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41. Tissues first time formed in phylum
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In animal evolution, structural complexity has increased in the evolution of animals so that there is considerable specialization and division of labor within body tissues.Some of the trends seen in animal evolution are:
I. Grades of Organization
An animal (or a plant for that matter) is composed of many units organized into successive units: Molecules are the units of organelles, Organelles are the units that make up cells, Cells are the units that make up tissues, Tissues are the units that make up organs, and Organs make up organ systems
Each level is more complex than the one before and, as a general rule, a more recent evolutionary product.
A. Protoplasmic grade of organization.
All life functions are confined within the boundaries of a single cell. Within the cell, the protoplasm is differentiated into organelles capable of carrying out specialized functions. (e.g., the protists)
B. Cellular grade of organization.
Cellular organization is an aggregation of cells that are functionally differentiated. A division of labor is evident, so that some cells are concerned with, for example, reproduction, others with nutrition. Such cells do not become organized into true tissues but may form definite patterns or layers. Sponges are at this level of organization.
C. Tissue grade of organization.
Cells all of one type begin to function in a unified way to accomplish a task. Cnidarians are usually considered to be at this level of organization.
D. Organ grade of organization.
The aggregation of different kinds of tissues into organs is a further advancement in the evolution of animals. Organs appear first in the Platyhelminthese (flatworms).
E. Organ system grade of organization.
When organs work together to perform some function (circulation, respiration, reproduction, digestion, etc.) we have the grade of organization seen in all animals that evolved after the flatworms.
II. Increased Complexity of Development
Embryology is the study of the progressive growth and differentiation that occurs during the transformation of a fertilized egg to a new individual. A brief summary of development is necessary for understanding the early evolution of animals.
A. Review: General Pattern of Development
1. Fertilization - In all animals, germ cells produce by meiosis eggs or sperm. The fusion of an egg or sperm to form a zygote is called fertilization. This is the starting point for development.
2. Cleavage - The division of the zygote into smaller and smaller cells.
3. Blastulation - cleavage eventually gives rise to a hollow ball of tiny cells called a blastula.
4. Gastrulation - The sorting out of cells of the blastula into layers (ectoderm, mesoderm, endoderm) that become committed to the formation of future body organs.
5. Differentiation - the formation of body tissues and organs. The basic body plan of the animal is established.
6. Growth - increased size of the animal.
Differentiation is a key feature of multicellular life:
Emergence of a New Field: Evolution and Development (Evo/Devo) - new insights into the origin and evolution of multicellular organisms.
One of the main differences between unicellular and multicellular organisms is cell differentiation. That is, cells become specialized for a specific function and, usually, take on a characteristic morphology. For example, your body has epithelial cells lining most of its surfaces. If you take an epithelial cell and grow them in tissue culture they will multiply but they will remain epithelial cells.
How do cells get locked into one type? In the early 1900's, scientists suggested that the genes that control a particular kind of cell (such as an epithelial cell, for example) are passed on to those cells during development and all other genes (such as those that control muscle cells) are filtered out. As you well know, that isn't the case - each of your cells has all the genes needed to make an exact copy of you. This means that some genes are not used in some cells -- for example, muscle cell genes are not used in epithelial cells.
Cell differentiation then depends on different genes being active in different cells. This occurs through a process called gene regulation. In gene regulation, one gene (called a regulator gene) acts a a switch that turns other genes on or off. A basic version of this process occurs in bacteria and protists as well as multicellular organisms.
For example, use of the milk sugar lactose by E. coli in your digestive tract. E coli has three genes that produce enzymes to break down lactose and release ATP. These genes are preceeded on the DNA strand by a promoter (a base sequence that signals the start of a gene) and an operator (an intervening sequence with an active binding site):
In phylum, Cnidaria first tissue level of the organization is formed.
- The animal body shows different levels of structural organization they may be of cells, tissues, organs, and organ systems.
- Tissues may be defined as a group of similar cells that have common functions.
- The animals belonging to the lower level of phylum have a lower grade of organization.
- Phylum cnidaria is the first phylum which shows the tissue level or organization.
- In phylum cnidaria, tissue organization takes place which is specialized for a particular function.
- Cnidarians are mostly marine entities while some are freshwater. Some are solitary and others are colonial.
- The body wall comprises an outer epithelium referred to as the epidermis and inner epithelium referred to as the gastrodermis.
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