The tissue

THE TISSUES

Four main groups of tissues are known in the body.  These are:

1-Epithelial tissues.

2-Connective tissues.

3-Muscular tissues.

4-Nervous tissues.

A- THE EPITHELIAL TISSUES

These tissues arise from any of the three primary germ layers, the ectoderm, endoderm or mesoderm. 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.

The epithelial tissues are characterized by having very little intercellular substance or matrix between their cells and by 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

-Simple, when it is one cell layer thick

-Stratified, when it is two or more cell layers

The individual cells that compose an epithelium are described as:

1-Squamous, where the width of the cell is greater than its height

2-Cuboidal , where the width, depth ,and height are approximately the same

3-Columnar , where the height of the cell appreciably exceeds the width (the term low columnar is often used where a cells height only slightly exceeds its other dimensions )

4-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.

5-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.

 Gland

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 or .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.

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. Goblet cells are located in the surface lining and glands of the intestines and in certain passages of the respiratory tract.

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 b