Chapter 5 Gastrulation 2024 PDF

2024 Chapter 5 Chapter 5: Gastrulation; establishment of embryonic germ layers and early patterning Key terms and concepts: Axis specification, body axes E-Cadherin Cilia Me...

2024 Chapter 5 Chapter 5: Gastrulation; establishment of embryonic germ layers and early patterning Key terms and concepts: Axis specification, body axes E-Cadherin Cilia Mesenchyme, mesenchymal Pattern Formation Epithelial cells, epithelial, epithelium Blastocyst (blastula) Epithelial-mesenchymal transition (EMT) Hypoblast Notochord Inner cell mass Anterior/posterior, dorsal/ventral Trophoblast (Trophectoderm) Extra-embryonic tissues (membranes) Embryonic germ layers Prechordal plate Mesoderm Primitive node Endoderm Primitive streak Ectoderm Cloacal membrane Gastrula, gastrulation Buccopharyngeal membrane Learning objectives: By the end of this unit, you should be able to: 1. Name the three germ layers of the embryo established during gastrulation and give 2 examples of adult tissues that arise from each. 2. Describe what is meant by the term “axis specification” and describe an example of axis specification that occurs during gastrulation. 3. Know what embryonic germ layer gives rise to the notochord and what the role of the notochord is during early development. 4. Understand the basic process that establishes right/left body axis patterning and how cilia are involved. 5. Distinguish between mesoderm and mesenchyme. 6. Describe the Epithelial- mesenchymal transition in broad terms and name one molecule important in this transition 7. Explain why “pattern formation” is crucial in embryogenesis and what some consequences of abnormal patterning could be. The following interactive exercise making a 3D “origami embryo” is a great way to understand the 3D process of gastrulation. Make an “origami embryo” for yourself! http://origamiembryo.cba.arizona.edu/ 1 2024 Chapter 5 I. Gastrulation “It is not birth, marriage, or death, but gastrulation, which is truly the most important time in your life.” Lewis Wolpert, British embryologist (1986) A. During gastrulation, specific groups of cells with different futures redistribute themselves in layers (germ layers). Gastrulation is a time of cell sorting and movement. The embryonic body axes are established during gastrulation. a. Gastrulation is the stage of embryonic development during which the embryo establishes the 3 embryonic germ layers (endoderm, ectoderm, and mesoderm) that will give rise to all the adult tissue types. b. Patterning of the body axes is a key process that occurs during gastrulation. The cranial/caudal and left/right body axes are defined during gastrulation. The dorsoventral axis is defined a bit earlier. Patterning lays out the groundwork for morphogenesis (structure building). Morphogenesis is the processes by which the embryo changes shape and establishes the more mature configuration of tissues and structures. c. Just prior to gastrulation, the embryo consists of the inner cell mass and the trophoblast/trophectoderm. The inner cell mass consists of pleuripotent cells that can give rise to all the tissues of the embryo and many of the tissues of the extra-embryonic membranes (all with the exception of the trophoblast/trophectoderm, which has already differentiated in the blastocyst stage embryo). d. After gastrulation, the embryo will be a complex being with a distinct body shape and several distinct constituent tissue types. 1. First step of gastrulation: a. Scattered cells of the inner cell mass migrate to the under-surface of the embryonic disc, detach (delaminate) and arrange themselves in a sheet called the hypoblast (later called endoderm) which expands to line the interior of the blastocoel. The blastocoel can also be called the lumen of the yolk sac or primitive gut once it is lined by hypoblast. The cells of the hypoblast are epithelial in their arrangement, as are the trophoblast cells. Epithelial cells form sheets or tubes, have very little intercellular space between cells, rest on or are associated with a basement membrane, cover surfaces or line cavities, and do not contain blood vessels (see histology notes). Epithelial cells are tightly linked together by a variety of intercellular junctions and generally have prominent regional (for example apical vs. basilar) specialization or polarization. They have little extracellular matrix. 2 2024 Chapter 5 b. Within the two-layered gastrula, the cells of the disc that remain after delamination of the hypoblast make up the epiblast. All of the cells of the embryo will come from the epiblast, while the hypoblast and trophoblast will form extraembryonic membranes. A basement membrane forms between the hypoblast and the epiblast. The epiblast also organizes into an epithelium. Cadherins and organization of Cells into Tissues: Each type of differentiated cell has a different set of proteins on its surface, and these proteins have key roles in establishing and maintaining the structure of tissues and organs during development. Cadherins are cell adhesion molecules that are important in this process. The Cadherins are a gene family with many members. They are Calcium dependent adhesion molecules that mediate cell adhesion and form adherens junctions between cells. They hold cells together and bind the actin cytoskeleton of cells. The various Cadherin family members have different binding properties, and differential cadherin expression (both type and amount) plays a role in the spatial segregation of cell types. c. Seen from its dorsal surface, the embryonic disc is a plate of cells. These cells are arranged in an epithelium. Many cells of the proliferating epiblast migrate toward the midline at one end of the disc and, upon reaching it, slip downwards. Movement of these cells creates a groove, the primitive streak, bordered by ridges of piled up migrating cells. The cranial end of the streak is the primitive (Hensen's) node. For those of you who have previously 3 2024 Chapter 5 studied amphibian development, Hensen’s node is equivalent to the “organizer.” Crucial signals emanate from the node that pattern (establish the body axes) of the embryo. The process of pattern formation in the mammalian embryo is complex and poorly understood. d. As yet unknown cues determine the location of the primitive streak within the embryonic disc. However, once it is formed, the longitudinal axis of the embryo becomes easily visible. The end of the disc occupied by the primitive streak becomes the caudal end of the embryo. The establishment of the body axes is a turning point in embryogenesis as it is the first step in creating the 3D form of the embryo. 2. The first cells to migrate down through the streak insert themselves in the center of the hypoblast layer and will become the endoderm of the embryo (“definitive endoderm”). A second wave of migrating cells inserts themselves between the epiblast and hypoblast layers, spreading first cranially, then laterally as a sheet. These cells form the mesoderm of both the embryo and the extraembryonic membranes. The cells that move directly through the node will migrate cranially to become the mesoderm of the notochord (see below), prechordal plate, somites, head and heart. 3. Cell migration into the primitive streak: a. The cells of the epiblast comprise an epithelium and rest on a basement membrane along their basilar aspect. E-cadherin is one mediator of cell adhesion among the epiblast cells. b. In order to migrate through the primitive streak, migratory epiblast cells must lose many of their epithelial characteristics and become migratory. This type of transition is called an Epithelial to Mesenchymal Transition (EMT). EMT is an important process in many aspects of development. c. Mesenchyme (definition)- Mesenchyme is a histologic term for loosely arrayed tissue with lots of matrix among cells. d. The EMT of gastrulation at the primitive streak is characterized by local loss of the basement membrane at the streak to allow streak cell to migrate beneath the epiblast layer. e. Loss of E-cadherin by migratory streak cells later in the EMT is important in allowing these cells to migrate. Failure to lose E- 4 2024 Chapter 5 cadherin expression disrupts the migratory movements of gastrulating cells. f. Cells that undergo EMT during gastrulation but end up contributing to endoderm (an epithelial layer) will undergo a reciprocal Mesenchymal to epithelial (MET) transition to transform back into an epithelium from their migratory mesenchymal phenotype. Somites: When viewing embryos beyond the gastrulation stage, it is difficult not to notice the “chunks” that extend along cranial-caudal axis. These structures, called somites, are bilaterally paired blocks of mesoderm and will give rise to cells that form vertebrae, ribs, skeletal muscles, and dermis. Somites form at specific intervals (every 2 hours in mouse) and can be to identify the specific stage of development of the embryo. For now, that’s all you need to know but you will learn much more about somites in the chapter on axial and musculoskeletal development. 4. At the completion of gastrulation, the embryonic disc is composed of three layers (trilaminar germ disc) and the body axes of the embryo have been defined. Cells remaining in the epiblast are the ectodermal layer. The intermediate layer is mesoderm, and hypoblast is now referred to as endoderm. In the extra-embryonic region, it takes a while for the mesoderm to complete its migration between the endoderm and the trophoblast. 5. Formation of the Notochord: The notochord is a key mesodermal structure in the embryo. The presence of a notochord is a defining characteristic of the Chordates (includes all vertebrates, but also sea squirts and some others). Some cells that ingress through the node and migrate cranially form the prechordal plate, a block of mesoderm important in patterning the head of the embryo. Subsequently, some of the mesodermal cells that migrate cranially from Hensen’s node form a rod-shaped mesodermal structure called the notochord that runs along the midline longitudinal axis just deep to the ectoderm and just caudal to the prechordal plate. Mesoderm, therefore, completely separates ectoderm from endoderm except at the far ends of the embryo at the buccopharyngeal membrane (prochordal plate) and the cloacal membrane. 5 2024 Chapter 5 a. Prechordal plate- The prechordal plate is an aggregate of mesoderm located just cranial of the cranial end of the notochord (hence the name PRE-chordal plate) in the future head region of the embryo. The prechordal plate lies just caudal to the buccopharyngeal membrane (also known as the PRO-chordal plate; see below). The Prechordal plate plays important roles in the patterning and formation of head structures in the embryo. b. Buccopharyngeal membrane (also referred to as the prochordal plate) i. The buccopharyngeal membrane/prochordal plate is an area of fusion of ectoderm and endoderm towards the cranial end of the embryonic disc. Believe it or not, the buccopharyngeal membrane will eventually become a membrane that separates the lumen of the digestive tube from the outside world at the oral cavity! Wait and see! ii. Mesodermal cells also migrate rostrally around the buccopharyngeal membrane and meet each other, forming the cardiogenic plate, important to heart formation. At this point, it doesn't look like a good place for the heart, does it? c. Cloacal membrane i. The fusion of ectoderm and endoderm forming the cloacal membrane establishes the caudal boundary of the embryo. The membrane formed here will separate the lumen of the digestive tube from the outside world at the anus. Obviously, both membranes normally degenerate before birth! ii. The notochord and prechordal plate form a midline mesodermal axis, which will induce formation of the central nervous system (Chapter 7) from the overlying ectoderm and will orient formation of the axial skeleton. It extends from the prechordal plate, just caudal to the buccopharyngeal membrane (prochordal plate), to the primitive node. iii. As time goes on, the primitive streak shortens until it finally disappears at the caudal end of the embryo. As the primitive node of the streak moves caudally during shortening, it lays down notochord and somites behind it, increasing the total length of these axial structures. Therefore, gastrulation is still occurring at the caudal end of the embryo after it has been completed at the cranial end. 6 2024 Chapter 5 II. Embryonic Period A. Completion of gastrulation marks the end of the "pre-embryonic period." During the embryonic period, each of the three germ layers begins to give rise to specific tissues and organs. By the end of this time, the major organ systems have been established. During the fetal period, these organs primarily grow and mature. Most of the semester will be spent discussing the events of the embryonic period. The following is a brief overview and "look ahead" at the tissues and organs formed by each of the three germ layers. We will discuss them in detail later. B. Ectodermal derivatives- In general, the ectodermal germ layer gives rise to those organs and structures that maintain contact with the outside world. 1. Central Nervous System 2. Peripheral Nervous System 3. Sensory Epithelium Of The Ear, Nose And Eye 4. Epidermis And Its Derivatives 5. Neural Crest Cells (These contribute to many structures including the peripheral nervous system and some mesenchyme of the head and heart). We will discuss these soon! C. Mesodermal Derivatives 1. Connective Tissues 2. Muscle, Bone, Cartilage 3. Vascular System 4. Most of Urogenital System (Including the kidney, the stromal elements of the gonads, and much of the “plumbing” of the embryo) 5. Spleen; Adrenal Glands 6. Mesoderm vs. Mesenchyme - "Mesoderm" refers to one of the three germ layers; “mesenchyme" is a histological description for very loosely arranged tissue with lots of matrix among the cells. Not all mesenchyme is derived from mesoderm. And mesoderm does not always become mesenchyme. D. Endodermal Derivatives 1. A major derivative of the endodermal layer is the epithelial lining of the gastrointestinal tract. 2. Epithelial lining of the respiratory tract, auditory tube and tympanic cavity; urinary bladder and urethra 3. Parenchyma (functional cells) of several glandular organs (pancreas, liver, etc.) 7