Tissues

A tissue can be conceptualized is a group of cells that have similar structure and that function together as a unit. When examining the histology of an organ, the orderly arrangement of cells is indicitave of a tissue.

There are four tissue types in the body: epithelium, connective tissue, muscle tissue, and nervous tissue.

Each tissue is designed for specific functions.

Histology of Epithelium

Epithelium lines body cavities and covers body surfaces.   Because of this, there is a free surface on epithelium. The histology of epithelium is cells that are packed close together with little space between cells.

Histology of Connective Tissue

Connective tissue is the most diverse tissue type. On a histology slide of connective tissue, the cells are separated by a large distance from each other.  There is a lot of intercellular matrix between the cells.

Histology of Muscle Tissue

Muscle tissue is specialized for contraction.

Histology of Nervous Tissue

Nervous tissue is specialized for conduction.  It consists of neurons and supporting cells.

HISTOLOGY OF EPITHELIUM TISSUE

Location of Epithelium

Epithelum is found throughout the body. Epithelium covers body surfaces, lines body cavities, and lines hollow organs.

Functions of Epithelium

Protection, absorption, secretion, excretion, filtration, and diffusion are all functions of epithelium.

Histology of Epithelium

When looking at the histology of epithelium, it can be seen that the cells in epithelial tissue are tightly packed.  There is very little intercellular space between the cells in epithelial tissue.

Since epithelium forms surface coverings and linings, there is one free surface that is not in contact with other cells. On the other side of the free surface, there is a basement membrane.  The basement membrane is non-cellular.  It is composed of carbohydrates and proteins which are secreted by the epithelial cells and connective tissue cells.

Histology hint from Sarah Bellham: The "free surface" on epithelum can be used as an aid in idenfying epithelium on a histology slide.

Classification of Epithelium

Epithelium is classified by the shape of the cells and the number of cell layers.

Epithelial cells may be flat, cuboidal, or columnar. If the cells are flat, the epithelium is classified as squamous. If the cells are as tall as they are wide, it is called cuboidal. If the cells are taller than they are wide, the epithelium is classified as columnar.

Epithelial cells may be arranged in a single layer or stacked upon one another in a multiple layers. Epithelium that is one cell thick is classified as simple. Epithelium that is more than one cell layer thick is classified as stratified.

llustration of the histology of epithelium

Histology of Simple Squamous Epithelium

Simple squamous epithelium is a single layer of flat cells.

Simple squamous epithelium is "simple" because it is one cell thick. "Squamous" refers to the fact that the cells are flat.

Endothelium is a type of simple squamous epithelium which lines blood vessels.  Mesothelium is a type of simple squamous epithelium which lines the body cavities.

Histology of Simple Cuboidal Epithelium

Simple cuboidal epithelium has cells which are as tall as they are deep and wide. Simple cuboidal epithelium is "simple" because it is one cell thick. "Cuboidal" refers to the shape of the cells.

The lining of most ducts is simple cuboidal epithelium. The kidney tubules are simple cuboidal epithelium.

Histology of Simple Columnar Epithelium

In simple columnar epithelium, the height of the cell is greater than the width and depth of the cell.  Simple columnar epithelium is "simple" because it is one cell thick. "Columnar" cells are taller than they are wide and have an oval nucleus.

The lining of the gastrointestinal tract is simple columnar epithelium. Goblet cells are associated with simple columnar epithelium of the gastrointestinal tract.

The mucosa of the gallbladder is made of simple columnar epithelium.

Histology hint from Sarah Bellham: the simple columnar epithelium of the gallbladder is very tall!

Histology of Stratified Squamous Epithelium

Stratified squamous epithelium is "stratified" because it is more than one cell layer thick. "Squamous" refers to the fact that the surface cells of the stratified squamous layer are flat. 

Histology hint from Sarah Bellham: In stratified squamous epithelium, the cells at the basal layer are cuboidal or even columnar. However, the epithelium is still classified as "squamous" based on the cells of the surface layer.

The epidermis of the skin is stratified squamous epithelium. The lining of the esophagus is stratified squamous epithelium. The cornea is covered by a non-keratinized stratified squamous epithelium.

Histology of Pseudostratified Columnar Epithelium

Pseudostratified columnar epithelium is a single layer of cells which looks like it is stratified. Pseudostratified squamous epithelium is "pseudostratified" because it is only one cell layer thick, yet it appears to be stratified. In reality, every cell touches the basement membrane.

Histology hint from Sarah Bellham: The prefix "pseudo" is of Greek origin and it means false or counterfeit. For example: pseudonym, pseudo-science or pseudostratified.

Pseudostratified columnar epithelium lines the trachea and respiratory tract as well as some of the male reproductive tract.

Histology of Transitional Epithelium

Transitional epithelium has domed shape cells on the apical surface.  It can be distended or stretched.

Transitional epithelium is found in the bladder and urinary tract.

Epithelial Modifications

Keratinization

Keratinization is seen in the epidermis. Cells in the stratum corneum are essentially just bags of keratin.

Microvilli

Microvilli are finger like projections seen on the surface of some cells. Microvilli form the brush border (striated border).

Stereocilia

Stereocilia are very long microvilli. Stereocilia are seen in the epididymis and the hair cells of the ear.

Cilia

Cilia is the hair like surface modification seen on some epithelia. Cilia are made of microtubules.

Glands

A simple gland has an unbranched duct. A compound gland has a branched duct.

If the secretory portion of a gland is shaped like a tube, it is called "tubular". If the secretory portion of a gland is shaped like a flask, it is called "alveolar" or "acinar". If the secretory portion of a gland is shape like a tube but at the end it is shaped like a flask, it is called "tubuloalveolar".

Epitheliod Tissue

If a tissue is composed of tightly packed epithelial-like cells, but it does not have a free surface, the tissue is called epithelioid tissue. An example of epithelioid tissue is the parenchyma of the adrenal gland.

HISTOLOGY OF CONNECTIVE TISSUE

Of the four basic tissue types (epithelium, connective tissue, muscle and nervous tissue), connective tissue is the most diverse. Connective tissue can be found all over the body. Connective tissue cells are capable of reproducing, however not as fast as epithelial cells.  Embryologically, connective tissue develops from mesenchyme.

Function of Connective Tissue

Connective tissues connects and holds tissues and organs to one another, forms a framework, provides support, stores fat, transports things, protects, and aids in tissue repair.

Histology of Connective Tissue

Connective tissue consists of cells and extracellular fibers in a ground substance and tissue fluid. The cells within connective are not tightly packed together. On a histology slide, it can be seen that there is generally abundant extracellular space in connective tissue.

Within connective tissue, the cells and fibers are scattered within the ground substance. The ground substance is amorphous material composed of proteoglycans and glycosaminoglycans. Hyaluronic acid is a glycosaminoglycan. Dermatan sulfate, chondroitin sulfate, and keratan sulfate are also glycosaminoglycans.

Most of the connective tissues in the body have a good vascular supply.

Cell Types in Connective Tissue

Numerous cell types are found in connective tissue. Fibroblasts, histiocytes, plasma cells, and mast cells are cell types that are found in loose connective tissue.

Fibroblast

A fibroblast is a connective tissue cell derived from mesenchyme.   Fibroblasts produce collagen. The fibroblast also produces the ground substance in connective tissue.

Histiocyte

The histiocyte is a connective tissue macrophage.  Macrophages are mononuclear phagocytes, derived from a monocyte. A macrophage is a phagocytic tissue cell of the reticuloendothelial system that may be fixed or freely motile. Many tissues have resident (fixed) macrophages. Fixed macrophages are given a unique name, depending on the tissue that they are located in. In connective tissue, the fixed macrophage is a histiocyte. The function of histiocytes is for the protection of the body against infection and other harmful things.  

Plasma Cell

Plasma cells are derived from B lymphocytes.

Mast Cell

A mast cell is a large cell seen in connective tissue.  A mast cell has basophilic granules containing histamine and heparin which mediate allergic reactions. Mast cells also secrete SRS-A (slow reacting substance of anaphylaxis and ECF-A (eosinophilic chemotactic factor of anaphylaxis).

Fibers in Connective Tissue

There are three types of fibers found in connective tissue: collagen fibers, elastic fibers, and reticular fibers. Collagen fibers are the most abundant fiber type in connective tissue.

Classification of Connective Tissue

Connective tissue can be sub-classified into connective tissue proper, specialized connective tissue and embryonic connective tissue.

Connective Tissue Proper

Connective tissue proper consists of loose irregular connective tissue and dense connective tissue. Dense connective tissue is subdivided into dense regular connective tissue and dense irregular connective tissue.

Loose Irregular Connective Tissue

Loose irregular connective tissue is areolar tissue.

Dense Regular Connective Tissue

Dense regular connective tissue comprises tendons and ligaments.

Dense Irregular Connective Tissue

Dense irregular connective tissue is seen in the dermis.

Specialized Connective Tissue

Specialized connective tissue includes cartilage, bone, adipose tissue, blood and hemopoietic tissue, and lymphatic tissue.

Cartilage

Bone

Adipose Tissue

Brown adipose tissue is multilocular adipose tissue. Brown adipose tissue is present during fetal development and then decreases after birth. White adipose tissue is unilocular adipose tissue. White adipose tissue persists into adulthood.

Blood and Hemopoietic Tissue

Lymphatic Tissue

Embryonic Connective Tissue

Embryonic connective tissue includes mesenchyme and mucous connective tissue.  Mesenchyme is embryonic connective tissue. It is an undifferentiated tissue found in the embryo. Mucous connective tissue is a type of embryonic connective tissue; it is a subset of mesenchyme. Wharton's jelly is mucous connective tissue.

Histotechniques

Collagen fibers stain green with Masson's trichrome stain. Collagen fibers can be differentiated from other fibers by staining with Masson's trichrome stain.  A trichrome stain is a mixture of three dyes.

Gomori's trichrome will stain connective tissue green, cytoplasm red, and nuclei gray blue black.

Verhoeff Elastic stain stains elastic fibers blue/black. Collagen stains pink/red.