Embryology - 2.1
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Questions and Answers

What is the chromosomal makeup of a human somatic cell?

46 (2 sex chromosomes, 44 autosomes and 22 homologous sets)

What is the male sex cell called?

Spermatozoa

What are female sex cells called?

Ova

What is the process that forms 4 non-identical haploid sex cells?

<p>Meiosis</p> Signup and view all the answers

Which of the following forms 2 identical diploid body cells?

<p>Mitosis</p> Signup and view all the answers

The ______ forms organs such as the colon and stomach.

<p>Endoderm</p> Signup and view all the answers

The ______ forms outer linings such as skin and the nervous system.

<p>Ectoderm</p> Signup and view all the answers

Meiosis results in diploid cells.

<p>False</p> Signup and view all the answers

What is the primary germ layer that forms the circulatory system?

<p>Mesoderm</p> Signup and view all the answers

What is Turner syndrome characterized by?

<p>XO chromosome pattern</p> Signup and view all the answers

Down syndrome is also known as which of the following?

<p>Trisomy 21</p> Signup and view all the answers

What is the main purpose of meiosis in human reproduction?

<p>To create haploid sex cells with genetic diversity</p> Signup and view all the answers

Which stage of prenatal development is primarily characterized by morphogenesis?

<p>Stage 2</p> Signup and view all the answers

What is the role of the ectoderm in embryonic development?

<p>To form outer structures such as skin and the nervous system</p> Signup and view all the answers

During which week does the formation of the blastocyst occur?

<p>Week 1</p> Signup and view all the answers

What type of genetic information is primarily achieved through the S phase in the cell cycle?

<p>The duplication of DNA for cell division</p> Signup and view all the answers

Which primary germ layer is responsible for forming the renal system, among other structures?

<p>Mesoderm</p> Signup and view all the answers

What type of chromosomal alteration is responsible for Down syndrome?

<p>Trisomy of chromosome 21</p> Signup and view all the answers

Which of the following structures is derived from the lateral plate mesoderm?

<p>Somatic layer of dermis</p> Signup and view all the answers

What is a common feature of Klinefelter syndrome?

<p>Presence of an extra X chromosome</p> Signup and view all the answers

Which condition is characterized by a single X chromosome, leading to distinctive features in females?

<p>Turner syndrome</p> Signup and view all the answers

At what stage of the menstrual cycle does the release of luteinizing hormone occur?

<p>Day 14</p> Signup and view all the answers

What role does estrogen play in the secretion of luteinizing hormone?

<p>It contributes to the positive feedback loop stimulating luteinizing hormone.</p> Signup and view all the answers

Which structure is primarily involved in the release of luteinizing hormone?

<p>Anterio pituitary</p> Signup and view all the answers

What is the primary function of luteinizing hormone during ovulation?

<p>To increase fluid production in the ovary.</p> Signup and view all the answers

Which phase does a secondary oocyte enter after ovulation but before fertilization?

<p>Metaphase II</p> Signup and view all the answers

What anatomical structure functions to guide the oocyte into the ampulla of the fallopian tube?

<p>Fimbriae</p> Signup and view all the answers

Which hormone directly stimulates the production of luteinizing hormone from the anterior pituitary?

<p>Ganadotropin-releasing hormone</p> Signup and view all the answers

What is the function of prostaglandins in the ovulation process?

<p>To dilate blood vessels around the follicle.</p> Signup and view all the answers

What initiates the process of ovulation in the ovarian follicle?

<p>Release of luteinizing hormone.</p> Signup and view all the answers

What is the main outcome of the action of metalloproteinases during ovulation?

<p>Degradation of connective tissue surrounding the follicle.</p> Signup and view all the answers

Which of the following is true about the secondary oocyte shortly after ovulation?

<p>It enters a resting phase until fertilization.</p> Signup and view all the answers

What anatomical position is the ampulla of the fallopian tube?

<p>At the midpoint of the fallopian tube.</p> Signup and view all the answers

In which phase does the oocyte remain until a sperm cell fertilizes it?

<p>Metaphase II</p> Signup and view all the answers

What are the two components that the bilaminar disk differentiates into?

<p>Epi blast and hypo blast</p> Signup and view all the answers

Which structure is chiefly responsible for forming the placenta?

<p>Trophoblast</p> Signup and view all the answers

During which developmental stage does the transition from the bilaminar disk to the trilaminar disk occur?

<p>Second week</p> Signup and view all the answers

What is the process by which the trophoblast penetrates the zona pellucida?

<p>Implantation</p> Signup and view all the answers

What does the term 'serial blast' refer to in the context of embryonic development?

<p>An early stage in the development of the trophoblast</p> Signup and view all the answers

What process must the sperm undergo before it can effectively fertilize the secondary oocyte?

<p>Capacitation</p> Signup and view all the answers

The fusion of paternal and maternal chromosomes occurs after the sperm releases which structure into the oocyte cytoplasm?

<p>Nucleus</p> Signup and view all the answers

How many chromosomes does the zygote contain after fertilization?

<p>46</p> Signup and view all the answers

What is the term used to describe the series of divisions that the zygote undergoes following fertilization?

<p>Cleavage</p> Signup and view all the answers

What is the initial stage of cellular development called after the zygote stage?

<p>Two-cell stage</p> Signup and view all the answers

What does the zona pellucida specifically facilitate during fertilization?

<p>Sperm attachment</p> Signup and view all the answers

As the zygote divides, what is the result of the cleavage process?

<p>Increase in cell number without growth</p> Signup and view all the answers

What occurs within the acrosome of the sperm cell during fertilization?

<p>Enzyme release</p> Signup and view all the answers

What are the two primary sources of chromosomes in a zygote?

<p>Egg and sperm</p> Signup and view all the answers

Which of the following best describes the initial phase after the zygote undergoes cleavage?

<p>Morula stage</p> Signup and view all the answers

What triggers the activation of the sperm once it contacts the oocyte?

<p>ZP3 receptors</p> Signup and view all the answers

How do the chromosomes from the sperm and oocyte contribute to the formation of the zygote?

<p>They combine to make a diploid set</p> Signup and view all the answers

After fertilization, the zygote goes through which of the following stages first?

<p>Two-cell stage</p> Signup and view all the answers

During which process does the zygote form a hollow structure of cells?

<p>Cleavage</p> Signup and view all the answers

What is the term for the hollow ball of cells with 16 or more cells during prenatal development?

<p>Morula</p> Signup and view all the answers

Which of the following cell types is formed from the inner cell mass of the blastocyst?

<p>Embryo blast</p> Signup and view all the answers

During the transition from the morula to the blastocyst, what process occurs?

<p>Blastulation</p> Signup and view all the answers

What is the outer cell mass of the blastocyst called after it becomes specialized?

<p>Trophoblast</p> Signup and view all the answers

Which specific structure is responsible for forming part of the placenta?

<p>Trophoblast</p> Signup and view all the answers

What is the term for the differentiation of the embryo blast into a flat structure?

<p>Bilaminar disk</p> Signup and view all the answers

What happens during the cleavage stage of prenatal development?

<p>Cell division without growth</p> Signup and view all the answers

Which type of trophoblast layer is responsible for implantation?

<p>Saito trophoblast</p> Signup and view all the answers

What is the initial step in the process of prenatal development?

<p>Fertilization</p> Signup and view all the answers

Which cell type is formed from the outer layer once it differentiates from the pre-blastocyst structure?

<p>Syncytium trophoblast</p> Signup and view all the answers

How many cell divisions occur during the cleavage stage?

<p>Multiple divisions occur</p> Signup and view all the answers

What type of cells are referred to as blastomeres?

<p>Cells in the morula stage</p> Signup and view all the answers

What does the term 'blastulation' refer to?

<p>Transformation of the morula into the blastocyst</p> Signup and view all the answers

What is the role of the outer cell mass in early embryonic development?

<p>Forms part of the placenta</p> Signup and view all the answers

What is the primary fluid-filled cavity that develops above the epiblast layer during embryogenesis?

<p>Amniotic cavity</p> Signup and view all the answers

Which structure forms the cranial orientation in the developing bilaminar disk?

<p>Precordal plate</p> Signup and view all the answers

What are the two layers that make up the bilaminar disk?

<p>Epiblast and hypoblast</p> Signup and view all the answers

What role does the primitive streak serve during the embryonic development process?

<p>It establishes the basic body plan.</p> Signup and view all the answers

What is the role of the syncytial trophoblast during implantation?

<p>To facilitate the connection between the embryo and maternal blood vessels</p> Signup and view all the answers

Which of the following correctly describes the hypoblast layer's function?

<p>It provides nutrients to the developing embryo.</p> Signup and view all the answers

What triggers the corpus luteum to continue producing progesterone?

<p>Release of human chorionic gonadotropin (beta hCG)</p> Signup and view all the answers

What structure develops at the caudal end of the bilaminar disk?

<p>Cloacal membrane</p> Signup and view all the answers

What is formed from the disintegration of the cytoplasm and nuclei of the cells in the finger-like processes of the trophoblast?

<p>Syncytium</p> Signup and view all the answers

What is the significance of the primitive yolk sac during embryonic development?

<p>It is involved in early blood cell production.</p> Signup and view all the answers

What is the significance of progesterone levels remaining elevated during the luteal phase?

<p>Prevents the spasm and rupture of uterine blood vessels</p> Signup and view all the answers

What embryonic structure provides early orientation for the body's cranial and caudal ends?

<p>Precordal plate</p> Signup and view all the answers

What is formed by the thickening of the epiblast cells as development progresses?

<p>Primitive streak</p> Signup and view all the answers

What happens to levels of progesterone as the luteal phase progresses and implantation has occurred?

<p>Levels must remain constant to prevent menstruation.</p> Signup and view all the answers

Which type of trophoblast forms defined cell boundaries with nuclei inside?

<p>Cytotrophoblast</p> Signup and view all the answers

What is the function of the buccopharyngeal membrane in embryonic development?

<p>It establishes the anterior opening of the digestive tract.</p> Signup and view all the answers

Which cell layer is primarily responsible for generating structures of the embryo while the hypoblast provides nutritional support?

<p>Epiblast</p> Signup and view all the answers

How does the syncytial trophoblast contribute to the embryo's nourishment?

<p>By facilitating maternal blood vessel connection</p> Signup and view all the answers

What initiates the process of implantation in the uterus?

<p>Contact between the embryo and endometrial cells</p> Signup and view all the answers

What does the term 'trilaminar disk' refer to in embryonic development?

<p>The structure after the formation of three germ layers.</p> Signup and view all the answers

How does the process of signaling influence the development of the epiblast layer?

<p>It causes cells to migrate to form the primitive streak.</p> Signup and view all the answers

What is the purpose of hydrolytic enzymes released by the syncytial trophoblast?

<p>To break down cellular membranes for implantation</p> Signup and view all the answers

What ultimately forms from the villi and maternal blood vessels during early pregnancy?

<p>Placenta</p> Signup and view all the answers

What important structure lies between the epiblast and the hypoblast layers in the bilaminar disk?

<p>Yolk sac</p> Signup and view all the answers

When does the secretion of beta hCG typically begin after implantation?

<p>Around day 21-24 post-implantation</p> Signup and view all the answers

Which process prevents the shredding of the endometrial lining in early pregnancy?

<p>Release of beta hCG by the syncytial trophoblast</p> Signup and view all the answers

What is the state of the blastocyst by the time of implantation?

<p>It reaches the uterine cavity with fluid accumulation.</p> Signup and view all the answers

What is the primary role of the trophoblast during early embryo development?

<p>It contributes to the formation of the placenta.</p> Signup and view all the answers

Which process ultimately leads to the formation of the trilaminar disk from the embryoblast?

<p>Gastrulation</p> Signup and view all the answers

What happens to the cell membranes of the syncytiotrophoblast during its development?

<p>They break down, allowing fusion of cytoplasms.</p> Signup and view all the answers

Which structure signifies the beginning of the implantation process?

<p>Blastocyst</p> Signup and view all the answers

What is the significance of selectins and integrins in embryo development?

<p>They enable the blastocyst to adhere to the uterine lining.</p> Signup and view all the answers

During which embryonic stage does the secondary oocyte remain in a suspended state?

<p>In metaphase II</p> Signup and view all the answers

What is primarily formed from the inner cell mass during the early stages of embryonic development?

<p>Bilaminar disk</p> Signup and view all the answers

What is the main characteristic of the inner cell mass after differentiation?

<p>It forms the bilaminar disk.</p> Signup and view all the answers

Which process contributes to the formation of the ectoderm during embryonic development?

<p>Gastrulation</p> Signup and view all the answers

What is the first structural formation that occurs after fertilization?

<p>Zygote</p> Signup and view all the answers

What is the role of growth factors released during the early stages of gastrulation?

<p>Induce formation of the neural tube</p> Signup and view all the answers

What significant event occurs as the trophoblast differentiates into the cytotrophoblast?

<p>Formation of the placenta</p> Signup and view all the answers

What significant structure forms from the mesoderm during embryonic development?

<p>Muscle and connective tissues</p> Signup and view all the answers

What initiates the cleavage process after fertilization?

<p>The activation of the zygote</p> Signup and view all the answers

What characterizes the syncytiotrophoblast compared to the cytotrophoblast?

<p>It is a multinucleated structure resulting from cell fusion.</p> Signup and view all the answers

Which embryonic layer is responsible for forming the lining of the gastrointestinal tract?

<p>Endoderm</p> Signup and view all the answers

During gastrulation, what happens to the hypoblast?

<p>It is replaced by endoderm</p> Signup and view all the answers

Which primary event occurs during early embryonic development immediately following cleavage?

<p>Blastocyst formation</p> Signup and view all the answers

What is the significance of the notochord in early embryo development?

<p>It serves as an early form of the spinal column</p> Signup and view all the answers

What leads to the multifactorial fusion of the cytoplasm in syncytiotrophoblast development?

<p>Cell membrane breakdown</p> Signup and view all the answers

What structure is formed as a result of the primitive groove during embryogenesis?

<p>Primitive streak</p> Signup and view all the answers

Which of the following layers is formed last during the process of gastrulation?

<p>Mesoderm</p> Signup and view all the answers

What major development occurs by week two of embryo development?

<p>Establishment of the trilaminar disk</p> Signup and view all the answers

Which structure is primarily formed by ectodermal cells migrating through the primitive pit?

<p>Notochord</p> Signup and view all the answers

What is the significance of the notochord during development?

<p>It induces neuralation.</p> Signup and view all the answers

What is the adult remnant of the notochord found in intervertebral discs?

<p>Nucleus pulposus</p> Signup and view all the answers

Which one of the following locations is devoid of mesoderm during embryonic development?

<p>Notochord</p> Signup and view all the answers

What does the lateral plate mesoderm primarily differentiate into?

<p>Splanchnic and somatic mesoderm</p> Signup and view all the answers

Which key feature defines the prochordal plate during embryonic development?

<p>It differentiates into the head region.</p> Signup and view all the answers

What does the term 'sagittal cut' refer to in the context of embryonic development?

<p>A slice that divides the body into right and left halves.</p> Signup and view all the answers

The three types of mesoderm derived from the lateral plate are involved in the formation of which structures?

<p>GI organs and connective tissue</p> Signup and view all the answers

What characteristic does the anterior end or cranial end of the embryo exhibit regarding mesoderm presence?

<p>It is lacking mesoderm.</p> Signup and view all the answers

What is the primary role of ectoderm during early development?

<p>Forming the nervous system and skin</p> Signup and view all the answers

Which structure is defined as the membrane that gives rise to the anal region?

<p>Cloacal membrane</p> Signup and view all the answers

Which part of the mesoderm is specifically important for kidney and gonad formation?

<p>Intermediate mesoderm</p> Signup and view all the answers

What is the primary component within the intervertebral disc derived from the notochord?

<p>Nucleus pulposus</p> Signup and view all the answers

Which structure is responsible for distinguishing cranial and caudal ends of an embryo?

<p>Prochordal plate</p> Signup and view all the answers

What happens to some of the cells in the center of the primitive streak as development progresses?

<p>They start dying, creating a cavity.</p> Signup and view all the answers

What primary chemical do the cells near the edge of the primitive streak secrete?

<p>Fibroblast growth factor 8.</p> Signup and view all the answers

What is the role of the protein snail one in embryonic development?

<p>Inhibits the formation of E-cadherins.</p> Signup and view all the answers

What term is used to describe the movement of epithelial cells as they detach from one another?

<p>Epithelial migration.</p> Signup and view all the answers

What is the name of the space formed in the primitive streak after some cell death occurs?

<p>Primitive groove.</p> Signup and view all the answers

What happens to the epiblast cells as they move through the primitive groove?

<p>They replace the hypoblast layer.</p> Signup and view all the answers

What is the designation of the layer that replaces the previous hypoblast cells after migration?

<p>Endoderm.</p> Signup and view all the answers

What takes place immediately after fibroblast growth factor 8 binds to its receptor on epiblast cells?

<p>An intracellular signaling process is initiated.</p> Signup and view all the answers

Which of the following defines the primitive node in embryonic development?

<p>A knob-like thickening of tissue.</p> Signup and view all the answers

How does the fibroblast growth factor 8 influence the fate of neighboring epithelial cells?

<p>It allows for their migration.</p> Signup and view all the answers

What is the significance of epithelial migration during embryonic development?

<p>It allows for the formation of new tissues and structures.</p> Signup and view all the answers

What change occurs to the primitive node following the activation of E-cadherins?

<p>It becomes a primitive groove.</p> Signup and view all the answers

Which process is primarily triggered by the activity of fibroblast growth factor 8?

<p>Cellular migration toward the primitive streak.</p> Signup and view all the answers

What is the ultimate outcome of the changes initiated by the fibroblast growth factor 8 during development?

<p>Replacement of tissue layers.</p> Signup and view all the answers

What is the primary function of fibroblast growth factor in the migration of epiblast cells?

<p>Inhibits adherence of epiblast cells to one another</p> Signup and view all the answers

Which new layer is formed after all hypoblast cells are replaced?

<p>Endoderm</p> Signup and view all the answers

How do epiblast cells primarily migrate during the gastrulation process?

<p>Downwards through the primitive groove and laterally</p> Signup and view all the answers

Which process transitions a bilaminar disk into a trilaminar disk?

<p>Gastrulation</p> Signup and view all the answers

Which layer is retained above the migrating epiblast cells during gastrulation?

<p>Ectoderm</p> Signup and view all the answers

What stimulates the continued migration of ectodermal cells during the gastrulation process?

<p>Release of additional growth factors</p> Signup and view all the answers

What type of cells are primarily involved in forming the mesoderm?

<p>Epiblast cells</p> Signup and view all the answers

What do the cells forming the mesoderm initially do before establishing this new layer?

<p>Fill in the area previously occupied by hypoblast cells</p> Signup and view all the answers

What is the significance of the primitive streak during embryonic development?

<p>It acts as a signaling center for tissue formation.</p> Signup and view all the answers

What role does the primitive pit play during the gastrulation phase?

<p>Serves as a point for cranial migration of cells</p> Signup and view all the answers

What outcome occurs when epiblast cells migrate through the primitive pit?

<p>They form a tube moving cranially.</p> Signup and view all the answers

Which growth factor is primarily mentioned as being secreted by ectodermal cells?

<p>Fibroblast growth factor type 8</p> Signup and view all the answers

What characterizes the transition from a bilaminar disk to a trilaminar disk?

<p>Establishment of three distinct germ layers.</p> Signup and view all the answers

What happens to the tissue in the center of the primitive node during gastrulation?

<p>It disintegrates to create the primitive groove.</p> Signup and view all the answers

what does the ectoderm form?

<p>Nervous syst Sense organs Epidermis Pituitary gland Olfactory epithelium Inner ear</p> Signup and view all the answers

what does the endoderm form

<p>Inner lining of dig tube Respiratory tracts Pancreas Liver</p> Signup and view all the answers

what does the mesoderm form?

<p>Paraxial mesoderm Somites Dermatome = spinal meninges, skin Myotome = epiaxial m, hypaxial m Sclerotome = vertebrae, discs and ribs</p> <p>Intermediate mesoderm Gonads - testes or ovaries and ductal system - uterus Renal system - kidneys and ureters</p> <p>Lateral plate mesoderm (somatic and splanchnic layer) Somatic layer - makes parietal layer Sternum Chondroblasts → Bones and hyaline cartilage of limbs</p> <p>Splanchnic layer - makes visceral layer Adrenal cortex LN Spleen Smooth muscle of GI tract Circulatory system Myeloid stem cells Lymphoid cells Erythroid stem cells</p> Signup and view all the answers

Study Notes

Human Somatic Cells

  • Human somatic cells contain 46 chromosomes: 2 sex chromosomes and 44 autosomes, organized into 22 homologous sets.
  • Sperm (male sex cell) and ova (female sex cell) are haploid, meaning they have one set of chromosomes (23).
  • Ova contain ZP3 receptors, crucial for fertilization.

Meiosis and Mitosis

  • Meiosis is a cell division process that produces four non-identical haploid sex cells.
  • Mitosis is a cell division process that produces two identical diploid body cells.

Prenatal Development

  • Prenatal development is divided into three stages:
    • Stage 1 (0-4 weeks): Cellular proliferation and migration occur, laying the foundation for the embryo.
    • Stage 2: Morphogenesis takes place, involving intricate embryological processes, giving shape to the developing embryo.
    • Stage 3: Growth and maturation happen as the embryo develops into a fetus.
  • Induction: The process where one group of cells influences the development of another group.
  • Competence: The ability of cells to respond to inductive signals.
  • Differentiation: The process where cells specialize into different types with unique functions.

Week 1: Fertilization to Blastocyst Formation

  • During week one, fertilization occurs, resulting in the formation of a zygote.
  • The zygote undergoes rapid cell division (cleavage), forming a morula.
  • The morula then transforms into a blastocyst, a hollow sphere of cells with an inner cell mass.
  • The inner cell mass is responsible for forming the embryo, while the surrounding outer layer (trophoblast) forms the placenta.

Week 2: Blastocyst Implantation

  • The blastocyst implants itself in the uterine lining, marking the start of pregnancy.
  • The trophoblast cells invade the uterine wall, creating a connection between the embryo and the mother.
  • The inner cell mass differentiates into two layers:
    • Epiblast: Forms the amniotic cavity and contributes to the embryo.
    • Hypoblast: Forms the yolk sac, which supports early development.

Week 3: Gastrulation

  • Gastrulation marks a significant stage in embryonic development, transforming the bilaminar disc into a trilaminar disc.
  • This process creates three primary germ layers:
    • Ectoderm: Develops into the skin, nervous system, and other external structures.
    • Endoderm: Forms internal linings like the digestive system, lungs, liver, and pancreas.
    • Mesoderm: Develops into musculoskeletal system, circulatory system, kidneys, and gonads.
  • The mesoderm further differentiates into:
    • Paraxial Mesoderm: Develops into skeletal muscle, vertebrae, and ribs.
    • Intermediate Mesoderm: Forms the urinary system and gonads.
    • Lateral Plate Mesoderm: Develops into connective tissues, blood vessels, and linings of the body cavities.
  • The head region is referred to as the prochordal region or prechordal plate, while the tail region is called the cecal plate.

Defects in Embryonic Development

  • Abnormalities in chromosome number can lead to congenital anomalies.
  • About 10% of developmental malfunctions are due to alterations in a single gene.
  • Autosomal dominant inheritance: A single copy of a mutated gene can cause a condition.
  • Autosomal recessive inheritance: Two copies of a mutated gene are needed for the condition to manifest.

Downs Syndrome (Trisomy 21)

  • Cause: Caused by an extra copy of chromosome 21.
  • Features: Characteristic physical traits, intellectual disabilities, and increased risk of health problems.

Turner's Syndrome (XO)

  • Common chromosomal disorder in Females.
  • Causes: Occurs when an X chromosome is missing or partially missing.
  • Features: Shorter stature, infertility, heart problems, and other developmental challenges.

Klinefelter Syndrome (XXY)

  • Causes: Caused by an extra X chromosome in males.
  • Features: Taller than average, low testosterone levels, reduced fertility, and other developmental features.

Human Somatic Cells

  • Contain 46 chromosomes: 2 sex chromosomes and 44 autosomes, organized into 22 homologous sets.
  • Diploid: have two sets of chromosomes.

Sex Cells

  • Spermatozoa: male sex cells.
  • Ova: female sex cells, containing zona pellucida receptors (ZP3).
  • Haploid: have one set of chromosomes.
  • Meiosis: cell division that produces four non-identical haploid sex cells.
  • Mitosis: cell division that produces two identical diploid body cells.

Prenatal Development Stages

  • Stage 1 (0-4 weeks): Cellular proliferation and migration.
  • Stage 2: Morphogenesis: intricate embryological processes.
  • Stage 3: Growth and maturation.

Week 1

  • Begins with fertilization and ends with the formation of a blastocyst.
  • Blastocyst: consists of an inner cell mass (ICM) that will develop into the embryo and an outer layer called the trophoblast, which will form the placenta.

Week 2

  • Implantation of the blastocyst in the uterine wall.
  • Formation of the bilaminar embryonic disc composed of epiblast and hypoblast.
  • Differentiation of the trophoblast into two layers: cytotrophoblast and syncytiotrophoblast.

Week 3: Gastrulation

  • Transition from a bilaminar to a trilaminar embryonic disc.
  • Primitive streak: forms along the dorsal midline, marking the beginning of gastrulation.
  • Ectoderm: the outermost layer, forms the skin, nervous system, and enamel
  • Mesoderm: the middle layer, forms muscles, bones, connective tissues, and blood vessels.
  • Endoderm: the innermost layer, forms the lining of the digestive system, respiratory system, liver, and pancreas.

Mesoderm Derivatives

  • Paraxial mesoderm: gives rise to somites, which develop into vertebrae, ribs, and skeletal muscles.
  • Intermediate mesoderm: forms the kidneys and gonads.
  • Lateral plate mesoderm:
    • Somatic layer: forms the lining of the body cavity (parietal layer).
    • Splanchnic layer: forms the lining of the digestive system (visceral layer).
    • Other derivatives: limbs, heart, blood vessels, smooth muscle of the gastrointestinal tract.

Defects in Embryonic Development

  • Abnormal chromosome number: causes congenital anomalies.
  • Alterations in single genes: account for 10% of developmental malfunctions.

Inheritance Patterns

  • Autosomal dominant inheritance: a single copy of a mutated gene is sufficient to cause the condition.
  • Autosomal recessive inheritance: two copies of a mutated gene are required for the condition to manifest.

Chromosomal Disorders

  • Down syndrome (Trisomy 21):
    • Cause: Presence of a third copy of chromosome 21.
    • Features: intellectual disability, distinctive facial features, heart defects, and increased risk of leukemia.
  • Turner syndrome (XO):
    • Common chromosomal disorder in females.
    • Cause: Absence of a second X chromosome.
    • Features: short stature, webbed neck, and heart defects.
  • Klinefelter syndrome (XXY):
    • Cause: Presence of an extra X chromosome.
    • Features: tall stature, underdeveloped testes, and reduced fertility.

Embryology: The First Week of Development

  • Embryology begins after fertilization and focuses on the development of the embryo within the first week.

  • The female reproductive system includes the vagina, cervix, uterus, fallopian tubes (with the ampulla being the site of fertilization), and ovaries.

  • Ovulation, typically occurring around day 14 or 15 of the menstrual cycle, is triggered by the release of luteinizing hormone (LH) from the anterior pituitary gland.

  • LH is stimulated by the hypothalamus, which releases gonadotropin-releasing hormone (GnRH).

  • Estrogen levels also play a role in stimulating LH release during the second half of the menstrual cycle.

  • During ovulation, LH causes the Graafian follicle to release a secondary oocyte, which is arrested in metaphase II of meiosis.

  • The secondary oocyte is then swept by the fimbriae towards the ampulla of the fallopian tube, awaiting fertilization by a sperm cell.

  • Capacitation, a process where sperm cells remove cholesterol molecules from their heads, enables them to attach to zp3 receptors on the surface of the oocyte membrane.

  • Once the sperm cell binds to zp3 receptors, it releases enzymes from its acrosome to penetrate the zona pellucida, eventually fusing its nucleus with the oocyte's nucleus.

  • This fusion of genetic material creates a diploid cell called the zygote, containing 46 chromosomes (23 paternal and 23 maternal).

  • The zygote undergoes a series of rapid cell divisions called cleavage, starting with the two-cell stage, then the four-cell stage, eight-cell stage, and sixteen-cell stage.

  • The sixteen-cell stage, known as the morula, is a hollow ball of cells called blastomeres.

  • The morula further develops into a blastocyst, a structure with a fluid-filled cavity and two distinct cell types:

    • Inner Cell Mass: A group of cells clumped together, destined to become the embryo blast.
    • Outer Cell Mass: Cells lining the edge of the blastocyst, destined to become the trophoblast.

  • The trophoblast further differentiates into two specialized layers:

    • Cytotrophoblast: A cellular layer responsible for forming part of the placenta.
    • Syncytiotrophoblast: A multi-nucleated layer that plays a critical role in implantation.

  • The embryo blast will eventually develop into the bilaminar embryo, consisting of two germ layers:

    • Epiblast: Forms the ectoderm, which gives rise to the skin, nervous system, and related tissues.
    • Hypoblast: Forms the endoderm, which gives rise to the lining of the digestive and respiratory tracts.

Key Points:

  • The first week of embryonic development involves a series of crucial steps: ovulation, fertilization, cleavage, blastulation, and the formation of the trophoblast and embryo blast.

  • The trophoblast forms part of the placenta, while the embryo blast will give rise to the embryo itself.

  • The trophoblast (placental precursor) plays a key role in implantation, while the embryo blast (embryo precursor) forms the bilaminar disc, the foundation of the embryo.

Embryonic Development: Week 2 - 3

  • Blastocyst's Journey: The blastocyst, formed through continuous cleavage, moves towards the uterine cavity.
  • Implantation: Specific proteins on the blastocyst, like selectins and integrins, enable it to attach and embed itself into the endometrium.
  • Trophoblast Differentiation: The trophoblast, initially an outer cell mass, differentiates into two layers: the cytotrophoblast and the syncytiotrophoblast.
    • Cytotrophoblast: Consists of well-defined cells with distinct cell membranes and nuclei.
    • Syncytiotrophoblast: Forms from the fusion of cytotrophoblast cells, resulting in a multinucleated mass with a shared cytoplasm.
  • Role of Trophoblast: The syncytiotrophoblast invades the uterine lining and establishes contact with maternal blood vessels.
    • This connection enables nutrient, oxygen, and hormonal exchange between the developing embryo and the mother.
  • Placenta Development: The syncytiotrophoblast continues to proliferate, eventually forming villi that extend towards maternal blood vessels, laying the groundwork for the placenta.
  • Hormonal Regulation: The syncytiotrophoblast releases human chorionic gonadotropin (hCG), a hormone that maintains the corpus luteum's progesterone production.
    • This sustained progesterone level prevents shedding of the endometrial lining, ensuring a stable environment for the implanted embryo.
  • Bilaminar Disk: The inner cell mass, also known as the embryoblast, differentiates into a bilaminar disk, composed of two layers:
    • Epiblast: The upper layer, closer to the amniotic cavity.
    • Hypoblast: The lower layer, closer to the primitive yolk sac.
  • Yolk Sac: A cavity below the hypoblast, crucial for nutrient provision and early red blood cell formation in the developing embryo.
  • Amniotic Cavity: A fluid-filled cavity above the epiblast, providing a protective environment for the developing embryo.
  • Prochordal Plate: A thickened area where the epiblast and hypoblast meet, serving as a landmark for establishing cranial and caudal ends of the developing embryo.
    • Cranial End: The end where the prochordal plate is located, representing the head region of the embryo.
    • Caudal End: The opposite end of the prochordal plate, representing the tail or posterior region of the embryo.
  • Primitive Streak Formation: As the embryo progresses towards week 3, signaling processes initiate the formation of the primitive streak.
    • It appears as a thickening of the epiblast cells, stretching along the midline of the bilaminar disk.
  • Primitive Node: A knob-like structure at the cranial end of the primitive streak, playing a crucial role in the formation of the neural tube.
  • Gastrulation: The process of converting the bilaminar disk into a trilaminar disk, the basis for the formation of the three primary germ layers (ectoderm, mesoderm, and endoderm).
    • Cells within the primitive node and streak undergo a process of invagination and migration, setting the stage for the formation of the vital organs and tissues of the embryo.
  • Fibroblast Growth Factor 8 (FGF8): A signaling molecule secreted from cells at the edge of the primitive streak.
    • FGF8 binds to receptors on epiblast cells, initiating intracellular processes that lead to the migration and differentiation of cells during gastrulation.

Gastrulation

  • Fibroblast growth factor 8 (FGF8) inhibits E-cadherin formation, allowing epithelial cells to migrate.
  • Migrating cells move through the primitive groove and replace the hypoblast, forming the endoderm.
  • Ectoderm continues to release FGF8, causing more cells to migrate through the primitive groove forming the mesoderm.
  • The three germ layers (ectoderm, mesoderm, and endoderm) are formed through gastrulation, transitioning from a bilaminar disc to a trilaminar disc.

Notochord Formation

  • Ectodermal cells migrate through the primitive pit and move cranially towards the prechordal plate, forming a tube called the notochord.
  • The notochord is crucial for neuralation, and its remnants form the nucleus pulposus of the intervertebral discs.

Germ Layer Differentiation

  • Ectoderm: Forms skin and nervous system.
  • Mesoderm: Forms connective tissues, muscles, ligaments, bones, and other structures.
  • Endoderm: Forms lining of the GI tract, accessory organs, and glands.

Location of No Mesoderm

  • Notochord
  • Prechordal plate
  • Cloacal plate

Key Terms

  • Primitive streak: Thickening of the epiblast tissue.
  • Primitive node: Thickening of the epiblast tissue at the anterior end of the primitive streak.
  • Primitive groove: Space within the primitive streak.
  • Primitive pit: Space within the primitive node.
  • Bilaminar disc: Two layered disc consisting of the epiblast and hypoblast.
  • Trilaminar disc: Three layered disc consisting of the ectoderm, mesoderm, and endoderm.
  • Gastrulation: Process of forming the three germ layers.
  • Neuralation: Formation of the neural tube.
  • Notochord: Tube that forms during gastrulation and induces neuralation.
  • Nucleus pulposus: Jellylike material in the center of the intervertebral discs, remnant of the notochord.

Summary

These detailed study notes cover the key concepts and terms associated with gastrulation, notochord formation, and the development of the three germ layers. The notes emphasize the importance of FGF8, the process of migration, the formation of the notochord, and the differentiation of the germ layers.

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Embryology 1 - 2.1 PDF

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This quiz covers human somatic cells, meiosis and mitosis processes, and the stages of prenatal development. Understand the differences between haploid and diploid cells, and the intricacies of embryonic growth. Test your knowledge on these essential biological concepts.

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