MESODERMAL DERIVATIVES

Intermediate Mesoderm, Chordamesoderm, Paraxial Mesoderm and Lateral Mesoderm

Major lineages of mesoderm

Paraxial Mesoderm, Intermediate Mesoderm and Lateral Plate Mesoderm

Three divisions of the Embryonic Mesoderm

Paraxial mesoderm

Breaks up into paired, repeating blocks of mesoderm called somites

Somites

Blocks of mesoderm

Kidneys and Gonads

Intermediate mesoderm gives rise to

Notochord

Chordamesoderm gives rise to

Head and Somite

Paraxial mesoderm gives rise to

Sclerotome, Myotome and Dermatome

Somites give rise to

Splanchic, Somatic and Extraembryonic

Lateral mesoderm give rise to

Notochord

Transient support structure that is eventually replaced by vertebral column

Nucleus pulposus

remnant of notochord

Paraxial mesoderm

After gastrulation, thick bands of mesodermal cells are located on each side of the neural tube called

Craniofacial musculature

Head mesoderm give rise to

Vertebrae and ribs, dermis of dorsal skin, skeletal musces of the back, skeletal muscle of the body wall and limbs

Somites give rise to

Segmental Plates

Birds

Unsegmental Plate

mammals

Somitogenesis

formation of somites

Periodicity, Separation, Epithelialization, Specification and Differentiation

5 developmental characteristics of somitogenesis

Periodicity

Paraxial mesoderm —> Somitomeres

Somitomeres

Whorl of cells

Separation

Somitomeres —> Somites via EPH4 and Ligand Ephrin B2

Chick

50 somites

Snake

500 somites

kinase Eph4 and ligand EphrinB2

causes cell-to-cell repulsion leading to separation of cells

Epithelialization

Conversion of paraxial mesoderm from randomly organized mesenchyme cells into epithelial blocks

Paraxis gene

synthesizes n-cadherin and fibronectin to epithelialize somites

Specification

Identical somites later differentiate according to position along anterposterior axis

Thoracic somite

thoracic vertebrae and ribs

Lumbar somite

Lumbar vertebrae

Differentiation

Somite forms 2 major regions

Sclerotome and Dermamyotome

2 major regions that comes from somite

Sclerotome

• Ventromedial portion

• mesenchymal form

• forms cartilage of vertebrae and parts of ribs

Dermamyotome

• remaining epithelial portion of somite

Dermatome

• Dermis of dorsal skin

Myotome

• forms myoblast

• Divides into epaxial myotome and hypaxial myotome

Epaxial myotome

forms deep back muscle

Hypaxial myotome

Forms body wall muscle, Intercostal muscle

Stak of the somite

Intermediate mesoderm is also known as

Intermediate mesoderm

• located between paraxial mesoderm and lateral plate mesoderm

• give rise to urogenital system, kidneys, gonads and respective duct system

Myocoel

Somite encloses a cavity called

Embryonic coelom

splitting of lateral plate mesoderm forms

Nephrocoel

nephrotome splits forming a cavity

Nephrocoel

communication with the myocoel and coelom,

Nephrostome

nephrocoel communicates with coelom via a cilliated funnel called

Pronephros, Mesonephros, Metanephros

Three distinct excretory organs

Pronephros

• first kidney

• degenerates

• functional in hag fishes

Pronephric tubules, Pronephric duct and Glomus

Structures present in Pronephros

Pronephric tubule

formed by evagination of nephrotome

Pronephrict Duct

Formed by fusion of tubules end

Glomus

Cluster of blood capillaries

Mesonephros

• functiona kidney for fishes and amphibians

Nephrogenic cord, mesonephric duct and mesonephric tubules

Structures present in Mesonephros

Nephrogenic cord

Precursor of mesonephric tubules

Mesonephric duct

hollow spheres forming tubules

Mesonephric tubules

S-shaped tubules

Filtration system

Glomerulus and Bowman Capsule

Metanephros

• most advanced

• functional kidneys of reptiles, birds and mammals

Metanephric diverticulum, Metanephrogenic tissue and metanephric tubules

Structures of metanephros

Metanephric Diverticulum

• derived from mesonephric duct

• forms ureter and renal pelvis

Metanephrogenic Tissue

forms nephrons

Metanephric tubules

Forms bowman capsule, Glomerulus and Convoluted Tubules

Afferent Blood Vessels

• arises from aorta

• carry unfiltered blood into kidney

Glomerulus

• Afferent blood vessels branch into this

• Dense capillary network located at end of mesonephric tubule

• Site of blood filtration

• Capillaries push against wall of tubule causing invagination

Bowman Capsule

• formed when glomerular capillaries pressed into tubule wall

• Cup-shaped sac surrounding glomerulus

• Collect filtrate from blood

Efferent blood vessels

• blood exists glomerulus through here

• carry filtered blood away from glomerulus

Mesothelium, mesenchyme and PGC

3 sources of indifferent Gonad

Germinal ridge

mesothelium proliferation

Germinal epithelium

mesothelium cell become spherical and increase in size

Tunica Albuginea

Cortex reduced to thin capsule

50-80

How many PGC in medulla

Sertoli cells

cells of the cord

Rete testes

• form when cell of cords extend deep in medulla

• series of reproductive duct for sperm transport

Ovarian follicle

Secondary sex cord breaks into this

Primodial follicle

• youngest ovarian folicle

• consists of oogonium and flat follicle cells

Mesonephrict Duct (Wolffian) and Paramesonephrict (Mullerian)

Indifferent duct system

Mesonephrict Duct/Wolffian

• mesonephros

• for excretion

• male reproductive system

Paramesonephric duct (Mullerian)

• invagination of coelomic epithelium

• lateral to mesonephric duct

• both sexes

Male reproductive Ducts

some mesonephric tubules, entire mesonephric ducts are retained while mullerian ducts degenerates

Efferent tubules

• mesonephric tubules + rete testis

Mesonephric duct

Becomes vas deferens (transport sperm into cloaca)

Epididymis

storage and maturation of sperm

Ductus deferens

Transport sperm

Seminal vesicle

produce nutritive fluid for sperm

Urethral epithelium

form prostate and bulbourethral gland

Female reproductive ducts

mullerian persists, wolffian degenerates

Ostium tuba

funnel-shaped opening

Uterus Duplex, Uterus Bipartitus, Uterus Bicornis and Uterus Simplex

Types of Uterus

Descent of Testis

as testis growth and mesonephros degenerates

Gubernaculum

Fibrous and connecting testis to future scrotum

Scrotal pouches

Gubernaculum is attached to this

Body Cavity

The final position of Testis descent is here

Ovary Descent

Descent slightly (caudal and ventral shift only)

Pelvic Cavity

Final position of Ovary Descent

Genital Tubercle

between umbilical cord and tail

Urogenital groove

shallow groove in posterior region

Urogenital membrane

Thin roof

Urogenital sinus

Opening

Genital swellings

Rounded ridges

Phallus

cylindrical structure

Scrotum swellings

Moves caudally