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Summary Human and Animal Biology I
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HUMAN AND ANIMAL BIOLOGYE-L1 – Amphibian embryology
Sequence of events of embryonic development:
1. Fertilization.
An unfertilized frog egg has polarity:
- animal pole → upper part, contains mostly cytoplasm and the
cell nucleus. The animal pole is pigmented and contains a polar
body. The outer layer of the cytoplasm contains the most
pigment, so it is darker. The polar body is a small fragment of
cytoplasm containing nuclear material, that has been expelled
during the first reduction division (during ovulation)
- vegetal pole → at the bottom, contains the yolk granula
Fertilization leads to:
- completion of meiosis and fusion of paternal and maternal genomes
- the egg membrane becomes impenetrable to other spermatozoa (to avoid polyspermy)
- the concentration of calcium in the cytosol increases, leading to the activation of the egg cell
- when the sperm fertilizes the egg (at the
penetration point), the egg cortex (the outer layer)
rotates. This creates a lighter spot opposite to the
penetration point in the cortex: this spot is the grey
crescent.
The plane through the animal pole and the
penetration point divides the grey crescent in two
halves; this plane is the symmetry plane of the
future embryo and of the first cleavage.
The grey crescent is the future dorsal side of the embryo, so the opposite side is the ventral side
(where the penetration point is). Consequently, also the cranial and caudal side can be
determined.
2. Cleavage
At the animal pole, before the first cleavage there is a dark spot called polar body. The symmetry
plane of the fertilised egg is the plane of the first cleavage. The cells formed after the first cleavage
are called blastomeres. Blastomeres at the animal side divide faster and more often than vegetal
blastomeres, because they contain less yolk. Eventually, the animal blastomeres are smaller and
more numerous; they are smaller because they do not grow, so they are called micromeres. The
larger vegetal blastomeres are called macromeres.
The plane of the second cleavage is perpendicular to the first. Both cleavages
are called meridional cleavages. After 2 cleavages there are four cells (2
ventral and 2 dorsal).
The third cleavage is perpendicular to the first cleavage plane (equatorial). The
large amount of yolk in the vegetal side pushes the third cleavage plane
towards the animal pole, so the third cleavage occurs above the equatorial
plane; for this reason, we call it inequal cleavage. At this point, the embryo has
, 8 cells: 2 ventral and 2 dorsal micromeres (animal cells) and 2 ventral and 2 dorsal macromeres
(vegetal cells, rich in yolk).
NB: in cleavage divisions, cells do not grow like in regular divisions.
The lump of cells formed after cleavages is called morula. On the animal pole,
cells are small and pigmented; on the vegetal side, they are large and not
pigmented. At the animal side of the morula, the blastocoel is formed (a
cavity containing fluid), so the embryo is now called blastula.
The wall of the blastula is made of many layers of cells. The cap of micromeres spreads across the
macromeres, in a process called epiboly. This process is the last phase of the cleavage phase.
3. Gastrulation
It is characterised by the migration of the surface cells towards the inside. Three phases:
1. Epiboly: micromeres spread and cover the macromeres.
2. Involution: the influx of cells that will fill the blastocoel.
3. Convergence: many cells (at the end of gastrulation) move towards the dorsal meridian to form
the dorsal side of the embryo.
The migration of cells results in a gastrula with three layers of cells: ectoderm (external), mesoderm
(middle) and entoderm (inside). The future entoderm is at the vegetal side, the future mesoderm
rolls inwards (involution) to form the dorsal blastopore lip. A new cavity is formed called primary gut
(archenteron), which is connected to the outside world and becomes larger.
Through the invagination of cells, the blastopore is created. First, the dorsal blastopore lip is formed
in the dorsal part of the blastopore; then, the blastopore lip extends laterally. Finally, the blastopore
forms a circle where macromeres bulge out (yolk plug) and becomes smaller until it nearly closes:
here the anus will later form.
The archenteron curves and is sealed off from the
outside world. It is continuously provided with new
mesoderm, which then spreads between ectoderm and
entoderm. The entoderm surrounds the archenteron and
forms a tubular structure called provisional gut. Now the
three layers are clearly distinguished.
The germ layers then differentiate in tissues and organs
through the process of induction.
,4 Neurulation
It consists in the formation and differentiation of the neural ectoderm,
that will differentiate into the nervous system. (the embryo is called
neurula during this phase).
The central part of the mesoderm that forms the roof of the archenteron
will differentiate into chordamesoderm, from which the notochord will
develop.
The neural plate is created; the borders of the neural plate push upwards
and form the neural folds and the neural groove. From one side on the
neural groove, the brain will form; from the other side, the spinal cord
develops. When the neural folds connect, they form the neural tube.
From the outer part of the neural folds, two groups of cells differentiate,
called neural crest cells. From these cells, the viscerocranium and the
peripheral nervous system will form.
5 Organogenesis
Phase of formation of the organs through the differentiation of ectoderm, mesoderm and entoderm.
- Ectoderm: nervous system, skin.
- Mesoderm: blood vessels, skeleton, urogenital system.
- Entoderm: gut, liver, pancreas, lungs.
Ectoderm discussed in neurulation phase.
The mesoderm extends laterally to enclose the entoderm, and develops into
4 regions:
- Axial mesoderm: develops into the notochord.
- Early paraxial mesoderm: forms the somites, which will develop into muscles, spinal
cord and connective tissue.
- Intermediate mesoderm: develops into kidneys and urogenital system.
- Plate mesoderm (lateral and ventral): develops into gonads,
connective tissue and intestine muscles, blood cells, heart, blood
vessels.
The lateral plate mesoderm splits and forms a cavity called
coelom. It divides the lateral mesoderm in a somatic layer and a
visceral layer.
The walls of the entoderm grow towards each other along the inside of the archenteron and close,
forming the gut cavity. Hence, the entoderm develops into the gut, lungs, urine bladder (vescica),
and epithelia of liver and pancreas.
,LECTURE LZ2 – PHENOTYPIC PLASTICITY
Phenotypic plasticity = the morphological/physiological capacity of an animal to change its phenotype in
order to adapt/respond to environmental factors. It is often reversable and does not depend on the
ontogenic stage (the stage of development, e.g. embryo, juvenile, adult, = non dipende dallo stato di sviluppo
dell’animale). E.g. the arctic fox is white in winter and grey-brown in summer.
Developmental plasticity = the environmental conditions during ontogeny determine the size, shape,
behaviour of the mature organism. This means that external factors present
when the animal is still an embryo (in general, when it is still developing) can
have an effect on it when it is adult. E.g. butterflies of the same species can
look different in the dry season (right) and wet season (left).
Subcategory: polyphenism = when there are many different phenotypes within the same population. They
can be seasonally dependent. E.g. in blue wrasse fish, there is often one male in a colony and a lot of females;
when the male dies, a female will develop into a male.
Another example is caste polyphenism = e.g. in bees, a colony works as a single
organism because every bee has a purpose and also a different shape/size (e.g. the queen is
bigger and is the only one that can lay eggs).
Evolution vs. phenotypic plasticity = evolution is a gradual change throughout millions of years
(e.g. evolution of the horse, it lost its fingers), because the information in the genotype
changes.
In phenotypic plasticity, the phenotype changes but the genome remains the same. Phenotypic
plasticity can start evolution.
Homeostasis vs. phenotypic plasticity = homeostasis means keeping the internal physiology stable, e.g.
regulation of body temperature, blood pH to keep them to a certain value.
Two types of organisms:
- Osmoconformers = it follows the environment, e.g. when the outside
temperature increases, the body temp. of the animal also increases.
- Osmoregulators = it keeps its condition constant regardless of the
environment.
Homeostasis is different from phenotypic plasticity.
Salmons in the Netherlands
Salmons used to spawn (deporre uova) near the Alps. In 1953 there was a flood in the North Sea in the NL so
they built dams (dighe), but they prevent fish from coming into rivers from the sea. The dams are now being
opened to let fish enter in Dutch rivers, including salmons. Salmons need brackish water (salmastra), but they
lived in sea water for years because of the dams.
Life cycle of the salmon:
1. alevins (baby fish) hatch in fresh water, so their body is adapted to fresh water
2. juvenile salmons swim down rivers towards the sea
3. they then prepare for smolting, which is the migration in brackish water → their color changes and
their ionocytes adapt. Ionocytes are cells that help the fish to maintain the salt balance in their body.
, 4. Adults live in sea/ocean for 3-4 years, then they migrate up to the rivers and spawn in fresh water
before dying. They usually spawn in the same river where they were born, because they recognise it
from the odours.
Salmon are anadromous fish = they live most of their life in salt water and spawn in fresh water.
Major challenge for salmon is osmoregulation → osmolarity refers to the salt concentration in the body, so
osmoregulation is its regulation.
Almost all fish have ionocytes → they are also called mitochondrion-rich cells because they contain a lot
of mitochondria and produce a lot of energy. They are needed to transport sodium and chloride for
osmolarity.
The gills of fish are covered by an operculum. A gill bar made of cartilage
is attached to filaments and lamellae under the operculum. The
ionocytes are embedded in the membrane of cells in the filaments and
lamellae. The distribution of ionocytes depends on the species.
What triggers the adaptation of ionocytes? The salt concentration in
water is the main reason of adaptation of ionocytes. This is an
environmental factor that influences phenotype → phenotypic plasticity. An increased salinity increases the
production of hormones by fish; if you give cortisol hormone to the fish, you can make it believe of being in
salt water.
Major challenge when migrating into fresh water? The salt in the body gets out of it into the water, so the
fish has to prevent it. The hormone prolactin is very important for this. The Na+ uptake into the body
increases. The stem cells into the gills differentiate into fresh water ionocytes thanks to cortisol, while salt-
water ionocytes undergo apoptosis.
Challenge when migrating into salt water? The water inside the body flows out of it, so the fish must prevent
water loss. The growth hormone is very important for this. The Na+ is excreted from the body. Cortisol
triggers the differentiation of stem cells into salt-water ionocytes, while fresh-water ionocytes die.
Phenotypic plasticity of salmon → they lose their scales so change color. When they go back to fresh water,
they stop eating, the organs shrink and have lower immunity. The osteoblasts in males form skeletal needles
to form a hook (osteoblasts are cells in bone tissue), used to fight to mate with females.
Bird migrations: red knots
Their preferred food on the European coast is bivalves (molluschi). The bill tip organ on the beak can detect
where the bivalves are located.
They do not have a 360° vision, so this suggests that they are predators. During the
breeding season they also eat other “walking” preys. When they are feeding on
bivalves, their gizzard increases in mass of 10% (gizzard=stomach in birds with
grinding stones) because bivalves are hard to digest. When red knots feed on
arthropods, larvae, or spiders (soft preys) the size of the gizzard decreases of 60%
in 8.5 days. When they start feeding on hard preys again, the gizzard returns to the
normal size in 6 days.
Why migration? The main aim is to find optimal conditions to raise offspring, to
escape severe conditions (e.g. too many predators, food scarcity), to get rid of
,pathogens that would not survive in a new habitat, or to find rich feeding grounds. Birds always know when
and where to go when they migrate.
Hibernation
It means that an animal can sleep for very long periods of time to escape severe conditions.
Grizzly bears go into “torpor” or deep sleep, so they can wake up easily if they need to (it’s not proper
hibernation). The heart rate is lower, they don’t produce faeces or urine (no waste at all) and they store more
fat. Muscle cells also remain active.
Deep hibernators, e.g. the Syrian hamster, cannot wake up. The body temperature is lowered and they do
not need to store more fat like bears. The TRMP8, an ion channel in their neurons, is responsible for detection
of cold temperatures but it is less active during hibernation, so they are more tolerant to cold temperatures.
LECTURE L3 AND L4 – EMBRYONIC DEVELOPMENT IN AMPHIBIANS AND MAMMALS
Phylotypic stage = stage during embryonic development where all vertebrates look very similar. E.g. during
the phylotypic pharyngula stage all vertebrates have gill slits (even mammals).
Epigenesis = process by which an organism develops from the egg cell through a sequence of steps in which
cells differentiate and organs form.
Development = progressive changes in an individual from the beginning to maturity. It usually begins with a
fertilised egg (fig. 8.3) that divides mitotically. The cells then specialise to form the various body structures.
The process is not reversible, so cells that start to differentiate are said to be determined.
Developmental stages in vertebrates:
1. Zygote: fusion of egg and sperm cell.
2. Cleavages, with formation of a blastula with a blastocoel.
3. Gastrulation: formation of 3 germ layers and archenteron
4. Neurulation (early organogenesis) with formation of the neural crest.
5. Organogenesis
6. Growth: until adult stage.
Cleavage and early development in amphibians
During cleavage, the embryo divides until it is made of several small cells called blastomeres. During each
division, a cleavage furrow is visible. During this phase, cells do not grow.
Note that the egg cell has a polarity: an animal pole, containing mostly cytoplasm, and a vegetal pole
containing mostly yolk. The first cleavage occurs on the vertical axis going from the animal to the vegetal
pole. The second is perpendicular to it.
The amount of yolk at the vegetal pole varies among species:
- Isolecithal eggs = very little yolk, evenly distributed
- Mesolecithal eggs = moderate amount of yolk, concentrated at the vegetal pole
, - Telolecithal eggs = a lot of yolk, concentrated at the vegetal pole
- Centrolecithal eggs = large mass of yolk at the centre of the egg
When much yolk is present, the cleavage furrow has difficulty in forming.
Hence, isolecithal eggs undergo cleavage most easily because they contain
little yolk.
- Cleavage is meroblastic when the egg is not cleaved completely
because there is much yolk.
- Cleavage is holoblastic when the egg cleaves completely because
there is little yolk.
- Cleavage is superficial when the cleavage only occurs at the borders of the egg because the yolk is
at the centre of the egg. (fig. 8.8 and 8.9)
The function of yolk is to nourish the embryo:
- Direct development occurs when much yolk is present, so the embryo directly develops into a
miniature adult.
- Indirect development occurs when there is little yolk, so the embryo develops into several larval
stages before becoming an adult.
Blastulation = Cleavage divides the zygote until a cluster of cells called blastula is
formed. In most mammals, the blastula cells form a layer around a cavity full of fluid,
called blastocoel. (fig. 8.11)
Development following cleavage in amphibians (see pictures of E-L1)
Gastrulation and formation of two germ layers = This process converts the blastula in an embryo with 2 or
3 layers of cells. The layers are called germ layers, from which the body parts will develop.
To form the second germ layer, one side of the blastula bends inwards in a process called invagination. It
forms an internal cavity called gut cavity or archenteron or gastrocoel. The opening of the gut is called
blastopore. If the gut only has the opening at the blastopore, it is called an incomplete gut. Most animals
have a complete gut with a second opening, which will be the anus or the mouth. When the embryo has two
layers it is called gastrula. The outer layer is the ectoderm, the inner layer is the endoderm which lines the
gut. (fig 8.11)
Formation of mesoderm, the third layer = Animals with only 2 layers of germ cells are diploblastic; animals
with 3 layers are triploblastic. The mesoderm lies between the ectoderm and the mesoderm. It forms from
cells that proliferate in the space between the archenteron and the body wall (fig. 8.20C)
Formation of the coelom = The coelom is a body cavity surrounded by mesoderm. It lies in the space
previously occupied by the blastocoel. When the formation of the blastocoel is complete, the body has 3
germ layers and two cavities (the coelom and the gut cavity). (fig 8.11)
Neurulation = Above the notochord (structure similar to spinal cord in embryos, the
red line), the ectoderm forms the neural plate (stage 1). The edges of the plate rise
and join together to form the neural tube (stage 3). This tube will develop into brain,
cranial nerves, spinal cord and spinal motor nerves. A group of cells constituting the
neural crest separate from the neural tube and migrate. They will form the
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