4.14:
Tissues
Multicellular organisms, like humans, are comprised of specialized cell communities called tissues, groups of cells that have similar structure to perform related functions.
They can be classified into four major types, nervous, controlling information, muscle, including skeletal, cardiac, and smooth for movement, epithelial, to cover and protect boundaries, and connective tissues, for further support.
Most organs contain all four types and their structural arrangements vary to provide distinguishable characteristics.
For instance, epithelial tissue, such as skin, displays an apical surface to the outside and are arranged either in single and simple layers or stratified, two or more layers stacked upon each other to protect against abrasion.
In addition, cell shape can vary, from squamous, flattened, to cuboidal and columnar, with the nucleus conforming to each form. These deeper sheets of epithelial cells are supported by connective tissue just below, where nutrients can diffuse up from blood vessels and provide the nourishment that cells need.
4.14:
Tissues
Cells with similar structure and function are grouped into tissues. A group of tissues with a specialized function is called an organ. There are four main types of tissue in vertebrates: epithelial, connective, muscle, and nervous.
Epithelial Tissue
Epithelial tissue consists of thin sheets of cells and includes the skin and the linings of internal organs and body cavities. Epithelial cells are tightly packed, providing a barrier against injury, infection, and water loss. Epithelial tissue can be a single layer called simple epithelium, or multiple layers called stratified epithelium. In stratified epithelium, such as the skin, the outer cells—which are subject to damage—are replaced through the division of cells underneath. Epithelial cells have a variety of shapes, including squamous (flattened), cuboid, and columnar. Some epithelial tissues absorb or secrete substances, such as the lining of the intestines.
Connective Tissue
Connective tissue is composed of cells within an extracellular matrix and includes loose connective tissue, fibrous connective tissue, adipose (fat) tissue, cartilage, bone, and blood. Although the characteristics of connective tissue vary greatly, their general function is to support and attach multiple tissues. For example, tendons are made of fibrous connective tissue and attach muscle to bone. Blood transports oxygen, nutrients and waste products, and exerts immune functions, to support the needs of other tissues.
Muscle Tissue
Muscle tissue consists of groups of long, thin muscle cells—called muscle fibers. Muscle cells can contract and expand, allowing the body and internal organs to move. The main types of muscle tissue are the cardiac muscle of the heart; the skeletal muscles that attach to bones, such as the limb muscles; and smooth muscle found in visceral organs such as the intestines.
Nervous Tissue
Nervous tissue is composed of neurons—specialized cells that send, transport, and receive information through electrochemical signaling—and supporting cells called glial cells. A bundle of neurons is a nerve. A brain is the concentration of nervous tissue. Besides controlling movement through the muscles, nervous tissue detects sensory stimuli and is responsible for directing many of the activities of the body.
Suggested Reading
Leberfinger, Ashley N., Dino J. Ravnic, Aman Dhawan, and Ibrahim T. Ozbolat. “Concise Review: Bioprinting of Stem Cells for Transplantable Tissue Fabrication.” Stem Cells Translational Medicine 6, no. 10 (October 2017): 1940–48. [Source]
Gonzalez, Ana Cristina de Oliveira, Tila Fortuna Costa, Zilton de Araújo Andrade, and Alena Ribeiro Alves Peixoto Medrado. “Wound Healing – A Literature Review.” Anais Brasileiros de Dermatologia 91, no. 5 (2016): 614–20. [Source]