7th lesson Medical students Medical Biology

Histology

Histology: is that branch of anatomy that studies tissues of animals and plants. Histology deals with the structure and function of the body.The cell: is the basic structural and functional unit of complex organisms.

Tissue: is a structure formed by collections of cells that have the similar morphologic characteristics and functions .The human body is composed of four types of tissues:

1 .Epithelial Tissue2 .Connective Tissue

3.Muscular Tissue4 .Nervous Tissue

Epithelial Tissue is a tightly bound cells with a very little intercellular substance forming sheets covering or lining the body . Epithelia are derived from all three embryonic germ layers. Most of the epithelium lining the skin, mouth, nose, and anus has an ectodermal origin .

The lining of the respiratory system, the digestive tract and the glands of the digestive tract such as the pancreas and the liver are derived from the endoderm. Other epithelia (e.g. the endothelial lining of the blood vessels) originate from mesoderm.Epithelial Tissue is presented in two forms:

1 .Covering Epithelia (Membranes)2 .Glandular Epithelia (Secretory elements )

characterizes of epithelial tissue 1-Epithelial tissue covers surfaces with layer of cells .

2-Epithelial cells are attached to one another . 3-Intercellular spaces in epithelium are small

4-Epithelial cells are polarized. 5-Epithelial cells are separated from the underlying tissue by a basement membrane.  

The Basement Membrane is a narrow, a cellular region and is the interface between epithelium and connective tissue, its well stained by PAS reaction (Periodic Acid Shiff) and other histological stains .

The basement membrane is shown by electron Microscopy to be composed of

1 .The basal lamina (nearest to the epithelial cells).2 .Reticular lamina (consisting of reticular tissue) .

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The Basal Lamina consist of: A. Lamina lucida (Lamina rarae): which appears to be transparent .B. Lamina densa: A delicate network of fine fibrils .Basal lamina are composed mainly of type IV collagen, a glycoprotein called Laminin.

Basal lamina is sometimes attached to the underlying connective tissue by anchoring fibrils of unknown composition

Functions of Epithelial Tissue:1-Protection, 2-Sensation, 3-Secretion, 4-Absorption , 5-Excretion ,

6-Diffusion, 7-Cleaning , 8-Reduces Friction

Covering Epithelia is classified according to the number of cell layers into:1 .Simple Epithelium:

2 .Stratified Epithelium:Simple Epithelium is classified according to the morphology of the cell into

1 .Simple Squamous Epithelium2 .Simple Cuboidal Epithelium3 .Simple Columnar Epithelium

4 .Pseudostratified Epithelium

Simple Squamous Epithelium is composed of a single layer of tightly packed thin polygonal cells.

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This tissue can be found lining pulmonary alveoli compose the loop of Henley and the partial layer of Bowman ś capsule in the kidney and form the endothelial lining of blood and lymph vessels .

Simple Cuboidal Epitheliumis a single layer of polygon-shaped cells. in vertical section the cells present a square with a centric round nucleus. Simple cuboidal epithelium make up the ducts of many glands of the body, form the covering of the ovary and compose some kidney tubules.

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Simple columnar epithelium: consists of a single layer of tall cells. Simple columnar epithelium is usually involved in active secretion and/or absorption of material across the single cell layer, Simple columnar epithelium lines the digestive tract and the female reproductive tract, Simple Columnar Epithelium that is found in the lining of much of the digestive tract, gallbladder and large ducts of glands is not ciliated, that lines the uterus, oviducts, and small bronchi is ciliated.

Pseudostratified epithelium appears to be stratified but it is actually composed of a single layer of cells, all of the cells in Pseudostratified epithelium are in contact with the basement membrane but only some cells reach the surface of the epithelium, their nuclei are located at different levels giving the impression of a stratified epithelium even though its composed of a single layer of cells, the cells are of three types: columnar, fusiform and basal cells .

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Pseudo stratified epithelium is ciliated and it’s found lining most of the trachea and primary bronchi and the nasal cavity Pseudo stratified epithelium is also found in the male urethra, epididymis and large excretory ducts of glands .

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Types of Epithelial tissue

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