FETAL SKELETON DEVELOPMENT
OBSERVE THE ARTICULATED, MOUNTED HUMAN FETAL SKELETON AND THE ALIZARIN RED STAINED 10 WEEK HUMAN EMBRYO SKELETON AND COMPARE THEM TO THE ADULT SKELETON
There are some very basic differences between fetal and adult skeletons.
1. proportions: the skull of the fetus is about one half the height at 2 months, more than one fourth the height at four months, one fourth at birth, about an eigth at age 25.
2. The legs are 1/8th the height at two fetal months, over 1/4th at 4 fetal months, 3/8ths at birth, 1/2 at age 25. Very early, the hips are very small, the eyes very large, in proportion.
SKULL In the early stained skull, you can only see a few regions of bone formation called ossification centers; jaws, frontal region just above the eye, and supraoccipital, occitital and perhaps a little of the temporal. On the mounted skeleton, you can see the regions between the skull bones called sutures and fontanels. There are several obvious fontanels, or soft spots on the fetal skull, the single frontal fontanel on the top, between the frontal and parietal bones, the bilateral (on both sides) sphenoid fontanel right above the eyes and jaws, the bilateral mastoid fontanel behind the ears, the single occipital fontanel at the back of the skull. You can tell these from the sutures between the rest of the bones because they are more triangular instead of just narrow elongate spaces. The roof and sides of the skull form as membrane bones without formation of cartilage first, whereas the base and palate form cartilage and then bone. The fontanels are in the membrane bones. What purpose do you think they serve? They are not present in skulls of adults.
VERTEBRAE There are similarities between all the vertebrae, with a vertebral canal where the spinal cord lies, and the neural arches which surround it; there is a centrum which stacks with other centra to support the body's weight. The transverse processes are sites for muscle attachment. Intevertebral discs of cartilage separate the centra and make a protective cushion between them. Notice the difference between the cervical, thoracic, lumbar, fused sacral vertebrae, in the mounted skeleton. Notice that only small dots of bone are present in the stained specimen. You can see an outline of the future bone as cartilage which does not stain, because vertebrae are formed first of cartilage which is replaced by bone (cartilage replacement bone.)
APPENDAGES THE STAINED SPECIMEN shows that the bone first forms in the center of the long bones, and girdles. This allows the growth of the bones at the two cartilaginous ends. Look for the humerus, radius, ulna, scapula, femur, tibia, fibula, ribs, phalanges. Compare them to the mounted skeleton.
10 and 20 mm PIG and HUMAN EMBRYO SECTIONS ON SLIDES: Find a section which has a forelimb bud on it. Using the diagram, find the apical ridge, and the limb mesenchyme. The apical ridge causes the outgrowth of the limb and the mesenchyme detemines what kind of limb develops, fore or hindlimb. See if there is a mass of cartilage in the center of the limbbud, and muscle cells forming along each side of it.
FETAL PIG SERIES Look at the pig series you looked at the last time. Examine the limbs again to compare the various stages. Is there a difference between the fore and hind limbs in terms of development and rate. Is there a change in flexibility of the limb with time? The first digit is missing in pigs, and the third and fourth digits develop hooves, as the pig walks on the tips of the digits, whereas man walks on the soles of the feet. See if you can find a stage early enough (between 20 and 30 mm or 2‑3 cm) that when you slit the lower jaw away by cutting at the angle of the jaw on one side only, you can see an incomplete palate. Then do the same thing on an older stage, (after 3 cm) to see when it is closed. Do this dissection carefully, so you can look for teeth, and tongue development. The palate separates the nasopharynx from the oral cavity or mouth so they (and we) can chew while breathing.
TEETH of FETAL PIGS You can see the appearance of two canine teeth and the third pair of incisors first to erupt. In older pigs you may see the other incisors and four pairs of premolars. The molars erupt after birth.
ANSWER SHEET FOR SKELETON EXERCISE NAME_________________________
1. Measure the height of the fetal skeleton_________________________
(in cm). Measure the height of the skull___________________, the legs_______________________. Do the same for the adult skeleton:__________________, _________________,_______________. Now figure out the proportions (divide each part by the total height). Compare the adult and fetal proportions.
Measure the size of the eye sockets in the fetal and adult skulls. What is the ratio?
Observe the fontanelles and sutures in adult and fetal skulls. What is the purpose of fontanelles, especially at birth?
2. Look at the chick skeletal stains for cartilage and bone. Where is the cartilage stain in the younger ones?
Where is it in the older ones?
What happens to the bone stain from younger to older (don't just say there is more, talk about specific regions which change.)
What is the relationship of cartilage and bone in the limbs?
How is the skull different in bone formation from the limbs?
3. Name some differences between the adult and fetal skeletons:
How do you think these differences are related to muscle strength in newborns, and ability to move?
How does the stained skeleton of the young human embryo differ from the mounted fetal skeleton?
4. Look at the limb buds developing in the slides and in the small pigs. What are the differences between the smaller and larger pigs. (Look back at your results from the external appearance lab, as well.)
a. what is the appearance before there are digits?
b. Now look at the stained specimens of chick, and observe the differences between the wing and leg development.
c. What is the difference in function of apical ridge and mesenchyme in the limb bud development? Read about recombination experiments.
5. Observe the palate formation as suggested in the written part. What is the first indication of palate formation, at which stage?
What is the purpose of the palate?
Why do you suppose the developmental pathway where teeth are not fully developed when humans are born has been selected by evolution?
Compare the teeth in fetal pigs and chicks with the human skeleton.