Download Embryology • Important as a process, the way the organism

Embryology
•
•
•
•
Important as a process, the way the organism develops –the early stages.
We can use embryology as evidence as phylogenic relationship; it will help us
decipher the relationships between organisms.
Urochordates for example, the young and adult stages have various
characteristics that disappeared.
The differences between the organisms for our purpose do not matter, as
the end result is essentially the same. The observed differences are largely
due to the amount of yolk presence in the egg.
Amount of Yolk:
•
•
Also gives an indication to the size of the egg.
We can summarize it into 3 types:
o Little to no yolk –microlecithal
o Moderate amount yolk –mesolecithal
o A lot of yolk –macrolecithal
Microlecithal:
•
•
•
•
Amphioxus
Two spheres, an animal and vegetal sphere.
The one that is less yolky is the animal pole (give rise to the organism),
contains the living protoplasm. The vegetal pole is made of yolk, it is inert
and not an actively dividing material.
Vertical and horizontal divisions occur until we get a spherical mass of cells,
in amphioxus they are 1 cell thick. The sphere (blastula) has a cavity within
it which can be seen with a section. The cavity is called the blastocoel.
Mesolecithal:
•
•
•
Similar to above.
Since there is a lot more yolk, the divisions are much slower (fast at the top,
but slower as it descending towards the vegetal) and not as even.
There is a large mass of cells in the animal pole (not evenly distributed).
Structure is more than 1 cell thick (amphibian eggs).
Macrolecithal:
•
•
•
•
Cell divisions are not easily comparable.
Very little protoplasm at the animal pole (chicken egg).
Wind up with a cap of many cells; if we section the egg we will still see the
blastocoel.
The surface of the blastula is termed the blastoderm.
Mammals:
•
•
Single cell layer, spherical blastula
Similar externally to amphioxus
•
•
It is called a trophoblast (as opposed to the blastula), which is destined to
become a part of the placenta (the structure in which the nutrient is
delivered from mother to egg).
Within the sphere is a cluster of cells, towards the animal pole. This is called
the inner cell mass. It is the division of this region that produces the
comparable structure, the formation of the three primary germ layers.
Gastrulation:
•
•
•
•
•
•
•
•
•
•
•
•
•
•
Two things happen, one the surface of the animal pole (VEGETAL?)
becomes invaginated to form a cup-like structure.
Continued cell division and the cell begin to migrate. Until we get a
somewhat elongated structure, which also has a cavity but not a closed cavity
but an open cavity –called an archenteron (gastrocoel).
The entrance into the gastrula (GASTROCOEL?) which is called the
blastopore.
A series of differentiation of cells occur, so that specific cells come to occupy
distinct places in the gastrula, these distinct regions are called the primary
germ layers: ectoderm, mesoderm and endoderm.
o Amphioxus:
§ Same main developmental processes as the craniates
(generally).
Endoderm surrounding the gastrocoel, ectoderm on the surface and the
mesoderm between the two.
Flattened portion of ectoderm that we call the neural plate and we see that
the mesoderm winds up subdividing into 3 portions. A central region called
chordamesoderm and lateral mesoderm –somite.
Initially as the animal begins to develop, the somites are segmentally
arranged. That is, the mesoderm actually develops as a series anteroposterior
sequential structures called somites on each side bilaterally of the
chordamesoderm.
Neuroectoderm folds in to that U-shape neural groove and begins to sink into
the embryo. They then fuse to encircle a neural tube (it will form spinal cord
and anterior end will form the brain). Between the ectoderm and
neuroectoderm, there forms neural crest cells (occurs in craniates) –which
migrate to another place in the embryo are things in the head. A feature that
distinguishes that from the protochordates (cephalochordates) which do not
have a true head.
Part of the lateral mesoderm subdivides so that we get a space starting to
open up within the mesoderm in which we call a coelom.
Chordamesoderm will form the notochord.
Dorsal ventral sections of the mesoderm –epimere, mesomere, hypomere.
Neural crest cells now become detached and begin to migrate, the neural tube
is isolated.
The lateral mesoderm, there is an expansion of the hypomere. Epimere
undergone a significant differentiation, dermatome, myotome and sclerotome.
Hypomere on each side begin to fuse together back to front, so we get on each
side of the body a continuous structure, which we called lateral plate.
•
•
•
The pharyngeal slits are formed at the anterior part of the gut. The mouth
develops the later (deuterostomata).
Movement and enlargement of these. Ectoderm forms the outer part of the
animal (part of the skin). Endoderm forms the inner lining of the gut tube.
The mesoderm forms everything else. For example, the dermatome (the
lateral or outer portion of the epimere) spans underneath the ectoderm and
loses its segmentation and becomes the dermis of the integument and also
helps to form the dermal muscle and dermal skeletal structures. The
sclerotome surround the notochord and the neural tube and will become
much of the somatic skeleton (vertebrae column). The part in between the
sclerotome and the dermatome is the myotome, it becomes subdivided into 2
blocks of muscle: dorsal portion (epaxial musculature), ventral portion
(hypaxial musculature) and into the head it differentiates into hypobranchial
and epibranchial musculature.
The hypomere (or the lateral plate mesoderm), it has a portion in 2 regions.
The coelom gets bigger and bigger, eventually they become one cavity by
reabsorption of the ventral parts of what you know as the mesentery. This is
where all of the structures that we have looked at in the shark will reside.
The visceral layer of the hypomere that closest to the viscera (guts) is going to
wind up forming the muscles of the gut as well as the heart and it will also
form the visceral serosa (the epithelial lining that surrounds those
structures). The somatic layer of the hypomere (the one towards the body, the
soma) is going to produce the peritoneal serosa (the epithelial layer that lines
the inside of the cavity –the peritoneum).
Meso-macrolecithal:
•
Complicated by the large amount of yolk. But the processes still lead to the
same arrangement of the three primary germ layers.