PP

lecture 3a: introduction to embryology

Introduction to Early Development in Developmental Biology

  • Early development, also known as embryology, focuses on the initial stages of life from fertilization to the formation of an embryo.

  • The term ontogeny refers to the complete life history of an organism, encompassing all stages from fertilized egg to death.

Early Stages of Ontogeny

  • Embryology specifically deals with the early part of ontogeny, from fertilization to embryo formation, including the differentiation of embryonic tissue layers into organs.

Major Processes in Early Development

  1. Cleavage

    • The zygote undergoes mitosis, dividing into multiple cells, leading to the formation of a hollow ball of cells known as a blastula.

    • The blastula typically consists of about a hundred cells, forming a hollow structure that is filled with fluid (blastocoel).

  2. Gastrulation

    • This stage follows the blastula formation, characterized by the development of a cup-shaped structure that indicates the beginning of the three germ layers (ectoderm, mesoderm, endoderm).

    • During gastrulation, a primitive gut starts to form, marking a transition towards the embryo stage.

  3. Neural Tube Formation

    • This process is essential for the development of the nervous system and involves further differentiation of the mesoderm.

The Process of Cleavage

  • Cleavage patterns dictate how the fertilized egg divides based on the amount of yolk present in the zygote.

Types of Eggs Based on Yolk Content

  1. Microlecithal Eggs (Little Yolks)

    • Found in primitive chordates (e.g., amphioxus) and euthelian mammals (placental mammals like humans).

    • Undergo holoblastic cleavage, where the cleavage furrows divide the egg completely; there is little to no impediment from yolk.

  2. Mesolecithal Eggs (Moderate Yolks)

    • Observed in lampreys and amphibians.

    • Also undergo holoblastic cleavage; yolk presence results in uneven cell division but cleavage remains complete.

    • Larval stages in these organisms can feed independently post-hatching, reducing reliance on yolk.

  3. Macrolecithal Eggs (Lots of Yolks)

    • Found in fishes, reptiles, birds, and monotremes (egg-laying mammals).

    • Experience meroblastic cleavage, where only a part of the egg divides due to yolk obstruction, leading to discoidal cleavage patterns.

Introduction to Gastrulation

  • Gastrulation is the transition from the blastula to the gastrula stage, signifying the start of organ and tissue development.

Characteristics of Gastrulation

  • The gastrula forms as the cells start migrating inward, establishing the three tissue layers.

  • The initial cavity in the blastula (blastocoel) is replaced by the primitive gut, crucial for future digestive system development.

Differentiation of Tissue Layers

  • The three germ layers formed are:

    1. Ectoderm - Outer layer, eventually forming the nervous system (neural tube formation).

    2. Mesoderm - Middle layer, which undergoes extensive differentiation into various organs and systems. It can be further divided into somatic, mesomere, and hypomere layers.

    3. Endoderm - Inner layer which surrounds the gut (gastroseal).

Conclusion and Next Steps

  • Upcoming lectures will address differences in early development among various vertebrate groups, including amphioxus.

  • A focus will be placed on how these tissue layers form and differentiate across different vertebrates.