Multicellular organisms such as sponges consist of multiple specialized cellular types cooperating together for a common goal. These cell types include Choanocytes, digestive cells; Sclerocytes, support-structure-secreting cells; Porocytes, tubular pore cells; and Pinacocytes, epidermal cells. Though the different cell types create an organized, macroscopic multicellular structure—the visible sponge—they are not organized into true interconnected tissues. This is illustrated by the fact that a sponge broken up using cheese cloth and a very specific ion cocktail (the classical blender experiment does NOT work) will reaggregate from the surviving cells. If individually separated, however, the particular cell types cannot survive alone. Simpler colonial organisms, such as Volvox, differ in that their individual cells are free-living and can survive on their own if separated from the colony.
More complex organisms such as jellyfish, coral, and sea anemones possess a tissue level of organization, in which differentiated, interconnected cells perform specialized functions as a group. For instance, jellyfish tissues include an epidermis and nerve net that perform protective and sensory functions, along with an inner gastrodermis that performs digestive functions. The overall spatial organization of differentiated cells is a topic of study in anatomy.
Even more complex organisms, while also possessing differentiated cells and tissues, possess an organ level of development, wherein multiple tissues group to form organs with a specific function or functions. Organs can be as primitive as the brain of a flatworm (merely a grouping of ganglion cells), as large as the stem of a sequoia (up to 90 meters (300 feet) in height), or as complex and multifunctional as a vertebrate liver.
The most complex organisms (such as mammals, trees, and flowers) have organ systems wherein groups of organs act together to perform complex related functions, with each organ focusing on a subset of the task. An example would be a vertebrate digestive system, in which the mouth and esophagus ingest food, the stomach crushes and liquifies it, the pancreas and gall bladder synthesize and release digestive enzymes, and the intestines absorb nutrients into the blood.
In order to reproduce, true multicellular organisms must solve the problem of regenerating a whole organism from germ cells (i.e. sperm and egg cells), an issue that is studied in developmental biology. Therefore, the development of sexual reproduction in unicellular organisms during the Ectasian period is thought to have precipitated the development and rise of multicellular life.
Multicellular organisms also face the challenge of cancer, which occurs when cells fail to regulate their growth within the normal program of development.
There are various mechanisms which are disputed as being the first responsible for the emergence of multicellularity, but it is difficult to say which is correct. This is because all the suggested mechanisms are viable, but establishing which was responsible for the first multicellular life requires mostly speculation.
One hypothesis is that a group of function-specific cells aggregated into a slug-like mass called a grex, which moved as a multicellular unit. Another hypothesis is that a primitive cell underwent nucleus division, thereby becoming a syncytium. A membrane would then form around each neucleus (and the cellular space and organelles occupied in the space), thereby resulting in a group of connected and specialised cells in one organism (this mechanism is observable in Drosophila). A third theory is that, as a unicellular organism divided, the daughter cells failed to separate, thereby resulting in a conglomeration of identical cells in one organism which could each then specialize.
Dynamics of thymocyte--stromal cell interactions visualized by two-photon microscopy. (Reports).(Statistical Data Included)
Jun 07, 2002; Tissue microenvironments are likely to have a profound impact on lymphocyte behavior, yet most studies of lymphocytes have relied...