The Tissue Level Of Organization

Tissues and homeostasis

The four basic types of tissues in the human body contribute to homeostasis by providing diverse functions including protection, support, communication among cells, and resistance .

A cell is a complex collection of compartments, each of which carries out a host of biochemical reactions that make life possible. However, a cell seldom functions as an isolated unit in the body. Instead, cells usually work together in groups called tissues. A tissue is a group of cells that usually have a common origin in an embryo and function together to carry out specialized activities. The structure and properties of a specific tissue are influenced by factors such as the nature of the extracellular material that surrounds the tissue cells and the connections between the cells that compose the tissue. Tissues may be hard, semisolid, or even liquid in their consistency, a range exemplified by bone, fat, and blood. In addition, tissues vary tremendously with respect to the kinds of cells present, how the cells are arranged, and the types of fibers present, if any.

Histology  the science that deals with the study of tissues. A pathologist  is a physician who examines cells and tissues to help other physicians make accurate diagnoses. One of the principal functions of a pathologist is to examine tissues for any  changes that might indicate disease.

Types of Tissues

 1. A tissue is a group of cells, usually with similar embryological origin, specialized for a particular function.

 2. The tissues of the body are classified into four basic types: epithelial, connective, muscular, and  nervous. 

Cell Junctions

 1. Cell junctions are points of contact between adjacent plasma membranes.

 2. Tight junctions form fluid-tight seals between cells; adherens junctions, desmosomes, and hemidesmosomes anchor cells to one another or to the basement membrane; and gap junctions permit electrical and  chemical signals to pass between cells.  

Comparison between Epithelial and Connective Tissues 

 1. Epithelial tissue has many cells tightly packed together and is avascular.

 2. Connective tissue has relatively few cells with lots of extracellular material.

Epithelial Tissue

 1. The subtypes of epithelial tissue include covering and lining epithelium and glandular epithelium.

 2. Epithelial tissue consists mostly of cells with little extracellular material between adjacent plasma membranes. The apical, lateral, and basal surfaces of epithelial cells are modified in various ways to carry  out specific functions. Epithelial tissue is arranged in sheets and attached to a basement membrane.  

Although epithelial tissue is avascular, it has a nerve supply. The high rate of cell division gives epithelial tissue a high capacity for renewal. 

 3. Epithelial layers can be simple, pseudostratified, or stratified. The cell shapes may be squamous (flat),  cuboidal (cubelike), columnar (rectangular), or transitional (variable). The subtypes of epithelial tissue  include covering and lining epithelium and glandular epithelium.  

Simple squamous epithelium, a single layer of flat cells , is found in parts of the body where  filtration or diffusion is a priority process. Endothelium lines the heart and blood vessels. Mesothelium forms  the serous membranes that line the thoracic and abdominopelvic cavities and covers the organs within them.

Simple cuboidal epithelium, a single layer of cube-shaped cells that function in secretion and absorption  is found covering the ovaries, in the kidneys and eyes, and lining some glandular ducts. 

Nonciliated simple columnar epithelium, a single layer of nonciliated rectangular cells lines most of the gastrointestinal tract and contains specialized cells that perform absorption and secrete  mucus. Ciliated simple columnar epithelium, a single layer of ciliated rectangular cells , is  found in a few portions of the upper respiratory tract, where it moves foreign particles trapped in mucus  out of the respiratory tract. A ciliated variety of pseudostratified columnar epithelium contains goblet cells and lines most of the upper respiratory tract; a nonciliated variety has no goblet cells  and lines ducts of many glands, the epididymis, and part of the male urethra. The ciliated variety moves  mucus in the respiratory tract. The nonciliated variety functions in absorption and protection. 

Stratified epithelium consists of several layers of cells: Cells of the apical layer of stratified squamous  epithelium and several layers deep to it are flat (Table 4.1F); a nonkeratinized variety lines the mouth, and a keratinized variety forms the epidermis. Cells at the apical layer of stratified cuboidal epithelium are cube-shaped found in adult sweat glands and in a portion of the male urethra, stratified cuboidal epithelium protects and provides limited secretion and absorption. Cells of the apical layer of stratified columnar epithelium have a columnar shape this type is found in a portion of the male urethra and in large excretory ducts of some glands, and functions in protection and secretion.

Transitional epithelium consists of several layers of cells whose appearance varies with the degree of stretching. It lines the urinary bladder.

 

***A gland is a single cell or a group of epithelial cells adapted for secretion. There are two types: endocrine and exocrine. Endocrine glands secrete hormones into interstitial fluid and then into the blood 

Exocrine glands secrete into ducts or directly onto a free surface 

**The structural classification of exocrine glands includes unicellular and multicellular glands. The functional classification of exocrine glands includes merocrine, apocrine, and holocrine glands. 

Connective Tissue

1. Connective tissue, one of the most abundant body tissues, consists of relatively few cells and an abundant extracellular matrix of ground substance and protein fibers. It does not usually occur on free  surfaces; it usually has a nerve supply and it is usually highly vascular.

2. Cells in connective tissue are derived primarily from mesenchymal cells. Cell types include fibroblasts (secrete extracellular matrix), macrophages (perform phagocytosis), plasma cells (secrete antibodies),  mast cells (produce histamine), adipocytes (store fat), and white blood cells (respond to infections). 

3. The ground substance and fibers make up the extracellular matrix. The ground substance supports and binds cells together, provides a medium for the exchange of materials, stores water, and actively influences cell functions. Substances found in the ground substance include water and polysaccharides. Also present are proteoglycans and adhesion proteins. 

4. The fibers in the extracellular matrix provide strength and support and are of three types: (a) Collagen fibers are found in large amounts in bone, tendons, and ligaments. (b) Elastic fibers are found in skin,  blood vessel walls, and lungs. (c) Reticular fibers are found around fat cells, nerve fibers, and skeletal  and smooth muscle cells. 

5. Two major subclasses of connective tissue are embryonic (found in embryo and fetus) and mature (present in the newborn). Embryonic connective tissues are mesenchyme, which  forms almost all other connective tissues, and mucous connective tissue, found in the umbilical cord  of the fetus, where it gives support. Mature connective tissue differentiates from mesenchyme and  is subdivided into several types: loose or dense connective tissue, cartilage, bone tissue, and liquid  connective tissue. 

6. Loose connective tissue includes areolar connective tissue, adipose tissue, and reticular connective tissue.

Areolar connective tissue consists of the three types of fibers (collagen, elastic, and reticular), several types of cells, and a semifluid ground substance it is found in the subcutaneous layer, in mucous membranes, and around blood vessels, nerves, and body organs. Adipose tissue consists of adipocytes,which store triglycerides . it is found in the subcutaneous layer, around organs, and in yellow bonemarrow. Brown adipose tissue (BAT) generates heat. Reticular connective tissue consists of reticular fibers and reticular cells and is found in the liver, spleen, and lymph nodes .

7. Dense connective tissue includes dense regular, dense irregular, and elastic. Dense regular connective tissue consists of parallel bundles of collagen fibers and fibroblasts; it forms tendons, most ligaments, and aponeuroses. Dense irregular connective tissue usually consists of collagen fibers and a few fibroblasts . it is found in fasciae, the dermis of skin, and membrane capsules around organs. Elastic connective tissue consists of branching elastic fibers and fibroblasts and is found in the walls of large arteries, lungs, trachea, and bronchial tubes. 

8. Cartilage contains chondrocytes and has a rubbery extracellular matrix (chondroitin sulfate) containing collagen and elastic fibers. Hyaline cartilage, which consists of a gel-like ground substance and appears bluish white in the body, is found in the embryonic skeleton, at the ends of bones, in the nose, and in respiratory structures; it is flexible, allows movement, provides support, and is usually surrounded by a perichondrium. Fibrocartilage is found in the pubic symphysis, intervertebral discs, and menisci (cartilage pads) of the knee joint; it contains chondrocytes scattered among clearly visible bundles of collagen fibers. Elastic cartilage maintains the shape of organs such as the epiglottis of the larynx, auditory (eustachian) tubes, and external ear; its chondrocytes are located within a threadlike network of elastic fibers, and it has a perichondrium.

9. Bone or osseous tissue consists of an extracellular matrix of mineral salts and collagen fibers that contribute to the hardness of bone, and osteocytes that are located in lacunae . It supports and protects the body, provides a surface area for muscle attachment, helps the body move, stores minerals,and houses blood-forming tissue.

10. There are two types of liquid connective tissue: blood and lymph. Blood consists of blood plasma and formed elements—red blood cells, white blood cells, and platelets; its cells transport oxygen and carbon dioxide, carry on phagocytosis, participate in allergic reactions, provide immunity, and bring about blood clotting. Lymph, the extracellular fluid that flows in lymphatic vessels, is a clear fluid similar to blood plasma but with less protein.

Membranes

 1. An epithelial membrane consists of an epithelial layer overlying a connective tissue layer. Types include

• mucous
• serous 
• cutaneous membranes.

Mucous membranes line cavities that open to the exterior, such as the gastrointestinal tract.

Serous membranes line closed cavities (pleura, pericardium, peritoneum) and cover the organs in the cavities. These membranes consist of parietal and visceral layers.

The cutaneous membrane is the skin. It covers the entire body and consists of a superficial epidermis (epithelium) and a deep dermis (connective tissue).

Synovial membranes line joint cavities and consist of areolar connective tissue; they do not have an epithelial layer. 

Muscular Tissue 

 1. Muscular tissue consists of cells called muscle fibers or myocytes that are specialized for contraction.

It provides motion, maintenance of posture, heat production, and protection.

 2. Skeletal muscle tissue is attached to bones and is striated and voluntary .

 3. The action of cardiac muscle tissue, which forms most of the heart wall and is striated, is involuntary .

 4. Smooth muscle tissue is found in the walls of hollow internal structures (blood vessels and viscera) and is nonstriated and involuntary.

Nervous Tissue

 1. The nervous system is composed of neurons (nerve cells) and neuroglia (protective and supporting cells).

 2. Neurons respond to stimuli by converting the stimuli into electrical signals called nerve action potentials (nerve impulses), and conducting nerve impulses to other cells. 

 3. Most neurons consist of a cell body and two types of processes: dendrites and axons.

Excitable Cells

 1. Electrical excitability is the ability to respond to certain stimuli by producing electrical signals such as action potentials. 

 2. Because neurons and muscle fibers exhibit electrical excitability, they are considered excitable cells.

Tissue Repair: Restoring Homeostasis

 1. Tissue repair is the replacement of worn-out, damaged, or dead cells by healthy ones.

 2. Stem cells may divide to replace lost or damaged cells.

 3. If the injury is superficial, tissue repair involves parenchymal regeneration; if damage is extensive, granulation tissue is involved. 

 4. Good nutrition and blood circulation are vital to tissue repair.

Aging and Tissues

 1. Tissues heal faster and leave less obvious scars in the young than in the aged; surgery performed on fetuses leaves no scars. 

 2. The extracellular components of tissues, such as collagen and elastic fibers, also change with age.