Histochemistry of the developing chick esophagus and trachea. I. Alkaline phosphatase

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Abstract
Tissues of White Leghorn embryos of stages 17–45, and chicks of one day, two days, and three weeks of age were frozen or fixed in 85% ethanol. Alkaline phosphatase was identified using the azo‐dye or Gomori techniques.Alkaline phosphatase activity is found in the ventral mesenchyme of the esophagus surrounding the epithelium to stage 34. At stage 38 activity appears in the basal layers of the epithelium and is subsequently lost in the mesenchymal tissues. This activity persists in the epithelium throughout development and following hatching. At 16 days the mucous glands arise as solid buds of epithelium projecting into the tunica propria. These buds are highly positive for alkaline phosphatase. As differentiation of the glands becomes complete they and their ducts lose their positive phosphatase reaction.On the fourth day of development the trachea is found lying ventral to the esophagus. Mesenchyme has condensed around the tracheal epithelium by the fifth day, at stage 37 (11 days) the first cartilaginous rings appear, and by stage 38 (12 days) all are completely formed. During the third week of development the trachea increases in size. Pseudo‐stratified columnar ciliated epithelium begins to appear at 15 days. Alkaline phosphatase is highly reactive in the mesenchyme surrounding the epithelium prior to cartilage formation. After the cartilages have formed the reaction diminishes and in late stages of development phosphatase is found only in the epithelium.