Chapter 112. Hematopoiesis and General Development of The Circulatory System - Review of Medical Embryology Book

THE YOLK SAC CAPILLARIES form a network which is drained by the vitelline veins. The latter flow directly into the venous sinus of the heart, forming first an anastomotic network around the duodenum, and then crossing the septum transversum (described later)
THE HEPATIC PRIMORDIUM, of entodermal origin, begins to invade the septum transversum in the middle of week 3 at about the same time that vascular primordia appear
THE HEPATIC EPITHELIAL CORDS proliferate and surround the vitelline veins and fragment them into many sinusoidal capillaries. Extension of this proliferation to the entire septum transversum carries the same process to the umbilical veins
THE HEPATIC CELLS or hepatocytes are next arranged into cords which surround the sinusoidal capillaries.
HEPATIC HEMATOPOIESIS begins during month 2 and attains its maximum in month 3. It then decreases and ceases about month 7 when it is assumed by the bone marrow, which has already become' functional beginning with month 4

The megaloblasts (primitive nucleated red cells) are replaced by erythroblasts (normoblasts) and finally by mature erythrocytes, without nuclei

PATHOLOGY: in fetal-maternal blood incompatibility, where there is immunization of the mother against the red blood cells of her fetus, the maternal antibodies destroy the fetal red cells. The fetus reacts against the anemia by an intense erythropoiesis.

The liver retains its hematopoietic function beyond the usual month 7, and the red cells appear in the circulation even before they are completely mature
Thus, the presence of erythroblasts in the blood of the newborn is one characteristic of this form of hemolytic anemia

INTRODUCTION: the many important rearrangements which the circulatory system undergoes during its development and the complex modifications of the general vascular plan are usually related to function. Thus, the organs of nutrition and excretion which are necessary for survival have a great deal of metabolic activity and show priority in development. The transition from fetal life to that after birth are not only marked by physiologic changes but also show marked circulatory modifications
THE ESSENTIAL STAGES OF CIRCULATORY DEVELOPMENT

The vitelline stage: where the embryo lives on its own small reserves, from week 3 to the beginning of month 2

The vitelline circulation of the yolk sac is predominant, and the primitive intraembryonic circulation and allantoic circulation are first forming

The placental stage: an intermediary organ, the placenta, develops between the fetus and the mother

The vitelline circulation disappears at the end of month 2, but from its only vestige arise the superior mesenteric vessels
The allantoic circulation becomes placental and is predominant after day 30
The umbilical vessels accomplish the change of circulation and become responsible for oxygenation, nutrition, and filtration
In addition, the above intraembryonic circulation development is marked by special enlargement of the liver, the brain, and the mesonephros

The neonatal stage: the organism assumes its postbirth or postnatal autonomous existenc Placental circulation is interrupte The placental circulation now is taken over by specialized organs

The lungs begin to function for oxygenation
The metanephros, or adult kidney, begins to function at about month 3 of fetal life and then takes over all renal function and filtration

The mesenteric network, which drains the digestive tract, takes over nutritional responsibilities