Histology Notes
Epithelial tissue- Are widespread throughout the body. Since epithelium covers organs, forms the inner lining of body cavities, and lines hollow organs, it always has a free surface- one that is exposed to the outside or to an open space internally. The underside of this tissue is anchored to connective tissue by a thin, nonliving layer called the basement membrane.
Function- Protection, secretion, absorption, excretion
Location- Cover body surface, cover and line internal organs, compose glands
Distinguishing Characteristics- Lack blood vessels, cells readily divide, cells are tightly packed
How are they classified- according to shape, and number of layers of cells.
Shape
- squamous; those that are composed of thin flattened cells, look like fried eggs from the top, flat, irregularly shaped cells with dark staining nuclei
- cuboidal; those with cube-like or square shapes and have a very round nuclei
- columnar; shaped like columns are stretched out and elongated cells have elongated nuclei located closer to the basement membrane than the apical surface.
Layers
- simple- one layer
- stratified- two layers
- pseudostratified- 1 layer but looks like it has many layers, all cells touch the basement membrane
The free surfaces of epithelial cells are modified to reflect their specialized functions.
1. Simple Squamous Epithelium-consists of a single layer of thin, flattened cells. These cells fit tightly together, somewhat like floor tiles, and their nuclei are usually broad and thin. Substances pass rather easily through simple squamous epithelium, which is common at sites of diffusion and filtration. Lines the air sacs (alveoli) of lungs where oxygen and carbon dioxide are exchanged. Also forms the walls of capillaries, lines the insides of blood and lymph vessels, and covers the membranes that line body cavities.
Because it is so thin and delicate it is easily damaged. Good for diffusion of gases.
LOCATED IN, (air sacs of lungs and capillaries)
Top view
Side view
2. Simple Cuboidal epithelium- consists of a single layer of cube-shaped cells. These cells usually have centrally located, spherical nuclei—large, round nuclei. Covers the ovaries and lines the kidney tubules and ducts of certain glands, such as the salivary glands, pancreas, and liver. Good for absorption and secretion, recognized by big round nucleus.
LOCATED IN- lining of kidney tubules and ducts of glands
3.Simple Columnar Epithelium- composed of a single layer of column shapes with elongated nuclei. All touch the base of the membrane. Primarily for absorption. Located in stomach and intestines.
Often have microvilli- tiny cylindrical, cellular processes extending from their surfaces. Microvilli increase the surface area of the cell for better absorption.
May be interrupted by goblet cells containing mucus.
Goblet cells have mucus inside to help lubricate things passing by.
LOCATED IN- stomach and intestines
4.Pseudostratified Ciliated Columnar Epithelium- Has a Single layer, but gives the appearance of more than one layer. All cells touch the basement membrane making it only one layer. A layered effect occurs because the nuclei are at two or more levels in the row of aligned cells. May also have goblet cells. Have cilia, hairlike extensions, to sweep things across the top of the cells. Lines the passages of the respiratory system. Located in Trachea. Mucous covered linings are sticky and trap dust. Cilia move mucus and captured particles upward and out of airways. Also found in fallopian tubes to help move eggs.
LOCATED IN- Respiratory tract, trachea (many goblets) and fallopian tubes, in fallopian tubes it doesn’t have goblets
5. Stratified Squamous Epithelium- Many layers
5a- Keratinized stratified squamous epithelium- Has a top layer of keratin and dead skin cells. Keratin is a protein coating for waterproofing. Only found in epidermis. To tell if it has keratin look for protein and dead skin cells on surface. LOCATED- only in epidermis
5b- Nonkeratinized Stratified Squamous epithelium- Does not have protein and dead cells on top and are not water proof. Located in Esophagus, oral mucosa, and anus, vagina, etc.
The many layers are for protection. Older cells will move toward the top and become flattened.
LOCATED IN- esophagus, oral mucosa, and anus
6.Transitional Epithelium- Changes shape and has many layers.
Has a special feature, umbrella cells are located next to the free space. These are rounded, bulging cells.. The top layer always has umbrella cells.
Found in urinary bladder, lines the uteters, and part of the urethra. Also forms a barrier that helps prevent the contents of the urinary tract from diffusing back into the internal environment. Transitional means it has many shapes and changes shape as the bladder empties and fills with urine.
LOCATED IN-urinary bladder
Unstretched Transitional epithelium
Stretched Transitional epithelium
**Stratified Cuboidal Epithelium –Consists of two or three layers of cuboidal cells that form the lining of a lumen. The layering of the cells provides more protection than the single layer affords. Stratified cuboidal epithelium lines the larger ducts of the mammary glands, sweat glands, salivary glands, and pancreas. It also forms the lining of developing ovarian follicles and siminiferous tubules, which are parts of the female and male reproductive systems.
**Stratified Columnar Epithelium- consists of several layers of cells. The superficial cells are elongated, whereas the basal layers consist of cube-shaped cells. Stratified columnar epithelium is in the vas deferens, part of the male urethra, and in parts of the pharynx.
Connective Tissue
There are two categories of connective tissues
1.Connective tissue proper- includes loose connective tissue, dense connective tissue, adipose tissue, reticular connective tissue, and elastic connective tissue.
2.Specialized connective tissues- includes cartilage, bone, and blood.
Tissue Fibers
Collagenous Fibers- Thick, threadlike fibers of collagen with great tensile strength. Hold structures together, such as tendons and ligaments. Sometimes called white fibers.
Elastic Fibers- Bundles of microfibrils embedded in elastin. Provide elastic quality to parts that stretch, such as vocal cords and air passages of the respiratory system. Sometimes called yellow fibers.
Connective Tissue Proper
1. Loose Connective Tissue or Areolar tissue- binds the skin to the underlying organs and fills spaces between muscles.
Location- Lies beneath most layers of epithelium, where its many blood vessels nourish nearby epithelial cells.
2. Dense Connective Tissue- Very strong tissue able to withstand pulling forces. Often binds body parts together.
Wavy collagenous fibers tightly packed with fibroblasts squeezed between them.
Location- In tendons (connecting muscles to bone), and in ligaments- (connecting bone to bone.)
3.Adipose Connective Tissue or Fat- At first, these cells resemble fibroblasts, but as they accumulate fat, they enlarge, and their nuclei are pushed to one side. Adipose CT cushions joints and some organs. Also insulates beneath the skin and stores energy in fat molecules.
Location- Beneath the skin, in the breasts, and around the heart and kidneys.
4. Reticular Connective Tissue- Composed of thin, collagenous fibers in a three-dimensional network. Supports the walls of certain internal organs.
(Hint- looks like a lot of black dots) thin, collagen fibers do not show up on the lab slide
Located in- Liver, spleen and lymphatic organs.
**5.Elastic Connective Tissue- consists mainly of elastic fibers in parallel strands. Between fibers are fibroblasts and collagenous fibers.
Locations- Found in attachments between vertebrae of the spinal column and in layers within the walls of hollow internal organs, including the larger arteries and some portions of the heart.
Specialized Connective Tissues
A. Cartilage- Provide support, frameworks, attachments, protects under-lying tissues, and form structural models for many developing bones. The cartilage matrix- (intercellular material) is largely composed of collagenous fibers embedded in a gel-like ground substance. Cartilage cells are called chondrocytes. Chondrocytes occupy small chambers called lacunae- (opening or space).
There are three types of cartilage- hyaline, elastic, and fibrocartilage.
1. Hyaline Cartilage- Strong Cartilage, matrix looks glassy – very few fibers.
Located- on the ends of bones in many joints, in the soft part of the nose, and in the supporting rings of the respiratory passages. The trachea also has C- rings of hyaline cartilage.
2. Elastic Cartilage- More flexible than hyaline because its matrix contains many elastic fibers.
Location- External ears and parts of the larynx.
**3. Fibrocartilage- Very tough contains many collagenous fibers. It is a shock absorber for structures that are subjected to pressure.
Location- Pads between the vertebrae of the spinal column and also cushions bones in the knees and in the pelvic girdle.
B. Compact Bone- Is rigid due to mineral salts, such as calcium phosphate and calcium carbonate, in its matrix. It protects vital structures and is an attachment for muscles. It contains red marrow, which forms blood cells, and it stores and releases inorganic salts. It’s osteocytes and layers of intercellular material, which are concentrically clustered around a central canal, form a cylinder-shaped unit called an Osteon- made of microcopic units also called a Haversian system.
In addition the bone cells have many cytoplasmic processes that extend outward and pass through minute tubes in the matrix called canaliculi- for cell-to-cell communication.
C. Blood – composed of cells that are suspended in a liquid matrix called Plasma. Include red blood cells, white blood cells, and platelets.
Red blood cells (erythrocyte), 7.5um- transport oxygen and nutrients, and have no nucleus.
White blood cells (leukocytes) 10-12um- are larger, they fight infection and can have a multi-lobed nucleus- the nucleus appear to be more than one, but are attached and are only one.
Platelets- (thrombocytes) cell fragments- are involved in blood clotting.
Muscular Tissue
Due to their elongated shape cells in muscle tissues are sometimes called muscle fibers. Muscle tissues are contractile: they can shorten and thicken. As they contract, muscle cells pull at their attached ends, which move body parts. There are three types of muscle tissues: Skeletal, Smooth, and Cardiac.
1. Skeletal Muscle Tissue- Forms muscles that usually attach to bones and are controlled by conscious effort (voluntary). Skeletal muscles are long, 40mm in length and narrow, less than 0.1 mm in width. These threadlike cells of skeletal muscle have alternating light and dark banding patterns called Striations. Each cell is multinucleate, (having many nuclei), that are peripherally located, (on the edge). The cells run parallel.
Location- attached to bones
2. Cardiac Muscle Tissue- Cells are striated and joined end-to-end. The resulting muscle cells are branched and interconnected in complex networks. Each fiber has a single nucleus that is centrally located. Where the cells join there is a special intercellular junction called intercalated discs used for cell-to-cell communication. Cardiac MT is involuntary.
Location- only in the heart
Hint- look for pink intercalated disks on lab slide. It looks different than text book pics.
3. Smooth Muscle Tissue- is called smooth because it is non-striated. Fibers are spindle shaped with one centrally located nucleus. Smooth MT is involuntary.
Location- walls of blood vessels and hollow internal organs.
Lab slide shows fibers teased apart
Nervous tissues
Nuerons- main conducting nerve cells. Conduct nerve impulses. Neurons sense certain types of changes in their surroundings and respond by transmitting nerve impulses along cellular processes to other neurons or to muscles or glands. They can coordinate, regulate, and integrate many body functions.
Nueroglial cells- cells support, nourish and protect neurons.
Motonueron- comes in contact with skeletal muscle tissue. Sends impulses for movement.
Dendrites- carry information to the cell body for processing.
Axon- carries information away from the cell body.
Myelin Sheath- formed by neuroglial cells wrapping around axon. Protects the axon and speeds up impulses.
Terminal Ends- synapse with skeletal muscles. Instructs skeletal muscles.
