Mitochondria are the energy-producing organelles of eukaryotic cells. They produce ATP through a series of complex redox reactions called oxidative phosphorylation. The process involves transferring reducing equivalents across the mitochondrial inner membrane.
The cytoplasm of sperm contains numerous mitochondria. They are arranged in a spiral around the axoneme and define a region known as the midpiece. Variations in midpiece morphometry affect sperm motility and deserve further investigation.
Mitochondria are located in the tail
Mitochondria are a kind of organelle in the cell that converts chemical energy into a form that can be used by cells. This process is known as oxidative phosphorylation (OXPHOS). The energy in mitochondria comes from oxygen molecules. These molecules are passed across a highly complex membrane inside the mitochondria to create adenosine triphosphate (ATP). This energy is then used by other enzymes in the cell to perform its functions.
Although ATP production is the primary function of mitochondria, they also play a significant role in other metabolic pathways. These include citric acid cycle (Krebs cycle), oxidative decarboxylation of a-keto acids, fatty acid synthesis and amino acid metabolism. In addition, they are involved in calcium signalling, iron metabolism and apoptosis.
During spermogenesis, mitochondria are surrounded by a structure called the mitochondrial sheath. The sheath is made up of proteins that make it mechanically stable and resistant to the hypo-osmotic environment. The sheath also protects the mitochondria from oxidative damage.
The most important protein in the sheath is the peptide phospholipid hydroperoxide glutathione peroxidase (PHGPx). This protein is thought to protect the mitochondria from oxidative damage by forming disulfide bonds between cysteine and proline residues. PHGPx is a critical part of the permeability barrier of the mitochondrial inner membrane. The permeability barrier impedes the passage of most polar molecules, including protons.
They are located in the axoneme
Mitochondria are cellular organelles that provide energy for sperm motility. They also play a role in regulating programmed cell death. Their vital functions make them the guardians of the gate between life and death. Similar to Janus, mitochondria appear to have two faces, one looking forward and the other backward. Their front face is involved in cellular bio-energetic and ion homeostasis, while their back is active in the regulation of apoptosis.
In sperm cells, mitochondria are arranged in a tight spiral that is strategically located in the tail of the sperm and close to the head, defining a region called the midpiece. This configuration enables efficient energy coupling in sperm, which is essential for a successful fertilization. Variations in the morphometry of the midpiece affect sperm competitiveness and motility. Mitochondria are among the few organelles retained throughout sperm maturation, during which they otherwise lose most of their cytoplasm and other organelles.
Researchers used a combination of imaging and image processing to analyze the structure of mitochondria in sperm cells. They discovered that neighbouring mitochondria were ‘glued’ together through an array of proteins on the outer membrane. These structures are likely to serve not only structural roles but also act as conducting channels for coordinating metabolite shuttling, ATP synthesis, and calcium signaling in the entire midpiece.
Moreover, the researchers found that a specialized protein array, known as the mitochondrial sheath, surrounded each mitochondrial cluster. The sheath is a matrix of cristae, which are narrow tubes that extend almost to the edge of the inner mitochondrial membrane. These tubes are lined with a special phospholipid, cardiolipin, which is capable of binding Ca2+ molecules and facilitating passage of ions across the mitochondrial membrane.
They are located in the cytoplasm
Sperm cells need lots of energy to reach the egg cell and transfer their genetic material during fertilization. They have many mitochondria, which are the powerhouses of the cell. Scientists have discovered that these organelles are arranged in different ways in sperm cells of different species. This could explain why some sperm cells are more fertile than others.
Researchers from Utrecht University have analyzed sperm mitochondria in unprecedented detail. They found that these vital energy producers have different shapes and internal arrangements in sperm cells of different species. They also discovered the molecular pins that hold these organelles together. The research has important implications for sperm health and fertility.
The mitochondria is a cytoplasmic organelle with two membranes that separate it into compartments. The outer membrane is thought to be derived from the cell membrane of the host that engulfed the mitochondrion, while the inner membrane has been extensively folded over time. The outer membrane is 60-70 nm thick and is permeable to molecules with low molecular weight, such as salts, adenine and nicotinamide nucleotides, sugars, and coenzyme A. The inner membrane is a thylakoid, which contains the enzymes that produce energy.
The ATP synthase, which produces adenosine triphosphate (ATP), is located inside the mitochondrial matrix. It is a protein complex with multiple subunits and several tetramers. ATP synthase is a critical component of the mitochondrial respiratory chain. The ATP synthase reacts with oxidative phosphorylation to generate energy for the cell.
They are located in the inner membrane
Despite their small size, mitochondria play a crucial role in the energy supply of mammalian sperm. They are responsible for generating ATP, and the energy needed to support motility. They also participate in the process of programmed cell death that occurs after sperm fuse with an egg. These processes are essential for sperm fertility. However, the exact mechanisms by which sperm generate their energy remain a mystery. It is believed that two pathways can account for the production of ATP in mammalian sperm: glycolysis and mitochondrial oxidative phosphorylation (OXPHOS). Oxidative OXPHOS requires high activation of complexes in the electron transport chain and ATP synthase, which produces up to 90% of cellular energy. Interestingly, the activity of these enzymes correlates with sperm motility. Low levels of ATP have been associated with male infertility.
Sperm cells undergo fundamental changes during spermatogenesis, which includes reorganization of the mitochondrial membrane. During this process, the mitochondrial DNA is conserved while the unnecessary components are destroyed. This makes it possible to study the structure of sperm mitochondria in unprecedented detail.
To determine the functionality of sperm mitochondria, researchers use a type of luminescent probe called MitoTracker or JC-121. These molecules have the ability to stain mitochondria in a sperm sample and detect their membrane potential. They can also be used to measure oxygen consumption. These data are usually compared with Respiratory Control Ratio (RCR) values, which provide a direct measurement of mitochondrial function.
