47 Animal Developement

Question 1

What does the “tinman” gene direct?

Answer 1

The location of the heart in both Drosophila and in humans.

(Tinman is a reference to the Wizard of Oz: if the tin man gene is defective the organism will have no functionally heart)

Question 2

Why is Drosophila a useful model organism?

Answer 2

It has short reproductive cycles and mutants can easily be identified.

Question 3

What organisms are used primarily as model organisms for embryonic development?

Answer 3

Sea urchin, frog, chickens and nematodes (roundworm)

Question 4

What is it called when multiple sperm fuse with one egg?

Answer 4

Polyspermy

Question 5

What is a consequence of polyspermy?

Answer 5

The offspring with have an unusual number of chromosomes and thus be genetically flawed

Question 6

What key stages occur during fertilisation?

Answer 6

The “acrosomal reaction”, the “cortical reaction” and “egg activation”

Question 7

What is the purpose of the key stages that occur during fertilisation?

Answer 7

To allow the sperm to dissolve or penetrate the plasma membrane so that the egg, which is also activated, can be fused with.

Preventing polyspermy is also important within these stages.

Question 8

What form of fertilisation do sea urchins employ?

Answer 8

External fertilisation.

Question 9

What adaption do sea urchin eggs have to maximise the chance of them begin fertilised?

Answer 9

They have a jelly coat surrounding them. This protects the eggs whilst also secreting chemicals that cause sperm to swim towards the egg.

Question 10

What is the primary purpose of the acrosomal reaction?

Answer 10

To act as a “fast block to polyspermy”

Question 11

What happens during the acrosomal reaction?

Answer 11

On contact with the jelly coat of the egg the acrosome, a vessel on the head of the sperm, releases hydrolytic enzymes through exocytosis. These enzymes form a hole in the jelly coat of the egg.

The “acrosomal process” (a structure) of the sperm elongates through this hole until it reaches the actual egg cell. Proteins on the end of “acrosomal proccess” bind to receptors on the plasma membrane of the egg if they are both from the species.

If the “acrosomal process” has the correct proteins this causes the plasma membranes of the sperm and egg to fuse. As the sperm nucleus enters the egg cytoplasm sodium ions diffuse into the cytoplasm causing it to depolarise. This increase in membrane potential prevents other sperm from binding and thus acts as a “fast block to polyspermy”

Question 12

How long does the “fast block to polyspermy” take?

Answer 12

1-3 seconds from when the sperm binds to the egg.

Question 13

Why is the acrosomal process’ role in matching species especially important in urchin?

Answer 13

As external fertilisers, the sperm of urchin could mix with that of other species. Thus to prevent genetic issues the species is checked using the acrosomal process and receptors on the plasma membrane of the egg.

Question 14

What occurs after the acrosomal reaction?

Answer 14

The cortical reaction.

Question 15

What is the purpose of the cortical reaction?

Answer 15

To prevent polyspermy.

Question 16

Why are both the acrosomal and the cortical reaction needed to prevent polyspermy?

Answer 16

The acrosomal reaction acts fast but the membrane depolarisation lasts around a minute.

The cortical reaction is more long-lasting but takes longer to occur.

Question 17

What happens during the cortical reaction?

Answer 17

Vessels in the egg cytoplasm called “cortical granules” bind with the plasma membrane of the egg. The contents of the cortical granules are released into the space be tween the plasma membrane and the surrounding vitelline layer (formed by the egg’s ECM)

Enzymes from the granules lift the vitelline layer away from the egg and harden it into a protective “fertilisation envelope”. Other enzymes remove the sperm binding receptors that “pin” the vitelline layer to the egg.

The formation of the “fertilisation envelope” and the removal of the sperm-binding receptors provides long term protection against polyspermy.

Question 18

What is the purpose of “egg activation”?

Answer 18

To initiate metabolic reactions that trigger the onset of embryonic development such as increasing the rate of cellular respiration and protein synthesis.

Question 19

What happens during “egg activation”?

Answer 19

The calcium ions released during the cortical reaction activate proteins and mRNA’s already present in the egg cytoplasm.

Question 20

In what animals does the cortical reaction occur?

Answer 20

Urchins and vertebrate including fishes and mammals

Question 21

Besides the cortical and acrosomal responses or egg activation, what happens when a sperm binds to an egg cell?

Answer 21

In humans this leads to the completion of meiosis as the egg cells are arrested at the metaphase of meiosis II.

Sea urchin eggs have already completed meiosis when they are released from the female.

Question 22

What roles does the actual digestive tract play in fertilisation?

Answer 22

In internally fertilising animals it regulates sperm movement by secreting chemical messengers while also providing a moist environment.

These chemical messengers are required for the sperm to even fertilise the egg. The release of these factors to ‘activate’ the sperm is called “capacitation” and occurs 6 hours after sperm enter the female reproductive tract.

Question 23

What happens during “capacitation”?

Answer 23

Support cells of the developing follicle surround the egg and remain with it during and after ovulation. A sperm must travel through this layer of follicle cells before it reaches the zona pellucida, the extracellular matrix of the egg. Within the zona pellucida is a component that functions as a sperm receptor. As sperm binds to this receptor it induces an acrosomal reaction (different to the one that prevents polyspermy) that facilitates sperm passage through the zona pellucida to the egg. This binding also exposes the “acrosomal process”

Question 24

What typically happens after the stages of fertilisation are complete?

Answer 24

The process “cleavage” in which the zygote rapidly divides.

Question 25

What happens during cleavage?

Answer 25

During the rapid cell division of the zygote the cell cycle basically skips the G1 and G2 phases of the cell cycle and thus only S phase and M phase are completed.

This means little protein synthesis occurs and thus the cell rapidly divides into smaller cells called “blastomeres”.

As the rapid cell division continues the blastomeres begin to form a hollow ball called a ‘blastula’. The inner cavity of the blastula is called the ‘blastocoel.’

Question 26

What causes cleavage to be non-symetrical?

Answer 26

Uneven distributions of yolk ’stored nutrients’ guides the asymmetry.

Where the yolk is concentrated the ‘vegetal pole’ forms. At the other end the ‘animal pole’ develops.

Question 27

How pronounced are the vegetal and animal poles in human blastulas?

Answer 27

Such polarity is almost non-existent as these eggs have very little yolk.

Question 28

Beside yolk concentration, how is cleavage regulated? Why is this the case?

Answer 28

Evidence suggests that cleavage occurs until a certain ratio of nucleus size to cytoplasm size is met.

This is important because the single nucleus in a newly fertilized egg has too little DNA to produce the amount of messenger RNA required to meet the cell’s need for new proteins during growth. Instead development is initially regulated by RNA and proteins deposited in the egg during oogenesis.

By diving into many small blastomeres that each have their own nucleus each one is capable of producing its individually needed proteins.

Question 29

What layers does the developing embryo form?

Answer 29

The Ectoderm (outer layer), the Mesoderm (middle layer) and an Endoderm (inner layer)

Question 30

What does the Ectoderm develop into?

Answer 30

• The epidermis of the skin
• Sweat glands and hair follicles of the epidermis
• Nervous and sensory systems
• Pituitary gland, adrenal medulla (not adrenal cortex)
• Jaws and teeth
• Germ cells i.e. haploid cells in the gonads

Question 31

What does the mesoderm develop into?

Answer 31

• Skeletal and muscle systems
• Circulatory and lymphatic system
• Excretory and reproductive systems (except germ cells)
• Dermis of the skin
• Adrenal cortex

Question 32

What does the endoderm develop into?

Answer 32

• Epithelial lining of digestive tract and associated organs (liver, pancreas)
• Epithelial lining of respiratory, excretory, and reproductive tracts and ducts
• Thymus, thyroid, and parathyroid glands

Question 33

What is formation of distinct features from the blastula called?

Answer 33

Morphogenesis.

Question 34

What stages of morphogensis occur in the embryo?

Answer 34

Gastrulation then organogensis.

Question 35

What are the layers of the embryo called?

Answer 35

Germ layers i.e. endoderm, ectoderm.

Question 36

How does the number of the embryonic ‘germ layers’ differ between species?

Answer 36

In radially symmetrical organism like starfish only two layers form: the endoderm and the ectoderm.

In bilaterally symmetrical organism a third layer, the mesoderm, is present in the middle.

Question 37

Put simply, what is gastrulation?

Answer 37

Invagination at the mouth and anus to form an alimentary canal between these openings.

Question 38

Based on gastrulation, how can organism be grouped?

Answer 38

Protostomes in which the mouth develops first and deuterostomes in which the anus develops before the mouth.

Question 39

Based on gastrulation, how are vertebrates grouped?

Answer 39

Deuterosomes

Question 40

How does the blastula appear prior to gastrulation?

Answer 40

It has a small region of future endosperm cells at the vegetal pole. This group of cells is called the vegetal plate. The rest of the blastula is future ectoderm

Some future mesoderm cells known as mesenchyme cells are found within the vegetal plate.

Question 41

What happens during gastrulation in sea urchins?

Answer 41

The mesenchyme cells migrate to the blastocoel from the vegetal plate, forming a hole called the ‘blastopore’ which will become the anus.

The vegetal plate invaginated as the mesenchyme cells continue to migrate throughout the blastocoel. This invagination of the vegetal plate forms a tube, known as an archenteron that will eventually become the digestive tube.

New mesenchyme cells at the tip of the archenteron send out thin extensions (filopodia) toward the blastocoel wall. As these extensions contract they grab the archenteron across the blastocoel.

Eventually the archenteron reaches the other side of blastocoel. It fuses will the wall to form the mouth.

Question 42

What does the ‘blastopore’ refer to?

Answer 42

The open end i.e. hole of the archenteron. Therefore in deuterostomes it will become the anus and in protostomes it will becomes the mouth.

Question 43

Besides the archenteron, what forms during gastrulation?

Answer 43

Cillia on the surface of the gastrula that will later be used for nutrition and movement.

Question 44

How many cells does a typical human embryo have before gastrulation?

Answer 44

100 cells.

Question 45

What is unique about the blastula of a human?

Answer 45

Unlike in urchins etc. where it is called a blastula, it is called a “blastocyst” humans and other mammals.

Question 46

How does a blastula differ from a blastocyst.

Answer 46

They are the same concept (very different structure) but it is called a ‘blastocyst’ in mammals and a blastula in other animals.

Question 47

What is the structure of the blastocyst prior to gastrulation?

Answer 47

The epithelium wall of the blastocyst is called the trophoblast. This structure is not part of the actual embryo but aids in its growth.

Like in other animals the blastocyst has a cavity called the ‘blastocoel’. On one side is a clump of cells that will develop into the embryo called the ‘inner cell mass’

Question 48

What stages happen in preparation of gastrulation in humans?

Answer 48

The trophoblast initiates implantation of the blastocyst into the “Endometrial lining” of the uterus by secreting enzymes. These enzymes break down a region of the uterine lining and thus allow the blastocyst to implant. This is called ‘invasion’

The trophoblast thickens through cell division it forms finger-like projection in to the surrounding tissue. This ruptures fine capillaries and thus causes bathes the trophoblast tissue with nutrient-rich blood.

The inner cell mass of the blastocyst forms a flat disk with an upper layer of cells, the epiblast, and a lower layer, the hypoblast. The embryo develops mostly from the epiblast cells.

The trophoblast continues to explained into the endometrium resulting in the formation of four membranes outside the embryo called the “extra embryonic membranes”

Technically gastrulation only begins as cells move inward from the epiblast through a “primitive streak” and form mesoderm and endoderm. The ectoderm arose form the epiblast. The endoderm formed form the Hypoblast. (Mesoderm forms in the middle)

Question 49

What happens during gastrulation in humans?

Answer 49

Technically gastrulation only begins as cells move inward from the epiblast through a “primitive streak” and form mesoderm and endoderm. The ectoderm arose form the epiblast. The endoderm formed form the Hypoblast. (Mesoderm forms in the middle from the extraembryonic membrane)

Question 50

Why is the expansion of the trophoblast before and during gastrulation important?

Answer 50

For the formation of the “extra embryonic mesoderm” and the four “extraembryonic membranes” surround the embryo.

The invading trophoblast, cells from the epiblast and adjacent endometrial tissue will all contribute to formation of the placenta that will allow the exchange of nutrients with the embryo.

Question 51

What is the basic purpose of organogenesis?

Answer 51

To take the three germ layers formed through gastrulation and morph them into distinct organs.

Question 52

What is an example of organogenesis?

Answer 52

“Neurulation”, the first initiation of brain and spinal cord development.

Question 53

How do gastrulation and organogenesis differ in terms of how the changes arise?

Answer 53

In gastrulation the change is from the mass movements of cells. In organogenesis arrises through more local change.

Question 54

What happens during ’neuralation’?

Answer 54

Cells from the dorsal mesoderm (at the top, near the archenteron) develop into the ’notochord’ that will eventually support the spinal cord. The dorsal ectoderm thickened to form the neural plate.

The neural plate folds inwards. The points at which the plate folds are located at the boundary between the rural plate and the mesoderm and are called the “neural folds”

As the neural plate continues to fold inwards it pinches off to form a neural tube which will later develop into the brain and spinal cord of the embryo. As the neural tube separates from the ectoderm “neural crest cells” are released which migrate and giver rise to numerous structures.

To be continued in “What happens after neurulation?”

Question 55

What happens after neurulation?

Answer 55

Two sets of cells develop near the neural tube before migrating elsewhere. These sets of cells are the “neural crest” and the “somites”

The neural crest develops along the borders where the neural tube pinches off from. They then migrate to various to special tissues such as peripheral nerves and part of teeth and skull bones.

Somites form when groups of cells located in strips of mesoderm lateral to the notochord separate into blocks. These somites are located all the way down the notochord. Part of the somites dissociate as mesenchyme cells which migrate and form the individual vertebra of the spinal column. These mesenchyme also form the muscle of the vertebral column and the ribs.

The notochord largely disappears before birth but some remains as the inner portions of the vertebral disks.

The somites are arranged serially on both sides along the length of the notochord. Parts of the somites dissociate into mes- enchyme cells, which migrate individually to new locations. Somites play a major role in organizing the segmented structure of the vertebrate body. One of the major functions of the mesenchyme cells that leave the somites is formation of the vertebrae. Although the notochord disappears before birth, parts of the notochord persist as the inner portions of the vertebral disks in adults. Somite cells that become mesenchymal later form the muscles associated with the ver- tebral column and the ribs. Through these processes, serially repeating structures of the embryo (somites) form repeated structures in the adult. Chordates can thus be described as segmented animals, although the segmentation becomes less obvious later in development. Lateral to the somites, the mesoderm splits into two layers that form the lining of the body cavity, or coelom.

Lateral to the somites, the mesoderm splits into two layers that form the lining of the body cavity, or coelom.

Question 56

Are chordates like humans segmented animals i.e. like worms?

Answer 56

Technically yes because of the serially repeating somites.