Lymph nodes: Police stations of the body

Lymph node as a police stationA lymph node is a bit like a jungle gym for our microscopic police force.  A filter or dragnet, evolved to catch unsavory characters like bacteria and viruses — drifters who are up to no good.  Our hundreds of lymph nodes function like district police headquarters, situated around the body with concentrations in the armpits, neck and other areas.

Tissue fluids throughout the body make their way into the lymphatic vessels, carrying any infectious microbes along.  Lymph nodes like the one shown above are stationed along those vessels, and here is where the “crooks” meet a concentration of leukocytes, or white “blood” cells.  (I add quotes around “blood” because they spend most of their time patrolling through other body tissues.  It’s a bit like calling someone a “streetwalker” just because you caught them crossing the street now and then.)

The “jungle gym” in a lymph node is built of reticular connective tissue, specially evolved to provide “networking” opportunities (“reticular” means “net-like”).  This facilitates interaction among the cops while also giving them a good chance of nabbing a microbe.  Some leukocytes may respond immediately (cop reaching for gun), and will later serve as an antigen-presenting cell to communicate their findings to the others (the “wanted” slide shown at top).  After the initial encounter, certain leukocytes with a receptor for the specific microbe (“I was born for this assignment, boss!”) divide into many identical clones that are specifically on the lookout for said crook.  The production of clones is what produces the swollen lymph nodes during an illness.  Memory cells (the cop with the desk job) remain on high alert (perhaps with the help of coffee and a donut) even after the crooks have been executed, in case they show up again, when the body will be prepared for a more rapid response (adaptive immunity).  Some memory cells can live for decades – very old “cops” indeed — and they never forget.  Leukocytes communicate with a variety of signals (cops joking around at right), some of which encourage them to “bulk up” for the encounter, like the cop doing pull-ups.

We run into the larger lymph nodes (around 1-2 cm) with every dissection in the cadaver lab.  They’re nondescript, and firm, but with a dense, spongy texture that, with a touch of imagination, is suggestive of their “cop station” function.

Interstitial fluid: Our internal life aquatic

I would love to shrink down to microscopic size and greet the cells in my own body face-to-face, like something out of the movie Fantastic Voyage.  Of course, I won’t “hold my breath” waiting for such an opportunity to come along.  But even if there were some way to do this, I would quickly drown without SCUBA equipment!  This underscores the fact that our internal environment is an aquatic one.  Our living cells depend on the special properties of water, both to exchange gases, nutrients and wastes, and to facilitate the chemical reactions necessary for life.  Evolution has not yet found a way to allow life to thrive without internal water (a few creatures are capable of surviving complete desiccation, but they do so in suspended animation).

Indeed, a cell out of water is like a fish out of water.  This means that the dry skin surface of the human body is all dead tissue.  It also means that all our internal tissues are moist.  Even bone tissue, despite being largely composed of calcium phosphate, is permeated with water and houses living bone cells that live comfortably within tiny water-filled “lagoons” known as lacunae.

Some tissues contain more water than others.  Blood is the tissue with the greatest water content, but all other tissues have, between their cells, a fluid known as tissue fluid, or interstitial fluid.  The tissue depicted above might represent areolar connective tissue, a ubiquitous tissue found in many parts of the body, which contains a large amount of interstitial fluid. Because it’s found adjacent to all blood vessels, it plays an important “middleman” role allowing molecules to make their way between the blood and nearby tissues.  The two typical cells shown here would be fibroblasts, which produce the protein fibers giving structure to the tissue (collagen fibers are shown in gray, and reticular fibers in magenta); these in turn provide attachment points for the cells, as they go about living their aquatic lives. Unfortunately, the interstitial fluid is also a benign environment for bacteria (shown in pink) — but that’s a story for another day.

Endomysium: A first-class seat for your muscle cells

Endomysium is like a first-class airplane seat for your muscle cells

Strapped into an economy seat as we fly across the country for the holidays, it’s hard not to appreciate life’s basic necessities — a cup of soda, a bag of pretzels, the relief of seeing “vacant” on the lavatory door. It’s also a good time to remember that the real protagonists in this story are your trillions of cells, each of whom has the same basic needs you have. Each muscle cell, for example, needs an oxygen supply, nutrients, a way to eliminate wastes, a command system telling it whether to contract (or just relax), and a physical attachment allowing it to work with the rest of the muscle. All of these things are provided by a thin sheath of connective tissue called the endomysium which surrounds each muscle cell. The endomysium, in effect, acts as a scaffolding to support the infrastructure of blood vessels and nerve cells that allow the muscle cell to function. What kind of airplane seat does a muscle cell occupy? Considering that the bloodstream keeps the cell supplied with a constant stream of goodies, I imagine it’s got to be a first-class seat.

The hierarchy of organ, tissue, cell in the human body

As another new semester begins, what better place to begin than the hierarchy of structure in the human body — organ systems, organs, tissues and cells. I hope this doesn’t sound old-fashioned, but it’s a strict hierarchy!

It sounds easy enough, but it’s easy to lose your bearings when you hear statements like “bone tissue has a lot of blood vessels”.  Sure, that’s true — and that’s why bone heals faster than, say, cartilage.  But this does not mean that blood vessels are a part of the bone tissue.  Blood vessels, in fact, are made of tissues like muscle, connective tissue, epithelium. So, if a blood vessel were a part of the bone tissue, then you’d have one tissue that’s part of another tissue — bad news for our hierarchy!