Tissue concept
The term tissue designates a set of cells having essentially the same structure and function. Tissues juxtapose, interpenetrate, and combine to form organs.
Tissue formation
All cells originate in the embryo and differentiate themselves through several cellular processes to form a tissue. These cellular processes are:
- direction and number of cell divisions,
- change in the way the cells,
- cell movements,
- cell growth,
- cell death and
- composition changes in the cell membrane and in the extracellular matrix.
These processes are controlled by: paracrine signaling, where neighboring cells release substances that influence cell development around them; or contact signaling, where the juxtaposed cells are in contact by ligand-receptor without producing/releasing substances to the surrounding environment.
The tissues have a compact structure due to the junctions between the cells: occlusion junction, adhesion junction and communicating junction. The existence of these junctions provides a polarity to the cells and, consequently, to the tissue. It is also through these junctions that the cells within a tissue communicate.
Types of tissue
In plants, in addition to meristems, the more common plant tissues are:
- epidermis,
- periderme,
- parenchyma,
- colenchyma,
- sclerenchyma and
- vascular tissues.
In animals, four types of tissues can be distinguished:
- connective:
- embryonic;
- connective tissue proper:
- loose;
- dense regular;
- dense irregular;
- special:
- adipose;
- cartilaginous;
- bone;
- hematopoietic;
- muscular:
- smooth;
- skeletal;
- cardiac;
- epithelial and
- nervous.
Tissue study
Histology refers to the study of tissues. Typically, in order to perform the study of intact tissues, it is first necessary to fix them with formaldehydein and after in a paraffin block. Subsequently, small slices of the paraffin block with the tissue are cut in a microtome (machine that cuts through micrometers of thickness). Thereafter, the cut is placed on a slide where the paraffin is withdrawn, and the cells are stained with dyes having affinity for different structures. At the end of the process, it is possible to visualize the tissue and its constituents using microscopy. It is possible to use optical, fluorescence or scanning microscopy, depending on what you want to see.
References:
- Alberts B., Johnson A., Lewis J., Raff M., Keith R., Walter P. (2007). Molecular Biology of the Cell (5th edition). Garland Science, New York.
- Kierszenbaum A. L. (2008). Histologia e Biologia Celular- Uma Introdução À Patologia (2nd edition). Elsevier.
- Gilbert S. F. (2013). Developmental Biology (10the edition). Sinauer Associates, Inc., Sunderland (MA).
