TRANSPORT FUNCTIONS OF CHICK CHORIO-ALLANTOIC MEMBRANE .I. NORMAL HISTOLOGY AND EVIDENCE FOR ACTIVE ELECTROLYTE TRANSPORT FROM ALLANTOIC FLUID, IN VIVO

A systematic study of blood and allantoic fluid electrolyte concentrations and chorio-allantoic membrane histology in chick embryos indicates that the chorio-allantois actively transports Na+ from allantoic fluid to plasma. From 8 to 19 days of incubation serum electrolyte values remained essentially constant and within the normal range for newly hatched chicks. In the allantoic fluid there was approximately at four-fold increase in Ca2+, K+ and PO4-3 concentrations coincident with water removal from the sac, while allantoic fluid Na+ and Cl- concentrations, initially 90 and 60 mEq/l respectively, decreased dramatically to levels one-third to one-fifth those of serum for the same stage. Allantoic fluid osmolality, originally 185 mOsm/l, remained essentially constant. In vivo trans-membrane potentials were 3-5 mV at 6-7 days and increased progressively to about 40 mV at day 17, with a sharp decrease thereafter. The outer surface of the membrane was always electrically positive relative to the allantoic cavity. Histological complexity of both the allantoic endoderm and the chorionic ectoderm increased during normal incubation. At 7 days of incubation, both components of the membrane were single layers of flattened cells. Subsequent differentiation produced characteristic cell layers, distinctly different in histological appearance, which related in time to physiological changes in the embryos. It is suggested that the endodermal cells have a metabolically active function in the reabsorption of allantoic fluid, while the ectodermal cells are actively engaged in the transport of calcium. © 1969.