A Morphological study of hepatic hemopoiesis in chicken embryos
Hemopoiesis in the liver of the chicken embryo begins on day 7 of incubation (Hamburger and Hamilton Stage 30) and peaks on day 14 (Stage 40). During this time frame, the differentiation of hepatic and hemopoietic cells was studied by light microscopy and by transmission and scanning electron microscopy. Morphometric analyses were performed on the light and electron micrographs. The avian liver is a closely packed array of dendriform cords and discontinuous sinusoids. Hepatocytes are pyramidal in shape, and they ring the bile canaliculi which run through the centers of the hepatic cords. Semithin sections, made possible by infiltration and embedding in glycol methacrylate, were stained with hematoxylin and eosin to reveal the general architecture of the liver, as well as the lipid content of the hepatocytes, and by the periodic acid-Schiff reaction and hematoxylin to visualize the cytoplasmic stores of glycogen. Differentiating hepatocytes were scored for the presence or absence of glycogen and lipid. Glycogen-containing cells fluctuate during early hemopoiesis, but the cell proportion progressively increases toward the hemopoietic peak. Most hepatocytes lack lipid droplets until Stages 39 and 40. From Stage 30 to Stage 35, hepatocyte volume falls to its lowest value. Subsequently (Stages 36 to 40), cell volume increases and hepatocytes achieve a relatively constant size. Ultrastructural changes in the differentiating hepatocytes, including alterations to the mitochondria, endoplasmic reticulum, and Golgi apparatus, were documented. This cell survey was extended to the sinusoidal linings (endothelial cells and Kupffer cells) and to the perisinusoidal cells between the cords and sinusoids. Glycol methacrylate sections were used in lieu of blood smears to study hemopoietic cells, thus overcoming such undesirable traits as cell shrinkage and/or rupture. Sections proved superior to smears for detailed examinations of nuclear and cytoplasmic morphologies and for precise localization of hemopoietic cells to intravascular and extravascular sites. The avian liver, it was found, is directly involved only in erythropoiesis and granulopoiesis. Erythropoietic cells were observed throughout the hemopoietic time frame, but blood islands with granulopoietic cells did not appear until Stage 35. Endothelial cells of the sinusoidal linings are important to erythropoiesis. Asymmetric cell division can release one daughter cell, a proerythroblast, into the circulation, while retaining the other daughter cell in the sinusoidal lining as an endothelial cell. Involvement of endothelial cells in no way discounts a contribution by erythropoietic stem cells. Granulopoiesis in the liver produces only eosinophilic leukocytes. Individual granulopoietic cells appear first in the connective tissue sheaths of hepatic blood vessels, and these cells later congregate into large blood islands. From a comparative standpoint, the elements deemed critical to hemopoiesis in the mammalian liver - the hepatic vasculature, the prehepatocyte population, and the compartments for stem cell differentiation - may not hold the same relevance in the bird. Owing to an inordinate reliance on intravascular hemopoiesis, the relative importance of a prehepatocyte population and individual stem cell compartments is diminished.
Bibliography: p. 135-141.
Wong, G. K. (1991). A Morphological study of hepatic hemopoiesis in chicken embryos (Master's thesis, University of Calgary, Calgary, Canada). Retrieved from https://prism.ucalgary.ca. doi:10.11575/PRISM/24166