Paraxial and Intermediate Mesoderm Lange BIOL 370 – Developmental Biology Topic #14

In this chapter we shall feel further development of the mesoderm and the endoderm: the endoderm will form the digestive and respiratory tubulature and organs the mesoderm generates all the intermediate organs between the ectoderm and endoderm

Figure 11.1 Major lineages of the amniote mesoderm (Part 1) Intermediate mesoderm  kidneys, gonads Chordamesoderm  notochord Paraxial mesoderm  head, somites somites  cartilage, tendons, skeletal muscle, dermis, endothelial cells Lateral plate mesoderm  circulatory system, body cavities

Figure 11.2 Gastrulation and neurulation in the chick embryo, focusing on the mesodermal component It is important at this stage to work on developing an ability to visualize the 3-D perspective of the organism from these sections.

Figure 11.4 Formation of new somites Somites are bilaterally paired blocks of mesoderm that form along the anterior-posterior axis of the developing embryo. An alternative term used in place of somites is metamere.

Figure 11.4 Formation of new somites (Part 2) New somites are formed in the process of somatogenesis, which is both molecular and cellular in origin. Of key interest in the somite formation below is the use of ephrins (also known as ephrin ligands (abbreviated Eph)). These are a family of proteins that serve as the ligands of the ephrin receptor..

Figure 11.5 Notch signaling and somite formation (Part 4) the Notch ligand is produced by the Delta-like 3 gene. “E” shows a Delta-like 3 gene knockout mouse with a clearly aberrant skeleton. White dots show the pattern of ossification centers in both mice.

Figure 11.7 Hypothetical pathway for regulation of the clock through which an Fgf8 gradient regulates a Wnt oscillating clock The pathway for control of the genes regulating somite formation are shown below. One of the most pressing issues we have poor understanding of currently is how the timing of these events is regulated and coordinated.

Figure When segmental plate mesoderm is transplanted it differentiates according to its original position In this chronologically backwards transplant study, older donor mesodermal tissue is transplanted to an earlier stage embryo in a different location. The resultant donor structure is already fixed and it continues development into its original presumptive structures… in this case, into vertebrae now developing in the cervical neck region.

Transverse section through the trunk of a chick embryo on days 2–4 Somites visible as the red structures are multipotent at this point and their specification is dependent upon their location relative to paracrine factors received from surrounding tissues (such as the neural tube, epidermis, etc.)

Figure Transverse section through the trunk of a chick embryo on days 2–4 (Part 1) Dermamyotome – the segment of the somite that is going to form the dermis (dermatome) and the skeletal muscle (myotome). The combined name is used because the initial development is slower than in other somite segments.

Figure Transverse section through the trunk of a chick embryo on days 2–4 (Part 2) Notice now the separate dermatome and myotome.

Figure Primaxial and abaxial domains of vertebrate mesoderm (Part 1) Primaxial simply refers to the portion of the mesoderm that is more “medial” and the abaxial is more distal to the center axis.

Remember from earlier discussions the term “epiblast” which is referring to the upper layer.

Figure  Ablating Noggin-secreting epiblast cells results in severe muscle defects Epiblastic mesodermal cells that are experimentally ablated (destroyed, usually chemolytically or electrolytically) result in severe muscle defects arising in the chick.

Figure Conversion of myoblasts into muscles in culture Notice the different paracrine factors that will facilitate the different steps. FGF = fibroblast growth factor CAM = cell adhesion molecule

Figure Schematic diagram of endochondral ossification Chondrocytes – cartilage producing cells Osteocytes - cells within the calcified aspect of bone Osteoblasts - cells that synthesize bone Osteoclasts – cells that degrade bone.

Figure 6.9 Endochondral ossification in a long bone Bone collar forms around hyaline cartilage model. Cartilage in the center of the diaphysis calcifies and then develops cavities. The periosteal bud invades the internal cavities and spongy bone begins to form. The diaphysis elongates and a medullary cavity forms as ossification continues. Secondary ossification centers appear in the epiphyses in preparation for stage 5. The epiphyses ossify. When completed, hyaline cartilage remains only in the epiphyseal plates and articular cartilages. Hyaline cartilage Area of deteriorating cartilage matrix Epiphyseal blood vessel Spongy bone formation Epiphyseal plate cartilage Secondary ossification center Blood vessel of periosteal bud Medullary cavity Articular cartilage Childhood to adolescence Birth Week 9 Month 3 Spongy bone Bone collar Primary ossification center

Figure 6.11 Long bone growth and remodeling during youth. Bone growth Bone remodeling Articular cartilage Epiphyseal plate Cartilage grows here. Cartilage is replaced by bone here. Cartilage grows here. Bone is resorbed here. Bone is resorbed here. Bone is added by appositional growth here. Cartilage is replaced by bone here.

Steel “Bone Cages” used to lengthen legs. These were originally developed in the Soviet Union in the 1950s to treat dwarfism.

An example of untreated acromegaly.

Figure Endochondral ossification In this image we see a potential pathway for the transition of cartilage into bone…. The formation of endochonral ossification.

Figure 6.17 Fetal primary ossification centers at 12 weeks. Parietal bone Radius Ulna Humerus Femur Occipital bone Clavicle Scapula Ribs Vertebra Ilium Tibia Frontal bone of skull Mandible

Figure Skeletal mineralization in 19-day chick embryos that developed (A) in shell-less culture and (B) inside an egg during normal incubation The shell of the egg is the primary source of calcium for ossification of the bird skeleton prior to hatching.

Figure Scleraxis is expressed in the progenitors of the tendons Scleraxis expressing progenitor cells lead to the eventual formation of tendon tissue and other muscle attachments. Scleraxis is also associated with embryonic tissues that develop into tendon and blood vessels. Aortic dissection is a disorder that is often due to an abnormality in scleraxis of the aorta. The late John Ritter is one person to have died due to this condition.

Figure Induction of scleraxis in the chick sclerotome by Fgf8 from the myotome Scleraxis expressing progenitor cells lead to the eventual formation of tendon tissue and other muscle attachments. [ [

Figure General scheme of development in the vertebrate kidney Pronephros - the most basic of the excretory organ that develops in vertebrates Mesonephric tubules - form to attach to the mesonephros as the pronephros degenerate Metanephros – the final mammalian kidney

Figure Signals from the paraxial mesoderm induce pronephros formation in the intermediate mesoderm of the chick embryo

Figure 25.3b

Figure 25.4b

Figure Reciprocal induction in the development of the mammalian kidney

Figure Kidney induction observed in vitro

Figure Development of the bladder and its connection to the kidney via the ureter

End.