Abstract
Recent findings strongly suggest that the molecular pathways involved in the development and function of blood cells are highly conserved among vertebrates and various invertebrate phyla. This has led to a renewed interest regarding homologies between blood cell types and their developmental origin among different animals. One way to address these areas of inquiry is to shed more light on the biology of blood cells in extant invertebrate taxa that have branched off the bilaterian tree in between insects and vertebrates. This review attempts, in a broadly comparative manner, to update the existing literature that deals with early blood cell development. I begin by providing a brief survey of the different types of blood cell lineages among metazoa. There is now good reason to believe that, in vertebrates and invertebrates alike, blood cell lineages diverge from a common type of progenitor cell, the hemocytoblast. I give a synopsis of the origin and determination of the hematocytoblast, beginning with a look at the hematopoietic organs that house hemocytoblasts in adult animals, followed by a more detailed overview of the embryonic development of the hematopoietic organ. Finally, I compare the process of blood lineage diversification in vertebrates and Drosophila.
Highlights
Moving cells with structural and functional properties of at least some of the blood cells characterized above for vertebrates can be found in all multicellular animals
In animals that have evolved a true body cavity along with a vascular system, these cells are commonly referred to as coelomocytes and/or hemocytes; in animals without a coelom, we speak of amebocytes, interstitial cells, or neoblasts
Moving cells are observed in the parenchyma of flatworms. These cells are known as neoblasts in the modern literature (Figure 1x,y) (Rieger et al 1991), early studies that attempted to trace the phylogenetic origin of blood cells applied terms such as lymphocyte and hemocytoblasts (Andrew 1965)
Summary
Moving cells with structural and functional properties of at least some of the blood cells characterized above for vertebrates can be found in all multicellular animals. In animals that have evolved a true body cavity (coleom) along with a vascular system, these cells are commonly referred to as coelomocytes and/or hemocytes; in animals without a coelom ( known as acoelomates or pseudocoelomates), we speak of amebocytes, interstitial cells, or neoblasts. Moving cells are observed in the parenchyma of flatworms These cells are known as neoblasts in the modern literature (Figure 1x,y) (Rieger et al 1991), early studies that attempted to trace the phylogenetic origin of blood cells applied terms such as lymphocyte and hemocytoblasts (Andrew 1965). The advent of molecular markers will help to clarify whether neoblasts in flatworms, or interstitial cells in coelenterates, are truly homologous to hemocytes in more derived animals
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