Gargi College Subject: Comparative Anatomy and Developmental Biology Class: Life Sciences 2 SEM Teacher: Dr Swati Bajaj Date: 17/3/2020 Time: 2:00 pm to 3:00 pm

EARLY EMBRYONIC DEVELOPMENT TILL GASTRULATION (HUMANS)

CLEAVAGE:

Cleavage in mammalian eggs are among the slowest in the animal kingdom, taking place some 12-24 hours apart. The first cleavage occurs along the journey of the embryo from oviduct toward the uterus. Several features distinguish mammalian cleavage: 1. Rotational cleavage: the first cleavage is normal meridional division; however, in the second cleavage, one of the two blastomeres divides meridionally and the other divides equatorially. 2. Mammalian blastomeres do not all divide at the same time. Thus the embryo frequently contains odd numbers of cells. 3. The mammalian genome is activated during early cleavage and zygotically transcribed proteins are necessary for cleavage and development. (In humans, the zygotic genes are activated around 8 cell stage) 4. Compaction: Until the eight-cell stage, they form a loosely arranged clump. Following the third cleavage, cell adhesion proteins such as E-cadherin become expressed, and the blastomeres huddle together and form a compact ball of cells.

Blatocyst: The descendents of the large group of external cells of Morula become trophoblast (trophoblast produce no embryonic structure but rather form tissues of chorion, extraembryonic membrane and portion of placenta) whereas the small group internal cells give rise to Inner Cell mass (ICM), (which will give rise to embryo proper). During the process of cavitation, the trophoblast cells secrete fluid into the Morula to create blastocoel. As the blastocoel expands, the inner cell mass become positioned on one side of the ring of trophoblast cells, resulting in the distinctive mammalian blastocyst.

[ Prior to blastocyst formation, each embryonic blastomere expresses both the Cdx2 and Oct4 transcription factors and appears to be capable of becoming either ICM or trophoblast. However, once the decision to become either trophoblast or ICM is made, the cell expresses a set of genes specific to each region. The pluripotency of the ICM is maintained by core of three transcription factors, Oct4, Sox2 and Nanog. Only the trophoblast cells synthesize the transcription factor Cdx2]

Totipotent: capable of everything (earliest blastomeres) Pluripotent: capable of many things (ICM)

Cells of the inner cell mass or embryoblast also differentiate into two layers:

(1) a layer of small cuboidal cells adjacent to the blastocyst cavity, known as the hypoblast layer (primitive endoderm). The primitive endoderm will form the yolk sac of the embryo. It is an extraembryonic layer and doesnot provide many cells to the actual embryo.

(2) a layer of high columnar cells adjacent to the amniotic cavity, the epiblast layer.

The epiblast and primitive endoderm together form bilaminar germ disc.

The primitive endoderm cells expand to line the blastocoel cavity, where they give rise to yolk sac. The primitive endodermal cells contacting the epiblast are the visceral endoderm, while those yolk sac cells contacting the trophoblast are the parietal endoderm.

The epiblast cell layer is split by small clefts that eventually separate the embryonic epiblast from other epiblast cells that form the amnion. Once the amnion is completed, it is filled with the amniotic fluid. The embryonic epiblast contain all the cells that will generate the actual embryo.

9-day human blastocyst

13 day human blastocyst

Gastrulation:

The most characteristic event occurring during the third week of gestation is gastrulation, the process that establishes all three germ layers (ectoderm, mesoderm, and endoderm) in the embryo. Gastrulation begins at the posterior end of the embryo and this is where the cells of the node arise. It begins with formation of the primitive streak on the surface of the epiblast. In a 15- to 16-day embryo, it is clearly visible as a narrow groove with slightly bulging regions on either side. The cephalic end of the streak, the primitive node (or Hensen’s node or organizer), consists of a slightly elevated area surrounding the small primitive pit (Fig. 5.2). Cells of the epiblast migrate toward the primitive streak (Fig. 5.2). Upon arrival in the region of the streak, they become flask-shaped, detach from the epiblast, and slip beneath it. This inward movement is known as invagination. Cell migration and specification are controlled by fibroblast growth factor 8 (FGF8), which is synthesized by streak cells themselves. This growth factor controls cell movement by downregulating E-cadherin, a protein that normally binds epiblast cells together. Once the cells have invaginated, some displace the hypoblast, creating the embryonic endoderm, and others come to lie between the epiblast and newly created endoderm to form mesoderm. Cells remaining in the epiblast then form ectoderm. Those cells arising from from the node give rise to notochord. Thus, the epiblast, through the process of gastrulation, is the source of all of the germ layers and cells in these layers will give rise to all of the tissues and organs in the embryo.

The primitive streak is a transient structure. Having given rise to the mesodermal layer and the notochordal rudiment, the primitive streak start shrinking.

Implantation site at the end of the second week

B. Cross section through the cranial region of the streak at 15 days showing invagination of epiblast cells.The first cells to move inward displace the hypoblast to create the definitive endoderm. Once definitive endoderm is established, inwardly moving epiblast forms mesoderm. C. Dorsal view of an embryo showing the primitive node and streak and a cross section through the streak.The view is similar to the illustration in B; arrow, detaching epiblast cells in the primitive streak.

Fate map during gastrulation:

Regions of epiblast that migrate and ingress through the primitive streak have been mapped, and their ultimate fates have been determined. Cells that ingress through the cranial region of node become prechordal plate and notochord. Those migrating at the lateral edges of the node and from the cranial end of the streak become paraxial mesoderm. Cells migrating through the midstreak region become intermediate mesoderm. Those migrating through the more caudal part of the streak form lateral plate mesoderm.