Medical Sciences - Bordeaux 2024-2025 UE11 Biology of Reproduction PDF

# Medical Sciences - Bordeaux 2024-2025 ## UE11 - Biology of reproduction - Embryology - Organogenesis ### The Third Week of Development **Notion tombée 1 fois au concours** **Notion tombée 2 fois au concours** **Notion tombée 3 fois ou plus au concours** ### Review * **Embryo in the form o...

# Medical Sciences - Bordeaux 2024-2025 ## UE11 - Biology of reproduction - Embryology - Organogenesis ### The Third Week of Development **Notion tombée 1 fois au concours** **Notion tombée 2 fois au concours** **Notion tombée 3 fois ou plus au concours** ### Review * **Embryo in the form of a didermic disc** - epiblast and hypoblast (visceral endoderm) * **Annexes in place** - with 3 cavities: amniotic, secondary vitelline and chorionic * **Prefiguration of the uteroplacental circulation** - Lacunas of maternal blood in the syncytiotrophoblast - Formation of the decidua * **Sagittal Section:** divides the body into 2 lateral sections, one right and one left * **Frontal (coronal) section:** divides the body into 2 parts, anterior and posterior * **Transverse section:** divides the body into 2 parts, superior and inferior ### Events of the Third Week * **Setting in place of the 3 primordial germ layers** - **Ectoderm (ectoblast)**: the superficial layer - **Mesoderm (mesoblast)**: the middle layer - **Endoderm (entoblast)**: the deep layer - All derived from the epiblast starting from the primitive line * **Setting in place of the body axes of the embryo** * **Neurulation**: the beginning of the neurulation * **Segmentation**: the beginning of the mesoderm segmentation ### Gastrulation - Setting in place of the primitive line * Begins on the _14th to 15th day_ - **Appearance at the epiblast surface**: on the longitudinal median axis - **Epiblast thickening**: at the caudal level progressing towards the cranial - **Forms the primitive groove bordered by epiblast ridges** ### Gastrulation - Formation of the primitive groove * Results from: - proliferation - recruitment - migration of the epiblast cells * **Towards the median axis** * **From the caudal part of the embryonic disc** * **Progression until it reaches the middle of the cephalic pole** ### Gastrulation - Formation of the primitive node * **Towards the 16th day** * **A deeper groove is formed, surrounded by an epiblast ridge** * **Located at the anterior end of the primitive line** ### Gastrulation - Establishment of the Planes of Symmetry of the Embryo * **Setting in place of the 3 orthogonal axes** - **Ventro-dorsal** - **Anterior-posterior** - **Right - left** ### Gastrulation - Ingression Through the Primitive Groove * **Epithelio-mesenchymal transition of the epiblast cells:** - loss of intercellular adhesion - reduction of cadherin expression - affinity for basal lamina proteins - expression of integrins - active movements: uses the cytoskeletal * **Migration in depth**: - towards the primitive endoderm - through the primitive groove - thanks to: - cell-extracellular matrix interactions - pseudopodia ### Gastrulation - Invagination and Migration of Epiblast Cells * **On the 15th day:** - **Formation of the definitive endoderm:** - replacement of all primitive endoderm * **On the 16th day:** - **Formation of the intra-embryonic mesoderm:** - invasion and migration of epiblast cells - colonization of all the surface of the embryo except: - the pharyngeal and cloacal membranes - the axial region * **The epiblast will become ectoblast at the end of gastrulation** ### Gastrulation - 2 Didermic Zones Devoid of Intra-embryonic Mesoblast * **Where the ectoblast and the entoblast remain tightly attached** * **Pharyngeal membrane:** - located at the cranial end of the embryonic disc - future mouth * **Cloacal membrane:** - located at the caudal end of the embryonic disc ### Gastrulation - Evolution of the Intra-embryonic Mesoblast * **Through the primitive node** - **Formation of the notochord:** - cells right behind the primitive node will give rise to definitive endoderm and the mesoblast - adjacent to the medio-sagittal plane - some cells will bypass the pharyngeal membrane to form the cardiogenic area which will give rise to the heart * **The most cranial cells:** - will give rise to the mesoderm * **The middle cells:** - will give rise to the mesoderm * **Caudal Cells** - will give rise to the extraembryonic **_mesoblast_** ### Setting in Place of the Notochord * **Origin:** migration and invagination of epiblast cells originating from the primitive node * **Progression in a cranial direction on the longitudinal median axis** * **Between the ectoderm and the primitive endoderm** * **On the 16th day** - **Invagination in glove-finger form:** - epiblast cells from the primitive node migrate - towards the pharyngeal end * **On the 17th day:** - **The anterior part will form a solid mass of cells:** - the prechordal plate - **The canal will elongate in a caudal direction :** - following the retreat of the primitive node * **The canal will be in place by the 17th day** - its lifespan is short * **On the 18th day:** - **Fusion and reabsorption of the ventral leaf of the chordal canal:** - on the medial line - gives rise to the notochord and endoderm - **The lumen of the chordal canal disappears:** - giving rise to the neurentric canal - **Temporary connection:** - from the primitive node, between the amniotic cavity and the secondary vitelline vesicle * **On the 20th day:** - **Proliferation and detachment of the entoblast:** - formation of a full tube - **Reformation of the entoblast:** - disappearance of the neurentric canal - **The notochord becomes hollow:** - the median axis around which the skeleton will be placed * **Completion around the 22nd day:** - involved in the induction of the vertebral bodies and the neurulation - disappearance: traces remain in the nucleus pulposus between the vertebral disks - remaining primitive line → caudal bud ### Primary Neurulation * **Origin of the cells:** from ectoderm cells * **Transformations of the medial ectoderm**: - **in a neural tube:** at the origin of the central nervous system - **flanked by the neural crests:** at the origin of the peripheral nervous system ### Primary Neurulation - Neural Plate * Formation of neurectoblast - Thickening of the medial ectoderm - Ahead of the primitive line - Behind the pharyngeal membrane - Cephalo-caudal gradient: - begins at a cranial position, - progresses toward the caudal part of the embryo - by the recruitment of neurectoblast cells - Appearance on the 18th day ### Primary Neurulation - Neural Groove * Raised edges of the neural plate forming the neural ridges * Invagination of the neural plate forming the neural groove ### Primary Neurulation - Neural Tube * The neural ridges meet and fuse together on the medial line * Neurectoderm detaches from the superficial ectoderm: - Formation of the neural tube and the neural crests * The lining formed by the neurectoblasts; - surrounds a lumen called the neural canal ### Primary Neurulation - Molecular Mechanisms of Neurulation * The cranial part of the notochord, just beneath the neural plate, secretes factors: - chordin, noggin and follistatin * **They inhibit BMP4 (Bone Morphogenesis Protein)**: - allowing neurectoderm differentiation - BMP4 usually inhibits the transformation of ectoderm cells into neural cells (default pathway) - BMP4 has a dorsalizing effect on the ectoblast * **FGFs also inhibit BMP4 (transcription)** * Inhibition of BMP4 also enables the formation of the neural crests, - thanks to the gradient of BMP4 inhibitors - **Near the notochord:** high inhibitor concentration: - neural tube formation - **Far from the notochord:** high activity of BMP4: - ectoderm cells which will give rise to the epidermis - **Intermediate region:** - formation of the neural crests ### Gastrulation - Growth of the Embryonic Disc * **Initial Aspect:** - goes from 0.2 mm to 2 mm in length by the end of the _3rd week_ * **Progressive elongation of the embryonic disc:** - Growth is uneven. - The cranial part is dominant compared to the caudal region. - Pear-shaped appearance - The primitive line retreats to the caudal end (disappears by the _4th week_) - Cephalo-caudal differentiation gradient: - Cranial part ceases to proliferate at the middle of the _3rd week_ - and differentiates - Caudal part ceases to proliferate at the end of the _4th week_ * **Pathology** - Abnormal persistence of the primitive line: sacrococcygeal teratoma containing tissues from the 3 germ layers ### Other Events of the Third Week - Setting in Place of the Allantoic Diverticulum and the Primordial Gonocytes * **Allantoic diverticulum:** - On the _16th day_ - Appears at the caudal level: posterior wall of the secondary vitelline vesicle - Evagination of the entoblast (endodermal origin) into the mesenchyme of the embryonic stalk - Only the intra-embryonic part will be involved in the formation of the bladder * **Primordial gonocytes:** - On the _18th day_ - Appear in the extra-embryonic splanchnopleure around the allantoid - Primordial germ cells that will migrate to the intra-embryonic position at the level of the genital crests (_4th week_) * **Vasculo-sanguineous islets: - On the _17th day_ - In the extra-embryonic mesoblast, then in the embryonic stalk - First in the splanchnopleure, then in the embryonic stalk to extend into the chorion.