Probably no more than ten people on Earth have a “favourite” texture, and these are almost certainly not people you would like to know. Conversely, pretty much everyone has a least-favourite texture. What do slugs, unsightly thigh boils, and disease-ridden swamps all have in common? They are all, of course, slimy. But how did they get that way? Where does slime—and/or sliminess—come from? For this week’s Giz Asks, we reached out to a number of slime experts to find out.
Assistant Professor, Molecular, Cellular and Biomedical Sciences, University of New Hampshire, who is currently in the Galapagos Islands collecting hagfish for the purpose of unravelling the molecular genetics and evolution of slime glands
and Douglas Fudge
Associate Professor, Biological Sciences, Chapman University, and a leading expert on hagfish and the slime they produce
In biology, slime is usually produced by glycoproteins. Glycoproteins are molecules of protein that have lots of additional carbohydrate groups added to them. (In science we also refer to carbohydrates as sugars). Glycoproteins of all shapes and sizes bind water owing to the interaction between water and their sugar molecules; however when such glycoproteins are really large (in molecular terms) they produce a consistency of what humans would call sliminess. In biology slime has evolved many, many times independently, and plays vital roles in a wide range of biological processes including respiration, feeding, reproduction and in the case of our hagfish, defence.
Your question also brings up a more fundamental question: what does it mean to be slimy? We already mentioned the fact that the glycoproteins that form slime tend to bind a lot of water. This lends a certain softness to their properties as a material and allows them to slip past each other easily. These large glycoproteins also interact with each other in a way that gives them elasticity. We humans call stuff with these sorts of properties slimy.
“In biology slime has evolved many, many times independently, and plays vital roles in a wide range of biological processes including respiration, feeding, reproduction and in the case of our hagfish, defence.”
Chairman, Cell and Developmental Biology, Centre for Genomic Regulation
Slime usually conjures up thoughts of unpleasantness because of the way it feels, but it is in fact very important because it has a protective function. Our bodies contain special cells called goblet cells in the lining of the airway and colon that secrete mucins. We secrete a litre of mucins per day and these secreted form a slimy coating on the surface of the cells that line the airway and colon.
Mucins are large proteins that are made in a compartment called endoplasmic reticulum (ER). These proteins are modified by sugars at the ER and then transported to another compartment called the Golgi apparatus. Within the Golgi, the mucins undergo a massive change in their size by addition of sugars. This increases the size of mucins by up to 5 fold. The highly glycosylated (a term used to denote addition of sugars) mucins are then packed into big containers and these containers then mature whereby the mucins almost become crystalline. The sugars added in the Golgi are ultimately the reason for sliminess of the secreted mucins. The containers release mucins to the extracellular space by fusion to the cell surface. This occurs by two different processes: 1, a basal level whereby a small number of mucin filled containers release their contents without a signal from the outside. 2, in the presence of an allergen or a foreign particle, there is a rapid burst of fusion of mucin containers to cell surface. The mucins released from by these processes mix with watery liquids outside the cells and become like a slimy gel, which is called mucous. The amounts and quality of mucins secreted is very important. Too much, too little, too thick, too runny quantity and quality of mucins is problematic and linked to a number of diseases like severe asthma, chronic obstructive pulmonary diseases (COPD), cystic fibrosis, Crohn’s disease and mucinous cancers.
The runny nose or slimy stuff that we cough is in fact a response to eject foreign irritants and pathogens from the airway.
So, in general: the slime we produce is composed of sugary proteins called mucins that are secreted by specialised goblet cells. This important slimy stuff provide the first layer of protection from foreign particles and pathogens in the airway and colon. My lab is working to understand how cells release the right quantity and quality of mucins.
“The runny nose or slimy stuff that we cough is in fact a response to eject foreign irritants and pathogens from the airway.”
Algae, whether they’re the one-celled, microscopic kind or multicellular seaweeds, feel slimy. When the first algae floated in the oceans more than three billion years ago, no ozone layer existed to protect the creatures from DNA-frying ultraviolet rays. Therefore, the algae evolved a slick layer of polysaccharides (long chains of sugar molecules) with a UV-absorbing mycosporine-like amino acids. The mucilage is clear so that the algae can capture the light wavelengths they need to photosynthesise.
Today, biochemists have created sunscreens using algae’s molecules. Algae sunscreen is effective and, unlike sunscreens containing oxybenzone and similar compounds, doesn’t harm corals, fish, and other marine life.
Professor of Head and Neck Surgery and Director of Paediatric Otolaryngology at UCLA, and the author of HYPE: A Doctor’s Guide to Medical Myths and Bad Advice: How to Tell What’s Real and What’s Not (2018)
Mucus in the nose and sinuses is slippery and slimy for good reason. The principal purposes of the nose are both to breathe some good clean air, but also to act as a filter for all of the contaminants present in that not-so-good nor clean air. If one is unable to breathe through the nose, for instance due to a bad cold or a physical obstruction, chronic breathing through the mouth leaves the congested individual without that important filtration of dust, allergens, and other particulate matter we breathe. The combination of slippery mucus and nasal hairs enable these particles to be trapped and eventually expelled with nose blowing and/or sneezing. One can also think of the nose as a greenhouse—providing moisture and warmth for the cold air we breathe. The filtration and warming functions of the nasal mucus enable the air to be filtered and warmed before it reaches the trachea (windpipe) and eventually the lungs. Nasal mucus is also important because it helps clear viruses and bacteria that inevitably land in this warm, wet environment, especially during the winter season.
Mucus is produced by mucous glands in the nose, middle ears, eyes, sinuses, throat, airways, and lungs. It consists of water, salt, proteins, and glycoproteins, which contain sugars. A substance called mucin gives mucus the tell-tale slippery quality. While all mucus is slippery or slimy, most recognise that there are varying types of nasal mucus. The mucus associated with allergies tends to be clear and more watery. When you have a cold, the mucus gets a little thicker and a darker colour, either yellow, green, or brown. These darker colours may be an indication of a bacterial infection, but do not necessarily indicate that there are bacteria growing. Viruses can grow in mucus, making it darker in colour. In addition, mucus tends to be more condensed (thicker and darker) first thing in the morning, but may thin out and become later as the day progresses. Mucus that’s a pale grey rubber cement consistency may indicate a nasal blockage. Newborns with absent development of an open nasal passage will have thick, pale rubber cement-like mucus filling their nose. And a newborn with both nostrils blocked is unable to breathe at all. While older infants develop the reflex to breathe through the mouth when stuffy, newborns are obligate nasal breathers, needing just the right amount of mucus (but not too much) to breathe at all.
Featured image: Illustration: Benjamin Currie (Gizmodo)