The human body is composed of trillions of cells and their products. The cell represents the most basic level of structure and function. From there, the level of organization increases in complexity to tissues, organs, organ systems and, ultimately, the organism as a total package. This article will focus on the key stages of this hierarchy and attempt to explain how a semblance of order can arise from apparent chaos.
The trillions of cells in the human body can be categorized into approximately 200 specialized cell types. In terms of structure, cells represent the most basic level of organization common to all living things. As for function, cells carry out the myriad chemical reactions essential to life. In terms of size, human cells run the gamut from tiny spermatocytes all the way to megakaryocytes, giant bone cells that give rise to platelets and are almost visible to the naked eye.
Amazingly, each of these cell types arises from a totipotent stem cell in the form of a fertilized egg. Over the course of development, most of the genes in each given cell type become silenced while others become activated. The end result is cellular progeny with a high degree of differentiation – red blood cells carry oxygen; B lymphocytes make antibodies; rods and cones capture photons; neurons conduct electrical signals and so on.
This transition is arguably the most fundamental step in the progression from the individual cell to multicellular existence. All tissues consist of at least one specific cell type and a surrounding extracellular matrix. Tissues run the spectrum from blood, individual cells floating in a liquid matrix; to brain, neurons surrounded by glial cells; to bone, osteocytes interspersed in a solid matrix of mineralized collagen. Cells lining the surface of the gut, airways, most solid organs and vasculature are collectively referred to as epithelial cells. In contrast, cells lining the thoracic and abdominal cavities are called mesothelium. The parenchyma of solid organs consists of specialized cell types such as hepatocytes in the liver; muscle fibers in the heart and skeletal muscles; and hematopoietic cells in the bone marrow.
Although histologists will no doubt continue to debate the finer nuances of tissue classification, it is often helpful to categorize tissues based on their embryological germ layer of origin. The ectoderm develops into the skin, eyes and nervous system. Mesoderm gives rise to the kidneys, genitalia and all major connective tissues, including the bones, muscles and blood. Finally, the respiratory and gastrointestinal tracts arise from the endoderm.
Organs are composed of two or more kinds of tissue. Organs can be classified by structural similarity. For example, solid organs include the heart, lungs, liver, kidneys and brain, whereas organs like the stomach, uterus and urinary bladder contain central cavities. A more familiar classification scheme is to divide organs into systems based on function – circulatory system, musculoskeletal system, digestive system, respiratory system, nervous system, endocrine and reproductive systems.
Some organs fall into more than one category. The pancreas, for example, consists of exocrine tissue that makes digestive enzymes, as well as endocrine tissue that makes insulin, glucagon and other metabolic hormones. The main function of the kidney is to filter waste out of the blood, but the kidney also has endocrine functions, including blood pressure regulation and the release of erythropoietin, a hormone that stimulates red blood cell production in the bone marrow.
In the final analysis, no organ system exists in isolation; at a fundamental level all systems interact and contribute to the maintenance of life.