One of the levels of organization of matter is tissue. A tissue is a group of similar cells that together with their corresponding cellular products are specialized to perform common functions . The science that studies tissues is histology and it is one of the branches of biology .
The cells that are part of a tissue do not have to be of the same type and, in addition, they can present many intercellular products that help them to develop their functions, such as neurotransmitters.
In embryonic development, the germ layers differentiate into four basic tissues: nervous, connective, muscular, and epithelial :
- epithelial tissue can be stratified in the epidermis, columnar in the intestine, and cuboidal in the kidneys
- muscle tissue can be smooth muscle in the intestinal wall, skeletal in the muscles that move voluntarily, and cardiac in the heart
- connective tissue can be bone, blood, or loose connective tissue
- nervous tissue are usually only nervous
Next we will describe the different types of fabrics a little more in depth.
When we talk about epithelium we are talking about a cellular layer that lines an external or internal surface.
The epithelium is lining the surface of the body taking a protective function. Inside the body it covers all the organs, channels and ducts through which secretions and materials are transported.
Epithelial cells can be modified to form glands that produce hormones, enzymes, or lubricating mucus.
The classification of epithelial tissue is carried out according to the shape of the cells and the number of layers and can be found:
- Simple epithelia : found in all metazoans:
- Simple squamous epithelium : it is composed of flattened cells that form a fine lining in the blood capillaries and lungs. It allows the passive diffusion of gases and tissue fluids into and out of the cavities it covers. In the case of the pleura they are called mesotheliums in the endothelial blood vessels.
- Simple cuboidal epithelium : it is made up of low cells with a square section. They occur in small ducts or tubules such as kidneys, salivary glands with secretory or absorption activity.
- Simple columnar epithelium : it is similar to the previous one but the cells are taller with elongated nuclei. It occurs on surfaces with great absorption capacity such as the intestine . It may present microvillosities that increase the absorption surface. It also appears in the female reproductive organ where the cells are ciliated.
- Stratified epithelia occur mainly in vertebrates:
- Squamous stratified epithelium : they are several layers (minimum two) of cells that protect against abrasion. The basal layer undergoes frequent mitosis . The cells resulting from this division move the pre-existing cells to the surface. These break off and are replaced by cells on the surface. This tissue appears mainly in the mouth, esophagus, and anal canal of various animals. In mammals, it also lines the vagina.
- Transitional epithelium : it is a special stratified epithelium that is adapted to undergo dilation or stretching. It appears in the urinary tract and bladder of vertebrates. Cell layers appear to shrink when tissue is stretched.
Connective tissue is very abundant and diverse. Its main function is of union and support giving shape to the body of the animal. The connective tissue is composed of few cells but with many extracellular fibers and the matrix or fundamental substance (it is a fluid).
The fundamental component is collagen fibers , a highly elastic protein that is the most abundant in the animal kingdom. It occurs in areas of the body where flexibility and resistance are important. The connective tissue of invertebrates is like that of vertebrates but with a less elaborate organization.
The types of connective tissue are:
- Loose connective tissue : Also called areolar connective tissue, it is the filling material of the body that holds blood vessels, nerves, and organs attached. It contains fibroblasts that synthesize the fibers, the fundamental connective substance, and the wandering macrophages that are responsible for phagocytizing pathogens. The fibers can be collagen, strong fibers and elastic fibers, thinner and branched formed by elastin. The adipose tissue is a type of connective tissue.
- Dense connective tissue : forms tendons, ligaments and fasciae. The fasciae are arranged as sheets of tissue around skeletal muscle. In the tendon the collagen fibers are very long and very closely arranged.
- Vascular tissue : made up of blood, lymph, and tissue fluids that are made up of special cells in an aqueous matrix, plasma.
- Cartilage : it is a semi-rigid form of connective tissue with closely grouped fibers embedded in a gelatinous matrix together with chondrocytes housed in small gaps called lagoons. Chondrocytes are a type of cell.
- Bone : calcified connective tissue containing calcium salts organized around collagen fibers. It is the strongest tissue. Osteocytes (bone cells) are housed in small gaps in the matrix and communicate with the blood vessels that penetrate the bone through a network of fine tubes called canaliculi. The bone remodels throughout the life of the animal and can repair itself as in fractures.
It is the most common in most animals. The unit of muscle is the muscle cell commonly called muscle fiber . These are elongated and specialized in contraction. It usually originates from the mesoderm.
The cytoplasm of muscles is called sarcoplasm and the elements that contract in muscle cells are the myofibrils .
Muscles can be smooth (where the fibers are not banded) or striated (the fibers have transverse bands when viewed under a microscope).
The types of muscle tissue are:
- Skeletal muscle : it is a type of striated muscle that appears in vertebrates and invertebrates. It is made up of very long, multinucleated cylindrical cells that can reach from one end of the muscle to the other. This is a voluntary muscle in vertebrates as it contracts at will.
- Cardiac muscle : it is another type of striated muscle that occurs, as its name indicates, in the heart of vertebrates. Cells are shorter with only one nucleus. This type of muscle is made up of a branched network of fibers where cells are connected to each other by intercalary discs . This muscle is called involuntary since it does not move at will. Its contraction is regulated by specialized cells in the heart itself.
- Smooth muscle : it is a non-striated muscle that appears in vertebrates and invertebrates. The cells of this muscle are long, tapered bands with a single nucleus. This type of muscle is the one that occurs most often in invertebrates . It acts as the musculature of the body wall and limits the ducts and sphincters. In vertebrates it lines the walls of blood vessels and surrounds organs such as the uterus and intestine. Its contraction does not occur consciously in vertebrates.
Nervous tissue is specialized in receiving and conducting impulses from one part of the body to another. The two cell types in this tissue are neurons and neuroglia.
The neurons are the basic functional unit and glia are cells that act as support and are not considered nerve cells.
Neurons are very important cells and are made up of the following parts:
- dendrites : extend from the soma or cell body, are shaped like tree branches and receive stimuli from other neurons, receive electrical impulses from receptors or other nerve cells
- axon : is the elongated part of the cell. It transmits electrical impulses from the cell body to synaptic terminals which can be another nerve cell or an effector organ. Also called nerve fiber.
- Synaptic Terminals : They are found at the end of the axon and release neurotransmitters at the synapse when an action potential arrives. The synapse is the point of contact of some nerves with each other or with effector organs.
- Schwann cells : form an insulating envelope in many peripheral nerves of vertebrates.
- Ranvier’s node : these are interruptions between the insulator of Schwann cells that allow action potentials to jump from node to node.