Isolecithal
Encyclopedia
Isolecithal refers to the even distribution of yolk in the cytoplasm
of ovum
s of mammal
s and other vertebrates—notably Petromyzontidae,Amiidae
, and Lepisosteidae. 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
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
s during the process of cytokinesis
. Coeloblastula is the next stage of development for eggs that undergo this radial cleavaging. In mammals, because the isolecithal cells have only a small amount of yolk, they require immediate implantation onto the uterine wall to receive nutrients.
Cytoplasm
The cytoplasm is a small gel-like substance residing between the cell membrane holding all the cell's internal sub-structures , except for the nucleus. All the contents of the cells of prokaryote organisms are contained within the cytoplasm...
of ovum
Ovum
An ovum is a haploid female reproductive cell or gamete. Both animals and embryophytes have ova. The term ovule is used for the young ovum of an animal, as well as the plant structure that carries the female gametophyte and egg cell and develops into a seed after fertilization...
s of mammal
Mammal
Mammals are members of a class of air-breathing vertebrate animals characterised by the possession of endothermy, hair, three middle ear bones, and mammary glands functional in mothers with young...
s and other vertebrates—notably Petromyzontidae,Amiidae
Amiidae
Amiidae is a family of primitive ray-finned fish. Only one species, Amia calva, the bowfin, survives today, although additional species in all four subfamilies are known from Jurassic, Cretaceous, and Eocene fossils....
, and Lepisosteidae. 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
Telolecithal , also known as macrolecithal, refers to the uneven distribution of yolk in the cytoplasm of ovums found in birds, reptiles, and fish...
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
Zygote
A zygote , or zygocyte, is the initial cell formed when two gamete cells are joined by means of sexual reproduction. In multicellular organisms, it is the earliest developmental stage of the embryo...
s during the process of cytokinesis
Cytokinesis
Cytokinesis is the process in which the cytoplasm of a single eukaryotic cell is divided to form two daughter cells. It usually initiates during the late stages of mitosis, and sometimes meiosis, splitting a binucleate cell in two, to ensure that chromosome number is maintained from one generation...
. Coeloblastula is the next stage of development for eggs that undergo this radial cleavaging. In mammals, because the isolecithal cells have only a small amount of yolk, they require immediate implantation onto the uterine wall to receive nutrients.