Embryology

·         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.

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·         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.

·         Notochord develops in the 3^rd week of gestation

·         Brain develops in the 8^th week of gestation

·         12th week of gestation the other organ systems begin to form

·         Lung develops in 1st trimester.

·         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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