Canadian Medical Guide > Anatomy > Embryonic Structures > Embryo Terms and Definitions




Embryo

Medical Definition: The early developmental stage of MAMMALS. It generally defines the period from cleavage into two BLASTOMERES to the end of embryonic differentiation of basic structures. In the human, the embryo represents the first two months of intrauterine development preceding the stages of the FETUS.
Guide Notes: mammalian only; EMBRYO, NON-MAMMALIAN is also available; human: 0-2 mos; IM when embryo itself as an organic entity or a tissue; prefer /embryol with organs or organisms; relation to /embryol: Manual 21.54; for plants, PLANT EMBRYOS see SEEDS is available
Embryo Categories.
Canadian Medical Guide Definition Blastocyst - Early preimplantation mammalian EMBRYO in the post-MORULA stage. Its structure consists of a TROPHOBLASTS-enclosed fluid-filled cavity with an inner cell mass. After EMBRYO IMPLANTATION, the inner cell mass of the blastocyst becomes the embryonic disc of the developing embryo.
Canadian Medical Guide Definition Branchial Region - Region containing paired arched columns that bear gills in lower aquatic vertebrates and appear in the embryos of higher vertebrates in comparable form. In higher vertebrates the arches are subsequently modified into structures of the neck and ear.
Canadian Medical Guide Definition Cleavage Stage, Ovum - The embryo in its earliest stage, lasting from the first mitotic division of the fertilized ovum (ZYGOTE) into two BLASTOMERES to the formation of the MORULA, a compact mass of blastomeres.
Canadian Medical Guide Definition Fetal Membranes - Thin layers of tissue which surround the embryo or fetus and provide for its nutrition, respiration, excretion and protection; they are the yolk sac, allantois, amnion, and chorion.
Canadian Medical Guide Definition Gastrula - The embryo in the early stage following the blastula, characterized by morphogenetic cell movements, cell differentiation, and the formation of the three germ layers.
Canadian Medical Guide Definition Germ Layers - The three layers of cells comprising the early embryo.
Canadian Medical Guide Definition Limb Bud - A swelling on the trunk of the vertebrate embryo that becomes a limb. Limb bud cultures are used in developmental, organogenesis, morphogenesis, and cell differentiation studies. The limb bud of the chick embryo is most commonly used but mouse and rat limb buds are also used.
Canadian Medical Guide Definition Mesonephros - The excretory organ of the embryo, collective Wolffian tubules, which forms the urogenital fold from which the reproductive organs develop. The mesonephros is the permanent kidney in fish and amphibians, but atrophies in reptiles, birds, and mammals.
Canadian Medical Guide Definition Mullerian Ducts - Two ducts of the embryo which empty into the cloaca, and which in the female develop into vagina, uterus and oviducts; in the male they disappear except for the vestigial vagina masculina and the appendix testis.
Canadian Medical Guide Definition Neural Crest - A strip of specialized ectoderm flanking each side of the embryonal neural plate, which after the closure of the neural tube, forms a column of isolated cells along the dorsal aspect of the neural tube. Most of the cranial and all of the spinal sensory ganglion cells arise by differentiation of neural crest cells.
Canadian Medical Guide Definition Notochord - The rod-shaped body, composed of cells derived from the mesoblast and defining the primitive axis of the embryo. In lower vertebrates, it persists throughout life as the main axial support of the body, but in higher vertebrates it is replaced by the vertebral column.
Canadian Medical Guide Definition Organizers, Embryonic - The regions within an embryo that control development and differentiation. In AMPHIBIA, the organizer forms at the dorsal-most lip of the blastopore during gastrulation and is named after its discoverer, Hans Spemann.
Canadian Medical Guide Definition Umbilical Cord - The flexible structure, giving passage to the umbilical arteries and vein, which connects the embryo or fetus to the placenta.
Canadian Medical Guide Definition Urachus - The urinary canal of the fetus; postnatally it is usually a fibrous cord but occasionally persists as a vesicoumbilical fistula.
Canadian Medical Guide Definition Vitelline Duct - The narrow tube connecting the YOLK SAC with the midgut of the EMBRYO; persistence of all or part of it in post-fetal life produces abnormalities, of which the commonest is MECKEL DIVERTICULUM.
Canadian Medical Guide Definition Wolffian Duct - An excretory duct of the embryonic kidney.

Embryo Medical Definitions and Terms

CANADIAN MEDICAL GUIDE

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