Embryology Lecture 2 random set not frmo our class

Gastrulation

Process by which the bilaminar embryo is transformed into one that has 3 germ layers (ectoderm, mesoderm and endoderm), The first sign of gastrulation is the formation of a primitive streak

Ectoderm

Will give rise to the skin and nervous system

Mesoderm

Will give rise to bones, muscles and mesenteries, second wave of cells migrating through the primitive streak and will place themselves between the endoderm and the epiblast, cells are loosely packed (forming a mesenchyme)

Endoderm

First cells to migrate through the primitive streak move first downwards towards the hypoblast, and then sideways underneath the epiblast; These cells push the hypoblast towards the sides of the embryo, towards the inner wall of the blastocyst cavity, i.e they replace the hypoblast in the embryonic disc; Most ventral of 3 germ layers; forms as the first wave of migrating cells during gastrulation form a flat sheet that occupies the roof of the yolk sac. As the embryo folds and bends this flat sheet becomes transformed into a tubular gut. As this happens, the gut becomes progressively distinguishable from the yolk sac. The embryonic gut and the yolk sac eventually remain connected at the level of the midgut.

Germ Layer

Each of the three layers of cells (ectoderm, mesoderm, and endoderm) that are formed in the early embryo.

Primitive Streak

Defines both the midline and the caudal end of the embryo, begins to form when cells in the epiblast start migrating first towards the midline on the caudal end of the epiblast and then 'downwards' towards the hypoblast; an elongated band of cells that forms along the axis of a developing fertilized egg early in gastrulation and that is considered a forerunner of the neural tube and nervous system

Primitive Node

Small, well-defined accumulation of cells at the anterior end of the primitive streak, determines and patterns the anterior-posterior axis of the embryo by directing the development of the chordamesoderm

Convergence Extension

Process by which the tissue of an embryo is restructured to converge (narrow) along one axis and extend (elongate) along a perpendicular axis by cellular movement; the anteroposterior axis (the axis drawn between the head and tail end of an embryo) becomes longer as the lateral tissues (those that make up the left and right sides of the embryo) move in towards the dorsal midline (the middle of the back of the animal); This process plays a crucial role in shaping the body plan; Occurs during gastrulation, neurulation, axis elongation, and organogenesis; Process by which embryonic tissue narrows in one axis and extends in the perpendicular direction

Organising Centres

Regions that contribute to establishing the 'head' and 'tail' ends of the embryo. They secrete molecules that act at a distance which influence the differentiation of different body parts

Morphogen

A signalling molecule that acts directly on cells to produce specific cellular responses depending on its local concentration; diffuse through tissues of an embryo so conc gradients are set up

Midline

A median line or plane of bilateral symmetry;

Rostral or Cranial

Head end of the embryo

Caudal

Tail end of embryo

Dorsal

Of, on, or relating to the upper side or back of an animal, plant, or organ

Ventral

Of, on, or relating to the underside of an animal or plant; abdominal

Tail Organiser

The primitive node of the primitive streak

Anterior Visceral Endoderm (AVE)

The head organiser. Is derived from the hypoblast

What is the difference between the blastocoel and the yolk sac?

Yolk sac is lined by cells derived from the hypoblast, in humans is small and relatively empty, Nourishes embryo in 2nd and 3rd week of development, provides blood cells to the embryo, Gives rise to germ line that will migrate into the gonads; Blastocoel can be described as the first cell cavity formed as the embryo enlarges. It is essential for later gastrulation. It is the cavity formed when the fluid moves through the trophoblast into the centre of the morula

What is the significance of the primitive streak?

Defines both the midline and the caudal end of the embryo, considered a forerunner of the neural tube and nervous system

What are the main differences between the formation of the amnion amniotic cavity and the formation of the yolk sac?

Formation of Amniotic Cavity:The roof of the amniotic cavity is formed by squamous cells derived from the epiblast (The epiblast migrates between the epiblastic disc and trophoblast) that form a membrane called the amnion around the cavity. The floor of the amniotic cavity is formed by the epiblast proper (which later on transforms to ectoderm). Eventually, as the embryo grows and folds, the amnion will surround the entire embryo.

Formation of yolk sac: Beneath the hypoblast lies the original blastocoel. Cells derived from the hypoblast will begin to migrate until they will eventually cover the inside surface of the blastocoel at which time this cavity begins to be called the yolk sac. The yolk sac starts forming during the second week of the embryonic development, at the same time of the shaping of the amniotic sac. The hypoblast starts proliferating laterally and descending. In the meantime Heuser's membrane (combination of hypoblast and extracellular matrix), located on the opposite pole of the developing vesicle, starts its upward proliferation and meets the hypoblast. So the yolk sac is in essence a hacked cavity (the cells of the hypoblast will migrate out to fully cover the inside of the blastocoel and form the Yolk Sac, so is formed from the Blastocoel, a pre-existing cavity) - the amniotic cavity is a newly made one (From the transformation of some of the epiblast cells into squamous and therefore differentiating the top and the bottom which eventually forms a cavity)

What is the role of the node and the AVE in patterning the AP axis?

These organising centres work together to influence the differentiation of body parts by secreting morphogens and antagonists to create a gradient where each cell is subjected to different levels of morphogens that will influence the expression of hox genes which will determine where body parts are formed

Which of the following tissues arises from cells passing through the primitive streak?

A. Embryonic endoderm

Which layer of the bilaminar (two-layered) embryo gives rise to all of the embryonic tissue proper?

Epiblast

Cells of which germ layer are not present in the oropharyngeal membrane?

Mesoderm

Amnion

Innermost membrane that encloses the embryo (amniotic cavity) of a mammal, bird, or reptile; It fills with the amniotic fluid which causes the amnion to expand and become the amniotic sac which serves to provide a protective environment for the developing embryo; stems from the extraembryonic somatic mesoderm on the outer side and the extraembryonic ectoderm on the inner side

Amniotic Sac

Thin but tough transparent pair of membranes, which hold a developing embryo (and later fetus) until shortly before birth. The inner membrane, the amnion, contains the amniotic fluid and the fetus. The outer membrane, the chorion, contains the amnion and is part of the placenta; provide a liquid that surrounds and cushions the fetus. It allows the fetus to move freely within the walls of the uterus. Buoyancy is also provided.

Bucchopharyngeal membrane

Region where the crescentic masses of the ectoderm and endoderm come into direct contact with each other constitutes a thin membrane, which forms a septum between the primitive mouth and pharynx.

Cloacal Membrane

Membrane that covers the embryonic cloaca when still in the development of the urinary and reproductive organs; formed by ectoderm and endoderm coming into contact with each other

Allantois

It helps the embryo exchange gases and handle liquid waste. Webbed with blood vessels; extra-embryonic membrane of reptiles, birds, and mammals arising as a pouch, or sac, from the hindgut

Neural Tube

A hollow structure from which the brain and spinal cord form (pre-cursor for Central Nervous System); a tube formed by the closure of ectodermal tissue in the early vertebrate embryo

Non Neural Ectoderm

Skin, hair, fingernails

Notochord/axial mesoderm

A cartilaginous skeletal rod supporting the body in all embryonic animals; It is composed of cells derived from the mesoderm and defines the primitive axis of the embryo.

Somite/Paraxial Mesoderm

The area of mesoderm in the neurulating embryo that flanks and forms simultaneously with the neural tube. Forms somites

Somites

Bilaterally paired blocks of paraxial mesoderm that form along the head to tail axis of the developing embryo in segmented animals; give rise to the vertebrae of the spine, rib cage, (and part of the occipital bone); skeletal muscle, cartilage, tendons, and skin.

Parietal lateral plate mesoderm

Somatic layer of mesoderm; depends on a continuous layer with mesoderm that covers the amnion. Along with the visceral layer, it covers the intraembryonic cavity. This layer along with overlying ectoderm forms the lateral body wall folds.

Splachnic lateral plate mesoderm

Depends on a continuous layer that covers the yolk sac. Along with the Parietal layer, it covers the intraembryonic cavity. Forms the walls of the gut tube

Endoderm

The innermost layer of cells or tissue of an embryo in early development, or the parts derived from this, which include the lining of the gut, the gastrointestinal tract, the lungs, and associated structures

Coelom

Refers to the main body cavity in most multicellular animals and is positioned inside the body to surround and contain the digestive tract and other organs. Formation begins in the gastrula stage; Space that forms between the parietal and visceral lateral plate mesoderm sheets will eventually become this