Embryo Project Encyclopedia

Fetus in fetu is a rare variety of parasitic twins , where the developmentally abnormal parasitic twin is completely encapsulated within the torso of the otherwise normally developed host twin. In the late eighteenth century, German anatomist Johann Friedrich Meckel was the first to described fetus in fetu, which translates to “fetus within fetus.” Fetus in fetu is thought to result from the unequal division of the totipotent inner cell mass , the mass of cells that is the ancestral precursor to all cells in the body. The unequal division is thought to occur during the formation of the blastocyst, which can also result in parasitic and conjoined twins . Fetus in fetu represents a developmental anomaly that has prompted developmental biologists to further examine the mechanisms for how twins arise.

For more than 2000 years, embryologists, biologists, and philosophers have studied and detailed the processes that follow fertilization. The fertilized egg proliferates into cells that begin to separate into distinct, identifiable zones that will eventually become adult structures through the process of morphogenesis. As the cells continue to multiply, patterns form and cells begin to differentiate, and eventually commit to their fate. This progression of events can be examined by following the developmental path of each progenitor cell and creating a two-dimensional representation where cell location and fate can be labeled and marked. Fate mapping is a method for tracing cell lineages and a fundamental tool of developmental biology and embryology.

Early development occurs in a highly organized and orchestrated manner and has long attracted the interest of developmental biologists and embryologists. Cell lineage, or the study of the developmental differentiation of a blastomere, involves tracing a particular cell (blastomere) forward from its position in one of the three germ layers. Labeling individual cells within their germ layers allows for a pictorial interpretation of gastrulation. This chart or graphical representation detailing the fate of each part of an early embryo is referred to as a fate map. In essence, each fate map portrays the developmental history of each cell.

Gastrulation is an early stage in embryo development in which the blastula reorganizes into three germ layers: the ectoderm, the mesoderm, and the endoderm. Gastrulation occurs after cleavage but before neurulation and organogenesis. Ernst Haeckel coined the term; gaster, meaning stomach in Latin, is the root for gastrulation, as the gut is one of the most unique creations of the gastrula.

A node, or primitive knot, is an enlarged group of cells located in the anterior portion of the primitive streak in a developing gastrula. The node is the site where gastrulation, the formation of the three germ layers, first begins. The node determines and patterns the anterior-posterior axis of the embryo by directing the development of the chordamesoderm. The chordamesoderm is a specific type of mesoderm that will differentiate into the notochord, somites, and neural tube. Those structures will later form the vertebral column. In the chick embryo, the node is referred to as Hensen's node because of its discoverer, Viktor Hensen, who first described the node in 1875. The discovery of Hensen's node has helped to answer questions of axis formation and has allowed experimental embryologists to further investigate vertebrate embryonic development.

Through various studies developmental biologists have been able to determine that the muscles of the back, ribs, and limbs derive from somites. Somites are blocks of cells that contain distinct sections that diverge into specific types (axial or limb) of musculature and are an essential part of early vertebrate development. For many years the musculature of vertebrates was known to derive from the somites, but the exact developmental lineage of axial and limb muscle progenitor cells remained a mystery until Nicole Le Douarin and Charles P. Ordahl published "Two Myogenic Lineagues within the Developing Somite" in 1991. This paper describes their experiment, which used chick-quail chimeras to demonstrate the exact lineage of the limb and back musculature.

Somites are blocks of mesoderm that are located on either side of the neural tube in the developing vertebrate embryo. Somites are precursor populations of cells that give rise to important structures associated with the vertebrate body plan and will eventually differentiate into dermis, skeletal muscle, cartilage, tendons, and vertebrae. Somites also determine the migratory paths of neural crest cells and of the axons of spinal nerves.

Laparoscopy, a subfield of endoscopy, is a minimally invasive surgical procedure used to examine and operate on the internal organs of the abdomen through a small incision in the abdominal wall. The term "laparoscopy" is derived from two Greek words: laparo, meaning the soft space between hips and ribs, and skopie, meaning to examine. Today laparoscopy has broad clinical applications including for diagnosis, fertility procedures, visual representation, and surgery.

Lennart Nilsson is a world-renowned photojournalist recognized for his exploratory images of the inside of the human body. Throughout his career, Nilsson has received a great deal of publicity for his images documenting the human reproductive system and the morphology of viruses. His photography was the first to capture early human development and the developmental stages of embryos and fetuses. These images have helped shape the way the public visualizes development.

In Birth without Violence (1975), French obstetrician Frederick Leboyer describes in poetic form the possible perceptions and feelings of embryos and fetuses before, during, and after birth. His work has helped to promote a gentler and more sensitive birthing method with the goal of easing the newborn's transition from the womb to the outside world. Leboyer's birthing method influenced later birth techniques such as water birth and unassisted childbirth.