EMBRYOLOGY
(FINAL EXAM REVIEW)
Lecture 1
Human Development- Continuous Process that begins with a sperm (male) and fuses with and
oocyte (female) for fertilization. (till death)
Embryology- Concerned with the development of the embryo to the birth of the infant.
Puberty- Capability of sexual reproduction (puberty ends with 1st mature sperm formed and also
with the first period).
Menarche- First menstruation.
Female- vagina (excretory passage for period, receives penis and forms inferior part of birth
canal), oocyte, uterus, body, cervix, fundus, os, perimetrium, myometrium, endometrium.
Uterine tube: horn fimbre, infindibulum, ampulla, isthmus.
Gametogenesis- formation of germ cells
Meiosis- reduction in chromosome number, and make another copy maintains the chromosome
number, random assortment of maternal and paternal chromosome, crossing over.
Spermatogenesis - metamorphosis (changing cell shape ): nuclear condensation, formation of
acrosome, and shedding of the cytoplasm (requires 2 months). Maturity takes place in the
epididymis. Spermatogenesis begins at puberty and continues basically for the life of the male.
Primary spermacyte 46; secondary spermatocyte 23; spermatids haploid 23.
Female reproductive cycle: monthly cycle: Hypothalamus to Gonadotropin releasing hormone to
pituitary gland to Gonadotropic hormones to ovaries to uterus to uterine tube to vagina to
mammary glands THESE STRUCTURES WILL PREPARE THE FEMALE FOR
PREGNANCY.
Oogonia- will enlarge to form a primary oocyte before birth-after birth the primary oocyte has
completed prophase of the 1st mitotic division where it remains there until puberty. Just prior to
ovulation the oocyte completes the 1st mitotic division
Here the secondary oocyte receives most of the cytoplasm. At ovulation the nucleus of the
second oocyte begins the second mitotic division - and goes into metaphase. If fertilized by
sperm, second mitotic is completed if not it degenerates.
The secondary oocyte released at ovulation is surrounded by a thin layer Zona Pellucida and the
layer of cells is referred to as Corona Radiata. Up to 2 million primary oocytes are present in the
ovaries or an new born female infant. These are reduced and by puberty only 40,000 remain
(only 400 mature).
Male
Sperm-Highly motile
little cytoplasm
Sperm two types
22 autosomes + X=23
22 autosomes + Y=23
Female
oocyte large and non-motile
mature oocyte an abundance of cytoplasm
Ovum
22 autosomes + X=23
Oogenesis - oogenia - oocytes begins at the fetal period and not completed until after puberty continues throughout the reproductive life of the female. Consists of sequence of events where
oogonia are transformed into and oocyte.
Gonadotropin Releasing Hormone (GnRH)- synthesized by the secretory cells of the
hypothalamus; stimulates the release of FSH and LH
Follicle-Stimulating Hormone (FSH)- Stimulates development of ovarian follicles and
production of estrogen.
Lutenizing Hormone(LH)- Trigger for ovulation; stimulates the follicular cells and corpus
luteum to produce progesterone. During cycle FSH promotes growth of several primary follicles.
Only one matures and ruptures through the surface of ovary, expels its oocyte (follicles
degenerate each month).
Ovulation- Formation of antrum (fluid filled space) the follicle is now referred to as a secondary
follicle - now the primary follicle is pushed to the side and is surrounded by a group of cells
cumulus oophorus. Mature ovarian follicle forms and bulge on the ovary surface and the
secondary oocyte detaches from the follicle
Mid-cycle- 14 days on average; The ovarian follicle develops into the corpus luteum under the
influence of FSH and LH, they undergo a sudden growth spurt producing a stigma (small
avascular spot or swelling); surge in estrogen; Stigma ruptures (expels the secondary oocyte
[resulting from pressure])
Ovarian follicle develops into the corpus luteum under the influence of LH, which secretes
(progesterone) which prepares the endometrium for implantation of blastocyst.
Phases of menstrual cycle -they are under the control of hormones approx. 23-25 days;
Menstrual: (4-5 days) sloughing of the uterine wall (period)
Proliferation: (9 days) growth of ovarian follicle (2-3 fold thickness of endometrium and also
water content epithelium reforms and covers the endometrium and artery’s.
Maturation of Sperm - Capacitation: Lasting 7 hrs. a glycoprotein coat and seminal proteins are
removed from the surface of the sperm acrosome by substances in the uterus or uterine tube.
Menopause - Permanent cessation of menses
Climacteric - Changes associated with menopause
Fertilization - Cleavage of a zygote leading to formation of a blastocyst; blastocysts begin
implantation on or about the 6th day of cycle; HCG (Human Chorionic Gonadotropin) keeps
corpus Luteum secreting estrogen and progesterone; the secretory phase continues and
menstruation does not occur.
No Fertilization - Corpus Luteum degenerates; Estrogen and progesterone levels fall;
menstruation occurs; endometrium degenerates (Ischemic phase).
Hypothalamus
GNRH
Pituitary
GnRH
FSH
primary
follicle
growing
follicle
estrogen
mature
follicle
LH
Ovulation
Developing
corpus luteum
Degenerating
corpus luteum
progesterone &
estrogen
Uterine Wall
Results of fertilization - Stimulates the secondary oocyte to complete the second meiotic
division, producing the second polar body; restores the normal diploid number (46) in zygote;
Results in variation of the human species through mingling of maternal & paternal
chromosomes; Determined the chromosomal sex of the embryo (X-female Y-male); causes
metabolic activation the oocyte which initiates cleavage (cell division of zygote)
2 an 3 lecture goes here!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
Lecture 4
Trophoblast - While blastocyst continues to implant divides into two layers cytotrophoblast
(cellular trophoblast) and syncytiotrophoblast (cellular attached toe the uterine wall).
Syncytiotrophoblast - continually invades the endometrial stroma (CT), while the blastocyst
invades and imbeds itself into the endometrium; It produces hCG (human Chorionic
Gonadotropin) maintains the corpus luteum and suppress menses.
cytotrophoblast
inner cell mass
Uterine Cell Wall
Syncytiotrophoblast
Cytotrophoblast Cells - Produce cellular extension that grow into the overlying
syncytiotrophoblast. These projections form the primary chorionic villi, and also begin induced
by extra embryonic somatic mesoderm.
Summary of 1st week of development
 Fertilization - union of sperm and egg
 Forms a mature oocyte and female pro-nucleus
 Tail of sperm degenerates and male pro-nucleus is formed
 Fertilization is complete when pronuclei unite and chromosomes intermingle
 Zygote undergoes cleavage as it passes along uterine tube.
 Formation of blastomeres and morula
 Blastocysts give rise to inner cell mass and trophoblast
 Inner cell mass becomes embryoblast and later hypoblast(endoderm) gives rise to embryo
and extraembryonic tissues.
 Trophoblasts closes the inner cell mass and blastocyst cavity and forms the extraembryonic
structures and embryonic part of the placenta.
5 days after fertilization implantation takes place. Here trophoblasts attach to the endometrial
epithelium. And differentiates into 2 layers, and outer syncytiotrophoblast (no distinct
boundaries) invades endometrial lining and CT. At the same time a hypoblast (cuboidal cells)
forms from inner cell mass
By the end of the 1st week blastocysts are implanted into endometrium
Summary of implantation
 Zona Pellucida degenerates (blastocyst enlarges) Day 5
 Blastocyst adheres toe endometrial epithelium Day 6
 Trophoblast differentiates into two layers syncytiotrophoblast and cytotrophoblast. Day 7
 The syncytiotrophoblast invades the endometrium and blastocyst embeds. Day 8
 Blood filled lacunae appear in the syncytiotrophoblast. Day 9
 Blastocyst sinks beneath the endometrium - filled by plug Day 10
 Lacunae networks formed Day 10-11
 Primitive uteroplacenta circulation. Day10-11
 Endometrium repaired defect disappears. Day 12-13
 Primary chorionic villi develop. Day 13-14
Summary of 2nd week
 Decidual reaction
 Formation of primary yolk sac
 Extraembryonic mesoderm
 Chorionic cavity
 Secondary yolk sac
 Amniotic cavity appears
 Bilaminar embryonic disc
 Prechordal plate
Notochord - Cellular rod formed from transformation of the notochordal process; defines the
primordial axis of the embryo and gives it form (rigidity). Serves of the bases for the
development of the axial skeleton. It indicates the future site of vertebral bodies.
Neural plate and tube - Neural plate formed thickened neuro-epithelial cells from embryonic
ectoderm, as the notochord develops. Day 18 the neural plate invaginates and forms the neural
groove (which is signs of brain development). By the end of the 3rd week, the neural folds have
begun to move together and fuse which converts the neural plate into a neural tube. The
ectoderm of the neural plates gives rise to the brain and spinal chord.
Neural Crest - Neural tubes separates from the surface ectoderm and neural crest cells and
migrates dorsolaterally on each side of the neural tube. They will form a flattened mass of neural
crest, which is between the neural tube and overlying surface ectoderm. These cells migrate into
different directions and disperse within the meschyme. They give rise to spinal ganglia of the
ANS, The CN’s V, VII, IX, & X, the sheaths of peripheral nerves, meningeal coverings of the
brain and spinal chord, pigment cells, medulla, skeletal and muscular components of the head.
Somites - Formed from paraxial mesoderm that comes from intraembryonic mesoderm. The end
of the 3rd week, the mesoderm differentiates and divides into paired cuboidal bodies, somites.
These are blocks of mesoderm on either side of the neural tube. 42-44 pair at the 5th week. They
are use to determine the age of the embryo. They extend craniocaudally and give rise to most of
the axial skeleton, associated musculature and adjacent dermis of the skin.
Angiogenesis - Mesenchymal cells: angioblast aggregate to form blood island (angiogenic cell
cluster). Cavities appear within the blood islands. Angioblasts flatten to form endothelial cells,
which are arranged around blood islands to form primitive endothelium. Cavities lined with
endothelium fuse to form networks of endothelium. Vessels extend into adjacent areas by
endothelial budding and fusion with other vessels. Blood vessels develop from endothelial. cells
of vessels on yolk sac and allantosis at the end of the 3rd week.
Cardiovascular system - 3rd week angiogenesis begins in the extra embryonic mesoderm of the
yolk sac, connecting stalk and chorion. This is correlated with the decrease in yolk in the ovum
and sac. Therefore, during the 3rd week a primitive uteroplacenta circulation is developed. HRT
development begins from mesenchymal cells in the cardiogenic area from endocardial tubes in
the HRT. They fuse during the 3rd week to form a primordial HRT tube. The tubular HRT joins
with the blood vessels in the embryo connecting stalk, chorion and yolk sac to form the
primordial cardio system. By the end of the 3rd week blood is circulating and the heart begins to
beat of the 21st or 22nd day. The cardio system if the 1st organ system to function!!!!!!
Summary of the 3rd week
 Primitive streak- caudal end of embryonic disc
 Notochord - primitive axis of the embryo around which the axial skeleton forms
 Neural tube appears as a thickening of embryonic ectoderm -neural plate--- then a groove
develops in the plate - neural folds are on each side and fusion of the folds forms the tube.
 Neural crest - made of neuroectoderm cells that migrate dorsolaterally to form a neural crest
between the surface ectoderm and the neutral tube.
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Somites - made of mesoderm of each side of notochord, which thickens to form columns of
paraxial mesoderm. They give rise to vertebrae, ribs and axial musculature
Intraembryonic coelom- gives rise to the body cavities
Blood vessels - primordia CV system
Chorionic villi --- development results in exchange of nutrients and other substances between
the maternal and embryonic circulation.
26-29 weeks - The fetus can survive if born prematurely and given Intensive Care (lung are
Cool). The CNS has matured to the stage of which it can direct breathing movements and control
temperature.
35-38(40 [or birth]) - Fetuses that are 35 weeks old exhibit spontaneous orientation to light. The
nervous system is mature; most fetuses are plump with a head circumference = to that of there
abdomen. Growth slows as the time of the birth approaches.
Factors that influence fetal growth
Glucose - primary source of energy - insulin secreted by the fetal pancreas. Maternal, fetal
environmental
Smoking: It is well known that cigarette smoking affects the growth rate of fetuses. In fact,
fetuses that are exposed toe cigarettes are much smaller (approx. 200gm’s less) The effect (low
birth weight) is additive if the mother has inadequate nutrition. It will constrict uterine blood
vessels also carboxyhemeglobin present of mothers blood.
Multiple pregnancies: requiring more nutrition and most affected during the 3rd trimester.
Social drugs: ETOH, Pot, Cocaine…etc…. can have a series of effects on the fetus.
Impaired uteroplacental blood flow: This leads to fetal starvation caused by small chorionic or
umbilical vessels impairment, severe hypotension or renal disease.
Genetic factors: growth retardation
Diagnostic Amniocentesis: Typically 20-30 ml’s of fluid is removed and analyzed. A needle is
inserted in the mother’s anterior abdominal and uterine wall into the amniotic cavity through the
chorion and amnion. Performed at the 14th week.
Aneuploidy - Any deviation from the human diploid number of 46 chromosomes. An aneuploidy
is an individual or cell that have a chromosome number that is not an exact multiple of the
haploid number of 23, where the principle cause id non-disjunction during cell division. This
results in an unequal pair of chromosomes: one has two and the other cell has neither
chromosomes of the pair and as a result the embryo’s cells maybe by hyperdiploid (trisomy 21)
downs syndrome (instead of 23 they have 24 or more). Embryo’s with monosomy (missing
chromosomes) usually die. Hypodiploid (45, X) (normal is 46) here leads to Turner syndrome ( 1
in 8,000 live births).
Anomalies caused by environmental factors
Teratogens: Are environmental agents that may cause developmental disruptions following
maternal exposure, but will not if cellular differentiation hasn’t begun. Moreover, these are
agents that can produce a congenital anomaly or raise the incidence of an anomaly in the
population. Additionally environmental factors, infections and drugs also may lead to genetic
conditions.
Organs and parts of embryo and most sensitive to teratogens during periods of rapid
differentiation.
Infectious Agents as Teratogens:
Rubella: (three day measles) virus causes cataracts, cardiac defects, and deafness.
Cytomegalovirus (CMV): Infection may result in IUGR, microphthalmia, chorioretinitis,
blindness, microcephaly, cerebral calcification, mental retardation, deafness, CP, and
hepatosplenomegaly (enlargement of liver and spleen).
Herpes Simplex Virus: Occurs late in pregnancy during delivery. Abnormalities observed are
cutaneous lesions and microcephaly, microphthalmia, spasticity, retinal dysplasia and mental
retardation.
Varicella (chicken pox): Brain damage, muscle atrophy, and hypoplasia of a limb, mental
retardation.
HIV: AIDS
Toxoplasma gondii: Protozoan (intracellular parasite) crosses the placental membrane and
infects the fetus causing destructive change in the brain and eyes, which result on mental
deficiency. Microcephaly and hydrocephaly.
Congenital Syphillis: Caused by treponema pallidum: a small, spiral micro organism. Fetus can
become infected at any stage of the disease or any stage of pregnancy. Usually causes severe
congenital anomalies, abnormal teeth and bones, hydrocephalus, mental retardation. Late
manifestations of syphilis are destructive lesions of the palate and nasal septum. Dental
abnormalities (centrally notched, wide spaced peg-shaped upper incisors) abnormal faces (frontal
bossing, saddle nose and poorly developed maxilla)
Radiation: Ionizing radiation may injure embryonic cells, which result on cell death,
chromosome injury and retardation or mental and physical growth.
The peritoneal cavity is connected with the extraembryonic coelom at the umbilicus. The
peritoneal cavity loses its connecting with the extraembryonic coelom during the 10th week as
the intestines return to the abdomen form the umbilical cord.
Mesenteries: A mesentery is a double layer of peritoneum that begins as an extension of the
visceral peritoneum covering an organ; it connects the organ to the body wall and conveys it as
vessels and nerves. The dorsal and ventral mesenteries divide the peritoneal cavity into right and
left halves. The ventral mesentery disappears except where it is attached to the caudal part of
fore gut. (arteries supplying gut are: celiac trunk (foregut [except the pharynx, respiratory tract
and most of the esophagus] ), superior (mid gut)and inferior (hind gut) mesenteric )
Composite from four embryonic structures:
septum transversum(ST): Composed of mesodermal tissue and is the primordial of the central
tendon of the diaphragm. As it grows it separates the HRT from the liver. Located caudal to the
pericardial cavity and partially separates it from peritoneal cavity. After the head fold ventrally
the (ST) forms a thin incomplete partition between the pericardial and abdominal cavities. (ST)
will expand and fuse with mesenchyme ventral to the esophagus (primitive medistinum) and
pleuroperitoneal membranes
Innervation of the diaphragm:
Myoblasts from the 3-5th cervical somite , migrate into the
developing diaphragm, bringing their nerve fibers with them. It is the VENTRAL rami of C-3, 4
& 5. The nerve also supplies the sensory fiber to the superior and inferior surfaces of the right
and left domes of the diaphragm.
1st pharyngeal arch:
It develops into two prominences:
Maxillary Prominence- gives rise to the maxilla, zygomatic bone, and squamous part of the
temporal bone.
Mandibular prominence- Forms the lower jaw.
2nd pharyngeal arch: (aka hyoid arch)
The Hyoid bone
The arches caudal to #2 are referred to by number only!!!!
All of the arches give support to the lateral walls and are derived from the cranial part of the fore
gut.
Components of a typical arch:
Aortic arch: an artery that arises form the truncus arteriousus of the primordial HRT and runs
around the primordial pharynx to enter the dorsal aorta.
Cartilaginous rod: Forms the skeleton of the arch (C-spine)
Muscular component: forms muscles in the head and neck
Nerve: supplies the mucosa and muscles derived from the arch. Nerves that grow into the arches
are derived from neuro-ectoderm of the primordial brain.
Derivatives of the Pharyngeal arch cartilage’s:
The first arch cartilage - becomes ossified to form two middle ear bones, the malleus and incus.
The middle part of the cartilage regresses, but its perichondrium forms the anterior ligament of
the malleus and the sphenomandibular ligament
The second arch cartilage - (Reichert Cartilage) related to the developing ear: ossified to form
the stapes of the middle ear and the styloid process of the temporal bone. The cartilage between
the styloid process and the hyoid bone regresses, its perichondrium forms the stylohyoid
ligament. The ventral end of the cartilage ossifies to form the lesser cornu and the superior part
of the hyoid bone.
The third arch cartilage: ossifies to form the greater cornu and the inferior part of the body of the
hyoid bone.
The fourth and sixth arches: fuse to form the laryngeal cartilage except for the epiglottis. It will
come from mesenchyme in the hypobranchial eminence in the 3rd & 4th arches.
Development of the tongue: Near the end of the 4th week, a medial triangular elevation appears
in the floor of the primordial pharynx, just rosteral to the foramen cecum. The swelling - the
median tongue bud - is the 1st indication of tongue development. Here, two oval distal tongue
buds form the anterior two thirds (oral part) of the tongue. The posterior third of the tongue is
indicated by two elevations that develop caudal to the foramen cecum. The anterior and posterior
part of the tongue is indicated by a V-shaped groove - the terminal sulcus.
Development of the salivary glands: During the 6th and 7th weeks, the salivary glands begin as
solid epithelial buds from the primordial oral cavity. The CT ion the glands is derives from the
neural crest cells. All parenchymal tissues arises by proliferation of the oral epithelium.
Five facial primordia appear as prominences around the stomodeum:
Single frontonasal prominences (1)
Paired Maxillary prominences (2)
Paired Mandibular prominences (2)
Development of the palate:
from two primordia
Primary palate- develops from the intermaxillary segments of the maxilla. About the 6th week
Secondary palate- two mesenchyme into the internal aspects of the maxillary prominences 6th
week.
Developmental anomalies:
Cleft lip: a common anomaly usually associated with a cleft palate. However, they are distinct.
Cleft lip results from the failure of mesenchymal masses in the medial nasal and maxillary
prominences to merge.
Cleft palate: Results from the failure of mesenchymal masses in the palatine processes to meet
and fuse. Both can be used by genetic and environmental factors. These factors interfere with the
migration of neural crest cells into the maxillary prominences of the 1st arch, and if the cells are
not sufficient, these anomalies can result.
Anomalies
Esophageal atresia: Blockage of the esophagus; results from a deviation of the tracheoesophageal
septum in a posterior direction. The fetus with esophageal atresia is unable to swallow amniotic
fluid. Therefore, this fluid cannot pass to the intestine for the absorption and transfer through the
placenta to the maternal blood for disposal. Here newborn infants appear healthy and their 1st
swallows are normal. Then, suddenly, fluid returns through their nose & mouth, and respiratory
distress occurs.
Development of the kidneys:
Three sets of excretory organs or kidneys
Pronephroi: Transitory, nonfunctional structures that appear in human embryo’s early in the 4th
week. It will soon degenerate.
Mesonephros: Large elongated organs that appear late in the 4th week, caudal to the
rudimentary pronephroi. They are well developed and function as interim kidneys until the
permanent kidneys develop. The mesonephric kidneys consist of glomeruli and mesonephric
tubules. The mesonephroi degenerate toward the end of the 1st trimester; however, their tubules
become the efferent ductules of the testes.
Metanephroi: Permanent kidneys begin to develop during the 5th week and start to function
about 4 weeks later. Urine formation continues throughout fetal life. Urine is excreted into the
amniotic cavity and mixes with the amniotic fluid.
Development of the genital system:
The gonads (testes and ovaries) are derived from three sources:
Mesothelium (mesodermal epithelium) linings of the posterior abdominal wall
The underlying mesenchyme (embryonic CT)
Primordial germ cells
Different Gonads:
The initial stages of gonadal development occur during the 5th week. A thickening mesodermal
epithelium develops on the medial side of the mesonephros. Proliferation of this epithelium and
the underlying mesenchyme produces a bulge on the medial side of the mesonephros- the
gonadal (genital ridge). Epithelial cords - primary sex cords grow into the underlying
mesenchyme. The different gonads now consist of an external cortex and an internal medulla. In
embryos with an XX sex chromosome complex, the cortex of the indifferent gonads
differentiates into ovary and medulla regresses. In embryo’s with an XY sex chromosome,
complex the medulla differentiates into a testis and the cortex regresses.
Development of the male glands:
Prostrate: Endodermal out growth arises from the prostatic part of the urethra and grows into the
surrounding mesenchyme. The glandular epithelium of the prostrate differentiates from these
endodermal cells, and the associated mesenchyme differentiated into the dense stroma and
smooth muscle of the prostrate
Bulbourethral Glands: From paired outgrowths from the spongy part of the urethra. Smooth
muscle fibers and the stroma differentiate from the adjacent mesenchyme.
Development of the Vagina:
The vaginal epithelium is derived from the endoderm of the fetal testes. As the phallus enlarges
and becomes the penis, the urogenital folds form the lateral walls of the uretheral groove of the
ventral surface of the penis. This groove is lined with endodermal cells. The urogenital folds fuse
together along with the ventral surface of the penis to form the spongy urethra. At the tip of the
glans penis and ectodermal in growth forms a cellular cord, the glandular plate, which grows
toward the root of the penis to meet the spongy urethra. During the 12th week a circular in
growth of ectoderm occurs at the periphery of the glans penis. When this in growth breaks down,
it forms the prepuce. The corpora cavernosa penis and corpus spongiosum penis develops from
mesenchyme in the phallus. The labial swellings grow toward each other and fuse to form the
scrotum.
Descent of the testes:
Testicular descent is associated with:
Enlargement of the testes and atrophy of the mesonephric kidney, which allow movement of the
testes caudally along the posterior abdominal wall.
Atrophy of the paramesonephric ducts, which enables the testes to move trans-abdominally to the
deep inguinal rings.
Enlargement of the process vaginalis, which guides the testis through the inguinal canal into the
scrotum.
Lecture 10
Cardiovascular System: 1st major system to function in the embryo!!!
The CVS is derived mainly from the following
Splanchnic mesoderm: forms primordial HRT
Paraxial and lateral mesoderm
Neural crest cells originate from the region between the otic vessels and the caudal part of the
3rd pair of somites.
The earliest sign of the HRT is the appearance of paired endothelial strand - the angioblastic
chords during the 3rd week. These chords canalize to form endocardial HRT tubes that fuse to
form the tubular HRT late in the 3rd week. The HRT begins to function at 22-23 days. An
inductive influence from the embryonic endoderm appears to stimulate early formation of the
HRT. Blood flow begins during the 4th week.
Partitioning of the AV canal:
By the end of the 4th week, the primordial atrium is divided into the right ant left atria by
formation and modification and fusion of 2 septa, the septum primum and septum secundum.
The septum primum, a thin crescent shaped membrane, grows toward the fusing endocardial
cushions from the roof of the primordial atrium that divided the atrium into right and left halves.
These canals partially separate the primordial atrium from the ventricle, and endocardial
cushions functions as AV valves.
Aortic stenosis and atresia:
The edges of the vale are fused to form a dome with a narrow opening. This
anomaly may be present at birth or it may develop after birth. Valvular stenosis causes extra
work for the HRT and results in hypertrophy of the left ventricle and abdominal HRT sounds
(HRT murmurs).
Aortic arches:
During the 4th week as the pharyngeal arches are developing, they are supplied by the arteries of
the aortic arches.
1st : Largely disappear; remaining parts form maxillary arteries (teeth, face and muscles of the
eye). Help to from the external carotid arteries.
2nd : Stapedial arteries: supply the stapes of the ears.
3rd : Proximal parts form the common carotid arteries, which supply the head. Distal parts of the
3rd pair of arches join with the dorsal aorta to form the internal carotid arteries, these supply the
ears, orbits and brain.
4th : the left arch form part of the arch of the aorta, the right 4th arch becomes the proximal part
of the right subclavian artery. The left subclavian artery is not derived from and aortic arch it
forms from the left 7th intersegmental artery.
5th : These remain rudimentary or do not develop.
6th The proximal part of the arch persists as the proximal part of the left pulmonary artery. The
distal part of the arch passes from left pulmonary artery to the dorsal aorta to form a prenatal
shunt, the ductus arteriosus (DA).
Fetal circulation:
Highly oxygenated blood returns from the placenta into the umbilical vein
Half of the blood passes directly into the ductus venosus (connects the umbilical vein to the
IVC).
The other half flows into the sinusoids of the liver and enters the IVC through the hepatic veins.
Because the IVC contains blood from the lower limbs, abdomen and pelvis, the blood entering
the right atrium is not well oxygenated as that in the umbilical vein.
After a short course in the IVC, the blood enters the right atrium of the heart. Fetal lungs extract
oxygen from the blood instead of providing it. From the left atrium, blood passes to the left
ventricle and leaves through the ascending aorta.
Arteries to the HRT, head, neck, and upper limbs receive well-oxygenated blood. The liver also
receives oxygenated blood from the umbilical vein. Most of the blood passes through the DA
into the aorta to profuse the caudal part of the fetal body. The ductus arteriosus protects the lungs
form circulatory overloading and allows the right ventricle to strengthen in preparation for
functioning at full capacity at birth.
Transitional Neonatal Circulation:
In the infant’s lungs 3 shunts that permitted much of the blood to pass by the liver and lungs
close and become obliterated. At birth the foramen ovale, ductus arteriosus, ductus venosus and
umbilical vessels are no longer needed.
The skeletal system:
Develops from the mesodermal and neural crest cells
Development of the bone and cartilage:
Most flat bones develop in mesenchyme within preexisting membranous sheaths. This type of
osteogenesis is intramembranous bone formation. Mesenchymal model of most limb bones are
transformed into cartilage bone models which later become ossified by endochondral bone
formation.
3 types of cartilage: Hyaline (joints), fibrocartilage (IVD’s), Elastic cartilage (auricle of the ear).
Intramembranous ossification:
This type of bone formation occurs in mesenchyme that has formed a membranous sheath. The
mesenchyme condenses and becomes highly vascular; some cells differentiate into osteoblasts.
And begin to deposit matrix or osteoid tissue (prebone). Calcium phosphate is deposited in the
osteoid tissue as it is organized into bone. Bone osteoblasts are trapped in the matrix and become
osteocytes. Concentric lamellae develop around blood vessels, forming haversian systems.
Between the surface plates, the intervening bone remains spiculated or spongy. This environment
is accentuated by the action of osteoclasts, which absorb bone. Mesenchyme also differentiates
into bone marrow. During fetal and neonatal life, remodeling of bone occurs by the actin of
osteoclasts and osteoblasts.
Development of joints:
Classified as: fibrous, cartilaginous and synovial joints.
Fibrous: During development the interzonal mesenchyme between the developing bones
differentiates into dense fibrous tissue. (Ex: skull)
Cartilaginous: The interzonal mesenchyme between the developing bones differentiates into
hyaline cartilage or fibrocartilage. (Ex: Pubic Symphysis)
Synovial joints: The interzonal mesenchyme between the developing bones differentiates
peripherally to form the capsular and other ligaments. Centrally it disappears, and the space
becomes the joint of synovial cavity. Where it lines the fibrous capsule and articular surfaces, it
forms the synovial membrane, a part of the articular capsule. (Ex: superior and inferior facets).
Skull:
Develops from mesenchyme around the developing brain; it consists of the Neurocranium (a
case for the brain).
Viscerocranium:
Skeleton of the face
Cartilaginous Viscerocranium:
These parts are derived from the cartilaginous skeleton of the 1st two parts of the pharyngeal
arches.
Dorsal end of 1st arch cartilage (Meckel’s) middle ear bones (malleus and incus)
Dorsal end of 2nd arch cartilage (Reichert) stapes of middle ear and styloid process.
3rd, 4th, and 6th cartilage’s form only the ventral parts of the arches. The 3rd arch cartilage gives
rise to the greater cornu and inferior part of the Hyoid bone. The 4th and 6th cartilage form the
laryngeal cartilage’s, except for the epiglottis.
Lecture 6
9- 12 weeks: The liver is a major site for erytheropoiesis, but towards the end of the period
process has begun in the spleen.
CAT scans and MRI’s: Generally used when planning treatment for an anomaly detected using
ultrasound. This is important for differentiating between monoamniotic and diamniotic twins (1
or 2 amniotic sac’s). This is important because the perinatal mortality of monoamniotic twins is
high 30-50% risk of death.
Three regions: named for there implantation site
Decidua basalis: Part of decidua deep to the conceptus forms maternal component of the
placenta.
Decidua capsularis: Superficial part of the decidual overlying the conceptus
Decidual parietalis: All of the remaining parts.
fetal component of the placentas formed by the villous chorion. Maternal component: formed
from decidua basalis; by the end of the 4th month, decidua basalis is almost entirely replaced by
fetal component of the placenta.
Placental circulation: It is through the numerous branched villi, which arise from the stem villi,
that the main exchange of material between the mother are separated by the placental
membrane, consisting of extrafetal tissues.
Fetal placental circulation: Poorly oxygenated blood leaves the fetus and passes through the
umbilical arteries to the placenta. At the attachment of the cord to the placenta, these arteries
divide into a number of radically disposed chorionic arteries that branch freely in the chorionic
villi. This network arterio-capillary-venous system brings fetal blood close to maternal blood and
provides a large area for the exchange of metabolic and gaseous produced between the maternal
and fetal blood streams.
Function of the placenta:
Metabolism (synthesis of glycogen): placenta synthesizes glycogen, cholesterol, and fatty acids
that serve as sources of nutrients and energy for the fetus.
Transport (of gases and nutrients): Transport of substances in both directions between the
placenta/maternal blood is facilitated by surface area of the placental membrane.
Endocrine secretions (human chorionic gonadotropin)
Stages of labor
Dilation
Expulsion
Placenta stage
Recovery
Yolk Sac: By week 10 the sac has shrunk to a pear shaped remnant and is connected to the mid
gut by a narrow yolk stalk. By week 20 the sac is very small. The yolk sac is significant because
it has a role in the transfer of nutrient to the embryo during the 2nd and 3rd weeks. Blood
development 1st occurs in the extraembryonic mesoderm covering the wall of the yolk sac in the
3rd week. During the 4th week the dorsal part of the mid gut. Primordial germ cells appear in the
endometrial lining of the walls of the yolk sac in the 3rd week.
Allantosis: During the 2nd month the extraembryonic ports of the allantosis degenerates.
However, it is important because blood formation occurs in its walls during the 3rd -5th weeks.
Its blood vessels become the umbilical vein and arteries.
Fluid from the amniotic cavity diffuses into the umbilical vein and enter the fetal circulation
from transfer to maternal blood through the placental membrane.
The intraembryonic portion of the allantosis run from the umbilicus to the urinary bladder - as
the bladder enlarges, allantosis involutes to form a thick tube the urachus and, eventually
becomes the median umbilical ligament.