Developmental Biology Terms Starting With M
Developmental Biology Glossary: M
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Mesoderm
/ MEZ-oh-derm / · Greek mesos, middle; derma, skin
Mesoderm is the middle of the three primary germ layers formed during gastrulation, giving rise to muscle, bone, blood, the circulatory system, kidneys, and connective tissues throughout the body.
The mesoderm arises during gastrulation when epiblast cells ingress through the primitive streak and migrate between the ectoderm and endoderm, a process that begins around week 3 in humans. Ingressing cells express the transcription factors Brachyury and Eomesodermin, which drive mesodermal identity, and then diverge into paraxial mesoderm flanking the notochord, lateral plate mesoderm forming the body wall and limb skeleton, and intermediate mesoderm generating the kidneys and urogenital ducts. Paraxial mesoderm segments into somites beginning at approximately day 20 in humans, with new somites added at a rate of roughly one pair every 6 hours; each somite then subdivides into dermomyotome, which activates MyoD to produce skeletal muscle, and sclerotome, which activates Runx2 to produce vertebrae and ribs.
Lateral plate mesoderm splits into somatic and splanchnic layers that together generate the heart, blood vessels, and blood cells through specification of cardiac progenitors by Mesp1 and Nkx2.5.
Mesodermal cells from the primitive streak of a quail embryo transplanted into a chick embryo at the equivalent stage migrate to the correct positions and differentiate into host-appropriate structures, yet retain quail-specific nuclear markers. This quail-chick chimera technique, developed by Nicole Le Douarin in the 1960s and 1970s, allowed researchers to trace the precise destinations of individual mesodermal populations for the first time.
Mesoderm only forms skeletal muscle. Mesoderm also generates cardiac muscle, smooth muscle, all bones, the entire blood and vascular system, kidneys, the adrenal cortex, and the connective tissue stroma of most organs.
In mouse embryos, Wnt3 signaling from the primitive streak maintains Tbx6 expression in paraxial mesoderm, and embryos homozygous for a Tbx6 null mutation convert prospective somite tissue into ectopic neural tubes. Mutant embryos form up to eight extra neural tube-like structures flanking the normal neural tube, each roughly the same diameter as the endogenous tube, demonstrating how a single transcription factor switch redirects an entire mesodermal population toward a neural fate.
Morphogen
/ MOR-foh-jen / · Greek morphe, form; gennan, to produce
Morphogen is a signaling substance that forms a concentration gradient across a tissue, with cells adopting different fates depending on the local concentration they detect.
Morphogens such as Sonic hedgehog, Bicoid, and bone morphogenetic proteins spread across developing tissues and establish positional information through graded concentration. Cells express different target genes depending on how much morphogen they receive, with high concentrations activating one set of genes and low concentrations activating another. Threshold concentrations trigger specific developmental switches that determine cell fate and tissue identity.
This gradient-reading mechanism lets cells determine their relative position in the embryo without requiring direct cell-to-cell contact at every step.
Bicoid protein in fruit fly (Drosophila melanogaster) embryos forms a gradient that halves in concentration roughly every 100 micrometers, and cells can detect concentration differences of as little as 10 to 20 percent between neighboring positions.
A morphogen gives the same instruction to every cell it reaches. Morphogens give different instructions depending on dose, so the same molecule can specify three or more distinct cell fates across a single tissue.
Sonic hedgehog forms a gradient in the vertebrate limb bud from the zone of polarizing activity, with high concentrations specifying the posterior digits and low concentrations specifying the anterior ones. At concentrations above roughly 10 nanomolar, SHH activates the transcription factor Gli3 activator form and drives expression of posterior digit genes including Hoxd13; cells receiving below 1 nanomolar SHH instead allow the Gli3 repressor form to dominate, specifying anterior identities. The gradient is established partly by diffusion and partly by transcytosis, with cells internalizing SHH and re-secreting it further from the source, spanning the roughly 300-micrometer width of the chick limb bud in under 12 hours.
Morphogenesis
/ mor-foh-JEN-eh-sis / · Greek morphe, form; genesis, origin
Morphogenesis is the set of developmental processes by which cells, tissues, and organs acquire their characteristic three-dimensional shapes.
Morphogenesis combines cell proliferation, apoptosis, migration, cytoskeletal-driven shape changes, and shifts in cell adhesion to sculpt tissues from relatively uniform sheets of cells. Mechanical forces generated by cells pulling on each other and on the extracellular matrix contribute to tissue folding and shape formation. Signaling molecules such as fibroblast growth factors and Wnt proteins coordinate these behaviors across developing tissues.
Body shape emerges from repeated interactions between cells and their mechanical environment rather than from any single genetic blueprint.
The fruit fly (Drosophila melanogaster) dorsal closure, in which two epithelial sheets zip together over roughly two hours, has been measured with enough precision to show that each leading-edge cell generates approximately 1 to 2 nanonewtons of contractile force, making it one of the best-quantified morphogenetic events in any animal.
Genes directly encode body shape the way a blueprint encodes a building plan. Genes encode proteins that guide cell behavior, and shape emerges from collective cell movements, adhesion changes, and mechanical feedback rather than from a pre-drawn structural plan.
Genetic Determinism →During gastrulation in the African clawed frog (Xenopus laevis), bottle cells at the blastopore lip constrict their apical surfaces to roughly one-tenth of their original diameter, generating the mechanical force that initiates involution of the mesoderm. This shape change is driven by circumferential actin-myosin contraction and occurs within 20 to 30 minutes of gastrulation onset, bending the tissue inward at a rate of approximately 10 micrometers per minute. Disrupting non-muscle myosin II activity with blebbistatin in Xenopus embryos prevents bottle cell formation and blocks gastrulation entirely, demonstrating that cytoskeletal morphogenesis is mechanistically indispensable rather than merely correlative.
Fun Facts About the Nervous System →Mosaic Development
/ moh-ZAY-ik deh-VEL-up-ment / · Greek mouseion, mosaic; Latin developmentum
Mosaic development is a mode of embryonic development in which cell fates are determined early by cytoplasmic determinants inherited from the egg, so that removing or destroying a blastomere causes permanent loss of the structures it would have formed.
In mosaic development, maternally deposited proteins and mRNAs are distributed asymmetrically in the egg cytoplasm and are partitioned unequally among daughter cells at each division, restricting developmental potential from the outset. Removing or relocating an early blastomere in a mosaic embryo typically prevents that cell’s descendants from forming their normal structures, because positional information came from inherited cytoplasm rather than from signals exchanged between cells. The nematode worm Caenorhabditis elegans shows strong mosaic development, and its invariant cell lineage of exactly 959 somatic cells in the adult hermaphrodite has been mapped completely.
Most animal embryos combine mosaic and regulative features to varying degrees, with pure mosaic or pure regulative development representing the extremes of a continuum.
Tunicate embryos carry a region of yellow crescent cytoplasm that contains maternal muscle determinants, and transplanting this cytoplasm into a non-muscle blastomere redirects that cell toward a muscle fate, demonstrating that the cytoplasm itself carries instructive information independent of cell position.
All embryos can fully compensate for lost early cells. Mosaic embryos such as those of tunicates and nematodes have limited flexibility because their cell fates are determined by inherited cytoplasmic factors rather than by cell-to-cell signals.
In the sea squirt (Ciona intestinalis), the yellow crescent cytoplasm contains the maternal determinant macho-1 mRNA, which segregates exclusively to the posterior vegetal blastomeres during the first two cleavages and autonomously specifies muscle cell fate. Injecting macho-1 mRNA into anterior blastomeres that normally never form muscle converts those cells into fully differentiated muscle, complete with sarcomere assembly and spontaneous contractions, without any additional inductive signals. Removing just the yellow crescent cytoplasm before the first cleavage abolishes muscle formation entirely across the embryo, while transplanting it into a different cell position redirects muscle fate to wherever it lands , a clean demonstration of cytoplasmic determinant logic.
Multipotency
/ mul-tee-POH-ten-see / · Latin multus, many; potentia, power, ability
Multipotency is the developmental capacity of a stem cell to generate multiple, but not all, cell types, with its potential restricted to the lineages of a single tissue or organ system.
Multipotent stem cells occupy the middle of the potency hierarchy, below totipotency and pluripotency but above unipotency. Hematopoietic stem cells are the classic example, generating all blood cell types including erythrocytes, platelets, and the full complement of immune cells, yet remaining unable to produce neurons, hepatocytes, or other non-blood lineages. Neural stem cells represent a second well-studied example, producing neurons, astrocytes, and oligodendrocytes but not cells outside the nervous system.
Each multipotent population is defined by the specific transcription factors and epigenetic marks that keep some lineage programs accessible while silencing others.
A single hematopoietic stem cell transplanted into a lethally irradiated mouse can reconstitute the entire blood and immune system of that animal within four to six weeks, a demonstration first achieved by Ernest McCulloch and James Till in 1961 that established the experimental definition of a stem cell.
Multipotent means a cell can become any cell type in the body. Multipotent cells are limited to a particular tissue family, so a hematopoietic stem cell can produce dozens of blood cell types but cannot generate a neuron or a liver cell.
Hematopoietic stem cells in human bone marrow are multipotent and generate roughly 200 billion red blood cells, 10 billion white blood cells, and 400 billion platelets daily throughout adult life from a pool of only 10,000 to 20,000 stem cells. Each stem cell divides on average every 40 weeks, producing one self-renewing daughter and one progenitor that amplifies through 20 to 30 transit-amplifying divisions before terminal differentiation. Single-cell RNA sequencing studies published between 2016 and 2022 revealed that this hierarchy is not a strict binary tree but a continuous landscape in which some progenitors retain plasticity, capable of switching lineage commitment in response to acute cytokine signals such as erythropoietin or thrombopoietin.
Hematopoietic Stem Cells →