| primary lymphoid tissue | <anatomy> Tissue that is particularly rich in lymphocytes (and accessory cells such as macrophages and reticular cells), particularly the lymph nodes, spleen, thymus, Peyer's patches, pharyngeal tonsils, adenoids and (in birds) the Bursa of Fabricius. Central lymphoid tissue: A term occasionally used as synonym for primary lymphoid tissue but should be avoided to prevent confusion between anatomical location, which is irrelevant and centrality in the system. Peripheral lymphoid tissue: Secondary lymphoid tissue, not necessarily located peripherally. Primary lymphoid tissue: Lymphoid tissues in which immune cells develop as opposed to the secondary or peripheral lymphoid tissues in which antigen independent or antigen dependent stages of maturation take place and in which responsive lymphocytes are found. Primary lymphoid tissues are foetal liver, adult bone marrow and thymus (and Bursa of Fabricius in birds). Secondary tissues are lymph nodes, spleen, tonsils and mucosa associated lymphoid tissue. (20 Mar 1998) |
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| primary lysosome | <cell biology> A lysosome before it has fused with a vesicle or vacuole. Origin: Gr. Soma = body (18 Nov 1997) |
| primary lysosomes | Lysosome's produced at the Golgi apparatus where hydrolytic enzymes are incorporated; they fuse with phagosomes or pinosomes to become secondary lysosome's. (05 Mar 2000) |
| primary macular atrophy of skin | Atrophoderma in which the skin becomes bag like and wrinkled. Synonym: atrophia maculosa varioliformis cutis, atrophoderma maculatum, macular atrophy, primary idiopathic macular atrophy, primary macular atrophy of skin. Origin: G. Anetos, relaxed, + derma, skin (05 Mar 2000) |
| primary medical care | Care of a patient by a member of the health care system who has initial contact with the patient. (05 Mar 2000) |
| primary megaureter | A peristaltic segment of distal ureter, that has no anatomical obstruction, but become dilated. It may be nonobstructive or related to congenital distal ureteral obstruction, UPJ obstruction or reflux. Occurs predominantly in males (80%), left more than right, 20% of cases are bilateral, and is really a diagnosis by exclusion. (21 Jun 2000) |
| primary meristem | <plant biology> The meristem at the tips of stems and roots. Composed of undifferentiated cells, many of which divide to add to the plant body but the central mass (the quiescent centre) remains inert and only becomes active if the meristem is damaged. Synonym: eumeristem. (18 Nov 1997) |
| primary mesoderm | Extraembryonic ectoderm cells or tissues which, though derived from the zygote, are not part of the embryo proper and along with the extraembryonic mesoderm form the foetal membranes (e.g., amnion). Synonym: primary mesoderm. (05 Mar 2000) |
| primary metabolism | Metabolic processes central to most cells; e.g., biosynthesis of macromolecules, energy production, turnover, etc. (05 Mar 2000) |
| primary metabolite | A metabolite excreted during the growth phase. (09 Oct 1997) |
| primary methemoglobinaemia | Methemoglobinaemia due to formation of any one of a group of abnormal a chain or b chain haemoglobins collectively known as haemoglobin M. Slate-gray cyanosis occurs in early infancy, without pulmonary or cardiac disease, and is resistant to ascorbic acid or methylene blue therapy; autosomal dominant inheritance, methemoglobinaemia due to deficiency of cytochrome b5 reductaseor methemoglobin reductase, the enzyme responsible for reduction of intraerythrocyte methemoglobin; cyanosis is improved by ascorbic acid or methylene blue; autosomal recessive inheritance, one case of methemoglobinaemia has been reported that apparently is due to a deficiency of cytochrome b5. Synonym: hereditary methemoglobinaemia, hereditary methemoglobinaemic cyanosis, primary methemoglobinaemia. (05 Mar 2000) |
| primary myeloid metaplasia | Myeloid metaplasia occurring as the primary condition, often in association with myelofibrosis. Synonym: agnogenic myeloid metaplasia. (05 Mar 2000) |
| primary narcissism | In psychoanalysis, the original psychic energy embodied or invested in the ego. (05 Mar 2000) |
| primary neurasthenia | A form of nervous exhaustion appearing in the adolescent period. Synonym: primary neurasthenia. (05 Mar 2000) |
| primary neuroendocrine carcinoma of the skin | A rare malignant cutaneous tumour seen in sun-exposed skin of elderly patients composed of dermal nodules of small round cells with scanty cytoplasm in a trabecular pattern; the tumour cells contain cytoplasmic dense core granules resembling neurosecretory granules seen in Merkel cells. Synonym: primary neuroendocrine carcinoma of the skin, trabecular carcinoma. (05 Mar 2000) |