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Development
(Campbell 6th & 7th Ed. Chapter 47)
The development of an organism involves an increase in mass and an elaboration of structures. Both these changes can be understood in terms of cell activity, namely an increase in cell number (due to mitosis),enlargement of cells, migration of cells, as well as differentiation and specialization of cells. In this laboratory we examine both growth as an increase in size and also embryonic development to gain a familiarity with these phenomena and interpret them in terms of cell activity.
When the mass, height, etc. of an organism is plotted against its age, we usually obtain an S-shaped, sigmoid, or logarithmic curve. This can be demonstrated in a wide variety of biological systems.
A population of cells initially passes through a lag phase before cell division occurs. This represents the time necessary for the cells to adjust to a new environment. Cell divisions gradually accelerate to a maximum which is called the log (logarithmic) or exponential phase, during which the cells increase at a rate proportional to the number present. The metabolic machinery is now fully adapted to the condition of the environment. Eventually the rate of division tapers off and the population reaches a plateau or stationary phase. The rate of growth equals that of death and the steady state is attained.
Embryonic development involves cell activities such as: mitosis, enlargement, movement, differentiation and specialization, both separately and simultaneously. Again our studies are observational and our intent is to find explanations in terms of cell activity.
Cleavage and
Blastulation:
(Campbell 6th Ed. 1002-1005; 7th Ed. 992-994)
All division starts from a single cell the zygote.
Cell division (mitosis) begins as the egg divides (cleaves) to form two cells. This first division or cleavage marks the initiation of development and the zygote is now known as an embryo. The two cells produced by the cleavage are known as blastomeres. The first and second cleavages, resulting in two and four blastomeres respectively, are in the vertical plane.The third cleavage, resulting in eight blastomeres, is horizontal and slightly above the equator of the embryo. The four upper and smaller blastomeres are known as micromeres and the bottom four as macromeres . The former become ectoderm and the latter are yolk cells. Cleavage continues until the embryo becomes a spherical hollow blastula of many cells. A cavity usually develops within the blastula and is known as the blastocoel.
Gradually the micromeres overgrow the yolk and form a dorsal lip over the yolk. The tissue tucks in or invaginates below the lip and an opening develops that is known as a blastopore. This process is known as gastrulation. It is similar to pushing your finger into an inflated balloon. A cavity, known as the archenteron, is formed internally which ultimately becomes the gut of the animal.
The frog embryo now has two germ layers, the ectoderm and the endoderm. The walls of the archenteron are endodermal cells. Gradually the blastocoel disappears with the expansion of the archenteron. Later in gastrulation irregular cells bud off from the advancing tip of the archenteron and give rise to the mesoderm or third germ layer. In the advanced gastrula, the anterior portion of the archenteron gives rise to a body cavity (known as a coelom) by a process of evagination. Thus, by the end of gastrulation, all three germ layers are formed.
Examine a cross section of a gastrula showing all three germ layers
The development of tissues is known as histogenesis and results from cell differentiation. The three germ layers will form and give rise to all the structures of the adult via organogenesis (organ formation) and eventually morphogenesis (establishment of form) as shown below.
Soon after gastulation, a layer of ectodermal cells lying on the dorsal surface of the developing embryo and distributed from anterior to posterior fold inward in a process known as neurulation . This folding results in a long neural groove, which folds completely over, eventually pinching off a tube of tissue which will differentiate into the spinal cord and brain.
