Introduction to the study of Embryology
• The fact is that all of those structures in all organisms are
derived from a single cell that was formed by the union of two
gametes.
• Every organism you see came originally from a single cell,
which divided and differentiated to form more complex
structures. Thus, everything that we will study during the
remaining part of the semester is dependent on the complicated
embryological process.
Reproduction starts with two cells, the
sperm and egg - haploid cells formed through the process of
meiosis which are specially designed for their own specific
purpose.
Sperm are:
• extremely small cells that lack most of their cytoplasm
• designed to travel through an aquatic medium (either internal
or external) to reach the egg cell
• travel by movements of one or more flagella that propel it
toward the egg
• all sperm consist of three basic pieces:
- the head, which contains the genetic material and is capped
by the acrosome (cap) at the apex that contains enzymes needed
for the sperm to penetrate the egg
- the middle piece contains the primary power source of
mitochondria that fuel the movements of the tail piece.
Eggs are:
• designed more for providing nutrient sources to the developing
young than for movement
• contains yolk that consists of lipids and protein for nutrients,
along with enzymes needed to initiate development
• Eggs are classified by the amount and the distribution of yolk
in the egg as
- microlecithal eggs (characteristic of the protochordates and
eutherian mammals) have a very small amount of yolk, and the
young hatch quickly as a result
- mesolecithal eggs (characteristic of lampreys and
amphibians) have an intermediate amount of yolk, and the
young hatch at a later stage of development
- macrolecithal eggs (characteristic of fishes, reptiles, birds
and monotremes) have a large amount of yolk, and the young
hatch at an even later stage
- isolecithal: distribution of yolk can be even through the egg
- telolecithal: yolk concentrated in one part of the egg - the
area with less yolk and prominent haploid nucleus as the animal
pole and the area with more yolk as the vegetal pole
FERTILIZATION
The process of union of the sperm and egg
At fertilization, enzymes in the acrosome of the sperm help to
penetrate the egg
• requires that the sperm break through the plasma and vitelline
membrane surrounding the egg
• to prevent more than one sperm from penetrating the egg
(polyspermy), the egg undergoes a cortical reaction to bring the
sperm head into the interior of the egg and change the vitelline
envelope to form the fertilization membrane
Just after fertilization the zygote (fertilized egg) undergoes
cleavage (mitotic cell divisions) and becomes subdivided into
smaller cells - the gross arrangement of cells differs greatly
among vertebrates, depending on the amount of yolk in the egg:
1. Holoblastic cleavage occurs when the cleavage furrows
pass through the entire egg
• cleavage can either be equal, where the resulting cells
contain the same amount of yolk, or unequal, in which
some cells contain more yolk than others:
- equal cleavage occurs in microlecithal eggs
- unequal cleavage occurs in mesolecithal eggs
• cleavage results in the formation of a ball of cells
(blastomeres) surrounding an internal cavity (blastocoel)
2. Meroblastic cleavage occurs more in macrolecithal eggs
• cleavage takes place only in a disk at the animal pole
• the cleavage furrows do not extend into the yolk
• results in the formation of the blastodisk that lies on the
top of the yolk
Gastrulation is characterized by cell movement and
reorganization within the embryo (morphogenetic movements)
to the interior of the embryo, forming three primary germ layers:
ectoderm, mesoderm, and endoderm.
The cells migrate inward at the blastopore, which forms, or is
close to, the location of the anus in the adult
• the ectoderm forms the outer tube of the embryo
• the endoderm is an inner tube that forms the alimentary canal
and all its derivative organs
• the mesoderm lies between these two layers.
At the end of gastrulation, the embryo is
bilaterally symmetrical, with three discrete cell layers, and
rudiments of the notochord and neural tube.
This blastopore-to-anus developmental pathway is found in
Chordata, Hemichordata, Echinodermata (starfish, sea urchins,
sea cucumbers, etc.), uniting these groups into a monophyletic
group called the Deuterostomes. The plesiomorphic condition,
found in the Protostomes, is for the blastopore to become the
mouth.