Riassunto esame Embriologia, Prof. Castagneti Lara, libro consigliato Carlson, Bruce M. Carlson

Development of Precardiac Mesoderm

Precardiac mesoderm is a zone of thickened mesoderm. As the mesoderm begins to split into the splanchnic and somatic layers, a cardiogenic plate is recognizable in the splanchnic mesoderm rostral to the oropharyngeal membrane. In this area, the space between the two layers of mesoderm is the forerunner of the pericardial cavity. The main layer of splanchnic mesoderm in the precardiac region thickens to become the myocardial primordium. Between this structure and the endoderm, isolated mesodermal vesicles appear, which soon fuse to form the tubular endocardial primordia. The endocardial primordia ultimately fuse and become the inner lining of the heart.

As the head of the embryo takes shape by lateral and ventral folding, the bilateral cardiac primordia come together in the midline ventral to the gut and fuse to form a primitive single tubular heart. This structure consists of an inner endocardial lining surrounded by a loose layer of specialized extracellular matrix that has historically.

been called cardiac jelly. Outside the cardiac jelly is the myocardium, which ultimately forms the muscular part of the heart. The outer lining of the heart, called the epicardium, and fibroblasts within the heart muscles are derived from the proepicardial primordium, which is located near the dorsal mesocardium. Cells migrating from the proepicardium cover the surface of the tubular heart. The entire tubular heart is located in the space known as the pericardial coelom. Shortly after the single tubular heart is formed, it begins to form a S-shaped loop. The heart is formed from a variety of cell lineages. Depending on their location within the cardiogenic mesoderm, N-cadherin-positive cells (express N-cadherin) go on to form either atrial or ventricular myocytes, whereas N-cadherin-negative cells form the endocardial lining and, later, cells of the endocardial cushions. Cells of the cardiac conduction system are derived from modified atrial and ventricular cardiac myocytes.

its caudal end, the endocardial tubes do not fuse, but rather extend toward the posterior part of the body as the venous inflow tract of the heart. Similarly, the endothelial tube leading out from the heart at its cranial end produces vascular arches that loop around the pharynx. Migrating neural crest cells form much of the walls of these vessels.

Extraembryonic mesoderm and the body stalk The posterior end of the embryo is connected with the trophoblast tissues (future placenta) by the mesodermal body stalk. As the embryo grows and a circulatory system becomes functional, blood vessels from the embryo grow through the body stalk to supply the placenta, and the body stalk itself becomes better defined as the umbilical cord. The extraembryonic mesoderm that lines the inner surface of the cytotrophoblast ultimately becomes the mesenchymal component of the placenta.

Around the 4th week, there is also the formation of the allantois, which is the extension of the yolk sac within the connecting stalk.

From the functional point of view, the allantois is not so important in humans, just mostly in the development of the urinary system. ⁴¹Letizia Hassan

Folding (development of endodermal germ layer)

Development of the endodermal germ layer continues with the transformation of the flat intraembryonic endodermal sheet into a tubular gut as a result of the lateral folding of the embryonic body and the ventral bending of the cranial and caudal ends of the embryo into a roughly C-shaped structure. A major morphological consequence of these folding processes is the sharp delineation of the yolk sac from the digestive tube.

The tremendous growth of the future brain region, results in the formation of the head fold and tail fold along the sagittal plane of the embryo. This process, along with concomitant lateral folding, results in the formation of the beginnings of the tubular foregut and hindgut.

This process also begins to delineate the yolk sac from the gut proper.

The sequence of steps in the

formation of the face and head structures, including the development of the nose, mouth, and jaw. The posterior end of the foregut gives rise to the respiratory system and the esophagus. The midgut undergoes a process called herniation, where it temporarily protrudes outside the body cavity. This allows for the formation of the umbilical cord, which connects the developing embryo to the placenta. Eventually, the midgut returns to the body cavity and undergoes further development to form the small intestine, large intestine, and other associated structures. The hindgut, located at the posterior end of the embryo, gives rise to the rectum and the lower part of the digestive tract. It also plays a role in the development of the urinary and reproductive systems. Overall, the formation of the gut is a complex process that involves the interaction of various signaling molecules and the coordinated growth and differentiation of different embryonic tissues.

Formation and specific morphology of the pharyngeal arches. There is a rapid bulging of the cephalic region, in conjunction with the constriction of the ventral region. In the early embryo, the cardiac primordia are located cephalic to the primitive gut. The forces that shape the tubular foregut cause the bilateral cardiac primordia to turn 180 degrees in the craniocaudal direction while the paired cardiac tubes are moving toward one another in the ventral midline.

A less exaggerated ventral folding also occurs in that region. Even as the earliest signs of the tail fold are taking shape, a tubular evagination of the hindgut extends into the mesoderm of the body stalk. This evagination is called the allantois. Caudal to the allantois is another ectodermal-endodermal bilayer called the cloacal plate, or proctodeal membrane. The proctodeal membrane, which ultimately breaks down, covers the cloaca, which in the early embryo represents a common outlet for the digestive and the urogenital systems.

The shallow depression outside the proctodeal membrane is called the proctodeum. As the gut becomes increasingly tubular, a series of local inductive interactions between the epithelium of the digestive tract and the surrounding mesenchyme initiates the formation of most of the major digestive and endocrine glands (e.g., thyroid gland, salivary glands, pancreas), the respiratory system, and the liver. In the region of the stomodeum, an induction between forebrain and stomodeal ectoderm initiates the formation of the anterior pituitary gland.

End of fourth week By the end of the fourth week of pregnancy, the embryo (about 4 mm long), has established the rudiments of most of the major organ systems except for the limbs (which are still absent) and the urogenital system (which has developed only the earliest traces of the embryonic kidneys). The embryo is C-shaped, with a prominent row of somites situated along either side of the neural tube. Except for the rudiments of the eyes and ears and the

oropharyngeal membrane, which is beginning to break down, the head is relatively featureless. In the cervical region, pharyngeal arches are prominent. The body stalk still occupies a significant part of the ventral body wall, and the heart and liver make prominent bulges in the contours of the ventral body wall. Posterior to the body stalk, the body tapers to a spiraled tail.

Another prominent but little understood feature of embryos of this age is a ring of thickened ectoderm, called the wolffian ridge, which encircles the lateral aspect of the body. Its function is not well understood, but it covers the primordia of many structures (e.g., nose, eye, inner ear, pharyngeal arches, limbs) that require tissue interactions for their early development.

Circulatory System: The embryo has a functioning two-chamber heart and a blood vascular system that consists of three separate circulatory arcs.

1. The intraembryonic circulatory arc is organized in a manner similar to that of a fish.

1. The ventral aortic outflow tract from the heart splits into a series of aortic arches passing around the pharynx through the pharyngeal arches and then collecting into a cephalically paired dorsal aorta that distributes blood throughout the body. A system of cardinal veins collects the blood and returns it to the heart via a common inflow tract. It is maintained after birth inside the body as vessels or ligaments.
2. The vitelline or omphalomesenteric arc is principally an extraembryonic circulatory loop that supplies the yolk sac.
3. Vessels associated with the allantois (extraembryonic). Umbilical vessels, which course through the body stalk and spread in an elaborate network in the placenta and chorionic tissues. This set of vessels represents the real lifeline between the embryo and mother.

Erythropoiesis in the embryo initially takes place in the yolk sac, the placenta (in less quantity), and the AGM (aortic gonadal mesoderm). Then, during fetal life,

The role is taken over by the liver, but nearly ends before birth. The spleen is important as well, producing RBC during the second trimester. During the second trimester, the production of RBC starts taking place in the bone marrow and will increase during the end of pregnancy and will continue after birth.

PLACENTA AND EXTRAEMBRYONIC MEMBRANES

To survive and grow, the embryo must maintain an essentially parasitic relationship with the body of the mother for acquiring oxygen and nutrients and eliminating wastes. It must also avoid being rejected as a foreign body by the immune system of its maternal host. These exacting requirements are met by the placenta and extraembryonic membranes that surround the embryo and serve as the interface between the embryo and the mother.

Derivatives

• Placenta of Chorion trophoblast
• Amnion - Ectodermal derivative, forms a protective fluid-filled capsule around the embryo
• Yolk sac - Endodermal derivative, no longer serves a primary nutritive function

1. innerAllantois: Endodermal derivative, helps remove cell mass wastes
2. Extraembryonic mesoderm: Bulk of umbilical cord, C.T. of extraembryonic membranes and the blood vessels that supply them.
3. Letizia Hassan: The amnion The amniotic membrane (or amnion) surrounds the body of the embryo like a fluid-filled balloon, thus allowing the embryo to be suspended in a liquid environment for the duration of pregnancy. The amniotic fluid serves as a buffer against mechanical injury to the fetus; in addition, it accommodates growth, allows normal fetal movements, and protects the fetus from adhesions.
4. THE MEMBRANE: consists of a single layer of extraembryonic ect