General Embryology

General Embryology I. Introduction to Embryonic Development Embryonic development is a highly orchestrated process involving the transformation of a single fertilized cell into a complex, multi-cellular organism. This process is divided into distinct periods: Germinal Period Weeks 12 Characterized by fertilization, cleavage, and implantation. Embryonic Period Weeks 38 Marked by gastrulation, neurulation, and organogenesis. Fetal Period Weeks 938 Involves growth and maturation of established structures. These periods are crucial as they lay the foundation for the formation of all major organs and tissues in the body. II. Germinal Period (Weeks 1-2) A. Formation and Implantation of the Blastocyst Early Cleavage Stages: After fertilization, the zygote undergoes a series of rapid cell divisions called cleavage. These divisions produce smaller cells (blastomeres) without increasing the overall size of the embryo. Morula Stage: As cleavage continues, a solid ball of cells known as the morula (composed of about 16 to 32 cells) is formed. This stage occurs approximately 3-4 days after fertilization. Fluid Accumulation: As the morula develops, fluid begins to accumulate in the intercellular spaces between the blastomeres. This fluid is derived from the uterine General Embryology 1 environment and is pumped into the morula by the cells. Formation of the Blastocoel: The accumulation of fluid leads to the formation of a central cavity called the blastocoel. The cells continue to pump fluid into this cavity, causing it to expand. Blastocyst Structure: As the blastocoel expands, the structure reorganizes into a hollow sphere known as the blastocyst. Blastocyst Formation: About five days after fertilization, the embryo reaches the blastocyst stage. A blastocyst consists of: Inner Cell Mass (ICM): A group of cells that will develop into the embryo itself. Trophoblast: An outer layer of cells that will form part of the placenta, facilitating nutrient and waste exchange between the mother and the embryo. Attachment to Uterine Wall: The blastocyst attaches to the endometrial lining of the uterus, initiating implantation. B. Differentiation of Inner Cell Mass and Trophoblast Once implantation begins, the cells of the blastocyst start to specialize into different types. General Embryology 2 Trophoblast Differentiation: The outer trophoblast differentiates into two cell types: Cytotrophoblast Definition: The inner layer of the trophoblast composed of mononucleated cells. Functions: Proliferation: Acts as a reservoir of proliferative cells that continuously divide and contribute to the growth of the trophoblast. Migration: These cells migrate into the outer layer, forming the syncytiotrophoblast. Syncytiotrophoblast Definition: A multinucleated layer formed by the fusion of cytotrophoblast cells. Key Features: Invasive Properties: Exhibits behavior similar to cancer cells, allowing for the invasion of the uterine epithelium. Hormone Secretion: Produces essential hormones, including human chorionic gonadotropin (hCG), which supports pregnancy by maintaining the corpus luteum and regulating maternal physiology. General Embryology 3 The syncytiotrophoblast invades the maternal tissue, creating spaces known as trophoblastic lacunae, which fill with maternal blood. Finger-like Projections: The syncytiotrophoblast forms projections that extend into the lacunae, covering a core of cytotrophoblast cells. These structures are known as primary villi, acting as the site of nutrient absorption and waste removal. 1. Inner Cell Mass (ICM) Differentiation: The ICM — embryoblast differentiates into two layers: Epiblast (Dorsal Layer): This layer will give rise to the embryo itself, forming all the tissues and organs of the body. Hypoblast (Ventral Layer): Contributes to the formation of extraembryonic structures, such as the yolk sac, which provides nutrients to the growing embryo. C. Formation of Bilaminar Germ Disc Bilaminar Structure: The ICM forms a bilaminar (two-layered) germ disc composed of the epiblast and hypoblast. General Embryology 4