Just like reading, one has to know which perspective for the letters to make sense.  For example, MOM can look like WOW depending on which perspective!

In histology, the same applies – but now consider tissue presented in both 2- and 3 dimensions.

In histology, we are looking at two dimensional (2D) sections of  structures that are in 3 dimensions (3D)! This could pose a challenge for us in the sense that structures that can be graphically depicted in its 3D form while being taught to us in an anatomy class can look very different as a section under the microscope. Through the exercise below, we hope to teach you how to think about 3D structures in 2D planes for every scenario. Of course, it could be helpful to keep a note of common cuts of organs, tissues and cells and how that looks under the microscope, but that should all be in the goal of accumulating experience in learning how to think about cuts and planes in histology as it’s impossible for us to anticipate every possible cut of all the structures in our body.

The most common planes used in cutting tissue are divided into horizontal (a.k.a. transverse, axial), sagittal (a.k.a. longitudinal), and frontal (a.k.a. coronal)

Because a 3D tissue specimen, such as a biopsy,  is prepared and sliced into 2D, hollow structures will appear as a mixture of the pattern illustrated in Figure 1.  When looking at hollow structures, the space within, the lumen, will appear differently based on the plane (Figure 2).

Figure 2 also represents how “front” and “back” applies when using the anatomical position.  IF there is a structure that is anterior to another structure, then you could say it’s ‘front of” or “anterior to”.  In  Figure 2, this image of the abdominal aorta is anterior to the spine as it is ‘in front of’ the spine in the anatomical position.  Note that ‘front’ and back’ doesn’t apply to tissue where structures can’t be easily recognized in the anatomical position.  For example, a skin biopsy doesn’t have an anterior or posterior perspective as one can’t tell if this biopsy tissue was taken with an identifiable ‘front’ surface.

Similarly, determining which way is “up” or “down’ really refers to your point of reference.  As with Figure 2, the abdominal aorta travels “down” in an inferior direction since the anatomical position has the body upright.  However, if you are looking at a biopsy of tissue, there is no “down” as there is not point of reference. Rather, you have “basal” and “apical”  In epithelial tissue, the apical surface faces the external environment or lumen of the tissue and the basal surface faces the basement membrane and deeper structures.  Consider the following analogy of ants walking near two buckets.

Both the blue and red buckets are empty which represent the lumen whereas the ground represents the basement membrane/basal layer.

From Ant A’s perspective, the apical (lumen) surface would be if it looked ‘up’ from the bottom of the bucket, into the empty bucket. Similarly, the basal surface for Ant A would be the ground under its legs.

From Ant B’s perspective, the same goes:  the apical surface is if Ant B looked up into the empty space of the bucket, even though the bucket bottom encloses Ant B.  The basal surface is still the ground under its legs

From Ant C’s perspective, there is no basal surface.    Ant C is perched between two open spaces:  the air above and the empty bucket below.

So let’s apply this to histology in Figures 3 & 4.

So always look for the luminal space and that helps you identify the apical surface.  Do not assume that apical is always ‘up’ on the image!