Chick Development Talk 3

Remember this?

Intermediate mesoderm • Our kidneys constitute the major foundation of our philosophical freedom. Only

because they work the way they do has it become possible for us to have bone, muscles, glands, and brains. – Homer Smith 1953

• Mammalian kidney development progresses through three major stages. The first two stages are transient; only the third and last persists as a functional kidney.

• Pronephric duct arises in the intermediate mesoderm just ventral to the anterior somites – anterior region of the duct induces the adjacent mesenchyme to form the pronephros - Fish and Amphibians – degrades in amniotes

• In amniotes As the pronephric tubules degenerate, the middle portion of the nephric duct induces a new set of kidney tubules in the adjacent mesenchyme. This set of tubules constitutes the mesonephros, or mesonephric kidney – some mammals

• Metanephrogenic mesenchyme is committed and forms in the posterior regions of the intermediate mesoderm, where it induces the formation of a branch from each of the paired nephric ducts. These epithelial branches are called the ureteric buds. The buds separate, enter metanephrogenic messenchyme, induce differentiation into nephrons.

Graphical representation

Lateral plate mesoderm Each plate splits horizontally into two layers. The dorsal layer is the somatic (parietal)

mesoderm, which underlies the ectoderm and, together with the ectoderm, forms the somatopleure. The ventral layer is the splanchnic (visceral) mesoderm, which overlies the endoderm and, together with the endoderm, forms the splanchnopleure

Heart development • Some heart progenitor cells migrate together through the streak and form two groups of

cells in the lateral plate mesoderm, at the level of the node. These are already defined cells (cardiogenic mesoderm, or heart field)

• The progenitors of the inflow tract (the left ventricle and the atria, which receive the blood) are located most laterally, while the outflow tract precursors (the conus arteriosus and truncus arteriosus, which become the base of the aorta and pulmonary arteries, and the right ventricle) are located medially, closest to the primitive streak.

Cardiac precursor cell migration 1 • When the chick embryo is 18-20 hours old, the presumptive heart cells move

anteriorly between the ectoderm and endoderm toward the middle of the embryo, remaining in close contact with the endodermal surface

Cardiac precursor cell migration 2 • When these cells reach the lateral walls of the anterior gut tube, migration ceases.

The directionality of this migration appears to be provided by the foregut endoderm. If the cardiac region endoderm is rotated with respect to the rest of the embryo, migration of the cardiogenic mesoderm cells is reversed.

Cardiac precursor cell migration 3 • In the chick, the fields are brought together around the 7-somite stage, when the

foregut is formed by the inward folding of the splanchnopleure • This movement places the two cardiac tubes together.

Cardiac precursor cell migration 4 • The two endocardial tubes lie within the common tube for a short time, but

eventually these two tubes also fuse. • The bilateral origin of the heart can be demonstrated by surgically preventing the

merger of the lateral plate mesoderm. This manipulation results in a condition called cardia bifida, in which two separate hearts form, one on each side of the body

The Extraembryonic Membranes • In reptiles, birds, and mammals, embryonic development took a new

evolutionary direction—the amniote egg • To cope with the challenges of terrestrial development, the amniote

embryo produces four sets of extraembryonic membranes to mediate between it and the environment.

• In developing amniotes, there initially is no distinction between the embryonic and extraembryonic domains. However, as the body of the embryo takes shape, the epithelia at the border between the embryo and the extraembryonic domain divide unequally to create body folds that isolate the embryo from the yolk and delineate which areas are to be embryonic and which extraembryonic

Step 1 & 2

Step 3 & 4

Detailed caudal part

Placentation 1

Placentation 2

Placentation 3

Classification – shape and contact points • Diffuse: Almost the entire surface of the allantochorion is involved in formation of

the placenta. Seen in horses and pigs. • Cotyledonary: Multiple, discrete areas of attachment called cotyledons are formed

by interaction of patches of allantochorion with endometrium. The fetal portions of this type of placenta are called cotyledons, the maternal contact sites (caruncles), and the cotyledon-caruncle complex a placentome. This type of placentation is observed in ruminants.

• Zonary: The placenta takes the form of a complete or incomplete band of tissue surrounding the fetus. Seen in carnivores like dogs and cats, seals, bears, and elephants.

• Discoid: A single placenta is formed and is discoid in shape. Seen in primates and rodents.

Shape and contact

Classification – layers between fetal and maternal blood

Fetal extraembryonic membranes • Endothelium lining allantoic capillaries • Connective tissue (chorioallantoic

mesoderm) • Chorionic epithelium (fetal membranes

derived from trophoblast)

Maternal extraembryonic membranes • Endothelium lining endometrial blood

vessels • Connective tissue of the endometrium • Endometrial epithelial cells

Fetal and maternal connection