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Note on differentiation of the epithelium of the small intestine in human embryos
We studied the development of the human embryonic intestine from the 6th to the 8th week of gestation (embryos with a crown-rump length of 16-25 mm). Semithin sections show that a smooth, simple intestinal lumen, which is often a mere slit, is elsewhere, in the same specimen, distinctly formed and in places is partly occluded by accumulations of cells of a somewhat different character from those of the actual intestinal lining. Whereas the epithelium proper is mainly high and pseudostratified, the cells which secondarily occlude the originally open intestinal lumen are smaller and are more cuboidal to polyhedric in shape. In the electron microscope, the cells of the primitive epithelium appear narrow and columnar, with an ovoid nucleus and a few nucleoli. The cytoplasm contains numerous mitochondria and a gradually developing granular endoplasmic reticulum and Golgi complex. These cells have small, irregular microvilli on their surface, but some have an almost straight surface without any microvilli. They display striking pinocytotic activity and contain a quantity of multivesicular bodies. Their cytoplasm further contains isolated osmiophilic granules and lysosomes and a small amount of glycogen. The accumulation of glycogen is typical of the more mature developmental stages. Cilia are a characteristic finding in embryos with a c-r length of 16-25 mm. There is only one cilium to a cell, but not all the cells have cilia. The cilia are relatively thick and not very long and they almost always grow in the centre of the cell surface. The apical surface of such cells is usually slightly or more deeply concave. Structurally, the ciliated cells closely resemble phylogenetically primitive entodermal collar cells (choanocytes). They are apparently a phylogenetically old type of cells, whose existence could have functional value, which appears for a time during ontogenetic development. The cilia may temporarily play a role in the movement of the intestinal fluid and thus coparticipate in resorption before the musculature of the intestinal wall has been formed.
We studied the development of the human embryonic intestine from the 6th to the 8th week of gestation (embryos with a crown-rump length of 16-25 mm). Semithin sections show that a smooth, simple intestinal lumen, which is often a mere slit, is elsewhere, in the same specimen, distinctly formed and in places is partly occluded by accumulations of cells of a somewhat different character from those of the actual intestinal lining. Whereas the epithelium proper is mainly high and pseudostratified, the cells which secondarily occlude the originally open intestinal lumen are smaller and are more cuboidal to polyhedric in shape. In the electron microscope, the cells of the primitive epithelium appear narrow and columnar, with an ovoid nucleus and a few nucleoli. The cytoplasm contains numerous mitochondria and a gradually developing granular endoplasmic reticulum and Golgi complex. These cells have small, irregular microvilli on their surface, but some have an almost straight surface without any microvilli. They display striking pinocytotic activity and contain a quantity of multivesicular bodies. Their cytoplasm further contains isolated osmiophilic granules and lysosomes and a small amount of glycogen. The accumulation of glycogen is typical of the more mature developmental stages. Cilia are a characteristic finding in embryos with a c-r length of 16-25 mm. There is only one cilium to a cell, but not all the cells have cilia. The cilia are relatively thick and not very long and they almost always grow in the centre of the cell surface. The apical surface of such cells is usually slightly or more deeply concave. Structurally, the ciliated cells closely resemble phylogenetically primitive entodermal collar cells (choanocytes). They are apparently a phylogenetically old type of cells, whose existence could have functional value, which appears for a time during ontogenetic development. The cilia may temporarily play a role in the movement of the intestinal fluid and thus coparticipate in resorption before the musculature of the intestinal wall has been formed.