Embryology and Early Development Quiz
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Questions and Answers

Which term describes a movement toward the midline of the body?

  • Lateral rotation
  • Medial rotation
  • Adduction (correct)
  • Abduction

    • What term is used to describe the position further from where a limb attaches to the body?

  • Distal (correct)
  • Medial
  • Lateral
  • Proximal

    • Which of the following congenital disorders occurs in approximately 1 in 1000 live births?

  • Cleft palate
  • Orofacial clefts (correct)
  • Trisomy 21
  • Spina bifida

    • At what stage does the conceptus exist during fertilization until week 2?

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

    Which type of rotation involves turning inwards towards the center of the body?

    <p>Medial rotation</p> Signup and view all the answers

    The posterior part of the body is referred to as which of the following?

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

    What is the embryological stage from week 3 until week 8 referred to as?

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

    What percentage of live births is affected by congenital disorders?

    <p>3%</p> Signup and view all the answers

    What is the origin of the mesenchymal core in the pharyngeal arches?

    <p>Neural crest cells and paraxial mesoderm</p> Signup and view all the answers

    Which structure forms between the pharyngeal arches?

    <p>Pharyngeal cleft</p> Signup and view all the answers

    What initiates the formation of primordial germ cells (PGC)?

    <p>Inductive signaling from surrounding cells</p> Signup and view all the answers

    How do primordial germ cells (PGC) reach the genital ridges?

    <p>Through the dorsal mesentery</p> Signup and view all the answers

    Which of the following is NOT a characteristic of each pharyngeal arch?

    <p>Develops into limb structures</p> Signup and view all the answers

    What is the role of the notochord in the formation of the neural plate?

    <p>It induces the formation of the neural plate.</p> Signup and view all the answers

    What gives rise to different structures such as dorsal root ganglia and Schwann cells?

    <p>Neural crest cells</p> Signup and view all the answers

    Which hypothesis suggests that molecules responsible for side determination in body axes are contained in vesicles?

    <p>Node vesicular parcel hypothesis</p> Signup and view all the answers

    What developmental disorder is characterized by organs being mirrored from their normal positions?

    <p>Situs invertus</p> Signup and view all the answers

    What regulates the segmentation of the neural tube, determining the pattern of expression in the embryo?

    <p>Hox genes</p> Signup and view all the answers

    During the third week of embryonic development, the embryo is in which shape?

    <p>Ovoid and trilaminar</p> Signup and view all the answers

    What embryonic structure is primarily responsible for the formation of skin?

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

    Which of the following is NOT a consequence of confusion regarding body axes during development?

    <p>Neural tube defects</p> Signup and view all the answers

    What initiates the folding of the embryo starting in the fourth week?

    <p>Differential growth rates</p> Signup and view all the answers

    The extension and invagination of the neural plate leads to the formation of what structure?

    <p>Neural groove</p> Signup and view all the answers

    What is the primary purpose of vasculogenesis during embryogenesis?

    <p>De novo assembly of blood vessels</p> Signup and view all the answers

    Which cells are responsible for forming blood vessels during angiogenesis?

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

    During which state does angiogenesis typically occur?

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

    What structure forms from the fusion of two endocardial tubes?

    <p>Primitive heart tube</p> Signup and view all the answers

    Which artery supplies blood to the midgut region?

    <p>Superior mesenteric artery</p> Signup and view all the answers

    What triggers the formation of lymph sacs during lymphangiogenesis?

    <p>VEGF-C</p> Signup and view all the answers

    Which membrane seals off the mouth during gut development?

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

    What is the main force driving the body folding process during embryogenesis?

    <p>Differential growth of various tissues</p> Signup and view all the answers

    The allantois becomes which of the following structures in the fetus?

    <p>Urinary bladder</p> Signup and view all the answers

    Which of the following is NOT a derivative of the foregut in gut development?

    <p>Proximal colon</p> Signup and view all the answers

    What is the primary role of the notochord in embryonic development?

    <p>Induces the formation of other organs</p> Signup and view all the answers

    What forms the skeletal elements of the limb during development?

    <p>Lateral plate mesoderm and somites</p> Signup and view all the answers

    Which of the following describes the fate of the paraxial mesoderm?

    <p>Forms somites contributing to muscle and bone</p> Signup and view all the answers

    What is the fate of the Müllerian duct in a male embryo?

    <p>Degenerates under the influence of AMH</p> Signup and view all the answers

    How do gonads differentiate into testes or ovaries?

    <p>Through the absence of the SRY gene</p> Signup and view all the answers

    What tissue type is primarily found in the intermediate mesoderm?

    <p>Epithelial tissue</p> Signup and view all the answers

    During somitogenesis, what transition occurs as somites develop?

    <p>Mesenchymal to epithelial transition</p> Signup and view all the answers

    What characterizes the lateral mesoderm?

    <p>Divided into somatic and splanchnic components</p> Signup and view all the answers

    What is the primary purpose of apoptosis in digit formation?

    <p>To sculpt the digits from paddle-like structures</p> Signup and view all the answers

    Which embryonic structures are responsible for forming the kidney?

    <p>Nephric duct and metanephric mesenchyme</p> Signup and view all the answers

    Which term describes the stage of development from week 3 to week 8?

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

    What percentage of live births is affected by congenital disorders?

    <p>3%</p> Signup and view all the answers

    Which of the following pairs describes terms that denote a direction towards the midline of the body?

    <p>Medial and Adduction</p> Signup and view all the answers

    Which congenital disorder has an occurrence rate of about 1 in 700 live births?

    <p>Trisomy 21 (Down syndrome)</p> Signup and view all the answers

    What anatomical term refers to the back of the body?

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

    What is the primary role of the notochord during early embryonic development?

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

    Which of the following best describes the position referred to as 'distal'?

    <p>Further from the attachment of the limb</p> Signup and view all the answers

    What does 'ventral' refer to in anatomical terms?

    <p>The front side of the body</p> Signup and view all the answers

    Which sequence correctly describes the steps of vasculogenesis?

    <p>Mesodermal cells cluster into haemangioblasts, haemangioblasts differentiate into angioblasts and haematopoetic cells, angioblasts form endothelial blood vessels.</p> Signup and view all the answers

    What is the primary factor that promotes angiogenesis in tumors?

    <p>Release of growth factor VEGF-A by the tumor itself during hypoxia.</p> Signup and view all the answers

    Which artery supplies blood to the distal half of the colon?

    <p>Inferior mesenteric artery.</p> Signup and view all the answers

    In which phase does the embryonic gut tube develop and differentiate into regions defined by their blood supply?

    <p>Beginning of the fourth week of embryonic development.</p> Signup and view all the answers

    What is the role of the septum transversum in embryonic development?

    <p>It separates the coelom into thoracic and abdominal cavities.</p> Signup and view all the answers

    What is the main role of the trophoblast during early embryonic development?

    <p>Contributes to the formation of the placenta</p> Signup and view all the answers

    During which stage does the process of gastrulation occur?

    <p>Immediately after implantation</p> Signup and view all the answers

    Which layer forms the ectoderm during the process of gastrulation?

    <p>Cells remaining in the epiblast</p> Signup and view all the answers

    What is indicated by the formation of the primitive streak during embryonic development?

    <p>The start of cell migration for germ layer formation</p> Signup and view all the answers

    At which point does the blastocyst increase in size significantly?

    <p>After implantation into the endometrium</p> Signup and view all the answers

    What is the embryonic disc, and what does it form?

    <p>A layer between the epiblast and hypoblast that develops into the embryo</p> Signup and view all the answers

    Which of the following correctly describes the roles of the epiblast and hypoblast?

    <p>Epiblast forms the embryo; hypoblast helps form the extra-embryonic structures</p> Signup and view all the answers

    What does the primitive node establish in embryonic development?

    <p>Left-right asymmetry of the body</p> Signup and view all the answers

    How does the process of cleavage contribute to early embryonic development?

    <p>By resulting in a rapid increase in cell number without growth</p> Signup and view all the answers

    What initiates the formation of the neural plate?

    <p>Induction by the notochord</p> Signup and view all the answers

    What is the primary result of the invagination and extension of the neural plate?

    <p>Creation of the neural groove</p> Signup and view all the answers

    Which cells give rise to structures such as dorsal root ganglia and Schwann cells?

    <p>Neural crest cells</p> Signup and view all the answers

    What role do Hox genes play in neural tube development?

    <p>Regulating the patterning of neural tube segmentation</p> Signup and view all the answers

    What is the consequence of situs invertus?

    <p>Mirroring of organs from normal positions</p> Signup and view all the answers

    Which of the following describes the fate of ectodermal cells in skin development?

    <p>They differentiate into melanocytes and immune cells</p> Signup and view all the answers

    What drives the folding of the embryo during the fourth week of development?

    <p>Differential growth rates of embryonic disc and amnion</p> Signup and view all the answers

    What is characterized by the lateral mesoderm during development?

    <p>Contribution to body cavities</p> Signup and view all the answers

    What is indicated by the term 'situs ambiguus'?

    <p>Partial organ reversal</p> Signup and view all the answers

    Which hypothesis explains the presence of molecules in vesicles impacting body axis determination?

    <p>Node vesicular parcel hypothesis</p> Signup and view all the answers

    What structure primarily drives the differentiation of somites into dermamyotomes and sclerotomes?

    <p>Hox genes</p> Signup and view all the answers

    Which tissue type does the splanchnic lateral plate mesoderm primarily form?

    <p>Circulatory system</p> Signup and view all the answers

    How do the gonads develop into testes in male embryos?

    <p>Presence of SRY gene</p> Signup and view all the answers

    What occurs during somitogenesis that involves a transition in cell types?

    <p>Mesenchymal to epithelial transition</p> Signup and view all the answers

    What is the primary development effect of the regression of the primitive streak during notochord growth?

    <p>Development of teratomas</p> Signup and view all the answers

    Which of the following structures is specifically formed from the paraxial mesoderm in the trunk?

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

    Which mesodermal layer contributes to the formation of the urogenital system?

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

    What is the result of the Wolffian duct in male embryonic development?

    <p>Development of male reproductive structures</p> Signup and view all the answers

    How does the notochord contribute to organ development?

    <p>It serves as a transient structure for support.</p> Signup and view all the answers

    Which segment of the paraxial mesoderm is crucial for forming the skeletal elements of the trunk?

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

    Study Notes

    Embryology

    • Congenital disorder: a condition present at or before birth, regardless of the cause.
    • Common congenital disorders: Orofacial clefts (1:1000) and Trisomy 21 (1:700-900).
    • Stages of Embryonic Development:
      • Conceptus: fertilization to 2 weeks
      • Embryo: weeks 3 to 8
      • Fetus: from the 3rd month onwards

    Early Development

    • Morphogen Hypothesis: a model explaining how cells determine their position in the body.
    • Node Vesicular Parcel Hypothesis: a model explaining the role of molecules in the fluid that are in vesicles in cell positioning.
    • Two Cilia Hypothesis: a model explaining how cilia in the node rotate and sense motion to signal cell positioning.
    • Situs Inversus: a condition where organs are mirrored from their normal position (1:8000 live births).
    • Situs Ambiguous or Heterotaxies: a condition where only some organs are partially mirrored from their normal position.
    • Early Development Summary: involves differentiation of the 3 germ layers (ectoderm, mesoderm, endoderm) and the folding of the embryonic disc.

    Ectoderm

    • Neurulation: the process by which the neural plate and neural tube are formed.
    • Neural Plate: thick plate of ectodermal cells above the notochord, induced by the notochord.
    • Neural Tube: formed via the closing of the neural groove.
    • Neural Crest: cells located where the neural folds meet that undergo epithelial to mesenchymal transition and migrate.
    • Neural Crest Cells: give rise to dorsal root ganglia, enteric ganglia, Schwann cells, melanocytes, parasympathetic and sympathetic ganglia, muscle, cartilage, and bone of the skull, face, jaw, pharynx, and dentine.
    • Segmentation of the Neural Tube: Formation of vesicles at the cranial end of the tube, which give rise to the brain, and the spinal cord from the remainder.
    • Hox Genes: regulate the patterning of the neural tube segmentation.
    • Development of Skin: epidermis arises from ectoderm and is colonized by melanocytes and Langerhans cells.
    • Dermis: in the face arises from neural crest cells and therefore is ectodermal.

    Body Folding

    • Third Week: Embryo is flat, ovoid, and trilaminar.
    • Fourth Week: Rapid embryo growth, especially in length.
    • Folding: initiates to generate body form, driven by different growth rates.
    • Yolk Sac: Almost no growth.
    • Folding Axes: Cranial-caudal and lateral axes are involved.

    Mesoderm

    • Mesoderm Differentiation: during body folding, the mesoderm differentiates into paraxial, intermediate, and lateral mesoderm.
    • Notochord: a cartilage-like, transient structure that is important for induction of other organs.
    • Notochord Formation: Cranial-midline extension of the primitive node to form a hollow tube.
    • Primitive Streak: Regresses as the notochord grows.

    Fate of Mesoderm

    • Paraxial Mesoderm:
      • Located closest to the neural tube.
      • Differentiates into different cells in the head vs. the trunk:
        • Head: forms bone, muscle, and connective tissue of the face and skull.
        • Trunk: forms somites, which give rise to dermis, muscle, and bone.
    • Lateral Mesoderm:
      • Divided into somatic (parietal) and splanchnic (visceral).
      • Somatic LP Mesoderm: forms ventro-lateral body wall.
      • Splanchnic LP Mesoderm: forms heart and vasculature, wall of the gut.
    • Somitogenesis: a mesenchymal to epithelial cell transition that occurs in a cranial to caudal direction.
    • Somites: differentiate into epithelial dermamyotomes (forming dermatomes and myotomes), and mesenchymal sclerotomes.
    • Hox Genes: drive somitogenesis differentiation.
    • Formation of Vertebrae: Each sclerotome splits into cranial and caudal sections.
    • Cranial and Caudal Sections: fuse to form a vertebra.
    • Limb Formation: limbs grow from somites at limb fields.
    • Mesenchymal Cells: from lateral plate mesoderm and somites migrate to limb fields.
    • Hox Genes: Drive limb segmentation, and digit formation.
    • Intermediate Mesoderm:
      • Located between lateral plate and paraxial mesoderm.
      • Forms the urogenital system (kidneys, gonads, and associated ducts).
    • Kidney Formation: Begins with the nephric duct (Wolffian duct).
    • Pronephros: tubules that quickly deteriorate.
    • Mesonephros: tubules that act as the embryonic kidney.
    • Ureteric Bud: forms out of the nephric duct and gives rise to the ureter.
    • Kidney Differentiation: The end of the uretric bud fuses with the metanephric mesenchyme to form the kidney.
    • Genital Tract Development: develops from the nephric (Wolffian duct) and Mullerian duct.
    • Mullerian Duct (Female Duct): forms the oviduct, uterus, and upper vagina.
    • Wolffian Duct (Male Duct): forms the epididymis, vas deferens, and seminal vesicles.
    • SRY Gene: on the Y chromosome drives the differentiation of the gonads into testes.
    • Gonads: form from the genital ridge, which is a bipotential precursor.
    • Anti-Mullerian Hormone (AMH): produced by the testes, causes degeneration of the Mullerian duct.
    • Testosterone: produced by the testes, causes the development of the Wolffian duct into the male reproductive tract.
    • Lateral Plate Mesoderm: located furthest from the neural tube, and divides into somatic (parietal) and splanchnic (visceral) LP mesoderm.
    • Somatic LP Mesoderm: lines the body cavity/coelom.
    • Splanchnic LP Mesoderm: lines the gut.
    • Lateral Plate Mesoderm Functions: forms the ventrolateral body wall, heart, and vasculature, and gut wall.

    Splanchnic LP Mesoderm

    • Circulatory System: forms from the splanchnic LP mesoderm.
    • Vasculogenesis: de novo assembly of blood vessels during embryogenesis.
    • Hemangioblasts: formed by mesodermal cells clustering.
    • Hemangioblasts Differentiation: into angioblasts and hematopoietic cells.
    • Angioblasts: form endothelial blood vessels.
    • Hematopoietic Cells: form blood cells.
    • Angiogenesis: blood vessel formation from pre-existing vasculature during embryogenesis and adulthood.
    • Hypoxia: a state that triggers angiogenesis.
    • Vascular Endothelial Growth Factor (VEGF-A): a growth factor released by vessels, causes sprouting of endothelial cells toward the GF.
    • Tumor Angiogenesis: when tumor cells release VEGF-A, causing blood vessel growth to provide oxygen and facilitate metastatic spread.
    • Heart Formation: two endocardial tubes fuse to form the primitive heart tube.
    • Primitive Heart Tube: has 4 vessels that will form the great vessels.
    • Heart Tube Twisting and Looping: forms septa that separate the 4 chambers.
    • Lymphangiogenesis: lymphatic system develops from pre-existing veins.
    • Cardinal Veins: bud towards VEGF-C to form lymph sacs.
    • Lymph Sacs: form their own vascular system.

    Septum Transversum

    • Mesodermal Cells: move under and inwards, separating the coelom.
    • Septum Transversum: separates the coelom into thoracic and abdominal cavities, and forms part of the diaphragm and ventral mesentery of the stomach and duodenum.

    Endoderm

    • Endoderm Function: important for inducing the formation of mesodermal organs, and forms the digestive tract lining.
    • Gut Tube: formed through body folding. Lateral folds fuse, except for the area where the yolk sac connects.
    • Splanchnic LP Mesoderm: lines the gut.
    • Somatic LP Mesoderm: lines the body cavity.
    • Buccopharyngeal Membrane: seals off the mouth.
    • Cloacal Membrane: seals off the anus.
    • Allantois: located at the caudal end of the gut tube and connected to the yolk sac.
    • Allantois Functions: gas exchange and excretion in the embryo, and becomes the urachus.

    Development of the Gut

    • Primitive Gut: develops at the beginning of the fourth week and is closed at both ends.
    • Gut Regions: foregut, midgut, and hindgut, defined by their blood supply.
    • Hox Genes: define the gut tube patterning.

    Regional Patterning of the Gut Tube

    • Regional patterning occurs along the foregut, midgut, and hindgut.

    Development of the Stomach

    • Distal Foregut: Slight dilation forms around the middle of the 4th week.
    • Enlargement: of the distal foregut.
    • Dorsal Growth: Grows quicker than ventral part, causing curvature.
    • Rotations: 2 x 90 degree rotations occur clockwise:
      • Dorsal part moves left, ventral part moves right about the dorsal-ventral axis.
      • Stomach bends into a C shape.

    Development of Other Endodermal Organs

    • Bud Growth: occurs from endodermal thickening and cell proliferation.
    • Lungs, Liver, Gall Bladder, and Pancreas: formed by the continued lengthening and bifurcating of buds.

    Development of the Lungs

    • Respiratory Diverticulum: formed from a ventral out-pocketing of the endoderm.
    • Branching: respiratory diverticulum bifurcates to form lung lobes.
    • Tracheal Buds: form the bronchi.
    • Secondary Bronchial Buds: form the lung lobes.
    • Bronchopulmonary Segments: formed from secondary bronchial bud bifurcations.
    • Terminal Bronchioles: formed through 14 more bifurcations.
    • Lengthening: between each branching.

    Gut Tube and Derivatives

    • Foregut: pharynx, esophagus, stomach, and proximal duodenum.
    • Midgut: distal duodenum to the proximal half of the colon.
    • Hindgut: distal half of the colon to anus.
    • Derivatives: thyroid, parathyroid, lungs, liver, gall bladder, pancreas, and urinary bladder.

    Head Structures

    • Neural Crest Cells: form many skeletal elements of the head.

    Pharyngeal Arches

    • Pharyngeal Arches: also known as brachial arches, are 4 pairs of arches that form many head structures.
    • Arches: have outer coverings of ectoderm, inner coverings of endoderm, and mesenchymal cores derived from paraxial and LP mesoderm and neural crest cells.
    • Pharyngeal Clefts: ridges between arches.
    • Pharyngeal Pouches: inner arches.
    • Arch Components: each arch contains:
      • Arch Specific Cranial Nerve: controls the arch.
      • Aortic Arch Artery: derived from splanchnic LP mesoderm.
      • Central Cartilaginous Elements: derived from neural crest.
      • Striated Muscle Rudiment: derived from head paraxial mesoderm.

    Germ Cells

    • Primordial Germ Cells (PGC): precursors for sperm and egg that are set aside prior to the differentiation of the 3 germ layers.
    • Pluripotent: PGC remain pluripotent.

    Migration of PGC

    • Allantois: PGC are specified at the base of the allantois.
    • Migration: PGC migrate along the hindgut, through the dorsal mesentery, and split into the left and right genital ridges.

    Anatomical Terminology

    • Distal refers to a structure that is further from the point of attachment of a limb to the body.
    • Proximal refers to a structure that is closer to the point of attachment of a limb to the body.
    • Adduction is the movement of a limb towards the midline of the body.
    • Abduction is the movement of a limb away from the midline of the body.
    • External rotation is the rotation of a limb along its longitudinal axis, moving outwards.
    • Internal rotation is the rotation of a limb along its longitudinal axis, moving inwards.
    • Anterior refers to the front, and Posterior refers to the back.
    • Ventral refers to the front, and Dorsal refers to the back.

    Congenital Disorders

    • A congenital disorder is a medical condition present at or before birth, regardless of the cause.
    • Congenital disorders occur in 3% of live births.
    • Examples of congenital disorders include orofacial clefts (lip, palate) and Trisomy 21 (Down syndrome).

    Stages of Embryological Development

    • The conceptus stage occurs from fertilization to week 2.
    • The embryo stage occurs from week 3 to week 8.
    • The fetus stage occurs from the 3rd month onwards.

    Early Development

    • Ovulation occurs at the end of the second week, where a secondary oocyte is released from the ovary and is swept into the oviduct.
    • Fertilization occurs at the end of the second week, where a single sperm penetrates the oocyte, fusing to form a zygote.
    • Cleavage begins with the zygote undergoing rapid mitotic cell division, moving along the oviduct and becoming a pre-embryo.
    • The morula, a solid ball of cells, forms on day 4 and enters the uterus.
    • The blastocyst, a hollow ball of cells with a fluid-filled cavity, forms on day 6, freed from the zona pellucida, and is able to increase in size.
    • Implantation occurs when the blastocyst attaches to the endometrium, leading to the formation of three germ layers.

    Blastocyst

    • The blastocyst contains two main cell types:
      • Trophoblast, the outer epithelial layer, forms extra-embryonic structures.
      • Embryoblast (inner cell mass), goes on to form the embryo.
    • Between 5 and 10 days, the blastocyst implants into the uterine wall, ensuring implantation, differentiation, and cavity formation.
    • The blastocyst forms two germ layers from the splitting of the inner cell mass:
      • Epiblast, the upper layer.
      • Hypoblast, the lower layer.
    • The embryonic disc, located between the two germ layers, forms the embryo.

    Gastrulation

    • During gastrulation, the blastula transforms into a gastrula, forming three germ layers: ectoderm, mesoderm, and endoderm.
    • The formation of the primitive streak defines all major body axes.
    • The primitive streak is a line of thickened cells that appears on the epiblast.
    • The primitive streak invaginates to form the primitive groove.
    • Cells migrate through the primitive groove to form all three germ layers:
      • The first cells to migrate replace the hypoblast, forming the endoderm.
      • The next cells to migrate form the mesoderm, located between the epiblast and endoderm.
      • The remaining cells in the epiblast form the ectoderm.
    • As cells migrate through the primitive groove, they distribute evenly.

    Primitive Node

    • The primitive node is located at the end of the primitive streak, filled with fluid, and is responsible for setting up left-right asymmetry.
    • Rotating cilia inside the node cause a leftward fluid flow, which helps to establish left-right asymmetry.
    • The morphogen hypothesis suggests that molecules in the fluid bind to receptors and signal to cells which side of the body they are on.
    • The node vesicular parcel hypothesis proposes that molecules in the fluid are found in vesicles.
    • The two cilia hypothesis posits that cilia at the bottom of the node rotate while cilia at the top sense motion to guide cell development.
    • Confusion about the side of body axes cells are on can cause sinus invertus disorders, such as Situs Invertus.

    Situs Invertus

    • Situs Invertus occurs in 1:8000 births.
    • In situs invertus, organs are mirrored from their normal position.
    • Some organs may only be partially inverted, known as Situs Ambiguous or Heterotaxies.
    • Situs invertus is often associated with other medical problems, particularly heart defects.

    Ectoderm

    • Neurulation is the process of forming the neural plate and neural tube.

    Neural Plate

    • The formation of the neural plate is induced by the notochord.
    • The neural plate, located from the cranial end to the primitive node, consists of thick pseudostratified epithelial cells from ectodermal cells.
    • The neural plate forms the neural tube.

    Neural Tube

    • The neural tube folds and invaginates to form the neural groove.
    • The neural groove closes to form the neural tube.

    Neural Crest

    • Neural crest cells are located where the neural folds meet.
    • Following the closure of the neural tube, cells undergo an epithelial to mesenchymal transition and migrate.
    • Neural crest cells give rise to diverse tissues, including:
      • Dorsal root ganglia.
      • Enteric ganglia.
      • Schwann cells.
      • Melanocytes.
      • Parasympathetic and Sympathetic ganglia.
      • Muscle, cartilage and bone of the skull, face, jaw, pharynx, and dentine.

    Segmentation of the Neural Tube

    • The cranial end of the neural tube develops into vesicles, which form the brain.
    • The remainder of the neural tube forms the spinal cord.
    • The patterning of segmentation is regulated by Hox genes.

    Hox genes

    • These genes control the order and location of body parts.
    • The location of a Hox gene on a chromosome predicts the pattern of expression in the embryo.
    • Areas of the central nervous system where a Hox gene is expressed correspond to its location on the chromosome.

    Development of Skin

    • The epidermis develops from ectoderm, colonized by melanocytes and Langerhans cells.
    • Melanocytes originate from neural crest cells.
    • Langerhans cells are immune cells from bone marrow.
    • Dermis in the face also arises from neural crest cells and is thus ectodermal.

    Body Folding

    • By the end of the third week, the embryo is a flat, ovoid, trilaminar disc.
    • During the fourth week, the embryo grows rapidly, especially in length.
    • Folding begins to create the body form.
    • Folding is driven by different growth rates:
      • The embryonic disc and the amnion grow faster than the yolk sac.
      • The yolk sac exhibits very little growth.
    • Folding occurs in the cranial-caudal and lateral axes.

    Mesoderm

    • Mesoderm differentiates through body folding into three types:
      • Paraxial Mesoderm.
      • Intermediate Mesoderm.
      • Lateral Mesoderm.

    Notochord

    • The notochord is a transient cartilage-like structure.
    • It is important for the induction of other organs, including the neural tube.
    • The notochord extends from the cranial midline of the primitive node to form a hollow tube.
    • The tube grows in length as cells are added from the primitive node.
    • The primitive streak regresses as the notochord grows. Failure of the primitive streak to regress can lead to teratomas.

    Fate of Mesoderm

    • Paraxial Mesoderm differentiates into different cells in the head versus the trunk.
      • In the head, it forms the bone, muscle, and connective tissue of the face and skull, in conjunction with neural crest cells.
      • In the trunk, paraxial mesoderm forms somites which give rise to dermis, muscle, and bone.

    Lateral Mesoderm

    • Lateral mesoderm is divided into somatic (parietal) and splanchnic (visceral) mesoderm.
    • Somatic mesoderm forms connective tissue in the ventrolateral body wall, not muscle.
    • Splanchnic mesoderm forms the heart, vasculature, wall of the gut, as well as bones of the limbs.

    Somitogenesis

    • Somites form through mesenchymal to epithelial cell transition.
    • As mesenchymal cells become more organized epithelial cells, somites fissure and close off.
    • Somite formation occurs in a cranial to caudal direction, in parallel on both sides of the neural tube.

    Somite Differentiation

    • Somites differentiate into epithelial dermatomes and myotomes, and mesenchymal sclerotomes.
      • Dermatomes form dermis.
      • Myotomes form muscle.
      • Sclerotomes form skeletal elements of the trunk, undergoing an epithelial to mesenchymal transition.
    • This differentiation is driven by Hox genes.

    Formation of Vertebrae

    • Each sclerotome splits into cranial and caudal sections.
    • Spinal nerves grow through to innervate myotomes and dermatomes.
    • The caudal section of one sclerotome fuses with the cranial section of the sclerotome below, forming a vertebra.

    Limb Formation

    • Limbs grow from somites.
    • Limb buds form at limb fields and develop laterally.
    • Mesenchymal cells from the lateral plate mesoderm and somites migrate toward these limb fields.
    • Lateral plate mesoderm forms the skeletal elements of the limb.
    • Somites form the muscle and dermis of the limb.
    • Limb segmentation is driven by Hox genes.
    • Digit formation starts with hand and foot paddles that are sculpted by apoptosis.

    Intermediate Mesoderm

    • Intermediate mesoderm is found between the lateral plate and paraxial mesoderm.
    • Intermediate mesoderm forms the urogenital system (kidneys, gonads, and associated ducts).

    Kidneys

    • The nephric duct (Wolffian duct) forms in the mesoderm.
    • At the cranial end, tubules (pronephros) form and connect to the nephric duct, but quickly degenerate.
    • At the caudal end, tubules (mesonephros) form as the embryonic kidneys.
    • A bud forms off the nephric duct (uretric bud) which gives rise to ureter.
    • The end of the uretric bud fuses with metanephric mesenchyme to form the kidney.

    Genital Tracts

    • Genital tracts develop from two ducts:
      • The nephric (mesonephric) or Wolffian duct.
      • The Mullerian duct.
    • The Mullerian duct (female duct) forms from invagination and develops into the oviduct, uterus, and upper vagina.
    • In females (XX), the genital ridge develops into the ovary, and no SRY gene is present:
      • No AMH is produced, allowing the Mullerian duct to develop.
      • No testosterone is produced, causing the Wolffian duct to disappear.
    • The Wolffian duct (male duct) develops into the epididymis, vas deferens, and seminal vesicles.
    • In males (XY), the SRY gene on the Y chromosome drives the genital ridge to develop into testes:
      • AMH production causes degeneration of the Mullerian duct.
      • Testosterone production causes the Wolffian duct to develop.

    Gonads

    • Gonads are located near the nephric duct, forming from the thickening of the surface of the mesonephros.
    • Gonads develop as the genital ridge, a bipotential precursor.
    • The SRY gene on the Y chromosome drives differentiation into testes.
    • In the absence of the SRY gene, differentiation into ovaries occurs.
    • Genital tracts run parallel to the mesonephros.
    • All embryos have both Wolffian and Mullerian ducts.
    • If the gonads develop into testes, testosterone and antimullerian hormone (AMH) cause degeneration of the Mullerian duct, and the Wolffian duct forms the male reproductive tract.
    • If the gonads develop into ovaries, the Wolffian duct degenerates, and the Mullerian duct forms the female reproductive tract.

    Lateral Plate Mesoderm

    • Lateral plate mesoderm is furthest from the neural tube and divides into somatic (parietal) and splanchnic (visceral) mesoderm.
    • Through body folding, splanchnic lateral plate mesoderm is positioned closer to the neural tube than the somatic lateral plate mesoderm.
    • Lateral plate mesoderm forms the ventrolateral body wall, heart and vasculature, and the gut wall.
    • Splanchnic lateral plate mesoderm forms the circulatory system.

    Vasculogenesis

    • De novo assembly of blood vessels from mesodermal progenitors
    • Occurs only during embryogenesis
    • Endoderm signals to mesoderm, forming haemangioblasts
    • Haemangioblasts differentiate into angioblasts (form vessels) and haematopoietic cells (form blood cells)
    • Vessels recruit pericytes for stability and contraction

    Angiogenesis

    • Formation of blood vessels from pre-existing vasculature
    • Occurs during embryogenesis and in adults
    • Requires hypoxia (low oxygen)
    • Vessels produce VEGF-A, causing sprouting of endothelial cells
    • Sprouts fuse and lumen forms
    • Pericytes strengthen the vessel

    Tumour Angiogenesis

    • Large tumours release VEGF-A, causing nearby vessels to sprout
    • Blood vessels provide oxygen and aid in metastatic spread

    Heart Formation

    • Two endocardial tubes fuse to form the primitive heart tube
    • Four vessels emerge from the heart tube, forming great vessels
    • The heart tube twists and loops, forming septa to separate the four chambers

    Lymphangiogenesis

    • Lymphatic system develops from pre-existing veins
    • Cardinal veins bud toward VEGF-C, forming lymph sacs
    • Lymph sacs form their own vascular system

    Septum Transversum

    • Mesodermal cells migrate inwards, forming the septum transversum
    • Separates the coelom into thoracic and abdominal cavities
    • Forms part of the diaphragm, ventral mesentery of the stomach and duodenum

    Body Folding

    • Occurs in the fourth week of embryogenesis
    • Driven by differential growth of tissues
    • Embryonic disc and amnion have high growth rates
    • Yolk sac has minimal growth
    • Cranial, caudal, and lateral body folds form

    Endoderm

    • Induces the formation of mesodermal organs
    • Forms the lining of the digestive tract
    • Lateral folds fuse to complete the gut tube, except where the yolk sac connects
    • Splanchnic mesoderm lines the gut, somatic lines the body cavity
    • Gut tube is sealed by the buccopharyngeal membrane (mouth) and cloacal membrane (anus)

    Allantois

    • Located at the caudal end of the gut tube
    • Sac-like structure
    • Endodermal, surrounded by blood vessels (umbilical arteries and veins)
    • Involved in gas exchange and excretion
    • Becomes the urachus (connects fetal bladder to the yolk sac)

    Gut Development

    • Primitive gut develops in the fourth week
    • Closed at both ends (oropharyngeal and cloacal membranes)
    • Three regions: foregut, midgut, and hindgut
    • Regional blood supply:
      • Foregut: celiac artery
      • Midgut: superior mesenteric artery
      • Hindgut: inferior mesenteric artery
    • Patterning defined by Hox genes

    Regional Patterning of the Gut Tube

    • Foregut: pharynx, esophagus, stomach, proximal duodenum
    • Midgut: distal duodenum to proximal ½ of the colon
    • Hindgut: distal ½ of the colon to anus

    Stomach Development

    • Develops from the distal foregut
    • Dilation in the distal foregut during the fourth week
    • Enlarges ventro-dorsally
    • Dorsal part grows faster, causing curvature
    • Two 90-degree clockwise rotations:
      • Dorsal part moves left, ventral part moves right
      • Stomach bends into a C shape

    Development of Other Endodermal Organs

    • Endodermal thickening leads to cell proliferation and bud growth
    • Lengthening and bifurcation form the lungs, liver, gall bladder, and pancreas

    Lung Development

    • Starts with a ventral outpocketing of the endoderm (respiratory diverticulum), forming the trachea
    • Bud grows ventro-caudally
    • Bifurcates into left and right tracheal buds (bronchi)
    • Bifurcates into secondary bronchial buds (lung lobes)
      • 3 right buds, 2 left buds
    • Secondary buds bifurcate into bronchopulmonary segments
    • Further bifurcations lead to terminal bronchioles
    • Lengthening occurs between each branching

    Gut Tube and Derivatives

    Gut Tube Derivative
    Foregut Pharynx, esophagus, stomach, proximal duodenum, thyroid, parathyroid, lungs, liver, gall bladder, pancreas
    Midgut Distal duodenum to proximal ½ of the colon
    Hindgut Distal ½ of the colon to anus, urinary bladder

    Head Structures

    • Many skeletal elements of the head are derived from neural crest cells, not mesoderm.

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    Test your knowledge on embryology and the stages of early development. This quiz covers congenital disorders, morphological models, and conditions related to organ positioning. Challenge yourself on these essential concepts in human development.

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