| 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) |
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| 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) |
| primitive node | 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 palate | In the early embryo, the mesoderm-filled shelf, formed from the medial nasal process, that anteriorly separates the oral cavity below from the primitive nasal cavities above. Synonym: primitive palate. (05 Mar 2000) |
| primitive perivisceral cavity | The space between the ectoderm and endoderm in the gastrula. Synonym: primitive perivisceral cavity. (05 Mar 2000) |
| primitive pit | The depression in the primitive node that serves to connect the notochordal canal with the surface ectoderm and the yolk sac. These connections are referred to as the neurenteric canal. (05 Mar 2000) |
| primitive reticular cell | A cell with processes making contact with those of other similar cell's to form a cellular network; along with the network of reticular fibres, the reticular cell's form the stroma of bone marrow and lymphatic tissues. (05 Mar 2000) |
| primitive ridge | One of the paired ridge's on either side of the primitive groove. (05 Mar 2000) |
| primitive streak | An ectodermal ridge in the midline at the caudal end of the embryonic disk from which arises the intraembryonic mesoderm; achieved by inward and then lateral migration of cells; in human embryos, it appears on day 15 and gives a cephalocaudal axis to the developing embryo. Synonym: germinal streak. (05 Mar 2000) |
| neuroectodermal tumour, primitive | A malignant brain tumour sharing common features and biologic properties with medulloblastoma. Some pathologists and clinicians use the terms interchangeably: they both can disseminate throughout the nervous system and, in some cases, systemically. most lesions arise from the posterior fossa in children under five years of age. The tumour often produces hydrocephalus and symptoms of increased intracranial pressure. Papilledema is often present. (12 Dec 1998) |