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Histology Notes J.Sasser
Histology is the study of tissues. A tissue is a group of cells with similar structure and function plus the extracellular substances located between the cells. The extracellular matrix is nonliving chemical substances located between cells.
Four basic tissue types: ๏ Epithelial tissue ๏ Connective tissue ๏ Muscular tissue ๏ Nervous tissue
Epithelial Tissue
Epithelial tissue covers surfaces, usually has a basement membrane, has little extracellular material, and has no blood vessels. A basement membrane attaches the epithelial cells to underlying tissues. Most epithelia have a free surface , which is not in contact with other cells. Epithelia are classified according to the number of cell layers and the shape of the cells.
Categories of epithelium based on cell layers:
๏ Simple epithelium has one layer of cells. ๏ Stratified epithelium has more than one layer of cells. ๏ Pseudostratified epithelium consists of a single layer of cells, in which some cells are tall and thin and reach the free surface and others do not.
Categories of epithelium based on cell shape: ๏ Squamous (flat) ๏ Cuboidal (cubelike) ๏ Columnar (tall and thin) ๏ Transitional (changes shape when relaxed or distented by fluid)
Functions of Epithelium:
๏ Simple epithelium involved with diffusion, filtration, secretion, or absorption ๏ Stratified epithelium protects from abrasion ๏ Squamous cells function in diffusion or filtration ๏ Cuboidal or columnar cells secrete or absorb
A gland is a single cell or a multicellular structure that secretes substances onto a surface, into a cavity, or into the blood. Most glands are composed primarily of epithelium. Exocrine glands have ducts. A duct is a tube or vessel which carries a secretion from a gland. Ducts can be simple or compound. The end of a duct can be tubular or expanded into a sac-Iike structure called an acinus or alveolus.
Types of exocrine glands: ๏ Unicellular (e.g., goblet cells in mucous membranes) ๏ Simple tubular (e.g., sweat glands and stomach glands) ๏ Simple acinar or alveolar (e.g., sebaceous glands) ๏ Compound tubular (e.g., duodenal glands) ๏ Compound acinar or alveolar (e.g., pancreas)
Endocrine glands - have no ducts and empty their secretions (hormones) directly into the blood.
Connective Tissue
Functions of Connective tissue: ๏ joins together other tissues ๏ supporting framework for the body (bone) ๏ fat stores energy ๏ blood transports substances
Connective tissue is usually characterized by large amounts of extracellular materials that separate cells from each other , whereas epithelial tissue is mostly cells with very little extracellular material. The extracellular substance of connective tissue consists of protein fibers which are embedded in ground substance containing tissue fluid.
Fibers in connective tissue can be divided into three types:
๏ Collagen fibers are the most abundant protein fibers in the body. ๏ Elastic fibers are made of elastin and have the ability to recoil to original shape. ๏ Reticular fibers are very fine collagen fibers that join connective tissues to other tissues.
Connective tissue cells are named according to their functions: ๏ Blast cells produce the matrix of connective tissues ๏ Cyte cells maintains the matrix of connective tissues ๏ Clast cells breaks down the matrix for remodeling (found in bone)
Word parts: ๏ fibro (fibrous connective tissue- tendons and ligaments) ๏ chondro (cartilage) ๏ osteo (bone)
Specific Types of connective tissue cells: ๏ fibroblasts ๏ fibrocytes ๏ chondroblasts
๏ chondrocytes ๏ osteoblasts ๏ osteocytes
๏ osteoclasts
Mast cells release chemicals promoting inflammation. Macrophages are large cells, capable of moving about and ingesting foreign substances, including microorganisms that are found in connective tissue.
Adipose - very little extracellular matrix; large cells filled with lipids
Bone - hard tissue with mineralized matrix; strong and rigid (two types- compact and cancellous)
๏ Compact bone - found in the shaft of a long bone ๏ Cancellous bone - found in the ends of long bones and in flat bones (contains red marrow).
Loose or areolar connective tissue - found covering muscles, glands, and nerves (has widely separated collagen fibers running in random directions)
Dense connective tissue - found in the dermis, also in tendons and ligaments (has an extracellular matrix consisting mostly of collagen fibers)
Blood - fluid matrix allows cells to travel freely
Inflammation
The inflammatory response occurs when tissues are damaged.
Major symptoms of inflammation:
- Redness
- Heat
- Swelling
- Pain
- Disturbance of function
Following an injury, chemicals (mediators of inflammation) are released or activated in the injured tissues.
Mediators of inflammation include: ๏ Histamine ๏ Kinins ๏ Prostaglandins
The presence of mediators of inflammation results in:
๏ vasodilation of blood vessels ๏ increased permeability of blood vessels ๏ pain ๏ symptoms of redness and heat
Edema (swelling of the tissues) results when water, proteins, and other substances from the blood move into the tissues.
Tissue Repair
Tissue repair is the substitution of viable cells for dead cells, and it can occur by regeneration or replacement. In regeneration , the new cells are the same type as those that were destroyed, and normal function is usually restored. In replacement , a new type of tissue develops that eventually causes scar production and the loss of some tissue function. When the edges of a wound are close together, the wound fills with blood, and a clot forms. The clot contains a threadlike protein, fibrin, which binds the edges of the wound together and stops the bleeding. The clot is replaced by granulation tissue , a delicate connective tissue which consists of fibroblasts, collagen, and capillaries.
Cells can be classified into three groups on the basis of their ability to divide and
produce new cells : ๏ Labile cells (skin and mucous membranes) continue to divide throughout life. ๏ Stable cells do not actively divide after growth ceases, but they do retain the ability to divide after an injury. For example, connective tissue and glands, including the liver and pancreas, are capable of regeneration. ๏ Permanent cells (neurons and skeletal muscle cells) have little or no ability to divide. If they are killed, they usually are replaced by connective tissue.
