Morphology of the developing down feathers of chick embryos
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Apico-basal gradients of differentiation and different cell types are recognizable at the ultrastructural level in the developing feather germ.
The hypothesis that keratin is synthesized de novo by ribosomes is probably correct, because the largest number of these organelles is present at the time when keratin formation is most prominent.
Intercellular gaps in the developing feather germs facilitate the reorientation and rearrangement of different cell types into definitive feather structures.
The sources of nutrition and energy for the completion of keratinization during later developmental stages of feather germs are the supportive and the barb medullary cells and large stores of glycogen.
Keratohyalin granules are not precursors of feather keratin, since no such structures were observed in feather germs.
Two distinct modes of keratinization occur in feather germs. Keratinization in sheath cells is similar to that which occurs in mammalian epidermal cells. Barb and barbule cell keratinization resembles that of hair.
The basal lamina is probably involved in transport of synthetic material from the pulp cavity to the epidermal cells. The lamina may also provide mechanically strong connections between the feather germ and the dermis.
It is suggested that desmosomal tonofilaments provide a framework which orients the synthesis of keratin. It is also suggested that the periderm granules provide mechanically weak areas in the sheath and facilitate the fragmentation of this structure.
Key wordsDifferentiation Down feathers Electron microscopy Keratinization Keratohyalin granules Periderm granules
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