An embryoblast, also known as an embryonic button or embryoblastema, is a large mass of undifferentiated cells that originates within the morula and continues until the primary blastula or blastocyst.
Its main function is to give rise to the embryo in vertebrates. Embryoblasts are distinguished as a collection of internal cells from the early 16-cell stage known as a morula.
Graphic diagram of a blastula embedded in the wall of the uterus. Inside the embryoblast is represented (Source: Sheldahl Sheldahl Via Wikimedia Commons)
While the cells of the embryoblast give rise to the embryo, the outer cells that surround it give rise to the placenta. Of the 107 cells that make up the blastocyst that forms later, only 8 form the embryoblast and 99 the trophoblast.
The trophoblast is what is attached to the uterine mucosa and is responsible for keeping the blastocyst in that cavity.
The scientists highlight the pluripotent characteristics of the eight cells that make up the embryoblast, since all the organs and tissues of the mature embryo and, later, of the neonate originate from these.
The relationships between the embryoblast and the trophectoderm are variable depending on the species of animal. In some cases, such as insectivorous primates, for example, the embryoblast is very well demarcated and surrounded by the trophectoderm.
However, in cases such as rabbit and pig, the boundary between both layers is difficult to distinguish and the trophoblast is simply a thickening embedded in the trophectoderm; moreover, this layer disappears in the upper region of the blastocyst.
Embryoblast development
Once fertilization of the oocell occurs and the zygote is formed, a series of successive mitotic divisions of the zygote begins, which results in a rapid increase in the number of cells, originating blastomeres. With each cell division, the resulting cells get smaller.
This exhaustive division of the zygote occurs 30 hours after fertilization. After the ninth division, the blastomeres change shape and line up neatly to form a compact sphere of cells.
The compaction of the mass of cells is necessary so that they can interact and communicate with each other, which is a prerequisite and necessary for the formation of the embryoblast.
Once the division of the blastomeres reaches 12 to 32 blastomeres, such a mass of cells is known as a morula. The cells inside the morula give rise to embryoblasts; while the external ones make up the trophoblast.
Differentiation of the zygote into the morula occurs approximately 3 days after fertilization, as it makes its way to the uterus.
Shortly after the formation of the morula, it enters the uterus. Successive cell divisions cause the blastocyst cavity to form within the morula. This cavity is filled with fluid through the zona pellucida; as the amount of fluid in said cavity increases, two parts are defined in said structure.
Most cells are organized into a thin layer of outer cells. These give rise to the trophoblast; meanwhile, a small group of blastomeres located in the center of the blastocyst give rise to the mass of cells known as the embryoblast.
Graphic diagram of the parts of a blastocyst (Source: Plinio vd Via Wikimedia Commons and modified by Román González)
Features
The function of the embryoblast is to give rise to an embryo. This, in turn, will give rise to a new individual. Development occurs through a series of complex changes that shape and differentiate the layers of cells that make up each of the tissues and organs.
The development of embryos and new individuals is due to the incredible totipotentiality that blastomeres have, which diminishes only after the third division of the embryoblast into the three layers known as the endoderm, the mesoderm and the ectoderm.
However, different organs and tissues of the embryo are formed from each layer, for example: the ectoderm gives rise to the central and peripheral nervous system, the epidermis and tooth enamel.
The mesoderm gives rise to the dermis, the smooth and striated muscles, the heart, the spleen, the blood and lymphatic vessels, the gonads and the kidneys. The endoderm gives rise to the digestive and respiratory tract, the bladder epithelium, the urethra, the thyroid, the parathyroid, the liver and pancreas, the tonsils and the thymus.
Layers
The embryoblast undergoes two divisions that give it a layered structure. In principle it is divided into two layers of cells and later into three.
Two-layer separation
On the eighth day of embryonic development and simultaneously with the process of fixation of the morula in the uterus, the embryoblast differentiates into two layers.
The upper layer is known as the epiblast and the lower layer as the hypoblast. The cells of the lower layer or hypoblast have two orientations, while those of the epiblast are all oriented in the same direction.
The epiblast layer is composed of long, radially arranged cylindrical cells, all oriented towards the upper pole of the embryo or embryonic pole. These, inside, form a new fluid-filled cavity called the "amniotic cavity."
The amniotic cavity houses a small amount of fluid and is found by separating one layer of epiblast cells from another. The cells that make up the wall facing the amniotic cavity in the epiblast layer are known as cytotrophoblasts.
Hypoblast cells have a small cubic structure, can be separated into two cell layers, and are oriented toward the blastocyst cavity (abembryonic pole).
A third thin layer of cells known as amnioblasts differentiates from the epiblast. Once these cells are observed, the cavity begins to widen, the cells surround the entire amniotic cavity and begin to synthesize amniotic fluid.
The division of the embryoblast into two layers culminates in the synthesis of amniotic fluid by the amnioblasts. Finally, the cells of the epiblast are oriented towards the embryonic pole and those of the hypoblast are oriented towards the abembryonic pole.
Graphic diagram of the separation of the embryoblast into two layers (Source: Ana Paula Felici de Camargo via Wikimedia Commons)
Three-layer separation
When the embryo reaches the third week of development, the embryoblast is seen as elongated in the craniocaudal direction, that is, the structure stops looking like a sphere and now resembles two ovals together. The upper oval is cranial in orientation and the lower oval is caudal in orientation.
The thick cells of the epiblast begin gastrulation, which will give rise to the three germ layers of the embryo: the ectoderm, the mesoderm and the endoderm.
From day 15, the epiblast cells proliferate and move towards the midline of the embryo. These form a cellular thickening known as "primitive line", this line manages to occupy the middle portion of the embryonic disc.
As the primitive streak grows to the caudal end by the addition of epiblastic cells, the cephalic region of the embryo begins to be clearly visualized. This region is called the primitive knot or Hensen's knot.
In the cephalic region the hypoblastic cells in a small area adopt a columnar arrangement. These establish a precise union with the nearby cells of the epiblast.
This region is called "oropharyngeal membrane", since it marks the site of the future oral cavity of the embryo. Epiblast cells of the primitive line invaginate and migrate between the epiblast and the hypoblast towards the lateral and cephalic region of the embryoblast.
The cells that displace the hypoblast cells during invagination give rise to the embryonic endoderm. The cells that are located between the epiblast and the embryonic endoderm form the intraembryonic mesoderm and the cells that remain in the epiblast give rise to the ectoderm.
References
- Bontovics, B., SLAMECKA, JS, Maraghechi, P., AV, AVM, CHRENEK, PC, Zsuzsanna, B. Å.,… & Gá, CZA (2012). Expression Pattern of Pluripotency Markers in Rabbit Embryoblast. Bulletin of University of Agricultural Sciences and Veterinary Medicine Cluj-Napoca. Veterinary Medicine, 69 (1-2).
- Denker, HW (1981). The determination of trophoblast and embryoblast cells during cleavage in the mammal: New trends in the interpretation of the mechanisms. Anat. Ges, 75, 435-448.
- Idkowiak, J., Weisheit, G., & Viebahn, C. (2004, October). Polarity in the rabbit embryo. In Seminars in cell & developmental biology (Vol. 15, No. 5, pp. 607-617). Academic Press.
- Manes, C., & Menzel, P. (1982). Spontaneous release of nucleosome cores from embryoblast chromatin. Developmental biology, 92 (2), 529-538.
- Moore, KL, Persaud, TVN, & Torchia, MG (2018). The Developing Human-E-Book: Clinically Oriented Embryology. Elsevier Health Sciences.