Eggshell Structure Research Articles

The tylenchid (Nematoda) egg shell: structure, composition and permeability

The fine structure of egg shells of four different genera belonging to the order Tylenchida has been examined. The species examined were Meloido-gyne javanica, Rotylenchulus reniformis, Tylenchulus semipenetrans and Pratylenchus minyus. They are all similar in their basic structure, being composed of vitelline membrane, chitin and lipid layers, but there is considerable variability in the thickness of these layers.We have retained the conventional nomenclature because of its convenience, but it is clear that these layers have a variety of chemical components. However, they do appear to contain the compounds from which they take their name. Thus chitin occurs in the chitin layer, and lipid in the lipid layer. The latter is removed by the technique used in isolating the shell from the egg. Chemical analysis of the hydrolysis products of these shells has revealed a high (35 %) proline content which appears to be a characteristic of those nematode egg shells which have been examined so far. These analyses and treatment with enzymes indicate that the chitin layer is a chitin-protein complex.Experiments on the permeability of eggs of M. javanica at different temperatures indicate that changes in permeability are not due to the melting of a single lipid with a distinct melting point as had been thought in the past. We have found that Arrhenius activation energies calculated from the two slopes of an Arrhenius plot were 17·8 kcal/mol and 43·0 kcal/mol respectively, the transition from one to the other taking place at 62°C. We think that these changes are due to changes in the properties of lipoprotein membranes in the lipid layer. These membranes appear to be of paramount importance in controlling the permeability of the nematode egg shell.

Further Morphological Studies on the Formation and Structure of Hen's Eggshell by Scanning Electron Microscopy

前報に続いて鶏卵殼の構造と卵殼形成の形態的観察を,走査電子顕微鏡を用いて行なった. 鶏卵殼は周知のように,内側の乳頭層(mammillary layer)と外側の海綿層(spongy layer)からなっている.乳頭層形成の最初は,有機質性の微小な凝塊が外卵殻膜上に播種状に沈着することによって始まった.この凝塊は,乳頭層形成の中心であるmammillary coreとなり,上方と側方に石灰沈着をともなってドーム状に発達し,最後にはお互いにゆ合して一層の乳頭層となった.乳頭は下方の卵殻膜側には生長しなかった. 完成した乳頭は,尖端のmammillary coreをわずかに外卵殼膜内に埋没していた.各乳頭は形と大きさがかなり異なり,1乳頭に1mammillary coreを有する典形的乳頭は円錐形を呈していた.大きな乳頭は,2～3のゆ合したmammmary coreを持っており,一般に臼歯形であった.このように乳頭はほぼ円錐形を呈し,大小不同であるために,各乳頭がゆ合して乳頭層を作くる際には,接触部に種々の形と広さの乳頭間隙が残る.この乳頭間隙から後に気孔が生じた. 乳頭はその周りを予備石灰沈着層ともいえる微細線維を含む有機質性の層で包まれていた.この層の形成とそれの石灰化が継続的に進行することによって,乳頭の生長発達が行われるものと考えられた.なお乳頭が卵殻膜側にのびないのは,この部が早く石灰化して予備石灰沈着層を持たないからのようであった. 乳頭は尖端のmammillary core部に,外卵殼膜線維が進入することによって,卵殻膜と強固に結ばれていた.一方,mammillary coreと乳頭固有部とは,微細な基質線維によって緩く結合していた.ふ卵中に卵殼膜と卵殻が分離するのは,このcoreと乳頭固有部の境界部であって,緩い結合様式のためと考えられた. 海綿層は乳頭部の完成後に引続いて形成された.海綿層の形成過程は大きく2段階に区別された.先ず基質線維からなる薄層が,乳頭層外表面の不平坦部を埋めるように積み重ねられた.この新生層の形成には,乳頭間隙を通って出る卵の内部の物質が関係しているように見えた.新生層は卵殻が一定の厚さに達すると,生長を止めた,その後はもっぱら,基質線維間隙に石灰沈着が進んだ. 卵殻の鋳型標本では,気孔の立体的構造がよく観察された.真の気孔は,前記の比較的広い乳頭間隙からおこり,狭い間隙からおこるものは,途中で閉鎖されて盲管となる傾向があった.気孔の形成機構について論議した.

The Structure of the Insect Egg-Shell in Relation to the Respiration of the Embryo

ABSTRACT Many insect eggs are able to avoid mechanical injury and desiccation because they are protected by an egg-shell or chorion which is tough, rigid and relatively waterproof. These properties tend to make it impermeable to gases, and since the egg contains a living embryo, there must be provision for the diffusion of an adequate volume of oxygen into the egg. The way in which this is achieved has never been satisfactorily explained. Specialized respiratory structures have been described in many species of insect egg, especially in those of parasites (Maple, 1937), and a respiratory function has sometimes been ascribed to the system of micropylar canals that penetrate the shell in hard-shell eggs, but until now there has been no direct evidence for the respiratory function of these structures. Indeed it seems unlikely, at first sight, that sufficient oxygen could diffuse through such small areas of the shell to satisfy the needs of the embryo, although in some species there can be little doubt that oxygen uptake is restricted to a small part of the shell; in certain species of capsid, for instance, the eggs are embedded in the tissues of a plant with only the caps exposed to the air.