mesoderm

The middle of the three primary germ layers of the embryo (the others being ectoderm and endoderm); m. is the origin of connective tissues, myoblasts, blood, the cardiovascular and lymphatic systems, most of the urogenital system, and the lining of the pericardial, pleural, and peritoneal cavities. SYN: mesoblast. [meso- + G. derma, skin]

- branchial m. m. surrounding the primitive stomodeum and pharynx; it contributes to the pharyngeal arches.

- extraembryonic m. extraembryonic cells which, though derived from the zygote, are not part of the embryo proper and contribute to the fetal membranes ( e.g., amnion). SYN: primary m..

- gastral m. m. in lower vertebrates formed by constriction from the roof of the archenteron or yolk sac.

- intermediate m. a continuous band of m. between the segmented paraxial m. medially and the lateral plate m. laterally; from it develops the nephrogenic cord.

- intraembryonic m. m. derived from the primitive streak and lying between the ectoderm and endoderm. SYN: secondary m..

- lateral m. SYN: lateral plate m..

- lateral plate m. the peripheral portion of intraembryonic m. that is continuous with the extraembryonic m. beyond the margins of the embryonic disk; it forms the somatic and splanchnic m. between which develops the intraembryonic celom. SYN: lateral m..

- paraxial m. the m. lying at either side of the midline embryonic notochord; on segmentation, it forms the paired somites.

- primary m. SYN: extraembryonic m..

- prostomial m. m. that arises in lower vertebrates by continued proliferation at the lateral lips of the blastopore.

- secondary m. SYN: intraembryonic m..

- somatic m. the m. adjacent to the ectoderm in the early embryo, after formation of the intraembryonic celom; the limbs and body wall are derived, in part, from it.

- somitic m. m. derived from cells situated in or derived from somites.

- splanchnic m. the layer of lateral plate m. adjacent to the endoderm.

- visceral m. the splanchnic m. or the branchial m..

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me·so·derm 'mez-ə-.dərm, 'mēz-, 'mēs-, 'mes- n the middle of the three primary germ layers of an embryo that is the source esp. of bone, muscle, connective tissue, and dermis broadly tissue derived from this germ layer

me·so·der·mal .mez-ə-'dər-məl, .mēz-, .mēs-, .mes- also me·so·der·mic -'dər-mik adj

me·so·der·mal·ly -ē adv

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n.

the middle germ layer of the early embryo. It gives rise to cartilage, muscle, bone, blood, kidneys, gonads and their ducts, and connective tissue. It separates into two layers - an outer somatic and an inner splanchnic mesoderm, separated by a cavity (coelom) that becomes the body cavity. The dorsal somatic mesoderm becomes segmented into a number of somite. See also mesenchyme.

• mesodermal adj.

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meso·derm (mezґo-) (meґzo-dərm″) [meso- + -derm] the middle layer of the three primary germ layers of the embryo, lying between the ectoderm and the endoderm. From it are derived the connective tissue, bone and cartilage, muscle, blood and blood vessels, lymphatics and lymphoid organs, notochord, pleura, pericardium, peritoneum, kidney, and gonads. Cf. ectoderm and endoderm. mesodermal, mesodermic adj