| ultimobranchial pouch | A transient fifth pharyngeal pouch; it is now considered to be incorporated into the caudal pharyngeal complex, the cells of which become the parafollicular cells (C cells) of the thyroid. Uterovesical pouch, a pocket formed by the deflection of the peritoneum from the bladder to the uterus in the female. Synonym: excavatio vesicouterina, cavum vesicouterinum, vesicouterine pouch. (05 Mar 2000) |
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| ultimum moriens | The right atrium of the heart, said to contract after the rest of the heart is still. Origin: L. The last thing dying (05 Mar 2000) |
| ultisols | <ecology> Highly weathered soils having significantly more clay in the B-horizon than in the A-horizon and having low base status, acidic soils common in the Southwest. (09 Oct 1997) |
| UlTma DNA polymerase | <enzyme> A thermostable DNA polymerase with putative proofreading activity Registry number: EC 2.7.7.- (26 Jun 1999) |
| ultra- | Excess, exaggeration, beyond. Origin: L. Beyond (05 Mar 2000) |
| ultrabrachycephalic | Denoting an extremely short skull, one with an index of at least 90. (05 Mar 2000) |
| ultracentrifugation | <procedure> Centrifugation at very high g forces: used to separate molecules for example mitochondrial from nuclear DNA on a caesium chloride gradient. (18 Nov 1997) |
| ultracentrifuge | A high-speed centrifuge (up to 100,000 rpm) by means of which large molecules, e.g., of protein or nucleic acids, are caused to sediment at practicable rates; used for determinations of molecular weights, separation of large molecules, criteria of homogeneity of large molecules, conformational studies, etc. (05 Mar 2000) |
| ultracytostome | Former name for micropore. Origin: ultra-+ G. Kytos, cell, + stoma, mouth (05 Mar 2000) |
| ultradian | Relating to biologic variations or rhythms occurring in cycles more frequent than every 24 hours. Compare: circadian, infradian. Origin: ultra-+ L. Dies, day (05 Mar 2000) |
| ultradian rhythm | See: ultradian. (05 Mar 2000) |
| ultradolichocephalic | Denoting a very long skull, one with a cephalic index of less than 65. (05 Mar 2000) |
| ultrafilter | A semipermeable membrane (collodion, fish bladder, or filter paper impregnated with gels) used as a filter to separate colloids and large molecules from water and small molecules, which pass through. (05 Mar 2000) |
| ultrafiltration | <procedure> Filtration under pressure. In the kidney, an ultrafiltrate is formed from plasma because the blood is at higher pressure than the lumen of the glomerulus. Also used experimentally to fractionate and concentrate solutions in the laboratory using selectively permeable artificial membranes. (18 Nov 1997) |
| ultrafiltration coefficient | The filtration coefficient of a semipermeable membrane. (05 Mar 2000) |