Introduction to Organismal Biology (BIOL221) - Dr. S.G. Saupe; Biology Department, College of St. Benedict/St. John's University, Collegeville, MN 56321; ssaupe@csbsju.edu; http://www.employees.csbsju.edu/ssaupe/

Introduction to Animal Structure & Function
 

I. Central Themes of Biology
    Recall in our introductory class we outlined some major themes for our course:

1. Life is hierarchically organized (levels of structural organization); and
2. Structure and function are correlated.

This unit will provide excellent examples of these themes.

III. Animal Tissues

A. Origin
    They are derived from the embryo, which in turn, is derived from the zygote. In other words, thank your mom and dad who provided your DNA which ultimately is the product of our evolutionary history.

B. Tissues are aggregates of cells
    Obvious, yes. But, how do these aggregates stick together? Recall that plants glue their cells together with pectins.  Mechanisms used to link animal cells include: (1) many cells have "sticky" surfaces (recall the glycocalyx?)and chemical interactions between membranes.  This is analagous the middle lamella of plants; (2) cells are woven together (note: the Latin word for tissue is weave); and (3) specialized cell connections including

• gap junctions - to enhance communication; analagous to plasmodesmata in plant cells
• tight junctions - seals, prevents leakage
• anchoring junctions - weld, especially tissue under stress; involve membrane proteins such as integrins and cadherins

    As an aside, just to highlight the importance of cell connections:  the failure of tumor cells to stick together is what causes some cancers to spread (metastatic) while others don’t (benign).

D. Tissue Types
    There are four major types of animal tissues; nervous, muscle, connective, and epithelial.

III. Nervous Tissue
    Duh, yup. Dis is da stuff en yur brayn. The function of nervous tissue is to sense stimuli and transmit signals. The functional unit is the neuron (axon, dendrite, cell body). Glial cells surround and support the neurons.

IV. Muscle Tissue
    These tissues are made of elongated cells, are excitable and can contract. Muscle tissue is primarily involved with movement and exerting pressure.  There are three major types of muscle tissue:

1. Skeletal
       Responsible for voluntary movements. Skeletal muscle cells are long and cylindrical, and when bundled together form fibers, which in turn are sheathed into a muscle. Skeletal muscle is striated. Attached to bone or skin.
   
2. Smooth
       Involved in involuntary movements. Smooth muscle cells are tapered (spindle shaped) and are important in blood vessels, stomach, bladder, etc.  Smooth muscle is not striated.
   
3. Cardiac
       Involuntary contractions. Found in the wall of the heart (where else?). Striated. The cells are branched and are fused (intercalated disks) for intimate contact.

V. Epithelial Tissue -  sheets of cells

1. Located on the outside of the body and lining body cavities; covers the inner and outer surfaces.
   
2. Epithelial tissues have one free side (no cells adjacent to it). The other side is a basement membrane or basal lamina (non-living matrix of material serving as a sort of glue).
   
3. Important in surface phenomena such as lubrication, maintaining moist surfaces, diffusion, absorption, transport, protection, support, secretion, receptor sites.
   
4. Can be one (simple epithelium, like in tubules of kidney) or several (stratified, like skin) cell layers thick.
   
5. The shape of epithelial cells is used as a means of classification. Epithelial cells can be: (1) squamous - flat, like a linoleum tile. Used to line blood vessels, important in diffusion; (2) cuboidal - like a cube. Common in the gut; and (3) columnar - line stomach/intestine. The latter two are especially important for secretion, absorption and transport.
   
6. Stratified epithelial tissues typically can divide quickly. The cells near the basement membrane are the ones dividing. These are especially common in tissues that tend to be sloughed off such as skin, lining of esophagus, anus, and vagina.

VI. Connective Tissue

A. Serves to bind and support other tissues. Connective tissues are comprised of an assortment of cells that are sparsely scattered through a gel-like matrix containing protein fibers.

B. Cell Types.  Some of the cells found in connective tissues include:

1. Fibroblasts - which make protein fibers.
2. Macrophages - which engulf bacteria and other invaders. They are scavengers.
3. Adipose (fat) cells
4. Mast cells – produce histamine in response to allergy
5. Other – there are a variety of other cells that make up the connective tissue

C. Protein Fibers - There are several types:

1. Collagenous fibers - made of collagen; most abundant protein in the body; forms large rope-like aggregates that provide strength (high tensile strength);
2. Elastic fibers - made of the protein elastin, which is obviously "elastic" and is responsible for such things as the skin on you arm snapping back into position when pulled; and
3. Reticular fibers - which are thin and branched fibers made of collagen.
4. Proteoglycans

D. Types of connective tissues. There are a variety of types of connective tissues, some obviously "connective" and the others not so. These include:

1. Loose connective tissue - common epithelial tissue. Serves to attach epithelium to the tissues underneath.
2. Fibrous Connective Tissue - made of parallel fibers of collagen. They resist stretching. Common in tendons (muscle to bone connections) and ligaments (bone to bone).
3. Blood - believe it or not, blood is a connective tissue. I envision blood "connecting" all of my cells.
4. Cartilage - functions in cushioning (i.e., vertebratal disks) and shape. Made of collagen fibers in a rubbery ground substance (called chondroitin sulfate, which is a protein/carbohydrate complex) secreted by chrondocytes.
5. Adipose - fat tissue. Important for cushioning, insulation, and fuel storage.
6. Bone - for support and movement. Mineralized with calcium salts. Produced by osteocytes.