Physiology -Chap 4- Outline.1

Chapter 4 (Pgs. 108-111)

Types of Tissues and their Origins

• Epithelial tissue:

covers body surfaces and lines hollow organs, body cavities, and ducts; it also forms glands

Connective tissue: protects and supports the body and its organs; binds organs together, stores energy reserves as fat, and helps provide immunity to disease-causing organisms

Muscular tissue: generates the physical force needed to make body structures move

Nervous tissue: detects changes in a variety of conditions inside and outside the body and response by generating nerve impulses that help maintain homeostasis

Tissues of the body develop from three primary germ layers

~Epithelial tissues develop from all three primary germ layers

~The first tissues formed in the human embryo are called:

-ectoderm

--nervous tissue develops from ectoderm

-endoderm

-mesoderm

--all connective tissue and most muscle tissues derive from mesoderm

• Normally, most cells within a tissue remain anchored to other cells or structures

Cell Junctions

• Cell Junctions: contact points between the plasma membranes of tissue cells

~5 types

-tight junctions

-adherens junctions

-desmosomes

-hemidesmosomes

-gap junctions

Tight Junctions

• Tight Junctions:

consist of web-like strands of transmembrane proteins that fuse the outer surfaces of adjacent plasma membranes together

~Ex: Cells of epithelial tissues that line the stomach, intestines, and urinary bladders

Adherens Junctions

• Adherens Junctions:

contain plaque, a dense layer of proteins on the inside of the plasma membrane that attaches to both membrane proteins and to microfilaments of the cytoskeleton

Cadherins: transmembrane glycoproteins that join the cells

In epithelial cells, adherens junctions often form extensive zones called adhesion belts

Help epithelial surfaces resist separation during various contractile activities (Ex: when food moves through the intestines)

Desmosomes

• Desmosomes:

contain plaque and have transmembrane glycoproteins (cadherins) that extend into the intracellular space between adjacent cell membranes and attach cells to one another

Unlike adherens junctions, the plaque of desmosomes does not attach to microfilaments; it attaches to the intermediate filaments that consist of the protein keratin

Desmosomes prevent epidermal cells from separating under tension and cardiac muscle cells from pulling apart during contraction

Hemidesmosomes

• Hemisesmosomes:

resemble desmosomes but they do not link adjacent cells

The transmembrane glycoproteins in hemidesmosomes are integrins rather than cadherins

~On the inside of the plasma membrane, the integrins attach to intermediate filaments made of the protein keratin

~On the outside of the plasma membrane, the integrins attach to the protein laminin

• Hemidesmosomes anchor cells not to each other, but to the basement membrane

Gap Junctions

• Gap Junctions:

membrane proteins called connexins form tiny fluid-filled tunnels called connexons that connect neighboring cells

The plasma membranes of gap junctions are not fused together as in tight junctions but are separated by very narrow intercellular gap

The transfer of nutrients, and perhaps wastes, takes place through gap junctions in avascular tissues

Gap junctions allow the cells in a tissue to communicate with one another

Enable nerve or muscle impulses to spread rapidly among cells, a process that is crucial for the normal operation of some parts of the nervous system

Epithelial Tissue

• Epithelial Tissue (a.k.a. Epithelium): consists of cells arranged in continuous sheets, in either single or multiple layers

~Little intercellular space between adjacent plasma membranes

• Apical (free) Surface:
• most superficial layer of cells
• Lateral Surfaces: face the adjacent cells on either side
• Basal Surface: deepest layer of cells
• Basement Membrane: a thin extra cellular layer that commonly consists of two layers, the basal lamina and reticular lamina

~Basal lamina: is close to and secreted by the epithelial cells

• The basement membrane functions as a point of attachment and support for the overlying epithelial tissue

Basement Membranes and Disease

• Become markedly thickened, due to increased production of callagen and laminin
• Epithelial tissue is avascular (lacks its own blood supply)
• Exchange of substances between epithelium and connective tissue occurs by diffusion
• Epithelial tissue constantly has to renew and repair itself by sloughing off dead or injured cells
• Epithelial tissue performs the roles of: protection, filtration, secretion, absorption, and excretion
• Epithelial tissue combines with nervous tissue to form special organs for smell, hearing, vision, and touch
• 2 types of epithelial tissue:

~Covering and lining epithelium: forms the outer covering of the skin and some internal organs

~Glandular epithelium: makes up the secreting portion of glands such as the thyroid gland, adrenal glands, and sweat glands