The bilaminar embryonic disc, bilaminar blastoderm or embryonic disc is the two-layered structure of

epiblast In amniote embryonic development, the epiblast (also known as the primitive ectoderm) is one of two distinct cell layers arising from the inner cell mass in the mammalian blastocyst, or from the blastula in reptiles and birds, the other layer is t ...

and

hypoblast In amniote embryology, the hypoblast, is one of two distinct layers arising from the inner cell mass in the mammalian blastocyst, or from the blastodisc in reptiles and birds. The hypoblast gives rise to the yolk sac, which in turn gives rise to ...

, differentiated from the

inner cell mass The inner cell mass (ICM) or embryoblast (known as the pluriblast in marsupials) is a structure in the early development of an embryo. It is the mass of cells inside the blastocyst that will eventually give rise to the definitive structures of ...

also known as the embryoblast. These two layers of cells lie between two cavities: the primitive yolk sac and the

amniotic cavity The amniotic sac, also called the bag of waters or the membranes, is the sac in which the embryo and later fetus develops in amniotes. It is a thin but tough transparent pair of membranes that hold a developing embryo (and later fetus) until shor ...

. The inner cell mass, begins to transform into two distinct epithelial layers just before implantation is completed. The epiblast, also known as the primitive ectoderm, is the outer layer that consists of columnar cells. The inner layer is called the hypoblast, also known as the primitive endoderm, which is composed of cuboidal cells. As the two layers become evident, a basement membrane presents itself between the layers. The final two layers of the embryoblast are known collectively as the bilaminar embryonic disc. This bilaminar disc also defines the primitive dorsal ventral axis.

Blastocyst The blastocyst is a structure formed in the early embryonic development of mammals. It possesses an inner cell mass (ICM) also known as the ''embryoblast'' which subsequently forms the embryo, and an outer layer of trophoblast cells called the t ...

implantation will occur during the second week of

embryonic development An embryo is an initial stage of development of a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male sperm ...

in the endometrium of the uterus;"Bilaminar Embryonic Disc." Atlas of Human Embryology. Chronolab A.G. Switzerland, n.d. Web. 27 Nov. 2012. . the epiblast is dorsal and the hypoblast is ventral.

Formation

The

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 multicellula ...

undergoes cleavage as it journeys from the

fallopian tube The fallopian tubes, also known as uterine tubes, oviducts or salpinges (singular salpinx), are paired tubes in the human female that stretch from the uterus to the ovaries. The fallopian tubes are part of the female reproductive system. In ot ...

to the

uterus The uterus (from Latin ''uterus'', plural ''uteri'') or womb () is the organ in the reproductive system of most female mammals, including humans that accommodates the embryonic and fetal development of one or more embryos until birth. The uter ...

. As it transforms from 2 to 4, to 8 to 16 cells, it becomes a ball of cells called a

morula A morula (Latin, ''morus'': mulberry) is an early-stage embryo consisting of a solid ball of cells called blastomeres, contained in mammals, and other animals within the zona pellucida shell. The blastomeres are the daughter cells of the zygote ...

. During these divisions, the zygote remains the same size, but the number of cells increases. The morula differentiates into an outer and inner group of cells: the peripheral outer cell layer, the

trophoblast The trophoblast (from Greek : to feed; and : germinator) is the outer layer of cells of the blastocyst. Trophoblasts are present four days after fertilization in humans. They provide nutrients to the embryo and develop into a large part of the pl ...

, and the central

, the embryoblast. The trophoblast goes on to become the fetal portion of the placenta and related

fetal membranes The fetal membranes are the four extraembryonic membranes, associated with the developing embryo, and fetus in humans and other mammals.. They are the amnion, chorion, allantois, and yolk sac. The amnion and the chorion are the chorioamniotic memb ...

. The

and

arise from the embryoblast and later give rise to the embryo proper and its affiliated fetal membranes. Once the zygote has differentiated into 16-32 cells, it starts to form a fluid-filled central cavity called the blastocyst cavity called the

blastocoel The blastocoel (), also spelled blastocoele and blastocele, and also called cleavage cavity, or segmentation cavity is a fluid-filled or yolk-filled cavity that forms in the blastula during very early embryonic development. At this stage in mammals ...

.Schoenwolf, Gary C., and William J. Larsen. Larsen's Human Embryology. 4th ed. Philadelphia: Churchill Livingstone/Elsevier, 2009. Print. This cavity is essential because as the cells continue to divide, the outer layer of cells increases making it difficult for the innermost cells to receiving adequate nutrients from the surrounding fluid. Therefore, the blastocyst cavity serves as a nutrient center and the fluid is able to reach and feed cells so that they can continue growing and dividing. The embryo is called a

blastocyst The blastocyst is a structure formed in the early embryonic development of mammals. It possesses an inner cell mass (ICM) also known as the ''embryoblast'' which subsequently forms the embryo, and an outer layer of trophoblast cells called the t ...

at about the 6th day of development once it has reached nearly 100 cells. Once formed, the blastocyst begins its journey into the uterus to start implantation in the

endometrium The endometrium is the inner epithelial layer, along with its mucous membrane, of the mammalian uterus. It has a basal layer and a functional layer: the basal layer contains stem cells which regenerate the functional layer. The functional laye ...

. During implantation the inner cell mass differentiates into two layers the

, and the

which are the two layers of the bilaminar embryonic disc. The epiblast gives rise to the embryo proper, and the hypoblast gives rise to the fetal membranes.

Establishment of the amniotic cavity

2907 Embroyonic Disc, Amniotic Cavity, Yolk Sac-02

Beginning on day 8, the

is the first new cavity to form during the second week of development. Fluid collects between the epiblast and the hypoblast, which splits the epiblast into two portions. The layer at the embryonic pole grows around the

, creating a barrier from the cytotrophoblast. This becomes known as the

amnion The amnion is a membrane that closely covers the human and various other embryos when first formed. It fills with amniotic fluid, which causes the amnion to expand and become the amniotic sac that provides a protective environment for the develo ...

, which is one of the four extraembyonic membranes and the cells it comprises are referred to as amnioblasts."10.1 Early Development and Implantation." The Embryoblast. N.p., n.d. Web. 29 Nov. 2012. Although, the

starts off small it eventually grows to be larger than the blastocyst and by week 8, the whole embryo is encompassed by the

Formation of the yolk sac and chorionic cavity

The formation of the chorionic cavity (

extraembryonic coelom The gestational sac is the large cavity of fluid surrounding the embryo. During early embryogenesis it consists of the extraembryonic coelom, also called the chorionic cavity. The gestational sac is normally contained within the uterus. It is the o ...

) and the

yolk sac The yolk sac is a membranous sac attached to an embryo, formed by cells of the hypoblast layer of the bilaminar embryonic disc. This is alternatively called the umbilical vesicle by the Terminologia Embryologica (TE), though ''yolk sac'' is far ...

(umbilical vesicle) is still debated. The thought of how the yolk sac membranes are formed begins with an increase in production of hypoblast cells, succeeded by different patterns of migration. On day 8, the first portion of hypoblast cells begin their migration and make what is known as the primary yolk sac, or

Heuser's membrane Heuser's membrane (or the exocoelomic membrane) is a short lived combination of hypoblast cells and extracellular matrix In biology, the extracellular matrix (ECM), also called intercellular matrix, is a three-dimensional network consisting of ...

(exocoelomic membrane). By day 12, the primary yolk sac has been disestablished by a new batch of migrating hypoblast cells that now contribute to the definitive yolk sac.
While the primary yolk sac is forming, extraembryonic mesoderm makes its way into the blastocyst cavity to fill it with loosely packed cells. When the extraembryonic mesoderm is separated into two portions, a new gap arises called the chorionic cavity, or the

extra-embryonic coelom The gestational sac is the large cavity of fluid surrounding the embryo. During early embryogenesis it consists of the extraembryonic coelom, also called the chorionic cavity. The gestational sac is normally contained within the uterus. It is the o ...

. This new cavity is responsible for detaching the embryo and its amnion and

from the far wall of the blastocyst, which is now named the

chorion The chorion is the outermost fetal membrane around the embryo in mammals, birds and reptiles (amniotes). It develops from an outer fold on the surface of the yolk sac, which lies outside the zona pellucida (in mammals), known as the vitelline ...

. When the extraembryonic mesoderm splits into two layers, the amnion,

, and

follow its lead and also become double layered. The

and amnion are composed of extraembryonic ectoderm and mesoderm, where as the

is made of extraembryonic endoderm and mesoderm. When day 13 rolls around, the

connecting stalk The connecting stalk, or body stalk is an embryonic structure that is formed by the third week of development and connects the embryo to its shell of trophoblasts. The connecting stalk is derived from the extraembryonic mesoderm. Initially it lie ...

, a dense portion of extraembryonic mesoderm, restrains the embryonic disc in the chorionic cavity.

Yolk sac during development

Like the amnion, the

is an

extraembryonic membrane The extraembryonic membranes are four membranes which assist in the development of an animal's embryo. Such membranes occur in a range of animals from humans to insects. They originate from the embryo, but are not considered part of it. They typ ...

that surrounds a cavity. Formation of the definitive yolk sac happens after the extraembryonic mesoderm splits, and it becomes a double layered structure with hypoblast-derived endoderm on the inside and mesoderm surrounding the outside. The definitive yolk sac contributes greatly to the embryo during the 4th week of development, and executes critical functions for the embryo. One of which being the formation of blood, or

hematopoiesis Haematopoiesis (, from Greek , 'blood' and 'to make'; also hematopoiesis in American English; sometimes also h(a)emopoiesis) is the formation of blood cellular components. All cellular blood components are derived from haematopoietic stem cells. ...

. Also, primordial

germ cell Germ or germs may refer to: Science * Germ (microorganism), an informal word for a pathogen * Germ cell, cell that gives rise to the gametes of an organism that reproduces sexually * Germ layer, a primary layer of cells that forms during embry ...

s are first found in the wall of the yolk sac. After the 4th week of development, the growing embryonic disc becomes a great deal larger than the

and its presence usually dies out before birth. However, the yolk sac may rarely remain as a deviation of the digestive tract named

Meckel's diverticulum A Meckel's diverticulum, a true congenital diverticulum, is a slight bulge in the small intestine present at birth and a vestigial remnant of the omphalomesenteric duct (also called the vitelline duct or yolk stalk). It is the most common malformat ...

Epiblast cells during gastrulation

The third week of development and the formation of the

primitive streak The primitive streak is a structure that forms in the early embryo in amniotes. In amphibians the equivalent structure is the blastopore. During early embryonic development, the embryonic disc becomes oval shaped, and then pear-shaped with the ...

sparks the beginning of

gastrulation Gastrulation is the stage in the early embryonic development of most animals, during which the blastula (a single-layered hollow sphere of cells), or in mammals the blastocyst is reorganized into a multilayered structure known as the gastrula. Be ...

Gastrulation Gastrulation is the stage in the early embryonic development of most animals, during which the blastula (a single-layered hollow sphere of cells), or in mammals the blastocyst is reorganized into a multilayered structure known as the gastrula. Be ...

is when the three germ cell layers develop as well as an organism’s body plan. During gastrulation, cells of the

, a layer of the bilaminar blastocyst, migrate towards the

, enter it, and then move apart from it through a process called ingression.

Definitive endoderm development

On day 16, epiblast cells that are next to the primitive streak experience epithelial-to-mesenchymal transformation as they ingress through the primitive streak. The first wave of epiblast cells takes over the hypoblast, which slowly becomes replaced by new cells that eventually constitute the definitive

endoderm Endoderm is the innermost of the three primary germ layers in the very early embryo. The other two layers are the ectoderm (outside layer) and mesoderm (middle layer). Cells migrating inward along the archenteron form the inner layer of the gast ...

. The definitive

is what makes the lining of the gut and other associated gut structures.

Intraembryonic mesoderm development

Also beginning on day 16, some of the ingressing epiblast cells make their way into the area between the epiblast and the newly forming definitive endoderm. This layer of cells becomes known as intraembryonic mesoderm. After the cells have moved bilaterally from the primitive streak and matured, four divisions of intraembryonic mesoderm are made; cardiogenic mesoderm,

paraxial mesoderm Paraxial mesoderm, also known as presomitic or somitic mesoderm is the area of mesoderm in the neurulating embryo that flanks and forms simultaneously with the neural tube. The cells of this region give rise to somites, blocks of tissue running ...

intermediate mesoderm Intermediate mesoderm or intermediate mesenchyme is a narrow section of the mesoderm (one of the three primary germ layers) located between the paraxial mesoderm and the lateral plate of the developing embryo. The intermediate mesoderm develops i ...

and

lateral plate mesoderm The lateral plate mesoderm is the mesoderm that is found at the periphery of the embryo. It is to the side of the paraxial mesoderm, and further to the axial mesoderm. The lateral plate mesoderm is separated from the paraxial mesoderm by a narrow ...

Ectoderm development

After the definitive endoderm and intraembryonic mesoderm formations are complete, the remaining epiblast cells do not ingress through the primitive streak; rather they remain on the outside and form the

ectoderm The ectoderm is one of the three primary germ layers formed in early embryonic development. It is the outermost layer, and is superficial to the mesoderm (the middle layer) and endoderm (the innermost layer). It emerges and originates from t ...

. It is not long until the ectoderm becomes the

neural plate The neural plate is a key developmental structure that serves as the basis for the nervous system. Cranial to the primitive node of the embryonic primitive streak, ectodermal tissue thickens and flattens to become the neural plate. The region ante ...

and

surface ectoderm The surface ectoderm (or external ectoderm) forms the following structures: *Skin (only epidermis; dermis is derived from mesoderm) (along with glands, hair, and nails) *Epithelium of the mouth and nasal cavity and glands of the mouth and nasal cav ...

. Due to the fact that an embryo develops cranial to caudal, the formation of ectoderm does not happen at the same rate during development. The more inferior portion of the primitive streak will still have epiblast cells ingressing to make intraembryonic mesoderm, while the more superior portion has stop ingressing. However, eventually gastrulation finishes and the three germ layers are complete.

References

External links

Diagram at manchester.ac.uk
{{Authority control Embryology