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Samenvatting The Developing Human week 1-8
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Filmpje human developingSECOND WEEK: BILAMINAR EMBRYO
By the end of the first week of development, the morula, formed from the cleavage of the zygote, develops into a
blastocyst. Two cellular areas are distinguished, the embryoblast, inner cell mass, and the trophoblast, outer cell
mass.
Throughout the second week of development, a number of events occur as the blastocyst continues to fully
implant within the endometrial wall of the uterus. These include the completion of implantation, establishment of
the uteromaternal circulation, and formation of the bilaminar embryonic disc, which gives rise to the germ layers
that form all the tissues and organs of the embryo.
Many of the events seen during this week occur in pairs, so the second week of development is often referred to as
the 'week of twos'.
During this period, there is considerable variation in the rate of development of the blastocyst; that is, embryos of
the same fertilization age may not necessarily develop at the same rate.
In this topic, a typical day-by-day overview of the major events of development is described, and a clear definition
of the formation of each individual embryonic structure is given.
STRUCTURES OF EMBRYONIC DEVELOPMENT
The production of many of the structures of embryonic development begins before implantation of the blastocyst
is complete. Over the course of the second week after fertilization, these structures grow rapidly at varying rates of
development.
The structures that develop during this week form both the origins of the germ layers of the embryo, and a series
of extraembryonic membranes and layers that support and nourish the embryo and fetus during the pregnancy.
Bilaminar embryonic disc
The bilaminar embryonic disc is a two-layered, flat disc, which gives rise to the germ layers that form all the
tissues and organs of the embryo. It is formed when the inner cell mass differentiates into two distinct layers:
Epiblast
Columnar cells, which form the upper layer of the inner cell mass, closest to the trophoblast.
These cells give rise to future cells of the embryo, and form the primitive ectoderm.
Hypoblast
Cuboidal cells, which form the lower layer of the inner cell mass, furthest from the trophoblast.
These cells are the embryonic precursor to the endoderm, and gives rise to many of the
extraembryonic structures.
Trophoblast
As it makes contact with the endometrium, the area of trophoblast directly over the inner cell mass differentiates
into two distinct layers:
Cytotrophoblast
Mononucleated cells forming the inner layer of the trophoblast.
As implantation progresses, cells of the cytotrophoblast are able to divide and migrate, forming a
new cell lineage, called the syncytiotrophoblast.
, The cytotrophoblast cell layer plays an important role in the implantation process of a blastocyst.
During implantation, the cytotrophoblast secretes proteolytic enzymes, which break down the
extracellular matrix surrounding the endometrial cells. This allows the blastocyst to move through
the lining of the uterus until it becomes fully embedded in the endometrium, and inner part of the
myometrium.
Cells of the cytotrophoblast will eventually contribute to the formation of the placenta.
Syncytiotrophoblast
Multinucleated zone forming the outer layer of the trophoblast.
The syncytiotrophoblast is formed from daughter cells of the cytotrophoblast. As the cells proliferate
and migrate towards the outer edges of the trophoblast, they lose their cell boundaries and combine
to form a syncytium, a mass of cytoplasm containing numerous nuclei.
Throughout the second week of development, the syncytiotrophoblast cell layer continues to
expand and actively invades the uterine wall by breaking down proteins and connective tissues via
the secretion of proteases.
As the syncytiotrophoblast breaks down the uterine wall, driving the implantation process, maternal
capillaries rupture forming an interface between the maternal blood supply and embryonic
extracellular fluid. As such, the syncytiotrophoblast plays an important role in gas exchange, nutrient
exchange, and immunological and metabolic functions between the mother and fetus.
In addition, the syncytiotrophoblast secretes a number of hormones, including human chorionic
gonadotropin, human placental lactogen, and progesterone, which maintains the integrity of the
uterine lining. By the fourth month, the progesterone secreted by the mature syncytiotrophoblast
also helps to support pregnancy.
Amniotic cavity
The amniotic cavity is the first cavity to form during the second week of development as a result of the build-up
of fluids between the epiblast cells, and those overlying the trophoblast.
As the amniotic cavity enlarges, the cells of the epiblast form the floor of the cavity, and a dome-like roof forms
called the amnion.
Amnion
Single layer of squamous cells that forms one of the four extraembryonic membranes.
At the beginning of the second week of development, the amnion is smaller than the blastocyst
cavity. As development continues, and folding of the embryo occurs, the amnion transforms into a
sac filled with amniotic fluid, which completely surrounds the entire embryo.
Amniotic fluid is very similar to blood plasma in composition. It is originally derived from maternal blood, but
later, importantly, contains the urine excretion from the fetus. This fluid is inhaled and exhaled by the fetus
during its development. It is this swallowed amniotic fluid that creates fetal urine, and contributes to the
production of meconium, feces produced from ingested materials in the uterus.
As well as a portal for urine excretion, amniotic fluid has a number of other functions that help to support the
developing fetus.
Body Regulation of fetal body temperature.
temperature
Shock absorber Acts as a protective mechanism to cushion the fetus against the mother's abdomen.
Adhesion Prevents adhesion between the skin of the fetus and the surrounding amnion.
Movement Allows for easier fetal movement, and promotes musculoskeletal development.
Hydrostatic During childbirth, the amniotic fluid creates a hydrostatic wedge that helps to dilate the cervix
wedge during childbirth.
, The amnion usually ruptures just before birth, and the release of amniotic fluid is often described as the 'waters
breaking'.
As the amniotic fluid contains embryonic cells, a procedure called amniocentesis can be carried out to test the
fetal cells and substances dissolved in the fluid for a number of genetic conditions.
Primitive yolk sac
Columnar cells at the abembryonic pole (the opposite side of the embryo), probably originating from
the hypoblast, migrate to the edge of the hypoblast, where they become squamous. They migrate,
forming a thin membrane that covers the inner surface of the cytotrophoblast, called the
exocoelomic membrane, or Heuser membrane.
The exocoelomic membrane and the hypoblast form the wall of the exocoelomic cavity, or primitive
yolk sac.
Extraembryonic mesoderm
The extraembryonic mesoderm is a layer of connective tissue that forms around the amnion and
primitive yolk sac.
During the second week of development, large cavities form in the extraembryonic mesoderm,
which then fuse to become a single larger cavity, called the extraembryonic coelom, which is later
called the chorionic cavity.
This cavity divides the extraembryonic mesoderm into two parts. The somatopleuric mesoderm lines
the cytotrophoblast and amnion, and the splanchnopleuric mesoderm covers the yolk sac.
Secondary yolk sac
The secondary yolk sac, also known as the definitive sac, is formed from a second migration of
squamous hypoblast cells. These cells migrate along the inside of the exocoelomic membrane and
proliferate to form a new cavity within the exocoelomic cavity.
As the definitive yolk sac is formed, large portions of the exocoelomic cavity are pinched off to form
exocoelomic cysts, which are eventually eliminated. As a result, the definitive yolk sac is much
smaller than the primitive yolk sac.
Chorion
The chorion is a membrane formed from the extraembryonic mesoderm, cytotrophoblast, and
syncytiotrophoblast. This membrane surrounds the embryo and continues to surround the fetus
during development, and the inner layer eventually fuses with the amnion.
Once the chorion is formed, the extraembryonic coelom is referred to as the chorionic cavity.
The chorion has several important functions that help in the development of the fetus.
Formation of The principal embryonic component of the placenta, which is responsible for the
the placenta exchange of materials between mother and fetus.
Protein Secretes proteins that block antibody production by the mother.
secretion
T lymphocyte Promotes production of T lymphocytes that suppress the normal immune
production response in the uterus.
hCG production The syncytiotrophoblast produces the hormone human chorionic gonadotropin
(hCG). It is responsible for maintaining the corpus luteum throughout pregnancy,
causing it to secrete progesterone.
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