Skin deep
The average person is covered by 16 to 21 square feet of skin. It’s the largest organ in (on?) the human body. A key role of skin (aside from simply helping keep us all together) is to serve as a physical barrier to our surrounding environment and its assorted, myriad pathogens.
Indeed, our skin (or more particularly, its outer layer called the epidermis) is home to its own ecosystem of microorganisms, including yeasts and bacteria that cannot be removed by any amount of cleaning. On average, it’s estimated that roughly 50 million microbes inhabit each square inch of epidermis, though that number varies greatly by location. Oily or moist skin, such as under the arm or between the toes, harbors higher numbers than a similarly sized patch of forearm.
The scanning electron micrograph above, created by Thomas Deerinck at UC San Diego’s National Center for Microscopy and Imaging Research, depicts a patch of skin. The epidermis contains no blood vessels, receiving all of its nutrients via diffusion from capillaries in the underlying dermis layer.
The bottom layer of the epidermis contains proliferating cells, which divide to form keratinocytes. These daughter cells migrate up through the epidermis and eventually die as their nutrient supply dwindles. The keratinocytes lose their cytoplasm, which is replaced with keratin, a structural protein that forms tough, insoluble filaments similar to those found in your hair and nails.
After 27 days or so, the dead keratinocytes reach the surface of the skin and are sloughed off. It’s estimated humans shed about 600,000 particles of skin per hour, about 1.5 pounds a year or 105 pounds of skin by the time they are 70 years old. This translates to an entirely new outer layer of skin cells every 27 days, almost 1,000 new skins in an average lifetime.
The worm turns
There are two main types of human intestinal parasite: Single-cell protozoa like giardia and cryptosporidium that live and multiply in the human gut, and multi-celled helminths, such as roundworms, pinworms and tapeworms (above). The latter may be appalling to look at, but at least in their adult form, they cannot multiply in the human body. And they tend to stay put. Perish the thought if, say, one had worms in the brain.
A mouthful
In this color-enhanced photomicrograph by Derren Ready of the Eastman Dental Institute, part of University College London, different species of periodontal bacteria are shown. These are some of the microbes that comprise the colorless plaque that forms on teeth. The human mouth is home to an extraordinarily rich and diverse microflora. More than 615 species of oral bacteria have been identified. These microbes – some good, some bad – have a complex relationship with their host, whose immune system must constantly guard against overabundance and the threat of invasive contamination of tissues, which can result in myriad diseases, including oral cancer. Image courtesy of the Wellcome Collection.
River of dreams
The hippocampus is a region of the mammalian brain involved in learning and memory. In this confocal microscopy image of an adult mouse’s hippocampus by Sandra Dieni of the Institute of Anatomy and Cell Biology at Albert-Ludwigs University in Germany, reactive astroglia (star-shaped cells that support neurons in the brain, here colored pale yellow) have proliferated and enlarged in response to neuronal activity over time.
What’s up your nose
Spring is in the air, and for folks with allergies, that means pollen too, signaling a season of sneezing, snuffling and sniveling. Seasonal allergies are an annoying sign that your body’s immune system is doing its job, which is basically to provide protection against environmental toxins that may be far more dangerous than pollen.
The immune response is roughly divided into two types: innate and adaptive.
Innate (or type 1) refers to a nonspecific form of immunity, a natural-born resistance to various pathogens, from viruses and bacteria to fungi and protozoa. It involves both barriers to keep pathogens out and internal methods for directly killing invaders or infected cells.
Adaptive (or type 2) immunity is specific, a learned response resulting from previous exposure to a toxin. It can be naturally acquired or artificially, as in vaccines.
In type 2 immunity, the body sends specialized T cells and antibodies to deal with a specific irritant. Sometimes, they go too far – at least for our immediate comfort.
In a paper published in the journal Nature recently, researchers at Yale University suggest that the nasty symptoms of seasonal allergies are the result of environmental allergens like pollen revving the body’s immune system into overdrive.
The result: Over-production of the neurotransmitter histamine and its consequent coughing, sneezing, runny noses and general suffering.
As much as we might hate them, though, seasonal allergies may be a sign of evolved status. “We believe that allergic hypersensitivity evolved to survey the environment for the presence of noxious substances,” said lead author Ruslan Medzhitov, a professor of immunobiology at Yale. “After the first exposure, the immune system gains a memory, and subsequent exposure to even minute amounts will induce an anticipatory response that helps minimize potentially harmful effects.”
Keep Medzhitov’s words in mind next time you’re insanely congested from airborne allergens. They might help.
Or not.
Above: A scanning electron micrograph of various pollen from common plants: sunflower, morning glory, hollyhock, lily, primrose and castor bean. The pollen has been magnified 500 times.
Baby’s bony body
Newborns are a bundle of bones – more than 300 to be more precise. Over time, many of these bones fuse together. One obvious example: The 44 original, separate components of the skull, whose loose confederation allows a newborn’s head to more easily pass through the birth canal and to accommodate dramatic brain and head growth during in the first year of life outside the womb. Generally, an infant’s skull fuses together by age two to provide better protection of the brain.
Overall, the total number of bones in the body is reduced to 206 by the time humans reach adulthood.
Above is a human fetus visualized in the third trimester of pregnancy using a computed tomographic scan and volume rendering software. Courtesy of Philipp Gunz and Jean-Jacques Hublin at the Max Planck Institute for Evolutionary Anthropology in Germany.
Logger heads
Toward the end of the 18th century, an influential Austrian neuroanatomist named Franz Joseph Gall developed a theory that suggested distinct areas of the brain’s cerebrum physically housed distinct mental faculties, such as emotions, moral impulses and intellect. Gall believed that these areas grew and shrank with use, resulting in bumps on the skull that could be felt, measured, analyzed and mapped.
The resulting study of phrenology became wildly popular for a time – and notorious as people used it to wrongly label others as “criminals” or to justify racist notions. Phrenology was eventually dismissed as quackery, but Gall did introduce the first modern theory ascribing different mental functions to different parts of the cerebrum. He got the details wildly wrong, but the overall concept was headed in the right direction. Today, researchers use a variety of technologies to map the location of cognitive functions throughout the brain, not the skull.
Photo courtesy of Eszter Blahak at the Semmelweis Museum in Budapest, Hungary.
Metabolomic Eye
Using a technique called computational molecular phenotyping (CMP), neuroscientist Bryan William Jones at the University of Utah depicts the diversity of cells inside a mouse eye retina. CMP maps different kinds of tissue by measuring concentrations of common organic molecules, in this case using antibodies that stained against taurine (red), glutamine (green) and glutamate (blue). The image, which captures both anatomical context and a view of normal tissue functioning, won Jones first place in photography in the 2011 International Science and Engineering Visualization Challenge, sponsored by the National Science Foundation and the journal Science.
A virus that’s hard to C
In this transmission electron micrograph, taken by Thomas Deerinck at the National Center for Microscopy and Imaging Research at UC San Diego, the rarely visualized hepatitis C virus (the yellow circles) is seen inside an infected liver cell. Magnification is approximately 100,000 times.
Hepatitis C is an infectious disease primarily transmitted by blood-to-blood contact associated with intravenous drug use, transfusions and poorly sterilized medical equipment. Infections are often asymptomatic, but chronic infections can lead to liver scarring, cirrhosis and liver cancer.
An estimated 130-170 million people worldwide are believed to be infected with hepatitis C. Persistent infections are treated with peginterferon and ribvirin. The cure rate is 50 to 80 percent. There is no current vaccine.
Eaten alive
In Greek, the word “macrophage” means “big eaters,” which is exactly what macrophages do. They are notably large cells whose job, as part of the innate immune response, is to engulf and digest cellular debris and invasive pathogens. They also stimulate other components of the immune system.
In this scanning electron micrograph from Nicole Ottawa and Oliver Meckes at eye of science, a macrophage (colored pale brown) interacts with Borrelia cells (colored blue), a spirochete bacteria that causes Lyme disease. The surface of Borrelia contains a strong antigen capable of provoking an immune response. The bacterium compensates by hiding in places where it’s less likely to be found by immune cells like macrophages, such as the central nervous system.
