Mesoderm II

Mesoderm II: Urogenital and Musculoskeletal Systems

Overview

3 successive stages of nephric structures form Þ pronephros, mesonephros, metonephros

By 6^th week, germ cells migrating from yolk sac arrive in the posterior body wall, medial to mesonephros

Sexual genotype is determined at fertilization, but sexual differentiation begins in the 6^th week due to actions of testis – determining factor

Urogenital System

Early after gastrulation

Notochord – gives embryo elongated form during folding

Kidneys

urinary system takes shape before gonadal development

Mesonephric ducts attach to the cloaca by end of 4^th week
Mesonephros is the functional kidney of the embryo
Tubules and simple glomeruli differentiate along the length of the nephrogenic cord

Mutual inductions are responsible for kidney formation (Fig 2)

Uteric bud induces surrounding mesoderm to form metanephric blastema which in turn induces the bud to branch

Uteric bud derivatives form the collecting function of the kidney Þ ureter, renal pelvis calyces, collecting tubules

A uteric bud branches, each arched collecting tubule induces formation of convoluted tubules and to form glomerulus

Gonads

4^th week: primary germ cells migrate from dorsal part of yolk sac near the allantois, along the dorsal mesentary of the hindgut to the gonadal ridges Þ gonads

If male, gonad Þ TDF – testis determining factor Þ testis Þ testosterone and Mullerian inhibiting factor

Testosterone acts on mesonephric duct (retains)

Mesonephric duct degenerates

Gonad has no germ cells until 6^th week, when germ cells come from yolk sac and migrates to gonads

ovaries are the female gonad – paramesonephric duct Þ superior end Þ fallopian tube; inferior portions fuse Þ uterus

Day 24, 25 – pronephros forms – degenerates quickly

Day 25 – nephoromes – forms urine

Musculoskeletal System

Somites (mostly build trunk) Þ sclerotome (backbone) + dermamyotome (skin, muscle)

Chorda Mesoderm

Notochord Þ induces ectoderm Þ neural groove Þ neural tube Þ brain and spinal chord, etc