BIU Human Embryology Lesson 3 Second and Third Weeks of Pregnancy PDF

### **Bilaminar Disc Formation** **Overview**: - **Bilaminar Disc Formation** occurs around **day 8-9** post-fertilization, shortly after implantation. The inner cell mass (embryoblast) differentiates into two distinct layers, creating a flat, two-layered structure known as the **bil...

### **Bilaminar Disc Formation** **Overview**: - **Bilaminar Disc Formation** occurs around **day 8-9** post-fertilization, shortly after implantation. The inner cell mass (embryoblast) differentiates into two distinct layers, creating a flat, two-layered structure known as the **bilaminar embryonic disc**. - This process is fundamental in establishing the basis for the embryo\'s body plan and is the precursor to more complex differentiation. **Two Layers of the Bilaminar Disc**: 1. **Epiblast**: - The epiblast is a columnar layer of cells located on the dorsal side of the bilaminar disc. - It plays a crucial role as it will eventually give rise to the entire embryo proper and form the basis of all three germ layers (ectoderm, mesoderm, and endoderm). 2. **Hypoblast**: - The hypoblast, a layer of cuboidal cells, lies on the ventral side of the bilaminar disc. - While it contributes to extraembryonic structures such as the yolk sac, it does not directly form part of the embryo. **Associated Structures**: - **Amniotic Cavity**: Forms within the epiblast layer. This cavity will expand to surround the embryo, providing a cushion and a stable environment for its development. - **Primary Umbilical Vesicle (Primary Yolk Sac)**: Formed by hypoblast-derived cells and serves as a site of initial nutrient exchange and hematopoiesis (blood formation) before the placenta fully forms. **Importance of Bilaminar Disc Formation**: - The bilaminar disc marks the establishment of the embryo\'s **dorsal-ventral polarity**. - It prepares the embryo for **gastrulation**, the next critical stage of development. ### 2. **Trilaminar Disc Formation (Gastrulation)** **Overview**: - **Gastrulation** is a transformative phase that occurs around **day 14-16** and involves the conversion of the bilaminar disc into a **trilaminar disc** with three primary germ layers. This process marks the beginning of the embryo\'s structural complexity and organization. - The three germ layers are **ectoderm**, **mesoderm**, and **endoderm**, each of which gives rise to specific tissues and organs. **Key Steps in Gastrulation**: - A structure called the **primitive streak** appears along the midline of the epiblast, extending from the posterior (caudal) to the anterior (cranial) end of the embryo. - The formation of the primitive streak is a critical event, as it determines the body's left-right and cranio-caudal (head-to-tail) axis. - Epiblast cells migrate toward and through the primitive streak, moving into the space between the epiblast and hypoblast layers. - These migrating cells displace hypoblast cells to form the **definitive endoderm**. - Some migrating cells form a middle layer, the **mesoderm**. - Remaining epiblast cells differentiate into the **ectoderm**. **The Three Germ Layers**: 1. **Ectoderm**: - Develops into the **central and peripheral nervous systems** (brain, spinal cord, and nerves), **epidermis** (outer skin layer), and **sense organs** (eyes, ears). 2. **Mesoderm**: - Gives rise to structures such as the **musculoskeletal system** (bones and muscles), **cardiovascular system**, **reproductive organs**, and **kidneys**. 3. **Endoderm**: - Forms the **epithelial lining** of the gastrointestinal and respiratory tracts and organs like the **liver, pancreas, and thyroid gland**. **Significance of Gastrulation**: - Gastrulation is essential for setting up the body's **anterior-posterior** and **dorsal-ventral axes**. - This process ensures that cells are positioned according to the body's future layout and are preprogrammed to develop into specific organs and tissues. ### 3. **Formation of the Amniotic Cavity** **Overview**: The **amniotic cavity** is a fluid-filled space that forms early in embryonic development, playing a vital role in protecting and nurturing the growing embryo. It develops within the **epiblast** layer of the bilaminar embryonic disc during the second week of gestation, shortly after implantation of the embryo in the uterine wall. ### Formation of the Amniotic Cavity - **Origin**: The amniotic cavity begins as a small space within the epiblast layer of the bilaminar disc. - **Amnioblast Cells**: Cells from the epiblast differentiate into **amnioblasts**, which line the roof of the cavity. These cells, along with the surrounding extraembryonic mesoderm, eventually form the **amniotic membrane** that encases the cavity. - **Expansion**: As the embryo develops, the amniotic cavity enlarges, filling with amniotic fluid, and eventually surrounds the entire embryo, providing a protective cushion. ### Functions of the Amniotic Cavity and Amniotic Fluid The amniotic cavity, along with its amniotic fluid, serves several critical functions: 1. **Protection**: The fluid-filled cavity cushions the embryo (and later, the fetus) from mechanical shocks and injuries, helping to prevent physical trauma. 2. **Temperature Regulation**: The amniotic fluid helps maintain a stable temperature, protecting the embryo from external temperature fluctuations. 3. **Prevention of Adhesions**: The amniotic membrane prevents the developing structures of the embryo from adhering to surrounding tissues, allowing for unrestricted growth and movement. 4. **Fluid Balance**: Amniotic fluid assists in maintaining the balance of fluids and electrolytes around the embryo and later, the fetus. 5. **Growth and Development**: It provides a medium for the embryo to move, which is important for musculoskeletal development. ### Developmental Changes - **Early Pregnancy**: The amniotic cavity remains relatively small during the early stages but expands quickly as the embryo grows. - **Later Stages**: By the end of the first trimester, the amniotic cavity grows to surround the fetus entirely, taking up most of the space within the gestational sac. The amniotic fluid volume increases throughout pregnancy, reaching its peak in the second trimester. ### Clinical Importance Monitoring the amniotic cavity and fluid is essential for assessing fetal well-being during pregnancy. Abnormalities in amniotic fluid volume---such as **oligohydramnios** (too little fluid) or **polyhydramnios** (too much fluid)---can indicate potential complications and may require medical intervention. In summary, the amniotic cavity is a crucial component of the early gestational environment, providing a safe, temperature-regulated, and cushioned space for the embryo and fetus to grow and develop throughout pregnancy. - The **amniotic cavity** forms soon after implantation, around **day 8-9**. - It is a fluid-filled cavity located between the epiblast and trophoblast, specifically within the epiblast layer. **Formation Process**: 1. Cells within the epiblast begin to separate and form a space filled with fluid, which becomes the amniotic cavity. 2. The **amniotic membrane** or **amnion** is established as a thin layer of cells, called **amnioblasts**, that lines the cavity. **Functions of the Amniotic Cavity**: - **Protection**: The amniotic fluid within the cavity acts as a cushion, protecting the developing embryo from mechanical injury. - **Hydration and Temperature Regulation**: The amniotic fluid maintains an optimal environment for fetal growth, preserving temperature and preventing dehydration. - **Freedom of Movement**: It allows the embryo to move, which is essential for proper musculoskeletal development. **Clinical Relevance**: - Abnormalities in the amniotic cavity, such as **oligohydramnios** (too little amniotic fluid) or **polyhydramnios** (too much amniotic fluid), can lead to developmental complications. ### 4. **Embryonic Disc** **Overview**: The **embryonic disc** is a flat, circular structure that forms in early embryonic development and plays a crucial role in establishing the body plan of the embryo. It emerges after implantation during the second week of gestation and is composed of a double-layered group of cells, which is why it is often referred to as the **bilaminar embryonic disc**. This disc undergoes further differentiation to give rise to all tissues and organs of the developing fetus. ### Structure of the Bilaminar Embryonic Disc The bilaminar disc consists of two distinct layers of cells: ### Formation and Function of the Embryonic Disc The embryonic disc forms from the inner cell mass of the blastocyst, which divides into the epiblast and hypoblast during the second week of gestation. This two-layered disc is initially flat and lies between two cavities: the **amniotic cavity** above and the **yolk sac** (umbilical vesicle) below. #### Key Functions of the Bilaminar Embryonic Disc: - **Foundation of the Embryo**: The bilaminar disc establishes the initial layout for the body axis of the embryo, with distinct dorsal (back) and ventral (front) sides. - **Support for Germ Layer Formation**: The epiblast layer of the disc serves as a precursor to all three germ layers (ectoderm, mesoderm, and endoderm), which will form during gastrulation in the third week of development. - **Support of Early Development**: The hypoblast and its derivatives help support early embryonic development by contributing to extraembryonic structures, such as the yolk sac. ### Transition to the Trilaminar Disc In the third week of gestation, the bilaminar disc undergoes a process called **gastrulation**, transforming into a **trilaminar embryonic disc** with three germ layers: - **Ectoderm** (which forms the skin, nervous system, and related structures), - **Mesoderm** (which forms muscles, bones, cardiovascular structures, and connective tissues), and - **Endoderm** (which forms the lining of the gastrointestinal and respiratory systems, among others). ### Clinical Relevance The embryonic disc's formation is a critical stage, as errors in this process can result in developmental abnormalities or early pregnancy loss. Its appearance and characteristics can also be observed via ultrasound in early pregnancy assessments. In summary, the embryonic disc represents an essential, foundational stage in human development, laying down the primary blueprint for the entire body structure and leading into the complex processes that form the tissues and organs of the developing embryo. - The **embryonic disc** is initially a bilaminar structure consisting of the epiblast and hypoblast layers. It is later transformed into a trilaminar disc during gastrulation. - This disc forms the foundational structure of the embryo, setting the stage for further growth and differentiation. **Stages of Development**: 1. **Bilaminar Disc**: Consists of the two germ layers (epiblast and hypoblast). 2. **Trilaminar Disc**: Develops from the bilaminar disc through gastrulation, resulting in three germ layers (ectoderm, mesoderm, and endoderm). **Importance of the Embryonic Disc**: - It represents the earliest stage of body plan organization and will undergo further folding to establish the basic structure of the body. - Each germ layer of the disc will later differentiate into specific organs and tissues. ### 5. **Umbilical Vesicle (Yolk Sac)** **Overview**: The **umbilical vesicle**, more commonly known as the **yolk sac**, is an essential structure in early embryonic development. It originates from the blastocyst and forms during the second week of gestation, playing crucial roles in nurturing and developing the embryo during the early stages before the placenta becomes fully functional. In humans, it has evolved beyond the nutrient-storage function seen in other species, serving mainly supportive roles in the initial phases of pregnancy. ### Structure and Formation of the Yolk Sac The yolk sac is initially formed from the **hypoblast cells** within the blastocyst. These cells line the blastocyst cavity and, together with the **extraembryonic mesoderm**, form the primary yolk sac, which later undergoes some changes and eventually forms the secondary yolk sac. This secondary yolk sac is the structure that persists throughout early embryogenesis. ### Functions of the Yolk Sac While the human yolk sac does not store yolk for nutrition like in birds or reptiles, it still performs several critical functions in the embryo\'s development: ### Importance and Clinical Significance The yolk sac is visible on early ultrasound and is often one of the first signs of a healthy early pregnancy. A normally sized and shaped yolk sac generally indicates a viable pregnancy, while abnormalities in size or structure can sometimes signal developmental issues. The yolk sac degenerates by the end of the first trimester as the placenta takes over its functions. By the time of birth, it has generally disappeared or remains only as a small remnant without functional importance. - Also known as the **primary yolk sac**, the umbilical vesicle is an early structure that forms from the hypoblast. - As development proceeds, it becomes the **secondary yolk sac** after extraembryonic mesoderm forms and the primitive yolk sac is pinched off. **Functions**: 1. **Nutrition**: Initially serves as a source of nutrients for the early embryo before the placental circulation fully develops. 2. **Hematopoiesis**: Acts as an early site for blood cell production until the liver takes over this function. 3. **Primordial Germ Cells**: Germ cells are initially housed here before migrating to the gonads. **Clinical Importance**: - Abnormal development or persistence of yolk sac structures, such as **vitelline duct anomalies** (e.g., Meckel's diverticulum), can result in congenital defects. ### 6. **Development of the Chorionic Sac** **Overview**: The **chorionic sac** is a membrane surrounding the developing embryo and the other embryonic structures, such as the amniotic cavity and yolk sac, during early pregnancy. It is formed from the **trophoblast** (the outer cell layer of the blastocyst) and the **extraembryonic mesoderm**, and it plays a critical role in early pregnancy development by contributing to the fetal portion of the **placenta**. ### Structure and Components The chorionic sac is made up of two main layers: ### Functions of the Chorionic Sac: ### Importance in Pregnancy The chorionic sac is typically visible on an early ultrasound and is one of the first signs of pregnancy. Its development is essential for the survival of the embryo, as it forms the basis for a functional placenta. Abnormal development or issues with the chorionic sac, such as chorionic sac detachment, can lead to complications in pregnancy, including potential miscarriage or placental insufficiency. - The **chorionic sac** is an essential component of early embryonic development, surrounding the amniotic cavity, yolk sac, and the embryo itself. - It is derived from both the trophoblast and the extraembryonic mesoderm and gives rise to the fetal portion of the placenta. **Structure and Formation**: 1. **Trophoblast Differentiation**: The trophoblast divides into two layers: - **Cytotrophoblast**: The inner layer that provides a source of cells for the expanding chorion. - **Syncytiotrophoblast**: The outer layer that invades the maternal endometrium and contributes to placental development. 2. **Extraembryonic Mesoderm**: This mesoderm forms a layer around the yolk sac and amniotic cavity, contributing to the chorionic structure. **Functions**: 1. **Protection**: It acts as a protective membrane around the embryo. 2. **Hormone Production**: The syncytiotrophoblast produces hormones like **hCG** to maintain pregnancy. 3. **Nutrient Exchange**: The chorion plays a crucial role in the formation of the placenta, facilitating maternal-fetal nutrient and gas exchange. ### 7. **Implantation Sites of Blastocysts** **Overview**: - **Implantation** is the process by which the blastocyst embeds itself into the uterine lining, a critical step in establishing a successful pregnancy. - Ideally, implantation occurs in the upper posterior wall of the uterus around **day 6-7 post-fertilization**. **Process of Implantation**: 1. **Attachment**: The blastocyst adheres to the endometrial epithelium, mediated by interactions between trophoblast cells and the uterine lining. 2. **Invasion**: The syncytiotrophoblast layer of the trophoblast actively invades the uterine tissue, embedding the blastocyst within the endometrium. 3. **Establishment of Blood Supply**: As the trophoblast invades further, it erodes maternal blood vessels, ### 1. **Scenario: A 24-year-old woman is undergoing an early pregnancy ultrasound at 6 weeks. The physician notices a well-formed bilaminar embryonic disc, but the development of the three germ layers has not yet occurred. Which of the following processes is expected to take place next?** - A\) Gastrulation - B\) Neurulation - C\) Organogenesis - D\) Somite formation - E\) Formation of the primitive streak - **Correct Answer: A) Gastrulation** ### 2. **Scenario: During an early ultrasound of a 6-week pregnant woman, the physician observes an abnormal yolk sac that is significantly larger than expected. Which of the following is the most likely outcome associated with this abnormality?** - A\) Ectopic pregnancy - B\) Healthy pregnancy with minor variation - C\) Chromosomal abnormality or miscarriage - D\) Multiple gestation - E\) Placental insufficiency - **Correct Answer: C) Chromosomal abnormality or miscarriage** ### 3. **Scenario: A 30-year-old woman reports missing her period and is found to have a positive pregnancy test. An ultrasound shows a bilaminar embryonic disc without visible yolk sac or fetal heart tones. What is the most likely diagnosis?** - A\) Ectopic pregnancy - B\) Complete molar pregnancy - C\) Intrauterine pregnancy with a delayed development - D\) Spontaneous abortion - E\) Blighted ovum - **Correct Answer: E) Blighted ovum** ### 4. **Scenario: A physician is examining a pregnant woman in her first trimester. She finds that the chorionic sac is well-formed, with the syncytiotrophoblast layer actively secreting hCG. What is the most likely role of the syncytiotrophoblast at this stage?** - A\) Embryonic development - B\) Initiation of blood circulation - C\) Maintaining the uterine lining for pregnancy continuation - D\) Formation of germ layers - E\) Formation of the amniotic fluid - **Correct Answer: C) Maintaining the uterine lining for pregnancy continuation** ### 5. **Scenario: A 28-year-old woman presents with abdominal pain and bleeding at 7 weeks of pregnancy. Ultrasound shows an irregular chorionic sac with abnormal trophoblast invasion. What is the likely complication?** - A\) Molar pregnancy - B\) Placenta accreta - C\) Ectopic pregnancy - D\) Threatened miscarriage - E\) Normal pregnancy - **Correct Answer: A) Molar pregnancy** ### 6. **Scenario: During a routine prenatal ultrasound, the physician notes that the developing embryo is situated near the fundus of the uterus. Where is the most likely implantation site of the blastocyst in a healthy pregnancy?** - A\) Anterior uterine wall - B\) Posterior uterine wall near the fundus - C\) Lower uterine segment - D\) Fallopian tube - E\) Cervix - **Correct Answer: B) Posterior uterine wall near the fundus** ### 7. **Scenario: A patient undergoing an ultrasound at 5 weeks of gestation shows a visible embryonic disc with a clear delineation between the epiblast and hypoblast layers. What is the most important role of the hypoblast at this stage?** - A\) Formation of the ectoderm - B\) Formation of the yolk sac - C\) Development of the placenta - D\) Formation of the heart - E\) Differentiation into mesoderm - **Correct Answer: B) Formation of the yolk sac** ### 8. **Scenario: A 32-year-old pregnant woman is diagnosed with a defect in the formation of her embryonic disc. This defect primarily affects the differentiation of the epiblast. Which of the following structures is most likely to be underdeveloped in this case?** - A\) Amniotic cavity - B\) Extraembryonic mesoderm - C\) Primitive streak - D\) Germ layers (ectoderm, mesoderm, endoderm) - E\) Yolk sac - **Correct Answer: D) Germ layers (ectoderm, mesoderm, endoderm)** ### 9. **Scenario: During early pregnancy, the trophoblast layer of a blastocyst begins to invade the uterine lining, and the syncytiotrophoblast begins secreting hCG. What is the primary function of hCG at this stage?** - A\) Support embryonic growth - B\) Inhibit ovulation - C\) Stimulate maternal immune response - D\) Maintain corpus luteum function - E\) Begin organogenesis - **Correct Answer: D) Maintain corpus luteum function** ### 10. **Scenario: A 25-year-old woman presents with an early pregnancy ultrasound at 6 weeks. The imaging shows a visible yolk sac, amniotic cavity, and trophoblast cells. Which of the following is the primary role of the yolk sac at this stage of pregnancy?** - A\) Nutrient storage - B\) Early blood cell formation - C\) Formation of the placenta - D\) Development of the central nervous system - E\) Induction of limb development - **Correct Answer: B) Early blood cell formation**