| reticular substance | A filamentous plasmatic material, beaded with granules, demonstrable by means of vital staining in the immature red blood cells. Synonym: alpha substance, filar mass, filar substance, substantia reticularis, substantia reticulofilamentosa. Synonym: reticular formation. (05 Mar 2000) |
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| reticular tissue | Retiform tissue, a tissue in which the argyrophilic collagenous fibres form a network and that usually has a network of reticular cells associated with the fibres. (05 Mar 2000) |
| lateral reticular nucleus | A group of cells in the medulla oblongata, located between the inferior olive and the descending trigeminal nucleus, receiving fibres from the spinal cord and motor cortex and projecting to the cerebellum. Synonym: nucleus lateralis medullae oblongatae, lateral reticular nucleus. (05 Mar 2000) |
| primitive | Undeveloped or in early stages of development, undifferentiated. (16 Dec 1997) |
| primitive aorta | The paired aortic primordia in young embryos. (05 Mar 2000) |
| primitive chorion | The chorion before its villi are well formed. Synonym: previllous chorion. (05 Mar 2000) |
| primitive costal arches | Arch's formed in the thoracic region of the vertebral column in the embryo from the costal processes or costal elements which give rise to the ribs. (05 Mar 2000) |
| primitive erythroblast | <cell biology> Large cell with euchromatic nucleus found in mammalian embryos. In the mouse, the cells are located in the yolk sac and are responsible for early production of erythrocytes with foetal haemoglobin. (18 Nov 1997) |
| primitive furrow | The median depression in the primitive streak flanked by the primitive ridges. Synonym: primitive furrow. (05 Mar 2000) |
| primitive groove | The median depression in the primitive streak flanked by the primitive ridges. Synonym: primitive furrow. (05 Mar 2000) |
| primitive gut | A flat sheet of intraembryonic endoderm that will change into a tubular gut due to the folding of embryonic body-head, tail and lateral body folds. Synonym: archenteron, celenteron, endodermal canal, subgerminal cavity. (05 Mar 2000) |
| primitive knot | A local thickening of the blastoderm at the cephalic end of the primitive streak of the embryo. Synonym: Hensen's knot, Hensen's node, Hubrecht's protochordal knot, primitive knot, protochordal knot. (05 Mar 2000) |
| primitive meninx | The embryonic loose mesenchymatous tissue surrounding the brain and spinal cord; from it the three definite meninges (arachnoidea, dura mater, and pia mater) are derived. Synonym: meninx primitiva. Meninx serosa, obsolete term for arachnoidea; the arachnoid is actually not a serous membrane. (05 Mar 2000) |
| primitive neuroectodermal tumour | A designation used to refer to a group of morphologically similar embryonal neoplasms that arise in intracranial and peripheral sites of the nervous system and which may show various degrees of cellular differentiation; includes medulloblastoma, pineoblastoma, etc. (05 Mar 2000) |
| primitive neuroectodermal tumours | A type of brain tumour. Prenatal diagnosis: diagnosis before birth. Methods for prenatal diagnosis include ultrasound (of the uterus, placenta and developing foetus), chorionic villus sampling to obtain tissue for chromosome or biochemical analysis, amniocentesis to obtain amniotic fluid for the analysis of chromosmes, enzymes, DNA, etc. A growing number of birth defects and diseases are now amenable to prenatal diagnosis. Also called antenatal diagnosis. (12 Dec 1998) |