Bio 205 Comparative Vertebrate Anatomy

Lecture 5 : Early Development

Major stages of early development

fertilization - (from Gilbert 1988, p. 30) The process where male and female gametes (sex cells) "fuse together to create a new individual with genetic potentials derived from both parents." The fertilized egg is the zygote. Fertilization includes at least four major activities:

  1. Contact and recognition between sperm and egg - "quality control" - only sperm of same species may enter
  2. Regulation of sperm entry into egg -"quantity control" - only 1 sperm may enter.
  3. Fusion of genetic material of both sperm and egg
  4. Activation of egg metabolism to start development

zygote - the fertilized egg. There is an animal pole (opposite the yolky end) and a vegetal pole (the yolky end) in the cytoplasm.

cleavage - the series of mitotic divisions that take the large zygote to a mass of smaller cells, the blastula. The individual cells during cleavage are called blastomeres. In early cleavage, the solid ball of cells looks like a blackberry and is called a morula (Latin for mulberry). In later stages, a fluid filled cavity, the blastocoel, forms. In most animals, the cell divisions do not increase the total volume of the cell mass. Instead, the volume remains the same because each generation of daughter cells simply gets smaller and smaller.

Group
type of egg
type of cleavage
blastula
blastula cavity
Amphioxus, mammals isolecithal, oligolecithal or microlecithal (little yolk) holoblastic (or complete): initial cleavage planes extend through egg) sphere with single layer wall large, central sphere
amphibians, lampreys, lungfish, paddlefish, sturgeon mesolecithal (moderate yolk) holoblastic sphere with layered wall small sphere
reptiles telolecithal meroblastic (incomplete): initial cleavages planes limited to animal pole, the region opposite the yolk) cell disk on surface of yolk, bilaminar flat space between epiblast and hypoblast

Holoblastic is primitive state for craniates while meroblastic is derived.

Note the blastula in reptiles is called a blastoderm or blastodisk because the divisions have been concentrated into a thin disk of cells at the animal pole.

Following cleavage, a series of events occurs that transforms the ball of cells into a stage of development (the pharygula stage) that looks like a vertebrate embryo. We will talk more about the pharyngula next lecture but first we need to summarize these steps. Gastrulation is the formation of the embryonic layers (ectoderm, mesoderm, endoderm) and the gut and notochord. Neurulation is the formation of the dorsal nerve chord. About the same time as neurulation, the mesoderm is differentiating both mediolaterally and anteroposteriorly into primitive organs. Note that these events are not perfectly sequential. neurulation and mesoderm formation are occuring near the end of gastrulation but in different parts of the embryo.

gastrulation - rearrangement of the cells by migration and division resulting in the formation of the gut, the notochord, and the three embryonic germ layers: ectoderm, endoderm, and mesoderm.

    The endoderm ultimately forms the primitive gut, or archenteron. The caudal end of the gut is either open at the blastopore or an opening (anus) will form near the blastopore. The cranial end of the archenteron is closed but an opening (mouth) will form later.

    The ectoderm will ultimately form the

    1. epithelial component of the skin
    2. the brain and spinal cord
    3. sensory organs in the body and special sensory organs in the head
    4. some cartilages and bones in the head.

    The mesoderm will form

    1. most of the skeleton
    2. the heart and blood vessels
    3. kidneys
    4. linings of internal body cavities

    Gastrulation is highly variable in vertebrates so the different types will be described.

    Primitive state - invagination and enterocoely in amphioxous (Fig. 5.7)

    1. invagination of vegetal pole wall creates archenteron
    2. dorsal wall of archenteron is the notochord rudiment
    3. mesodermal pouches pinch off by a process called enterocoely
    4. ectoderm derived from animal pole cells that now cover the ball

    Derived state in amphibians - involution through blastopore (Fig. 5.11)

    1. Sheets of cells in wall at junction of vegetal and animal pole begin migrating in along the blastocoel. The site of this involution is the blastopore.
    2. the involuting sheets of cells create the archenteron, which displaces the blastocoel
    3. migrating cells on the surface (a process called epiboly) migrate toward the blastopore but many continue past this and cover the vegetal pole, only a small plug of yolk cells (the yolk plug) can be seen at the surface through the bastopore. All of these covering cells become ectoderm
    4. cells in the dorsal wall of the developing archenteron become the notochord
    5. cells migrating through the blastopore laterally become mesoderm

    Gastrulation in the frog, Xenopus laevis

 

 

 

 

 

Derived state in amniotes - involution through the primitive streak

Gastrulation in the chick

  1. In reptiles (as well as in teleost fishes) the blastula is a small disk of cells (blastoderm) on a large yolk. Cells in the blastoderm migrate ventrally to form the hypoblast. The remaining cells in the disk are the epiblast. The blastocoel is the thin space between the epiblast and hypoblast
  2. Cells migrate medially along the epiblast to form a thickened sheet of cells, the primitive streak. A depression that forms in the streak is the primitive groove. At the anterior end of the streak is a local thickening of cells, the primitive knot, or Henson's node, which is effectively the equivalent of the dorsal lip of the amphibian blastopore.
  3. Epiblast cells migrate across and down into the blastocoel through the primitive streak. Initially, the cells migrate anteriorly and ventrally forming the foregut endoderm. These cells moving ventrally forming the foregut displace the hypoblast cells. More epiblast cells ingress and form the head mesoderm and the anterior part of the notochord. Following this, the primitive streak regresses, moving Hensen's node posteriorly. As it moves, the more posterior regions of the notochord develop. More epiblastic cells move in forming the endoderm ventrally and mesoderm in an intermediate position. At its most posterior position, Hensen's node forms the anal region. At the end we have three layers, the endoderm which has displaced the hypoblast, the ectoderm, which is what is left of the epiblast, and lots of mesoderm in between. Note that the gut is not formed yet (no archenteron). The gut forms as a fold of the endoderm during neurulation.

Neurulation - the development of the neural tube from the neurectoderm. About the same time as neurulation, many other features occur. Neural crest cells migrate away from the developing neural tube to form much of the head skeleton, sensory and autonomic cells and pigment containing cells of the epidermis.

Chick neurulation (see movie)

Mesoderm and coelom formation (see movie)