1. The Making of a Body and a Field

Developmental biology seeks to elucidate the cellular and molecular mechanisms that drive changes in cells, tissues, and organs over time–a timescale that spans all of life, from fertilization to again. Development generates a cellular diversity and order within the individual organism and it insures the continuity of life from one generation to the next. Developmental biologists asks questions of differentiation, pattern formation, morphogenesis, growth, reproduction, regeneration, environmental integration, and evolution.

Most animals pass through similar stages of development:

The stages of development between fertilization and hatching (or birth) are collectively called embryogenesis. The life cycle of land plants is different from that of animals in having two alternating stages, a diploid sporophytic (spore-bearing) stage and a haploid gametophytic (gamete-producing) stage.

Cleavage can be either holoblastic (complete) or meroblastic (incomplete). Holoblastic cleavage can be isolecithal (sparse, evenly distributed yolk) or mesolecithal (moderate vegetal yolk disposition). Meroblastic cleavage can be telolecithal (dense yolk throughout most of the cell) or centrolecithal (yolk in center of the egg).

During gastrulation, the three germ layers are first produced:

  • The ectoderm generates the outer layer of the embryo. It produces the surface layer (epidermis) of the skin and forms the brain and nervous system.
  • The endoderm becomes the innermost layer of the embryo and produces the epithelium of the digestive tube and its associated organs (including the lungs).
  • The mesoderm becomes sandwiched between the ectoderm and endoderm. It generates the blood, heart, kidney, gonads, bones, muscles, and connective tissues.

The germ cells are set aside early in development and do not arise from any particular germ layer.

Embryonic cells do not remain in one place, nor do they keep the same shape. Five basic types of cell movements occur – invagination, involution, ingression, delamination, and epiboly. Embryos also develop the anterior-posterior axis, the dorsal-ventral axis, and the right-left axis.

There are two major types of cells in the animal embryo – epithelial cells, which are tightly connected to one another sheets or tubes, and mesenchymal cells, which are unconnected or loosely connected to one another and can operate as independent units. Morphogenesis si brought about through direction and number of cell divisions, cell shape changes, cell migration, cell growth, cell death, and changes in the composition of the cell membrane or secreted products.

Von Baer’s laws of vertebrate embryology state:

  1. The general features of a large group of animals appear earlier in development than do the specialized features of a smaller group.
  2. Less general characters develop from the more general, until finally the most specialized appear.
  3. The embryo of a given species, instead of passing through the adult stages of lower animals, departs more and more from them.
  4. Therefore, the early embryo of a higher animals is never like a lower animal, but only like its early embryo.

Homologous structures are those whose underlying similarity arises from their being derived from a common ancestral structure. Analogous structures are those whose similarity comes from their performing a similar function rather than their arising from a common ancestor.

The most fundamental evolutionary step required to build an animal was multicellularity (going from one cell to many different cells).

All life on this planet is related in one way or another. Direct tweaks in developmental mechanisms largely associated with embryogenesis, in concert with natural selection, generated today’s diversity of life.

—August 2020