Lec 10 Dev Bio

The Intermediate Mesoderm and Urogenital Tract Formation

Development of the Pronephros, Mesonephros, and Metanephros

The intermediate mesoderm is essential for the formation of the urogenital system, including the kidneys and associated ducts. Kidney development proceeds through three main stages: pronephros, mesonephros, and metanephros.

• Pronephros: The earliest kidney form, consisting of the pronephric duct and some tubules. It is transient and degenerates early in development.

• Mesonephros: Forms as the pronephros degenerates. The mesonephric duct (Wolffian duct) persists and induces new tubules. The mesonephros also degenerates but contributes to reproductive tract structures.

• Metanephros: The permanent kidney. The metanephrogenic mesenchyme in the posterior intermediate mesoderm induces the branching of ureteric buds from the nephric duct, leading to kidney formation.

Reciprocal induction between the metanephrogenic mesenchyme and ureteric buds is crucial for further kidney development.

Example:

The Wolffian duct in males becomes part of the reproductive tract, while in females it regresses.

The Lateral Plate Mesoderm

Structure and Subdivision

The lateral plate mesoderm splits into two layers, each contributing to different body structures.

• Somatic (parietal) mesoderm: Dorsal layer, combines with ectoderm to form the somatopleure (body wall).

• Splanchnic (visceral) mesoderm: Ventral layer, combines with endoderm to form the splanchnopleure (gut wall).

• Coelom: The space between these layers becomes the body cavity, subdivided into pleural, pericardial, and peritoneal cavities.

Development of Heart and Circulatory System

Heart Field Formation and Fusion

The vertebrate heart develops from two regions of splanchnic mesoderm (left and right heart fields) that fuse at the midline.

• Pluripotent stem cells: Heart field cells are specified to form heart tissue but can differentiate into cardiomyocytes, endothelial cells, epicardial cells, or Purkinje fiber cells.

• Induction: Differentiation of cell types from pluripotent progenitors is regulated by signaling pathways and transcription factors.

Regulation of Heart Development

• BMP2: Required for heart development, secreted by endoderm and mesoderm.

• Noggin and Wnt proteins: Block BMP activity, ensuring proper heart field induction.

• Wnt inhibitors (Cerberus, Dickkopf): Secreted by anterior endoderm, allow cardiac precursor cell formation.

Heart Tube Formation and Migration

• Cardiac tubes form on each side and fuse at the midline.

• Migration is influenced by retinoic acid gradients, determining inflow regions (atria, sinus venosus).

Prevention of Heart Primordia Fusion

• Failure of fusion leads to two separate hearts, as seen in experimental models (chick, zebrafish, mouse mutants).

Adult Human Heart Anatomy

• Chambers: Right atrium, right ventricle, left atrium, left ventricle.

• Major vessels: Aorta, pulmonary trunk, superior/inferior vena cava.

Heart Chamber Formation and Looping

• Looping: Converts anterior-posterior polarity to left-right polarity, controlled by Nodal and Pitx2 proteins.

• Atria and ventricles are specified before looping, visualized by myosin expression.

Septum and Outflow Tract Formation

• Septum primum and secundum: Separate left and right heart chambers.

• Cardiac neural crest cells: Migrate to form endothelium of aortic arch arteries and septum between aorta and pulmonary trunk.

Vasculogenesis and Angiogenesis

• Vasculogenesis: Mesenchyme cells differentiate into hemangioblasts, forming blood islands. Endothelial cells form tubes and capillary plexus.

• Angiogenesis: Remodeling of capillary networks, differentiation of arteries/veins, and sprouting of new vessels from existing ones.

Hematopoiesis

Hematopoiesis is the process of blood cell differentiation from pluripotent stem cells.

• Lineages: Myeloid (erythrocytes, monocytes, granulocytes, megakaryocytes) and lymphoid (B cells, T cells, plasma cells).

Table: Hematopoietic Lineages

Stem Cell	Progenitor	Final Cell Types
Pluripotent stem cell	CFU-GEMM	Erythrocyte, Monocyte, Granulocyte, Megakaryocyte
Pluripotent stem cell	CFU-L	B cell, T cell, Plasma cell

Conversion from Prenatal to Postnatal Circulation

• Prenatal circulation bypasses the lungs via the foramen ovale and ductus arteriosus.

• Postnatal circulation closes these shunts, directing blood through the lungs for oxygenation.

The Endoderm

Endodermal Derivatives

The endoderm forms the linings of the digestive and respiratory tubes and induces development of mesodermal organs.

• Digestive tube: Forms the gut lining and associated glands.

• Respiratory tube: Develops as an outgrowth of the digestive tube.

• Inductive role: Signals to mesoderm for heart, blood vessel, and notochord formation.

Early Endoderm Development

• Formation of the gut tube and pharynx from the anterior endoderm.

• Pharyngeal pouches and arches form, with neural crest cell migration contributing to further development.

Pharyngeal Pouch Derivatives

• First pair: Auditory cavities and eustachian tubes.

• Second pair: Walls of the tonsils.

• Third pair: Thymus and one pair of parathyroid glands.

• Fourth pair: Second pair of parathyroid glands.

• Thyroid gland: Derived from a central diverticulum between the second pouches.

Gut Development

Regional Specification and Inductive Interactions

Gut tissue specification is regulated by gradients of retinoic acid and inductive signaling between endoderm and mesoderm.

• Endoderm: Forms gut lining and glands.

• Mesoderm: Forms smooth muscle for peristalsis.

• Neural crest (ectoderm): Forms nerve plexus within the gut.

Development of Liver and Pancreas

Organogenesis from the Digestive Tube

• Liver and pancreas arise as outgrowths of the digestive tube.

• Pancreas has ventral and dorsal anlagen that rotate and fuse.

• Fusion variations result in different pancreatic duct arrangements.

• Ectoderm and notochord inhibit liver gene expression; cardiogenic mesoderm promotes it via Fgf1/Fgf2.

Table: Pancreatic Duct Variations (Inferred)

Fusion Type	Duct Arrangement
Complete fusion	Single main duct
Partial fusion	Accessory duct present
No fusion	Separate ducts

Development of the Respiratory Tube

Lung Formation and Differentiation

• Lungs develop as derivatives of the digestive tube.

• Respiratory diverticulum bulges from the pharynx and branches to form bronchi and lungs.

• Lungs differentiate late in embryonic development; surfactant production is essential for postnatal lung function.

Role of Surfactant

• Surfactant: A film of proteins and phospholipids that reduces surface tension in alveoli.

• Facilitates alveolar expansion, prevents collapse, enables first breaths, and defends against microorganisms.

• Surfactant production begins around 33-34 weeks in humans; may signal initiation of birth.

Functions of Surfactant

• Facilitates expansion of alveoli during inspiration

• Prevents collapse of small alveoli

• Enables lungs to expand during first breaths after birth

• Defense against microorganisms

Additional info: These notes cover key aspects of organogenesis and tissue specification relevant to developmental biology, with emphasis on mesodermal and endodermal derivatives, and their inductive interactions. While not directly matching the Cell Biology chapter list, the content is foundational for understanding cell differentiation, tissue formation, and organ development.