IV. The Tissue Level of Organization

Although all of the functions of life can be executed by a single cell, most organisms have evolved multicellularity, largely due to the increased efficiency that multicellularity permits. Multicellularity permits division of labor and specialization. Certain cells take on special functions, and by doing so, become extremely efficient at these functions. The first level of multicellular organization is the tissue.

A. Tissue - A group of related cells and extracellular material which work together to perform a common function.

B. Intercellular membrane junctions - Multicellularity requires that cells be connected to each other in some way. The major means of connecting cells are as follows.

l. Tight junctions - In this type of connection the membranes of adjacent cells are fused together thereby eliminating any extracellular space.

2. Desmosomes - These are spots where adjacent membranes are held together by intermediate filaments and a proteoglycan layer. They may be thought of as be similar to spot welds. There is a small extracellular space between the membranes of cells that are attached in this fashion.

3. Gap junctions - The cell membranes are separated by a small space which is bridged by tubular channels that link the cytoplasm of adjacent cells. Gap junctions provide for rapid movement of small molecules from one cell to another and in the transmission of electrical impulses.

C. Tissue types - There are four fundamental types of tissues that are found in the human body. All organs are made up of some combination of these four tissue types. The four types are as follows.

1. Epithelial

2. Connective

3. Muscle

4. Nervous

Muscle and nerve are collectively referred to as excitable tissues because of their ability to rapidly respond to changes in the environment. They will be discussed later when the muscular and nervous systems are considered.

D. Epithelial tissues

l. General characteristics

a. Epithelial tissues are found covering the body and lining the hollow organs of the body. Because of their location, epithelial tissues always have an unattached or free (apical) surface. They exhibit polarity as the apical (free surface) is as the cell region differs from the basal surface (attached surface).

b. Epithelial tissues are predominantly cellular, with only small amounts of extracellular material. They rest on a thin basement membrane. This membrane is composed of basal lamina, a layer of glycoproteins secreted by the epithelial cells, and the reticular lamina, derived from connective tissues beneath the epithelial layer.

c. Epithelial tissues are avascular and have good regenerative power.

d. Epithelial tissues function in:

(l) protection, especially where abrasion occurs.

(2) transport of materials such as occurs in the capillaries, intestine, kidney, and glands.

(3) sensation, in the form of sensory cells that provide information to the nervous system.

2. Classification of epithelial tissues - These tissues are classified both by the number of layers of cells and by the shape of the cell.

a. Number of layers

(l) Simple - A sheet of cells composed of only one layer. Most of the transport epithelia are of this type.

(2) Stratified - Here there are at least two layers of cells (usually several). The protective epithelia are of this type.

b. Cell shape

(l) Squamous - These epithelial cells are highly flattened and resemble fried eggs sunny side up. Simple squamous epithelia compose the walls of the capillaries and alveoli while stratified squamous epithelia are found in the skin and the lining of the mouth, anus, and vagina.

(2) Cuboidal - Here the cell dimensions are roughly equal with a central nucleus. The cells therefore resemble cubes. Simple cuboidal epithelium composes the walls of many of the tubules found in the kidney, liver, pancreas, and salivary glands. Stratified cuboidal tissues are rare, and are found in ducts of the sweat glands and mammary glands.

(3) Columnar - These cells are rectangular. Most of the digestive tract is line by a simple columnar epithelium. Stratified columnar tissues are uncommon but do provide protection along portions of the pharynx, urethra, and anus.

c. Variations - While most epithelial tissues fall into the classification system above, there are two that are different in certain respects.

(l) Pseudostratified ciliated columnar - This is a simple columnar type of epithelium, but under the microscope it appears stratified because the nuclei are located at different levels within the cells. The free surface is ciliated. This type of epithelium is found lining the respiratory tract as well as the sperm ducts and the oviducts.

(2) Transitional epithelium - This is a stratified cuboidal epithelium where each of the cells in the outer layer extends over several cells of the inner layers. This is found only in the urinary bladder and the ureters. Unlike most epithelia, it can tolerate stretching, and its appearance will change at the cells rearrange themeselves during the stretching process.

d. Goblet cells - These are mucous secreting cells that are frequently found mixed in with columnar and pseudostratified ciliated columnar epithelia. They function to produce a layer of mucous that coats the epithelium's free surface.

3. Membranes - Most of the epithelial tissues which line the hollow organs and body cavities are arranged into membranes. An epithelial membrane consists of the epithelial cells plus a thin basement membrane composed of connective tissue to which the cells are attached. The major epithelial membranes are as follows.

a. Mucous membranes - These are moist epithelial tissues that organ systems which open to the outside. These include the digestive, respiratory, urinary, and reproductive tracts. The basement membrane of loose connective tissue is known as the lamina propria. The epithelium may be simple columnar as in the intestine, transitional as in the bladder and ureters, or stratified squamous as in the mouth and nasal cavity.

(l) Mucous membranes are both absorptive and secretory.

(2) Mucous membranes secrete mucous, a viscous fluid which moistens and lubricates the free surface.

b. Serous membranes - These line the ventral body cavities which do not open to the outside. Serous membranes secrete a watery fluid that moistens their free surface. The epithelium is a simply squamous type known as a mesothelium. It rests on a loose connective tissue basement membrane. There are three major serous membranes, the peritoneum that lines the peritoneal cavity, the pleura which line the pleural cavity, and the pericardium which lines the pericardial cavity.

c. Endothelium - This is a simple squamous epithelial membrane that lines the circulatory system and is the sole component of the smallest blood vessels, the capillaries.

d. Cutaneous membrane - This is the stratified squamous epithelium and loose connective tissue layer that covers the body. It is the major portion of the skin.

4. Glands - Most glands are composed largely of secretory epithelium. All glands are divided into two groups based upon how their secretion reaches their target areas.

(l) Endocrine - The secretion of these glands is carried to the target tissue by the blood which they secrete directly into.

(2) Exocrine - These secrete into a tube or duct which conveys the secretion to the target area. Exocrine glands are further subdivided according to their mode of secretion.

(a) Merocrine - In these glands the secretions do not accumulate in the cells but pass directly out by exocytosis. The secreting cells do not die during the secretion process. Merocrine glands include the exocrine part of the pancreas, the salivary glands, and most of the sweat glands.

(b) Holocrine - Cells of these glands accumulate the secretions. The secretions are released when the cell dies and disintegrates. The sebaceous (oil) glands of the skin are of this type.

(c) Apocrine - Secretions accumulate at the apical end of the cell. The apical end then pinches off, releasing the secretion, but leaving the cells alive. Some of the sweat glands are of this type as are the mammary glands.

E. Connective tissues

1. General characteristics

a. Connective tissues are found all over the body. They are the most widely distributed of the primary tissue types.

b. As a rule connective tissues have few cells and a large amount of extracellular material (matrix). The cells of connective tissues vary widely in appearance, but they all have a common origin from an embryonic tissue known as mesenchyme. The extracellular material consistsof protein fibers embedded along with the cells in a ground substance.

c. Connective tissues function to:

(1) connect and bind other tissues into organs.

(2) establish the structural framework of the body.

(3) store energy reserves.

(4) defend against microbial invasion.

(5) transporting dissolved substances throughout the body.

2. Connective tissue fibers - There are three types of fibers found in connective tissues. All three are proteins. Some connective tissues have a single type of fiber while others may have two, or even all three types. The fiber types are as follows.

a. Collagenous (white) - These are composed of the protein collagen. These fibers are tough and flexible, but they are not elastic. They are termed white because that is their color in fresh material.

b. Elastic (yellow) - These are composed of the protein elastin. They are yellow in color and are highly elastic. Mixed with collagenous fibers in most cases, they predominate in organs with elastic recoil such as the skin and arteries.

c. Reticular - These are very fine and delicate fibers. They form highly branched network called a reticulum. Reticular fibers are composed of a variety of highly branching collagen.

3. Ground substance - This second component of connective tissue is a complex mixture of proteins that can range from a liquid to a solid. It is in the ground substance in which the cells and fibers are embedded.

4. Connective tissue cell types - There are several different cell types associated with connective tissue. The principal ones are listed below.

a. Mesenchymal cells - These are unspecialized cells which resemble embryonic cells. They function in regeneration and repair, giving rise to other types of connective tissue cells and scar tissue.

b. Fibroblasts - These are the basic fiber forming cell. They are the most abundant of connective tissue cells and can differentiate into more specialized cell types.

c. Macrophages - These are phagocytic cells, second in abundance to the fibroblast. They serve a protective role. Some are fixed, that is, they do not move about while others are free, that is, they wander throughout the tissue.

d. Adipose cells - These are fat storage cells. They are derived from fibroblasts.

e. Melanocytes - These cells contain the brown pigment melanin, and give the tissues which are found in a dark appearnce. They are found in both connective tissues and epithelia.

f. Mast cells - These are small, mobile cells, which are usually associated with blood vessels. They contain vasoactive substances such has histamine and heparin which play roles in altering blood circulation and promoting inflammation.

g. Blood cells - Both the red and white cells are connective tissue cells. Red cells transport oxygen to the tissues while white cells play a role in fighting infection.

h. Chondrocytes - These are the cells of cartilage.

i. Osteocytes -- The cells that are found in bone.

5. Classification of connective tissues

a. Loose (areolar)

(l) Contains both elastic and collagenous fibers.

(2) The fibroblast is the basic cell type.

(3) The fibers are arranged randomly within a delicate, jelly-like ground substance.

(4) In the body it is located beneath the skin and around organs where it serves as "filler" for tissue spaces. It also supports epithelia.

(5) This is the most abundant connective tissue in the body.

b. Dense Collagenous

(1) Fibers are very dense and mostly collagenous.

(2) Fibroblasts are few and far between.

(3) The fibers may be regularly arranged in parallel bundles as in the case of tendons, or irregularly arranged as in the case of joint capsules, organ capsules, and the heart valves.

(4) This tissue is found where great strength is necessary, tendons, ligaments, and heart valves.

c. Dense elastic

(1) This is similiar in organizing to dense collagneous except that elastic fibers now outnumber collagenous fibers.

(2) It is found where elastic stretch and recoil are required, such as in the walls of the large arteries.

d. Adipose

(1) This develops from loose connective tissue when fibroblasts become adipose or fat cells.

(2) Adipose tissue functions in energy storage, insulation, and padding.

(3) It is located in a subcutaneous layer and on internal membranes.

(4) Much of the difference between the male and female body torso has to do with the amount and distribution of fat. Females have more adipose tissue in general and a thicker subcutaneous layer.

e. Reticular tissue

(1) This consists of reticular fibers along with macrophages and lymphocytes.

(2) It is the principal component of the body defense system known as the reticuloendothelial system.

(3) Reticular tissue is found in the lymph nodes, thymus gland, spleen, and other areas.

f. Cartilage

(l) The ground substance is firm, but not rigid.

(2) The basic cartilage forming cell is the chondroblast.

(a) The cells are found in holes in the cartilage

which are termed lacunae.

(b) A mature cartilage cell which is no longer forming

cartilage is termed a chondrocyte.

(3) Cartilage is surrounded by a nourishing membrane known as the perichondrium. Cartilage is avascular, and therefore all nutrients must diffuse to the cells from the perichondrium. The perichondrium also buds off chondroblasts.

(4) Cartilage types - There are three basic types of cartilage in the body.

(a) Hyaline - This is the most abundant cartilage in the body. It has a glassy appearance and is found covering the articular surfaces of bones, in the trachea, bronchial tubes, nose, and connecting the ribs to the sternum or breastbone.

(b) Elastic - This cartilage has a large percentage of elastic fibers and is therefore resilient. It is found in the auditory tube, the epiglottis, and the outer ear.

(c) Fibrocartilage - This contains collagenous fibers as does hyaline, but it appears fibrous. It composes the disks found between the vertebrae and is the connecting substance in symphysis joints.

g. Bone

(l) The ground substance if both firm and rigid, due to the deposition of inorganic salts.

(2) The cells are termed osteocytes, and like cartilage, lie in lacunae.

(3) There is a membrane surrounding bone which is known as the periosteum, but its function is different from the membrane surrounding cartilage as bone is vascularized.

(4) The structure and organization of bone will be discussed in detail along with the skeletal system.

h. Blood and lymph

(1) This liquid connective tissue is sometimes considered to be a fifth tissue type.

(2) The cells are derived from connective tissue type cells, but there are no fibers.

(3) The ground substance is a liquid termed plasma.

(4) Lymph is derived from tissue fluid which in turn is derived from plasma.

(5) Lymph is associated with reticular tissue and differs from tissue fluid mainly in that it contains lymphocytes.

6. Connective tissue framework - Connective tissue membranes from a framework termed fascia that provide strength, maintain the positions of the internal organs, and provide a route for blood vessel and nerve distribution. There are three categories of fascia.

a. Superficial fascia (hypodermis) - This is a loose connective tissue layer that separates the skin from the underlying muscles. It permits the skin to move independently of the muscles.

b. Deep fascia - These are sheets of dense collagenous connective tissue. They form capsules around internal organs. They surround the muscles and then blend in with the tendons which connect the muscles to the bones. The periosteum and perichondrium are also part of the deep fascia.

c. Subserous fascia - This is a layer of loose connective tissue that lies between the deep fascia and the serous membranes that line the body cavity. This separation prevents movements of the muscles from distorting the serous membranes.

F. Muscle tissue - Muscle is an excitable tissue that responds by shortening. It is responsible for both the internal and external movements of the body. There are three types of muscle, skeletal, smooth, and cardiac. The muscle tissues will be discussed in detail with the muscular and circulatory systems.

G. Nervous tissue - This is an excitable tissue that is responsible for communication between cells and also with the external environment. The basic cell type is the neuron. Nervous tissue will be discussed in detail along with the nervous system.