Embryoblast and Trophoblast Development

Formation of the Blastocyst

• Blastocyst Blastocyst A post-morula preimplantation mammalian embryo that develops from a 32-cell stage into a fluid-filled hollow ball of over a hundred cells. A blastocyst has two distinctive tissues. The outer layer of trophoblasts gives rise to extra-embryonic tissues. The inner cell mass gives rise to the embryonic disc and eventual embryo proper. Fertilization and First Week formation begins 5 days after fertilization Fertilization To undergo fertilization, the sperm enters the uterus, travels towards the ampulla of the fallopian tube, and encounters the oocyte. The zona pellucida (the outer layer of the oocyte) deteriorates along with the zygote, which travels towards the uterus and eventually forms a blastocyst, allowing for implantation to occur. Fertilization and First Week.
• The blastocyst Blastocyst A post-morula preimplantation mammalian embryo that develops from a 32-cell stage into a fluid-filled hollow ball of over a hundred cells. A blastocyst has two distinctive tissues. The outer layer of trophoblasts gives rise to extra-embryonic tissues. The inner cell mass gives rise to the embryonic disc and eventual embryo proper. Fertilization and First Week is approximately 70–100 cells.
• The blastocyst Blastocyst A post-morula preimplantation mammalian embryo that develops from a 32-cell stage into a fluid-filled hollow ball of over a hundred cells. A blastocyst has two distinctive tissues. The outer layer of trophoblasts gives rise to extra-embryonic tissues. The inner cell mass gives rise to the embryonic disc and eventual embryo proper. Fertilization and First Week cavity is formed by uterine fluid, which penetrates the morula Morula An early embryo that is a compact mass of about 16 blastomeres. It resembles a cluster of mulberries with two types of cells, outer cells and inner cells. Morula is the stage before blastula in non-mammalian animals or a blastocyst in mammals. Fertilization and First Week after deterioration of the zona pellucida Zona pellucida A tough transparent membrane surrounding the ovum. It is penetrated by the sperm during fertilization. Fertilization and First Week.
• As fluid increases, the morula Morula An early embryo that is a compact mass of about 16 blastomeres. It resembles a cluster of mulberries with two types of cells, outer cells and inner cells. Morula is the stage before blastula in non-mammalian animals or a blastocyst in mammals. Fertilization and First Week differentiates into the trophoblast Trophoblast Cells lining the outside of the blastocyst. After binding to the endometrium, trophoblasts develop into two distinct layers, an inner layer of mononuclear cytotrophoblasts and an outer layer of continuous multinuclear cytoplasm, the syncytiotrophoblasts, which form the early fetal-maternal interface (placenta). Fertilization and First Week and embryoblast.
  • Trophoblast Trophoblast Cells lining the outside of the blastocyst. After binding to the endometrium, trophoblasts develop into two distinct layers, an inner layer of mononuclear cytotrophoblasts and an outer layer of continuous multinuclear cytoplasm, the syncytiotrophoblasts, which form the early fetal-maternal interface (placenta). Fertilization and First Week (outer cell mass Mass Three-dimensional lesion that occupies a space within the breast Imaging of the Breast): layer of flattened blastomeres that surrounds the embryoblast
  • Embryoblast (inner cell mass Mass Three-dimensional lesion that occupies a space within the breast Imaging of the Breast): primordium to the embryo Embryo The entity of a developing mammal, generally from the cleavage of a zygote to the end of embryonic differentiation of basic structures. For the human embryo, this represents the first two months of intrauterine development preceding the stages of the fetus. Fertilization and First Week and fetus

Implantation of the Blastocyst

The trophoblast Trophoblast Cells lining the outside of the blastocyst. After binding to the endometrium, trophoblasts develop into two distinct layers, an inner layer of mononuclear cytotrophoblasts and an outer layer of continuous multinuclear cytoplasm, the syncytiotrophoblasts, which form the early fetal-maternal interface (placenta). Fertilization and First Week is primarily responsible for implantation Implantation Endometrial implantation of embryo, mammalian at the blastocyst stage. Fertilization and First Week of the blastocyst Blastocyst A post-morula preimplantation mammalian embryo that develops from a 32-cell stage into a fluid-filled hollow ball of over a hundred cells. A blastocyst has two distinctive tissues. The outer layer of trophoblasts gives rise to extra-embryonic tissues. The inner cell mass gives rise to the embryonic disc and eventual embryo proper. Fertilization and First Week.

• Implantation Implantation Endometrial implantation of embryo, mammalian at the blastocyst stage. Fertilization and First Week occurs 6–10 days after fertilization Fertilization To undergo fertilization, the sperm enters the uterus, travels towards the ampulla of the fallopian tube, and encounters the oocyte. The zona pellucida (the outer layer of the oocyte) deteriorates along with the zygote, which travels towards the uterus and eventually forms a blastocyst, allowing for implantation to occur. Fertilization and First Week.
• The trophoblast Trophoblast Cells lining the outside of the blastocyst. After binding to the endometrium, trophoblasts develop into two distinct layers, an inner layer of mononuclear cytotrophoblasts and an outer layer of continuous multinuclear cytoplasm, the syncytiotrophoblasts, which form the early fetal-maternal interface (placenta). Fertilization and First Week differentiates into the cytotrophoblast and syncytiotrophoblast.
  • Cytotrophoblast
    • Inner cell layer
    • Essential for placenta Placenta A highly vascularized mammalian fetal-maternal organ and major site of transport of oxygen, nutrients, and fetal waste products. It includes a fetal portion (chorionic villi) derived from trophoblasts and a maternal portion (decidua) derived from the uterine endometrium. The placenta produces an array of steroid, protein and peptide hormones (placental hormones). Placenta, Umbilical Cord, and Amniotic Cavity formation
  • Syncytiotrophoblast
    • Outer cell layer
    • Invades endometrial connective tissue Connective tissue Connective tissues originate from embryonic mesenchyme and are present throughout the body except inside the brain and spinal cord. The main function of connective tissues is to provide structural support to organs. Connective tissues consist of cells and an extracellular matrix. Connective Tissue: Histology by producing proteolytic enzymes Proteolytic enzymes Proteins and Peptides
    • Secretes hCG
• Decidual cells—polygonal fibroblasts Fibroblasts Connective tissue cells which secrete an extracellular matrix rich in collagen and other macromolecules. Sarcoidosis in the endometrium that control depth of blastocyst Blastocyst A post-morula preimplantation mammalian embryo that develops from a 32-cell stage into a fluid-filled hollow ball of over a hundred cells. A blastocyst has two distinctive tissues. The outer layer of trophoblasts gives rise to extra-embryonic tissues. The inner cell mass gives rise to the embryonic disc and eventual embryo proper. Fertilization and First Week penetration Penetration X-rays—are enlarged.
• Completely implanted 12 days after fertilization Fertilization To undergo fertilization, the sperm enters the uterus, travels towards the ampulla of the fallopian tube, and encounters the oocyte. The zona pellucida (the outer layer of the oocyte) deteriorates along with the zygote, which travels towards the uterus and eventually forms a blastocyst, allowing for implantation to occur. Fertilization and First Week

Formation of Bilaminar Disk and Amniotic Cavity

Embryoblast differentiation

The embryoblast gives rise to the bilaminar disk and amniotic cavity. The embryoblast differentiates into the epiblast and hypoblast. Together, they are referred to as the bilaminar disc. The process begins around the 8th day of gestation.

Hypoblast:

• Located ventrally near the blastocyst Blastocyst A post-morula preimplantation mammalian embryo that develops from a 32-cell stage into a fluid-filled hollow ball of over a hundred cells. A blastocyst has two distinctive tissues. The outer layer of trophoblasts gives rise to extra-embryonic tissues. The inner cell mass gives rise to the embryonic disc and eventual embryo proper. Fertilization and First Week cavity
• Some hypoblast cells will form the anterior visceral endoderm Endoderm The inner of the three germ layers of an embryo. Gastrulation and Neurulation, which becomes the cranial end of the embryo Embryo The entity of a developing mammal, generally from the cleavage of a zygote to the end of embryonic differentiation of basic structures. For the human embryo, this represents the first two months of intrauterine development preceding the stages of the fetus. Fertilization and First Week.
• A cavity emerges within the hypoblast to form the primary yolk sac.
• On the ventral side of the yolk sac, the hypoblast expands to form the extraembryonic mesoderm Mesoderm The middle germ layer of an embryo derived from three paired mesenchymal aggregates along the neural tube. Gastrulation and Neurulation.
  • Somatic mesoderm Mesoderm The middle germ layer of an embryo derived from three paired mesenchymal aggregates along the neural tube. Gastrulation and Neurulation: lines the cytotrophoblast and amnion Amnion The innermost membranous sac that surrounds and protects the developing embryo which is bathed in the amniotic fluid. Amnion cells are secretory epithelial cells and contribute to the amniotic fluid. Placenta, Umbilical Cord, and Amniotic Cavity
  • Splanchnic mesoderm Mesoderm The middle germ layer of an embryo derived from three paired mesenchymal aggregates along the neural tube. Gastrulation and Neurulation: surrounds the yolk sac

Epiblast:

• Located dorsally
• A small cavity will emerge within the epiblast to form the amniotic cavity.

Mnemonic

The 2nd week of development is known as the week of 2s: 

• The trophoblast Trophoblast Cells lining the outside of the blastocyst. After binding to the endometrium, trophoblasts develop into two distinct layers, an inner layer of mononuclear cytotrophoblasts and an outer layer of continuous multinuclear cytoplasm, the syncytiotrophoblasts, which form the early fetal-maternal interface (placenta). Fertilization and First Week differentiates into 2 layers: the cytotrophoblast and syncytiotrophoblast.
• The embryoblast forms 2 layers: the epiblast and hypoblast.
• The extraembryonic mesoderm Mesoderm The middle germ layer of an embryo derived from three paired mesenchymal aggregates along the neural tube. Gastrulation and Neurulation splits into 2 layers: the somatic and splanchnic layers.
• 2 cavities form: the amniotic and yolk sac cavities.

Clinical Relevance

Spontaneous abortion Abortion Expulsion of the product of fertilization before completing the term of gestation and without deliberate interference. Spontaneous Abortion: caused by inadequate production of estrogens and progesterone Progesterone The major progestational steroid that is secreted primarily by the corpus luteum and the placenta. Progesterone acts on the uterus, the mammary glands and the brain. It is required in embryo implantation; pregnancy maintenance, and the development of mammary tissue for milk production. Progesterone, converted from pregnenolone, also serves as an intermediate in the biosynthesis of gonadal steroid hormones and adrenal corticosteroids. Gonadal Hormones by the corpus luteum Corpus Luteum The yellow body derived from the ruptured ovarian follicle after ovulation. The process of corpus luteum formation, luteinization, is regulated by luteinizing hormone. Ovaries: Anatomy. Other causes include chromosomal abnormalities and physical trauma. The major symptom of early spontaneous abortion Abortion Expulsion of the product of fertilization before completing the term of gestation and without deliberate interference. Spontaneous Abortion is vaginal bleeding. Treatment of early spontaneous abortion Abortion Expulsion of the product of fertilization before completing the term of gestation and without deliberate interference. Spontaneous Abortion is with supportive care.