electrons are removed from four molecules of cytochrome c and transferred to molecular oxygen (O2), producing two molecules of water. The complex contains coordinated copper ions and several heme groups. At the same time, eight protons are removed from the mitochondrial matrix (although only four are translocated across the membrane), contributing to the proton gradient. The exact details of proton pumping in complex IV are still under study.  Cyanide is an inhibitor of complex 4.
Coupling with oxidative phosphorylation Edit
The chemiosmotic coupling hypothesis, proposed by Nobel Prize in Chemistry winner Peter D. Mitchell, the electron transport chain and oxidative phosphorylation are coupled by a proton gradient across the inner mitochondrial membrane. The efflux of protons from the mitochondrial matrix creates an electrochemical gradient (proton gradient). This gradient is used by the FOF1 ATP synthase complex to make ATP via oxidative phosphorylation. ATP synthase is sometimes described as Complex V of the electron transport chain.  The FO component of ATP synthase acts as an ion channel that provides for a proton flux back into the mitochondrial matrix. It is composed of a, b and c subunits. Protons in the inter-membranous space of mitochondria first enters the ATP synthase complex through a subunit channel. Then protons move to the c subunits.  The number of c subunits it has determines how many protons it will require to make the FO turn one full revolution. For example, in humans, there are 8 c subunits, thus 8 protons are required.  After c subunits, protons finally enters matrix using a subunit channel that opens into the mitochondrial matrix.  This reflux releases free energy produced during the generation of the oxidized forms of the electron carriers (NAD + and Q). The free energy is used to drive ATP synthesis, catalyzed by the F1 component of the complex. 
Coupling with oxidative phosphorylation is a key step for ATP production. However, in specific cases, uncoupling the two processes may be biologically useful. The uncoupling protein, thermogenin—present in the inner mitochondrial membrane of brown adipose tissue—provides for an alternative flow of protons back to the inner mitochondrial matrix. Thyroxine is also a natural uncoupler. This alternative flow results in thermogenesis rather than ATP production. 
Reverse electron flow Edit
Reverse electron flow, is the transfer of electrons through the electron transport chain through the reverse redox reactions. Usually requiring a significant amount of energy to be used, this can result in reducing the oxidised form of electron donors. For example, NAD+ can be reduced to NADH by complex I.  There are several factors that have been shown to induce reverse electron flow. However, more work needs to be done to confirm this. One such example is blockage of ATP production by ATP synthase, resulting in a build-up of protons and therefore a higher proton-motive force, inducing reverse electron flow. 
In eukaryotes, NADH is the most important electron donor. The associated electron transport chain is
NADH → Complex I→ Q → Complex III→ cytochrome c→ Complex IV→ O2 where Complexes I, III and IV are proton pumps, while Q and cytochrome c are mobile electron carriers. The electron acceptor is molecular oxygen.
In prokaryotes (bacteria and archaea) the situation is more complicated, because there are several different electron donors and several different electron acceptors. The generalized electron transport chain in bacteria is:
Electrons can enter the chain at three levels: at the level of a dehydrogenase, at the level of the quinone pool, or at the level of a mobile cytochrome electron carrier. These levels correspond to successively more positive redox potentials, or to successively decreased potential differences relative to the terminal electron acceptor. In other words, they correspond to successively smaller Gibbs free energy changes for the overall redox reaction Donor → Acceptor.
Individual bacteria use multiple electron transport chains, often simultaneously. Bacteria can use a number of different electron donors, a number of different dehydrogenases, a number of different oxidases and reductases, and a number of different electron acceptors. For example, E. coli (when growing aerobically using glucose as an energy source) uses two different NADH dehydrogenases and two different quinol oxidases, for a total of four different electron transport chains operating simultaneously.
A common feature of all electron transport chains is the presence of a proton pump to create an electrochemical gradient over a membrane. Bacterial electron transport chains may contain as many as three proton pumps, like mitochondria, or they may contain only one or two. They always contain at least one proton pump.
Electron donors Edit
In the present day biosphere, the most common electron donors are organic molecules. Organisms that use organic molecules as an electron source are called organotrophs. Organotrophs (animals, fungi, protists) and phototrophs (plants and algae) constitute the vast majority of all familiar life forms.
Some prokaryotes can use inorganic matter as an energy source. Such an organism is called a lithotroph ("rock-eater"). Inorganic electron donors include hydrogen, carbon monoxide, ammonia, nitrite, sulfur, sulfide, manganese oxide, and ferrous iron. Lithotrophs have been found growing in rock formations thousands of meters below the surface of Earth. Because of their volume of distribution, lithotrophs may actually outnumber organotrophs and phototrophs in our biosphere.
The use of inorganic electron donors as an energy source is of particular interest in the study of evolution. This type of metabolism must logically have preceded the use of organic molecules as an energy source.
Complex I and II Edit
Bacteria can use a number of different electron donors. When organic matter is the energy source, the donor may be NADH or succinate, in which case electrons enter the electron transport chain via NADH dehydrogenase (similar to Complex I in mitochondria) or succinate dehydrogenase (similar to Complex II). Other dehydrogenases may be used to process different energy sources: formate dehydrogenase, lactate dehydrogenase, glyceraldehyde-3-phosphate dehydrogenase, H2 dehydrogenase (hydrogenase), electron transport chain. Some dehydrogenases are also proton pumps others funnel electrons into the quinone pool. Most dehydrogenases show induced expression in the bacterial cell in response to metabolic needs triggered by the environment in which the cells grow. In the case of lactate dehydrogenase in E.coli, the enzyme is used aerobically and in combination with other dehydrogenases. It is inducible and is expressed when there is high concentration of DL- lactate present in the cell. [ citation needed ]
Quinone carriers Edit
Quinones are mobile, lipid-soluble carriers that shuttle electrons (and protons) between large, relatively immobile macromolecular complexes embedded in the membrane. Bacteria use ubiquinone (Coenzyme Q, the same quinone that mitochondria use) and related quinones such as menaquinone (Vitamin K2). Archaea in the genus Sulfolobus use caldariellaquinone.  The use of different quinones is due to slightly altered redox potentials. These changes in redox potential are caused by changes in structure of quinone. The Change in redox potentials of these quinones may be suited to changes in the electron acceptors or variations of redox potentials in bacterial complexes. 
Proton pumps Edit
A proton pump is any process that creates a proton gradient across a membrane. Protons can be physically moved across a membrane this is seen in mitochondrial Complexes I and IV. The same effect can be produced by moving electrons in the opposite direction. The result is the disappearance of a proton from the cytoplasm and the appearance of a proton in the periplasm. Mitochondrial Complex III uses this second type of proton pump, which is mediated by a quinone (the Q cycle).
Some dehydrogenases are proton pumps others are not. Most oxidases and reductases are proton pumps, but some are not. Cytochrome bc1 is a proton pump found in many, but not all, bacteria (it is not found in E. coli). As the name implies, bacterial bc1 is similar to mitochondrial bc1 (Complex III).
Cytochrome electron carriers Edit
Cytochromes are pigments that contain iron. They are found in two very different environments.
Some cytochromes are water-soluble carriers that shuttle electrons to and from large, immobile macromolecular structures imbedded in the membrane. The mobile cytochrome electron carrier in mitochondria is cytochrome c. Bacteria use a number of different mobile cytochrome electron carriers.
Other cytochromes are found within macromolecules such as Complex III and Complex IV. They also function as electron carriers, but in a very different, intramolecular, solid-state environment.
Electrons may enter an electron transport chain at the level of a mobile cytochrome or quinone carrier. For example, electrons from inorganic electron donors (nitrite, ferrous iron, electron transport chain.) enter the electron transport chain at the cytochrome level. When electrons enter at a redox level greater than NADH, the electron transport chain must operate in reverse to produce this necessary, higher-energy molecule.
Terminal oxidases and reductases Edit
When bacteria grow in aerobic environments, the terminal electron acceptor (O2) is reduced to water by an enzyme called an oxidase. When bacteria grow in anaerobic environments, the terminal electron acceptor is reduced by an enzyme called a reductase. In mitochondria the terminal membrane complex (Complex IV) is cytochrome oxidase. Aerobic bacteria use a number of different terminal oxidases. For example, E. coli (a facultative anaerobe) does not have a cytochrome oxidase or a bc1 complex. Under aerobic conditions, it uses two different terminal quinol oxidases (both proton pumps) to reduce oxygen to water.
Bacterial Complex IV can be split into classes according to the molecules act as terminal electron acceptors. Class I oxidases are cytochrome oxidases and use oxygen as the terminal electron acceptor. Class II oxidases are Quinol oxidases and can use a variety of terminal electron acceptors. Both of these classes can be subdivided into categories based on what redox active components they contain. E.g. Heme aa3 Class 1 terminal oxidases are much more efficient than Class 2 terminal oxidases 
Anaerobic bacteria, which do not use oxygen as a terminal electron acceptor, have terminal reductases individualized to their terminal acceptor. For example, E. coli can use fumarate reductase, nitrate reductase, nitrite reductase, DMSO reductase, or trimethylamine-N-oxide reductase, depending on the availability of these acceptors in the environment.
Most terminal oxidases and reductases are inducible. They are synthesized by the organism as needed, in response to specific environmental conditions.
Electron acceptors Edit
Just as there are a number of different electron donors (organic matter in organotrophs, inorganic matter in lithotrophs), there are a number of different electron acceptors, both organic and inorganic. In aerobic bacteria and facultative anaerobes if oxygen is available, it is invariably used as the terminal electron acceptor, because it generates the greatest Gibbs free energy change and produces the most energy. 
In anaerobic environments, different electron acceptors are used, including nitrate, nitrite, ferric iron, sulfate, carbon dioxide, and small organic molecules such as fumarate.
In oxidative phosphorylation, electrons are transferred from a low-energy electron donor such as NADH to an acceptor such as O2) through an electron transport chain. In photophosphorylation, the energy of sunlight is used to create a high-energy electron donor which can subsequently reduce redox active components. These components are then coupled to ATP synthesis via proton translocation by the electron transport chain. 
Photosynthetic electron transport chains, like the mitochondrial chain, can be considered as a special case of the bacterial systems. They use mobile, lipid-soluble quinone carriers (phylloquinone and plastoquinone) and mobile, water-soluble carriers (cytochromes, electron transport chain.). They also contain a proton pump. The proton pump in all photosynthetic chains resembles mitochondrial Complex III. The commonly-held theory of symbiogenesis believes that both organelles descended from bacteria.
COVID-19 Prognosis: Mitochondrial Perspective
Higher age (>65 years), type 2 diabetes, obesity (body mass index [BMI] >30), hypertension, and comorbid illness (chronic renal disease, liver disease, cancer, immunosuppression, heart disease) have consistently been shown to be associated with poor prognosis (Wolff et al., 2021). Not surprisingly, most of these conditions are associated with mitochondrial dysfunction. Aging and age related diseases are associated with chronic sterile systemic inflammation also called “inflammaging” (Ferrucci and Fabbri, 2018). It is characterized by increased pro-inflammatory markers in blood. Chronic inflammation is sustained by various stimuli, including increased gut permeability, chronic infections, and cellular senescence (Ferrucci and Fabbri, 2018 Franceschi et al., 2018). Declining mitochondrial function with age and consequently increasing oxidative stress is an important contributor to inflammaging (Conte et al., 2020). Similarly, obesity, fatty liver disease, and type 2 diabetes (particularly when they are together as components of metabolic syndrome) are associated with mitochondrial dysfunction, oxidative stress, and systemic inflammation (Galvan et al., 2017 Lahera et al., 2017 de Mello et al., 2018 Silzer and Phillips, 2018 Simões et al., 2018 Yaribeygi et al., 2019 Prasun, 2020). In addition, these conditions are characterized by altered mitochondrial dynamics and exaggerated mitochondrial fission leading to fragmented dysfunctional mitochondria (Liu et al., 2020). These relatively unhealthy mitochondria are unlikely to produce effective interferon response as explained above in the pathogenesis section. Moreover, hijacking by SARS-CoV-2 may further compromise mitochondrial integrity, deteriorate mitochondrial function, and increase oxidative stress. There is increased ROS generation and release of oxidized mitochondrial DNA into cytoplasm, which aggravate inflammation by activating inflammasome (Sorbara and Girardin, 2011 Liu et al., 2018). Inflammasome activation leads to production of pro-inflammatory cytokines (Interleukin 1β and 12) and predisposition to pyroptosis (Kesavardhana and Kanneganti, 2017). Pyroptosis is inflammation generating form of cell death. Cell-free mitochondrial DNA released after pyroptosis further aggravates local and systemic inflammation (Riley and Tait, 2020). Thus, the preexisting hyperinflammatory condition is aggravated by mitochondrial dysfunction and its worsening during the course of infection. SARS-CoV-2 infection in these high risk conditions is akin to adding fuel to the fire (Fig. 2).
Apart from the abovementioned risk factors, male gender is independently associated with poor prognosis in severe COVID-19 (Jin et al., 2020). Estrogen, the female sex hormone, promotes mitochondrial biogenesis (Lejri et al., 2018), while a low estrogen status is associated with dysfunctional fragmented mitochondria (López-Lluch, 2017). This may explain the gender difference in prognosis.
In one case series, high lactate was observed in all fatal cases of COVID-19 (Li et al., 2020b). Mitochondrial dysfunction and decline in oxidative phosphorylation together with hypoxia due to pulmonary involvement favor glycolysis and lactate accumulation. Accumulation of lactate is detrimental for MAVS function and may further decrease interferon response (Zhang et al., 2019). Thus, mitochondrial dysfunction favors viral replication and simultaneously aggravates inflammation (Burtscher et al., 2020).
Steps of Oxidative Phosphorylation
Before the Electron Transport Chain
For the electron transport chain to be able to pump protons to one side of the mitochondrial inner membrane, it must first have a source of those electrons and protons. There are several cellular processes which lead to the oxidation (“burning”) of various cellular food sources. These processes include glycolysis, the citric acid cycle, the fatty acid beta-oxidation metabolism, and the oxidation of amino acids.
All of these processes involve the transfer of electrons and protons to coenzymes. The most common coenzymes are nicotinamide adenine dinucleotide (NAD) and flavin adenine dinucleotide (FAD). NAD can be reduced with electrons and a proton to become NADH, while FAD can take on two protons and four electrons to become FADH2. These coenzymes can bind to the proteins of the electron transport chain, and transfer their electrons and protons. This becomes the first stage in the electron transport chain.
Within the Electron Transport Chain
The electron transport chain consists of four protein complexes, simply named complex I, complex II, complex III, and complex IV. Each complex is designed to receive electrons from a coenzyme or one of the other complexes in the chain. The actions each complex takes can be seen in the image below.
Complex I is responsible for relieving NADH of its hydrogen and electrons. The energy received by taking the electrons allows complex I to pump the hydrogen atom through the inner mitochondrial membrane, which concentrates hydrogens in the intermembrane space. The electrons are then passed to coenzyme Q (CoQ). CoQ can take on hydrogens and electrons, and can be reduced to CoQH2. The coenzyme transfers the electrons to complex III.
At this point, the electron transport chain has built up a large number of hydrogen ions in the intermembrane space. It did this with the energy it received through passing electrons through a series of energy releasing reactions. The final step of oxidative phosphorylation is the production of ATP, or the process of phosphorylation.
This process takes place in a complex called ATP synthase. This large complex uses the proton-motive force to attach phosphate groups to ADP molecules. Because there are so many protons built up in the intermembrane space, they want to push their way to the other side. ATP synthase uses this energy to undergo a conformational change. In doing so, it forces the ATD and phosphate group together, and reduces the energy they need to bond. ATP can then go on to fuel reactions all over the cell, when it is exported from the mitochondria.
Video Sequence 1: Movement of Lycopodium Spores in Oviductal Ciliary Currents
A dissected hamster infundibulum has been mounted in a tissue culture dish to examine movement of Lycopodium spores on its surface (Knoll and Talbot, 1998). Cilia cover the entire surface of the infundibulum but are in focus only at the periphery. In this video sequence, most spores tumble over the surface of the infundibulum in ciliary currents, while several spores are either stuck in crevices or are attached to cilia and remain stationary (Figure2A). Oviductal cilia beat at 6–8 hertz when cultured at this temperature (DiCarlantonio et al., 1995). Because spores are relatively small particles, they are transferred rapidly to the ostium. Spores move over the surface of the hamster infundibulum at a rate of ∼70–100 μm/s when incubation is done at ambient temperature.
Fig. 2. Movies showing movement ofLycopodium spores and OCCs on the surface of hamster infundibula. (A) Lycopodium spores being picked up by currents above hamster oviductal cilia. (B) Stained OCC traveling over the surface of a hamster infundibulum. (C) Bird’s eye view of the ostium showing entry of a stained OCC. (D) Higher-magnification video showing movement of an OCC into the ostium. (E) Video showing pickup of an OCC that has been recovered from inside an infundibulum. The matrix of the OCC has been compacted and is no longer elastic. (F) Pair of movies showing the effect of cigarette smoke on OCC pickup. The infundibulum on the left has been exposed to sidestream smoke solution, whereas the one on the right was incubated in control medium only.
Video Sequence 2: Movement of an OCC on the Surface of the Oviduct
Cilia on the outer surface of the infundibulum normally function in picking up the OCC after ovulation and transferring it rapidly into the lumen of the oviduct (Figure 2B). A methylene blue–stained OCC has been placed on a hamster infundibulum and viewed with a stereoscopic microscope. Cells of the OCC are stained blue and are separated by extracellular matrix, which is unstained. Pickup of the OCC by the oviduct occurs in two phases. First the OCC attaches to the tips of the cilia and glides over the surface of the infundibulum to the ostium at a rate of 50–60 μm/s at ambient temperature (Huang et al., 1997). The video shows this event in approximately real time. Then the OCC enters the ostium and undergoes churning activity as it becomes compacted to a size that can be accommodated by the lumen of the infundibulum. The video shows entrance of the OCC into the ostium approximately five times faster than real time. When positioned at the same starting place on an infundibulum, OCCs will repeatedly travel over the same path on the surface each time (Huang et al., 1997).
Only a small fraction of the OCC surface touches the cilia at any one time. Because the matrix is elastic, it stretches out in front and behind the main body of the OCC, thereby increasing the area of contact and improving adhesion. The extension of cells and matrix at the leading edge of the OCC is characteristic of normal pickup, and the extension is easily able to penetrate into the narrow opening of the ostium. The cells and matrix trailing the OCC remain attached to cilia as the main body of the OCC moves away from them toward the ostium. Eventually the trailing matrix snaps either because it releases from the cilia or because it tears. Tearing is known to occur as scanning electron micrographs have revealed matrix trails left by OCCs on infundibular surfaces. Movement of the matrix extending from the leading edge into the ostium is important in anchoring the OCC to the oviduct as the main body of the OCC makes a 180° turn and enters the ostium. The main body of the OCC is usually too large to move directly into the lumen of the infundibulum, and the OCC churns while the cilia inside the infundibulum pull on the its matrix and compact the OCC into a smaller size that can pass through the ostium.
Video Sequence 3: Movement of an OCC into the Ostium
This camera view looks down the oviduct and presents a bird’s eye view of an OCC entering the ostium (Figure 2C). This process takes longer than movement of the OCC over the surface of the oviduct and is therefore shown approximately six times faster than real time. The OCC is large relative to the size of the ostium, which often is completely closed as entry begins. At the beginning of this sequence, several cells and the matrix at the leading edge enter the right side of the ostium. As cilia pull the matrix, the OCC churns until it becomes spherical and sufficiently compacted to enter the ostium completely. The oocyte, which is located in the center of the OCC at the beginning of this sequence, is squeezed to the periphery during churning and enters the ostium last. This explains why OCCs recovered from the ampulla of the oviduct often have eccentrically located oocytes (Corselli and Talbot, 1987). Repositioning the oocyte to the periphery of the OCC potentially shortens the distance a sperm must pass through the matrix before reaching the zona pellucida of the oocyte. As a result of churning, the OCC has a smaller diameter, and the matrix is compacted and less elastic.
Video Sequence 4: Entrance into the Ostium
This video was made at higher magnification to better show the interface between cells of the OCC and the surface of the infundibulum as the OCC is entering the ostium (Figure 2D). Because this infundibulum has already picked up an OCC, the ostium is partially open and easier to visualize. Cumulus cells, but not the matrix between them, are stained with methylene blue. Cumulus cells glide over the surface of the cilia. Cells and matrix at the leading edge move out ahead of the main body of the OCC and enter the ostium first. As the leading cells and matrix penetrate deeper into the lumen of the infundibulum, adhesion between the main body of the OCC and cilia on the surface is disrupted, and the main body snaps forward and is drawn down into the infundibulum. Adhesion between the cilia and matrix of the OCC ensures that the OCC successfully makes a 180° turn as it enters the ostium without falling off the oviduct.
Video Sequence 5: Effect of Insufficient Adhesion on OCC Pickup
This sequence shows pickup of an OCC that has been recovered from the inside of an infundibulum (Figure 2E). Because the OCC has been compacted, it is smaller in diameter, its cells are difficult to resolve, and its matrix is less elastic. The matrix is still somewhat adhesive and can attach to the cilia well enough to allow the OCC to be pulled along an edge of the infundibulum. However, the matrix is not elastic enough to enable an extension of matrix to form at the leading edge of the OCC. As a result, the OCC is not adhering well to the cilia when it approaches the ostium, and rather than make a 180° angle turn to enter the ostium, the OCC falls off the oviduct. This sequence illustrates that matrix elasticity is necessary for formation of an adhesive extension at the leading edge of the OCC and that this extension is important for turning into the entrance of the oviduct.
Video Sequence 6: Effect of Cigarette Smoke on OCC Pickup
Cigarette smoke causes the oviductal ciliary to beat frequency and oocyte pickup rate to decrease (Knoll et al., 1995 Knoll and Talbot, 1998). When smoke-treated infundibula are placed in fresh culture media, the ciliary beat frequency recovers, but unexpectedly, the OCC pickup rate does not recover and may continue to decrease (Knoll and Talbot, 1998). This decrease in pickup rate appears to be related to a change in the adhesive interaction between the cilia and OCC matrix. The video sequence on the right shows a control infundibulum and OCC moving at normal speed during a 10-s interval. The sequence on the left shows an infundibulum pretreated with sidestream smoke solution. Although the ciliary beat frequency has recovered from the smoke treatment, the OCC pickup rate continues to be depressed, and in this 10-s sequence, the OCC on the treated infundibulum moves very little. The OCC on the treated infundibulum eventually moved to the ostium, but it took >3.5 min. After longer exposures to smoke solutions, OCCs are not picked up at all, despite the fact that cilia beat at normal frequency (Knoll and Talbot, 1998).
This research was supported by the NSF (MCB-1158571) and US Department of Agriculture National Institute of Food and Agriculture Hatch projects 0119 and Hatch Umbrella Project 1015621. The EM work was supported by a Department of Energy (DOE)–Basic Energy Sciences grant (DOE-DE-SC0017160). M.H. gratefully acknowledges institutional support (RVO:60077344) and financial support by the ALGAMAN Project CZ.1.07/2.3.00/20.0203 cofinanced by the European Social Fund. M.P. and D.L. acknowledge support from Deutsche Forschungsgemeinschaft: LE 1265/29-1 (FOR 2092) and TRR 175. Finally, we thank Leonardo Curiel for his help with fluorescence induction measurements.