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Lateral Plate Mesoderm and Endoderm

July 30, 2008

Developmental Biology

Major Mesoderm Lineages

Lateral Plate Mesoderm

Lateral Plate Mesoderm/Coelom

Lateral Plate Mesoderm ­ Coelomic Cavities

Lateral plate layers

Somatic – associates with ectoderm ­ contributes to connective tissue of body wall & limbs

Visceral – associates with endoderm

Lateral plate derivatives surround coelomic cavities

­ pleural ­ pericardial ­ peritoneal

Coelomic Cavity Formation

­ mesentery – peritoneal membrane lining abdominal cavity ­ dorsal mesentery attaches gut to body wall ­ pericardial cavity closes off in ventral anterior coelom

Coelomic Cavity Formation

­ pleuropericardial folds displaced by growing lungs

­ pericardium forms as separate sac

Lateral Plate Mesoderm and Endoderm ­ Overview

Endoderm

Digestive system

Pharyngeal pouches

Liver, lung, etc.

Extraembryonic membranes

Heart

Blood vessel

Vasculogenesis

Angiogenesis

Blood cells

Cardiovascular development

Coelomic cavity formation

Heart Forming Cells Cardiogenic mesoderm ­ early through primitive streak ­ induced by endoderm signals – BMPs ­ inhibited by notochord – Noggin, chordin ­ inhibited by neural tube – Wnts ­ Wnts promote blood vessel formation (hemangiogenic mesoderm)

­ cells migrate towards midline

Cardiogenic Mesoderm

Endocardium – forms endothelial lining; ­ cushion cells – form valves

Atrial and ventricular myocytes – heart muscle

Formation of the Chick Heart

Somatopleure

Splanchnopleure Endocardial primordia

Neural tube

Foregut

Pericardial cavity

Endocardial tube

Endocardium

Cardiac Looping / Chamber Formation Looping converts anterior­posterior polarity into right­left polarity

Looping is dependent on L­R patterning proteins; e.g. Nodal

After looping, differential expression in L and R chambers

Human day 21 day 28 newborn

RA

Blood Vessel Formation Blood vessels form independently from the heart ­ link with the heart soon after formation ­ heart primordia starts beating after first circulatory loop is established

Blood Vessel Formation

Constraints:

Every individual’s circulatory system is unique (genetic pattern?); however, each develops in a similar way because of certain constraints.

1. Physiological – embryos need to function as they develop: ­ food absorption from yolk or placenta ­ oxygen and waste exchange from chorionic or allantoic membranes

Blood Vessel Formation

2. Evolutionary – e.g. aortic arches

Constraints:

Vertebrate model (e.g. human – 29 days)

Primitive fish

Every individual’s circulatory system is unique (genetic pattern?); however, each develops in a similar way because of certain constraints.

Human Aortic Arches

Mammals and birds convert 6 pairs of aortic arches into single aortic arch

Blood Vessel Formation

2. Evolutionary – e.g. aortic arches

3. Physical – laws of fluid movement and diffusion constrain the size and number of vessels ­ most effective fluid transport in large tubes ­ however, diffusion of nutrients and gases can take place only through small tube and at slow flow

Constraints:

1. Physiological – embryos need to function as they develop ­ food absorption from yolk or placenta ­ oxygen and waste exchange from chorionic or allantoic membranes

Every individual’s circulatory system is unique (genetic pattern?); however, each develops in a similar way because of certain constraints.

Blood Vessel Formation

Vasculogenesis – first process: capillary network formed from lateral plate mesoderm

Angiogenesis – second process: primary capillary networks are remodeled; veins and arteries made

Vasculogenesis and Angiogenesis

Angiogenesis

hemangioblasts Vasculogenesis

Splanchnic mesoderm

Model for Blood Vessel Formation

VEGF* gradient (green) from mesenchyme near blood islands induces endothelial cells to form arteries (red) ­ arteries induce veins (blue) *VEGF – vascular endothelial growth factor

Arterial and Venous Differentiation Angiogenesis

How to make sure veins attach just to arteries, and vice versa?

Artery precursor endothelial cells contain ephrin­B

Vein precursors contain ephrin receptor Ephb4 tyrosine kinase

Blood Cell Development Stem cells:

HSCs generate a series of intermediate stem cells with potencies restricted to certain lineages

Embryonic hematopoiesis takes place in the blood islands; ­ mostly RBC production

~10 11 RBCs are replaced daily

Pluripotential hematopoietic stem cells (HSC) are capable of generating all blood and lymph cells

Definitive hematopoietic cells derived from splanchnic lateral mesoderm surrounding the aorta ­ later move to the fetal liver; later to the bone marrow

Blood Stem Cells

Lateral Plate Mesoderm and Endoderm ­ Overview

Mesoderm lineages

Endoderm

Digestive system

Pharyngeal pouches

Liver, lung, etc.

Extraembryonic membranes

Heart

Blood vessel

Vasculogenesis

Angiogenesis

Blood cells

Cardiovascular development

Endoderm

Endoderm has two functions:

1. Instructions for the formation of notochord, heart, blood vessels, mesoderm

2. Construction of linings of the digestive tube (including liver, gall bladder, and pancreas), and respiratory tube ­ both the respiratory and digestive tubes are products of the primitive gut

Human Digestive System

Human Digestive System

Pharyngeal Pouches – Glandular Primorida

Pharyngeal Pouches

Regional Specification of the Gut

­ specified by regionally specific mesenchymal mesoderm

­ endoderm specified early; possibly before tube formation

transcription factors

Liver, Pancreas, Gall Bladder

The Respiratory Tube Lungs are derived from the digestive tube ­ laryngeotracheal groove occurs in the center of the pharyngeal floor (between 4 th pharyngeal pouch pair)

Lung Maturation ­ Surfactant Lungs fully differentiate late in development; ­ alveolar (air sac) cells secrete a surfactant; allows cells to slide against one another

Lungs fully differentiate late in development; ­ alveolar (air sac) cells secrete a surfactant; allows cells to slide against one another ­ secreted late in gestation; ~ wk 34 (human)

Lung Maturation ­ Surfactant Lungs fully differentiate late in development; ­ alveolar (air sac) cells secrete a surfactant; allows cells to slide against one another ­ secreted late in gestation; ~ wk 34 (human) ­ birth may be triggered by maternal immune response to surfactant

Extraembryonic Membranes Reptiles, birds, and mammals are amniotes ­ eggs are adapted to develop on dry land ­ adaptations use combinations of ectoderm, endoderm, &mesoderm to solve specific problems of land eggs

desiccation → amnion gas exchange → chorion waste disposal → allantois nutrition → yolk sac

Problem → Solution

Splanchnopleure

Somatopleure

desiccation → amnion gas exchange → chorion waste disposal → allantois nutrition → yolk sac

Problem → Solution

Somatopleure forms: Amnion Chorion

Splanchnopleure forms: Allantois Yolk sac

Chick / Placental Mammals

Allantois – forms portion of the umbilicus ­ vascularizes umbilicus ­ forms urachus – empties fetal bladder