February 7, 2007

Embryology Biology 441 Spring 2007 Albert Harris


Embryonic artery, wrapped in collagen fibers (yellow)

Finish Mesoderm, and then Endoderm

First sub-topic, blood vessels: which are formed from mesoderm.

Capillaries; Veins; Arteries; + also Lymphatic ducts.

In all of these, the innermost lining is a sheet of a special kind of epithelial cell, called "endothelial cells" "endothelium"

(Don't be fooled by the name, these are NOT formed by endoderm!)

Capillaries are narrow hollow tubes of endothelial cells, with little or no other reinforcement. Despite their very thin walls, the pressure in capillaries is more than in veins, and in many capillaries the pressure is almost as strong as in arteries!!?
(90+% of biologists don't really understand how this is possible!)

But earlier in the course you learned about P = C*T + c*t
Capillaries have very high Curvature, around their circumference.
So Pressure can be very large, & Tension is not too strong.
(The same principle is used in backhoes and other heavy equipment
Sometime, notice the thin pipes that carry their high pressure fluid)

Veins are much wider tubes, in which the inner-most lining is a tube of endothelial cells, but they are surrounded by many layers of fibroblasts (mesenchymal cells), and wrapped by Type I collagen fibers (which are mostly secreted by the fibroblasts).

Arteries: innermost layer of endothelial cells
Wrapped around that is a thick, strong layer made mostly of smooth muscle cells, with their long axes, and directions of contraction, oriented in the circumferential direction.
Also fibroblasts and collagen fibers are wrapped around circumferentially; plus another fibrous protein "Elastin"

Elastin is rubber-like, 'stretchy'; In contrast to collagen which forms strong nylon-like fibers and sheets.
Veins also have some elastin, but I am trying to keep this simple.

The aorta differs from other arteries in that its walls have many fewer smooth muscle cells, and many more fibroblasts, and LOTs more elastin, along with the collagen fibers.

Here is a photograph I took of a cross section through a vein and an artery. (a normal artery)

The artery is in the upper-middle.
The vein is below it and somewhat to the left.
The large solid red blob in the lower right is a skeletal muscle.
The smaller red blob to the right of the artery is a bunch of nerves

One of the biggest causes of human death and disability is the disease "atherosclerosis". It is true that if the concentration of cholesterol in your blood is high, then you are more likely to have your arteries blocked by atherosclerotic "plaque".

A cross section of a human artery blocked by atherosclerosis.

(with another blocked artery to the left, and a vein to the lower right)

and that these are located outside the endothelium tube.

It is definitely NOT true that cholesterol sticks to the inside of anyone's arteries. Clotted blood & blood platelets do stick there.
(some clotting is easily visible in this section)

Why does nearly everyone tell you that cholesterol sticks inside your arteries? I don't know why! It's just an Aesop's fable!

* It IS TRUE that if your body has too much cholesterol, then you will get more and worse arterial blockages of this kind!

* Among all those extra smooth muscle cells, that are bulging into the space where the blood should flow, cholesterol does accumulate. So there is some kind of connection.

But many of you will be going into medicine or medical research, so you should be told the truth, not Aesop's fables, and in this case the truth is that nobody really understands what's happening.

One theory: The blockages are tumors of the smooth muscle cells, analogous to many small cancers, caused by somatic mutations.

Another theory: Turbulence in the blood stimulates blood platelets to secrete PDGF (Platelet-Derived Growth Factor), a protein which is known to stimulate smooth muscle cells to grow and divide and also is a chemotactic attractant for them.

Notice that neither of these theories predicts the (true) fact that these over-growths of smooth muscle cells are larger and worse in animals and people who have more cholesterol in their blood.

If you could invent some possible reasons, and also how to test them, then that would be a HUGE contribution to medical science.

Research science is full of riddles and paradoxes of this kind.

Usually you can't just observe complex phenomena.
Almost always, you have to INVENT THEM, and then look for predictions
(surprising predictions will convince people more)

Inventing theories tells you what details to be curious about!

Please invent as many possibilities as you can, and send or email them to me, and I will make a long list of every explanation any of us can think of. (but NOT on the exams)

The following are entirely imaginary; there is no evidence for them. (yet)

    1) When the smooth muscle cells move and divide in response to the PDGF, then cholesterol precipitates between them, and either makes them grow or move more, or maybe it somehow locks them in position.
    2) Cholesterol deposits make smooth muscle cells more sensitive to PDGF.
    3) Cholesterol stimulates platelets to secrete more PDGF, and it's just a coincidence that cholesterol also gets deposited in among the extra smooth muscle cells.
    4) Cholesterol deposits prevents apoptosis of extra smooth muscle cells, so that excess accumulations of smooth muscle are not eliminated..
    5) Cholesterol somehow causes or allows smooth muscle cells to pull more strongly on collagen fibers, or elastin fibers, or to contract more strongly.
    6) Cholesterol deposits between smooth muscle somehow stimulates them to divide.
    10) please invent more theories!!!

The vertebrate circulatory system begins as a series of unconnected endothelial sacks, called blood islands.
Most of these are in the yolk sac, in birds and mammals.
Maybe that's why mammal embryos continue to have yolk sacs?

Red blood cells and white blood cells differentiate inside;
Endothelial cells crawl and connect other to form hollow tubes: the first capillaries. Only much later does bone marrow become the place where blood formation occurs
Early embryos don't have bones; and when the skeleton does form it is nearly all made out of cartilage, which is solid, without a marrow, and isn't penetrated by blood vessels ("vascularized")

The yolk sac is one of several different organs that are used as locations for the hemopoietic stem cells which form red blood cells (and also white blood cells). These stem cells move to the liver, and later to the bone marrow.

Embryonic and fetal hemoglobins are different from (and coded for by different genes than) the hemoglobin we have in the red blood cells we make after birth. Fetal hemoglobin binds oxygen slightly more strongly than adult hemoglobin, which increases the amount of oxygen that is transferred across the placenta from the blood of a pregnant woman to the blood of the fetus developing inside her. Two other interesting aspects are:

#1) The genes for embryonic and fetal hemoglobin proteins are directly adjacent to the genes for adult hemoglobins, with the early blood cells activating one gene, and then later blood cells activating the gene next to that one. The mechanism isn't known, nor whether there is any similarity to the mechanisms that cause adjacent Hox genes to be transcribed in adjacent tissues.

#2) A few people continue to make fetal hemoglobin all their life, instead of switching to the adult hemoglobin genes, and the symptoms of this are almost unnoticeable! So far as I know, it hasn't yet been discovered what mechanism causes this; but it could become a new and better treatment for sickle-cell anemia (and other genetic defects of adult hemoglobins), if you could discover a drug or other treatment that switches their hemopoietic stem cells back to transcribing the genes for fetal hemoglobin.

The Skeleton: formed by mesodermal cells, except the face, where skeleton is neural crest cells
(and if you figure out some logical reason for that, please tell me what it is, because to me it's very surprising.

Vertebrate skeletons are made partly of bone and partly cartilage.

Bone compared with Cartilage

Bone is stronger and more rigid,
       but can grow only by surface deposition.

cross section of bone

Cartilage is lighter, flexible, weaker,
      but can grow by internal swelling (& also by surface addition).

cross section of a typical cartilage

Bone is a tightly woven composite of 1/3 type I collagen fibers, embedded among crystals of calcium phosphate (2/3rds by weight) (& also fluoride and hydroxide) (dentine is almost the same)
If horses are "sent to the glue factory", it's for bones not hoofs.

Bone is made (but not really secreted!) by osteocytes. (= osteoblasts) Deposition of bone is called "ossification".
Osteocytes do secrete collagen, but I think it is still an unsolved problem exactly what mechanism they use to create the calcium phosphate crystals!

Another special differentiated cell type (osteoclasts) constantly dissolves and destroys bones. Bone is a very dynamic tissue, being constantly destroyed and remade. When osteocytes don't keep up with the osteoclasts the result is osteoporosis. There are many unsolved problems here.
Is it over-activity of osteoclasts? Or underactivity of osteocytes?
How can you prove which is the reason? How to cure it?

Cartilage is a mixture of type II collagen and chains of sugar molecules that have sulfate groups covalently bound to them. The sulfates ionize, and their negative charges keep a cloud of cations (like Na+) nearby.
This excess concentration of cations causes an osmosis-like inflow of water ("electro-osmosis"), and this swelling pressure of water is what makes cartilages stiff and makes them tend to expand.
(Very few biologists understand the physics of cartilage swelling!)

Cartilages grow partly by increased synthesis of sulfated sugars, and partly by selective cutting of both the internal and surface collagen.




As an analogy, imagine controlling the shape of a balloon partly by forming more gas inside, and partly by weakening the rubber.
There is a big need for better researchers in this whole area.


Elastic cartilage of ear Elastic cartilage seen by polarization microscopy

The support of your ears and tip of your nose are elastic cartilage, which is more flexible than hyaline cartilage which constitutes most of the rest of the cartilage.

Hyaline cartilage by polarization microscopy

In polarization microscopy of tissues, the colors show you where collagen fibers (or other fibers) are lined up in a particular direction. Fibers in one direction produce the color blue, and fibers oriented perpendicular to them produce complementary colors, like red and yellow. There has to be a net majority of fibers in a certain direction to produce either color, because the effects cancel each other out. Nevertheless, some parts of embryonic development can look like a 4th of July fireworks display when studied by these (simple and relatively inexpensive) methods.

In some vertebrates (sharks and salamanders) the skeleton is mostly made of cartilage (instead of bone). Much of this cartilage becomes calcified and can be quite stiff and strong (but weaker than bone).

The articular cartilages serve for what amounts to lubrication.
They feel and look somewhat like sheets of wet teflon.

Articular cartilage, and bone at right side
Notice the directional orientation of the cells

Bone formation = ossification.
During embryonic development of mammals and birds (including humans), nearly all the "bones" start out being made of cartilage!

Chicken leg skeleton at a stage at which it is all cartilage

In other words, when you were an embryo, your femur, pelvis, radius, ulna, etc. were all made out of cartilage instead of bone! Only certain skull bones, especially the flat ones, are made out of bone from the very beginning. Later, during development, little-by-little, and continuing up to about age 20, this cartilage is dissolved and replaced by bone. NOTE: the cartilage isn't changed into bone; the cartilage is replaced by bone.

Polarized light view of a chicken leg cartilage begining to ossify
The bone appears yellow
The two blue lines are collagen


Replacement of cartilage by bone used to be considered as a recapitulation (in the sense proposed by Ernst Haeckel).
He believed that our distant ancestors evolved cartilage first, and then millions of years later our less-distant ancestors evolved bone; and he believed that embryos are (for some reason!) obligated to repeat such evolutionary sequences. This concept dominated embryology for about 50 years, but has been considered misguided for many more years than that.

Now it is believed that bone evolved before cartilage, and that replacement is a means of allowing growth.
Bones that undergo this process are called "replacement bones";
A synonymous term is "endochondral bone".

The term "dermal bone" refers to those (skull roof, etc.) bones that are made of bone from the start.

The last parts to ossify are the epiphysial plates;
when they ossify, the bone can't grow any longer.
(Rabbits, chickens, etc. just have an epiphysis instead of a plate)


Cross section of rabbit epiphysis seen by polarization microscopy


Up to around 20-21, the age of a skeleton at death can be dated within +/- a few months by noticing which epiphysial plates had "closed" and which ones had not. Ossification is very stereotyped.

Mutations in certain genes can cause premature ossification "closure" of epiphysial plates. Basset hounds and dachshunds have short legs because of this type of mutation; also some breeds of sheep.

So-called "chondroplastic dwarfism" in humans results from an equivalent (dominant!) mutation.
(no one knows what protein these genes normally code for: an enzyme?)

During the healing of broken bones, bone around the break is dissolved, cartilage is formed in its place, and then this cartilage is gradually replaced by bone, as occurred during development!

Intermediate mesoderm: between somites & lateral plate : forms kidneys and male sex ducts

This tissue differentiates into excretory tubules, first the anterior part, then the middle, and finally the most posterior.
Embryos need to filter wastes out of their blood, even very early. They don't need to walk, or think, but they do need to excrete. Heart and kidneys develop much earlier than brain or limbs.

Heart and kidney solve this problem in very different ways.

Embryos form a succession of different kidneys, and destroy the old ones They make the first kidney out of the most anterior parts of the intermediate mesoderm; and they make the final, adult kidney out of the most posterior part of the intermediate mesoderm. Mammals & birds use different kidneys after birth than before.

The heart uses the same organ, and keeps improving it!

Vertebrate embryos first make a very simple pair of kidneys called the pronephros, to filter the blood of the early embryo.

Then they build a better pair of kidneys, mesonephros, and bring them on line during most of embryonic development.

In mammals, birds and reptiles we then build a third, even better pair of kidneys metanephros, which we use from then on.
The first 2 pairs of kidneys are then destroyed.

Special ducts (= pipes) carry urine from these different kidneys.

The pronephric ducts "grow" rearward from the two pronephros kidneys, mostly by cell locomotion, analogous to the formation of capillaries, and sort of like a multicellular equivalent of axon formation, except that the structure being formed is multicellular and becomes hollow.

Experiments support the idea it is guided by an adhesion gradient.

The pronephric duct connects to the cloaca (rearmost part of the digestive tract). In mammals, the cloaca later becomes split into the rectum(dorsal) and the bladder (ventral), and of course the kidney ducts connect to the latter.
"Wolffian duct" is another name for the pronephric duct.

Mesonephric kidney tubules connect to these two pronephric ducts.

In males, the seminferous tubules of the testis become connected to some of these mesonephric tubules.
Sperm eventually exit the body via these tubules and via the pronephric duct, which will then be called the vas deferens.

The sperm ducts were embryonic kidney ducts.

A different pair of ducts, called the ureters, develop to connect the mesonephros to the bladder.

In Amphibians, the sperm still go down the urine duct;
Their adult kidneys are equivalent to our mesonephros.
They don't form ureters or metanephric kidneys.

In female mammals, etc., the pronephric duct degenerates.

Female sex ducts (oviducts, Fallopian tubules and ureter) develop from entirely separate tissue of the lateral plate mesoderm.

They are called "Mullerian ducts".
In males, these start development, but then degenerate.

Notice how the sex ducts of both sexes begin development in the other sex, but then degenerate.
Male sex ducts are NOT homologous to the female sex ducts.

Gonads themselves are formed from the same tissue in both sexes:

Testis (testes plural) in males and ovaries in females.
They are both formed from thickenings in the wall of the coelomic cavity, which makes then lateral plate mesoderm.
These thickenings are called genital ridges.

The actual future sperm and egg cells do NOT develop from cells of the genital ridge. Sperm and egg cells develop from special cells called "primordial germ cells", that migrate to the genital ridges from some other part of the body

The places from which the primordial germ cells start out differs widely between different kinds of animals.

In mammals, PGCs originate in the yolk sac.
In birds PGCs originate from a crescent shaped region of the zona pellucida in front of where the body forms

In flies and nematodes PGCs come from special cells at the extreme posterior end.

There may or may not be any kinds of animals in which the future egg & sperm start out located in the same place as the gonads.
(& please tell me if you hear of any.)

If you kill or remove the PGCs from an embryo, or keep them from getting to the genital ridges, that animal will be sterile.

Grafts of chicken primordial germ cells from one egg to another will result in animals that produce eggs or sperm that have different genes than the parent.

One of the world leaders of research on primordial germ cells is out at the NIEHS in the Res. Triangle Park, and one year a student in this course did research with him, and we had PGCs crawling around in dishes. We were trying to prove whether or not they were guided by chemotaxis, but we didn't succeed.

The Stomodeum is an infolding of the somatic ectoderm.

So the epithelia of the inside of your mouth is considered to be part of the somatic ectoderm.
The ameloblasts, that secrete the enamel of your teeth are also derived from the stomodeal part of the somatic ectoderm.


Salivary glands: formed as epithelial outfoldings

Pharyngeal pouches:

(the following is not a misprint)  

Thyroid gland forms as an outpocketing of the floor or the endoderm behind the throat, that then disconnects from the surface, but remains as a series of hollow sacks, full of mucus.

trachea; bronchi; alveoli (in mammals) (air capillaries in birds)

Hepatic diverticulum (--> liver) gallbladder


Cloaca --> Bladder (these separate in mammals)
--> Rectum

Birds, reptiles & amphibia continue to have cloacas

In amphibian embryos, the anus develops from the blastopore.
But in reptiles birds and mammals, there is no blastopore.

Instead, they form a posterior infolding called the proctodeum
This fuses with and then opens into the rear of the archenteron

The proctodeum is an infolding of the somatic ectoderm, equivalent to the stomodeum at the other end.


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