Tissues & Junctions

Lecture 34Chapter 21 - Part 1

A) TISSUES

What is tissue?

• Biological tissue is a collection of • similar cells • and the intercellular substances surrounding them.

• There are four basic kinds of tissue in the body: 1. epithelium

2. connective tissues– including adipose tissue, blood, bone, and cartilage

3. muscle tissue

4. nerve tissue

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With most multicellular organisms there are different types of cells, which are organized in to discrete groupings called tissues

Plants

21_05_model_plantwall.jpgPlant cells produce a primary cell wall followed by a secondary cell wall. The cell wall - which is on the outside of the plasma membrane - is made of cellulose - a polysaccharide

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The cell wall will only allow the cell to grow in one direction, which is dependant upon the orientation of the fibers that were laid down during the production of the primary cell wall.

A) Here the plant tissue may only expand in a vertical direction

B) Here the plant tissue may only expand in a horizontal direction

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The celulose synthase complex, that is embedded in the plasma membrane, oozes out cellulose microfibrils

Animals

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Animal cells do not use cellulose. They instead have a protein extracellular matrix of collagen, which has great tensile strength.

Animals have just 4 classes of tissue - connective, epithelial, nervous, and muscular (the last three are basically similar).

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Collagen is made of many bundles of a simple polypeptide

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21_11_elastic_skin.jpgMutations in the arrangement of these fibers may result in various abnormal outcomes.

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21_13_align_collagen.jpgFibroblasts are the cells that secrete collagen and also the ones that maintain it.

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The interlinks between cells is made between collagen fibers and the actin filaments of cells using transmembrane and adaptor proteins known as integrins

How are cells held together?How are cells held together?

Fibronectin - is a linker protein

21_17_epithelial_sheet.jpgEpithelial sheets and cell-cell junctions

21_18_sheet_polarized.jpgMost tissue cells have a polarized structure. They rest on the basal lamina.The basal lamina is just a layer of type IV collagen, plus other molecules.

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Lung Tissue

21_20_polarize_line_gut.jpgThe intestine is made of two main cell types. The absorptive cells take in nutrients and the goblet cells secrete mucus.

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Animal cells are held together by a number of types of cell-cell junctions.

a) Tight Junctions - b) Adherens junction - c) desmosomes d) gap juctions - e) hemidesmosomes

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Tight junctions prevent the passage of solutes between cells. They also allow the cell to maintain discrete types of plasma membrane regions

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Tight junctions consist of proteins, for example, claudins, occludins and junctional adhesion molecules that are anchored in the membranes of two adjacent cells and interact with each other to hold the cells together and prevent other molecules from passing between them.

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21_23_Cadherin.jpgAdheren junctions and desmosome junctions are build on a common theme - they make use of transmembrane proteins that belong to the cadherin family - these molecules from adjacent cells link directly to each other.

actin or intermediate filament

21_24_Adherens_junct.jpgAdheren junctions form large deposits as belts permitting adjacent cells to transmit mechanical forces

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Vital part in development

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Desmosomes

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Hemidesmosomes

Gap junctions permit the free passage of small ions and small water soluble molecules to pass. However, they can be closed on demand. Plants have these too.

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A little perspective…

The Inner Life of a Cell - Short Version

Annotated Version