Embryonic envelopes

The egg and reproductive system thus present an elegant model for the study of a number of areas of physiological interest, especially the ultrastructure and cytochemistry of the spermatozoa, ova and embryonic envelopes (see p. 166), as well as the physiological processes involved in egg hatching and the subsequent activation of the released oncosphere. 

The three basic embryonic envelopes are as follows (Figs. 7.4 and 7.11) ... 

Table 7.3. Hymenolepis nana histochemical reactions of the embryonic envelopes of the egg. (Data from Fairweather ... 

Although the embryonic envelopes of all cyclophyllideans appear to be formed in a similar manner, this may not always be evident from the final form of the egg. This divergence undoubtedly reflects specific functional demands placed on cestode species by the habitats of eggs outside the host or by the ecology of their intermediate hosts, both of which may have important implications in the transmission strategies of various species (148). 

Conn, D. B. (1985a). Fine structure of the embryonic envelopes of Oochoristica anolis (Cestoda Linstowiidae). Zeitschrift fur Parasitenkunde, 71 639-48. 

Scanning electron microscopy and histochemistry of embryonic envelopes of the porcupine tapeworm Monoecocestus americanus (Cyclophyllidea Anoplocephalidae). Canadian Journal of Zoology, 63 1194—5. 

Fairweather, I. Threadgold, L. T. (1981a). Hymenolepis nana the fine structure of the embryonic envelopes. Parasitology, 82 429-43. 

Ultrastructure of embryonic envelopes and their differentiation in Hymenolepis diminuta (Cestoda). Journal of Parasitology, 58 849-63. 

III. The inner envelope a syncytial layer showing much variation. Some workers divide this layer into two zones - zone I, a cytoplasmic layer and zone II, a gelatinous layer (Fig. 7.14). Part of this embryonic layer gives rise to the embryophore (Fig. 7.4) and also to the oncospheral membrane (Figs. 7.4, 7.11 and 7.14) (a very thin layer surrounding the oncosphere), which is often counted as a fourth layer. Additional layers, which may be further derived from the above basic envelopes have been reported in some species (e.g. H. nana 204), but it is beyond the scope of this text to discuss all the various modifications which can occur. Only those features which have a special physiological significance are discussed below. 

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). 

Fluorescence microphotolysis can be used to measure diffusion in single cellsand nuclear envelope permeability. Pyranine probes have been used to measure internal pH changes in Escherichia coli membrane vesicles. Probe measurements of intralysosomal pH in living cells and perturbation of pH has been described by Ohkuma and Poole.Fluorescence polarization of six membrane probes in embryonal carcinoma cells have been measured by a cell sorter,Fluorescence of pancreatic islets labelled with fluorescein diacetate has been used to show the effects of cations, ionophores, and hypoglycaemic sulphonylureas. ... 

Each muscle fiber, or cell, is enveloped by an elastic membrane (sarcolemma), immediately beneath which lie large numbers of ovoid, longitudinally oriented nuclei, leaving the interior of the fiber occupied by the sarcoplasm and myofibrils. In embryonic muscle, the nuclei occupy a central position. 

Prior to cell division, the nucleus replicates itself so that the two new cells will each contain genetic information. Several nuclear enzymes coordinate the replication of DNA. During cell division, the nuclear envelope breaks down, and equal copies of DNA and cytoplasm are partitioned into two daughter cells. After division, the nuclear envelopes reform in each daughter cell around its own copy of DNA. This fundamental sequence of events allows for the continuation of eukaryotic life during embryonic development and cellular regeneration throughout life. 

Mouse P19 embryonic carcinoma stem cells have been xenotransplanted into an emptied chorion (the transparent envelope of a fertilized zebrafish egg) [3]. The chorion is a useful biological vessel for developing stem cells, owing to its relatively large size of 1 mm in diameter and also to the presence of penetrating nanopores of dimensions... 

The vitelline envelops are removed with Dumont 5 watchmakers forceps. For cutting embryos, we use eyebrow hairs embedded in wax in the tips of pulled, disposable Pasteur pipets. Hairloops, which are used to position and hold embryonic tissues, are also embedded in pipets. Pipets are pulled to a diameter... 

In 1955, Ris [120], working with sectioned meiotic and mitotic chromosomes of rat, salamander, lily, and onion, was the first to observe that long fibers about 200 A in diameter were fundamental units of structure in both plant and animal chromosomes. During mitosis and meiosis, these 200-A fibers are apparently coiled around each other, forming large bundles. From his studies on honey bee embryonic cells, DuPraw [121] demonstrated that 230-A fundamental fibers can be found in vivo and that such fibers are firmly attached to the nuclear envelope at the edges of the annuli. 

Turner F.R. and Mahowald A.P. 1976. Scanning electron microscopy of Drosophila embryogenesis. 1. The structure of the egg envelopes and the formation of the cellular blastoderm. Dev. Biol. 50 95-108. Zhang C.X., Lee M.P., Chen A.D., Brown S.D., and Hsieh T. 1996. Isolation and characterization of a Drosophila gene essential for early embryonic development and formation of cortical cleavage furrows. /. Cell Biol. 134 923-934. 

---