The embryonic period
Frank J. Dye in Human Life Before Birth, 2019
Gastrulation and neurulation occur during the early embryonic period. Both of these processes involve cell movements, including invagination, evagination, epiboly, involution, convergence extension, migration, and ingression. Invagination is the movement of a sheet of cells into a preformed cavity. Evagination is the movement of a sheet of cells away from a preformed cavity, for example, the evaginations of the developing retinas from the lateral walls of the early forebrain. Epiboly is the spreading of cells upon a surface. Involution is the turning in of cells over a rim, for example, the involution of epiblast cells into the primitive streak. Convergence is the movement of cells toward each other. Extension is the elongation of a structure as a result of cell convergence. Migration is the movement of single cells, for example, the movement of neural crest cells throughout the developing embryo to give rise to a wide diversity of structures, such that the neural crest has been referred to as the fourth germ layer. Ingression is the movement of single cells out of a cell layer into a preformed cavity, for example, ingression of epiblast cells along the primitive streak to give rise to endoderm and mesoderm, or formation of the notochord by cells that ingress through the primitive (Hensen's) node.
Soft and Hard Tissue Repair
John C Watkinson, Raymond W Clarke, Louise Jayne Clark, Adam J Donne, R James A England, Hisham M Mehanna, Gerald William McGarry, Sean Carrie in Basic Sciences Endocrine Surgery Rhinology, 2018
This process involves replication and movement of the epidermal cells from the wound edges in order to reconstitute an organized, keratinized, stratified squamous epithelium. Increased mitotic activity within the basal cells of the wound edges occurs within 12 hours of wounding. Initially, migration of epidermal cells creates a delicate covering over the raw area, a process also known as ‘epiboly’. These cells then migrate, usually as a sheet, by extending lamellipodia (from the Latin lamina – thin sheet, pod – foot) from the free edge of the cut epidermis, across the defect. This process of migration is dependent on the oxygen tension present in the wound and is most rapid in hyperbaric conditions.7 ‘Contact inhibition’ prevents movement when epithelial sheets meet. Gradually, a continuous squamous cell epithelium is restored.
Integrin Function in Early Vertebrate Development: Perspectives from Studies of Amphibian Embryos
Yoshikazu Takada in Integrins: The Biological Problems, 2017
Mechanistically, the process of gastrulation may be considered on several levels that include: (1) the “behavior” of the individual cells that collectively serve to “drive” morphogenesis at gastrulation, (2) the molecules mediating the shape changes and adhesive properties of these cells, and (3) the control of the timing and patterning of the cellular movements involved. Following the progression through early cleavage and blastula stages, gastrulation begins with the appearance of a slit-like invagination of bottle cells, termed the blastopore, on the dorsal side of the embryo as illustrated in Figure 1. The involuting mesoderm subsequently comes in contact with the blastocoel roof and travels along it in the direction of the animal pole. The zone of involution initiated at the dorsal lip of the blastopore spreads laterally and ventrally to enclose the endoderm, which remains visible as a yolk plug through late gastrulation. The superficial cells of the animal pole and equatorial marginal zone spread by epiboly during this process, thus covering the entire outer surface of the embryo. Inside the embryo, the endodermally derived archenteron forms as the mesoderm advances, resulting in the displacement of the blastocoel (Figure 1).
Toxicity assessment of biological suspensions using the dielectric impedance spectroscopy technique
Published in International Journal of Radiation Biology, 2018
S. Muñoz, J. L. Sebastián, P. Antoranz, J. P. García-Cambero, A. Sanchis-Otero
Regarding to the conductivity, the results showed a similar trend for the three viable suspensions, with a larger dispersion corresponding to the sample V1 and slightly higher values through almost the whole spectra from the sample V3. These differences would be related to the variations in structure and composition of the embryo interior during the development, considering both the animal and vegetal poles. Apart from the aforementioned differences in the animal pole among the three observed embryo stages, during the embryogenesis also the vegetal pole suffers some alterations. In fact, at earlier stages, the yolk cell is composed of yolk granules packed densely underlying the blastodisc, appearing later a peripheral layer, the yolk syncytial layer (YSL) (sample V1). Then, this layer will spread underneath the blastodisc, becoming an internal layer between animal and vegetal poles at sphere stage (embryos of V2). In the late blastula, epiboly begins and the YSL accompanies the thinning and spreading of the blastodisc over the yolk cell (embryos in V3).