Why are the third to eighth weeks of embryogenesis so important?
This 3–8 week period is the time of organogenesis when the anlage of each organ system is formed. Graph showing susceptible period of teratogenesis when birth defects can be induced. Originally it was thought that the third to eighth weeks after fertilization were the most sensitive time (blue).
What happens in 3rd week of development?
The third week is concerned with establishing left and right handedness, the craniocaudal axis, as well as the conversion of the bilaminar disc into a trilaminar embryo. Additionally, there is further specialization of the extraembryonic structures that will continue to support the embryo during the intrauterine phase.
What are the 3 stages of embryology?
The process of prenatal development occurs in three main stages. The first two weeks after conception are known as the germinal stage, the third through the eighth week is known as the embryonic period, and the time from the ninth week until birth is known as the fetal period.
What is the 3rd week of gestation?
During the third week of gestation the notochord sends signals to the overlying ectoderm, inducing it to become neuroectoderm. This results in a strip of neuronal stem cells that runs along the back of the fetus. This strip is called the neural plate, and it is the origin of the entire nervous system.
How does the blastocyst develop?
In humans, blastocyst formation begins about 5 days after fertilization when a fluid-filled cavity opens up in the morula, the early embryonic stage of a ball of 16 cells. About seven days after fertilization, the blastocyst undergoes implantation, embedding into the endometrium of the uterine wall.
What is the purpose of the notochord?
The notochord is the defining structure of the chordates, and has essential roles in vertebrate development. It serves as a source of midline signals that pattern surrounding tissues and as a major skeletal element of the developing embryo.
What is connecting stalk in embryology?
Anatomical terminology. The connecting stalk, or body stalk also known as the allantoic stalk is a yolk sac diverticulum, that by the third week of development connects the embryo to its shell of trophoblasts.
What happens if gastrulation goes wrong?
When gastrulation is COMPLETE, the primitive streak DISAPPEARS. what can happen if gastrulation goes wrong? conjoined twins result from partial splitting of the primitive node and streak.
What are the 4 stages of embryonic development?
Contents
- 1.1 Fertilization.
- 1.2 Cleavage.
- 1.3 Blastulation.
- 1.4 Implantation.
- 1.5 Embryonic disc.
What are the 6 stages of embryonic development in order?
The early stages of embryonic development, such as fertilization, cleavage, blastula formation, gastrulation, and neurulation, are crucial for ensuring the fitness of the organism. Fertilization is the process in which gametes (an egg and sperm) fuse to form a zygote.
Are you pregnant at 3 weeks?
Sperm met egg last week, and voila—you’ve made a baby! It’s so early that when you’re 3 weeks pregnant, you may have no idea that you’re actually pregnant. Conception just happened a few days ago, and there probably hasn’t been time for you to miss a period yet at week 3 of pregnancy.
What size is a fetus at 3 weeks?
At a Glance Your soon-to-be fetus is still a cluster of cells that are growing and multiplying. It’s about the size of a pinhead. It takes about four days for your fertilized egg — now dubbed a blastocyst — to reach your uterus and another two to three days to implant. Boy or girl?
When does the embryonic period begin and end?
The embryonic period, or period of organogenesis, occurs from the third to the eighth weeks of development. This is the time when each of the three germ layers, ectoderm, mesoderm, and endoderm, gives rise to several specific tissues and organs.
What happens in the eighth week of development?
The eighth week represents the last phase of the embryonic period. The fingers and toes are still connected with webbing. Through the apoptosis(physiologic, programmed death of cells) that occurs in the ectoderm as well as in the mesoderm that lies below it, this gradually disappears (interdigital necrosis zones [INZ]).
Where is the conceptus located in the embryo?
The embryonic disc and cavities are enclosed by the chorionic membrane within the chorionic cavity. If everything went according to plan, then the conceptus would have successfully embedded in the endometrium.
When does the primitive streak disappear in the embryo?
As mesoderm production declines, the primitive streak also begins to regress, becoming a small structure in the sacrococcygeal part of the embryo. Eventually, it completely disappears by the end of week 4. As the primitive node and streak are formed, invaginating mesodermal cells migrate cranially through the structure.