Notes on histology
Histology Lecture 1
Histology is the study of tissues: in our case, it means study of tissues which make up the human body. Tissue is defined as a group of cells which perform a common function. All the cells in a given tissue are not necessarily identical or similar e.g. blood is a type of connective tissue but it has many different types of cells. However, tissue also contains two other components i.e. ground substance (or matrix) and fibres.
Despite the complexity of the human body and the diversity of organs it contains; there are only four tissues called basic or principal tissues - these are epithelium, connective tissue, muscle and nerve tissue.
The following paragraphs describe the main structural features, functions and sub-divisions of each type of tissue.
Epithelium is characterised by closely packed cells and has very little extra-cellular material (ground substance or fibres). Cells are held together by means of desmosomes and tight junctions and form continuous sheets or layers which either cover the body’s outer surface (skin) or internally line various body cavities and vessels. Epithelial cells rest on a basement membrane which separates them from other tissues. Epithelium continues to regenerate throughout life, making it particularly susceptible to disorders of cellular growth such as cancer (90% of all human cancers are epithelial in origin).
Epithelium is classified on the basis of two criteria
(i) Cell shape: epithelial cells can be squamous (thin and flat, spread out flat like a fried egg), cuboidal (cell height = cell width) and columnar (shaped like a brick).
(ii) Number of layer of cells: epithelium which is formed by only one layer of cells is called simple epithelium; if two or more layers of cells are present, it is known as stratified epithelium.
Below is a list of common types of simple and stratified epithelia and their location(s) in the human body. Students are strongly encouraged to link the structure with the function and think why a particular type of epithelium is present in a given organ and how its structure enables it to carry out the functions it performs.
|Type of epithelial tissue |Location | |Simple squamous epithelium |Alveoli (air sacs) in the lungs and the inner lining of the | | |vessels | |Simple cuboidal epithelium |Kidney tubules | |Simple columnar epithelium |Inner lining of the stomach and the intestine | |Simple columnar ciliated epithelium |Fallopian tube | |Stratified squamous epithelium |Mouth, throat and oesophagus | |Stratified squamous keratinised epithelium |Skin | |Transitional epithelium |Urinary bladder |
Histology Lecture 2
Connective tissue (CT) is the supporting, binding and packaging tissue of the body; it consists of (a) ground substance (b) fibres and (c) cells. The characteristic feature of connective tissue is that it contains relatively few cells but an abundance of extra cellular material (i.e. fibres and ground substance). Remember that the opposite is true in case of the epithelium.
Ground substance (also called matrix) fills the spaces between cells and fibres. It is an amorphous (unstructured) material and is composed of tissue fluid, cell...
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