HISTOLOGY

Histology: Study of Tissues

Tissues: Aggregations of similar cells that perform a function.

Tissue Types: Defined by Structure and Function

Epithelial: Line body cavities and surfaces, glands.
Connective: Bind, support and protect.
Muscle: Contraction and Movement.
Nervous: Conduct impulses; integrate information; sensory and motor (controls movement)

Cells in tissues are associated with an "extracellular matrix"

Matrix can be liquid (plasma of blood), semi-solid (gelatinous or fibrous, such as basement membranes, or solid (bone).

EPITHELIAL TISSUE

External and Internal Linings

Single or Multiple layers of cells that are bonded together by:

1) Glycoproteins: "Sticky," extracellular matrix

2) Junctions : Membranes fused or linked by specialized proteins. Associated with basement membrane.

Epithelial Types: Defined by Layering and Shape of cells.

Layering

Simple Epithelia: Single layer of cells. Found on surfaces across which diffusion, secretion, or filtration take place.

Examples: Linings of lungs; small ducts of kidney.

Stratified Epithelia: Multiple layers of cells. Found on surfaces that are protective or have to distend or stretch.

Examples: Skin; lining of bladder; large ducts.

Shape

1.) Squamous: flat shaped (Simple and Stratified)

2.) Cuboidal: cube shaped (Simple and Stratified)

3.) Columnar: rectangular (Simple and rarely stratified)

Columnar cells can be ciliated or non-ciliated. Cilia are fine extensions from cell that are used to increase surface area (digestive system) or are motile (move particles along surface-- as in the bronchial tubes and reproductive tract).

4.) Transitional (essentially stratified, but variable cell shapes)

Layering and Shape terms are combined:

Examples:

Simple Squamous: in lungs, kidney, and linings of vessels: endothelium.
Simple Cuboidal: in small ducts and tubules.
Simple Columnar: found in digestive system -- lining of stomach, gall bladder, intestine.

Stratified Squamous:

Found in skin and some linings.

Keratinized form makes up outer layer of skin. Waterproofing.

Non-keratinized is found lining body orifices (nose, mouth, esophagus, anus, cervix, vagina).

Stratified Cuboidal: larger ducts of sweat, salivary, and pancreatic glands.

Transitional Epithelium consists of layers of rounded cells and is found in luminal walls of ureters and urinary bladder. Change shape when distended.

Glandular Epithelium.

Unicellular glands: mucus-secreting goblet cells

Multicellular glands: Simple and compound.

Sweat glands, oil (sebaceous) glands of skin, salivary glands, some digestive glands, mammary glands.

Glands secrete their products in different ways:

Merocrine: products secreted via exocytosis. Sweat glands, exocrine portion of pancreas.

Holocrine: cells accumulate products and then rupture, e.g., sebaceous (oil) glands.

Apocrine: cells accumulate products and then bleb off pieces of cytoplasm. (It is debated whether humans have apocrine glands. In the past, mammary glands were considered apocrine).

CONNECTIVE TISSUE

Binds, Supports, or provides for metabolic needs of other cells

Defined by Cell Type and the type of Matrix (the material surrounding the cells).

Mesenchyme: Embryonic form. Gives rise to all types of Connective tissue.

Connective Tissue Proper:

5 Subtypes. Matrix is loose and flexible.

Fibroblasts are the common cell type in this tissue.

Fibroblasts secrete two important proteins: collagen and elastin, which are long, thin fibers.

Collagen and elastin fibers are generally not branched.

Reticular fibers (a type of fine collagen fiber) are highly branched.

Packing density of these two proteins (collagen and elastin) gives connective tissues their strength or cushioning ability.

Low density: Cushioning function rather than high strength.

High density: High strength.

Subtypes

1) Loose Connective Tissue or Areolar

Binding and packing material (binds skin to muscles).

Low density and irregular patterns of fibers allows stretch and passage of nutrients.

2) Dense Fibrous Connective Tissue

Dense packing of collagenous fibers; some elastic fibers.

Tendons: Connect muscles and bones.

Ligaments: Connect bones.

Sclera (White of eye)

3) Elastic Connective Tissue

More elastin than collagen fibers. Can stretch more than dense fibrous.

Found in walls of large arteries that must expand and contract in response to blood pressure.

4) Reticular Connective Tissue

Has network of reticular fibers (a type of collagen). Spongy.

Found in liver, spleen, thymus and lymph nodes.

Fat containing cells. For food storage and protective (insulation; pads of fat cushion organs). Like loose fibrous, but fibroblasts specialize into fat-containing cells rather than protein secreting cells.

Other Connective Tissues:

Cartilage: Cells (chondrocytes) are embedded in semi-solid matrix having high amounts of "mucopolysaccharides" (glycosaminoglycans).

Supportive and protective. Has strength yet flexible. Found in nasal septum, ear, joints, trachea, bronchi, intervertebral discs and in growing bones.

Generally avascular.

Structure: chondrocytes within lacunae (spaces). Surrounded by perichondrium (dense fibrous connective tissue).

3 Subtypes (Matrix & Fiber content vary):

1) Hyaline cartilage: Most abundant. Found in joints (articular surfaces), growing bones (cartilage transformed into bone), "rings" of supportive tissue of trachea. Has little fibrous content.

2) Fibrocartilage: Has high fibrous (collagen) content. Withstands compression. Found in intervertebral discs, knee joint.

3) Elastic: High fibrous (elastin) content. Strong yet elastic (can be bent easily). Found in external ear (pinna) and epiglottis.

Bone

Hard, rigid tissue. Calcium salts deposited in matrix give hardness.

Some collagen fibers give bone flexibility.

Two categories: Based on porosity. Most bones are composites.

Dense or Compact bone (outer layer of most bones)

Spongy or Cancellous. Porous and vascular. Site of red marrow.

Structure of Bone

Haversian systems: Central Haversian canal with blood vessels and nerve fibers, with concentrically arranged layers (lamellae) of osteocytes in lacunae.

Canaliculi allow movement of substances between Haversian canal and osteocytes.

Blood (or Vascular Tissue)

Connective tissue in that it metabolically supports other tissues.

Matrix is plasma. Cell types are red blood cells, white blood cells and platelets.

MUSCLE

Responsible for movement. Contraction is their specialization.

Muscle fibers or cells are collected into bundles and bundles form "muscles."

They are "excitable" cells in that they respond to electrical changes in their cell membrane potential (they generate and conduct action potentials).

Types:

Smooth Muscle

Muscle fibers are long, spindle-shaped cells. No striations and have one nucleus per cell.

Found associated with gut, blood vessels, respiratory, urinary and reproductive tracts. Generally "involuntary."

Found in heart. Has striated appearance. Fibers are branched with 1 or 2 nuclei.

Intercalated discs connect adjacent cells and help to transmit force of contraction and help synchronize contraction.

Striated appearance. Long, unbranched, multi-nucleate cells.

Associated with movement of skeleton.

"Voluntary." Individual cells are grouped into bundles (fascicles).

NERVOUS TISSUE

Composed of Neurons and Neuroglia.

Neurons: Nerve cells.

Conduct and integrate signals.

(Pop Quiz: since neurons have lots of RER, they must produce lots of _____________________

Dendrites are processes that bring signals to cell body

Neuroglia: often just called "glia" (means "glue").