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TISSUES
A tissue is a group of cells that usually have a common embryonic origin and function together to carry out specialized activities. Histology (histo= tissue; - logy= study of) is the science that deals with the study of tissues. A pathologist is a physician who specializes in laboratory studies of cells and tissues to help other physicians make accurate diagnoses. Types of Tissues in Body: Body tissues can be classified into four basic types according to structure and function:
- 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. Various types of connective tissue bind organs together, store energy reserves as fat, and help provide immunity to disease-causing organisms.
- Muscular tissue generates the physical force needed to make body structures move and generates body heat.
- Nervous tissue detects changes in a variety of conditions inside and outside the body and responds by generating action potentials (nerve impulses) that activate muscular contractions and glandular secretions. 1. EPITHELIAL TISSUE: An epithelial tissue or epithelium (plural is epithelia) consists of cells arranged in continuous sheets, in either single or multiple layers. Because the cells are closely packed and are held tightly together by many cell junctions, there is little intercellular space between adjacent plasma membranes. Epithelial tissue forms coverings and linings throughout the body. It is never covered by another tissue, so it always has a free surface. Epithelial tissues have three major functions: ➢ Selective barriers that limit or aid the transfer of substances into and out of the body; ➢ Secretory surfaces that release products produced by the cells onto their free surfaces ➢ Protective surfaces that resist the abrasive influences of the environment.
Structure of Epithelial Tissue: ➢ The apical (free) surface of an epithelial cell faces the body surface, a body cavity, the lumen (interior space) of an internal organ, or a tubular duct that receives cell secretions. Apical surfaces may contain cilia or microvilli. ➢ The lateral surfaces of an epithelial cell face the adjacent cells on either side. ➢ The basal surface of an epithelial cell is opposite the apical surface, and the basal surfaces of the deepest layer of cells adhere to extracellular materials. ➢ The basement membrane is a thin extracellular layer that commonly consists of two layers, the basal lamina and reticular lamina. ➢ The basal lamina (thin layer) is closer to and secreted by the epithelial cells. It contains proteins such as laminin and collagen, as well as glycoproteins and proteoglycans. ➢ The reticular lamina is closer to the underlying connective tissue and contains proteins such as collagen produced by connective tissue cells called fibroblasts. ➢ The basement membrane functions as a point of attachment and support for the overlying epithelial tissue. ➢ Epithelial tissue has its own nerve supply, but is avascular; it lacks its own blood supply. Epithelial tissue may be divided into two types. (1) Covering and lining epithelium forms the outer covering of the skin and some internal organs. It also forms the inner lining of blood vessels, ducts, and body cavities, and the interior of the respiratory, digestive, urinary, and reproductive systems. (2) Glandular epithelium makes up the secreting portion of glands such as the thyroid gland, adrenal glands, and sweat glands.
Combining the two characteristics (arrangements of layers and cell shapes), the types of covering and lining epithelia are as follows:
A. Simple squamous epithelium: Description: Single layer of flat cells; centrally located nucleus. Location: Lines heart, blood vessels, lymphatic vessels, air sacs of lungs, glomerular (Bowman’s) capsule of kidneys, and inner surface of the tympanic membrane (eardrum); forms epithelial layer of serous membranes, such as the peritoneum, pericardium, and pleura. Function: Filtration, diffusion, osmosis, and secretion in serous membranes. B. Simple cuboidal epithelium: Description: Single layer of cube-shaped cells; centrally located nucleus. Location: Covers surface of ovary, lines anterior surface of capsule of the lens of the eye, forms the pigmented epithelium at the posterior surface of the eye, lines kidney tubules and smaller ducts of many glands, and makes up the secreting portion of some glands such as the thyroid gland and the ducts of some glands such as the pancreas. Function: Secretion and absorption.
E. Pseudostratified columnar epithelium: Description: Not a true stratified tissue; nuclei of cells are at different levels; all cells are attached to basement membrane, but not all reach the apical surface. Location: Pseudostratified ciliated columnar epithelium lines the airways of most of upper respiratory tract; pseudostratified nonciliated columnar epithelium lines larger ducts of many glands, epididymis, and part of male urethra. Function: Secretion and movement of mucus by ciliary action . F. Stratified squamous epithelium: Description: Several layers of cells; cuboidal to columnar shape in deep layers; squamous cells form the apical layer and several layers deep to it; cells from the basal layer replace surface cells as they are lost. Location: Keratinized variety forms superficial layer of skin; nonkeratinized variety lines wet surfaces, such as lining of the mouth, esophagus, part of larynx, part of pharynx, and vagina, and covers the tongue. Function: Protection.
G. Stratified cuboidal epithelium: Description: Two or more layers of cells in which the cells in the apical layer are cube-shaped. Location: Ducts of adult sweat glands and esophageal glands and part of male urethra. Function: Protection and limited secretion and absorption. H. Stratified columnar epithelium: Description: Several layers of irregularly shaped cells; only the apical layer has columnar cells. Location: Lines part of urethra, large excretory ducts of some glands, such as esophageal glands, small areas in anal mucous membrane, and part of the conjunctiva of the eye. Function: Protection and secretion. I. Transitional epithelium: Description: Appearance is variable (transitional); shape of cells in apical layer ranges from squamous (when stretched) to cuboidal (when relaxed). Location: Lines urinary bladder and portions of ureters and urethra.
B. Exocrine glands: Description: Secretory products released into ducts. Location: Sweat, oil, and earwax glands of the skin; digestive glands such as salivary glands, which secrete into mouth cavity, and pancreas, which secretes into the small intestine. Function: Produce substances such as sweat, oil, earwax, saliva, or digestive enzymes. SUMMARY OF EPITHELIAL TISSUE:
2. CONNECTIVE TISSUES:
Connective tissue is one of the most abundant and widely distributed tissues in the body.
It binds together, supports, and strengthens other body tissues; protects and insulates
internal organs; compartmentalizes structures such as skeletal muscles; serves as the
major transport system within the body (blood, a fluid connective tissue); is the primary
location of stored energy reserves (adipose, or fat, tissue); and is the main source of
immune responses.
1. Fibroblasts: large, flat cells with branching processes. They are present in several
connective tissues, and usually are the most numerous. Fibroblasts migrate through the
connective tissue, secreting the fibers and certain components of the ground substance
of the extracellular matrix.
2. Adipocytes/fat cells/adipose cells: store triglycerides (fats). They are found deep to
the skin and around organs such as the heart and kidneys.
3. Mast cells: abundant alongside the blood vessels that supply connective tissue. They
produce histamine, can bind to, ingest, and kill bacteria.
4. White blood cells
5. Macrophages develop from monocytes, a type of white blood cell. Macrophages have
an irregular shape with short branching projections and are capable of engulfing
bacteria and cellular debris by phagocytosis.
Fixed macrophages reside in a particular tissue; examples include alveolar
macrophages in the lungs or splenic macrophages in the spleen.
Wandering macrophages have the ability to move throughout the tissue and gather at
sites of infection or inflammation to carry on phagocytosis.
6. Plasma cells : small cells that develop from a type of white blood cell called a B
lymphocyte. Plasma cells secrete antibodies, proteins that attack or neutralize foreign
substances in the body. Thus, plasma cells are an important part of the body’s immune
response.
Extracellular matrix of connective Tissues:
The extracellular matrix consists of two major components:
(1) Ground substance
(2) fibers.
Ground substance: The ground substance is the component of a connective tissue
between the cells and fibers.
It may be fluid, semifluid, gelatinous, or calcified.
The ground substance supports cells, binds them together, stores water, and provides a
medium through which substances are exchanged between the blood and cells. It plays an
active role in how tissues develop, migrate, proliferate, and change shape, and in how they
carry out their metabolic functions.
Ground substance contains water and an assortment of large organic molecules, many of
which are complex combinations of polysaccharides and proteins.
The polysaccharides include:
➢ Glycosaminoglycans or GAGs: hyaluronic acid, chondroitin sulfate, dermatan
sulfate, and keratan sulfate. They trap water, making the ground substance more
jellylike.
Hyaluronic acid: viscous, slippery substance that binds cells together lubricates joints,
and helps maintain the shape of the eyeballs.
Chondroitin sulfate: provides support and adhesiveness in cartilage, bone, skin, and
blood vessels.
Dermatan sulfate: Present in skin, tendons, blood vessels, and heart valves.
Keratan sulfate: Present in bone, cartilage, and the cornea of the eye.
Except for hyaluronic acid, the GAGs are associated with proteins to form large molecules
called proteoglycans.
Also present in the ground substance are adhesion proteins , which are responsible for
linking components of the ground substance to one another and to the surfaces of cells.
The main adhesion protein of connective tissue is fibronectin, which binds to both
collagen fibers (discussed shortly) and ground substance, linking them together. It also
attaches cells to the ground substance.
Fibres in connective tissue:
Three types of fibers are embedded in the extracellular matrix between the cells:
B. Mucous connective tissue
II. Mature connective tissue:
A. Loose connective tissue
1. Areolar connective tissue
2. Adipose tissue
3. Reticular connective tissue
B. Dense connective tissue:
1. Dense regular connective tissue
2. Dense irregular connective tissue
3. Elastic connective tissue
C. Cartilage
1. Hyaline cartilage
2. Fibrocartilage
3. Elastic cartilage
D. Bone tissue
E. Liquid connective tissue
1. Blood tissue
2. Lymph
EMBRYONIC CONNECTIVE TISSUES:
I. Mesenchyme: Description: Consists of irregularly shaped mesenchymal cells embedded in a semifluid ground substance that contains reticular fibers. Location: Under skin and along developing bones of embryo; some mesenchymal cells are found in adult connective tissue, especially along blood vessels. Function: Forms all other types of connective tissue.
II. Mucous connective tissue: Description: Consists of widely scattered fibroblasts embedded in a viscous, jellylike ground substance that contains fine collagen fibers. Location: Umbilical cord of fetus. Function: Support. MATURE CONNECTIVE TISSUES LOOSE CONNECTIVE TISSUE: A. Areolar connective tissue: Description: Consists of fibers (collagen, elastic, and reticular) and several kinds of cells (fibroblasts, macrophages, plasma cells, adipocytes, and mast cells) embedded in a semifluid ground substance. Location: Subcutaneous layer deep to skin; papillary (superficial) region of dermis of skin; lamina propria of mucous membranes; and around blood vessels, nerves, and body organs.
Description: A network of interlacing reticular fibers and reticular cells. Location: Stroma (supporting framework) of liver, spleen, lymph nodes; red bone marrow, which gives rise to blood cells; reticular lamina of the basement membrane; and around blood vessels and muscles. Function: Forms stroma of organs; binds together smooth muscle tissue cells; filters and removes worn- out blood cells in the spleen and microbes in lymph nodes. DENSE CONNECTIVE TISSUE: D. Dense regular connective tissue: Description: Extracellular matrix looks shiny white; consists mainly of collagen fibers regularly arranged in bundles; fibroblasts present in rows between bundles. Location: Forms tendons (attach muscle to bone), most ligaments (attach bone to bone), and aponeuroses (sheetlike tendons that attach muscle to muscle or muscle to bone). Function: Provides strong attachment between various structures. E. Dense irregular connective tissue:
Description: Consists predominantly of collagen fibers randomly arranged and a few fibroblasts. Location: Fasciae (tissue beneath skin and around muscles and other organs), reticular (deeper) region of dermis of skin, periosteum of bone, perichondrium of cartilage, joint capsules, membrane capsules around various organs (kidneys, liver, testes, lymph nodes), pericardium of the heart, and heart valves. Function: Provides strength. F.Elastic connective tissue: Description: Consists predominantly of freely branching elastic fibers; fibroblasts are present in spaces between fibers. Location: Lung tissue, walls of elastic arteries, trachea, bronchial tubes, true vocal cords, suspensory ligament of penis, and some ligaments between vertebrae. Function: Allows stretching of various organs.
