Eukaryotic Cell Types
Diversification of cell types afforded the evolution of multicellular organisms. The simplest are made of a few dozen different cells, while humans are composed of over 200 kinds of cells.
Somatic cells are the ordinary cells of a multicellular eukaryote. These cells are the basis of the 4 primary animal tissue types: epithelium, muscle, connective, and intelligence tissues.
Epithelial tissues line the surfaces and cavities of bodily structures and form many glands. These cells secrete, selectively absorb, protect, and transport.
Muscle cells, being capable of contraction, provide for movement. Muscle is the most abundant tissue in most animals.
Connective tissue supports, separates, or connects other tissues. Connective tissue, immersed in body fluids, is composed of cells, fibers, and extracellular matrices.
In some animals, including vertebrates, the body physically centers its intelligence system in glial cells, which are networked together via neurons. 85% of human brain cells are glia. (Whether cell or organism, an intelligence system is energetically driven by a mind.) Glia nurture and control neural growth and activity within the brain, as well as receiving neutrally transmitted information from external stimuli for further processing.
Stem cells, found in all multicellular life, are cells of flexible form, able to differentiate into diverse specialized cell types, including somatic cells. Stem cells guide organism development. Stem cells can also self-renew: stir up more stem cells.
There is a fitness advantage to renewing your mitochondria. Stem cells know this and have figured out a way to discard their older components. ~ American biologist David Sabatini
Stem cells intelligently manage their resources: providing the highest-quality organelles to daughter stem cells. Such strategic thinking lessens cellular damage that can lead to stem cell exhaustion, thereby aging an organism from reduced tissue renewal.
By dividing asymmetrically, stem cells can generate two daughter cells with distinct fates. Stem cells segregate their old mitochondria to the daughter cell that will differentiate, whereas a new stem cell will receive only young mitochondria. ~ Finnish cytologist Pekka Katajisto
If there is a shortage of stem cells, differentiated cells can take their place: generating various cell types, including more stem cells. Genes dormant in differentiated cells again become active when reverting back to stem cell status.
Germline cells are the special cells of sexual reproduction, producing gametes. In animals, the gametes are eggs and sperm. Plant germ cells produce ovules and pollen.
Animal germ cells develop in the embryotic stage. In flowering plants, germ cells come from somatic cells in adult floral meristem: the plant tissue where growth occurs.
