Ultrastructural aspects of early development of the fore-limb buds in the chick and the mouse

Abstract

Light microscope investigations of the early development of the fore-limb buds in chick and mouse were made to guide electron microscope studies with these tissues. At the time of maximal development of the ectodermal apical ridge there is a higher concentration of cytoplasmic RNA in the apical ridge cells than in the other cells of the limb bud. Ultrastructural investigations showed that, in the mesoblast cells at the earliest stages, profiles of endoplasmic reticulum are often found attached to the outer nuclear membrane. Some what later, discontinuities of nuclear envelope occur by which the content of the nucleus may communicate with the endoplasmic reticulum. In the cytoplasm of the mesoblast cells at these stages there were many granules similar in form and size to secretory granules of gland cells. Ribosomes are in the polysomal condition. At stages later than 20 in chick and in 11-day-old mouse embryos, the mesoblast shows the character of a syncytial tissue. Epiblast cells possess all the characters of an epithelium with well developed junctional complexes. The desmosomes form a chain consisting of units equipped with individual dense plaques, but connected by continuous bundles of fibres running parallel to the chain. The free cell membrane of the epiblastic cells, particularly at early stages, forms numerous microvilli and single cilia. In later stages during the form action of the ectodermal apical ridge, cilia have been found between the cells. This fact indicates that when the apical ridge is formed ectodermal cells migrate towards the margin of the limb bud. At these stages microvilli are also found between the apical ridge cells where they contribute to the cell-to-cell adhesion. Beginning at stage 22 in chick embryos and from the 12th day in mouse embryos there are in cells of the apical ridge numerous and extensive Golgi systems spread throughout the cytoplasm. Some what later there appear successively lysosomes, cytolysomes and extranuclear necrotic centres. All these organelles manifest acid phosphatase activity and are thoughtto initiate the involutive process in the apical ridge. Pycnosis and karyorrhexis appear as the last stage of cellular degeneration. Degenerating cells undergo phagocytosis by neighbouring epithelial cells. A basement membrane is present at all stages of development of the chick and mouse limb buds. It is an acellular continuous structure lining the internal (basal) surface of the epiblast, but in chick embryos it shows discontinuities immediately under the apical ectodermal ridge at the time of its maximum development.