3. Tissues and organs
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Inside the cell each organelle fulfills a function and they all work in a coordinated way. However, in the human body there are different types of cells that have a different morphology.
3.1. Cell differentiation
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As cells multiply during their embryonic development, a process of cell differentiation occurs. Cell groups specialise so they can perform different tasks and different types of cells are created.
The process used by cells to specialise is referred to as cell differentiation.
Cell differentiation involves changes at different levels:
Cell shape : the shape of cells is related to the different specific functions they perform. Most cells have a spherical shape. However, their adaptations to perform concrete functions or the pressure from adjacent cells can make different shapes: polyhedral, prismatic, elongated, starry, amongst others.
Function : because of cell differentiation, some cells are programmed to perform certain functions and not others. For example, a muscle cell can contract, but they are not able to create antibodies. A nerve cell can transport a nerve impulse but does not transport oxygen.
Activity of the cytoplasmic organelles : cell specialisation also requires changes at cytoplasmic level that affect the number and activity of the different organelles. In this way, muscle cells have a great amount of mitochondria to obtain the energy they need; glandular cells have a very developed Golgi apparatus, capable of releasing substances to the external medium and a red blood cell does not contain organelles, so they have more space to transport oxygen.
Cell differentiation
3.2. Types of tissues
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In order to increase their efficiency, cells of the same type group together. They collaborate in a common task and so they form a tissue. Tissues complete specific processes cells have specialised for.
A tissue is a cluster of cells with the same morphology and function.
Tissues are characterised by the type of cells that constitute them as well as the intercellular substance between them. A basic classification of tissues groups them according to the function they perform: epithelial, connective, muscular and nervous.
3.2.1. Epithelial tissues
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They cover the surface of the body, lining internal cavities and enveloping the different organs. Cells are arranged very closely together (there is almost no intercellular substance between them). According to their function, epithelial tissues are classified into various types: the epithelium tissue and the glandular epithelium.
Epithelium tissue : Lining the surface of the body, both the outside (skin) as well as hollow cavities that communicate with it, such as the digestive and respiratory tract (mucus).
Glandular epithelium : produces and releases substances for different purposes. In certain occasions, cells group forming structures called glands , such as salivary glands.
3.2.2. Connective tissues
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The main function of connective tissues is to support, unify and connect systems in the organism.
Connective tissues have a large amount of extracellular matrix (set of substances found outside cells) in which they are embedded.
There are two types of connective tissues: non-specialised , which are connective, and specialised , including adipose, cartilage and bone tissue.
Connective tissue : located in the deepest layer of our skin, it constitutes the ‘stuffing’ between organs. The extracellular matrix contains many fibres, such as collagen.
Adipose tissue : composed of cells that accumulate fat. Their main function is to be an energy reserve, although it also has other functions.
Cartilaginous tissue : is a firm and elastic tissue, that protects joints from the bones, strengthens the larynx, trachea and forms structures such as the pinna or nose.
Bone tissue : is a rigid tissue, due to the great amount of minerals that are deposited in its matrix. It supports the organism.
3.2.3. Muscle tissue
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Muscle tissue is composed of special types of elongated cells called muscle fibres . Their most important characteristic is that they are able to contract and for this reason they are responsible for the organism's movement.
Muscle tissue that allows the skeleton to move is referred to as striated muscle tissue and they contract voluntarily. Smooth muscle tissue , which is responsible for the movements of organs such as the stomach or uterus, contracts involuntarily. Cardiac muscle tissue is a special type of tissue because it has striated fibres, but contracts involuntarily.
3.2.4. Nervous tissue
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Nervous tissue is composed of very specialised cells called neurons, which transmit nerve impulses. The nervous system coordinates the functioning of the organism.
3.3. Organs
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Tissues group together to form organs ; as they do so, the functions each organ performs complement each other leading to even more complex functions. Displayed in the images below you can see some of the many organs humans have.
Cell differentiation allows the development of cell types that are morphologically different.
Cells that are programmed for the same function group together forming tissues.
There are different types of tissues: epithelial, connective, muscle and nervous tissues.
Tissues group together forming organs and so they can coordinate and perform more complex functions.