“The study of Tissues”

This video may be all you need—  “Histology for Beginners

Histology is the study of tissues—  the types of cells that can be found in the tissue as well as the extracellular components (molecules and other substances around the cells) also called extracellular matrix or ECM.

Extracellular matrix (ECM):  All tissues have an extracellular matrix of some kind.  In some tissues there is very little ECM that is mostly fluid and a bit of protein or glycoprotein, in other tissues there is a lot of ECM that is dense with collagen fibers and a gel like ground substance.  In all tissues there is a constant movement of fluid from and to the vascular systems.

You will most likely be studying tissue under a light microscope, so here is a blog about: The Light Microscope

The human body has four primary tissue types…           (click on each for more detail)

  • Epithelial tissue —  makes up linings and coverings and also most exocrine glands.
  • Muscle tissue —  the only tissue in the body that is capable of quick movement in the form contraction (shortening).  Muscle is also one of two tissues considered “excitable” meaning capable of quick reactions to certain stimuli.  Three types:  skeletal, smooth, and cardiac.
  • Nervous tissue — performs communication and data processing (thinking, memory, controlling muscles and other structures, etc.).  This is the second type of “excitable” tissue.
  • Connective tissue — connects adjacent tissues, fills spaces, stores materials and nutrients.  This is the most variable of the primary tissue types ranging from the hardness of bone connective tissue to the liquid of blood connective tissue.

Tissue healing is also important to understand.  There are two major forms of healing, the first and usually preferred is regeneration and the second is fibrotic healing.  Regenerative healing basically replaces the original tissue with the same tissue that has all the same properties and functions as the original tissue (obvious to see why this is usually preferred).  Fibrotic healing involves replacing the original tissue with fibrotic tissue.  Fibrotic tissue is very dense with collagen fibers, so it is not very flexible and it does not have all of the same properties or functions of the original tissue, it basically just binds local tissues together.

Some tissues heal better than others; here are some examples:

  • Smooth muscle tissue can take a lot of damage and tends to heal by regeneration, and rather quickly.
  • Epithelial tissues tend to heal quickly by regeneration.
  • Cardiac muscle tissue tends to heal by fibrosis because cardiac myocytes can not be replaced once destroyed.  This means that after a heart attack some portion of the person’s heart wall gets replaced by fibrotic tissue that can’t contract…  it changes the person’s heart forever and may lead to more problems down the road.
  • Skeletal muscle cells also get replaced by fibrotic tissue if a large number of cells are killed.
  • Nervous tissues of the brain and spinal cord heal by liquefaction if a large area of neurons die.  The tissue is literally replaced by liquid.

Here is a blog on the basics of wound healing using a cut in skin as an example:  Wound Healing

Other terms to understand regarding cells and tissues:

  • Atrophy — this is tissue shrinkage due to a decrease in cell size (the number of cells stays the same).  This is how skeletal muscles shrink if the muscle stops exercising for example.
  • Hypertrophy — this is the opposite of atrophy.  The tissue grows due to an increase in cell size.  This is how muscles grow when we lift weights for example.
  • Necrosis — this is abnormal and premature cell death due to toxins, infarction (loss of blood flow), trauma, infection, or other causes.
  • Hyperplasia — this is tissue growth due to cells dividing.  Here the tissue grows because more cells are being added–  the size if the cells is the same, there are just more of them.  The can happen normally as when we develop a callous on our hand, or it can be part of a disorder such as hemihyperplasia where one side of a childs body grows faster, leading to limbs that are longer than the other side of the body.
  • Dysplasia — this is not only cell division, but abnormal appearing cell division.  Dysplasia can be caused by viral infection, mutation of a cells DNA, or both.  Dysplasias may develop into cancers.
  • Metaplasia — this the reversible transformation of a tissue from one type to another type of fully developed normal tissue.  Most often metaplasia occurs because of a change of the tissue’s environment  that requires the tissue to become more tough.  Example:  Lining the trachea we have pseudostratified columnar epithelium, but if it gets exposed to the irritants of cigarette smoke this epithelium can change into stratified squamous epithelium.  This change allows the epithelium to withstand the irritants better, but it no longer has cilia to move mucus (major cause of long term smoker’s cough).
  • Tissue Engineering — this is the artificial production of tissues and/or organs.  Right now research is being done to increase the possible treatments for this kind of research, but so far here are some of the real world human parts that have been grown in a lab and implanted into a person:  cartilage, trachea, and small blood vessels.
  • Stem cells — Stem cells are cells that are capable of dividing and giving rise to one or multiple types of mature differentiated cells.  Some stem cells are unipotent (can only give rise to one cell type), multipotent (can give rise to multiple cell types), totipotent (can give rise to all cell types–  embryonic stem cells are the only cells that can do this).  The fact that embryonic stem cells can give rise to any tissue or organ is the reason that we are interested in studying them (and also the reason that studying them is controversial).  If we can learn the secrets of embryonic stem cells we might be able to take skin cells from a person that needs a kidney for example, turn those skin stem cells from being unipotent skin stem cells into being totipotent embryonic stem cells and then signal them to produce a kidney.  If we could do this kind of thing with stem cells it could help many people.  The controversy stems from the fact an embryonic stem cell, capable of producing all tissues and organs, could also produce a fully formed human being…  And for some people that makes the embryonic stem cell a human being.


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