the embryonic period is weeks 3-8 of development; the three germ layers- ectoderm, mesoderm, and endoderm- form the tissues and main organ systems and the body becomes recognizably human
When is the period in which the conceptus is most likely to develop a major congenital malformation?
the period during which the conceptus is most likely to develop a major congenital malformation is during the embryonic period
What does the ectodermal germ layer give rise to?
the ectodermal germ layer gives rise to the central nervous system; peripheral nervous system; sensory epithelium of teh eye, nose, and ear; epidermis of the skin, hair, nails, and subcutaneous glands (e.g. sweat glands); the pituitary gland; and tooth enamel
the development of a pear-shaped thickening of ectoderm begins the formation of the central nervous system (CNS), due to induction by the notochord and prechordal mesoderm
the neural folds approach each other in the midline and begin to fuse to form the neural tube; fusion proceeds cranially and caudally from the cervical region
Cranial and Caudal Neuropores
fusion of neural folds is temporarily incomplete at cranial and caudal neuropores, which close around days 25-27 respectively; neural tube forms the large brain and spinal cord
*special band of ectodermal cells on each side of the neural folds that elevate and fuse, then separate from the junction of neural tube and surface ectoderm; neural crest cells migrate into underlying mesoderm, proliferate, and form a number of important structures
Neural Crest Derivatives
neural crest derivatives include spinal (posterior root) ganglia and sensory ganglia of cranial nerves V, VII, IX, and X; autonomic ganglia; the andrenal medulla; schwann cells; connective tissues of anterior part of the skull and meninges; melanocytes; C cells of thyroid; conotruncal septum of heart
neural crest cells migrate into pharyngeal arches, which are responsible as part of the formation of the face and anterior neck, and form connective tissues
The mesodermal germ layer differentiate into what three parts from medial to lateral?
the mesodermal germ layer differentiates into three parts on each side of the neural tube; from medial to lateral- paraxial, mesoderm, intermediate mesoderm, lateral plate mesoderm
paraxial mesoderm is the thickened band of tissue on each side of the neural tube; becomes segmented into 42-44 blocks of tissue called "somites" from the occipital region caudally
The number of somites can be used to determine what about the embryo?
the age of the embryo can be expressed by the number of somites (table slide 14)
What do somites divide into?
ventromedial sclerotome: forms cartilage and bone of the axial skeleton, including vertebral column; dorsomedial (epimeric) and dorsolateral (hypomeric) skeletal muscle-forming regions; dorsal dermatome, which forms dermis of skin
temporarily connects paraxial and lateral plate of mesoderm; differentiates into urogenital structures (e.g. kidneys, gonads)
The lateral plate mesoderm divides into what two layers separated by a cavity?
the lateral plate mesoderm divides into somatic/parietal layer, which joins overlying surface ectoderm to form ventral and lateral body walls, the splanchnic/visceral layer, which joins underlying endoderm to form wall of gut, and the intraembryonic cavity
lined by mesothelial/ serous membranes- secrete serous fluid; intraembryonic cavity is later partitioned to form the pericardial, pleural, and peritoneal cavities of the adult.
Blood Vessel Formation
blood vessel formation occurs in the mesoderm; first in extraembryonic mesoderm surrounding the yolk sac at *3 weeks; slightly later in lateral plate mesoderm of the embryo
The Liver & Hematopoiesis
the liver becomes the major hematopoietic organ of the fetus by week 6; sends stem cells to colonize bone marrow (the definitive blood-forming organ) after 7th month of gestation
Endodermal Germ Layer
main organ system derivative of the endodermal germ layer is the gastrointestinal tract
How is the GI tract formed?
the GI tract is formed due to lateral and cephalocaudal folding of the fetal trilaminar germ disc, incorporating part of the yolk sac into the body.
lateral folding of fetal trilaminar germ disc occurs due to growth of somites; along with cephalocaudal folding, it closes ventral body wall around umbilical ring and helps pull on the amnion, expanding it so that the *amniotic cavity surrounds the embryo
What causes cephalocaudal folding?
Cephalocaudal folding of the fetal trilaminar germ disc is due to growth of brain vesicles and lengthening of the embryonic axis; causes formation of head fold and tail fold w/ embryo assuming curved "fetal position"
What are the three parts of the endoderm-derived gastrointestinal tract?
in the cephalic part- foregut, extending from buccopharyngeal (oropharyngeal) membrane to the liver bud; hindgut in the caudal region; midgut between the foregut and hindgut- temporarily connected to yolk sac by the vitelline duct (yolk stalk)
Formation of the Respiratory System
the respiratory system appears as an outgrowth, the *respiratory diverticulum (lung bud), from the ventral wall of the foregut
What does the endodermal germ layer contribute to?
the endodermal germ layer contributes to the urinary bladder and urethra; thyroid and parathyroid glands; liver and pancreas; tonsils and thymus; and lining of tympanic (middle ear) cavity and auditory (eustachian) tube
The Fetal Period
week 9 of gestation until birth (38 weeks after fertilization or 40 weeks after first day of last period); time of growth and functional maturation of tissues and organs
Primary Ossification Centers
present in all long bones and skull by 12th week; bone first starts to appear in embryo in week 8
During what months does the head grow more slowly than the rest of the body?
during the 4th and 5th months the head/ skull grows more slowly than the rest of the body, but still has the largest circumference than any other part of the body at birth
When does formation of the placenta begin?
formation of the placenta begins at the morula stage; the outer cell mass (trophoblast) forms the fetal portion of the placenta
What happens during the second week of a development?
cells of the trophoblast differentiate into two layers- outer multinucleated layer (syncytiotrophoblast) and the inner cellular layer (cytotrophoblast)
What layer erodes into the endometrium of the uterus?
the syncytiotrophoblast erodes into the endometrium of the uterus; cells in the cytotrophoblast divide and enter the syncitiotrophoblast
What is the decidua reaction?
the endometrium of the uterus undergoes the decidua reaction, in which endometrial cells around the conceptus become loaded with glycogen and lipids and the tissue becomes endematous
How is uteroplacental circulation established?
large spaces (lacunae) appear within the syncytiotrophoblast as it invades congested, dilated capillaries (sinusoids) of the endometrium; flow of maternal blood into syncytiotrophoblast establishes the uteroplacental ciruclation.
What are the large spaces called that appear within the syncytiotrophoblast as it invades the sinusoids of the endometrium?
the large spaces that appear within the syncytiotrophoblast as it invades the sinusoids of the endometrium are called lacunae
Where does the chorionic cavity develop?
the chorionic cavity develops within the extraembryonic mesoderm between the primitive yolk sac and the cytotrophoblast
What happens to the secondary yolk sac during the formation of the chorionic cavity?
the secondary yolk sac, lined by hypoblast cells, is pinched off from the primary yolk sac during formation of the chorionic cavity
consists of extraembryonic mesoderm plus the two layers of trophoblast, is attached to the amnion and yolk sac across the chorionic cavity by the *connecting stalk; connecting stalk later contributes to the umbilical cord
What happens to the trophoblast during the third week of development?
during the third week of development the trophoblast forms an *outer cytotrophoblast shell when cytotrophoblast cells penetrate the syncytiotrophoblast to entirely surround the trophoblast and firmly attach it to the endometrium
Small finger-like *villi bathed in maternal blood become thinned until the fetal and maternal circulations are separated in places only by, what?
fetal and maternal circulations are separated in places only by fetal capillary endothelium and syncytiotrophoblast, which facilitates the exchange of gases and nutrients
What is the placental barrier formed by?
the placental barrier is formed by the fetal capillary endothelium and syncytiotrophoblast
What are the two components that form the placenta?
the placenta is formed by two components: chorion frondosum- the fetal portion, part of the chorion where villi remain; decidua basalis- maternal portion of decidua overlying the chorion frondosum
What are the 4 functions of the placenta?
the functions of the placenta include: exchange of gases, exchange of nutrients and electrolytes, transmission of maternal antibodies (IgG), and production of hormones
What hormone is being tested for in a pregnancy test?
human chorionic gonoadotropin (hCG) is secreted during the first two months by the syncytiotrophoblast to maintain the corpus luteum
What hormones does the placenta produce and when does it produce enough to maintain pregnancy?
by the end of the fourth month the placenta produces enough *progesterone to maintain pregnancy if the corpus luteum were removed; estrogens produced by the placenta stimulate uterine growth and mammary gland development
What are amniotic bands? What are their implications?
amniotic bands are strips of amnion that tear away from the uterine wall; they can encircle an extremity of the fetus resulting in constriction or amputation
How is the primitive umbilical cord formed?
lateral and cephalocaudal folding of the trilaminar germ disk closes the body wall around the *umbilical ring to form the primitive umbilical
What is the primitive umbilical cord formed from?
the primitive umbilical cord is formed from the connecting stalk (contains allantois and umbilical vessels) and the vitelline duct (yolk stalk)
What are the important functions of amniotic fluid?
important functions of amniotic fluid includes: shock absorption, preventing embryonic adhesion to the amnion, allowing fetal movements, allowing fetal growth (e.g., lung expansion), providing barrier to infection
The act of collecting and analyzing amniotic fluid for fetal anomalies is called what?
amniocentesis is the collection of amnion for the purpose of analyzing for fetal anomalies
When does the fetus begin to swallow amnion? What does it contribute to?
at five months the fetus begins to swallow amnion, which contributes hypotonic urine to it
Anencephaly and intestinal atresia prevents swallowing and absorption, respectively, which results in what?
polyhydramnios is present in excess due to inability of the fetus to ingest amnion, caused by anencephaly or intestinal atresia; poly- not as bad as oligohydramnios
What is intestinal atresia?
intestinal atresia is the narrowing or absence of a portion of the intestine
What is oligohydramnios, and what can it be caused by?
oligohydramnios is the deficient amount of amnion, which can be due to renal agenesis or amnion rupture, resulting in clubfoot or lung hypoplasia; more lethal than polyhydramnios
What can cause clubfoot or lung hypoplasia?
clubfoot or lung hypoplasia can be caused by oligohydramnios