Metamerism (biology)

Metamerism is the segmentation of body parts. It is why animals can move, wiggle, and bend. Many animals have a segmented anatomical structure. In mammals, this segmentation occurs in embryos. A classic example is segmented worms (annelids). Annelids have a series of repeating segments, like a train. At either end of this repeating series are either a mouth with sensory organs or a back end, which is the end of the digestive system. Each segment has the same inner tube like shape which resembles bagels or doughnuts. Each segment also has similar construction with a network of nerves, blood vessels, muscles and a doughnut-shaped fluid-filled chamber. In addition to the chamber there are bristles (setae) and other structures (parapods). These blocks of tissue are called somites. The pattern of repetition is called serial homology; it is why each person has two arms with the exact same structure. This is a feature of vertebrates and invertebrates. The somites reproduce in pairs and eventually make up the skeleton and skeletal muscle.

Brief History

The repeated segments in the body (think of the repetition of the spine, ribs, etc.) are an example of repeated metameric segments. This is a type of organizational pattern the body uses to create movement. Ancient naturalists like Aristotle observed this feature in animals, such as snakes, maggots, worms, and chicken embryos. In the sixteenth century this process was observed in chicken eggs, linking the presence of segmentation in embryos. It was called the "division of the embryo.""

Somites are also known as metameres, and they undergo somitogensis (metamerism). Somites were first described in the seventeenth century, however, scant attention was paid until the scientific community focused on evolution and biology in the nineteenth century. The similarities of structure in both vertebrates and invertebrates became of great interest to early scientists. Advances in technology, such as early microscopes and laboratories, led to detailed descriptions of developing embryos. These early scientists asked very important questions that led to the discovery of somites and their function. They described how somites form and separate and how they give rise to cartilage, dermis, and muscles.

In the eighteenth century scientists believed the process of human embryology occurred through preformation. Preformation is the concept of a preformed infant which simply grows larger over time. Another idea put forth by Aristotle nearly 2,000 years prior is epigenesis. Epigenesis entails development through which an egg goes through several steps, cells differentiate, and organs form. Another term for this is neoformationism (in contrast with preformationism). Aristotle explains this theory in his On the Generation of Animals (c. 350 BCE). This theory is now confirmed through modern science and may seem obvious; however, the theory did not receive much attention for many years because of competing theories. In particular, epigenesis was overshadowed for centuries by creationist theories. In the late eighteenth century a debate by biologists led to the acceptance of epigenesis over the formerly held preformationism. Much of what scientists knew about embryologists came from the research of Italian anatomists such as Leonardo da Vinci, Aldrovandi, Aranzio, Emilio Parisano, Girolamo Cardano, and others. As technology, especially microscopy, improved, scientists could see the shape of embryos and how development took place.

Overview

The discovery of somites and the understanding of embryology can be divided into four phases: (1) In the 1800s scientists surmised the basic structure of somites. This understanding was clarified in the twentieth century by the use of transmission and scanning electron microscopy. This is the foundation of what is known of somites. (2) The experimental phase of scientific discovery of somites occurred through the use of transplantations and other procedures of the mesoderm with adjacent tissues. Scientists observed that somites undergo several phases of development and interact with other tissues. This phase of discovery and research is ongoing. (3) In the twentieth and twenty-first centuries, the phase of theoretical models attempts to explain the process of somite patterning inside the embryo. This area of study has sparked more research. (4) The phase of analysis of the changing patterns of extracellular materials and gene expression and how it effects the development of somites is ongoing. Scientists believe these changing patterns may indicate their importance in somite development. There is considerable interest in this line of study because it links genetics and embryological development.

Somites are the first segmented tissues to appear in embryo development; they create the foundation for all other segmented structures which follow, including the spinal nerves, vertebrae and ribs, and skeletal muscles of the trunk. The importance of somites lies in the fact that they give rise to important structures in the body which turn into skin, cartilage, muscle, and the spinal column. Somites determine how cells will split and reproduce and what their function will become. Somites undergo somatogenesis, a process of how they are formed, which is composed of five parts: periodicity, tissue formation, epithelialization, specification, and differentiation. In the end, specific regions of each somite become specific types of tissue and cells as the body matures. When somitogenesis concludes, it is followed by four more processes: myogensis (muscle formation), osteogensis (bone formation), tendon formation, and specification of the intermediate mesoderm (this becomes the urogenital system). Somites are a fundamental part of the developing body of vertebrates, as such, an interruption in the process of segmentation can cause anomalies such as congenital defects. Somites are the blueprint for the vertebrate body, directly responsible for the development of the musculoskeletal system as well as the nervous system.

Bibliography

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DeRuiter, Corinne. "Somites: Formation and Role in Developing the Body Plan." Embryo Project Encyclopedia, 25 Sept. 2013. Web. 6 Dec. 2015. http://embryo.asu.edu/handle/10776/2065.

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Moormann, Stephanie, Debbie Guatelli‐Steinberg, and John Hunter. "Metamerism, Morphogenesis, and the Expression of Carabelli and Other Dental Traits in Humans." American Journal of Physical Anthropology 150.3 (2013): 400–08. Print.

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Takahashi, Yu, et al. "Metameric Pattern of Intervertebral Disc/Vertebral Body Is Generated Independently of Mesp2/Ripply-mediated Rostro-caudal Patterning of Somites in the Mouse Embryo" Developmental Biology 380.2 (2013): 172–84. Print.