If a non inquisitive person were shown a heart, he or she might ask, “What is that?”  The easiest answer would be “It is a heart,” and that might suffice.  If, on the other hand, the same heart were shown to a curious minded or analytically minded person, he or she may counter with   “How do you know?” or “Why is it a heart?”  The answer would include describing its unique and characteristic sizeshapeposition, and character.  We may speak to its parts, and its relationships to other structures to further characterize it.  If the heart were in vivo (inside the body) we could speak to how the heart connects to other biological structures and also how it changes with time and different cyclical events.  The way it functions in vivowould describe its physiological characteristics. These descriptors that we use are universal, used for all biological structures, whether a heart, the glial cell of the brain or the pancreas of a mountain rat.   We can use these descriptors for describing the cells, tissues, and anatomic structures.  We also apply them to non biological structures, whether the atoms of a stone in the Amazon or the Rose Famosa marble pillars of the Empire State building in the centre of Manhattan.

We use these same features when evaluating tissues under the microscope, or the organs under the window of the CTscan to determine whether they are normal or abnormal.  Take size, for example.  An abnormally big mitochondrion, nucleus, or liver are key findings that will define a structure as being abnormal and lead to a specific differential diagnosis.

A physician is a judge of biology who is required to preside on a clinical situation and decide whether the patient is in a state of health or disease. Is a situation “thumbs up” or “thumbs down”? – “Guilty!” or “Not guilty!”of disease?   In order to make this decision doctors rely on the rules of biology.  Some of these rules are black and white, and the decision is an easy one.  More often the situation is complex, and the diagnostic evolution involves a series of progressive steps that culminate in accurate evaluation and treatment.  In order for prudent judgment, it is imperative to know the rules.

Here are the fundamental rules of biological structure.

(1)  All structures have component parts.

(2)  All structures have a size or a dimension.

(3)  All structures have shape.

(4)  All structures occupy a position in space.

(5) All structures have character, or a nature. For example, structures may exhibit elastic, rubbery or liquid properties that define them.

(6)  Biological structures exist in relationship with other structures, and are connected within an environment.  The connections often take the form of conduits that bring products to the structure (arteries or ducts for example), and conduits that take products away from the structure (veins, capillaries or lymphatics).

(7) All structures change with time

Note that the only element that differentiates a structure such as a stone from a biological structure (in this set of structural rules) is the form of relationship and connections that it has with other structures. Obviously the nucleus and protoplasm of the cell have very different natures that allow them to function in a biological way, that are precluded in an inert stone.

3.8mm fetus  – 6 weeks
49786b01 fetal pole size dates time 6 weeks USscan obstetrics gestational sac amniotic cavity pregnancy Davidoff MD
Death based on Size Shape and Position
46592c01 uterus OB pregnancy fetal demise spontaneous abortion shape size position heart rate USscan Davidoff MD death

The idea of space is key in this discussion on structural concepts.    Each cell, tissue, and organ has to have space to work and to breathe.  The position and relations of the units has to be optimal in order for them to be able to work and communicate with each other, and in the end come together as an empowered unit.  The spatial organization of the liver desacribed above is a great example.  The  plates and cords are organized around the linear shape of the sinusoids and spaces of Disse, with the portal triads and central veins having their spatial arrangement to allow for optimal exposure of the liver cell surfaces to the circulation.  We see this organizational pattern in our communities.  There are industrial sites where the factoies are located and centered around access roads, ports or railway stations, and then we have our neighbourhoods where we live, centered around schools, playgrounds and grocery stores. 

Our property lines are well defined and have to be respected.  I had often wondered why cancer killed people and it came to me that cancer was a a disorder where property lines were not respected.  Cancer cells are radicals and bullies  in the community who have no respect for the community at large.  They eat what they want to eat, provide no essential function, take all they can get, give nothing, disregard the rules, but most importantly they move into the space of others and take over, essentially throttling the well being of well meaning citizens of the community at large.  The community cannot survive once their space and hence their function is taken over.

Structure changes with  time.  We go from the sperm and an egg to an elderly frail individual in the time of a lifetime.  The changes are progressive and are embedded in a variety of cycles that we live within. Knowledge of the effect of time on structure gives us a tremendous tool  that helps evaluate structure and its aberrances.

Uniqueness of structure

No matter how structurally perfectly two side by side cells resemble each other, it is in our ignorance and our expedience that we  lump them together as liver cells for example.  For one, they will be made unique to us by the different space they occupy, but if we carefully analyse the two cells we would find that they are unique even in their makeup.  This uniqueness to each structure gives one pause to remember and to respect the beauty of each structure and – to label it a liver cell but knowing that in fact it is quite unique.  For practical purposes we have to group the structural and functional characteristics of the biological units and study and describe them in generalities such as the descriptors above and in their functional commonalities.

 A few adjectives  can be used to describe any structure whether it is the tiniest of tinies – the atom, or largest of largest – the universe.  Size shape position and character are four universal adjectives that can describe any structure, be they of biological origin or non biological origin – all matter – no matter.

For size we have applied units that can measure the linear extents, the volume, the mass, weight or the area for example.  Other units of measuring size include angles, vectors, velocity and acceleration.  Of  all the structural descriptors, size is the easiest to agree upon since we have tools that can measure a structure accurately.

Shape is well understood by us all but more subjective under many circumstances.  In the case of a circle or a sphere we can all imagine and apprciate the perfect sphere.  In the biological world perfect spheres or cuboids rarely exist .  We use the word cuboidal for example to transmit th idea of a cube like structure.  Either way we can communicate shape imagine All structures are also made up of parts.

In the biological world a few other concepts of structure have to be considered.  No biological structure is an island – it has to coexist and relate to other structures to exist.  While size, shape, and position, are easily understood, character (or nature) requires some explanation.

In summary – biological structures have the following defining principles;   

 (1)  All structures have component parts.

(2)  All structures have a size or a dimension.

(3)  All structures have shape.

4)  All structures have a position in space.

(5) All structures have character, i.e. when a structure interacts with its environment there are characteristic ways in which it will behave.  For example, elastic behavior, liquid behavior, hard soft, green blue.

(6)  Structures have relationships with other structures, and are connected to an environment.  The connections take the form of conduits that bring products in to the structure, conduits that take products out of the structure (arteries, veins, capillaries and lymphatics), or connect to the environment directly in  intimate contact. 

(7) All structures change with time