mammal gastrulation

Gastrulation in Mammals

  • Process Overview

    • Gastrulation is similar between mammals and birds, but with specific differences in structures formed.

    • Key structures include the placenta, inner cell mass, and tropoblast.

  • Embryo Structure

    • At the blastula stage, the inner cell mass contains embryonic stem cells essential for embryo development.

    • Inner cell mass differentiates into:

      • Epiblast: Contributes to the embryo itself.

      • Hypoblast: Initially contributes to the embryo but becomes nonfunctional in mammals as epiblast takes over.

    • The structure formed by these cells is called the bilaminar germ disc.

  • Amniotic Cavity

    • Formed from the inner cell mass, filled with amniotic fluid, serving as a shock absorber for the developing fetus.

Mesoderm and Germ Layers Formation

  • Gastrulation leads to the formation of the mesoderm and notochord from epiblast cells.

  • Cells closer to the primitive streak contribute to the neural plate, somites, heart, and kidney.

    • Note: Both hypoblast and epiblast are crucial during this stage in mammals but vary in functionality compared to other organisms.

  • Rodent Gastrulation Variations

    • Differentiates from other mammals in the formation of a cup-shaped structure due to the infolding of the bilaminar disc, establishing signaling centers such as the anterior visceral endoderm (AVE) and the node.

    • These structures guide dorsal-ventral patterning and organization of germ layers through signaling molecules like beta catenin and BMP.

Asymmetry in Development

  • Asymmetry in mammals is influenced by mechanical forces within the uterus, contrasting gravity and rotation in birds.

  • Initial symmetrical organs gradually become asymmetrical as development progresses: grow in all directions

    • Organs such as the liver, heart, and lungs shift localization and structure.

  • initially most organs are symmetrical, they become assymetrical and localized the one side of the body

  • Cilia's Role in Asymmetry

    • Displacement and rotation of cilia lead to asymmetric calcium fluxes, causing differential gene expression (e.g., nodal and PIX2), which directs organ development.

    • Cilia establish left-right orientation in the embryonic body plan.

Neurulation Process

  • Neurulation follows gastrulation, focusing on ectoderm differentiation into neural structures:

    • Ectoderm forms a plate that undergoes morphological changes to develop into various structures.

  • Steps of Neurulation

    1. Elongation of Neural Plate through convergent extension.

    2. Elevation of Neural Folds at the medial hinge point involving distinct cadherin levels (E-cadherin vs. N-cadherin).

    3. Convergence of Neural Folds to form a single layer, essential for forming the neural tube.

    4. Closure of Neural Tube, which separates from the epidermis and establishes neural crest cells.

  • Signaling during neurulation primarily derived from mesoderm, influencing ectodermal cell differentiation and closure.

Neural Tube Defects

  • Failure of neural tube closure can lead to congenital conditions such as spina bifida and anencephaly, often influenced by environmental factors like alcohol consumption.

  • Proper formation requires understanding the interactions of BMP, noggin, and hedgehog signaling in the underlying tissues.

Review and Questions

  • The quiz concluded with an invitation for any remaining exam questions.