What is the hypoblast?
The hypoblast is a thin tissue present in the first stages of the embryo of mammalian animals that comprises the lower layer of the so-called embryonic disc . It contributes to the formation of different extra-embryonic structures, as well as to the establishment of the body plan of the embryo itself.
Like the epiblast, which is the tissue above it, the hypoblast is a one-cell-thick tissue that forms during the division of the cell mass that forms the blastocyst.
Embryonic development and origin of the hypoblast
The hypoblast is a tissue that is formed during the early stages of embryonic development in mammalian animals, including humans.
Embryonic development is the set of processes that take place from fertilization – the union of an ovum with a sperm – until the delivery or birth of a new living being, which is a consequence of sexual reproduction.
Fertilization and / or fertilization depends, in the first instance, on the process known as gametogenesis, which has to do with the production of haploid sex cells – ovule (female) and sperm (male) – which fuse to form a new diploid cell. known as a zygote .
Initially, the zygote undergoes several mitotic divisions that result in the formation of a mass of cells smaller than this first one.
After the first 4 divisions, the structure that is formed is called a morula and is made up of between 12 and 16 cells, also called blastomeres , which are arranged in such a way that an external group surrounds another internal group.
As the blastomeres of the morula continue to divide, the transition occurs from this stage to another known as the blastula or blastocyst , where a fluid-filled internal cavity known as a blastocele forms .
Developmental progression then involves the transition from the blastula to the gastrula , through a process called gastrulation . During gastrulation, the most important embryonic structures are formed.
Trophoblast and embryoblast
As the blastocyst cavity fills with fluid, two parts known as trophoblast and embryoblast differentiate in it .
The trophoblast is formed from the mass of external cells that surround the epithelial wall of the blastocyst, meanwhile the embryoblast consists of the group of internal cells that project towards the blastocele, that is, towards the internal cavity of the blastocyst.
The trophoblast differentiates into the cytotrophoblast and the syncytiotrophoblast , which participate in the communication of the embryo with the maternal tissue.
The embryoblast, on the other hand, differentiates into a structure known as the bilaminar embryonic disc, which is made up of two layers: the hypoblast and the epiblast .
Hypoblast and epiblast
The three germ layers of the embryo then derive from the epiblast, ie ectoderm, mesoderm and endoderm; while the hypoblast forms the yolk sac and the chorion.
In the epiblast, after the formation of the embryonic disc, a cavity appears that gradually increases in size, originating the amniotic cavity, which is the site where the embryo and later the fetus will grow as gestation progresses.
Meanwhile, cells derived from the hypoblast also divide and delimit the inner surface of the blastocyst cavity, forming the primitive yolk sac , which later differentiates into the secondary or definitive yolk sac . This sac is kept in the outer region of the developing embryo.
A set of cells derived from the primitive yolk sac proliferates in the space between the cytotrophoblast and the yolk sac cavity, forming the membrane known as the chorion , which covers the embryo and later participates in the formation of what will later become the placenta.
The hypoblast is made up of a monolayer of cuboidal cells, a layer one cell thick. Its primary function is to form the yolk sac and the chorionic membrane.
The yolk sac is a thin membranous structure found on the periphery of the embryo and on which its development strongly depends.
This sac connects to the embryo through a thin tube known as the yolk stalk or omphalomesenteric duct , which joins the primitive midgut of the embryo, exactly where the ileum will be later.
The yolk sac has critical functions for the early stages of embryonic development, among which are primitive hematopoiesis (formation of blood cells) and the production of germ cells (from which the gametic cells will later derive).
Before the placenta forms, the yolk sac also functions in the exchange of gases and nutrients between the mother and the embryo.
It participates in the formation of the umbilical cord, in the formation of stem cells and primitive macrophages, in the metabolic regulation of embryonic cells and in the synthesis of important proteins such as albumin, alpha-fetoprotein and some apolipoproteins.
This is another structure derived from hypoblast cells. It is described as the outermost membrane of the embryo and, in mammals, it develops abundant blood vessels and forms an intimate association with the endometrium of the uterus where the fetus grows.
The placenta is made up of the chorion and the endometrium, and represents the main organ of respiration, nutrition and excretion of the embryo.
The chorion also participates in the formation of the amniotic sac, which is the “bag” that contains the embryo and which is made up of the chorion and the amnion, which are the two membranes that separate the embryo from maternal tissues.
Thus, the main functions of the chorion are the protection and nourishment of the embryo while the gestation takes place in the womb.
Other functions of the hypoblast
Although initially it was considered that it did not participate in any subsequent event in the development of embryonic structures, today it is known that this tissue has important functions in:
- The establishment of the body plan of the embryo.
- The control of the movement of cells derived from the epiblast, which give rise to the three germ layers of the embryo: endoderm, ectoderm and mesoderm.
- The establishment of bilateral symmetry of the embryo.
- The transient induction of pre-neural markers in the epiblast.
- After gastrulation, the hypoblast also protects forebrain cells against ” flow” signals .
- Donovan MF, Bordoni B. Embryology, Yolk Sac. [Updated 2020 Mar 29]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2020 Jan-. Available from: ncbi.nlm.nih.gov
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- Stern, CD, & Downs, KM (2012). The hypoblast (visceral endoderm): an evo-devo perspective. Development, 139 (6), 1059-1069.