Birds and Mammals: Early Development and Axis Formation Lange BIOL 370 – Developmental Biology Topic #11

The amniotic egg…. eggs of those vertebrates that have an amnion ( a water sac). This group includes birds, reptiles and mammals. Amniotic eggs contain membranes that are designed to allow survival on land: amnion – the membrane that allows the embryo to float in a fluid environment to avoid desiccation yolk sac – enables nutrient uptake and development of the circulatory system chorion – contains blood vessels that exchange gasses with the external environment.

Allantois - helps the embryo exchange gases and handle liquid waste.

Figure 8.1 Discoidal meroblastic cleavage in a chick egg area pellucida - the pellucid (translucently clear) central area that immediately surrounds a vertebrate embryo (as of a bird) formed by discoidal cleavage. area opaca - the opaque region of yolk

In the following slide, you will see midsagital sections of chick blastoderm tail region and then dorsal and ventral views of the whole organism. Look for the Koller’s Sickle (in avian gastrulation, Koller's sickle is a local thickening of cells that acts as a margin separating sheets of cells) Look for the Posterior Marginal Zone (PMZ) which can induce a primitive streak Look for Hensen's Node which acts as an organizer for gastrulation in vertebrates. This node starts at the region where the head will form and regresses thereafter The term Hensen’s Node is used mainly in birds. In mammals, it is usually called the primitive knot (or primitive node) is the organizer for gastrulation in vertebrates.

Figure 8.2 Formation of the chick blastoderm

Figure 8.3 Cell movements of the primitive streak and fate map of the chick embryo A representation of the typical development of the chick across the first 24 – 26 hours of normal development. Additionally, there is a display of the fate map seen in the embryo at the 18 – 20 hour stage which is when the PRIMITIVE STREAK is seen. 12 – 14 hours 15 – 17 hours 18 – 20 hours 20 – 22 hours 23 – 25 hours Four somite stage

Figure 8.3 Cell movements of the primitive streak and fate map of the chick embryo (Part 1)

Figure 8.3 Cell movements of the primitive streak and fate map of the chick embryo (Part 2) Notice here how the Primitive Groove and the Hensen’s Node are both visible at this stage. The primitive groove appears on the surface of the primitive streak, and the anterior end of this groove communicates (via an aperture called the blastophore) with the yolk- sac.

Figure 8.3 Cell movements of the primitive streak and fate map of the chick embryo (Part 3) Notice how the Hensen’s node regresses as the head process forms.

Figure 8.3 Cell movements of the primitive streak and fate map of the chick embryo (Part 4)

Figure 8.4 Migration of endodermal and mesodermal cells through the primitive streak (Part 2) Significant migration occurs through the primitive streak for a variety of cells. In this slide, focus on the hypoblast cells. These cells are regulatory and when absent would result in multiple primitive streaks forming. The primitive streak is a structure that forms in the blastula during the early stages of avian, reptilian and mammalian embryonic development. The presence of the primitive streak will establish bilateral symmetry.

The absence of hypoblast results in multiple primitive streaks in chicken embryos.

Figure 8.6 Hox gene activation begins when the mesodermal precursor cells are still in the epiblast Hox genes (homeobox genes) are a group of 1 or a few related genes that control the development of the body plan of the embryo along the anterior-posterior (head-tail) axis.

Figure 8.7 Chick gastrulation 24–28 hours after fertilization

Figure 8.7 Chick gastrulation 24–28 hours after fertilization (Part 1) Hensen’s Node – specific name give to the primitive knot in birds. The primitive knot (or primitive node) is the organizer for gastrulation in vertebrates. Somites - In vertebrates, somites give rise to skeletal muscle, cartilage, tendons, endothelial cells, and dermis. The term “metamere” is sometimes used alternatively for the term "somite“.

Figure 8.7 Chick gastrulation 24–28 hours after fertilization (Part 2)

Figure 8.8 Specification of the chick anterior-posterior axis by gravity Notice how this affects the blastodisc position, and how this then is portrayed in the egg that you break open in lab.

Figure 8.10 Induction of a new embryo by transplantation of Hensen’s node The Hensen’s node induces a new embryo with transplantation.

Figure 8.15 Development of a human embryo from fertilization to implantation

Figure 8.16 Comparison of early cleavage in (A) echinoderms and amphibians and (B) mammals

Figure 8.22 Tissue formation in the early mammalian embryo

Figure 8.22 Tissue formation in the early mammalian embryo (Part 2) Trophoblasts are cells forming the outer layer of a blastocyst, which provide nutrients to the embryo and develop into a large part of the placenta. Decidua is the term for the uterine lining (endometrium) during a pregnancy, which forms the maternal part of the placenta. It is formed under the influence of progesterone and forms highly characteristic cells. The placenta is an organ that connects the developing fetus to the uterine wall to allow nutrient uptake, waste elimination, and gas exchange via the mother's blood supply.

Figure 8.22 Tissue formation in the early mammalian embryo (Part 3)

Figure 8.22 Tissue formation in the early mammalian embryo (Part 4) The cytotrophoblast (also called the Layer of Langhans) is the inner layer of the trophoblast. It is interior to the syncytiotrophoblast and external to the wall of the blastocyst in a developing embryo. The cytotrophoblast is a trophoblastic stem cell because the layer surrounding the blastocyst remains while daughter cells differentiate and proliferate to function in multiple roles. Syncytiotrophoblast is the epithelial covering of the embryonic placental villi, which invades the wall of the uterus to establish nutrient circulation between the embryo and the mother.

Figure 8.22 Tissue formation in the early mammalian embryo (Part 5)

Figure 8.24 (A) Human embryo and placenta after 50 days of gestation. (B) Relationship of the chorionic villi to the maternal blood supply in the primate uterus

Figure 8.25 The timing of human monozygotic twinning with relation to extraembryonic membranes

Figure 8.36 Early development of four vertebrate classes

End.