Embryology Problem Based Questions (Langman - Q10)

ANATOMY AIIMS, GROSS ANATOMY, EMBRYOLOGY, NEUROANATOMY, MICROANATOMY, APPLIED/ CLINICAL ANATOMY

The second week of development is known as 2’s. Formation of what structures support this statement?

Answer:

At the beginning of the second week of development, the blastocyst has already implanted into the endometrium of the uterus. The inner cell mass (ICM) of the blastocyst divides into two layers, the epiblast and the hypoblast, forming the bilaminar embryonic disc.

The epiblast gives rise to the three primary germ layers (ectoderm, mesoderm, and endoderm) and the hypoblast contributes to the formation of the yolk sac.

The amniotic cavity begins to form during the second week of development as the epiblast cells at the cranial (head) and caudal (tail) poles of the embryonic disc proliferate and migrate towards the center. These cells form a membrane called the amniotic membrane, which expands and eventually fuses to form the amniotic cavity. The amniotic cavity is lined by the amnioblasts, which are derived from the epiblast.

The amniotic cavity is important for protecting the developing embryo from mechanical shocks and temperature changes, and also allows for free movement and growth of the embryo.

The yolk sac cavity forms from the hypoblast cells that line the bilaminar embryonic disc. These cells migrate and form a layer of cells beneath the embryonic disc, creating a space between the hypoblast layer and the exocoelomic cavity called the primary yolk sac.

As the embryonic disc continues to grow and differentiate, the yolk sac is gradually incorporated into the developing embryo. The hypoblast cells of the yolk sac give rise to a variety of structures, including the gut, the respiratory tract, and the germ cells.

The chorionic sacs also begin to develop during the second week of development. The chorion is formed by the trophoblast, which surrounds the developing embryo and gives rise to the fetal portion of the placenta. The chorionic sacs are initially bilateral, but eventually fuse to form a single chorionic sac.

The chorionic sacs are important for the exchange of gases and nutrients between the embryo and the mother, and also play a role in protecting the embryo from the mother's immune system.

Overall, the "week of twos" is a critical time for the establishment of the bilateral body plan and the formation of the embryonic germ layers, which will go on to form all of the organs and tissues of the body. The development of the amniotic cavity, yolk sac, and chorionic sacs are also crucial for the growth and protection of the developing embryo.

The extraembryonic mesoderm is a layer of cells that surrounds the bilaminar embryonic disc during the second week of development, and it plays a critical role in supporting the developing embryo.

The extraembryonic mesoderm arises from the epiblast cells that migrate towards the outer edge of the bilaminar embryonic disc. These cells form a layer of mesoderm that surrounds the amniotic cavity, yolk sac, and exocoelomic cavity. The extraembryonic mesoderm is divided into two layers, the somatic (parietal) layer and the splanchnic (visceral) layer, which are separated by a space called the extraembryonic coelom.

The somatic layer of the extraembryonic mesoderm is in contact with the cytotrophoblast layer of the chorionic sacs and eventually gives rise to the fetal part of the placenta. The splanchnic layer is in contact with the yolk sac and will eventually form the blood vessels that supply the yolk sac and embryo.

The extraembryonic mesoderm also plays a critical role in the formation of the umbilical cord, which connects the developing embryo to the placenta. As the connecting stalk elongates, it incorporates the extraembryonic mesoderm and the yolk sac into its structure. The extraembryonic mesoderm will eventually differentiate into the umbilical cord, which contains blood vessels that transport nutrients, gases, and wastes between the embryo and the placenta.

Overall, the extraembryonic mesoderm is an essential tissue that provides mechanical and nutritional support for the developing embryo during the second week of development, and its contribution is crucial for the proper growth and development of the embryo.

Reference:

Sadler, T. W. (2019). Langman's medical embryology. Wolters Kluwer.