A complex organism consists of thousands, millions, billions, or even trillions of cells, each of which performs a specific function. On the cellular level, the cells of the skin and intestines might seem similar, but they perform completely different functions. This is because different types of cells come together to form tissues, which in turn form organs and give them their complex and specific functions.
What is a tissue
In the process of evolution, some cells began cohabitating and colonies appeared. Colonies are not a multicellular organism because each individual cell in it can live independently and all of the cells are the same type and perform the same functions. As evolution continued, colonies became more complex and the cells began to differentiate, meaning that the cells began to perform different functions and look different. This is how the first multicellular organisms appeared, the cells of which abandoned the possibility of independent existence and switched to performing individual functions.
The human, plant, or animal organism develops from a single cell that divides and replicates. Up to a certain point, all its descendants look and behave the same. But then some of the offspring cells acquire characteristic features and the ability to perform functions specific only to them. This process is called differentiation.
Tissue is an organizational term used to describe a group of cells that are similar in shape, size, function, and metabolic products within a larger multicellular organism. One or more tissue types combine to form an organ. Tissues can be simple when they consist of cells of the same type and complex when they contain cells of different types. Complex tissues are quite common: for example, the intestinal mucosa and nervous tissue both contain many different types of cells. The term tissue also encompasses the intercellular substance or matrix that exists between and around the cells. Histology is a science that deals with the study of tissues.
Tissue classification
Tissues are usually classified into four main groups: epithelial, muscle, connective, and nervous. Each tissue can be distinguished from others by the characteristics of their cells and their relationships with each other. Many organs are composed of several types of tissues, which can be recognized by their characteristic microscopic structure.
Undifferentiated cells that can "turn" into any other type of cell are called stem cells. These are usually round with a large nucleus. Such cells have two features — the ability to divide and maintain a continuous supply of stem cells, and potency. Potency is the ability of stem cells to give rise to mature (specialized, differentiated) cell lines. In this case, with each division, the cell changes more and more, choosing a certain line of development. With each stage of development, the cell loses potency [pic 2]. The main function of somatic stem cells is to maintain homeostasis by regenerating and replacing old, damaged, or dying cells.
Different types of tissue are united by common properties. By these properties, it is usually easy to distinguish a cell belonging to a particular tissue.
Epithelial tissue
The main properties of epithelial tissue are the almost complete absence of intercellular matrix and very tight contacts between cells. All the epithelial cells in a tissue are usually located on a thin plate called the basement membrane. Epithelial cells are polar: that is, the different ends of the cell are morphologically (in appearance) different from each other due to the specific orientation of different cellular structures and processes.
The epithelium can be multi-layered, ie. consisting of several layers of cells (this is how the outer layer of the skin looks like) or single-layer, i.e. all of its cells are associated with the underlying basement membrane (such as the epithelium that lines the gastrointestinal tract). Epithelial tissue primarily serves as a protective membrane for the body and acts as a barrier to all substances and molecules. Many important tissues in the body are epithelial, including the skin and the lining of the intestine and the respiratory tract. Epithelial tissue also forms some vital regulatory organs, such as glands and secretory areas.
Muscular tissue
Muscle tissue is made up of cells that can contract. Contraction is performed by specific muscle fibers that consist of cells containing contractile proteins (most often actin and myosin). Muscle tissue is responsible for the movement of the body and its parts, as well as for changing the size and shape of internal organs. Muscle cells consist of special protein filaments (myofilaments): thin and thick. Muscle cells are usually longer than they are wide, so they are often called muscle fibers or myofibrils. An important feature of muscle tissue is not only contraction, but also relaxation. Muscle tissue makes up the skeletal muscles, as well as the heart and many internal structures in organs (for example, the muscular lining of blood vessels so that they can contract).
Connective tissue
Connective tissue is a very diverse family of tissues, ranging from bone tissue to blood. The main distinguishing feature of connective tissue is a large amount of intercellular substance, that might actually make up more of this tissue than the cells themselves. Cartilage cells (chondrocytes) secrete a dense elastic ground substance (matrix) around themselves. Bone cells (osteoclasts) are surrounded by a ground substance containing salt deposits, mainly calcium phosphate. In addition, connective tissue often plays the role of a layer in many organs. Blood and lymph are the only liquid tissues in the body, since, despite the presence of cells, they mainly consist of water with substances dissolved in them. Blood cells, unlike any other tissue, are not connected to other cells and "work" alone.
It is the connective tissue that forms scars in the affected areas of other tissues — for example, when muscle cells die in the heart muscle, their place is taken by a rough seam of connective tissue.
Nerve tissue
Nervous tissue is made up of neurons. Neurons have a characteristic long part called an axon, which stretches for long distances. The axons of many neurons form nerves. Neurons also have dendrites, which are shorter and present in large numbers on a single neuron. Neurons are supported by auxiliary cells — neuroglia.
The main characteristic of the nervous tissue is the ability to transmit an impulse. The transmission of excitation occurs in the nerve endings (synapses), which are the place of contact between neurons, as well as between neurons and muscle cells. To transmit an impulse, neurons secrete neurotransmitters, signaling molecules. When a nerve impulse arrives, neurotransmitters are released into the synaptic cleft, transmitting excitation to neurons or muscle cells.
Conclusion
Tissues are one of the most important "inventions" of evolution and enabled the development of complex organisms. There are four types of tissues in the human body — epithelial, muscle, connective and nervous.