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Development of the embryonic disc

This image shows the development of the amniotic cavity and the location of the embryonic disc.
Formation of the embryonic disc leaves spaces on either side that develop into the amniotic cavity and the yolk sac.

On the ventral side of the embryonic disc, opposite the amnion, cells in the lower layer of the embryonic disk (the hypoblast    ) extend into the blastocyst cavity and form a yolk sac    . The yolk sac supplies some nutrients absorbed from the trophoblast and also provides primitive blood circulation to the developing embryo for the second and third week of development. When the placenta takes over nourishing the embryo at approximately week 4, the yolk sac has been greatly reduced in size and its main function is to serve as the source of blood cells and germ cells (cells that will give rise to gametes). During week 3, a finger-like outpocketing of the yolk sac develops into the allantois    , a primitive excretory duct of the embryo that will become part of the urinary bladder. Together, the stalks of the yolk sac and allantois establish the outer structure of the umbilical cord.

The last of the extra-embryonic membranes is the chorion    , which is the one membrane that surrounds all others. The development of the chorion will be discussed in more detail shortly, as it relates to the growth and development of the placenta.

Embryogenesis

As the third week of development begins, the two-layered disc of cells becomes a three-layered disc through the process of gastrulation    , during which the cells transition from totipotency to multipotency. The embryo, which takes the shape of an oval-shaped disc, forms an indentation called the primitive streak    along the dorsal surface of the epiblast. A node at the caudal or “tail” end of the primitive streak emits growth factors that direct cells to multiply and migrate. Cells migrate toward and through the primitive streak and then move laterally to create two new layers of cells. The first layer is the endoderm    , a sheet of cells that displaces the hypoblast and lies adjacent to the yolk sac. The second layer of cells fills in as the middle layer, or mesoderm    . The cells of the epiblast that remain (not having migrated through the primitive streak) become the ectoderm    ( [link] ).

Germ layers

This image shows the different germ layers. The top panel shows the epiblast and trophoblast cells in the early stages of development. The bottom panel shows the three germ layers: the endoderm, ectoderm, and mesoderm. All the other major parts are also labeled.
Formation of the three primary germ layers occurs during the first 2 weeks of development. The embryo at this stage is only a few millimeters in length.

Each of these germ layers will develop into specific structures in the embryo. Whereas the ectoderm and endoderm form tightly connected epithelial sheets, the mesodermal cells are less organized and exist as a loosely connected cell community. The ectoderm gives rise to cell lineages that differentiate to become the central and peripheral nervous systems, sensory organs, epidermis, hair, and nails. Mesodermal cells ultimately become the skeleton, muscles, connective tissue, heart, blood vessels, and kidneys. The endoderm goes on to form the epithelial lining of the gastrointestinal tract, liver, and pancreas, as well as the lungs ( [link] ).

Fates of germ layers in embryo

This image shows the structure of the embryo in the third week of development. Under the image, three callouts list the different organ systems into which each germ layer develops.
Following gastrulation of the embryo in the third week, embryonic cells of the ectoderm, mesoderm, and endoderm begin to migrate and differentiate into the cell lineages that will give rise to mature organs and organ systems in the infant.

Questions & Answers

What is ELISA
POULAMI Reply
(enzyme linked immunosorbent assay) is a test that uses antibodies and color change to identify a substance.
luke
tr
Mohamed
difference between apocrine sweat glands and merocrine sweat glands
Binkheir Reply
I believe the apocrine sweat gland uses a sac under the hair follicle and the merocrine sweat gland releases directly on to the surface of the skin
Mark
normal blood volume in our body
pankaj Reply
5Litres
Albert
Normal blood volume in adults is 6 litres
Kedha's
4.7 to 5ltr.. normal for adult
Clangbhelle
what are the advantages of the concave shape of red blood cells?
Amy Reply
This structure is VERY flexible. It can allow these cells to get into the most tiny places in our bodies. a VERY good design! The advantage of red blood cells' biconcave shape is that the surface area is increased to allow more haemoglobin to be stored in the cell.
Saafi
They can stack so that they can move to capillaries
Nejat
action of gluteus medius and minimus
Green Reply
Lateral rotation of the hip joint
Hertzo
Briefly explain location of ecg on a patient
Prince Reply
it is a machine that gives a graphical representation of heart beat
Nani
Briefly explain location of ecg leads on a patient?
Prince
in ecg we use electrical leads over the chest ,ancle and wrist
Nani
what is the anatomical and function difference between paravertebral and prevertebral ganglia ?
Rada Reply
types of tissue in human
Preety Reply
charactetistic Of cartilaginous tissue
Preety
what is theRecurrent infection?
pankaj Reply
what do you mean about recurrent infection
pankaj
Recurrent or persistent infection is a manifestation of primary immuno deficiency
Kedha's
weakens the immune system, allowing infections and other health problems to occur more easily
Kedha's
lysis of RBC
Abdirizack
What is barometric pressure
Kedha's Reply
what is the agglutination advantage
Gopal Reply
the functions of the liver
Nana Reply
it produces bile juice which is used to make the food smaller
Kedha's
it also plays an important role in conversion of amino acid into urea
Komal
it also has role in gluconeogenesis in which amino acids and lipids convert into glucose.
Komal
during fetal life it's a center for hemopoiesis (formation of blood cells)
Komal
it filters, or removes, harmful substances from the blood
Kedha's
It stores nutrients, such as vitamins and iron,for the body
Kedha's
what is the largest gland in human body
Shahid Reply
liver
rachna
correct
Said
correct
dominic
thyroid gland
Kedha's
thyroid is largest endocrine gland
Komal
describe microscopic structures of the kidney
Nana Reply
kidney is covered by fibrous capsule, consists of an outer cortex and inner medulla with medullary pyramids. The microscopic structure is seen as 1-2 millions of nephrons and collecting tubule.
Komal
identify the four major tissue types
Binkheir Reply
connective epithelial
Nana
two ramining
Binkheir
muscle nervous
Nana
epithelial, connective, muscle, and nervous tissue
Mel

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Source:  OpenStax, Anatomy & Physiology. OpenStax CNX. Feb 04, 2016 Download for free at http://legacy.cnx.org/content/col11496/1.8
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