Isolecithal (''Greek'' iso = equal, lekithos = yolk) refers to the even distribution of
yolk
Among animals which produce eggs, the yolk (; also known as the vitellus) is the nutrient-bearing portion of the egg whose primary function is to supply food for the development of the embryo. Some types of egg contain no yolk, for example bec ...
in the
cytoplasm
In cell biology, the cytoplasm is all of the material within a eukaryotic cell, enclosed by the cell membrane, except for the cell nucleus. The material inside the nucleus and contained within the nuclear membrane is termed the nucleoplasm. ...
of
ova of
mammals and other
vertebrates
Vertebrates () comprise all animal taxa within the subphylum Vertebrata () ( chordates with backbones), including all mammals, birds, reptiles, amphibians, and fish. Vertebrates represent the overwhelming majority of the phylum Chordata, wi ...
, notably fishes of the families
Petromyzontidae,
Amiidae, and
Lepisosteidae
Gars are members of the family Lepisosteidae, which are the only surviving members of the Ginglymodi, an ancient holosteian group of ray-finned fish, which first appeared during the Triassic, over 240 million years ago. Gars comprise seven l ...
.
Isolecithal cells have two equal hemispheres of yolk. However, during cellular development, normally under the influence of gravity, some of the yolk settles to the bottom of the egg, producing an uneven distribution of yolky hemispheres. Such uneven cells are known as
telolecithal Telolecithal (Greek: ''τέλος'' (telos) = end, ''λέκιθος'' (lekithos) = yolk), refers to the uneven distribution of yolk in the cytoplasm of ova found in birds, reptiles, fish, and monotremes. The yolk is concentrated at one pole of the ...
and are common where there is sufficient yolk mass.
In the absence of a large concentration of yolk, four major cleavage types can be observed in isolecithal cells: radial
holoblastic, spiral holoblastic, bilateral holoblastic, and rotational holoblastic cleavage. These holoblastic cleavage planes pass all the way through isolecithal
zygote
A zygote (, ) is a eukaryotic cell formed by a fertilization event between two gametes. The zygote's genome is a combination of the DNA in each gamete, and contains all of the genetic information of a new individual organism.
In multicell ...
s during the process of
cytokinesis
Cytokinesis () is the part of the cell division process during which the cytoplasm of a single eukaryotic cell divides into two daughter cells. Cytoplasmic division begins during or after the late stages of nuclear division in mitosis and me ...
. Coeloblastula is the next stage of development for eggs that undergo this radial cleavage. In mammals, because the isolecithal cells have only a small amount of yolk, they require immediate implantation onto the uterine wall to receive nutrients.
See also
*
Cell cycle
The cell cycle, or cell-division cycle, is the series of events that take place in a cell that cause it to divide into two daughter cells. These events include the duplication of its DNA ( DNA replication) and some of its organelles, and sub ...
*
Centrolecithal Centrolecithal (''Greek'' kentron = center of a circle, lekithos = yolk) describes the placement of the yolk in the centre of the cytoplasm of ova. Many arthropod eggs are centrolecithal.
During cytokinesis, centrolecithal zygotes undergo merobla ...
*
Telolecithal Telolecithal (Greek: ''τέλος'' (telos) = end, ''λέκιθος'' (lekithos) = yolk), refers to the uneven distribution of yolk in the cytoplasm of ova found in birds, reptiles, fish, and monotremes. The yolk is concentrated at one pole of the ...
References
Cell biology
{{Cell-biology-stub