Embriología del aparato digestivo

Welcome to Embryology of the Digestive System

Introduction to Developmental Stages

• Estela Maris Roma introduces the class on embryology, using Leonardo's "The Last Supper" fresco as a welcoming visual.

• Overview of intrauterine development stages: preembryonic (weeks 1-3), embryonic (weeks 4-8), and fetal (week 9 to birth).

Preembryonic Period

• The preembryonic period includes the formation of three germ layers: ectoderm, mesoderm, and endoderm.

• These germ layers represent fundamental divisions for future organ systems.

Embryonic Period

• The embryonic period is critical for forming all organ systems and is highly susceptible to teratogenesis (malformation).

Fetal Period

• The fetal stage focuses on differentiation and maturation of organs in preparation for life outside the womb.

Detailed Weekly Breakdown

Second Week Developments

• In the second week, the preembryo consists of two layers: epiblast (floor of amniotic cavity) and hypoblast (roof of yolk sac).

Third Week Key Events

• Formation of the primitive streak marks a significant event; it serves as a pathway for cell migration during gastrulation.

Gastrulation Process

• Cells entering the primitive streak lose intercellular connections, leading to invagination—a process crucial for layer formation.

Germ Layer Naming Changes

• Post-gastrulation, epiblast becomes ectoderm; hypoblast transforms into endoderm. Mesoderm forms as an additional layer between them.

Germ Layers Functions

Ectoderm Contributions

• Ectoderm develops into structures that interface with the environment, including skin epidermis and nervous system components.

Mesoderm Functions

• Mesoderm gives rise to connective tissues, muscles, cartilage, bone, and adipose tissue through amoeboid movements.

Endoderm Role

• Endoderm contributes to internal organs involved in metabolism such as respiratory lining and digestive tract structures like liver and pancreas.

Digestive System Development Insights

Digestive Tract Formation

• The epithelial lining related to digestion derives from endoderm while supporting structures come from adjacent mesoderm.

Neural Plexus Development

• Nervous plexuses processing sensory information arise from neural crest cells linked with ectodermal origins.

This structured overview captures key insights from Estela Maris Roma's lecture on embryology concerning digestive system development. Each section highlights essential concepts tied directly to specific timestamps for easy reference.

Development of the Intraembryonic Coelom and Primitive Gut

Formation of the Intraembryonic Coelom

• The intraembryonic coelom begins to form in the third week, characterized by the separation of lateral mesoderm into two layers, creating a space known as the intraembryonic coelom.

Cylindrization Process

• Transition from a flat body shape in the third week to a cylindrical form occurs through folding processes in both cephalocaudal and transverse planes due to excessive growth of the nervous system and somite structures.

Incorporation of Yolk Sac Structures

• The roof of the yolk sac, which contains endodermal cells from early cell waves, becomes integrated into the embryo's body, now referred to as primitive gut. This structure is divided into three sectors: anterior (foregut), middle (midgut), and posterior (hindgut).

Communication with Yolk Sac

• The midgut connects with the yolk sac via a structure called vitelline duct or omphalomesenteric duct, facilitating nutrient transfer during early development. The anterior and posterior intestines are accessed through specific intestinal portals that play crucial roles in signaling between endoderm and neighboring mesoderm.

Developmental Signaling Sites

• Both anterior and posterior intestinal portals serve as critical sites for gene expression regulation along anteroposterior and dorsoventral axes, influencing future organ development distinctively across these regions. This differentiation process continues beyond week four as organs prepare for extrauterine life.

Primitive Mouth Formation

Stomodeum Development

• A primitive mouth structure known as stomodeum develops at this stage; it is lined with ectoderm and plays a significant role in forming oral cavity structures including parotid gland parenchyma through interactions with adjacent mesoderm.

Membrane Perforation Events

• The buccopharyngeal membrane perforates around week four, establishing communication between oral cavity and pharynx—this event is essential for proper digestive tract formation leading to functional anatomy in adults.

This structured summary captures key developmental milestones discussed within specified timestamps while maintaining clarity for study purposes.

Development of the Digestive System

Formation of the Pharyngeal Gut

• The pharyngeal gut, extending to the laryngotracheal bud, is referred to as the pharyngeal intestine. This area involves endoderm and neighboring mesoderm contributing to the formation of the floor of the mouth.

• The stomodeum primarily contributes to forming the pharynx and buds for sublingual and submandibular glands, while also giving rise to organs unrelated to digestion such as tympanic cavity and palatine tonsils.

Development of Respiratory Structures

• A thyroid bud emerges from the ventral side, leading to thyroid gland formation. The laryngotracheal bud marks a boundary between pharyngeal intestine and true anterior intestine, which will form respiratory tract linings.

Anterior Intestine Development

• The anterior intestine extends from the laryngotracheal bud to hepatopancreatic bud, giving rise to esophagus, stomach, duodenum (first two portions), liver, pancreas, and gallbladder; further details will be discussed later in class.

Midgut Characteristics

• The midgut stretches from hepatopancreatic bud to an imaginary limit corresponding with adult colon anatomy; this demarcation separates territories supplied by superior mesenteric artery from those supplied by inferior mesenteric artery.

• Organs derived from midgut include third and fourth portions of duodenum, jejunum, ileum, ascending colon, cecum with appendix, and transverse colon. This section must accommodate long organs during development stages when abdominal cavity is small due to large developing organs like liver and kidneys.

Physiological Hernia Event

• Between weeks six through ten of development, midgut connected via vitelline duct displaces into umbilical coelom (umbilical cord space) resulting in physiological hernia; this process allows growth within umbilical region rather than confined abdominal space.

Intestinal Rotation Dynamics

Growth Patterns During Herniation

• As midgut forms a loop (cephalic limb before vitelline duct), it rotates 90 degrees clockwise around superior mesenteric artery axis during its time in umbilical cord space; cephalic limb grows more than caudal limb during this phase.

Return to Abdominal Cavity

• By week ten when abdominal cavity expands sufficiently for organ accommodation:

• Cephalic limb returns first undergoing another rotation positioning it predominantly on left posterior side.

• Caudal limb follows occupying right anterior position upon return.

Posterior Intestine Development

Proctodeum Formation

• At caudal level exists proctodeum entry covered by embryonic tail; cloacal membrane at proctodeum's end is set for perforation after buccopharyngeal membrane rupture following cephalocaudal developmental pattern observed earlier in class.

Derivatives of Posterior Intestine

• Posterior intestine spans from junction of proximal two-thirds transverse colon with distal third; derivatives include transverse colon descending colon rectum part of anus along with cloacal membrane that perforates around week seven leading towards anal opening formation alongside urethral openings in both sexes followed by partitioning into separate structures over time.

Accessory Glands Development

Liver and Pancreas Origin

• Liver originates from an endodermal bud emerging on ventral side between anterior and middle intestines; similar associations occur between endoderm and adjacent mesoderm throughout digestive system development processes which will be elaborated using visual aids later in class session.

Development of the Liver and Pancreas

Formation of the Liver

• The liver develops from an endodermal hepatic bud that penetrates into the mesoderm of the transverse septum, which will eventually form the diaphragm's central tendon.

• Initially, the liver does not perform its well-known metabolic functions; instead, it has two critical roles: collecting blood to send to the developing heart and forming blood elements.

• Hematopoietic stem cells (hemocytoblasts) colonize the developing liver, allowing it to function as a precursor to bone marrow until around three months of intrauterine life.

• The liver grows significantly during development, occupying almost all abdominal cavity space. This growth causes displacement of the midgut into the umbilical cord's celom.

• By this stage, there is no formed bone marrow; thus, the liver takes on essential functions in blood circulation and cell formation.

Development of the Pancreas

• The pancreas originates from two buds: a dorsal bud emerging from the primitive intestine's dorsal side and a ventral bud associated with bile duct development.

• As development progresses, growth disparity between duodenal walls leads to posterior displacement of both pancreatic buds, facilitating their fusion.

• The ventral bud contributes to parts of the pancreas such as its head and uncinate process while also forming part of its duct system.

• The dorsal bud forms other parts including body and tail sections along with endocrine components (islets of Langerhans) and exocrine components necessary for digestion.

Summary Insights

• The liver has three origins: hepatocytes from endodermal tissue for functional metabolism; Kupffer cells from mesoderm for support; and hepatic sinusoids originating from vitelline veins.

• Understanding these developmental processes is crucial for grasping how organs like the liver and pancreas evolve structurally and functionally during embryogenesis.