Week 8 - Developmental biology

What is it meant by development (otogeny)? What does the process start and end with? Which processes does this idea include?

• Development (ontogeny) is the process by which multicellular organisms grow and differentiate tissues and organs. (goes from single cell to an adult with multiple cells)

• Starts with a one undifferentiated cell embryo (zygote), ends with a mature adult with multiple differentiated cells.

• It also includes regeneration, asexual reproduction, metamorphosis (i.e. insects), and stem cells differentiation

Summarise the role of molecules in successful fertilisation.

–      Sperm penetrate the protective layer around the egg

–      Receptors on the egg surface bind to molecules on the sperm surface

–      Changes at the egg surface prevent polyspermy, the entry of multiple sperm nuclei into the egg

What is fertilisation?

•       Fertilization is the formation of a diploid zygote from a haploid egg and sperm

What is the acrosome? What happens in the acrosomal reaction (stage 1 of fertilisation)?

The acrosome is at the tip of the sperm. It releases hydrolytic enzymes that digest the material surrounding the egg (the jelly coat). This process is known as the acrosomal reaction.

The acrosomal reaction is triggered when the sperm meets the egg.

What happens after the acrosomal reaction (step 2 of fertilisation)?

1.     Once the jelly coat is gone, a cytoskeleton bridge will from between the sperm and the egg (through membrane fusion)

2.     The sperm nucleus then enters the egg

What happens after the sperm nucleus has entered the egg? (step 3 of fertilisation)

Fast block to polyspermy (to prevent more sperm nuclei to enter)

Describe the process of “fast block to polyspermy”

Seconds after the sperm binds to the egg, ion channels open in the egg plasma membrane.

Sodium ions influx (into the egg) causes depolarisation of the egg membrane, as the charges are changed (the inside becomes more positive).

Because the egg is now more positive on the inside, and the sperm are also positively charged, they basically repel one another.

Which 2 processes prevent polyspermy

Fast block process and slow block.

Describe the process of “slow block to polyspermy” (aka the cortical reaction).

Fusion of the egg and sperm also initiates the cortical reaction.

Seconds after the sperm binds to the egg, vesicles in the egg release their contents (causing a Ca+2 wave) and form a fertilisation envelope (acting as a protective barrier).

The fertilisation envelope acts as a slow block to polyspermy.

What is the standard life cycle processes of an animal?

Fertilisation → Cleavage → Gastrulation → Organogenesis → Metamorphosis.

What is cleavage?

A period of rapid cell division without growth.

Cleavage partitions the embryo into smaller cells called blastomeres

What is the blastula (in cleavage)?

The blastula is a ball of cells with a fluid-filled cavity called a blastocoel

What happens in gastrulation?

Gastrulation rearranges the cells of a blastula into a layered embryo called a gastrula.

Which of the embryonic (germ) layers are produced by gastrulation

Germ layer

Ectoderm

Endoderm

What is the germ layer (produced by gastrulation)?

The germ layer is the embryonic tissue (a collection of specialised cells)

What is the ectoderm (produced by gastrulation)?

The ectoderm is the germ layer covering the embryos surface (its skin).

What is the endoderm (produced by gastrulation)?

The endoderm is the innermost germ layer, and lines the developing digestive tube, called the archenteron (the gut).

Describe the difference between diploblastic and tripoblastic animals?

• Diploblastic animals have ectoderm and endoderm, include non-bilaterians.

• Tripblastic animals also have an intervening third layer, the mesoderm; these include all bilaterians.

Why is the mesoderm present sometimes?

Mesoderm is there for organism to produce a higher diversity of organs. This explains why a lot of people have the mesoderm.

Give examples of where the different embryonic layers are found.

Endoderm: digestive tract (e.g., guts) and organs derived from it (e.g., lungs) as well as secretory organs (e.g., liver, pancreas).

Ectoderm: epidermis and nervous system.

Mesoderm: coelom, notochord in chordates, muscles, bone/cartilage, circulatory, spleen, kidney, reproductive, etc.

What happens in cell differentiation?

Cells become specialised 

What happens in morphogenesis?

Tissue formation. Differentiated cells form tissues and organs and the shape and structure of the organism emerges.

How does the blastopore fate differ between the protostomes and the deuterostomes?

• In protostomes (“mouth first”) the blastopore becomes the mouth.

• In deuterostomes (“mouth second”) the blastopore becomes the anus.

What is the structure of a chick (a vertebrate) embryo before gastrulation?

Before gastrulation, the embryo has two layers:

• Epiblast (upper layer → ectoderm)

• Hypoblast (lower layer → endoderm)

What major movements occur during gastrulation in chicks?

• Epiblast cells move toward the midline of the blastoderm, then migrate inward toward the yolk.

• This movement forms the primitive streak, a thickened structure at the midline.

• Movement of epiblast cells and formation of the primitive streak are essential because they initiate gastrulation, establish body axes, and generate the germ layers that give rise to every organ and tissue in the organism.

How do human eggs differ from many other vertebrates?

• Human eggs contain very little yolk, unlike birds or reptiles.

• The blastocyst is the human equivalent of the blastula (different from other animals).

What is the role of the trophoblast (found in the blastocyst) in early human development?

• The trophoblast is the outer layer of the blastocyst.

• It does not contribute to the embryo itself, but instead initiates implantation into the uterine lining.

• After implantation, the trophoblast expands and helps form extraembryonic membranes.

How does gastrulation occur in humans?

• Gastrulation involves cells of the epiblast moving inward through a primitive streak, similar to the process in chick embryos.

• After gastrulation, all three embryonic germ layers are established.

Summarise the 4 stages of gastrulation and what occurs at each stage (and at what time this occurs).

1. The blastocyst reaches the uterus

2. It then implants (7 days after fertilisation)

3. The extraembryonic membranes then start to form (10-11 days), and gastrulation begins (13 days).

4. Gastrulation has then produced a 3 layered embryo with 4 extraembryonic membranes: the amnion, chorion, yolk sac, and allantois.

Describe and name the 4 extraembryonic membranes.

•       Chorion: outermost layer, with two sublayers, trophoblast and mesoderm. Forms the placenta. Gas exchange.

•       Allantois: “sausage”, full of blood vessels, nutrition, excretion and gas exchange.

•       Amnion: full of amniotic fluid, embryo protection.

•       Yolk sac: made of hypoblast (endoderm) cells, initial circulation, encloses the yolk (food).

Which animals are classified as amniotes and why?

• Reptiles, birds, and mammals are amniotes.

• They are given this name because their embryos are surrounded by fluid inside a membrane called the amnion.

What developmental feature do all amniotes share?

They form four extraembryonic membranes, which act as a life-support system for the developing embryo, handling protection, nourishment, waste removal, and gas exchange

How did amniotes adapt to reproduce outside water?

Reproduction on land required new structures:

• Shelled eggs in birds, other reptiles, and monotremes.

• A uterus in marsupial and eutherian (placental) mammals.

What role does the amnion play in amniote reproduction?

• In both shelled eggs and the uterus, the embryo is surrounded by fluid in the amnion, which protects it and prevents drying out.

• This adaptation enables successful reproduction on dry land.

What is organogenesis?

Where various regions of the germ layers develop into rudimentary organs.

(Organogenesis is the stage of embryonic development in which the three germ layers (ectoderm, mesoderm, and endoderm) develop into the body’s organs and tissues. It occurs after gastrulation).

What can the adoption of particular developmental fates cause the cells to do?

Adoption of particular developmental fates may cause cells to change shape or even migrate to a new location in the body

What is neurulation?

Neurulation is the formation of the brain and spinal cord in vertebrates

What forms the notochord during neurulation?

Mesodermal cells develop into the notochord, a rod running along the dorsal side of the embryo.

How does the neural plate form during neurulation?

The notochord and nearby tissues secrete signaling molecules that cause the overlying ectoderm to thicken and become the neural plate.

What is induction in embryonic development?

Induction is when one group of cells or tissues triggers developmental changes in nearby cells, such as the notochord inducing formation of the neural plate.

What happens to the neural plate during neurulation?

The neural plate curves inward and folds, eventually forming the neural tube.

What does the neural tube become?

The neural tube develops into the central nervous system, including the brain and spinal cord.

What happens to the notochord after neurulation?

The notochord disappears before birth, but contributes to parts of the intervertebral discs in the spine.

Where do neural crest cells form?

Neural crest cells form along the edges of the developing neural tube during early development.

What structures do neural crest cells give rise to?

Neural crest cells migrate throughout the embryo and form:

• Nerves

• Parts of the teeth

• Bones of the skull

• Other specialized tissues

What forms somites during development?

Mesoderm on either side of the notochord organizes into segmented blocks called somites.

What structures do somites eventually form?

Somites develop into vertebrae, ribs, and the muscles associated with the vertebral column.

What is a major mechanism controlling changes in cell shape during development?

Reorganization of the cytoskeleton is a key force driving cell shape changes.

How can a cell become wedge-shaped during development?

Contraction of actin filaments at one end of the cell pulls the cell inward, making it wedge-shaped.

What developmental process can result from many cells becoming wedge-shaped?

When many cells undergo this change together, the cell layer invaginates, which is a common mechanism in early embryonic morphogenesis.

What is the extracellullar matrix (ECM)? What is it important in?

Migration also involves the extracellular matrix (ECM), a meshwork of secreted glycoproteins and other molecules lying outside the plasma membrane of cells.

What is apoptosis?

Programmed cell death.

What is determination in cells fate specification?

Determination is a term referring to the process by which a cell or group of cells becomes committed to a particular fate.

What are fate maps?

Fate maps are diagrams showing organs and other structures that arise from each region of an embryo.

How can the nematode be used in cell/fate mapping?

•       Studies of the nematode Caenorhabditis elegans used the ablation (destruction) of single cells to determine the structures that normally arise from each cell

•       The researchers were able to determine the lineage of each of the 959 somatic cells in the worm

Why does cell differentiation occur?

Cell differentiation occurs as a result of the expression of different genes in different cells.

Cell-specific sets of genes are “switched on” by proteins called Transcription Factors, the resulting proteins define the type of cell.

How does MyoD demonstrate cell differentiation in muscle development?

• Muscle precursor cells (myoblasts) begin producing muscle-specific proteins, showing differentiation, which later leads to the formation of muscle tissue (morphogenesis).

• MyoD is a transcription factor that binds to promoters and activates target genes involved in muscle development.

• MyoD is one of several master regulatory genes that switches on the genetic program required for muscle formation.

What are gene cascades?

Transcription factors that regulate the expression of other transcription factors, and so on.

What are morphogens?

Morphogens are proteins that establish the embryos axes and other features using gradients of concentration. (Morphogens are molecules that pattern tissues by providing positional information through a concentration gradient, telling cells what to become based on how much they detect).

What is the French flag model and how does it explain cell fate determination?

• Proposed by Lewis Wolpert (1969) to explain how morphogen gradients specify cell fates.

• Morphogen spreads from a localized source, forming a concentration gradient across a tissue.

• Cells “read” the local morphogen concentration and differentiate based on threshold levels:

  • High concentration → blue fate

  • Intermediate concentration → white fate

  • Low concentration → red fate

• Demonstrates how one gradient can pattern multiple cell types along a developmental axis.

What does the Bicoid gene determine in Drosophila embryos, and how?

• Bicoid is a maternal effect gene that determines anterior (head and thorax) structures.

• Bicoid mRNA is deposited at the anterior end of the egg, then translated into protein after fertilization.

• The Bicoid protein forms a gradient from anterior to posterior.

• Cells read the local concentration:

  • High → head

  • Intermediate → thorax

  • Low → posterior

• Provides positional information along the anterior–posterior axis.

What are homeotic genes? Provide an example.

Homeotic genes are genes which regulate the development of anatomical structures, usually specifying the position in an axis. An example is: Hox genes.

What are homeotic mutants?

Mutants in which the position of organs along the anterior-posterior axis is altered.

Name an example of 2 genes that perform similar functions in plants.

MADS-box genes do similar functions to homeobox (but are not evolutionary relationships)

Describe the main differences in plant and animal development?

Animals:

• Grow by cell division

• Cell movement determines shape

• Cell death determines shape

• Embryonic shape determination

• Grow to a fixed size and shape

Plants:

• Grow by cell expansion

• Cell growth determines shape

• Cell division plane determines shape

• Post-embyronic shape determination

• Grow to variable size and shape

What is evolutionary developmental biology?

The study of the evolution of developmental processes in multicellular organisms.