Structure of the glandular layer and koilin membrane in the gizzard of the adult domestic fowl (Gallus gallus domesticus).

The koilin membrane is formed by the secretions of gland, crypt and surface epithelial cells. Glands form a continuous layer and are arranged in groups of 10-20. They are straight tubes about 500 microns long and 15 microns in diameter and produce rodlets of hard koilin. Hard koilin rodlets (5 microns diameter) form clusters of five or six as they pass through the crypts and enter the koilin membrane. Each rodlet hardens within its gland and maintains its individuality throughout its entire length. Rodlet clusters have previously been called 'rods'. Most of the softer koilin, which fills the spaces between the rodlet clusters, is produced by the surface epithelial cells. These cells form gentle arches between the cavities of adjacent crypts. Horizontal branches between rodlet clusters ('rods') do not exist. There is approximately twice as much surface koilin as rodlet koilin within the membrane. Abrasion of the koilin membrane is not uniform but occurs in a patchy fashion.

Intercalated discs, nexuses, sarcoplasmic reticulum and transitional cells in the heart of the adult domestic fowl (Gallus gallus domesticus).

Intercalated discs usually consisted of three or four transverse steps of folded cell membrane connected by straight parts running longitudinally. The transverse folded parts contained a variable number of interlocking processes from contiguous cells. Large processes were frequently flanked by smaller ones. Nexuses appeared to be better developed on the straight parts of intercalated discs than on the folded parts. They varied in size, shape and particle-packing density. The largest nexus observed was almost perfectly circular. It covered an area of 0.087 micron2, contained 530 particles and had a particle-packing density of 6078 particles/micron2. The sarcoplasmic reticulum formed a uniform mesh of fine tubes which was distributed in sheets between bundles of myofilaments and longitudinally aligned mitochondria. Neither extended junctional sarcoplasmic reticulum nor fenestrations in the sarcoplasmic reticulum were observed. Peripheral couplings were numerous and widespread. They were not restricted to the sarcolemma in the regions of the Z-line, H- or I-bands. Transitional cells combine the characteristics of contractile and conducting cells in varying proportions. There is some speculation about the mechanism of nexus formation and the interconvertibility of contractile and conducting cells via the intermediary of the transitional cell.