LABORATORY #22 BIOLOGY 441L
TITLE
: FETAL PIG DEVELOPMENT
THIS INVESTIGATION REQUIRES:
SLIDES 6 AND 10 MM PIG SLIDES; TRANSVERSE SECTIONS AND SAGITAL; FETAL
PIGS‑1‑8INCH; HUMAN SAGITAL SECTION DIAGRAMS:DRAWINGS OF THE PIG SECTIONS TO BE
LABELLED, AND FOUR DRAWINGS TO BE MADE.
REFERENCES:PATTEN: HUMAN EMBRYOLOGY ;Embryology of the pig.
PRELAB;
OBJECTIVES To learn to
envision a three dimensional embryo from 2‑D cross sections; to see the
changes in organs systems from the 6 to the 10 mm pig.
REQUIRED TASKS:examine slides and fetal pigs and make drawings, label
diagrams.
STUDENTS TO WORK INDIVIDUALLY AND TURN IN COMPLETED LAB SHEET AT THE END
OF THE LAB PROCEDURES.
IN THIS SECTION YOU WILL DETERMINE THE RELATIONSHIP BETWEEN the heart
and the bulbus as they become compartmented into right and left sides
which are systemic and pulmonary; the development of the vena cavae, vitelline and umbilical
derivatives; the regions of the brain, and the cranial nerves;
the various evaginating organs of the digestive system; the urogenital
sustem.
Great use can be made of the stereoptican cards of rat embryos which are
labelled to tell you which structures are which and which give a 3D view of
dissections. The pig embryo
sectioned model is also useful, with digestive system in yellow, heart in
red, kidneys in green. This will help correlate external appearance which we
have become familiar with, to the internal structure which you need to find in
the slides and later identify on the older fetal pigs.
These pictures of 6 mm sections are to
give you an idea of which areas you need to study since these are the exam
slides.
T
Notice the visceral arches in the region of the head, with
the first one giving rise to the jaws. In the sections, the visceral arches
look like blocks with holes in the center which are the aortic arches. Between
the arches are the visceral pouches of the pharynx which extend
outward to the ectoderm. The first pouch gives rise to the middle ear cavity
and the eardrum. The ventral part of it will give rise to the thyroid. Notice what the brain looks like in section.
The rest of the pictures are of 10 mm slides from the test. label the brain
regions and cranial nerves in the photo.
The shape of the section of
the brain varies according to which part of the brain you are in, so you can
tell where you are when you become familiar with how it is laid out. The
forebrain is divided into three parts; the two anterior telencephalon bulbs
which will give rise to the cerebral cortex and the single diencephalon
region behind that to which the eyes (optic stalks) and pituitary (infundibulum and Rathke's
pouch) are attached. The midbrain is the large round bulb which is
the top of the head in the embryo. The hindbrain is the thin roofed part
behind that and will give rise to the cerebellum, pons, from the
metencephalon, and medulla from the myelencephalon. Rhombomeres are visible
in the hindbrain.
The cranial nerves are nerves coming to or from
the brain and the sensory ones often go the the special sense organs
such as eyes, ears, nose, taste buds.
Find nerves 3,5,7,8,9,10. Nerve 3 is a motor nerve, going
to eye muscles, and therefore comes out the ventral part of the brain, in the
mesencephalon region Nerve 5 is just behind the eyes, next to the midbrain‑hindbrain
transition. the sensory part of it is in a ganglion next to the first
neuromere of the metencephalon. Nerves 7 and 8 are just in front of the ear
vesicles, 9 is just behind the ear vesicle. The ear vesicle becomes the inner
ear and the endolymphatic ducts and the semicircular canals can
be seen as knobs on it. There is one large nerve of the parasympathetic
nervous system (cranial and caudal nerves), the vagus (nerve 10)
which goes to all the viscera (the soft digestive and reproductive organs and
the heart and lungs.) 10 is just behind 9. Use the chick models to see the
placement of the nerves, then try to find them in the pig sections. Go posteriorly until you get out of the
brain, and you will see how much smaller the spinal cord is. You can
also see the dorsal spinal gangion next to it on each side. That is where all
the sensory cell are found, and they extend processes out to the sense organs,
and other cell processes into the spinal cord to the association neurons of
the alar plate. The motor areas of the basal plate are seen as
enlargements, and their nerve fibers extend out the ventral root to
combine with the dorsal root fibers to
break up into dorsal and ventral ramus which go out to the dorsal and
ventral parts of the outer body tube. The grey or sympathetic ramus can
be seen in the thoracic region going out to the sympathetic ganglia
which look like eyebrows over the paired dorsal aortae. In this region
you can also see the paddle‑like limb buds. Can you see the nerves
coming out to the limbs as the cervical plexus? Just trace them from the
spinal cord. They are pink because nerves just contain fibers, the ganglia
contain the cell bodies and you can see the nuclei there, which are purple.
Motor nerves grow out from the ventral half of the spinal cord. Then both
sensory and motor nerves grow out into the limb together, but motor nerves only
attach to muscles, and sensory nerves only attach to sense organs.
In the fetal pig dissections with sagital cuts, the choroid plexus of
the brain ventricles can be seen
hanging down into
Them. This is a 36 mm whole pig cut
sagitally.
them. 
The
enlargement of the telencephalon growing over the rest of the brain can also be
seen. 
DIGESTIVE SYSTEM: EXAMINE THE PHARYNX. Find the visceral
pouches, arches, grooves. Notice that a developing tongue appears in the
floor of the pharynx, made of two rudiments, the tuberculum impar and the
copula. The pharynx extends back to the laryngo‑tracheal groove,
where the trachea separates off and ultimately gives off the lung buds. Find
the esophagus, and the trachea and lung buds. Notice the pleural
cavities, the pericardial cavity, and the peritoneal cavity, which
we saw in our dissections. The esophagus leads to the stomach which is larger
and thicker walled. Below the stomach, you can see the huge liver which
is full of sinusoids making it like a honeycomb. Label a section with
the liver and the pancreas, spinal cord, body cavities 
The liver
is connected to the duodenum, find this connection at the common bile
duct, notice the unbranched part of the liver which will become the gall
bladder.
In the 10 mm pig sections, in this same region, you can see the pancreas
growing out from the duodenum. It looks like a wavy flag, coming out from it,
just posterior to where the bile duct enters the duodenum, and more dorsally,
in the vicinity of the portal vein, which develops from the vitelline vein. It
is not hollow yet. You can see it branching into the splanchnic mesenchyme.
HEART Compare the 6mm and 10 mm
slides in the heart region. The heart
has become more fully separated into right and left sides in this period of
development.
Draw the
section including heart of the 10 mm pig on the lab sheet to be turned in. The sinus
venosus is becoming incorporated into the wall of the right atrium,
and there are valves there still, at the junction. The SV will become the pacemaker
of the heart since its cells beat the fastest. Can you find the foramen
ovale? It is not seen in the section where the rest of the heart is seen in
its characteristic view with the four chambers. You have to go a little bit
forward until you can only see the atria and the bulbus. Notice the folds in
the bulbous which are coming together to make the division in it. Now compare
the heart to the 6 mm to see the changes, especially in the bulbous. The
bulbous region is splitting into the pulmonary artery and the systemic artery,
just as the heart split into right and left sides. Trace the arteries out of
the heart and see if you can find the ductus arteriosus on the left side.
Remember it is a part of the 6th aortic arch.
Look at the aortic arches and find which ones are present.
Find the internal
carotid, stapedial arteries. Find the large anterior cardinal which
becomes the superior vena cava and takes blood to the sinus venosus at this
stage. The left common cardinal is in the process of becoming the coronary
sinus. Where does it empty? Now
look at the differences of the umbilical circulation in the 6 and 10 mm. In the
6 mm, both of the umbilical veins are present and both go through the
body wall and the liver. Trace them from the allantoic region, the umbilical
cord to the heart. In the 10 mm, the left umbilical is much larger and crosses
into the left bottom quadrant of the liver, then up to the vena cava in the right
top quadrant. The right one will be lost.
Vena cava development as far posterior as the subcardinal anastomosis
can be seen at these stages. Trace it from the subcardinal anastomosis right
between the two mesonephric kidneys across the caval mesentery, a
part of the dorsal mesentery of the intestine to the liver (top right
quadrant.) Don't get it confused with the hepatic portal which we saw with the
pancreas, and which also goes to the liver to break up into the sinusoids.
Trace the vena cava to the sinus venosus.
Find the coeliac and mesonephric arteries off the dorsal aorta. Find
glomerular arterioles. Find the umbilical arteries.
.
Label the diagrams or drawings you make with
all underlined structures in the text.
draw the heart of the 10 mm pig; label sinus venosus,
rt. and lft. atrium, rt, and lft. ventricle, interatrial septum,
interventricular septum