The primitive streak is a structure that forms in the blastocyst during the third week of development. It forms on the caudal end of the embryonic disc. The primitive streak determines the site of gastrulation and initiates the gastrulation. Gastrulation is the process in which three germ layers ectoderm, endoderm, and mesoderm are formed. The primitive streak extends through the midline of the embryonic disc and forms the cranial-caudal and left-right body axis. The cells of the primitive streak are pluripotent which means that they have the ability to transform into any types of cells
The knowledge of primitive streak, notochord, and a neural tube is very important for the students to understand various congenital anomalies such as teratomas, neural tube defects, and gastroschisis. The proper understanding of the intraembryonic mesoderm helps the student to understand the development of somites, urogenital system, cardiovascular system, and body cavities.
Formation of the primitive streak
Primitive streak formation depends on various signaling pathways. The primitive streak formation is a highly regulated process ensure by the signaling pathways. Various created factors and transcription factors are required for the process. In addition, the hypoblast also plays a major role in this process.
It is formed due to the proliferation and migration of cells of ectoderm on the superior surface of the embryonic disc in the midline.
Primitive streak becomes visible on the dorsal surface of the embryonic disc on day 15 of embryonic development as a narrow groove by a slight bulge on either side.
At the cranial end of the primitive streak, a rounded and elevated area is present called primitive node that surrounds the primitive pit.
Functions of the primitive streak
The primitive streak give rise to the following structures
1. Intraembryonic mesoderm
2. Septum transversum
4. Determines the future craniocaudal axis of the embryo.
5. Divide the embryo into right and left halves.
The fate of primitive streak
The primitive streak forms intraembryonic mesoderm by ingressions of its cell up to the end of the third week of development. Thereafter it progresses craniocaudally and completely disappears by the end of the fourth week of development or becomes an insignificant structure in the sacrococcygeal region of the embryo.
What is Sacrococcygeal teratoma?
Normally the streak undergoes degenerative changes and disappears by the end of the fourth week of development. If the cells of the primitive streak remain after the fourth week it gives rise to a large pre coccygeal tumor called sacrococcygeal teratoma.
It is an unusual tumor in newborn located at the base of the tail bone. Sacrococcygeal teratoma is more common in females than in males. The tumors can grow large but are not malignant. The sacrococcygeal teratoma can be cured by surgery but they cause trouble before birth.
It can be discovered by a blood test or a sonogram. Its diagnosis can be done by an ultrasound examination.
Its large size may cause an obstructed labor or even death of the baby. It is the most common tumor in the newborn and occurs at the ratio of 1:37000 in pregnancies.
What are the outcomes of sacrococcygeal teratoma?
Most babies with this condition do well with the surgical treatment done after birth. Babies that contain these tumors are passed through a surgical procedure in which these tumors along with the coccyx bone are removed after birth. The babies that pass from the surgical procedure can be expected to live normal lives.
Babies with large tumors that go inside the abdomen require more complex surgery but do well.
Studies show that sacrococcygeal tumors that are fluid-filled do not cause problems before birth but the sacrococcygeal tumors that contain mostly solid tissues with a lot of blood flow can cause adverse effects. The effects occur because of the heart of the fetus heart that supplies blood to the fetus body and to the vessels of the tumors which can be as big as the fetus. Due to the above reason, the heart works twice from its normal amount of work. Due to this work hydrops may develop.
If a hydrops develops then the fetus will not survive without intervention.
If the hydrops is very serious the mother is at high risk for maternal mirror syndrome.
Maternal mirror syndrome
Maternal mirror syndrome is a condition in which the mother condition mimics the suck fetus condition. In this condition, the mother may develop symptoms similar to preeclampsia and might include peripheral edema, hypertension, vomiting, pulmonary edema, and proteinuria.
How to find the severity of these tumors?
To understand the severity of these tumors it is important to collect information from various tests. The tests will help you to determine
1. The type of defect
2. The severity of the defect
3. Associated defect
The severity of these tumors depends on the size and blood flow to the tumors. Scientists, researchers, and anatomists show that small or medium-sized tumors without excessive blood flow will not cause a problem but they say that in this case, the fetus needs special care to ensure that the tumors do not enlarge or the blood flow does not increase.
If the tumors are large sized and receive excessive blood flow then it will cause heart failure.
To measure the size of the tumor and blood flow to the tumor doctors use sonography and echocardiography techniques.
What to do?
In most cases, the baby born with this condition survives and does well. As we know that this condition requires a surgical procedure after birth, therefore, it is necessary that arrangements should be made for infants to be born in a specialized hospital with pediatric surgery expertise.
It is another type of treatment which is offered to the female on whom there is evidence of heart failure in the fetus. Two possible fetal interventions for the fetus are minimally invasive surgery and open fetal surgery.
What will happen after birth?
Fetuses with this condition will be delivered in a well-equipped hospital in which pediatric surgeons are present. The fetus in which the tumors are larger than 10 cm will require C-section delivery. After the birth surgical removal of these tumors can be done.
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