Alibardi L, Sawyer R H
Dipartimento di Biologia evoluzionistica sperimentale, University of Bologna, Italy.
J Anat. 2006 May;208(5):621-42. doi: 10.1111/j.1469-7580.2006.00580.x.
The present ultrastructural and immunocytochemical study on the embryonic feathers of the zebrafinch, an altricial passerine bird, describes cellular differentiation of developing downfeathers. Barb ridges are folds of the original epidermis of the embryonic feather germ in which the basal-apical polarity of epidermal cells is upset. The result is the loss of most germinal activity of basal cells of the barb ridges so that only the embryonic epidermal layers remain. The more external layer is the primary periderm, followed by 4-6 layers of inner-periderm cells that mature into feather sheath and barb vane ridge cells. The following layer, the subperiderm, produces a small type of beta-keratin typical of feathers. In barb ridges, the subperiderm layer is displaced to form barbule plates and barb cells. The formation of branching barbules occurs by the presence of barb vane ridge cells that function as spacers between barbule cells. The fourth layer is homologous to the germinal layer of the epidermis, but in barb ridges it rapidly loses the germinal capability and becomes the cyclindrical layer of marginal plates. The study indicates that a necrotic process determines the carving out of the final feather shape, although apoptosis may also play a role. In fact, after barb and barbule cells have formed a keratinized syncitium, retraction of the vascular bed determines anoxia with the resultant necrosis of all feather cells. Only those of the keratinized syncitium remain to form the feather while supportive cells disappear. The sheath covering the barb and barbule syncitium is lost by the formation of a sloughing layer following degeneration of external barb ridge vane cells and loss of the sheath. It is proposed that the evolution of the morphogenetic process of barb ridge formation was peculiar to tubular outgrowths of the integument of archosaurian reptiles that evolved into birds. Once established in the embryonic programmes of skin morphogenesis of ancient birds, variations in the process of barb ridge morphogenesis allowed the fusion of ridges into large or branched ridges that originated the rachis. This process produced pennaceous feathers, among which were those later used for flight. The present study stresses that the morphogenetic process of barb ridge formation determines the concomitant appearance of barbs and barbules. As a consequence, intermediate forms of evolving feathers with only barbs but not barbules are unlikely or are derived from alteration of the above basic morphogenetic mechanism.
本项对斑胸草雀（一种晚成鸟雀形目鸟类）胚胎羽毛的超微结构和免疫细胞化学研究，描述了发育中的绒羽的细胞分化。羽枝嵴是胚胎羽毛原基原始表皮的褶皱，其中表皮细胞的基顶极性被打乱。结果是羽枝嵴基底细胞的大多数生发活性丧失，以至于仅保留胚胎表皮层。最外层是初级周皮，接着是4 - 6层内周皮细胞，它们会成熟为羽鞘和羽枝叶片嵴细胞。接下来的一层，即周皮下层，产生一种羽毛特有的小型β - 角蛋白。在羽枝嵴中，周皮下层移位形成羽小枝板和羽枝细胞。分支羽小枝的形成是由于羽枝叶片嵴细胞的存在，它们在羽小枝细胞之间起到间隔作用。第四层与表皮的生发层同源，但在羽枝嵴中它很快失去生发能力并变成边缘板的圆柱形层。该研究表明，坏死过程决定了最终羽毛形状的雕琢，尽管细胞凋亡可能也起作用。事实上，在羽枝和羽小枝细胞形成角质化多核体之后，血管床的退缩导致缺氧，进而所有羽毛细胞坏死。只有角质化多核体的细胞留存下来形成羽毛，而支持细胞消失。覆盖羽枝和羽小枝多核体的鞘，由于外部羽枝嵴叶片细胞退化和鞘的丧失后形成的脱落层而消失。有人提出，羽枝嵴形成的形态发生过程的演化，是主龙类爬行动物体表管状突出物所特有的，这些突出物后来进化成了鸟类。一旦在古代鸟类皮肤形态发生的胚胎程序中确立，羽枝嵴形态发生过程中的变化使得嵴融合成大的或分支的嵴，从而形成羽轴。这个过程产生了正羽，其中就有后来用于飞行的羽毛。本研究强调，羽枝嵴形成的形态发生过程决定了羽枝和羽小枝的同时出现。因此，仅有羽枝而没有羽小枝的进化中羽毛的中间形式不太可能存在，或者是上述基本形态发生机制改变的结果。
