Written by Elijah Dave M. Cordova

Reviewed by Dr. Reuben J C. Los Baños, Ph.D.

Histology is a fascinating field of study. It helps you understand how normal tissues appear. It teaches you how they function. It helps confirm the presence of several diseases. Histology allows you to trace the causes of diseases. It can also teach you how to treat them.

The term “histology” comes from two Greek words. “Histos” means tissue, and “-logos” is a field of study. Hence, it deals with the study of the tissues in the body. It studies how they constitute the different body organs. It focuses on how cells’ order and structure optimize the organs’ functions.

Recall the different levels of organization in organisms.

• Cells are the most basic structures that constitute life.
• Tissues comprise cells similar in function and form. They work together to perform the same functions.
• Organs are tissues that work together to do specific activities.
• Organ systems are many organs that work together to perform needed roles. These functions benefit the organism in normal conditions.

The interconnectedness of histology to other study fields is obvious here. Among these fields are the following:

• Biology
• Medicine
• Genetics
• Immunology

Histology grants a microlevel perspective on these subjects and more. Many students who prefer more generalized and applicable studies might find it boring. Yet, without it, these other studies would fail to make sense.

Through this article, you will get to learn more about histology. You will discover its main goals and concepts. You will appreciate its relationship with other fields in medicine. Moreover, you will realize the privilege that is the chance to learn this subject.

What are the basic concepts of histology?

Now that you know its importance, you might wonder how to start studying histology. It begins by learning what makes up tissues. Next, you will learn how to prepare tissue sections for examination. What follows is a discussion on the different microscopes you can use.

Cells and the extracellular matrix (ECM) make up tissues. Several macromolecules form the ECM. Among them are collagen fibrils which are the building blocks of tendons.

The ECM aids the cell and contains the fluid that serves three purposes.

1. It transports nutrients to the cells;
2. It provides mechanical support for the cells; and
3. It transports cell wastes and other secretory products.

Cells produce ECM components. These components, in turn, influence cells. This relationship causes intense interactions between these two components. These interactions create different types of tissues, each having unique features.

The minute size of the cells and ECM elicits the need for microscopes. Hence, the most common procedure in histology is the preparation of tissue sections. These are thin and translucent organ slices. The basic steps to prepare tissues for examination under light microscopy are here:

1.Fixation is placing tissue sections in fixatives. These are solutions with cross-link proteins that inactivate degradative enzymes. Hence, they preserve the tissue structures.

A buffered isotonic solution of 37% formaldehyde is a usual light microscopy fixative. Meanwhile, glutaraldehyde is a prevalent electron microscope fixative.

2. Dehydration is placing the sections through increasing concentrations of alcohol solutions. The final solution is 100% alcohol.

3. Clearing is removing the alcohol in the samples through organic solvents. These solvents are miscible in alcohol and embedding mediums. Common embedding mediums are paraffin and xylene.

4. Infiltration is placing the sample in melted paraffin. You perform this at 52°- 60°C.

5. Embedding is placing the paraffin-infiltrated tissue in a mold. It allows your samples to harden. It is often done at room temperature.

6. Trimming is slicing or sectioning the hardened sample. It does this through an instrument called a microtome. This microtome creates 3-10 μm thick tissue sections for light microscopy.

7. Staining (dyeing) makes tissues distinguishable from one another. Its necessity is due to most cells and ECM components being colorless.

Cell components with an anionic (negative) charge react well with basic dyes or stains. These components are basophilic. Examples include DNA and RNA.

Cationic (positive) substances react with acidic dyes. These substances are acidophilic. Examples are collagen and several cytoplasmic proteins.

Hematoxylin and eosin (H&E) are the most common staining dyes. The former is a basic dye, and the latter is acidic.

8. Mounting a glass coverslip on the slide is the last step. The coverslip must have a clear adhesive.

Slide preparation (steps 1 through 8) takes around 12 hours to 2½ days. Its duration depends on tissue size, staining method, and embedding medium.

There are two types of microscopes you can use in histology. They are as follows:

1. Light microscopy uses the interaction of light with tissue components.
   1. Bright-field microscopy identifies tissues through colors caused by staining. Students and pathologists use this the most.
   2. Fluorescence microscopy uses UV light to visualize fluorescent molecules alone. It allows fluorescent probe localization that is more specific than routine stains.
   3. Phase-contrast microscopy produces stain-less images. Hence, it allows you to observe living cells. It does this through refractive index differences among tissue components.
   4. Confocal microscopy scans samples at successive focal planes to generate images. It often uses a laser. Confocal microscopy produces a 3D reconstruction with the image.
2. Electron microscopy uses the interaction of electron beams with tissue components.
   1. Transmission electron microscopy (TEM) permits resolutions around 3 nm. It allows up to 400,000x magnification. You can add compounds with heavy metal ions to the fixatives here to improve resolution. You can also perform cryofracture and freeze etching. Here, you freeze the specimen in liquid nitrogen before cutting it.
   2. Scanning electron microscopy (SEM) produces black-and-white images. It presents a 3D view of the specimen.

Other ways of studying histology samples are the following:

1. Autoradiography uses radioactive precursors to localized synthesized cell components.
2. Cell and tissue culture is growing cells in vitro. You will often use the phase-contrast microscope here.
3. Enzyme histochemistry (cytochemistry) uses specific enzyme activities in samples. It yields visible products in specific enzyme locations. You will often use a cryostat here.
4. Immunohistochemistry visualizes specific molecules in samples. It uses antigen-antibody reactions with visible markers.
5. Hybridization techniques localize DNA sequences on chromosomes. They also detect specific RNA targets in cells. In situ hybridization (ISH) is a common hybridization technique.

What are the four types of tissue? Describe each.

You now know the basic concepts in histology. You may next wonder what kinds of tissues there are in our bodies. The epithelium and connective tissue are among them. The muscle and nervous tissues complete this list.

The epithelial cells (epithelium) have the following functions:

1. Absorption. It includes the gut (intestinal lining) and the stomach.
2. Filtration. An example is a kidney.
3. Protection. It involves the skin that protects us from the outside world.
4. Secretion. It includes the different glands in our bodies.

They have the following characteristics:

1. A free (apical) surface open to outside the body. It can also be in internal organ cavities in the body.
2. A fixed (basal) area connected to underlying connective tissue
3. Can have several nerves in them (innervated)
4. Excellent regeneration as seen in sunburn
5. Forms a protective barrier through close attachment to each other
6. No blood vessels

They, too, have the following classifications:

1.By cell arrangement.

a. Simple. They are single-cell layers often used for absorption and filtration.

b. Stratified. They have several layers often used for protection.

2. By shape.

a. Columnar. These are column-shaped and tall.

b. Cuboidal. These take the form of a cube.

c. Squamous. These are scale-like and flat.

Meanwhile, connective tissues have the following functions, elements, and classifications.

A. Functions

1.Cushioning and internal support for organs.

2. Protects and attaches body parts. Examples include tendons and ligaments.

3. Stores nutrients.

4. Strengthens the skin.

2. Elements

1.Cells.

2. Fibers provide elasticity, support, and strength.

3. Ground substance is a gel around the cells and fibers.

Classifications

1. Loose connective.
   1. Adipose. These have blood vessels and cells. They also store nutrients.
   2. Areolar connective. These are a loose arrangement of cells and fibers. They provide cushions to organs
   3. Reticular connective. These are delicate fiber-cell networks. They provide internal support to organs.
2. Dense connective

Dense regular connective. These include tendons and ligaments. They often have fibers and provide strength.

Dense irregular connective. These include the skin and organ capsules. They often have fibers going in all directions.

Next are the jobs and elements of nervous tissues.

A. Functions

1.Conducts impulses to and from organs through neurons.

B. Elements

1.Brain.

2.Nerves.

3.Spinal cord.

Last are the jobs and types of muscle tissues.

A. Functions

1. In charge of body movement.

2. Transports waste, food, and blood through the body organs.

3. Responsible for mechanical digestion.

Types

1. Cardiac muscle. It is the muscles that make up the heart. These muscles are involuntary and striated. They have intercalated discs that cause synchronous heart contractions.

2. Skeletal muscle. It includes all the body muscles. They are voluntary and striated. They connect to bones to aid movement. They, too, come in bundles.

3. Smooth muscle. It includes all blood vessels and organ walls. These muscles are involuntary and spindle-shaped. They push materials through organs.

What is the difference between histology and biopsy?

You now know the basic types of tissues. Let us now discuss the terms associated with histology. The first is a biopsy. A biopsy is a procedure of taking small tissue samples. It also refers to the tissue sample itself. Meanwhile, histology is a study field that uses biopsies in diagnosing diseases.

Biopsies often diagnose or rule out the following diseases:

1. Cancer.
2. Peptic ulcers.
3. Hepatitis.
4. Kidney diseases.

Also, among the types of biopsies are as follows:

1. Endoscopic biopsy. You can use it during an endoscopy which visualizes the upper GI tract. It uses an endoscope to remove tissues.

2. Excisional biopsy. You can use this in surgery to remove larger tissue sections.

3. Needle biopsy. It uses a hollow needle to get organ or skin tissue sections.

What is the difference between histology and pathology?

Next to a biopsy is pathology. Pathology is a branch in the field of medicine. It studies the nature and causes of diseases. It, too, investigates their development and effects. Histology has roots in biology. It involves the small tissue structures and their composition.

Histology can benefit the study of pathology. It helps explain how and why certain diseases afflict humans. Pathology can also benefit histology. Its study provides applications of what histologists identify under their microscopes.

What is the difference between biopsy and histopathology?

Last is the term histopathology. It combines histology and pathology. Histology studies tissues, and pathology studies diseases. Hence, histopathology studies tissues related to disease. A biopsy is the removal of tissue samples for clinical examination. It also refers to the actual tissue samples.

Biopsies are crucial to the field of histopathology. They help in diagnosing diseases from tissue samples.

Why should I study histology?

Histology is interesting. It allows you to see and appreciate the world in a different way. It gives you a thorough understanding of the microstructures that make you, you. Histology also sits at the crossroads of anatomy, pathology, and physiology.

Aside from this, there are other applications for histology:

1. Autopsies. Studying tissues from a deceased human or animal can help determine the cause of death.

2. Forensic investigations. Tissue sample studies can clarify several forensic issues. Hence, they can help solve crimes.

3. Veterinary diagnosis. Animal samples can suggest ways to treat and manage their conditions.

REFERENCES:

Ali, R. (n.d.). Introduction to Basic Histology. UO Babylon. Retrieved February 20, 2022 from uobabylon

Brown,    R.    (2021).    What    is    a    biopsy?    Rcpath.org.    pathology/news/fact-sheets/what-is-a-biopsy.html#:~:text=Histopathologists%20examine%20biopsies%20(tissue%20or,more% 20detail%20under%20the%20microscope.

Exploring Nature. (n.d.). from exploring nature. The 4 Basic Tissue Types in the Human Body

Histology vs. Pathology – What’s the difference? | Ask Difference. (2018). Askdifference.com.

Introduction to Histology – Applications & Importance – Anatomy Notes. (2020, January 14). Anatomy Notes. https://anatomynotes.org/histology/introduction-to-histology- applications-importance/

Mescher, A. (2018). Junqueira’s basic histology : text and atlas (15th ed.). Mcgraw Hill Education.

Thompson, V. (2014). Why Is the Study of Histology Important in Your Overall Understanding of Anatomy & Physiology? Education – Seattle PI. https://education.seattlepi.com/study- histology-important-overall-understanding-anatomy-physiology-5337.html