· sperm, which lack the acrosome (globozoospermia) and therefore the enzymes necessary for penetration of the ovum are missing. The acrosome develops from Golgi vesicles just like any other secretory granules. It contains acrosin, a serine protease, hyaluronidase, and neuraminidase, responsible for the penetration ability of the sperm.
·
Capacitation,
the acrosome reaction and penetration are required for the hamster sperm
penetration assay (SPA).
·
Primary villi consist of
syncytiotrophoblast with a core of cytotrophoblast cells. In secondary villi, the cytotrophoblast core is
invaded by mesoderm and subsequently by umbilical blood vessels in tertiary villi.
·
Retinoic acid directs the
polarity of development in the central nervous system, the axial skeleton
(vertebral column), and probably the appendicular skeleton. Retinoic acid
induces transcription of various combinations of homeobox
genes, depending on tissue type and location (distance and direction
from the source of retinoic acid). Exogenous sources of retinoic acid may
induce the wrong sequence or combination of homeobox genes, leading to structural abnormalities in nervous and skeletal systems.
·
Mesoderm Derivatives
The mesoderm is divided into four regions (from medial to lateral): axial,
paraxial, intermediate, and lateral plate.
Axial mesoderm is located in the midline and forms
the notochord. Paraxial mesoderm forms somites. Somites are divided into
sclerotomes (bone formation), myotomes (muscle precursors), and dermatomes
(precursor of dermis). Intermediate mesoderm gives rise to components of the
genitourinary system. Lateral plate mesoderm forms bones and connective tissue
of the limbs and limb girdles (somatic layer, also known as somatopleure) and
the smooth muscle lining viscera and the serosae of body cavities (splanchnic
layer, also known as splanchnopleure).
Intermediate mesoderm is not found
in the head region, and the lateral plate mesoderm is not divided into layers
there.
·
EMBRYONIC
DEVELOPMENT
The embryo forms
one germ layer during each of the first 3 weeks. During the second week, the blastocyst
differentiates into two germ layers, the epiblast and the hypoblast. This
establishes the dorsal (epiblast)–ventral (hypoblast) body axis. During the
third week, the process of gastrulation occurs by which epiblast cells
migrate toward the primitive streak and ingress to form the endoderm and
mesoderm germ layers below the remaining epiblast cells (ectoderm). Lateral
body folding at the end of the third week causes the germ layers to form
three concentric tubes with the innermost layer being the endoderm, the
mesoderm in the middle, and the ectoderm on the surface.
·
LIMB FORMATION
The limbs form as
ventrolateral buds under the mutual induction of ectoderm [apical ectodermal
ridge (AER)] and underlying mesoderm beginning in the fifth week. The AER
influences proximal-distal development.
Somatic lateral
plate mesoderm (somatopleure) forms the bony and connective tissue elements of
the limbs and limb girdles while skeletal muscle of the appendages is derived
from somites.
Cranio-caudal polarity is determined by
specialized mesoderm cells [zone of polarizing activity (ZPA)] that
release inducing signals such as retinoic acid.
Homeobox genes
are the targets of induction signals. They are named after their homeodomain
called the homeobox which is a DNA-binding motif. Homeobox genes encode
trancription factors that regulate processes such as segmentation and axis
formation.
Rotation of the
limb buds establishes the position of the joints, the location of muscle groups,
and the pattern of sensory innervation (dermatome map).
·
Eye
The eye is derived
from three different germ layers: Neuroectoderm: Vesicular outgrowths of
the forebrain differentiate into retina and optic nerve. Surface ectoderm:
Contributes to the lens, cornea, and epithelial coverings of the lacrimal
glands, eyelids, and conjunctiva. Mesoderm: The sclera and choroid are
derived from lateral plate mesoderm. The extraocular muscles are derived from
myoblasts of the cranial somitomeres.
·
Ear
Structures of the
outer and middle ear are derived from the first and second pharyngeal arches
and the first pharyngeal cleft. Structures of the inner ear are derived
from the ectodermal otic placode, not neuroectoderm.
Maternal rubella can cause defects in both eye (fourth to sixth weeks of
gestation) and ear (seventh to eight weeks).
·
Vasculogenesis
is the de novo formation of blood vessels and differs from angiogenesis, initiated in a pre-existing
vessel. Both of those processes are regulated in part by vascular
endothelial growth factor (VEGF), which induces chemotactic (migratory) and
proliferative responses in endothelial cells.
·
All components of
hematopoietic organs are derived from mesoderm except for the epithelium of the
thymus, which is derived from endoderm of the third pharyngeal pouch.
·
·
Mesodermal defects = VACTERL: Vertebral
defect, Anal atresia, Cardiac defects, Tracheo-Esophageal
fistula, Renal defects, Limb defects (bone and
muscle).
·
Teeth
originate from both ectodermal (enamel) and
neurectodermal (neural crest: dentin, pulp, cementum, and
periodontal ligament) derivatives.
·
Endochondral ossification involves development of
hyaline cartilage models that are replaced by bone, except at epiphyseal plates
and articular cartilages, whereas intramembranous ossification involves direct
ossification of mesenchyme and lacks a cartilaginous precursor.
·
The midgut loop
Rotates 270 degrees counterclockwise around the superior mesenteric artery
as it returns to the abdominal cavity.The cranial limb of the midgut loop forms
the jejunum and ileum (cranial portion).The caudal limb forms the caudal
portion of the ileum, cecum, appendix, ascending colon, and the transverse
colon (proximal two thirds).
·
Notochord is
derived from MESODERM.
·
Ventral mesentery forms the lesser omentum,
falciform, coronary, and triangular ligaments.
Dorsal mesentery forms the greater
omentum, mesentery of the small intestine, mesoappendix, transverse mesocolon,
and sigmoid mesocolon.
·
Fate of five dilations of the primitive heart tube:
1.
Truncus arteriosus (ventral aorta) forms aorta
and pulmonary trunk by formation of the aorticopulmonary (AP) septum.
2.
Bulbus cordis forms conus arteriosus (smooth
part of right ventricle) and aortic vestibule (left ventricle).
3.
Primitive ventricle forms trabeculated part of
right and left ventricles.
4.
Primitive atrium forms trabeculated part of
right and left atrium.
5.
Sinus venosus forms sinus venarum (smooth part
of right atrium), coronary sinus, and oblique vein of left atrium.
·
Aortic arch derivatives
1.
Aortic arch 1 has no derivative because it
disappears soon after development.
2.
Aortic arch 2 has no derivative because it
persists only during the early development.
3.
Aortic arch 3 forms the common carotid arteries and the proximal part of the internal
carotid arteries.
4.
Aortic arch 4 forms the aortic arch on the left and the brachiocephalic artery and the
proximal subclavian artery on the right.
5.
Aortic arch 5 has no derivative.
6.
Aortic arch 6 forms the proximal pulmonary arteries and ductus arteriosus.
·
Development of the Venous
System
The sinus venosus
receives the veins (cardinal) from the body. Originally the caudal end of the
heart tube, the sinus venosus rotates cranially and dorsally during the looping
process.
The left horn of
the sinus becomes a narrow channel located in the groove between the left
atrium and ventricle. The coronary sinus empties into the right atrium.
The right horn of
the sinus forms the entrance of the two venae cavae into the right atrium and
becomes the smooth-walled portion of the right atrium.
The primitive
atrium becomes the entire left atrium and the trabeculated (rough-walled)
portion of the right atrium. The bulbus cordis gives rise to the right
ventricle and the muscular portions of the outflow tracts of both ventricles.
The venous system
develops from the vitelline, umbilical, and cardinal veins, which drain into
the sinus venosus.
A. Vitelline veins (HIPS)
Return poorly oxygenated blood from the yolk sac.
Right vein forms the hepatic veins and sinusoids, ductus
venosus, hepatic portal, superior mesenteric, inferior mesenteric, and splenic
veins and part of the IVC.
Left vein forms the hepatic veins and sinusoids and ductus
venosus.
B. Umbilical veins
Carry well-oxygenated blood from the placenta.
Right vein degenerates during early development.
Left vein forms the ligamentum teres hepatis.
C. Cardinal veins
Return poorly oxygenated blood from the body of the embryo.
Anterior cardinal vein forms the internal jugular veins and
SVC.
Posterior cardinal vein forms a part of the IVC and common
iliac veins.
Subcardinal vein forms a part of the IVC, renal veins, and
gonadal veins.
Supracardinal vein forms a part of the IVC, intercostal,
azygos, and hemiazygos veins.
·
Genitalia
·
Mesonephros
o Testes
o Vas
deferens
o Seminal
vesicles
o Epididymis
·
Paranephros
o Ovaries
o Fallopian
tube
o Uterus
o Upper
vagina
Male
|
Embryo structure
|
Female
|
Prostate
Prostatic urethra
Bulbourethral Gland
|
Urogenital Sinus
|
Labia vagina
Labia minora
|
Penis
|
Urogenital tubercule
|
Clitoris
|
Scrotum
|
Labioscrotal swelling
|
Labia majora
|
·
The fusion of the dorsal aortae occurs through lateral
folding.
Craniocaudal
folding establishes the definitive head and tail regions of the
embryo.
Fusion
is already complete at the time that looping of the heart tube occurs. Fusion
of the endocardial heart tube and incorporation of the yolk sac into the
primitive gut also occurs as a result of lateral folding.
Gastrulation
establishes the three germ layers (trilaminar disk), and
Neurulation
establishes the neural groove with two neural folds.
·
The heart forms during the third week by the
apposition of left and right endocardial tubes as the head fold progresses
caudally. The endocardial tubes fuse to form a single-tube heart. This fusion
begins cranially in the region of the bulbus cordis (outflow trunks) and
proceeds caudally through the ventricles and the atria to the sinus venosus,
which is incorporated into the atrium after loop formation. Rapid proliferation
of the ventricular region results in the single-tube heart bending into an
S-shaped loop. During this process, the dorsal mesocardium partially breaks
down, which leaves the heart suspended only at the cranial and caudal ends; the
discontinuity in the mesocardium is the transverse sinus. The left and right sides of the heart are
established by the subsequent division of the single-tube heart, not by the
apposition of left and right endocardial tubes.
·
Tracheal and Laryngeal cartilage is of
neural crest origin (septum).
·
Muscles develop from mesodermal cell
populations arising in the somite. The connective tissues around the
muscles have a different embryologic origin and are derived from the somatopleural
mesoderm.
·
Epimeres
(epaxial): form true or “intrinsic” back muscles (e.g., erector
spinae) that are innervated by a dorsal ramus of the spinal nerve.
Hypomeres (hypaxial): form the remainder of
the trunk and limb musculature and are innervated by a ventral ramus of the
spinal nerve.
·
Gray matter of
Spinal Cord develops from MANTLE ZONE.
·
Each of the
embryonic germ layers is continuous with an extraembryonic structure.
Ectoderm is continuous with the amniotic membrane, endoderm with
the lining of the yolk sac, and embryonic mesoderm with the
extraembryonic mesoderm.
The chorion consists
of two parts, smooth (laeve) and villous. The villous chorion attaches to the
decidua basalis of the placenta. There is no tunica
adventitia in the umbilical vessels.
·
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