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

走査電子顕微鏡下の鶏卵殻の構造と卵殻形成に関する知見補遺

Fujii Shunsaku

広島大学水畜産学部紀要

Journal of the Faculty of Fisheries and Animal Husbandry, Hiroshima University

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 three-dimensional structure of the hen's eggshell was studied by scanning electron microscopy. The morphological subjects of this study included the process of formation of the eggshell, the fine structure of the mammilla and air pore, and the morphological changes of the hatched eggshell.

The first step of shell formation began in the uterus of the oviduct with the formation of small organic concretions on the shell membrane. After these concretions had grown to some extent, they were encrusted with the deposition of calcium and became cone-shaped mammillae. They enlarged gradually upwards and in the lateral direction, while they were prevented from growing inwards, because the site of their presence had been mineralized early. The mammillae were enveloped by a cuticle-like covering. The enlargement of the mammillae proceeded by alternative formation of the covering and the subsequent mineralization of this covering. Finally, the mammillae were fused with one another to form a single layer in the mammillary stratum. The completed mammillae rested on the shell membrane, with their tips embedded in this stratum. They were connected firmly with the shell membrane by means of fibers derived from its outer layer. The mammillary core was connected loosely with the rest of the mammilla by a small amount of fibrils. The liberation of the shell membrane from the shell during the incubation period took place at the boundary between the core and the rest of the mammillae. It was considered to be induced due to the loose connection at this boundary.

After the completion of the mammillary layer, spaces varying in size and form were left behind in places where mammillae had come together. These inter-mammillary spaces seemed to play a certain role in the formation of a spongy layer. This laver was formed on the mammillary layer by the superimposition of new calcite layers. The process of calcification of the spongy layer consisted in two phases. The first step was the formation of a network of matrix fibers, and the second one the cementation of the meshes of the network with calcium.

The air pores of the eggshell showed themselves as mushroom-like projections in the resin cast. They arose from the large intermammillary spaces and opened in shallow grooves on the shell surface. In addition to true air pores, there existed a number of blind or aborted canals which had been closed on the way of the entire thickness of the shell. These canals arose from small intermammillary spaces. The formation mechanism of the air pore was discussed.

Zoology [ 480 ]

eng

広島大学水畜産学部

[ISSN] 0440-8756

[NCID] AN00213563

[NAID] 40018463840