✅ PERIODO EMBRIONARIO | DE LA TERCERA A LA OCTAVA SEMANA 📚 ORGANOGÉNESIS | CAPAS GERMINALES

Embryonic Development: From Flat to Cylindrical Structures

Overview of Embryonic Period

• The transition from a flat structure to a cylindrical trilaminar disc occurs during the embryonic period, involving key processes such as neurulation, somite formation, angiogenesis, and embryo folding.

• Focus will be on derivatives of germ layers formed during gastrulation; detailed exploration based on Langman and Moore's texts.

Prenatal Development Stages

• Prenatal development is divided into three periods: preembryonic, embryonic, and fetal.

• Different sources define the start of the embryonic period variably; Langman and Moore state it spans from week 3 to week 8, while Hib and Carlson extend it from week 4 to week 8.

Organogenesis Phase

• The embryonic period is also known as organogenesis when ectoderm, mesoderm, and endoderm give rise to various tissues and organs.

• Although organs begin forming during this phase, they require further maturation in the fetal stage for postnatal life preparation.

Teratogenic Susceptibility

• This developmental stage makes embryos susceptible to teratogens (e.g., certain medications or viruses), which can lead to developmental delays or anomalies.

Visual Aids for Understanding Development

• Utilization of sagittal sections of a gravid uterus helps visualize implantation locations within the maternal endometrium.

Germ Layer Derivatives: Ectodermal Developments

Neurulation Process

• As notochord develops, it induces overlying ectoderm thickening into the neural plate.

• Cells in this plate are termed neuroectoderm; their induction marks the beginning of neurulation—the formation of the neural tube.

Neural Tube Formation

• Initially equal in length with notochord but grows cephalocaudally as notochord elongates; lateral edges elevate forming neural folds while central region sinks creating a neural groove.

Fusion Dynamics

• Neural folds converge at midline starting cervical fusion extending both cranially and caudally.

• Completion results in neuropores communicating with amniotic cavity—anterior (rostral) closes around day 25; posterior (caudal), day 28.

Consequences of Neurulation Completion

• Successful closure leads to formation of central nervous system components including retina and pineal gland.

Ectoderm Segmentation Post-Neurulation

Ectoderm Division Outcomes

Neural Crest Cells and Their Contributions

Transition from Epithelium to Mesenchyme

• Neural crest cells undergo an epithelial-to-mesenchymal transition, actively migrating from the neuroectoderm into the underlying mesoderm.

• These cells differentiate into various structures including connective tissue, cranial bones, thyroid C cells, dermis of the face and neck, and more.

Ectomesenchyme Formation

• The mesenchyme derived from neural crest cells is referred to as ectomesenchyme, contributing significantly to organ and tissue formation.

• Key derivatives include epidermis, hair, nails, skin glands, pituitary gland (adenohypophysis), and dental enamel.

Development of Sensory Structures

• The superficial ectoderm gives rise to otic placodes that invaginate to form optic vesicles for hearing and balance.

• Lens placodes also invaginate during week five to form lens vesicles which develop into the eye's lens.

Mesodermal Development Stages

Initial Mesoderm Structure

• By day 17 of development, paraxial mesoderm forms a thickened plate adjacent to the notochord.

• This structure organizes into somitomeres in the cephalic region before transitioning into somites along the body axis.

Somite Formation

• Somites appear sequentially at a rate of three pairs per day until reaching 42–44 pairs by week five.

• Distribution includes occipital (4), cervical (8), thoracic (12), lumbar (5), sacral (5), and coccygeal (8–10).

Somite Subclassification

Structure of Somites

• Each somite has a triangular shape divided into three segments: sclerotome, myotome, and dermatome.

• Sclerotome develops into axial skeleton components such as ribs and vertebrae; it also contributes to tendon formation.

Myotome and Dermatome Functions

• Myotome differentiates into skeletal muscle while dermatome gives rise to dermis layers in skin.

Intermediate Mesoderm Contributions

Urogenital System Development

• The intermediate mesoderm connects paraxial mesoderm with lateral mesoderm; it differentiates into urogenital structures.

• Nefrotomas arise in cervical/upper thoracic regions while caudally forming nephrogenic cords marking early genital system development.

Lateral Mesoderm Layers

Development of Mesoderm and Its Contributions to Embryonic Structures

Formation of Parietal and Visceral Layers

• The parietal mesoderm gives rise to the parietal layers of the peritoneal, pleural, and pericardial cavities, including bones of the limbs, shoulder girdle, pelvic girdle, and sternum.

• Splanchnic mesoderm forms visceral layers of these cavities and contributes to the stroma of some organs, smooth muscle covering certain viscera, cardiac muscle, and initiates blood formation by invading the yolk sac in week three.

Importance of Intraembryonic Coelom

• The coalescence of celomic spaces results in a single longitudinal space called intraembryonic coelom which is crucial for forming three types of body cavities: pericardial cavity, pleural cavities, and peritoneal cavity.

Cardiovascular Development Initiation

• Notably in week three, development begins for the heart, blood vessels, and blood cells; an 18-day old embryo is referenced for visualization.

• Cells destined to form the heart originate from splanchnic mesoderm located cranially before the buccopharyngeal membrane and neural folds. This area is termed Primary Cardiogenic Field.

Hematopoiesis Begins

• Blood cell development starts at week three when extraembryonic splanchnic mesoderm surrounding the yolk sac differentiates into hemangioblast precursors that cluster into cell islands known as Wolf-Pander or blood islands.

• Central cells within these islands become blood cell precursors while peripheral cells flatten to form endothelial cells lining developing blood vessels.

Vascular Network Formation

• These blood islands undergo budding through a process called budding (gemación), interconnecting to create a network of small vessels.

Transition from Primitive Blood Cells

• Primitive blood cells originating from the yolk sac enter circulation by day 22 but are replaced by fetal-derived cells around weeks six or seven due to programmed cell death. The yolk sac serves as the first hematopoietic organ but is extraembryonic.

Mechanisms of Vessel Formation

• Important concepts include:

• Vasculogenesis: formation of new vessels from blood islands.

• Angiogenesis: budding from pre-existing vessels.

Endoderm Derivatives in Embryo Development

Overview of Endoderm Functionality

• The endoderm covers the ventral surface of a 19-day old embryo; its primary derivative is the gastrointestinal tract formed during cephalocaudal and lateroventral folding processes.

Embryo Folding Dynamics

• Cephalocaudal folding occurs mainly due to longitudinal growth in central nervous system leading to an arched shape; this folding is more pronounced at cephalic and caudal regions where future head and tail folds will develop.

Tubular Structure Formation

• As days progress with lateral folding during somite formation, embryos take on cylindrical shapes with endoderm enclosed inside forming primitive intestine or intestinal tube.

Communication Between Yolk Sac & Intestine

• A narrow connection called omphalomesenteric duct links primitive intestine with yolk sac as communication narrows progressively due to embryonic movements.

Distinct Portions Within Primitive Intestine

• Anterior portion develops into foregut,

• Caudal portion becomes hindgut,

• Intermediate section forms midgut temporarily connected via omphalomesenteric duct.

Membranes Associated with Intestinal Development

Key Membrane Developments

• At anterior end (foregut), buccopharyngeal membrane fenestrates during week four establishing connection between amniotic cavity and primitive intestine.

Posterior End Developments

• Cloacal membrane ruptures in week seven forming anal opening,

• Partial incorporation of allantois into embryo creates cloaca while distal allantois remains attached at umbilical ring along with yolk sac and umbilical vessels by fifth week.

Endoderm's Role Beyond Gastrointestinal Tract

Derivatives of Germ Layers in Human Embryology

Overview of Epithelial Linings

• The epithelial lining of the urinary bladder and urethra, as well as the tympanic cavity and auditory canal, are discussed.

• Visual aids summarizing the derivatives of germ layers are presented, encouraging viewers to pause and take screenshots for study purposes.

Ectoderm Derivatives

• The ectoderm is divided into superficial ectoderm and neuroectoderm.

• Neuroectoderm gives rise to the neural tube and neural crest cells.

Mesoderm Classification

• Mesoderm is categorized into paraxial, intermediate, and lateral mesoderm.

• Some authors differentiate a fourth type called head mesoderm, which originates from neuromeres.

Endoderm Characteristics

• Unlike ectoderm and mesoderm, endoderm does not have sub-classifications; its derivatives are listed without further division.

Conclusion & Future Content

• A promise for future videos covering external aspects observed during embryonic development is made.