Hypoblast

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Hypoblast
Human Embryo Day9.png
Human embryo on day 9. Hypoblast (brown) is beneath the epiblast (pink)
Details
Days8
Precursor Inner cell mass
Gives rise to Endoderm
Identifiers
Latin hypoblastus
TE E6.0.1.1.3.0.4
Anatomical terminology

In amniote embryology, the hypoblast is one of two distinct layers arising from the inner cell mass in the mammalian blastocyst, [1] [2] or from the blastodisc in reptiles and birds. The hypoblast gives rise to the yolk sac. [3]

Contents

The hypoblast is a layer of cells in fish and amniote embryos. The hypoblast helps determine the embryo's body axes, and its migration determines the cell movements that accompany the formation of the primitive streak, and helps to orient the embryo, and create bilateral symmetry.

The other layer of the inner cell mass, the epiblast, differentiates into the three primary germ layers, ectoderm, mesoderm, and endoderm.

Structure

The hypoblast lies beneath the epiblast and consists of small cuboidal cells. [4] The hypoblast in fish (but not in birds and mammals) contains the precursors of both the endoderm and mesoderm. [5] In birds and mammals, it contains precursors to the extraembryonic endoderm of the yolk sac. [3] [5]

In chick embryos, early cleavage forms an area opaca and an area pellucida, and the region between these is called the marginal zone. [5] Area opaca is the blastoderm's peripheral part where the cells remain unseparated from the yolk. It is a white area that transmits light. [5]

Function

Although the hypoblast does not contribute to the embryo, it influences the orientation of the embryo. [5] The hypoblast also inhibits primitive streak formation. [6] The absence of hypoblast results in multiple primitive streaks in chicken embryos. [7] The primitive endoderm derived yolk sac ensures the proper organogenesis of the fetus and the exchange of nutrients, gases, and wastes. Hypoblast cells also provide chemical signals that specify the migration of epiblast cells. [5]

Amniotes

Birds

In birds, the primitive streak formation is generated by a thickening of the epiblast called the Koller's sickle [5] The Koller's sickle is created at the posterior edge of the area pellucida while the rest of the cells of the area pellucida remain at the surface, forming the epiblast. [5] In chicks, the mesoderm cells don't invaginate, like in amphibians, but they migrate medially and caudally from both sides and create a midline thickening called primitive streak. The primitive streak grows rapidly in length as more presumptive mesoderm cells continue to aggregate inward. Gastrulation begins in the area pellucida next to the posterior marginal zone, as the hypoblast and primitive streak both start there. [5] The avian embryo comes entirely from the epiblast, and the hypoblast does not contribute to any cells. [5] The hypoblast cells form parts of the other membranes such as the yolk sac and the stalk linking the yolk mass to the endodermal digestive tube. [5] [8] In between the area opaca and Koller's sickle is a belt-like region called the posterior marginal zone (PMZ). [5] The PMZ organizes the Hensen's center in amniotes.

Meanwhile, cells in more anterior regions of the epiblast delaminate and stay attached to the epiblast to form hypoblast "islands." These islands are clusters of 5–20 cells that migrate and become the primary hypoblast. [5] The sheet of cells that grows anteriorly from Koller's sickle combines with the primary hypoblast to form the secondary hypoblast (also called the endoblast). [5]

The resulting two-layered blastoderm (epiblast and hypoblast) is joined at the marginal zone of the area opaca, and the space between the layers forms a blastocoel-like a cavity. Cell division adds to the length produced by convergent extension. Some of the cells from the anterior portion of the epiblast contribute to the formation of Hensen's node. The Hensen's node is the organizer for gastrulation in the vertebrate embryo. Simultaneously, the secondary hypoblast (endoblast) cells continue to migrate anteriorly from the blastoderm's posterior marginal zone. [5] The elongation of the primitive streak is coextensive with the anterior migration of these secondary hypoblast cells, and the hypoblast directs the movement of the primitive streak. [5] The streak eventually extends to about ¾ of the length of the area pellucida. [5]

Cells migrate to the primitive streak, and as they enter the embryo the cells separate into two layers. The deep layer joins the hypoblast along its midline, displacing the hypoblast cells to the sides. [5] The first cells to migrate through Hensen's node are destined to become the foregut's pharyngeal endoderm. [5] Once deep within the embryo, the endodermal cells migrate anteriorly and eventually displace the hypoblast cells, causing the hypoblast cells to be confined to a region in the area's anterior portion pellucida.

This pattern appears similar to that of amphibian embryos. Nodal activity is needed to initiate the primitive streak, and that it is the secretion of Cerberus—an antagonist of Nodal—by the primary hypoblast cells that prevent primitive streak formation. [5] As the primary hypoblast cells move away from the PMZ, Cerberus protein is no longer present, allowing Nodal activity (and, therefore, forming the primitive streak) in the posterior epiblast. [5] Once formed, however, the streak secretes its Nodal antagonist—the Lefty protein—which prevents further primitive streaks from forming. [5] Eventually, the Cerberus-secreting hypoblast cells are pushed to the future anterior of the embryo, where they contribute to ensuring that neural cells in this region become forebrain rather than more posterior structures the nervous system. [5] As the primitive streak reaches its maximum length, transcription of the Sonic hedgehog gene (Shh) becomes restricted to the embryo's left side, controlled by activin and its receptor. [5]

Mammals

In mammalian embryogenesis, differentiation and segregation of cells in the inner cell mass of the blastocyst produces two different layers—the epiblast ("primitive ectoderm") and the hypoblast ("primitive endoderm"). [5]

The first segregation of cells within the inner cell mass forms two layers. In contact with the blastocoel, the lower layer is called the primitive endoderm, and it is homologous to the chick embryo hypoblast. [5] While hypoblast cells delaminate ventrally, away from the embryonic pole, to line the blastocoele the remaining cells of the inner cell mass, situated between the hypoblast and the polar trophoblast, become the epiblast. [5]

In the mouse primordial germ cells are from epiblast cells. [9] This specification is accompanied by extensive epigenetic reprogramming that involves global DNA demethylation, chromatin reorganization, and imprint erasure which results in totipotency. [9] The mammalian equivalent to the chick hypoblast is called the anterior visceral endoderm (AVE) [10] and creates an anterior region by secreting antagonists of Nodal. [5] In the mouse, (the most studied mammalian model organism for this) the hypoblast restricts Nodal activity using Cerberus and Lefty1 while birds use only Cerberus. [5]

Fish

In fish, the hypoblast is the inner layer of the thickened margin of the epibolizing blastoderm in the gastrulating fish embryo. [5] The hypoblast in fish (but not in birds or mammals) contains the precursors of both the endoderm and mesoderm. [5]

Genetics

The signal transduction pathway, the Wnt pathway, is activated by fibroblast growth factors (FGF) produced by the hypoblast. [5] If the hypoblast is rotated, the orientation of the primitive streak follows the rotation. If FGF signaling is activated in the epiblast margin, Wnt signaling will occur there. The orientation of the primitive streak will change as if the hypoblast had been placed there. The cell migrations that form the primitive streak appear to be regulated by FGFs from the hypoblast, which activates the Wnt planar cell polarity pathway in the epiblast. [5] The Wnt pathway, in turn, is activated by FGFs produced by the hypoblast. [5]

Related Research Articles

<span class="mw-page-title-main">Mesoderm</span> Middle germ layer of embryonic development

The mesoderm is the middle layer of the three germ layers that develops during gastrulation in the very early development of the embryo of most animals. The outer layer is the ectoderm, and the inner layer is the endoderm.

<span class="mw-page-title-main">Gastrulation</span> Stage in embryonic development in which germ layers form

Gastrulation is the stage in the early embryonic development of most animals, during which the blastula, or in mammals the blastocyst, is reorganized into a two-layered or three-layered embryo known as the gastrula. Before gastrulation, the embryo is a continuous epithelial sheet of cells; by the end of gastrulation, the embryo has begun differentiation to establish distinct cell lineages, set up the basic axes of the body, and internalized one or more cell types including the prospective gut.

<span class="mw-page-title-main">Blastocoel</span> Fluid-filled or yolk-filled cavity that forms in the blastula

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 the blastula is called the blastocyst, which consists of an outer epithelium, the trophectoderm, enveloping the inner cell mass and the blastocoel.

A germ layer is a primary layer of cells that forms during embryonic development. The three germ layers in vertebrates are particularly pronounced; however, all eumetazoans produce two or three primary germ layers. Some animals, like cnidarians, produce two germ layers making them diploblastic. Other animals such as bilaterians produce a third layer between these two layers, making them triploblastic. Germ layers eventually give rise to all of an animal's tissues and organs through the process of organogenesis.

<span class="mw-page-title-main">Yolk sac</span> Membranous sac attached to an embryo

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 more widely used. In humans, the yolk sac is important in early embryonic blood supply, and much of it is incorporated into the primordial gut during the fourth week of embryonic development.

<span class="mw-page-title-main">Animal embryonic development</span> Process by which the embryo forms and develops

In developmental biology, animal embryonic development, also known as animal embryogenesis, is the developmental stage of an animal embryo. Embryonic development starts with the fertilization of an egg cell (ovum) by a sperm cell (spermatozoon). Once fertilized, the ovum becomes a single diploid cell known as a zygote. The zygote undergoes mitotic divisions with no significant growth and cellular differentiation, leading to development of a multicellular embryo after passing through an organizational checkpoint during mid-embryogenesis. In mammals, the term refers chiefly to the early stages of prenatal development, whereas the terms fetus and fetal development describe later stages.

<span class="mw-page-title-main">Neurula</span> Embryo at the early stage of development in which neurulation occurs

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The primitive node is the organizer for gastrulation in most amniote embryos. In birds, it is known as Hensen's node, and in amphibians, it is known as the Spemann-Mangold organizer. It is induced by the Nieuwkoop center in amphibians, or by the posterior marginal zone in amniotes including birds.

<span class="mw-page-title-main">Primitive streak</span> Structure in early amniote embryogenesis

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 broad end towards the anterior, and the narrower region projected to the posterior. The primitive streak forms a longitudinal midline structure in the narrower posterior (caudal) region of the developing embryo on its dorsal side. At first formation, the primitive streak extends for half the length of the embryo. In the human embryo, this appears by stage 6, about 17 days.

<span class="mw-page-title-main">Epiblast</span> Embryonic inner cell mass tissue that forms the embryo itself, through the three germ layers

In amniote embryonic development, the epiblast 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 the hypoblast. It drives the embryo proper through its differentiation into the three primary germ layers, ectoderm, mesoderm and endoderm, during gastrulation. The amniotic ectoderm and extraembryonic mesoderm also originate from the epiblast.

<span class="mw-page-title-main">Mesenchyme</span> Type of animal embryonic connective tissue

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<span class="mw-page-title-main">Bilaminar embryonic disc</span>

The bilaminar embryonic disc, bilaminar blastoderm or embryonic disc is the distinct two-layered structure of cells formed in an embryo. In the development of the human embryo this takes place by day eight. It is formed when the inner cell mass, also known as the embryoblast, forms a bilaminar disc of two layers, an upper layer called the epiblast and a lower layer called the hypoblast, which will eventually form into fetus. These two layers of cells are stretched between two fluid-filled cavities at either end: the primitive yolk sac and the amniotic sac.

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<span class="mw-page-title-main">Fish development</span>

The development of fishes is unique in some specific aspects compared to the development of other animals.

<span class="mw-page-title-main">Cerberus (protein)</span> Protein found in humans

Cerberus is a protein that in humans is encoded by the CER1 gene. Cerberus is a signaling molecule which contributes to the formation of the head, heart and left-right asymmetry of internal organs. This gene varies slightly from species to species but its overall functions seem to be similar.

<span class="mw-page-title-main">Human embryonic development</span> Development and formation of the human embryo

Human embryonic development or human embryogenesis is the development and formation of the human embryo. It is characterised by the processes of cell division and cellular differentiation of the embryo that occurs during the early stages of development. In biological terms, the development of the human body entails growth from a one-celled zygote to an adult human being. Fertilization occurs when the sperm cell successfully enters and fuses with an egg cell (ovum). The genetic material of the sperm and egg then combine to form the single cell zygote and the germinal stage of development commences. Human embryonic development covers the first eight weeks of development, which have 23 stages, called Carnegie stages. At the beginning of the ninth week, the embryo is termed a fetus. In comparison to the embryo, the fetus has more recognizable external features and a more complete set of developing organs.

<span class="mw-page-title-main">Nodal homolog</span> Mammalian protein found in Homo sapiens

Nodal homolog is a secretory protein that in humans is encoded by the NODAL gene which is located on chromosome 10q22.1. It belongs to the transforming growth factor beta superfamily. Like many other members of this superfamily it is involved in cell differentiation in early embryogenesis, playing a key role in signal transfer from the primitive node, in the anterior primitive streak, to lateral plate mesoderm (LPM).

<span class="mw-page-title-main">Koller's sickle</span>

In avian gastrulation, Koller's sickle is a local thickening of cells at the posterior edge of the upper layer of the area pellucida called the epiblast. Koller's sickle is crucial for avian development, due to its critical role in inducing the differentiation of various avian body parts. Koller's sickle induces primitive streak and Hensen's node, which are major components of avian gastrulation. Avian gastrulation is a process by which developing cells in an avian embryo move relative to one another in order to form the three germ layers.

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References

  1. ↑ Palmer, N.; Kaldis, P. (2016-01-01), DePamphilis, Melvin L. (ed.), "Chapter One - Regulation of the Embryonic Cell Cycle During Mammalian Preimplantation Development", Current Topics in Developmental Biology, Mammalian Preimplantation Development, 120, Academic Press: 1–53, doi:10.1016/bs.ctdb.2016.05.001, PMID   27475848 , retrieved 2020-10-16
  2. ↑ Keefe, David L.; Winkler, Nurit (2007-01-01), Sokol, Andrew I.; Sokol, Eric R. (eds.), "Chapter 1 - Embryology", General Gynecology, Philadelphia: Mosby, pp. 1–20, doi:10.1016/b978-032303247-6.10001-2, ISBN   978-0-323-03247-6 , retrieved 2020-10-16
  3. 1 2 Hafez, S. (2017-01-01), Huckle, William R. (ed.), "Chapter One - Comparative Placental Anatomy: Divergent Structures Serving a Common Purpose", Progress in Molecular Biology and Translational Science, Molecular Biology of Placental Development and Disease, 145, Academic Press: 1–28, doi:10.1016/bs.pmbts.2016.12.001, PMID   28110748 , retrieved 2020-10-16
  4. ↑ Moore, K. L., and Persaud, T. V. N. (2003). The Developing Human: Clinically Oriented Embryology. 7th Ed. Philadelphia: Elsevier. ISBN   0-7216-9412-8.
  5. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 Barresi, Michael; Gilbert, Scott (July 2019). Developmental Biology (12th ed.). Oxford University Press. ISBN   978-1605358222.
  6. ↑ Egea J., Erlacher C., Montanez E., Burtscher I., Yamagishi S., Hess M., Hampel F., Sanchez R., Rodriguez-Manzaneque M. T., Bosl M. R., et al. (2008). Genetic ablation of FLRT3 reveals a novel morphogenetic function for the anterior visceral endoderm in suppressing mesoderm differentiation. Genes Dev. 22, 3349–3362.
  7. ↑ Perea-Gomez A, Vella FD, Shawlot W, Oulad-Abdelghani M, Chazaud C, Meno C, Pfister V, Chen L, Robertson E, Hamada H, Behringer RR, Ang SL (2002). "Nodal antagonists in the anterior visceral endoderm prevent the formation of multiple primitive streaks". Dev Cell. 3 (5): 745–56. doi: 10.1016/S1534-5807(02)00321-0 . PMID   12431380.
  8. ↑ Charles, A. K.; Faye-Petersen, O. M. (2014-01-01), McManus, Linda M.; Mitchell, Richard N. (eds.), "Human Placental Development from Conception to Term", Pathobiology of Human Disease, San Diego: Academic Press, pp. 2322–2341, doi:10.1016/b978-0-12-386456-7.05002-4, ISBN   978-0-12-386457-4 , retrieved 2020-10-16
  9. 1 2 Hackett JA, Sengupta R, Zylicz JJ, Murakami K, Lee C, Down TA, Surani MA (January 2013). "Germline DNA demethylation dynamics and imprint erasure through 5-hydroxymethylcytosine". Science. 339 (6118): 448–52. Bibcode:2013Sci...339..448H. doi:10.1126/science.1229277. PMC   3847602 . PMID   23223451.
  10. ↑ Stower, Matthew J.; Srinivas, Shankar (5 December 2014). "Heading forwards: anterior visceral endoderm migration in patterning the mouse embryo". Philosophical Transactions of the Royal Society B: Biological Sciences. 369 (1657): 20130546. doi:10.1098/rstb.2013.0546. PMC   4216468 . PMID   25349454.
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