finally, the definitive cell line arises. Thrombo- 

 cyte stages of development in the embryo differ 

 morphologically also from those found in the de- 

 finitive stages. Therefore, to cover all these vari- 

 ations, definitions must be broad. 



ERYTHROCYTE SERIES 



Erythroblast. — A large cell with more cytoplasm in 

 relation to the nucleus than in most blast cells. The 

 cytoplasm of the primary generation is strongly baso- 

 philic, but in the definitive cell line it stains less in- 

 tensely. The cytosonie shows mitochondrial spaces 

 and may have an amoeboid shape. The nucleus is an 

 open, coarse network with chromatin that is clumped 

 more than usually found in other blast cells. The plas- 

 mosome nucleolus is large and more conspicuous than 

 in the thromboblast. 



Early polychromatic erythrocyte. — A smaller cell 

 than the blast stage, and the cytosome is rounded. 

 Mitochondrial spaces are largely replaced by a more 

 homogeneous cytojilasm. The cytosome has a strong 

 basophilic color. The chromatin of the nucleus is defi- 

 nitely clumped. The nucleolus is smaller than in the 

 blast stage but is often visible. 



M id-polychromatic erythrocyte. — This cell is usually 

 smaller than in the preceding stage, of rounded shape, 

 and has a gray cytoplasmic color that ranges from 

 nearly basophilic to slightly eosinophilic. No mito- 

 chondrial spaces are visible but the cytosome often 

 shows an artifact of fractured spaces usually concentric 

 to the cell perimeter. The nucleus is small relative to 

 the cytoplasm. It is round and has a clumped chro- 

 matin pattern. No nucleolus is seen in the definitive 

 cells. 



Late polychromatic erythrocyte. — This cell is round 

 to slightly oval. Staining of the cytoplasm varies from 

 an eosinophilic gray color to a pale eosin and. in the 

 older cells, to a moderate eosin color. Fracture arti- 

 facts tend to appear in tlie younger forms of this stage. 

 The luicleus is round to slightly oval and there is ir- 

 regular clumping of the chromatin. 



Reticulocyte. — Without special stain, this cell ap- 

 pears as a late polychromatic erythrocyte with almost 

 full development of hemoglobin or even as a mature 

 erythrocyte. A reticulocyte stain reveals the presence 

 of granules in the cytosonie. concentric to the nucleus. 

 The basophilic granules characteristic of a ty])ical re- 

 ticulocyte may be found at earlier stages of develop- 

 ment, after the appearance of hemoglobin in the cell. 



Mature erythrocyte. — This cell has an oval shape and 

 a cytosome of uniform texture, colored a reddish 

 orange. The nucleus is elongated, oval, and sometimes 

 rodlike with rounded ends. The nucleus is lepto- 

 chromatic to pachychroniatic. Aged cells have dense 

 homogeneous or nearlv structureless nuclei. 



THROMBOCYTE SERIES 



Thromboblast. — A cell with basophilic cytoplasm 

 that is amoeboid in the early embryonic generations. 

 The cytosome forms a narrow rim around the nucleus. 

 Spaces are present in the cytosome. It has a round 

 nucleus with a plasmosome nucleolus that may be 

 masked by the density of chromatin granules. The 

 chromatin tends to he punctate in contrast to that of 

 the erythroblast. where it is more angular, merging 

 into the linin network. 



Thrombocytes appear to be functional in the embryo 

 from an early stage and hence probably should not be 

 called immature; thus, in table 2 two columns of terms 

 are given for the thrombocytes — one for the embryo 

 and one for the bone marrow of the adult and circu- 

 lating blood. 



Embryo Thrombocytes 



Large embryo thrombocyte. — This is a large cell with 

 a moderate amount of cytoplasm around the nucleus. 

 The cytosome shows partial to nearlv complete vacu- 

 olization and stains more lightly than in the preceding 

 stage. The nucleus has large irregular chromatin 

 cluni|)s and no visible nucleolus. 



Medium embryo thrombocyte. — A cell of medium 

 size with about the same nucleocell ratio as before. 

 The cytosome is fully vacuolated and S|)ecific granules 

 sometimes are present. Early degeneration is marked 

 by pinkish coloration and by crumpling of the borders 

 of the c)tosome. The nucleus is pachychroniatic. 



Small embryo thrombocyte. — A small cell of which 

 the cytosonie is highly vacuolated and pale staining. 

 The nucleus is dense and individual chromatin clumps 

 are fairly distinct. Nearly all cells of this type, as 

 seen in the smear of embryo blood, have lost most of 

 the cytosome and are clumjied. 



Definitive Thrombocytes 



Early immature thrombocyte. — A large cell, often 

 not round, and with the nucleocell ratio less than in 

 the preceding stage. The cytoplasm is basophilic but 

 the overall color is lightened by the presence of vacu- 

 oles. The nucleolus may still be visible and the 

 chromatin is aggregated into clumps of nonuniform 

 size. 



Mid-immature thrombocyte. — A cell of medium size 

 that is often elongated slightly or has an irregular 

 shape. The cytoplasm is vacuolated and stains a light 

 blue color. Specific granules may be present. The 

 chromatin is clumjied extensively but linin network 

 is still visible. 



Late immature thrombocyte. — This cell is elongated 

 but the nucleus fills up a larger proportion of it than 

 at the mature stage. Clumps of chromatin are still 

 clearly visible. Specific cytoplasmic granules are gen- 

 erally present. 



11 



