Developmental Biology Terms Starting With T
Developmental Biology Glossary: T
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Totipotency
/ toh-TIP-oh-ten-see / · Latin totus, whole; potentia, power
Totipotency is the developmental capacity of a single cell to give rise to all cell types of a complete organism, including both the embryo proper and the extraembryonic tissues such as the placenta.
Totipotent cells possess the broadest possible developmental potential, exceeding that of pluripotent cells by retaining the ability to form extraembryonic lineages. In mammals, the zygote is totipotent at fertilization, and individual blastomeres isolated from the two-cell and four-cell stages can still develop into complete embryos, as demonstrated experimentally in sheep and mice. Totipotency is progressively restricted as cells divide and receive positional signals; the first major restriction occurs at the blastocyst stage, when cells commit to either the inner cell mass or the trophoblast lineage, with trophoblast cells forming the placenta and inner cell mass cells giving rise to the embryo.
Plant somatic cells retain remarkable totipotency compared to most animal cells: a single carrot (Daucus carota) mesophyll cell placed in culture with appropriate auxin and cytokinin concentrations can regenerate a complete plant.
In 2022, researchers identified a rare population of cells in mouse blastocysts, called totipotent-like blastomere cells, that expressed markers of the two-cell stage even days after that stage had passed, suggesting that totipotency is not lost abruptly but can persist in a small subset of cells well into early embryogenesis.
All stem cells can generate any cell type in the body. Only totipotent cells produce both the embryo and the placenta; pluripotent stem cells, including embryonic stem cells derived from the inner cell mass, cannot form trophoblast and therefore cannot support a complete pregnancy on their own.
In sheep (Ovis aries), embryologist Steen Willadsen demonstrated in 1986 that a single blastomere isolated from a two-cell embryo could develop into a complete lamb when transferred to an enucleated egg and then to a surrogate mother. Each of the two resulting lambs was genetically identical to the other, confirming that individual blastomeres at this stage retain full totipotency.
Trophoblast
/ TROH-foh-blast / · Greek trophe, nourishment; blastos, germ
Trophoblast is the outer cell layer of the mammalian blastocyst that attaches the embryo to the uterine wall and gives rise to the placenta and associated extraembryonic membranes.
The trophoblast differentiates by day 5 to 6 of human development into two distinct populations: cytotrophoblast cells, which remain mononucleate and supply precursor cells, and syncytiotrophoblast cells, which fuse into a continuous multinucleate layer that directly contacts maternal blood to mediate gas and nutrient exchange. Invasion of the uterine endometrium depends on trophoblast expression of matrix metalloproteinases MMP-2 and MMP-9, which remodel the spiral arteries and increase maternal blood flow to the developing placenta. Trophoblast cells also secrete human chorionic gonadotropin, the hormone detected by pregnancy tests, which sustains progesterone production from the corpus luteum during the first trimester.
Critically, trophoblast cells lack classical MHC class I proteins on their surface, preventing maternal immune cells from mounting a rejection response against the genetically foreign tissue.
In cases of complete hydatidiform mole, trophoblast cells proliferate without forming any embryonic tissue at all, producing a mass of placental-like villi that can fill the uterus and, in rare cases, transform into a malignant tumor called choriocarcinoma. This condition arises most often when an egg lacking a nucleus is fertilized by a sperm that then duplicates its own chromosomes.
How To Become A Neonatologist? →Every blastocyst cell becomes the embryo body. Trophoblast cells form the placenta and extraembryonic membranes rather than any part of the embryo itself, which develops exclusively from the inner cell mass.
In mouse embryos, trophoblast giant cells at the implantation site secrete proteases that digest the uterine decidua, allowing the embryo to embed by embryonic day 4.5. A single mouse trophoblast giant cell can reach 100 micrometers in diameter, roughly ten times the size of a typical somatic cell, as it undergoes repeated rounds of DNA replication without cell division.
Tubulogenesis
/ tyoo-byoo-loh-JEN-eh-sis / · Latin tubulus, small pipe; Greek genesis, origin
Tubulogenesis is the developmental process by which epithelial cells organize into hollow tubes through coordinated polarization, lumen formation, and branching, shaping organs such as the kidney nephron, lung airways, and vascular system.
Epithelial cells can open a lumen by two distinct mechanisms: cavitation, in which interior cells die by apoptosis to leave a hollow core, and hollowing, in which living central cells redirect their plasma membranes to create an internal space without cell death. Branching morphogenesis then extends primary tubes into secondary and tertiary branches through localized proliferation and directed invasion guided by fibroblast growth factor and VEGF signaling. In the developing mouse lung, branching generates roughly 17 million terminal airways through stereotyped bifurcations controlled by FGF10 gradients from the surrounding mesenchyme.
Tube diameter and length are regulated by cell-cell adhesion molecules including E-cadherin and by tension generated through actin-myosin contractility, so disrupting either system produces tubes of abnormal caliber.
In the fruit fly Drosophila melanogaster, the tracheal system forms entirely by tubulogenesis without any cell division after the initial invagination, meaning a fixed pool of about 80 cells per tracheal pit rearranges itself into a branched network spanning the entire larval body. Mutations in the Drosophila gene breathless, which encodes an FGF receptor, halt tracheal branching at the primary tube stage and cause larval suffocation.
Organs start as solid blocks and stay that way. Many organs form by hollowing, folding, branching, or wrapping tissues into tubes, and the specific mechanism of lumen formation differs between organ systems even within the same embryo.
During kidney development in the mouse, the ureteric bud undergoes roughly 11 rounds of branching tubulogenesis between embryonic days 10.5 and 14.5 to generate the collecting duct tree. Each branch tip induces the surrounding metanephric mesenchyme to form nephrons, so defects in tubulogenesis at this stage can reduce nephron number by more than 50 percent.
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