Embryonic membranes

At this point, the yolk sac and all extra embryonic membranes are... 

The structure, ultrastructure and formation of the hymenolepidid egg has been reviewed in detail by Ubelaker (888). Its general morphology is shown in Fig. 7.14). Although there are only the usual three basic embryonic membranes (p. 179) in the developing egg - shell/capsule, outer envelope, and inner envelope - the fully formed egg often appears to be more complex due to further differentiation of these layers. The following structures can be recognised (Fig. 7.11). 

Yet other methods have been proposed (Fig. 9.6) whereby the CAM vascular networks can be displayed in greater detail, except that the embryo with the extra-embryonic membranes and yolk must be transferred to an in vitro system during the early stages of development (day 3 or 4 of incubation). The system consists of a Petri dish (Auerbach et al., 1974), or an inert plastic container, previously equipped with a parafilm ring (4—5 cm inside depth) to provide a support for the embryo and adnexa (Dugan et al., 1991). The embryo in the petri dish or in the container is then incubated in an humidified CO2 atmosphere at 37°C. This CAM in petri dish, or cracked eggs method is among the most frequently used CAM assays. 

Beck, F., Erler, T., Russell, A., James, R. 1995. Expression of Cdx-2 in the mouse embryo and placenta possible role in patterning of the extra-embryonic membranes. Dev. Dyn. 204, 219-227. 

FIG. 15.1 Electron micrograph of the eggshell of the nematode Haemonchus contortus (from sheep) showing the vitelline (v), chitin (ch) and lipid (1) layers and the embryo bounded by the embryonic membrane (em). 

Glucocorticoids. Glucocorticoids can be mediators of the action of stress on PG and in turn on other PG-dependent reproductive events. Stress induces an increase in glucocorticoid formation in maternal and embryonal tissues. The glucocorticoids (mainly corticosterone) in turn activate PGF production by both embryonal membranes and the uterus, and promote the PGF-induced labor in different animal species and humans (2,19). 

The adult female parasitoid may transfer one or more type(s) of regulatory factor(s) to the host during parasitism. Some genera of braconid and scelionid families produce teratocytes. These cells originate from an embryonic membrane in the parasitoid egg and are released into the host hemocoel. Certain parasitoid females of the families Ichneumonidae and Braconidae contain symbiotic viruses that are injected into the host with the egg. Diverse functions have been attributed to the teratocytes and the symbiotic viruses. Both play a major role in the interaction of the parasitoid with their respective hosts. 

Teratocytes and Serosa. Teratocytes, which originate from the disintegration of an embryonic membrane or serosa (= trophamnion) of some parasitic Hymenoptera, appear to have a significant role in parasitoid-host interactions (, 77-80, Dahlman, D. L. Arch. Insect Biochem. Physiol., in press). A discussion of teratocytes is... 

Holland, P.W.H. and Hogan, B.L.M., Spatial expression of the alpha-fetoprotein gene in mouse extra-embryonic membranes. Unpublished, 1985. 

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