Embryonic Development: Week 2

• During the second week of development, implantation of the blastocyst is completed.
• This process results in a bilaminar embryonic disc composed of the epiblast and hypoblast layers.
• The embryonic disc leads to germ layers, which form all the body's tissues and organs.
• Structures that form during the second week include the amniotic cavity, amnion, umbilical vesicle (yolk sac), connecting stalk, and chorionic sac.
• Implantation typically occurs in the endometrium.
• As the blastocyst implants, more trophoblast contacts the endometrium and differentiates into two layers: the cytotrophoblast and the syncytiotrophoblast.

Cytotrophoblast

• Layer of mitotically active mononucleated cells.
• Forms new trophoblastic cells that migrate into the increasing mass of syncytiotrophoblast where they fuse and lose their cell membranes.

Syncytiotrophoblast

• Rapidly expanding, multinucleated mass without discernible cell boundaries.
• Actively erodes endometrial connective tissue that supports the uterine capillaries and glands.
• Produces proteolytic enzymes involved in implantation
• Displaces endometrial cells in the central part of the implantation site.
• Produces human chorionic gonadotropin (hCG), that enters the maternal blood through syncytiotrophoblast lacunae.
• The development of spiral arteries in the myometrium and the formation of syncytiotrophoblast is maintained by hCG.
• Forms the basis for pregnancy tests.
• Can be detected as early as the end of the second week of pregnancy.

Days 7-8

• After 8 days, the didermic embryonic disc forms (hypoblast and epiblast).
• The ST continues its invasive, lytic activity into the maternal tissue

Blastocyst Implantation

• As implantation progresses, changes in the embryoblast result in the formation of a circular, bilaminar plate of cells that consists of two layers that form the embryonic disc.
• The Amniotic cavity starts to form around day 8

Epiblast

• The thicker layer is composed of columnar cells related to the amniotic cavity.
• Forms the the floor of the amniotic cavity and is continuous peripherally with the amnion.
• Amniogenic (amnion-forming) amnioblasts separate from here.

Hypoblast

• Thinner layer of small, cuboidal cells adjacent to the exocoelomic cavity.
• Forms the roof of the exocoelomic cavity.
• Continuous with the cells that migrated from the hypoblast to form the exocoelomic membrane
• Begins to multiply around the 9th day.

Amnion

• A small cavity appears in the embryoblast, the primordium of this.
• Amniogenic cells organize to form this thin membrane.
• Encloses the amniotic cavity

Exocoelomic Cavity

• The hypoblast forms its roof.

Second Week Details

• The exocoelomic membrane and cavity change to form the primary umbilical vesicle (yolk sac).
• The embryonic disc is located between the amniotic cavity and primary umbilical vesicle.
• The outer layer of cells from the umbilical vesicle creates a layer of loosely organized connective tissue called the extraembryonic mesoderm.

Day 9 Details

• Implantation location covered by a fibrin plug
• Expanding amniotic cavity, a cellular layer (amnioblasts) separates it from the CT.
• Extracellular vacuoles appear in the ST and join to form lacunae.

Days 9-10

• The destructive lytic activity of the ST reaches the capillaries of the endometrium.
• Maternal blood flows into the lacunae.

Maternal Blood Exchange

• Lacunae fill with of maternal blood, ruptured endometrial capillaries, and cellular debris from eroded uterine glands.
• Embryotroph passes to the embryonic disc by diffusion from fluid.
• The communication of the eroded uterine vessels with the lacuna represents the beginning of the primordial uteroplacental circulation.
• Oxygenated blood from spiral endometrial arteries and nutritive substances becomes available to the extraembryonic tissues over the large surface of the syncytiotrophoblast when maternal blood flows into the lacunae.
• Oxygenated blood passes into the lacunae from the spiral endometrial arteries.
• Deoxygenated blood is removed from the lacunae through endometrial veins.

Decidual Cells

• The uterine connective tissue cells around the implantation site are loaded with glycogen and lipids.
• Some of these cells (decidual cells) degenerate adjacent to the penetrating syncytiotrophoblast.
• Syncytiotrophoblast engulfs these degenerating cells providing a rich source of embryonic nutrition.
• Decidual cells are large round, oval, or polygonal cells.
• Decidua is the term for the uterine lining (endometrium) during pregnancy, which forms the maternal part of the placenta.
• Formed under the influence of progesterone and forms highly characteristic cells.
• Provides an immunologically privileged site for the conceptus.

Closing Plug

• The embryo and extraembryonic membrane is completely embedded in the endometrium.
• A defect exists in the endometrial epithelium for ~2 days is filled by a closing plug, a fibrinous coagulum of blood.
• Around day 12, an almost completely regenerated uterine epithelium covers the closing plug

Days10-11 Details

• The ST envelops the maternal capillaries
• Expands its lacunae network
• Forms an arterial inflow and a venous outflow system.

Days 11-12 Details

• Adjacent syncytiotrophoblastic lacunae fuse to form lacunar networks (primordia of the intervillous space of the placenta).
• Endometrial capillaries around the implanted embryo become congested, dilating to become sinusoids (thin-walled terminal vessels that are larger than capillaries).
• The syncytiotrophoblast then erodes the sinusoids and maternal blood flows into the lacunar networks.
• Growth of the bilaminar embryonic disc is slow compared with the trophoblast.
• The degenerated endometrial stromal cells and glands, together with the maternal blood, provide a rich source of material for embryonic nutrition.

Extraembryonic Mesoderm

• The extraembryonic mesoderm increases and isolated extraembryonic coelomic spaces appear within the trophoblast and endometrium.
• These spaces rapidly fuse to form a large, isolated cavity called the extraembryonic coelom (fluid-filled cavity that surrounds the amnion and umbilical vesicle, except where they are attached to the chorion by the connecting stalk).
• As the extraembryonic coelom forms, the primary umbilical vesicle decreases in size where a smaller secondary umbilical vesicle forms.
• The umbilical vesicle contains no yolk, but may play a role in the selection of nutritive materials to the embryonic disc.
• The extraembryonic coelom splits the extraembryonic mesoderm into two layers: extraembryonic somatic mesoderm and extraembryonic splanchnic mesoderm.
• Cytotrophoblastic extensions are induced by the underlying extraembryonic somatic mesoderm.

Extraembryonic Somatic Mesoderm

• Lines the trophoblast and covers the amnion.

Extraembryonic Splanchnic Mesoderm

• Surrounds the umbilical vesicle

Chorionic Sac Development

• The extraembryonic somatic mesoderm and the two layers of trophoblast form the chorion; thus, the chorion forms the wall of the chorionic sac.
• Transvaginal ultrasonography (endovaginal sonography) is used to measure the diameter of the chorionic sac.
• Measurement of the chorionic sac is useful for evaluating early embryonic development and pregnancy outcome.

Chorionic Villi Development

• Characterized by the appearance of primary chorionic villi towards the end of the second week.
• Proliferation of the cytotrophoblastic cells results in cellular extensions that grow into the overlying syncytiotrophoblast forming primitive chorionic villi, the first stage in the development of the chorionic villi of the placenta.

Final Details

• The embryo, amniotic sac, and umbilical vesicle are suspended in the chorionic cavity by the connecting stalk.