Codex @codex  0

A signalling region that becomes positioned on the prospective anterior side of the mouse embryo and antagonizes posteriorizing signals, helping position the primitive streak.

In angiosperms, apomixis produces an embryo through asexual reproduction within a seed, bypassing the usual combination of meiosis and fusion of gametes. It can preserve a maternal genetic combination across generations. It differs from vegetative reproduction, which propagates an individual through structures such as shoots or rhizomes rather than through seeds.

Producing a clonal seed requires coordinating more than one developmental event: retaining the maternal chromosome complement and initiating an embryo without ordinary fertilization are separate requirements. Rice research combining altered meiosis with redirected embryogenic gene activity demonstrated clonal propagation through seeds.^[ref] In some forms of apomixis, fertilization is still needed to produce supporting endosperm, so an asexual embryo does not imply that every component of the seed develops without fertilization.

BABY BOOM1 is a rice transcription factor that can initiate embryonic development. Its early expression in the zygote normally involves the paternal allele, connecting a contribution from the male gamete to the start of a new developmental programme.

Imtiyaz Khanday and colleagues redirected BABY BOOM1 expression into the unfertilized ovum. This was sufficient to initiate an embryo without fertilization. Combining that trigger with altered meiosis enabled clonal seed production in an engineered system.^[ref]

The components solve different problems: initiating an embryo does not by itself prevent the genetic reshuffling and chromosome-number reduction of ordinary meiosis. This separation explains why a single developmental trigger and a complete clonal propagation system are different experimental achievements.

A maternally supplied Drosophila transcript localized at the anterior of the oocyte and early embryo, providing a spatial source of Bicoid protein.

Biological development is the organized change of an organism through growth, differentiation and maturation. It can change cell identities, spatial relationships and function, rather than merely increasing size.

For example, gastrulation reorganizes an embryo's cells into a new arrangement, while later cell differentiation produces specialized tissues. Developmental stage is consequently more informative than chronological age alone when an intervention changes the rate or sequence of events.

An early arrangement of cardiogenic tissue before formation of the linear heart tube in the mammalian embryo.

Researcher in early mammalian development. Gerri and collaborators investigated initiation of a conserved trophectoderm program in human, cow and mouse embryos, connecting comparative measurements with the origin of an extraembryonic lineage.^[ref]

A cloaca is a common chamber receiving outputs from more than one of the digestive, urinary and reproductive systems. It occurs in many vertebrates, while the early human embryo has a cloacal region that becomes partitioned during development.

The developmental chamber and the adult arrangement must be distinguished: a shared embryonic space does not mean the mature human rectum, urinary tract and reproductive tract all empty through one opening.

The connecting stalk is an early extraembryonic attachment between the developing embryo and the chorion. As the body folds and extraembryonic structures are reorganized, this attachment contributes to the umbilical cord.

Its position helps explain the arrangement of the chorionic cavity: most early embryonic and associated structures can be surrounded by that cavity while remaining attached at the connecting stalk.^[ref]

Developmental biology investigates how an organism's structures and cell types arise and become organized. Cell division, cell differentiation, movement, signaling and mechanical interactions connect a starting state such as a zygote to a patterned body. Development also includes later growth, maturation and regeneration.

A central distinction is between what a biological cell normally becomes and what it can become after an intervention. Fate mapping measures the former, while transplantation or changed signaling tests developmental potential. Live imaging can follow divisions and movements in the same embryo, making a developmental sequence directly observable rather than reconstructed solely from different fixed specimens.^[ref]

The characteristic early postimplantation mouse embryo arrangement, with cup-shaped epiblast, extraembryonic ectoderm and surrounding visceral endoderm. It differs geometrically from a flat embryonic disc.

An embryologist studies embryos and their development. Work can compare stages of cell differentiation and morphogenesis, or test how an intervention changes later developmental outcomes. The scientific role therefore connects observation of structure with experimental tests of developmental organization.

Study of embryos and their formation. Historical embryology combined dissection, optical microscopy, comparison of developmental stages and, increasingly, experimental intervention.

Embryonic development is the organized sequence through which an embryo forms and differentiates before the later juvenile or fetal stages defined for its species. Cell division, movement, changing gene expression and interactions among neighbouring cells jointly build tissues and body organization. Developmental stage therefore describes more than elapsed time or total cell number.

Light-sheet imaging of living zebrafish embryos enabled repeated three-dimensional observation and tracking of nuclei across early development. ^[ref] Following one developing specimen distinguishes movement and ancestry from an ordering inferred solely from fixed specimens of different ages. A lineage record still does not by itself identify the molecular signals determining every descendant cell fate.

A superficial epithelial cell layer of the zebrafish embryo that spreads over the yolk during epiboly.

The embryonic lineage that generates the embryo proper and, in mammals, also contributes to certain extraembryonic tissues. It is distinct from the hypoblast.

Spreading and thinning of a tissue to cover a larger area of the embryo, often requiring intercalation and forces generated by adjacent tissues.

A fallopian tube is one of the paired ducts connecting the region near an ovary to the uterus. Its ovarian end opens into the peritoneal cavity rather than forming a sealed attachment to the ovary. Fimbriae help receive the released oocyte, which is transported along the tube by ciliary activity and smooth-muscle movement.^[ref]

The tube is therefore an active transport environment, not a passive storage vessel. It can bring gametes into the same region and transport an early embryo toward the uterus. These tasks distinguish ovarian release, fertilization and uterine implantation: they are different events with different locations, even when they occur in a connected sequence.

Authors include Minjung Kang.

Development 140, pp. 267–279. FGF4-null embryos distinguish initial marker coexpression from stable lineage restriction.

The coordinated rearrangements that establish the principal germ layers and their relative positions in an early embryo.