Embryo Development: Weeks 1 & 2

Development of the Embryo: Week 1 & 2 Overview

• Initially covers development up to week one, including fertilization, cleavage, and blastocyst formation.
• Then focuses on development through week two, emphasizing gastrulation and a brief look at implantation.
• The trophoblast, initially the outer cell mass, differentiates into the cytotrophoblast and syncytiotrophoblast.
• The inner cell mass differentiates into the embryoblast, which then forms the bilaminar disc.
• The trophoblast contributes to the chorion and placenta, facilitating oxygen and nutrient transfer and waste removal between mother and fetus.
• The speaker prefers to focus on one aspect at a time, starting with the trophoblast's significance before discussing the embryoblast's development into the trilaminar disc via gastrulation.

Blastocyst Implantation and Trophoblast Differentiation

• After ovulation triggered by LH, the secondary oocyte is fertilized by sperm in the ampulla, initiating cleavage as it moves toward the uterine cavity.
• The blastocyst implants in the uterine cavity using selectins and integrins to attach to the endometrium.
• The outer cell layer of the blastocyst, the trophoblast, differentiates into the cytotrophoblast and syncytiotrophoblast.
• Cytotrophoblast cells have well-defined cell margins and a nucleus, acting as a base for the syncytiotrophoblast.
• Some cytotrophoblast cells proliferate, losing their cell membranes, and their cytoplasm fuses to form a syncytium called the syncytiotrophoblast.
• Syncytiotrophoblast forms finger-like processes extending into the uterine lining.
• The syncytiotrophoblast releases hydrolytic enzymes to penetrate the uterine lining and connect with maternal blood vessels.
• This connection allows the embryo to receive oxygen, nutrients, and hormones from the mother's vascular system.
• Around days 21-24, the syncytiotrophoblast begins producing beta-HCG (human chorionic gonadotropin).
• Beta-HCG stimulates the corpus luteum to continue producing progesterone, preventing the shedding of the endometrial lining.
• Progesterone maintains the endometrial lining by preventing spasms in the uterine blood vessels, ensuring proper implantation and nourishment of the embryo.

Bilaminar Disc Formation and Gastrulation Initiation

• The bilaminar disc consists of two layers: the epiblast (top layer) and the hypoblast (bottom layer).

• Below the bilaminar disc is the primitive yolk sac, and above it is the amniotic cavity, filled with fluid.

• The prochordal plate, where the epiblast and hypoblast are fused, indicates the cranial end of the embryo; the caudal end is opposite it.

• Signaling processes cause epiblast cells to thicken, forming the primitive streak, with a knob-like structure called the primitive node at its cranial end.

• Cells in the center of the primitive streak and node die, creating a space/cavity.

• • Epiblast cells located in proximity to the primitive streak play a crucial role in early embryonic development by secreting significant signaling molecules, including fibroblast growth factor 8 (FGF8) and transforming growth factor beta (TGF-β). These factors are essential to orchestrate the differentiation and migration of embryonic cells during the gastrulation process.
  • FGF8 interacts specifically with tyrosine kinase receptors on neighboring epiblast cells. This interaction results in the activation of various intracellular signaling pathways, one of which leads to the expression and activation of the snail 1 protein. The activation of snail 1 is a vital step in the transformation of epithelial cells into a more migratory and invasive phenotype.
  • The snail 1 protein functions as a transcription factor that represses the expression of E-cadherin, an important protein that mediates cell-cell adhesion within epithelial tissues. The inhibition of E-cadherin is a key regulatory mechanism that facilitates the breakdown of cellular adhesiveness, making it an essential process for the cells undergoing epithelial-to-mesenchymal transition (EMT).

  Formation of the Three Germ Layers

  • The downregulation of E-cadherin allows epiblast cells to detach from their neighbors and migrate toward the primitive streak through a process known as epithelial migration. This cellular motility is a critical feature of gastrulation, wherein the epiblast cells undergo coordinated movement, proliferate, and begin to differentiate into the three primary germ layers: ectoderm, mesoderm, and endoderm.

  • This migration process is essential not only for the formation of the embryonic germ layers but also for the overall organization of the developing embryo, as it sets the foundation for various structural and functional tissues that will emerge later in development.

  • Further exploration reveals that the interplay of FGF8 and TGF-β signaling pathways is instrumental in refining the behavior of epiblast cells. These signals not only facilitate migration but also help in determining the fate of cells as they transition towards becoming specialized cell types in the respective germ layers.

  • The specific mechanisms employed by snail 1 in mediating these changes underscore the intricate control of gene expression during early development, highlighting the significance of both extrinsic factors, like FGF8 and TGF-β, and intrinsic factors, such as snail 1, in shaping embryonic development. Understanding the dynamics of these interactions is pivotal for grasping the complexities of developmental biology and related fields.

  Conclusion

  • The processes initiated by FGF8 and TGF-β, alongside the modulation of E-cadherin through snail 1, collectively contribute to key morphological and functional outcomes during gastrulation, setting the stage for subsequent embryonic development and establishing the layers from which all tissues and organs will arise.

  The primitive streak forms the primitive groove as cells die within the streak, and the primitive node forms the primitive pit.

• Epiblast cells migrate through the primitive groove, displacing the hypoblast cells and forming the endoderm.

• After the endoderm forms, more epiblast cells migrate through the primitive groove, forming the mesoderm, leaving the remaining epiblast as the ectoderm.

• Gastrulation is the process of transforming the bilaminar disc into a trilaminar disc.

Notochord Development and Mesoderm Differentiation

• Ectodermal cells migrate through the primitive pit, moving cranially towards the prochordal plate and forming the notochord, a tubular structure.
• There is no mesoderm present where the notochord is located.
• There is also no mesoderm at the prochordal plate or the cloaca plate.
• The notochord induces neuralation, the process of forming the neural tube, and its adult remnant is the nucleus pulposus of the intervertebral discs.
• The intervertebral disc is comprised of the annulus fibrosus, and the nucleus pulposus, which is a jelly-like material, is the adult remnant of the notochord.
• The mesoderm differentiates into three components: paraxial (central), intermediate (kidneys and gonads), and lateral plate.
• Lateral plate mesoderm differentiates into splanchnic (surrounding GI organs) and somatic (forming body structures).
• The ectoderm forms the skin and nervous system, and the endoderm forms the lining of the GI tract and accessory organs/glands.