Title: Feralis-Booster Chapter 4 - Developmental Biology
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Developmental Biology 1
> DAT Booster | Booster Prep TM
Gonads describe the reproductive structures
responsible for the production of gametes. In
males, the gonads are the testis, while in
females, the gonads are the ovaries. The
gonads are responsible for primary sex
characteristics, and are directly involved in
reproduction. Secondary sex characteristics
differ from primary characteristics in that they
indicate sexual maturity but are not involved in
reproduction, such as breast development or
increased body fat in females during puberty.
Female Reproductive System
The following structures are essential in the
female reproductive system:
1. Ovary - ova, or eggs, are produced here,
and each female has two ovaries
2. Oviduct (fallopian tube/uterine tube) -
allows for eggs to move from the ovary
to the uterus, with one oviduct for each
ovary (2). Fertilization occurs here!
i. The ovary isnt actually directly
connected to its adjacent oviduct, so
the egg is swept into the oviduct via
fi nger-like fi mbriae.
3. Uterus - muscular chamber where
development of the embryo occurs until
birth. A fertilized ovum implants
(attaches) on the inside uterine wall, or
endometrium, on day 6 after fertilization
# Developmental Biology
> Animal reproduction and development is a heavily tested area of biology on the DAT, and requires strong
> understanding of the concepts and processes listed in this chapter. Rest assured these notes cover everything
> youll need to know, and be sure to supplement these notes with the DAT Booster Biology question bank!
Asexual Reproduction 1
Before we delve into the intricate human
reproductive system, we must understand how
reproduction occurs in non-animal organisms:
asexual reproduction. Organisms that rely on
asexual reproduction produce offspring that are
clones, while sexual reproduction produces
offspring that are genetically different from the
parents. The following terms are important to
understand:
1. Fission - separation of an organism into
two new cells (amoeba)
2. Budding - occurs when a new individual
grows from an existing one and then
splits off (hydra)
3. Fragmentation + Regeneration - when a
single parent breaks into parts that
regenerate into new individuals (sponge/
planaria/star fi sh)
4. Parthenogenesis - process in which egg
develops without fertilization, resulting in
an adult that is either haploid or diploid
(honeybees, wasps, ants, some lizards,
and hammerhead sharks)
Human Reproductive Anatomy 2
DAT Pro-Tip: Testosterone results in the secondary
sex characteristics in men, but also closes the
epiphyses of long bones
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> CC BY 4.0
Developmental Biology 2
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4. Vagina - at birth, the fetus passes
through the cervix, an opening in the
uterus, and out of the body
Male Reproductive System
The male reproductive system contains more
structures than the female system:
1. Testis - each consists of seminiferous
tubules for production of sperm, and
interstitial cells (Leydig cells) that
produce male sex hormones
testosterone and androgens at the
beginning of puberty. These hormones
are secreted in the presence of LH.
i. Sertoli cells - are stimulated by FSH
and serve to surround and nurture
sperm, as well as secrete peptide
hormone inhibin, which acts on the
anterior pituitary to inhibit FSH
release
ii. Scrotum - testis are located here;
provides an external cavity, about
2oC lower than the body
temperature, for sperm production.
2. Epididymis - a coiled tube attached to
each testis that serves as the site for
fi nal maturation and storage of sperm
3. Vas deferens - transfers sperm from
one epididymis to the urethra
4. Seminal vesicles - two glands that, during
ejaculation, secrete into vas deferens and
provide mucus (liquid for sperm), fructose
(energy for sperm), and prostaglandins
(stimulate uterine contractions that help
sperm move into uterus).
5. Prostate gland - secretes milky alkaline
fl uid into urethra and neutralizes acidity of
urine that may still be in the urethra, as
well as acidity of the vagina. This gland
also neutralizes seminal fl uid, which is too
acidic from metabolic waste of sperm
6. Bulbourethral (Cowpers) glands -
secrete small amount of thick mucus of
unknown function into urethra
7. Penis - transports semen ( fl uid containing
sperm and secretions) into the vagina
8. Sperm - are compact packages of DNA
specialized for effective male genome
delivery. Contains the following three
segments:
i. Sperm head - haploid (23
chromosomes); at tip is acrosome, a
lysosome-like organelle containing
enzymes (hyaluronidase), which are
used to penetrate the egg. The
acrosome originates from Golgi body
vesicles that fuse together, and only
the nuclear portion of the sperm
enters the egg
ii. Mid-piece - fl agellum (9+2
microtubule array) arising from one
member of centriole pair, and contains
lots of mitochondria
iii. Tail - remainder of fl agellum; sperm is
propelled by whiplike motion of tail
and mid-piece
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Developmental Biology 3
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Gametogenesis in Humans
Gametogenesis describes the meiotic cell
divisions that produce eggs (oogenesis) and
sperm (spermatogenesis). The egg contains
most of the cytoplasm, RNA, organelles, and
nutrients needed by the developing embryo,
which explains why the egg is much larger in size
than the sperm. The sperm contributes very little
cytoplasm during fertilization. Lets go through
the processes of oogenesis and
spermatogenesis:
1. Oogenesis - begins during the female
embryonic development (before birth).
Oogonia (fetal cells) undergo mitosis and
become primary oocytes. These primary
oocytes begin meiosis, but remain in
prophase I until puberty. During puberty,
one primary oocyte is selected and
stimulated via FSH to continue its
development through meiosis I during
the ~28 day menstrual cycle.
i. This primary oocyte continues its
development within a follicle, which is
formed via encircling cells that
protect and nourish the oocyte.
ii. Within the follicle, the oocyte
completes meiosis I and becomes
ii. The secondary oocyte, which
consists mostly of cytoplasm. The
secondary oocyte also contains a
polar body, which has much
smaller cytoplasm content, and
may or may not divide, but its
products disintegrate.
iii. The secondary oocyte remains
arrested in metaphase II until
ovulation occurs.
2. Ovulation - releases the secondary
oocyte from a vesicular follicle, which
is stimulated by an LH surge. If this
oocyte is fertilized by a sperm, the
oocyte completes meiosis II, and the
resulting ovum/egg becomes diploid.
The polar body degenerates.
i. At puberty, FSH stimulates the
growth of granulosa cells around
the primary oocyte, which release
a viscous substance that forms the
zone pellucida, a jelly like layer
around the egg. The structure at
this stage is still a primary follicle.
> CC BY 4.0
> DAT Pro-Tip: Use the mnemonic SEVEn UP to
> memorize the path of sperm: Seminiferous tubules >
> Epididymis > Vas deferens > Ejaculatory duct >
> Urethra > Penis (pause before the V for maturation!)
> CC BY 4.0
Developmental Biology 4
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ii. Next, theca cells differentiate from
the interstitial tissue and grow
around the follicle to form a
secondary follicle. Upon stimulation,
by LH, theca cells secrete androgen,
which is converted to estradiol (a
type of estrogen) by the granulosa
cells in the presence of FSH and is
secreted into the blood.
iii. Typically, estradiol inhibits LH
secretion by the anterior pituitary,
but just before ovulation, estradiol
levels rise rapidly, causing a
dramatic increase in LH secretion.
Spermatogenesis begins at puberty within the
seminiferous tubules of testes. This differs from
oogenesis, which begins during fetal
development.
1. Spermatogonia cells - undergo mitosis
and become primary spermatocytes.
These primary spermatocytes undergo
meiosis I and form two secondary
spermatocytes, which undergo meiosis
II to become four spermatids.
2. Sertoli cells - are found in seminiferous
tubules, provide nourishment, and
nurse spermatids as they differentiate
into mature spermatozoa (sperm). The
sperm complete maturation (gain
motility and are stored) in the
epididymis.
3. Semen - the combination of
spermatozoa and fl uids that leave the
penis upon ejaculation.
4. Capacitation - the penultimate step in the
maturation of the spermatozoa while in
the vagina that allows for egg
penetration. This is the fi nal maturation of
spermatozoa.
i. During capacitation, physiological
changes occur to the sperm, including
changes in intracellular ion
concentration, motility, and
metabolism.
> DAT Pro-Tip: A common misconception regarding a
> developing human ovum is that the route from ovary
> to uterus is completely enclosed. After ovulation, a
> secondary oocyte is briefly located in the body cavity
> prior to entering the oviduct.
> CC BY 4.0
> DAT Pro-Tip: eggs release progesterone which
> aids in sperm motility and increases the
> likelihood of fertilization. Also, spermatogenesis
> is a continuous process, while oogenesis is
> discontinuous.
Developmental Biology 5
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Hormonal Control of Human Reproduction
Lets cover the different hormonal cycles that
control reproduction!
1. Female reproductive cycle - consists of
ovarian cycle (in the ovary) and the
menstrual cycle (involves uterus)
i. Menstrual cycle - is divided into the
proliferative, secretory, and
menstruation (menstrual fl ow)
phases. Menstruation begins with
the disintegration of the
endometrium (menstrual fl ow phase).
a. Hypothalamus and anterior
pituitary initiate - monitor
estrogen and progesterone in
blood. Low levels of estrogen
and progesterone stimulate
hypothalamus secretes GnRH
stimulates anterior pituitary to
release FSH and LH
b. Follicle develops - FSH
stimulates follicle to secrete
estrogen lots of estrogen
(positive feedback on
hypothalamus) produces
GnRH anterior pituitary
produces sudden mid cycle surge
of LH
c. LH surge - triggers ovulation
(follicle is now the corpus luteum,
which develops and is
maintained by LH, which, along
with estrogen, begins to
decrease after ovulation),
secretes estrogen and
progesterone, which stimulate
d. Development of endometrium -
thickens in preparation for
implantation of fertilized egg. If
no implantation occurs, then
negative feedback on anterior
pituitary from increased estrogen
and progesterone terminates
production of FSH and LH, due to
low GnRH from hypothalamus
e. Corpus luteum disintegrates (no
longer maintained by LH) -
becomes corpus albicans; no
estrogen and progesterone
results in the endometrium shed
during the menstruation fl ow
phase!
If implantation occurs, then the embryo, and
later the placenta, secretes chorionic
gonadotropin (hCG), which maintains the corpus
luteum.
> CC BY 4.0
Developmental Biology 6
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The production of estrogen and progesterone
remains high, so the endometrium is not shed.
The placenta still continues to make hCG
throughout pregnancy, just at lower levels
after the fi rst trimester. Without hCG,
menstruation would begin, and the embryo
would abort, as hCG maintains pregnancy.
This is why pregnancy tests check the
presence of hCG in urine to deduce if a female
is pregnant or not. If a fertilized egg implants
anywhere other than the endometrium of the
uterus, it is considered an ectopic pregnancy,
and usually spontaneously aborts.
ii. Ovarian cycle -
a. Follicular phase - development
of egg and secretion of
estrogen from follicle [ends at
ovulation]
b. Ovulation - mid-cycle release of
egg
c. Luteal phase - secretion of
estrogen and progesterone
from corpus luteum after
ovulation [shedding of the
uterine lining lasting
approximately 5 days]
a. Note that the secretory
phase of the menstrual cycle
overlaps with the luteal
phase of the ovarian cycle
Remember, estrogen serves to thicken the
endometrium, while progesterone serves to
develop and maintain the endometrial wall.
Progesterone also inhibits lactation during
pregnancy. The fall in progesterone after
delivery allows for milk production.
2. Male reproductive cycle -
i. GnRH FSH + LH (also known as
ICSH, interstitial cell stimulating
hormone testosterone and
androgens from testes)
ii. FSH and testosterone in fl uence
Sertoli cells to promote development
of sperm (nourish sperm during
development, or spermatogenesis).
Hormone and gamete production are
constant unlike in females.
iii. LH stimulates Leydig cells (in the
interstitium between seminiferous
tubules) to release testosterone +
androgens that promote
spermatogenesis in tubules. Sertoli
cells secrete inhibin that acts on the
anterior pituitary to inhibit FSH
secretion.
3. Contraceptive methods -
i. Female hormonal contraception -
estradiol and/or progesterone are
spiked arti fi cially high negative
feedback suppresses LH/FSH surge
no ovulation can occur no
fertilization possible
ii. Male contraception - a pill would
interfere with LH and FSH to
decrease sperm production
Embryonic Development
Animal embryos follow four stages in growth
and development: gametogenesis (sperm/egg
formation), embryonic development (fertilization
of egg until birth), reproductive maturity
(puberty), and aging process to death. In
mammals, development occurs in two stages:
embryonic development followed by fetal
development.
> DAT Pro-Tip: The follicle that releases the secondary
> oocyte is also called the Graafian follicle
> CC BY 4.0
Developmental Biology 7
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A fetus is an embryo that resembles the human
infant form, and in humans, an embryo is called
a fetus at about 8 weeks.
1. Stages of embryonic development
(sea urchin, echinoderm) -
i. Fertilization - sperm penetrates
plasma membrane of secondary
oocyte
a. Recognition - before
penetrating, the sperm secretes
proteins that bind with
receptors that reside on a
glycoprotein layer surrounding
the plasma membrane of the
oocyte. In non-mammals, this
layer is called the vitelline
membrane. In mammals, this
layer is the zona pellucida. In
both organisms, the layer
ensures same species
fertilization.
2. Zona pellucida - external glycoprotein
membrane surrounding the plasma
membrane (jelly coat) of an oocyte. This
fi rst appears in unilaminar oocytes, and
is secreted by both the oocyte and
follicular cells. At puberty, FSH
stimulates growth of granulosa cells
around the primary oocyte that secrete
the viscous zona pellucida.
i. When the zona pellucida binds
sperm, the acrosome reaction is
initiated.
ii. The sperm releases the contents of
its acrosome as it approaches the
egg, and contributes to a charge-
based fast block of polyspermy.
iii. 5 days after fertilization, the
blastocyst undergoes zona hatching
(zona pellucida degenerates and is
replaced by the underlying later of
trophoblastic cells so it can implant
in the uterus)
Fertilization cannot occur until capacitation and
acrosomal reaction have taken place. In
capacitation, secretions from the uterus wall and
uterine tube destabilize the plasma membrane
surrounding the head of the sperm (acrosome),
making the head more fl uid, which helps prepare
it for fertilization and makes the sperm
hyperactive (faster and wiggle more).
The capacitated sperm moves through the
corona radiata (dense layer of granulosa cells
surrounding the oocyte) and comes into contact
with the zona pellucida. The zona pellucida
expresses speci fi c receptor proteins called ZP3,
which bind to proteins expressed in the head of
the sperm. The binding of ZP3 triggers the
acrosome reaction, during which the enzymatic
contents of the acrosome are released.
These enzymes help digest a path through the
zona pellucida, allowing the sperm to enter the
perivitelline space (space between the plasma
membrane of the secondary oocyte and the
zona pellucida), which then fuses with the
oocytes plasma membrane.
To ensure only one sperm penetrates the zona
pellucida and fuses with the oocyte membrane,
this fusion activates a fast block and a slow block
to polyspermy. First, during the fast block, which
takes place after fusion, the oocyte membrane
depolarizes, preventing other sperm from fusing
with it. Slow block to polyspermy is then
stimulated by this depolarization during slow
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Developmental Biology 8
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block to polyspermy, a wave of intracellular
calcium is released, causing small cortical
granules beneath the oocyte membrane to
release their contents outward, rendering ZP3
in the zona pellucida inactivate and making it
impermeable. Note that intracellular Ca 2+
triggers cortical granule release, but the
granules themselves dont contain Ca 2+ .
In non-mammals, the zona pellucida is called
vitelline membrane, and it plays an important
role in preventing cross-breeding of different
species, especially in species where fertilization
occurs outside of the body. The vitelline
membrane is also commonly used to control
wildlife population via immunocontraception.
When the vitelline membrane of one animal
species is injected into the bloodstream of
another, sterility of the second animal occurs
due to an immune response. Fertilization
cannot occur because antibodies have already
bound to the vitelline membrane, thus
preventing sperm from binding.
After capacitation and the acrosome reaction,
the following steps occur:
2. Penetration - plasma membrane of
sperm and oocyte fuse, and the sperm
nucleus enters the oocyte
3. Formation of fertilization membrane -
the vitelline layer forms a fertilization
membrane that blocks additional sperm
(remember, this is due to the cortical
reaction, which is the exocytosis of
enzymes produced by cortical granules in
egg cytoplasm during fertilization
called slow block in mammals)
4. Completion of meiosis II in secondary
oocyte - sperm penetration triggers
meiosis II to complete. The oocyte was
previously arrested in metaphase in
humans. The result is an ovum and a
second polar body, which is discharged
through the plasma membrane
5. Fusion of nuclei and replication of DNA
- sperm and ovum nuclei fuse diploid
zygote forms. This is associated with a
sharp increase in protein synthesis and
metabolic activity.
Cleavage
The cleavage stage involves rapid cell division of
the zygote without cell growth. Each cell is a
blastomere, which has less cytoplasm than the
original zygote. The transition from fertilization
to cleavage is caused by mitosis promoting
factor, which is now being transcribed.
> CC BY 4.0
> DAT Pro-Tip: Fertilization can be external in water
> (lots of eggs laid since the chance of fertilization is
> lower - frogs/amphibians) or internal (terrestrial
> vertebrates). The # of eggs laid is affected by the
> following factors: internal vs external fertilization,
> early development, and amount of parental care (less
> care = more eggs)
> DAT Pro-Tip: Fertilization takes place in the
> oviduct (fallopian tubes); cleavage while swept;
> and the embryo is at the blastula stage by the
> time it reaches the uterus for implantation
Developmental Biology 9
> DAT Booster | Booster Prep TM
1. Embryo polarity - an egg has
an upper, animal pole and lower, vegetal
pole. Depending on the species, the
vegetal pole can contain more yolk
material, which is denser than the
cytoplasm and settles at the bottom. In
general, the vegetal pole differentiates
into extra-embryonic membranes that
protect and nourish the embryo. Polarity
is critical in setting up body axes.
i. Note the grey crescent in frogs, which
is a region of non-pigmented
cytoplasm formed at the opposite
side of sperm entry. The grey
crescent is a marker of the future
dorsal side that is always bisected by
the fi rst cleavage plane.
2. Polar and equatorial cleavages - early
cleavages are polar, and divide the egg
into segments that stretch from pole to
pole, like the segments of an orange.
Others are parallel with the equator.
Note that in frogs, the horizontal
cleavage is closer to the animal pole.
3. Radial and spiral cleavages - radial
cleavage occurs in deuterostomes. Radial
cleavage forms indeterminate cells at
animal and vegetal poles that are aligned
together, with top cells directly above
bottom cells. In protostomes, spiral
cleavage occurs, and determinate cells
are formed on top and are shifted
relative to those below.
i. Recall that the fi rst opening in
protostomes forms the mouth, while
the fi rst opening in deuterostomes
forms the anus.
4. Indeterminate and determinate
cleavages - in indeterminate cleavage,
blastomeres can individually complete
normal development if separated. In
determinate cleavage, blastomeres
cannot develop into a complete embryo
if separated; each is differentiated into
part of the embryo. Thus, if blastomeres
of determinate cleavages are separated
from each other, they will die/arrest.
5. Morula - successive cleavage results in a
solid ball of ~8 cells, where the fi rst 8
cells are totipotent, meaning the cells are
capable of giving rise to any cell type or
embryo
6. Blastula - cell division continues, and
liquid fi lls the morula and pushes cells
outward to form a circular cavity
surrounded by a single layer of cells. The
blastocoel is the fl uid fi lled cavity.
> CC BY 4.0
> CC BY 4.0
> DAT Pro-Tip: There are two cleavage types:
> holoblastic and meroblastic. In holoblastic, cleavage
> is complete (passes all the way through the zygote),
> and occurs in most vertebrate ancestors and
> descendants. In meroblastic, cleavage is only partial,
> and occurs in birds, reptiles, fish, monotremes, and
> mollusks.
Developmental Biology 10
> DAT Booster | Booster Prep TM
i. Note that there are ~128 cells at the
blastula stage
ii. In humans, the blastula is called the
blastocyst when it implants into the
endometrium, and this blastocyst
contains an inner cell mass. Frogs
and sea urchins have similar blastula
to one another, but frogs have a
built up vegetal hemisphere
iii. Note that all embryos have an
animal and vegetal pole, but
amphibians, reptiles, fi sh, and birds
are distinct in that their yolk, and
therefore nutrients, is not evenly
distributed.
iv. Blastula cells are pluripotent, and
can develop into any cell type but
not a complete organism like morula
cells can
7. Gastrula - the formation of
the gastrula occurs with the invagination
of a group of cells into the blastula,
forming a two-layered embryo with an
opening from the outside into a center
cavity. The gastrula forms ~14 days post
fertilization. Actin fi lament contractions
changing the shape of migrating cells
causes the invagination of the blastula
tissue. Three features associated with
the gastrula are as follows:
i. Three germ layers - ectoderm,
mesoderm, and endoderm. The third
layer is formed between the outer
and inner layer of the invaginated
embryo. These germ layers give rise
to all subsequent tissues.
a. Ectoderm - gives rise to the
following:
Nervous system (brain and
spinal cord)
Integument (epidermis, hair,
epithelium of nose, mouth,
and anal canal)
Sensory structures (lens of
eye, retina)
Neural tube
Via neural crest cells, gives rise
to teeth, jaws, and bones of
face and skull
Adrenal medulla
b. Mesoderm - gives rise to the
following:
Musculoskeletal system
Circulatory/lymphatic system
Excretory system
Gonads
Connective tissue
Portions of digestive and
respiratory system
Notochord
Somites
Kidney
Dermis of skin
Adrenal cortex
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Developmental Biology 11
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c. Endoderm - gives rise to the
following:
Epithelial lining of digestive
and respiratory tract
Parts of liver
Pancreas
Gall bladder
Thyroid and parathyroid
Thymus
Urinary bladder lining
ii. Archenteron - the center cavity
formed by gastrulation that is
completely surrounded by
endoderm cells and gives rise to the
gut
iii. Blastopore - opening into the
archenteron, becomes the mouth in
protostomes or the anus in
deuterostomes
Note that cells undergo morphogenesis as
they differentiate.
8. Extra-embryonic membrane
development - in birds, reptiles, and
humans (collectively, amniotes),
membranes develop outside of the
embryo proper. Note that amphibians
are not amniotes.
i. Chorion - outer membrane
a. Birds and reptiles - functions
as a membrane for gas
exchange
b. Mammals - chorion implants
into endometrium, and later,
the chorion and maternal tissue
(which is modi fi ed endometrial
tissue called deciduas basalis)
form the placenta. The placenta
is a blend of maternal and
embryonic tissue across which
gases, nutrients, and wastes are
exchanged.
ii. Allantois - sac that buds off from
archenteron that eventually
encircles the embryo, forming
below the chorion.
> CC BY 4.0
DAT Pro-Tip: Some primitive animals like sponges
and cnidaria will develop mesoglea, a noncellular
layer, instead of the mesoderm
> CC BY 4.0
> CC BY 4.0
Developmental Biology 12
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a. Birds and reptiles - initially
stores waste products as uric
acid, and later fuses with the
chorion to form a membrane for
gas exchange with blood vessels
beneath it
b. Mammals - allantois functions to
transport waste products to
placenta, and eventually forms
the umbilical cord between the
embryo and placenta. The
umbilical cord transports gases,
nutrients, and wastes. In adults,
the allantois becomes the
urinary bladder. The allantois is
only found in reptiles, birds, and
mammals.
iii. Amnion - encloses the
amniotic cavity, and is a fl uid-fi lled
cavity that cushions the developing
embryo, much like the coelom that
cushions internal organs in
coelomates. Amphibians do not
have an amnion.
iv. Yolk sac -
a. Birds and reptiles - digests
enclosed yolk, and blood vessels
transfer nutrients to embryo
b. Mammals - is empty and
contains no yolk as the umbilical
cord and placenta deliver
nutrients instead. Major function
involves aiding in the formation
of developing RBCs.
v. Organogenesis - cells
continue to divide after gastrulation
differentiate into speci fi c tissues
and organs. Once gastrulation is
complete, evidence of cell
differentiation can be observed. In
chordates:
a. Notochord - cells along the
dorsal surface of the mesoderm
layer form the notochord, a stiff
cartilaginous rod that provides
support in lower chordates.
Vertebrae of higher chordates are
formed from nearby cells in
mesoderm.
b. Neural tube - in ectoderm layer
directly above the notochord, a
layer of cells forms the neural
plate. When the plate indents, the
neural groove forms, which rolls
up into a cylinder forming the
neural tube. The neural tube
develops into the CNS, and
additional cells roll off the top of
the neural tube and form neural
crest cells, which form teeth,
bones, muscles of skull, pigment
cells in skin, and nerve tissue.
> CC BY 4.0
> CC BY 4.0
Developmental Biology 13
> DAT Booster | Booster Prep TM
donor to a recipient embryo at a
location corresponding to its
future belly. The embryo
developed two notochords - a
normal dorsal one, and a second
one along the belly that could
give rise to a second embryo,
formed partly from recipient
tissue! This showed that the cells
which ordinarily form the belly
could be induced to develop
structures different from their
ordinary fate.
2. Bird -
i. Blastodisc - fl attened, disc shaped
region that sits on top of the yolk;
cleavage occurs here. The yolk of a
bird egg is very large and is not
involved in cleavages.
ii. Primitive streak - (also found in
mammalian embryos) when
gastrulation begins, invagination
occurs along a line called the
primitive streak. As cells migrate into
here, the result is an elongated
blastopore rather than a circular one
as in sea urchins and frogs.
3. Humans and most other mammals -
i. Blastocyst - the blastula stage
consists of two parts: an outer ring of
cells (trophoblast) and inner cell mass
(embryonic disc)
a. The inner cell mass forms the
epiblast and hypoblast
Notable Exceptions to General Embryonic
Patterns
The following are exceptions that may show up
on your DAT
1. Frog - note that this information is largely
redundant if youve thoroughly learned
frog gastrulation earlier in this chapter
i. Gray crescent - sperm penetrates
frog egg reorganization of
cytoplasm pigmented cap of
animal pole rotates towards the point
of penetration while a gray crescent
shape region forms opposite of the
point of penetration.
a. The gray crescent experiment -
in early cleavage, each individual
cell could develop into a frog only
if it had some portion of the gray
crescent
ii. Gastrulation - blastopore forms at
the border between the gray crescent
and the vegetal pole. During
gastrulation, cells migrate over the
top edge of, and into, the blastopore
through a process called involution,
forming the dorsal lip in the same
region previously occupied by the
gray crescent. The blastocoel
disappears and is replaced by the
archenteron. The bottom edge of the
blastopore ventral lip, side
lateral lip
iii. Yolk - more extensive than sea urchin;
cells from the vegetal pole rich in yolk
material form a yolk plug near the
dorsal lip
a. Note that the dorsal lip is
important experimentally.
Normally, these cells give rise to
the notochord. In the dorsal lip
transplanted experiment, dorsal
lip cells were translated from a
> CC BY 4.0
Developmental Biology 14
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The epiblast gives rise to the
endoderm, ectoderm, and
mesoderm. The trophoblast
consists of a double layer of cells,
and is the precursor of the
placenta.
ii. Trophoblast - accomplishes
implantation by embedding into the
endometrium.
a. Produces human chorionic
gonadotropin (hCG) to maintain
estrogen and progesterone
production from the corpus
luteum, which maintains the
endometrium
b. The trophoblast later forms the
chorion, which eventually forms
the placenta by fusing with the
endometrium. By the end of the
fi rst trimester, the placenta
reaches full development and
secretes its own estrogen and
progesterone so hCG lowers.
iii. Embryonic disc - within the cavity
created by the trophoblast, the inner
cell mass clusters at one pole and
fl attens into the embryonic disc,
analogous to the blastodisc of birds
and reptiles.
i. The primitive streak develops
gastrulation development of
embryo + extra-embryonic
membranes (except the chorion)
ii. Basically, aquatic vertebrates
form the blastopore, and
terrestrial vertebrates form the
primitive streak!
Factors that In fl uence Development
The following four factors are de fi nite must-
knows for the DAT!
1. In fl uence of egg cytoplasm -
cytoplasmic material is distributed
unequally in the egg (think gray crescent
in frogs and yolk in bird eggs), which
results in embryonic axes, such as animal
and vegetal poles. When cleavages
divide the egg, daughter cells have
different quality of cytoplasmic
substances, or cytoplasmic determinants.
These determinants are unique
substances that in fl uence subsequent
development of each daughter cell.
2. Embryonic induction - in fl uence of one
cell/group of cells over neighboring cells.
Organizers (controller cells) secrete
chemicals that diffuse among
neighboring cells, and in fl uence their
development. The dorsal lip of the
blastopore, functioning as a primary
organizer, induces notochord
development in nearby cells.
i. Example within human
development - the lens of the eye is
formed from the ectoderm of the
head. The optic vesicle (part of the
brain) touching a portion of the
ectoderm of the head induces lens
formation.
> DAT Pro-Tip: Contrary to what some illustrations
> may imply, the hypoblast actually only gives rise to
> the extra-embryonic endoderm (umbilical vesicle/
> allantois/yolk sac). The definitive (intra-embryonic)
> endoderm is actually formed from invagination of the
> epiblasts displacing the hypoblast. Note that the
> chorion develops from the trophoblast + extra
> embryonic mesoderm layers
> CC BY 4.0
Developmental Biology 15
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3. Homeotic (Hox) genes - control
development by turning on/off other
genes that code for substances that
directly affect development of body
segments. An experiment in fruit fl ies
found that mutant homeotic genes
resulted in wrong body parts in wrong
places.
i. Homeobox (unique DNA segments
of 180 nucleotides) - identi fi es a
particular class of genes that control
development (encodes
homeodomain of protein that can
bind DNA). The homeobox
sequence is highly preserved across
species.
ii. Embryonic lethals - mutations that
affect a process as fundamental as
segmentation, and cause death at
embryo/larval stage
4. Apoptosis - programmed cell death that
is part of normal cell development.
Apoptosis is essential for development
of the nervous system, operation of
immune system, and destruction of
tissue (webbing) between fi ngers and
toes.
i. Damaged cells also undergo
apoptosis; if not, cancer may
develop. The process is regulated by
protein activity, rather than at the
transcriptional/translational level.
Apoptosis proteins are present
but inactive in normal cells.
ii. Characteristics of apoptosis - changes
include blebbing, cell shrinkage,
nuclear fragmentation, chromatin
condensation, and chromosomal
DNA fragmentation. There is no
cellular rupturing, and no
in fl ammatory response
A cell is said to be determined if its fi nal form
cannot be changed. Cytoplasmic in fl uences
become diminished with each successive cell
division, and the cells become determined later
rather than sooner. Cells can be traced during
development to build a lineage map.
Cell differentiation is the process by which cells
become specialized in structure and function.
Recent research has shown that even fully
differentiated cells can be altered under proper
conditions. These are referred to as induced
pluripotent stem cells. Induction occurs when
one cell type affects the direction of
differentiation of another cell type. In this case,
cytoplasmic determinants still play a role. In the
example of a frog, it takes just four days for cell
division, differentiation, and morphogenesis,
which describes the shaping of an organism, to
transform a fertilized frog egg into a tadpole.
Morphogenesis can be traced back to changes
in shape, motility, and other characteristics of
the cells that make up various parts of the
embryo.
Extra Information About Human Reproduction
Here are some additional concepts that can arise
on your DAT!
1. Labor (three stages) - a series of strong
uterine contractions
> CC BY 4.0
> DAT Pro-Tip: In mammals, mitochondria play an
> important role in apoptosis, which typically affects
> single cells.
Developmental Biology 16
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i. Cervix thins out and dilates,
amniotic sac ruptures and releases
fl uids
ii. Rapid contractions followed by
birth
iii. Uterus contracts and expels
umbilical cord and placenta
2. Twins -
i. Fraternal/dizygotic twins - two
separate eggs are fertilized by two
different sperm, so the eggs are
not genetically identical, and are no
more related than ordinary siblings
ii. Identical/monozygotic twins -
result from indeterminate cleavage;
a single fertilized egg splits into
two, forming two genetically
identical offspring
This section is complex, and can be dif fi cult to
understand at fi rst. I personally fi nd
visualization helpful in general, but especially
so with embryology. The DATBooster
Developmental Biology videos explain early
human development, and are very helpful in
understanding how each event takes place,
and is connected to other events.
Because this is a common area of confusion,
remember that the notochord is derived from
the mesoderm. The mesoderm tissue initially
forms a notochordal process that fuses with
the underlying endoderm to form the
notochordal plate, and this plate is what rolls
upward to form the notochord. The developing
notochord induces neural tube formation. On
either side of the neural tube, blocks of tissue
called somites form that go on to produce
vertebrae of the backbone and muscles of the
axial skeleton. The somites are also derived
from the mesoderm.
3. Stem cell types -
i. Totipotent stem cells - can give rise
to any and all human cells, and even
an entire functional organism (morula
stage)
ii. Pluripotent - can give rise to all tissue
types, but not an entire organism
(blastula stage)
iii. Multipotent - can give rise to limited
range of cells within a tissue type
iv. Unipotent - just one single cell type
> CC BY 4.0
> DAT Pro-Tip: Embryonic stem cells are pluripotent
> (isolated from blastula/blastocyst), and have more
> therapeutic value. Adult stem cells (and umbilical
> cord blood cells) are multipotent. Induced
> pluripotent stem cells are formed by
> deprogramming a fully differentiated cell
Developmental Biology 17
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Differences in Development
Lets do a quick overview of how different
organisms develop:
1. External development - fi sh and
amphibians have external fertilization
(ovuliparity) in water, to prevent
gametes from drying out, and to allow
sperm to swim to the egg. This requires
cooperative mating behaviors to ensure
simultaneous egg and sperm release.
i. There is no copulation in external
fertilization!
ii. Ovuliparity - external fertilization
2. Internal development - reptiles, birds,
and some mammals (monotremes) have
internal fertilization, which requires
cooperative behavior leading to
copulation. These organisms then lay
eggs (oviparous; if the egg hatches to
live young internally or immediately
after release, ovoviviparous). There are
no placenta or tropic interactions
between the zygote and parent.
i. Viviparity - birth to live young that
was given nutrients during
development
ii. Oviparity - egg is laid and hatches
later
iii. Ovoviviparous - eggs are internal
and birthed as live young, but the
egg is not nourished in any way by
the parent
3. Non-placental internal development -
certain animals like marsupials and
tropical fi sh spend a short time in the
uterus as embryos, then crawl out and
complete development attached to a
mammary gland in the mothers pouch!
As the name suggests, there is no
placenta, so there is limited exchange
of food and oxygen between mother
and young
4. Placental internal development - major
components of this development in
humans include the umbilical cord and
placenta system. The oxygen is received
directly from the mother (as fetal lungs
are not functional until birth), as well as
nutrients. CO 2 and metabolic wastes are
removed. The placenta and umbilical
cord form from outgrowths of amnion,
chorion, allantois, and the yolk sac.
Amnion contains amniotic fl uid as a shock
absorber.
i. Placenta formation - begins with
chorion, and blood vessels of allantois
wall enlarge and become umbilical
vessels that connect the fetus with
the developing placenta. The yolk sac
becomes associated with umbilical
vessels.
5. Amniotes - group of tetrapods, four-
limbed animals with backbones or spinal
columns that have terrestrially adapted
eggs that are supported by several extra
embryonic membranes.
i. Does not include amphibians or frogs
6. Monotremes - mammals that lay leathery
eggs, lack nipples, and are endothermic
(but have an unusually low body
temperature and metabolic rate
compared to other mammals)
i. Includes platypuses and echidnas
> DAT Pro-Tip: Internal fertilization is typically
> associated with production of fewer games than
> external fertilization, but a higher % of survival of
> zygotes. This is because zygotes are sheltered from
> predators and have greater protection and care
> from mother. This is true even in oviparous animals
> (reptiles and birds lay eggs with shells and internal
> membranes that fish and amphibians lack).
Developmental Biology 18
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Placental Circulation
As weve covered, gas and nutrient exchange
between fetal and maternal blood occurs
across the villi that extend from the chorionic
membrane surrounding the fetus. Oxygen
depleted fetal blood travels to the villi through
the fetal arteries, and returns as oxygenated
through the fetal veins. Blood fl owing from the
mother fi lls in pools that surround the chorionic
villi, and allow the passage of nutrients and gas
between the fetal and maternal circulatory
systems. It is important to recognize that
maternal and fetal circulatory systems are not
continuous (not directly connected).
In Vitro (IVF) vs In Vivo Conception
1. In vitro - looks at cells and biological
molecules outside their normal biological
context, such as in a lab.
i. IVF (in vitro fertilization) with
microinjection is more effective than
regular IVF. IVF with microinjection
involves delivering sperm directly to
the egg via a needle, whereas in
regular IVF the sperm and egg are
mixed together in a dish for natural
fertilization
a. Microinjection success rate is
~75% and helps with male fertility
because low sperm count or
abnormally swimming sperm are
no longer a problem
2. In vivo - normal biological environment
i. Natural conception (in vivo) normally
has a higher success rate than IVF
Pregnancy Trimesters
Pregnancy is split into trimesters, 3 months each.
Organs of the fetus develop in the fi rst trimester,
which is known as the critical development
period. During pregnancy, it is important that
the mother maintains adequate calcium levels,
or the developing fetus will resorb it from the
mothers skeleton. Also note that the fetus is
very active during the second trimester and the
uterus grows enough for the pregnancy to be
noticeable. The fetus has a fi nal growth of ~1.6
feet/7 lbs during the third trimester.