Chapter Summary

Another important model organism in developmental biology is the African clawed frog, Xenopus laevis. The oocytes are very large, and develop from the primary oogonium [fig. 4.3]. In contrast to Drosophila, each of the 16 cells generated by four rounds of mitoses in the primary oogonium will become a primary oocyte. Each primary oocyte will enter meiosis, during which huge extended chromosomes, called lampbrush chromosomes [fig. 4.2] will form, indicating early activation of transcription. After being surrounded by somatic follicle cells, these lampbrush chromosomes will recompact prior to the onset of vitellogenesis, the process by which yolk is imported from the liver via the circulation into the growing oocyte. The yolk platelets accumulate in the vegetal hemisphere of the oocyte, and the pigment granules accumulate along the cortex of the animal pole along with the germinal vesicle. Thus, the animal vegetal polarity of the oocyte can be easily determined visually [fig. 4.1].

The maturation of the oocyte is induced by progesterone [fig. 4.4], a hormone which is secreted in response to environmental signals and which can be induced artificially in the laboratory by the injection of pituitary hormones. The process of oocyte maturation involves the completion of the first phase of meiosis: germinal vesicle breakdown (GVBD), polar body formation and arrest during the early prophase of meiosis II. The oocyte will stay in this phase of meiosis II until fertilization [fig. 4.5] when the necessary centrosome provided by the sperm will enter the egg.

The entry point of the sperm during fertilization in the animal hemisphere will determine the future dorsal ventral axis of the zygote. The sperm centrosome initiates formation of the sperm aster [fig. 4.6], and cortical rotation follows via a microtubule dependant process [fig. 4.7]. The rotation of a fertilized frog egg results in formation of the grey crescent, which will become the Nieuwkoop centre. Rotation can be measured experimentally by using dye to mark positions of the cortex, and can be inactivated by UV light. Cortical rotation is necessary for establishing the dorsal-ventral axis of the egg, in the absence of rotation no dorsal structures form (muscles, nervous system, etc.), and in the case of overrotation no ventral structures form (gut, endoderm, etc.) [fig. 4.8].

The grey crescent is also the site of the first cleavage plane, which bisects the gray crescent in an animal-vegetal direction. The early cleavages of the zygote occur very rapidly, and cells in the animal hemisphere cleave more rapidly than cells in the yolky vegetal hemisphere [fig. 4.9]. During cleavage, tight junctions form [fig 4.10] and ions are pumped inside the embryo, raising osmotic pressure and causing the blastocoel to form. The outer surface of the blastula forms an epithelium. These early cleavages occur in the absence of maternal transcription. The mid-blastula transition (MBT) occurs at this stage, beginning the onset of zygotic transcription, increasing cell motility, and slowing the rate of cell division.

At this stage, maps can be made that describe the developmental potential of regions of the blastula in different ways [fig. 4.11]. Fate maps can be constructed that describe what various regions of the blastula will become under normal developmental circumstances. Alternatively, a specification map can be constructed that describes which cells of the late blastula are committed to certain fates. The specification maps were constructed by a series of explant studies that measured developmental potential of different regions of the embryo. Through these studies several regions were identified, the animal cap, the dorsal and ventral marginal zones (DMZ and VMZ respectively), and the vegetal cap.

While the fate maps show that regions of the late blastula can be predicted to follow a particular fate during normal development, those regions may not yet be committed to that exact fate if developed in isolation. Thus the fate map and specification maps differ slightly. The specification map is more general and describes regions of future ectoderm, mesoderm, mesendoderm, and endoderm. Whereas the fate maps show that under normal conditions, cells can be predicted to form the epidermis, neural tube, notochord, blood, somatic muscle, and gut. A third type of map, called a competence map, that describes the developmental potential of different regions of the blastula when developed in different developmental contexts via transplantation experiments can also be constructed.

The different regions of the blastula have different inductive capabilities [fig. 4.12]. Nieuwkoop described induction of the animal cap by the vegetal cap to form mesoderm. This inductive event has been shown to occur via a combination of signaling and localization of certain molecules, resulting in the formation of the Nieuwkoop center from prospective dorsal l regions of the vegetal hemisphere . The Nieuwkoop center signals the dorsal marginal zone to form the Spemann organizer. This induction involves both the general induction of mesoderm and the formation of dorsal mesoderm (dorsalization) near to the Spemann organizer.

Gastrulation in the frog involves a series of coordinated morphogenic movements [fig. 4.13]: invagination, involution, epiboly, and convergent extension [fig. 4.14]. Invagination is caused by the formation of bottle cells, which change shape by constricting their apical surface to initiate formation of the blastopore in the dorsal marginal zone (DMZ). The dorsal lip will then begin the process of involution at the site of invagination, causing movement of cells from the dorsal marginal zone to the interior and forming the archenteron. As involution continues, the cells of the involuting marginal zone (IMZ) and the overlying non-involuting marginal zone (NIMZ) will undergo convergent extension, resulting in elongation of the embryo in an anterior-posterior direction.

When the process of gastrulation is complete, the embryo has three germ layers, a dorsal-ventral axis, and an anterior-posterior axis. The gastrulation movements are carefully orchestrated and are initiated by inductive signaling from the Spemann organizer, which corresponds to the dorsal lip of the blastopore [fig. 4.15]. This signalling is also important for neurulation and in the event of dorsal lip transplantation a secondary site of involution and a subsequent secondary axis can be induced. Thus, the dorsal mesoderm, which forms the notochord, is important for inducing neural structures from the overlying ectoderm after gastrulation. The process of neurulation, like gastrulation, involves coordinated morphogenetic movements [fig. 4.16]. These movements include apical constriction of the neural plate, followed by elevation and fusion of the neural folds. This process creates a neural tube overlying the notochord that is flanked on both sides by somitic mesoderm. Thus the body plan of the frog embryo is established by a coordinated series of morphogenetic movements and inductive events [fig. 4.17].

Chapter Summary

Further Reading