Conidium

Tiny mold bits float in the air.

These bits are called spores. They stay very still. They wait for water and air to grow.

When they grow, they can make a long tube. This tube helps the mold grow more. Some spores grow in a chain.

Spores can live for a long time. They can stay still for a year. They can survive in heat or dry air.

Some spores grow under the skin of plants. They can push out to spread. This helps the mold move to new spots.

People breathe in these tiny bits every day. Most of the time, they are not a problem. They are just part of our world.

A conidium is a tiny spore from a fungus.

These spores do not move on their own. They are made through mitosis. This is a way cells divide to make new ones. The new spores are just like the parent. They grow on special stalks called conidiophores.

Some spores grow in long chains.

To grow, a spore must break its sleep. This state is called dormancy. Spores can stay dormant for a year. They can survive heat and dry air. When they find water and food, they swell up. This is called isotropic growth. Then, they grow a tube on one side. This is called polarized growth.

Some fungi make big structures to spread spores. These are called conidiomata. One type is a pycnidium. This is a shape like a small vase. Another type is an acervulus. This looks like a small cushion. These can push through plant skin to let spores out.

People breathe in many spores every day. Most are not a problem, but some can cause illness.

A conidium is a tiny, non-moving spore made by a fungus.

The name comes from an Ancient Greek word meaning dust. Scientists also call them mitospores. This is because they are made through mitosis, which is how cells divide. These spores are genetically identical to their parent. They are produced exogenously, meaning they grow on the outside of the fungus. They often grow on special stalks called conidiophores. These stalks have unique shapes that help scientists identify different species.

There are two main ways these spores develop. In blastic development, the spore is visible before it even leaves the stalk. In thallic development, a cross-wall forms first to create the spore cell. Once a spore is ready, it might go through germination. This is the way it starts to grow into a new fungus. Some spores grow germ tubes to build larger fungal networks. Others grow conidial anastomosis tubes, or CATs. These tubes grow toward each other to fuse together. This fusion helps the fungus establish a new colony.

Many fungi use special structures to release their spores. These structures are called conidiomata. They can be very small, sometimes only 1 mm wide. One type is a pycnidium, which is shaped like a bulging vase. It releases spores through a tiny hole called an ostiole. Another type is an acervulus, which looks like a small cushion. These can grow under the skin of a plant. As they grow, they create pressure that splits the plant's surface. This allows wind and rain to carry the spores away.

Some spores, like those from the mold Aspergillus, are very tough. They can stay in a state called dormancy for over a year. They can survive heat, dry air, and even UV light. To wake up, they often need water, air, or a food source like glucose. When they wake up, they go through isotropic growth. This is when the spore takes in water and swells up. Next comes polarized growth, where a tube grows out from one side. This growth uses a material called chitin to build the new cell wall.

Conidia are always in the air around us. An average person breathes in at least 40 conidia every hour. Most people breathe in thousands of these spores every single day. For most people, this is not a problem. However, they can be dangerous for people with weak immune systems. For example, inhaling Aspergillus can cause a lung infection called aspergillosis. Some spores can even cause lung issues for forest workers. It is a reminder of how much tiny life is moving around us.

A conidium, or conidia in the plural, is an asexual, non-motile spore produced by a fungus.

The term originates from the Ancient Greek word for dust. These spores are also known as mitospores because they are generated through mitosis, a cellular process of division. Because they are produced through this process, the two new haploid cells are genetically identical to the haploid parent. This allows them to serve as a primary method for biological dispersal. They are produced exogenously, which means they develop on the outside of the fungus. In many species, such as ascomycetes and basidiomycetes, they are borne on specialized stalks called conidiophores.

There are two primary methods of conidiogenesis, which is the process of spore development. The first is blastic conidiogenesis. In this method, the spore is already visible before it separates from the conidiogenic hypha. The second method is thallic conidiogenesis. In this process, a cross-wall appears first, and the newly created cell then develops into a spore. Before the rise of molecular techniques at the end of the 20th century, scientists used the unique morphology of these conidiophores to identify different species. This remains a vital way to distinguish between fungi like the genus Metarhizium.

Once a conidium finds favorable conditions, it may begin germination. This process can involve the formation of germ tubes, which grow to create hyphae and fungal mycelia. Some conidia also produce conidial anastomosis tubes, or CATs. These tubes are morphologically and physiologically different from standard germ tubes. After these tubes are induced, they grow toward one another and eventually fuse. This fusion allows nuclei to pass through the fused tubes. This event is part of vegetative growth rather than sexual reproduction. Such fusion is important for some fungi when they are establishing a new colony. This specific type of tube production has been suggested to occur in 73 different fungal species.

In the genus Aspergillus, a common mold, germination follows a very specific three-stage sequence. The first stage is dormancy. During dormancy, conidia are incredibly resilient. They can remain dormant for over a year at room temperature. They can survive dehydration, changes in osmotic pressure, oxidation, and varying levels of acidity or UV exposure. This resilience is driven by central regulatory proteins. One essential protein is called wetA. In mutants where wetA is defective, the cell wall synthesis is weak, and the spore has less tolerance for harsh environments. Other proteins, such as velvet regulator proteins and heat shock proteins, also help manage growth and environmental stress.

The second stage is isotropic growth. This occurs when the conidium takes up water, causing increased intracellular osmotic pressure. This causes the spore to swell and increases its cellular diameter. During this time, the cell undergoes remodeling of the cell wall. The third stage is polarized growth. The swelling from the isotropic stage directs growth toward one side of the cell. This leads to the formation of a germ tube. This stage involves the upregulation of proteins used for DNA processing and mitosis. It also involves the synthesis of chitin, which is a major component of the fungal cell wall.

Many plant pathogens use specialized structures called conidiomata to release their spores. These structures can be about 1 mm in diameter and form masses of conidia under a host's skin. As pressure builds, they erupt through the surface to be spread by wind and rain. One type is a pycnidium, which is a flask-shaped structure. These form in the fungal tissue and release spores through a small opening called an ostiole. The other type is an acervulus, which is a simpler, cushion-like structure. Acervuli can form in various layers of a host, such as the subcuticular or intraepidermal layers. They develop a flat layer of short conidiophores that eventually split the plant's epidermis or cuticle to release the spores.

Conidia are a constant presence in our environment. An average person inhales at least 40 conidia every hour. In total, humans inhale between $10^3$ and $10^{10}$ mold conidia every day. While usually harmless, they can cause serious health issues. For example, species like Cryptostroma corticale can cause hypersensitivity pneumonitis in forest workers. In immunocompromised individuals, such as those with AIDS or organ transplants, inhaling Aspergillus can lead to aspergillosis. This is a pulmonary infection that can be very dangerous. The mortality rate for invasive fungal infections can exceed 50%, even when antifungal drugs are used. This makes the study of conidial germination and resistance a critical area of medical research.