the epithial tissues

THE TISSUES
Four main groups of tissues are known in the body. These are:
? Epithelial tissues.
? Connective tissues.
? Muscular tissues.
? Nervous tissues.
A- THE EPITHELIAL TISSUES
1-These tissues arise from any of the three primary germ layers,
the ectoderm, endoderm or mesoderm.
2-They are almost always found covering a surface, external or
internal, thus mainly performing a protective function. But
some of them are specialized in various ways to perform
different other functions.
3- The epithelial tissues are characterized by having very little
intercellular substance or matrix between their cells and by
4- Resting, in the majority of cases, on a basement membrane
formed of the underlying connective tissues.
Classification of Epithelium
Cell arrangement and cell shape classify the
epithelium, not by function
1- Simple, when it is one cell layer thick
2 -Stratified, when it is two or more cell layers
The individual cells that compose an epithelium are described as:
? Squamous, where the width of the cell is greater
than its height
Lining of vascular system (endothelium) Bowman ,s capsule (kidney)
Lining of respiratory spaces in lung
? Cuboidal , where the width, depth ,and height are
approximately the same
Small ducts of exocrine glands
Surface of ovary (germinal epithelium)
Kidney tubules
? Columnar , where the height of the cell appreciably
exceeds the width.
Lining of small intestine and colon stomach
Lining and gastric glands
lining of gallbladder
? Pseudostratified epithelium has the appearance of
being stratified. Some of cells do not reach the free
surface; however, all rest on the basement membrane.
Thus, it is actually a simple epithelium.
Lining of trachea and bronchi
Lining of differences efferent ductules of epididymis
? Transitional epithelium has several layers of cells,
characterized by large, dome-shaped cells at the free
surface, that help maintain the integrity of the epithelium
during distention of the various components of the urinary
tract.
Renal calyces Ureters Bladder Urethra
Glands
Glands are composed of epithelial cells specialized to
synthesize and secrete a specific product. Typically, glands are
classified into two major groups reflecting how their products
are distributed:
? Exocrine glands, secrete their products onto a
surface through ducts.The ducts, also composed of
epithelial cells.
? Endocrine glands, lack a duct system. They secrete
their products into the connective tissue from which they
enter the blood stream in order to reach their target cells.
The products of endocrine glands are hormones.
Exocrine glands are classified as either unicellular or
multicellular:
? Unicellular glands: are the simplest in structure, the
secretory component consists of single cells distributed
among other cells that are not secretory. A typical
example is the goblet cell, mucus-secreting cell
positioned among other columnar cells.
? Multicelluar glands: are composed of more than one
cell and exhibit varying degrees of complexity. Their
structural organization allows for subclassification
according to the arrangement of the secretory cells and
the presence or absence of branching of the duct
elements.
? If the duct is unbranched, the gland is called
simple;
? if the duct is branched, it is called compound.
? If the secretory portion is shaped like a tube,
the gland is tubular;
? if it is shaped like a flask, the gland is alveolar
or acinar;
? if the tube ends in a sac - like dilation, the
gland is tubuloalveolar.
Tubular secretory portions may be single or branched. Thus,
exocrine glands may be described as:
Simple tubular, as in the
intestinal glands of the colon
Simple coiled tubular, as in
the eccrine sweat glands
Simple branched tubular,
as in the submucosal glands of
Brunner in the duodenum
Simple branched acinar,
as in the cardiac glands of the
stomach
Compound acinar, as in the
pancreas
Compound tubuloacinar,
as in the submandibular gland
Mucous and serous glands, are so named
because of the type of secretion produced
.The secretory cells of exocrine glands
associated with the various body tubes, i.e.,
the alimentary canal, respiratory passages,
and urogenital system, are often described as
being mucous, serous, or both.