A Simple Introduction to Tissue Identification in Preparation for the USMLE
Through the Microscope
I have put together this guide for the identification of tissue types and where they are found within the body to help students of Histology.
The following is an attempt at organizing information necessary to not only pass a graduate level Histology course but to aid with the studying for the United States Medical Licensing Examination (USMLE).
This information is provided from my personal experience in Histology and a beleif that I can organize the information in a way that will be useful for those who are about to study for the above mentioned exams.
For those of you who have no experience with Histology and are not currently working on a Medical License or the lay person who wants to learn more about Anatomy and Physiology, take your time, and use the comment section to ask as many questions you may have.
Basic Cell Structure
The cell is a structurally independent unit and may function independently or as a group. There are two major parts of the cell, the nucleus and the cytoplasm.
The cells have organelles that are responsible for the specialized functions of the cell. for example the ribosomes for protein synthesis, or vacoules for storage, or mitochondria for respiration. These organelles are primarily found in the cytoplasm.
The nuclear area of the cell contains the nucleus and the components of the nucleus.
All living organisms are composed of cells as their basic functional units.
Cytology is the study of cells, in this article we are more concerned with Histology or the study of tissue.
Tissue Layers and Organs
Our body is made of organ systems, organs, tissues, and cells. Sometimes to understand the physiology of an organ system, or an organ within a system, it is necessary to look at the cells that make up the tissue within the organ.
I will be going over the different types of epithelial tissue, connective tissue (including bone and blood cells), muscle tissue, and nervous tissue, and where these tissues are located in the body. These are considered the big four when looking at tissue types.
A discussion of function may not be included with the tissue types or an indepth look at physiology. There is far too much information to fit here, but it is important to remember that form and function lie hand in hand.
Epithelium covers all external surfaces of the body and lines the internal surfaces of organs. The free surface of epithelium has contact with either air or fluid.
The major function of epithelium is to cover and line body surfaces. Modifications on the surface of epithelial cells is dependent on their functions. They can be either in single layers or multiple layers and always rest upon a basement membrane.
Simple Squamous Epithelium is a single layer of flat or elongated cells. It is located in:
- the lining of the Heart
- the Alveoli of the Lungs
- the Glomerulus of the Kidney
- the inner surfaces of the Tympanic Membrane
- the layer of Peritonium
- and Lymph Vessels
Simple Squamous Endothelium is a single layer of flat or elongated cells having a lumen which contain blood. It is located in arteries and veins.
Simple Cuboidal Epithelium is a single layer of round to square shaped cells attached to a basement membrane. It is located on:
- the surface of the Ovary
- the pigment epithelium at the posterior surface of the Eye
- the Kidney tubules
- the ducts of many glands
- and the secretory portions of some endocrine glands including the Thyroid and Pancreas
Simple Columnar Epithelium is a single layer of cells that are taller than they are wide. They may be ciliated or nonciliated.
Ciliated Simple Columnar Epithelium are located in:
- the Upper Respiratory Tract
- the Fallopian Tubes
- Brain Ventricles
- and the Central Canal of Spinal Cord
Nonciliated Simple Columnar Epithelium are located on the surface of the gastrintestinal tract, the ducts of many glands, and the Gallbladder.
Psuedostratified Columnar Epithelium is actually a single layer of cells, taller than they are wide, which appear to be arranged in multiple layers. Individual cells can be seen extending from the basement membrane to the lumen. They are found in the Epidiymis and parts of the male Urethra.
Ciliated Pseudostratified Columnar Epithelium are the same as the above mentioned yet have cilia on the outer border facing the lumen. These are found in the Upper Respiratory Tract.
Stratified Squamous Epithelium are multiple layers of cells flattened at the luminal surface. They may be keratinized or nonkeratonized.
The keratinized cells are located in thick and thin skin.
The nonkeratonized cells are located on:
- the lining of the Mouth
- the Larynx
- the Epiglottis
- the Vagina
- and on the Tongue
Stratified Cuboidal Epithelium are two or three layers of round to square cells on the luminal surface. These are located in adult sweat glands, in Esophageal glands, and in parts of the male Urethra.
Stratified Columnar Epithelium are two or more layers of cells elongated on the luminal surface. They line parts of the Urethra, the excretory ducts of some glands, anal mucous membranes, and part of the conjuctiva of the Eye.
Transitional Epithelium are multiple layers of cells whose luminal surface is characterized by loose rounded to cuboidal appearing cells. They are located on the lining of the Urinary Bladder, Ureters, and parts of the Urethra.
Connective Tissues are tissues containing cells which connect one type of tissue to another or one body area to another. Some have characteristic fibers or cells, and some have both. It provides support to the overlying Epithelium, provides form for the body, provides storage centers, and provides defense mechanisms.
Embryonic Connective Tissue Mesenchymal Tissue has many irregular shaped Mesenchymal cells in a ground substance containing Reticular fibers. These are found in skin and the developing bone of embryos. It can also be found in adult connective tissue along blood vessels.
Embryonic Connective Tissue Mucoid Tissue is scattered Fibroblasts dispersed within a viscous jellylike ground substance containing Collagen fibers. These are located in the Umbilical Cord of the Fetus.
Loose Areolar Connective Tissue can be identified by irregularly arranged fibers of Collagen, Elastin, and Reticulan. There is also the presence of characteristic cells such as Fibroblasts, Macrophages, Plasma cells, Adipocytes, and Mast cells in a semifluid matrix. It is located in:
- the Subcutaneous Layers of the Skin
- deep to the basement layers of the Epidermis
- the Lamina Propria of mucous membranes
- and around the outer portion of arteries, veins, nerves, and most body organs
Adipose tissue is identified as a collection of Adipocytes with the cytoplasm and nucleus located peripherally. Each Adipocyte has a large central droplet of fat. Adipose It is located in:
- the Subcutaneous Layer of Skin
- around the Heart, Kidneys, Adrenal Glands, Pancreas, and Intestinal Tracts
- Posterior to the Eyeball in the Orbit
- in Yellow Bone Marrow
- and between Skeletal Muscle Fiber Bundles
Reticular Connective Tissue or Fibers are identified as thin network of interlacing fibers and cells made up of Glycoprotein coated fine Collagen fibers. It is located in:
- Lymph Nodes
- Red Bone Marrow
- and associated with basement membranes around blood vessels and muscles
Dense Regular Connective Tissue or White Fibrous Connective Tissue is the strongest material in our bodies. It is identified as bundles of Collagen fibers in rows with Fibroblasts present between the rows. It is found in Ligaments, and Tendons.
Dense Irregular Connective Tissue is randomly arranged Collagen Fibers with periodic Fibroblasts. It provides flexibility and strength and is located in:
- Fascia beneath Skin
- around muscles and organs
- the Dermis of Skin
- in capsules around many organs
- in Pericardium of Heart
- the Peristeum of Bone
- the Perichondrium of Cartilage
- and in Joint Capsules
Elastic Connective Tissues are freely distributed Elastic Fibers and Fibroblasts. It provides maximal elasticity and is located in walls of arteries, Lungs, Trachea, Bronchi, Vocal Chords, and Ligaments between Vertebrae.
Cartilage and Bone
Hyaline Cartilage is identified as numerous Chondrocytes located at the ends of long bones, anterior ends of Ribs, parts of Larynx, Trachea, and Bronchi, and the Fetal Skeleton.
Fibrocartilage is identified as Chondrocytes surrounded by dense bundles of Collagen fibers within the extracellular matrix. It is located in the Pubic Symphosis, Intervertebral Discs, Mensci, and portions of Tendon attached to bone.
Elastic Cartilage is identified as Chondrocytes located in a matrix containing a threadlike network of Elastic fibers. It is located in the Epiglottis, the Ear, and the Eustachian Tubes.
Compact bone is identified as a collection of Osteocytes within Lacunae, arranged in concentric Lamellae. it is very dense material and is used a a storage site for Calcium. It is found in the dense portions of bones.
Spongy Bone or Bone Marrow is Hemopoeitic tissue, or the blood forming portion of bone. It is made up of Trabeculae, Megakaryocytes, and Adipocytes. It is located in the deep central portions of bones particularly long bones or the distal and proximal Epiphyses and Metaphyses.
Blood tissue is made up of Erythrocytes or red blood cells and Leukocytes or white blood cells. White blood cells are either granular like Neutrophils, Basophils, and Eosinophils or nongranular like Lymphocytes, T-cells and B-cells, and Monocytes.
Blood tissue is an integral part of the circulatory system which transports blood cells, contains clotting components, and transports nutrients and gases.
Some other constituents of blood are the platelets or Thrombocytes, and plasma proteins.
Blood tissue is located inside Arteries and Veins, Red Bone Marrow, and highly vascular organs such as the Liver, Spleen, Lungs, and Kidneys.
Lymph is an aggregation of lymphocytes, T-cells and B-cells, and are located in Lymph nodes, Thymus, Tonsils, and Peyer's Patches in the Intestinal Tract.
There are three major types of muscle tissue:
- Smooth Muscle
- Skeletal Muscle
- Cardiac Muscle
Muscle tissue helps with body movement, and with movement of bodily secretions.
Smooth Muscle is located in the skin, for example the Arrector Pili muscle in Dermis, the deep layers of the Gastrointestinal Tract, the deep layers of Arteries and Veins, the Uterus, the Gallbladder, the Urinary Bladder, and the Iris of the Eyes. It can be identified by it's central elongated nucleus and it elongated fusiform cellular shape.
Skeletal Muscle are the Muscles attached to bones by tendons and attached to other muscles. It can be identified by an eccentric nuclei, the presence of striations, and an elongated cellular shape.
Cardiac Muscle is located exclusively in the Heart. This muscle can be identified by a central ovoid nucleus, striations, intercalated discs, and irregular cellular branching patterns.
Nervous tissue is ectodermal in nature, which describes where the tissue would begin differentiation in the embryo.
Nervous tissue detects, analyzes, and transmits information received by sensory stimuli. It then coordinates motor, visceral, endocrine, and mental activities.
The area made up of neurons and glial cells is called the matrix.
The conductive cells in the matrix are called neurons and can be unipolar, bipolar, and multipolar.
The supportive cells in the matrix are called Glial Cells. The Glial Cells are Astrocyes, Microglia, Oligodendroglia, and Ependymal cells.
The nervous tissue located in the Brain and Spinal Cord can be Grey Matter or White Matter.
Grey matter contains primarily neurons and their dendrites.
Aggregations of neurons in the brain are called nuclei. Aggregations of neurons outside the Central Nervous System are called ganglia.
White Matter consists of myelinated and unmyelinated axons travelling from one area to another.
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© 2012 Jamie Lee Hamann