Embryonic Development
Embryonic Development - includes: 1) cleavage
2) growth
3) differentiation ( specialization)
Cleavage
- zygote undergoes a series of cell divisions by mitosis ( 1-2-4-8- 16, etc.) forming a ball of cells called a morula.
- cells do not grow between divisions so these cells decrease in size.
- ova are much larger than the average cell size in the organism. Cleavage continues until the size of the cells in the developing embryo are reduced to the size of the cells in the adult organism.
- as cleavage continues the morula forms a hollow ball called a blastula with fluid inside. The arrangement of cells in the developing embryo depends on the amount and distribution of yolk. Eggs with little yolk (Ex.- humans) have cells that are of nearly equal size.
Growth
- when the cells of the embryo begin to grow before dividing, the cells of the blastula also begin to move causing a change in shape. This point is called gastrulation. Cells on one side of the blastula move inward (like a deflating beach ball) to form a two layered embryo called a gastrula. A third layer grows between these two layers producing three germ layers.
- gastrula - outer layer - ectoderm becomes: skin, hair, scales, feathers, brain & nervous system
- middle layer - mesoderm becomes: skeletal, muscular, circulatory, excretory, reproductive ( most of the ) digestive systems plus connective tissue.
- inner layer - endoderm becomes: internal lining of organs, the lungs and various glands, liver
Differentiation
- a series of changes that transforms the unspecialized embryonic cells of the gastrula into specialized cells, tissues and organs.
- differentiation is accomplished by the DNA of some cells turning “off” or “on” different segments so that the specific protein structures of those cells are produced. Exactly how embryonic cells do this is not known. It is known that by the early gastrula stage, cells in specific regions are committed to specific lines of development. Another known fact is that certain parts of the developing embryo act as organizers, influencing the development of adjacent cells. This is known as embryonic induction. How does this happen? Maybe cell contact or cell chemicals. This is what turns “off” or “on” DNA segments.