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Development of the Mesoderm and Endoderm: A Comprehensive Guide to Embryonic Development, Study notes of Biology

California State University (CSU) - SacramentoBiology

Material Type: Notes; Class: VERTEBRATE EMBRYOLOGY; Subject: Biological Sciences; University: California State University - Sacramento; Term: Fall 2004;

Typology: Study notes

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BIO 127: Vertebrate Embryology
Fall 2004
Lecture Outline for Section 3
IV. Development of the Mesoderm and the Endoderm: In this section we will look
at the development of the other two primary germ layers. The ectoderm starts on the top
surface, so next we’ll look at the middle layer, or mesoderm, and then the bottom layer, or
endoderm. It is often easier to look at the tissues that these latter two structures form
simultaneously. This is true because, while they form quite different cell types, many adult
tissues contain derivatives of both. Many of the same concepts that we saw in our
discussions of the ectoderm will also be obvious here, such as induction events between
germinal components, transfer of organizing power and progressive specification. A key
difference between these layers and the ectoderm is that they start as a loose mesenchymal
migration of cells, thus their “plates” are quite different. For ease of visualization we’ll
again use the nice discoidal avian embryos as the primary model in our discussions – just
keep in mind that the same things occur in our curved embryos, it’s just way harder to
draw on the board!
A. The Paraxial Mesoderm: The primary distinctions in mesodermal tissues are their
position relative to the midline. “Paraxial” means “along the axis” or near the midline and, thus,
along the developing neural tube. This distinguishes it from regions of the mesoderm farther out
– the intermediate mesoderm and the lateral plate mesoderm.
1. The Three Subdivisions of the Paraxial Mesoderm.
a. The Prechordal Plate Mesoderm.
1. Forms the connective tissue and musculature of the head.
b. The Chordamesoderm.
1. Forms the notochord.
c. The somitic dorsal mesoderm, or paraxial mesoderm proper.
1. Forms the somites, which give rise to:
a. Vertebrae and ribs.
b. Dermis of the skin of the back.
c. Skeletal muscle of the back and the body walls.
d. Skeletal muscle of the limbs.
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Download Development of the Mesoderm and Endoderm: A Comprehensive Guide to Embryonic Development and more Study notes Biology in PDF only on Docsity!

BIO 127: Vertebrate Embryology

Fall 2004

Lecture Outline for Section 3

IV. Development of the Mesoderm and the Endoderm: In this section we will look

at the development of the other two primary germ layers. The ectoderm starts on the top

surface, so next we’ll look at the middle layer, or mesoderm, and then the bottom layer, or

endoderm. It is often easier to look at the tissues that these latter two structures form

simultaneously. This is true because, while they form quite different cell types, many adult

tissues contain derivatives of both. Many of the same concepts that we saw in our

discussions of the ectoderm will also be obvious here, such as induction events between

germinal components, transfer of organizing power and progressive specification. A key

difference between these layers and the ectoderm is that they start as a loose mesenchymal

migration of cells, thus their “plates” are quite different. For ease of visualization we’ll

again use the nice discoidal avian embryos as the primary model in our discussions – just

keep in mind that the same things occur in our curved embryos, it’s just way harder to

draw on the board!

A. The Paraxial Mesoderm: The primary distinctions in mesodermal tissues are their

position relative to the midline. “Paraxial” means “along the axis” or near the midline and, thus,

along the developing neural tube. This distinguishes it from regions of the mesoderm farther out

– the intermediate mesoderm and the lateral plate mesoderm.

1. The Three Subdivisions of the Paraxial Mesoderm. a. The Prechordal Plate Mesoderm. 1. Forms the connective tissue and musculature of the head. b. The Chordamesoderm. 1. Forms the notochord. c. The somitic dorsal mesoderm, or paraxial mesoderm proper. 1. Forms the somites, which give rise to: a. Vertebrae and ribs. b. Dermis of the skin of the back. c. Skeletal muscle of the back and the body walls. d. Skeletal muscle of the limbs.

2. The Formation of the Somites (p. 467). The main mesodermal components that form along the developing neural tube are the somites. We used them in the last section to count out our position along the neural tube but they are much more than place markers. They are the source of the non-cranial axial skeleton and of the musculature of the thorax, abdomen, tongue and limbs. They also provide the cartilage of the spinal cord and ribs, and the dermal layer of the skin of the back. a. The timing and periodicity of somite formation. 1. As the primitive streak moves forward. a. Mesenchymal cells migrate to form the middle layer. b. Simultaneous with start of neural tube formation. c. Presomite mesoderm due to noggin antagonism of BMP 2. As the primitive streak moves backward. a. Henson’s node secretes FGF8 - goes away as node does. b. FGF8 blocks Lunatic Fringe protein expression c. Lunatic Fringe (TF) causes expression of Notch. d. Notch causes expression of Hairy1 in 90 second waves. e. Each wave leads to a new somite forming. b. Somite epithelialization. 1. Mesenchymal to epithelial transformation. c. Specification of the somite along the Anterior-Posterior axis. 1. Somites that form cervical and lumbar vertebrae don’t form ribs. 2. Somites that form thoracic vertebrae also form ribs. 3. Primarily Hox gene dependent like the neural tube. d. Determination of the derivatives of the somite. 1. The sclerotome forms in the ventral-medial portion of the somite. a. Forms the cartilage of the vertebrae and ribs. b. Undergoes a reversal to mesenchymal cells.

4. Osteogenesis: The Development of Bone. (p. 474) a. Three cell lineages give rise to bones. 1. Cranial neural crest gives face bones. 2. Lateral plate mesoderm gives bones of the limbs. 3. Somites of the paraxial mesoderm give the axial skeleton. b. Two methods of bone formation. 1. Intramembraneous ossification. a. Mesenchymal progenitors are converted directly into bone. 2. Endochondral ossification. a. Mesenchymal cells go through a cartilage phase first. c. Intramembraneous ossification of flat bones. 1. Migrating neural crest or paraxial mesoderm cells condense into nodes 2. Condensation leads to osteoblast formation - bone cell commitment. a. Osteoblasts secrete collagen-based matrix specialized for Ca++ b. Attachment to their own product gives final differentiation. 1. Osteocytes 3. Calcification proceeds outward from osteocytes. a. Makes more mesenchymal cells condense – can happen fast! d. Endochondral ossification of long bones. 1. Migrating paraxial or lateral plate mesoderm cells commit to cartilage. 2. Mesenchymal cells condense into nodes of pre-chondrocytes. 3. Rapid proliferation produces cartilage model of the bone to come. a. Produces the exact shape of the future bone. 4. Chondrocytes hypertrophy and change their metabolism.

a. Causes a secretion of a “calcifiable” matrix. b. Also cause a little thing called apoptosis!

  1. Cells hanging out around the cartilage model become osteoblasts. a. Invade model and calcify the matrix. b. Accompanied by chondroclasts – eat the dying. e. Bone remodeling.
  2. Osteoclasts differentiate from macrophage progenitors.
  3. Reside in bone and balance activity of osteocytes.

B. The Intermediate Mesoderm: The portion of the mesodermal layer just lateral to the

paraxial mesoderm is the intermediate mesoderm. The main claim to fame of this tissue is the

formation of the urogenital system - the kidneys, gonads and their respective duct systems. We’ll

save the gonads for a later discussion of sex-determination (might as well follow the book for

once). The formation of these organs is a highly informative introduction into the formation of

organs in general. As you’ll see it is not surprising that such a relatively large area of embryonic

tissue is devoted to such a seemingly small pursuit.

1. The Specification of the Intermediate Mesoderm. (p. 478) a. Develops next to paraxial mesoderm which instructs it to form kidneys. 1. Cut the tissue connection – no kidneys will form. 2. Co-culture of the two tissues – kidneys will form 3. Don’t know what the signaling molecule is, yet! 2. Progression of Kidney Types. (p. 479) a. The kidney is very complex. 1. The nephron has over 10,000 cells of at least 12 different types. a. Each is exactly located to the appropriate spot or the organ fails. b. Three stages of development, the latter persists as the functioning organ. 1. The pronephros forms very early in development a. Arises ventral to the most anterior somites b. Mesenchyme of the intermediate mesoderm condense into tubules

1. The Structural Divide in the Lateral Plate. a. The Somatopleure. b. The Splanchnopleure. 2. The Development of the Heart. (p. 492) a. A Little Anatomy Review. 1. The heart is a muscular hollow ball about the size of your fist. 2. Four chambers contract synchronously – two atria, two ventricles. a. The right and left atrium contract together to fill ventricles. b. The right and left ventricles contract together to send blood out. 1. The right side fills the lungs, the left the rest of the body. 3. The vena cava flows into the right atrium 4. The pulmonary artery out of the right ventricle, the aorta out of the left. 5. Valves form between atria and ventricles, ventricles and arteries. b. Specification and Migration of Heart Progenitors. 1. The heart forms as two tubes in the splanchnic mesoderm. a. Some of the first cells into the streak, right behind the node. b. Cardiogenic mesoderm is mesenchymal cells bilateral to notochord. 1. Progenitors for muscle, valves, endothelium, Purkinje fibers 2. Specification is from endodermal induction 3. Migration of specified cells to the anterior is along the endodermal lining. a. Epithelial condensation then follows 4. The inward folding of the foregut endoderm then brings the two tubes close. a. Interestingly, zebrafish have an active migration of cardiac cells.

c. Determination and Fusion of Cardiac Primordia.

  1. Endocardium separate out from epithelium and then reepithelialize a. They migrate into the center of each tube to line the muscle
  2. Tubes come together and fuse at ~29 hours in the chick, 3 weeks in humans a. Both muscle and endocardium b. Spontaneous contractions begin before the tubes are fully fused!
  3. Unfused portions at each end become presumptive inflow and outflow tracts d. Looping of the Heart Tube and Chamber Development.
  4. A very widely studied phenomenon – check it out if you’re curious (p. 495) a. Hand-1 and Hand-2 transcription factors are key regulators b. Looping is how the ventricles and atria move into superior-anterior.
  5. Myocardium induces endocardium to EMT and make endocardial cushions. a. Cushions separate the tube in half. b. Two septa then grow toward cushion to partition the atria. 3. The Formation of Blood Vessels. (p. 500) a. Vasculogenesis: de novo formation of endothelium
  6. Blood islands throughout the splanchnic mesoderm c. Angiogenesis: budding and extension of existing blood vessels. d. Secondary Vasculogenesis in the coronary arteries. e. Invasion and Replacement. 4. The Development of Blood Cells. (p. 505) a. Stem Cells Revisited. b. Two Stage Hematopoiesis. c. Progressive Specification.

2. The Gastrointestinal Tract and its Derivatives. (p. 511) a. Constrictions in the tube form esophagus, stomach, small and large intestine. 1. Different regions are induced by different splanchnic mesoderm. 2. Mesodermal mesenchyme recruited to form smooth muscle layer. 3. The back end also starts covered by ectoderm – cloacal membrane. b. The derivative organs. 1. The liver forms much like the kidney – buds induced by mesenchyme. a. The key mesenchyme is heart-forming region. b. Mesenchyme also induces branching and differentiation. c. The gallbladder forms as early drainage duct but remains. d. Interstingly, the liver induces proepicardial formation in turn. 2. Two distinct endodermal buds fuse to form pancreas. a. Notochord is key inducer, heart cells seem antogonistic. 3. The Respiratory Tract. (p. 515) a. Bud extends from pharyngeal floor as laryngotracheal groove. 1. The tube bifurcates into paired bronchi and lungs. 2. Mesodermal mesenchyme implicated here as well. a. Also recruits smooth muscle layer. b. Alveolar development is last to develop in terrestrial animals. 1. Surfactant is finally secreted as late as 34 weeks in humans. 4. The Extraembryonic Membranes of Terrestrial Vertebrates. (p. 517) a. The embryo must avoid dessication – forms the amnion. 1. Secretes amniotic fluid.

b. The embryo must exchange gasses from an enclosed place (egg or uterus).

  1. Forms the chorion. a. The membrane inside the shell in reptiles and birds b. The placenta in humans. c. The embryo must remove waste – forms the allantois.
  2. A membraneous sack that holds waste until hatch or birth.
  3. Ours is vestigial, pigs use it tremendously. d. The embryos gotta eat – forms the yolk sac.
  4. surrounds the entire yolk, connects to the midgut and blood.

E. Development of the Tetrapod Limb: The limb is a pretty amazing thing. It always

forms as two mirror image pairs directly opposite each other on the sides of the trunk. It

changes from the shoulder or hip to the fingers or toes (proximal-distal axis). It changes

from the thumb or big toe in “front” to the pinky finger or little toe in the “back”

(anterior-posterior axis). It also changes from the knuckles to the palm or bottom of your

foot (dorsal-ventral axis). How does such a complex pattern form so regularly (look

around, it’s pretty remarkably consistent)? This process is a microcosm of embryonic

Pattern Formation – development of coordinate structure in four dimensions of space and time.

1. Specification of the Limb Bud – Getting It All Started. a. Specification along the animals’ anterior-posterior axis. 1. Forelimb buds always form at anterior-most Hox d-6 in midline. 2. Mesoderm in limb-forming region then recruits somitic myoblasts. 3. Retinoic acid from Henson’s Node is critical. a. Remove tadpole’s tail, give RA – get several legs! b. The limb field has complete potential for all limb components. 1. Transplantation gives ectopic limbs. 2. Separation gives multiple limbs.

F. Sexual Development: This is an area of Developmental Biology that often surprises

people. We tend to be familiar with the chromosomal determination of sex (at least in

mammals) and of the hormonal influences that drive the development of secondary sexual

characteristics during puberty. These are the beginning and the end of the process.

Chromosomal determination starts at the point of conception and the development of

secondary sexual characteristics occurs well after birth. During the embryonic period some

rather odd things are known to happen…

1. Chromosomal Determination of Sex. a. Mammals 1. Females have a matched pair of sex chromosomes: XX a. Haploid sex cells (eggs) have an single X 2. Males have an unmatched pair of sex chromosomes: XY a. Haploid sex cells (sperm) have an X or a Y 3. Offspring pick up one from each parent. b. Birds 1. Females have an unmatched pair: ZW 2. Males have a matched pair: ZZ c. Bees 1. Females are fertilized diploids. 2. Males are unfertilized haploids. d. Flies 1. Determined by the numeric ratio of X and somatic chromosomes 2. Development of the Gonads. a. Develop from the intermediate mesoderm near the developing kidney. b. Early development is uniform in males and females: “indifferent stage”. 1. Selected mesenchyme condenses into sex cords. a. Development is by mutual induction with mesenchyme. b. Forms the Mullerian duct by interconnection of cords.

c. There is no default stage in humans.

  1. Need two X’s to develop functional ovaries. a. With one X ovarian follicles develop but can’t be maintained.
  2. Need two Y’s to develop testes. a. Y carries a gene SRY that provides testes determination. d. Gonad development prepares for the final differentiation of germ cells.
  3. Granulosa and thecal cells are specified in females.
  4. Leydig cells in males. 3. Development of the Primordial Germ Cells. a. Germ line cells are determined in the epiblast during cleavage phase!
  5. In the posterior portion near the start site of the primitive streak. b. They have to walk to the gonads – in humans it can take weeks.
  6. Migrate through the hind gut in a little row.
  7. Reach the gonads determined, differentiate there based on hormones. 4. Development of Secondary Sexual Characteristics. a. External genitalia, internal duct-work, hair, breasts, larynx (you know the list). b. All of this is dependent on the gonadal hormonal milieu.
  8. Female characteristics dependent on ovarian estrogen expression. a. Mullerian duct becomes uterus, oviducts, upper end of vagina. b. In puberty all the rest of the effects kick in.
  9. Male dependent on testosterone and Mullerian Inhibiting Substance. a. Mullerian duct degenerates. b. Penis, seminiferous tubules and ducts develop.
  10. Vas deferens, epididymous are mesonephric remnants

c. In puberty all the rest of the effects kick in.