Tissues are present exclusively in vascular animals and plants, which have acquired them with the course of evolution independently and, therefore, they present differences between them although, in essence, they are homologous.
Animal tissues: what are they and their characteristics
In biology, tissue is a level of biological organization consisting of a material made up of an organized series of cells (the same or different from each other) of the same nature, with an embryonic origin and a common physiological behavior.
Animals are characterized by highly specialized cells. The more evolved an organism is, the more differentiation its cells will present. The association of these specialized cells gives rise to the tissues that, in turn, make up the organs of animals. These have specific functions and can be categorized into four types of animal tissues : epithelial, connective, muscular and nervous.
The 4 types of animal tissues are listed below, along with their functions and characteristics.
It covers the body’s surface, lines its internal cavities, and forms the secretory portions of the body’s glands, which secrete substances such as hormones and enzymes. It is a non-vascularized tissue (to nourish itself, it depends on the underlying connective tissue), whose constituent cells are characterized by:
- Being attached to each other by means of specific molecules that establish intercellular bonds.
- Possess morphological and functional polarity.
- Present a basal surface attached to a basement membrane, which separates them from the connective tissue, while its apical part is exposed to the external environment or the body cavity.
Epithelia are classified according to the amount of cell strata in simple (if they only have one layer of cells) or stratified (if it has several layers) and according to the shape of the epithelial cells (which can be flat, cubic or cylindrical).
Originating in the mesenchyme, the term “connective tissue” includes various types of tissues with different functions (mechanical and functional support, immunological protection, energy reserve and transport). The cells that form them are separated from each other by collagen, reticulin and elastin fibers and by an extracellular matrix, these three elements being the components of these tissues. The extracellular matrix is a complex structural network secreted by connective tissue cells (which it surrounds and supports), which influences extracellular communication and is composed of protein fibers (collagen and elastic) and the so-called fundamental substance (which consists of proteoglycans, multi-adhesive glycoproteins and glycosaminoglycans, in addition to water,
There are several types of connective tissue: adipose, bone, blood (liquid type, driven by the heart and vascular walls), cartilage, hematopoietic and lymphatic.
Its function is the body movement and the change in size and shape of the organs (muscle contraction). It has mesodermal origin and is made up of elongated cells called muscle fibers, which contain actin and myosin and whose functional characteristics are excitability, conductivity, and contractility. These muscle cells have a cytoplasm called the sarcoplasm, bounded by a membrane called the sarcolemma. In sarcoplasm, microfibrils stand out, which can be smooth or striated. There are three types of muscle tissue:
- Striated muscle tissue: it is associated with bones and moves the different parts of the body.
- Cardiac muscle tissue: appears in the heart, promoting blood and lymphatic circulation throughout the body.
- Smooth muscle tissue: it is found in the walls of hollow organs and is involved in the internal movements of the body.
Composed of neurons, which receive stimuli from others, conduct the electrical impulse to other tissues and store information, and also formed by support cells, which are in contact with the former and provide protection, electrical isolation and metabolic exchange mechanisms between blood vessels and neurons.
The nervous system is divided into the central nervous system , which is made up of the brain and spinal cord, and the peripheral nervous system , made up of cranial, spinal, and peripheral nerves (which transmit and receive impulses to and from the central nervous system) and the ganglia. Supporting cells form neuroglia in the central nervous system, while in the peripheral nervous system they are represented by Schwann cells and satellite cells. This tissue coordinates the functions of the body, since the nervous system allows the response to environmental stimuli and controls the activities of organs and devices.
The component cells of animal and plant tissues are different, since, obviously, the former are animal cells and the latter are plant cells, which implies a series of intrinsic differences. These are the main differences between the cells of animal and plant tissues :
- Animal cells are mobile and plant cells are not.
- Animal cells lack chlorophyll, unlike plant cells.
- Animal cells are heterotrophic (and consuming) and plant cells are autotrophic (and producing).
- Animal cells are of limited growth and plant cells are of unlimited growth.
- Animal cells have only a plasma membrane, while plant cells also have a cell wall that gives them rigidity and turgor.
- Plant cells have vacuoles in the cytoplasm that do not appear in animals.
- Animal cells have centrosomes and plant cells, for the most part, do not.
- In plant tissues, most cells can differentiate from one tissue to another, whereas, in animal tissues, cells are usually not capable of this.