Anamniotic vertebrates, such as amphibians, first form a gut tube (or archenteron ) by invaginating tissue from the outside into the inside (think of the way you might stick your fist into a balloon to create a tube-like structure.) As a deuterostome (“second-mouth”), like all vertebrates, this original hole (or blastopore ) becomes the anus of the animal. The inside of the archenteron becomes the gut tube. The cells directly contacting the archenteron become the endoderm; they will form several outpockets (or ceca, singular cecum) to produce the lungs, pancreas, liver, vermiform appendix, and bladder. The cells that form between the endoderm and outer tissue are the mesoderm, which forms tissues that become muscles and bones. The outer cells form the ectoderm, which creates the skin and the nervous system. A major chordate character, the notochord , forms from the dorsal-most portion of the mesoderm along the central axis of the organism. Amniotic gastrulation is slightly more complicated. A primitive streak appears on the embryo. This structure is homologous to the blastopore, and is the site of cell migration from the outside to the inside of the embryo. Those that migrate in become the endoderm and mesoderm, while those left on the outside are the ectoderm. (This process will be described in detail in lecture.) In this section of the lab you will look at slides of amniotic frog development, which is holoblastic and involves gastrulation via a blastopore.
3 Lab Exercises Exercise #1 A. Examine a cross section of a slide labeled frog blastula (slide #1; Fig. 85 in the lab appendix). Draw and label the blastocoel (the central cavity of the embryo), the animal pole , and the vegetal pole. B. Examine the cross section of the frog gastrula (slide #2; Fig 86 in the lab appendix). Draw and label the blastopore , and the developing archenteron. Step 3: Neurulation After gastrulation, vertebrates undergo neurulation, during which they develop a neural tube . The neural tube forms by a process called invagination, which is similar in both amniotic and amniotic vertebrates. Cells from the ectoderm just dorsal to the notochord form a groove. As the groove deepens, the edges fuse together to form a tube that will become the dorsal, hollow nerve tube. The lumen of this tube fills with cerebrospinal fluid later during development, eventually differentiating into the central nervous system. The derivatives of the neural tube persist throughout the entire life of the organism. Special cells from the most dorsal region of the neural tube, the “crest”, are among the most important in vertebrate development. These neural crest cells are highly mobile and colonize nearly every tissue in the body to form or direct the formation of many of the major tissues Neural crest cells develop into the cartilage and bones of the face, as well as many aspects of the peripheral nervous system.
4 Exercise #2 A. Examine transverse sections of an embryo at two points during neurulation (slide #3; labeled frog early neural tube and frog late neural groove; Fig. 94 & 95 in the lab
- Fall '10
- Embryology, Neural tube, Mesoderm