Paraxial & Intermediate Mesoderm

Mesoderm of the Neurula Stage

• 1. Chordamesoderm - forms the notochord

• 2. Paraxial Mesoderm (somitic dorsal mesoderm) - here dorsal refers to structures that will form in the back of the embryo along the spine

– Cells will form somites - blocks of mesoderm cells on both sides of the neural tube - produce connective tissues of the back (bone, muscle cartilage and dermis)

• 3. Intermediate mesoderm - forms the urogenital system

Mesoderm of the Neurula Stage

• 4. Lateral late mesoderm- forms the heart, blood vessels & blood cells - the lining of the body cavities - and also all mesodermal components of the limb except the muscles

– Also forms extraembryonic membranes important in transporting nutrients to the embryo

• 5. Head mesenchyme - forms connective tissues and musculature of the face

Lineages of Mesoderm

Paraxial Mesoderm

• Mesodermal cell formation occurs synchronously with neural tube formation

• Thick bands of mesodermal cells form on either side of the neural tube

– This is paraxial mesoderm - referred to as segmental plate in birds and unsegmented mesoderm in mammals

• As the primitive streak regresses & neural folds begin to gather at the center of the embryo - the paraxial mesoderm divides into block

– Somites

Somites: Overview

• Somites are transient - but important in organizing the segmental pattern of vertebrate embryos

• Important in migration paths for neural crest cells and spinal nerve axons

• Give rise to the cells that form the vertebrae and ribs, dermis of the dorsal skin, skeletal muscle of the back and the skeletal muscle of the body wall and limbs

Gastrulation - Neurulation in the Chick

Periodicity - Somite Formation

• First somites - anterior portion of the trunk - new somites bud off from the rostral end of the paraxial mesoderm at regular intervals

• Somite formation begins as cells become organized into whorls of cells - somitomeres

– Somitomeres become compacted - bond together by a epithelium & will separate off as a individual somite

• Somite number is a good indicator of how far along an embryo is in development

– 50 in chicks, 65 in m ice and 500 in snakes

Neural Tube and Somites

Hairy Gene and Somite Formation

• First expressed in the caudal portion of each somite

• Second - it is expressed in the presomitic segmental plate or unsegmented mesoderm in a cyclic wavelike manner cresting every 90 minutes

– Starts caudally moves anteriorly as the somite forms

– Caudalmost region of this anterior expression band correlates with the posterior terminus of the next somite to be formed

Separation - Somite Formation

• Hairy gene encodes a transcription factor - target unknown

• One possible set of targets - genes for ephrin and its receptor

– Remember ephrin ligands cause repulsion between the migrating neural crest cells and the posterior somite

– Ephrin may be critical for somite separation

Epithelialization - Somite Formation

• Conversion from mesenchyme tissue into an epithelial block occurs even before each somite splits off

• Fibronectin and N-cadherin links these cells into an arrays that will form tight junctions - and generate their own basal laminae

• These extracellular proteins are likely regulated by the expression of the Paraxis gene

Specification and Commitment of Somitic Cell Types

• Axial specification - although somites look identical they will form different structures at different positions along the anterior to posterior axis

– Hox gene involved in this specification

Specification and Commitment of Somitic Cell Types

• Differentiation within the somite

– Sclerotome - mesenchymal cells - forms vertebral cartilage - in the thorax also form the ribs

– Dermamyotome - muscle forming region

• Epaxial muscles - closest to the neural tube (deep muscles of the back)

• Hypaxial muscles - body wall, limb and tongue musculature

• Dermatome - central region of the demamyotome - form the mesenchymal connective tissue of the back skin (dermis)

• Also forms the distal cartilage of the ribs & most ventral portion of the ribs

Trunk Area - 2 and 3 Day Embryos

Trunk Area - 4 and Late 4 Day Embryos

Specification of Somitic Regions

• Specification by paracrine factors secreted by neighboring tissue

• The sclerotome specified by sonic hedgehog protein from the notochord and floor plate cells

• The dermatome is specified by neurotrophin-3 - secreted by the roof plate cells of the neural tube

Specification of Somitic Regions

• The two myotome regions specified by different factors

• The epaxial myotome is specified by Wnt proteins from the dorsal neural tube

• The hypaxial myotome is specified by BMP4 (others?) from the lateral plate mesoderm

Patterning of the Somite

Myogenesis

• Myogenic bHLH family of transcription factors are induce in the cells that will be come muscles (myoblasts)

• These cells align themselves & fuse into myotubes characteristic of muscle tissue

– Thus the multinucleate myotubes are the products of several myoblasts joining together & dissolving cells membranes between themselves

Conversion of Myoblasts Into Muscles

Skeletal Lineages

• Three distinct lineages that generate the skeleton

• Somites generate the axial skeleton

• Lateral Plate mesoderm generates the limb skeleton

• Cranial neural crest gives rise to the brachial arch (gill apparatus in fish & jaws and ears in mammals)and craniofacial bones and cartilage

Osteogenesis

• Intramembranous ossification - direct conversion of mesenchymal tissue into bone

– Occurs primarily in the bones of the skull & turtle shells

• Endochondral ossification - mesenchymal cells differentiate into cartilage - then the cartilage is replaced by bone

Intramembranous Ossification

• Neural crest derived mesenchymal cells proliferate and condense into compact nodules

• Some of the cells develop into osteoblasts - committed bone precursor cells

• These cells secrete a collagen-proteoglycan matrix that is able to bind calcium salts

• Osteoblasts that get caught in the calcified matrix become osteocytes - bone cells

• Eventually a compact mesenchymal cell layer forms around the region of calcification - called the periosteum

Intramembranous Ossification

Endochondral Ossification

• 1. Mesenchymal cells are committed to become cartilage cells

• 2. Committed mesenchymal cells condense into compact nodules & differentiate into chrondocytes

• 3. Chrondrocytes proliferate rapidly to from the model for the bone

Endochondral Ossification

• 4. Chrondrocytes stop dividing & increase their volume dramatically

– Hypertrophic chrondrocytes - alter matrix to enable it to become mineralized by calcium carbonate

• 5. Invasion of the cartilage model by blood vessels - hypertrophic chrondrocytes die - spaces become bone marrow

– Group of cells surrounding the cartilage model differentiate into osteoblasts

– They begin to form bone on the partially degraded cartilage

Endochondral Ossification

Epiphyseal Growth Plates

• Long bones - ossification spreads outward in both directions

• As the endochondral ossification front nears the ends of the cartilage model - chrondrocytes proliferate prior to undergoing hypertrophy - pushing out the cartilage ends of the bone

• This is the epiphyseal growth plate - three regions

– 1. Region of chrondrocyte proliferation

– 2. Region of mature chrondrocytes

– 3. Region of hypertrophic chrondrocytes

Epiphyseal Growth Plates

Osteoclasts

• Multinucleate cells that enter the bone through blood vessels

• These cells pump out hydrogen ions into the surround area and acidifying and solubilizing it

• These cells continually remodel bone throughout adult life

• Osteoblasts lay down new bone to undue the damage caused by osteoclasts

Intermediate Mesoderm - Progression of Kidney Types

• Pronephric duct (day 22 in humans; day 8 in mice)- arises in the intermediate mesoderm just ventral to the anterior somites

• The cells of the duct migrate caudally - anterior region of the duct induces the adjacent mesenchyme to form tubules of the initial kidney - the pronephros

• Form functional kidneys in fish and amphibian larvae

Intermediate Mesoderm - Progression of Kidney Types

• In mammals - anterior portion of the pronephric duct degenerates - more caudal portions persist - serve as central component of the excretory system throughout development

• called the nephric or Wolffian duct

Intermediate Mesoderm - Progression of Kidney Types

• As the pronephric tubules degenerate - the middle of the duct induces a new set of kidney tubules to from - mesonephros

• In humans and mice it never functions in urine formation

• However it is the source of hematopoietic stem cells necessary for blood cell formation

• In male mammals some of these tubules become sperm carrying tubes - the vas deferens & efferent ducts

Intermediate Mesoderm - Progression of Kidney Types

• Permanent kidney of amniotes - metanephros

• Metanephrogenic mesenchyme forms in posteriorly located region of intermediate mesoderm

• It induces formation of a branch from each of the paired nephric ducts

• Called ureteric buds - separate from the nephric duct to become the ureters

• As they pass through the metanephrogenic mesenchyme - induces formation nephrons

• Called reciprocal induction - ureteric bud & metanephrogenic mesenchyme form the kidney

• 8 sets of signals appear to be involved

Reciprocal Induction- Kidney Development