Cephalization

Cephalization is considered an evolutionary trend,^[1] whereby nervous tissue, over many generations, becomes concentrated toward one end of an organism. This process eventually produces a head region with sensory organs.^[2]

Cephalization is intrinsically connected with a change in symmetry. It accompanied the move to bilateral symmetry made in flatworms, with ocelli and pinnae placed in the head region. In addition to a concentration of sense organs, all animals from annelids on also place the mouth in the head region. This process is also tied to the development of an anterior brain in the chordates from the notochord. A notable exception to the trend of cephalization throughout evolutionary advancement is phylum Echinodermata, which, although having a bilateral ancestor, as evidenced by their embryology, develop into a pentaradial animal with no concentrated neural ganglia or sensory head region. However, some echinoderms have developed bilateral symmetry secondarily.

In neuroembryology, neural induction of the ectoderm forms a neural tube which undergoes cephalization to form initially three, then five vesicles as a developing embryo. It is the internalized ectoderm which goes on to become the central nervous system, peripheral nervous system and epidermis.

In respirology, cephalization refers to the distribution of pulmonary flow from the bases to the apices of the lung due to pulmonary edema.^[3]

Cephalization in the Animal Kingdom

Even hydras, which are primitive, radially symmetrical cnidarians, show some degree of cephalization. They have a “head” where their mouth, photoreceptive cells, and a concentration of neural cells are located.

Flatworms (phylum Platyhelminthes) are the most primitive animals with bilateral symmetry. They also have a fairly advanced degree of cephalization, with sense organs (photosensory and chemosensory cells) and a brain concentrated at the anterior end. Consequently, scientists believe that cephalization characterized all bilaterally symmetrical animals from their origins. However, flatworms differ from more advanced animals in that their mouths are in the center of their bodies, not at the anterior end.

In arthropods, cephalization progressed with the incorporation of more and more trunk segments into the head region. Scientists believe this was advantageous because it allowed for the evolution of more effective mouth-parts for capturing and processing food.

Cephalization in vertebrates, the group that includes mammals, birds, and fishes, has been studied extensively. The heads of vertebrates are complex structures with many features not found in close relatives such as the cephalochordates. The cephalochordate Branchiostoma (formerly called Amphioxus), which is the closest relative of vertebrates, is a burrowing marine creature which lacks most of the head structures that are so distinct in vertebrates, such as distinct sense organs; a large, multilobed brain; teeth; and a tongue.

There was a persistent debate during the twentieth century as to whether the vertebrate head is “old” or “new”. Scientists who champion the idea of an “old” head suggest that the vertebrate head resulted from the evolution of important modifications to a previously existing head. The idea of a "new" vertebrate head was proposed originally by American vertebrate morphologists Carl Gans and Glenn Northcutt in 1983. They suggested that the vertebrate head is a new structure, which has no corresponding structure in close relatives such as Branchiostoma.

Evidence to support a “new” vertebrate head comes from the observation that most important features of the head are derived from neural crest cells, embryonic cells found only in vertebrates. The neural crest cells are of ectodermal origin—rather than mesodermal or endodermal—and arise during the process of neurulation, the time at which the dorsal hollow nerve cord forms.

Neural crest cells are exceptional in that they are highly mobile, migrating in streams throughout the head region and the rest of the body, and because they give rise to an unusual diversity of features. The neural crest cells are responsible for forming the bones of the face and jaws, the structures of the tongue and larynx, the teeth, and portions of the eye. Experiments in which the neural crest was removed from developing animals confirmed that these critical head structures failed to develop without it.

Scientists hypothesize that increased cephalization in vertebrates, including the evolution of many of their novel head features, is related to adaptations for predation. Sensory structures—the jaw and large brain—are all requirements for a successful existence as a predator.

References