Above is an illustration of the spinal nerves and the dermatome regions that they innervate. Dermatomes are embryonic components of somites that differentiate into neurally designated sections of the body. Viruses such as varicella-zoster (shingles) are capable of of lying dormant in individual nerve ganglions and resurfacing later on, affecting only the skin area innervated by that was infected.

Image Credit: Mikael Häggström

Placodes (depicted above), like neural crest cells, are multipotent cells unique to vertebrates. Placodes are ectodermal in origin and can either be neurogenic or non-neurogenic. Neurogenic placodes give rise to nervous or sensory cranial structures. Non-neurogenic placodes, such as epidermal placodes, give rise to hair follicles, teeth, and feathers.

Photo Credit: Dr Mark Hill, UNSW Embryology

Bacteriophages (bacteria-infecting viruses) target a single bacterial cell. The viruses anchor themselves to the bacteria’s cellular surface using proteinaceous tails before injecting their genomes into the bacterial host.

Using the natural evolutionary specificity of bacteriophages could contribute to medical biology by targeting and curing increasingly resistant bacterial infections such as tuberculosis and streptococcus.

Photo Credit: Lee D. Simon/Photo Researchers

Above is a popular scanning electrom micrograph of numerous HIV-1 virions (green) emerging from a cultured white blood cell. HIV has caused millions of deaths since the early 80s, but some individuals are actually immune to contracting the strain of the virus depicted above. Note: This isn’t a recommendation for risky behavior to see if you’re immune; HIV/AIDS is devastating and incurable.

The HIV-1 virus operates by detecting a protein on the surface of T-lymphocytes called CCR5. In some individuals, a 32 base pair deletion in the gene leads to defective CCR5 proteins that are absent from the white blood cell surfaces. If there is no CCR5 protein to bind to, the HIV virus cannot begin its attack on the immune system. Thus, these individuals will be resistant to HIV-1.

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Model for the DNA binding and cleavage fragment of yeast topoisomerase II (blue/red spheres) bound to a separated DNA strand (green spheres). One protomer is shaded darker than the other to highlight the protein’s two subunits. A second DNA (viewed end-on, yellow sticks) is modeled into a large internal hole present in the enzyme. This figure represents a likely conformational and substrate-bound intermediate of this region of the topoisomerase during its duplex DNA passage reaction.

A collection of scanning electron micrographs of untreated water from a wild stream, revealing a myriad of bacteria, protozoa, and algae. 

Many of the organisms in untreated waters are harmless, but the presence of those that are pathogenic, such as Giardia and virulent strains of E. coli make drinking water from natural streams typically unsafe to both humans and other animals. Furthermore, in our modern age, it isn’t uncommon to find toxic chemicals and debris present among the natural infusoria of freshwater streams.

Courtesy: CDC Public Health Image Library

The genome of Monosiga brevicollis, the choanoflagellates photographed above, was recently sequenced.

Choanoflagellates,single-celled, aquatic, eukaryotic microbes are currently biological giants in unlocking the key to the evolutionary transition from unicellular to multicellular organisms, particularly in animals.

Sponges, or animals of the phylum Porifera , serve as the most primitive extant group of animals. (You can see this on this  previously posted phylogenetic tree of the history of life). Interestingly enough, aquatic sponges possess flagellated feeding structures  called choanocytes, which are nearly morphologically identical to the single-celled organisms we call choanoflagellates.

Photo Courtesy: National Science Foundation