MESODERM FORMATION / SEGMENTATION - …

1 FORMATION / SEGMENTATIONREADING ASSIGNMENT: Larsen Human Embryology, 3rd edition , pp. 79-85, :During the third and fourth weeks of embryonic development the MESODERM is established as the 2ndgerm layer. The mesodermal cells are organized into 4 regions: the axial MESODERM of the prechordalplate and notochord, paraxial MESODERM , intermediate MESODERM and lateral plate MESODERM . Each ofthese undergoes some form of SEGMENTATION . The most evident and complete SEGMENTATION occurs in theparaxial trunk MESODERM , where each segment becomes an entirely separate somite. Much of theparaxial and lateral plate MESODERM develops into mesenchyme, an embryonic connective tissue.

2 Thederivatives of mesenchyme are connective tissue proper, cartilage, bone and blood. The cardiovascularand lymphatic systems are derived from MESODERM as well. Part of the paraxial MESODERM gives rise toall skeletal muscle cells. The intermediate MESODERM gives rise to most of the urogenital system. Part ofthe lateral plate MESODERM develops into the lining of the pericardial, pleural and peritoneal OBJECTIVE:At the conclusion of this segment of the course you should be able to:1. Define the 4 regions of MESODERM and describe their spatial relationships within the Define Describe for each of the 4 regions of MESODERM their further development during the embryonicperiod and their specific fate in the adult human Describe these structures: somitomeres, somites, dermatomes, myotomes and Discuss the steps in the process of Discuss the development and fate of the different segments of a Describe the effect of the specific combinatorial codes of Hox genes on the patterning of Discuss the meaning of the clock-wave gene expression pattern and its role in somite :Axial - Medial/midline.

3 Site of the prechordal plate and - Toward the tail end of the - Relating to the head or toward the Ann -Judith SilvermanDepartment of Anatomy & Cell BiologyTelephone: 305-3540E-mail: - The dorsolateral part of the somite that will contribute to the dermis of the - Segment of myotome that is dorsal to the body - Segment of myotome that is ventral to the body - A primordial embryonic connective tissue, consisting of stellate cells and a loose, fluidextracellular matrix. Can originate from the MESODERM or from neural - The middle of the 3 germ layers. It gives rise to all connective tissues (except in the headand neck regions), all body musculature, blood, cardiovascular and lymphatic systems, most of theurogenital system and the lining of pericardial, pleural and peritoneal - That part of the somite that gives rise to the skeletal muscle - Mesodermal rod-like structure, defining the cephalic-caudal body - Next to the body - Located rostral to the - Toward the snout end of the - A group of mesenchymal cells that emerge from the ventromedial part of a somite.

4 Thesecells migrate toward the notochord where they form the - Mesodermal segments, formed in pairs in the early embryonic paraxial MESODERM , along thelength of the - The development of a - Metameric pattern in the preaxial MESODERM that precedes the development of a the head region 7 somitomeres are formed, which do not go on to form : MESODERM FormationGastrulation is a series of cell movementsthat transforms the bilaminar germ disc (epiblastand hypoblast) into a 3 layered embryo (ectoderm, MESODERM , and endoderm, Fig. 5-1). Not only arethe 3 germ layers established but cells also becomecommitted to endodermal or mesodermal lineagesduring this process.

5 The critical factors that deter-mine the different fates of the mesodermal cellpopulations are: 1) the point of entrance of theepiblast cells into the primitive streak and 2) thedirection of their subsequent migration. Dependingon these 2 events, mesodermal cells can formtissues as varied as muscle, heart, kidney, or gastrulation the mesodermal sheet oneither side of the notochord is a connected layer ofundifferentiated mesenchymal cells (Fig. 5-1).During the third week, this undifferentiated meso-derm will begin to condense on both sides of thenotochord to form the 1) paraxial, 2) intermediate,and 3) lateral plate MESODERM (Fig. 5-2).

6 Startingon day 20 at what will be the base of the skull, theparaxial MESODERM (just lateral to the notochord;Fig. 5-1. Diagrammatic view with cross sections ofembryo during 5-2. Sections through a 17-dayembryo showing the differentiation ofthe MESODERM on either side of themidline. (A) Early on day 17, themesoderm has begun to differentiateinto paraxial, intermediate, and lateralplate MESODERM . (B) Sagittal cutawayshowing the rod-like condensations ofparaxial and intermediate dotted line marks the plane of thetwo transverse sections. (C) Later onday 17, the lateral plate begins tovacuolate to form the rudiment of theintraembryonic 5-3.)

7 Scanning electron micro-graph of an embryo with the ectodermremoved to show somites and, morecaudally, the paraxial MESODERM thathas not yet segmented. Arrowsindicate the region of 5-4. Dorsal view of a humanembryo of about 3 weeks. The firstsomite pairs are externally visible,just caudal to the occipital arrow indicates the rostro-caudal sequence of called the segmental plate) begins to condense in a cranial to caudal direction (Fig. 5-3). Thesecondensations will become the somites which are the focus of this to the paraxial MESODERM is the intermediate MESODERM . As the embryo begins to fold(see Lecture 3 on embryonic folding) the intermediate MESODERM will lose its connection with thesegmental plate and condense to form a solid mass of tissue running most of the length of the intermediate MESODERM will give rise to the kidneys (two embryonic forms and the final adult form;see lectures 13/14) and most of the uro-genital tract, including gonadal tissue but excluding the primor-dial germ cells (see lectures 13/14).

8 Somite FORMATION : SegmentationOne of the mechanisms by which animals generate form and diversity is first to establish asegmental pattern followed by homeotic transformations of each segment. Homeotic transformation isthe differentiation of initially identical repeating segments into unique structures; the nature of thehomeotic transformation is dependent on the expression pattern of homeotic genes (see Fig. 5-9). Thisstrategy is conserved throughout the animal kingdom and vertebrates are recognized as a separate animalphylum because of their most conspicuous SEGMENTATION , the vertebral segmental plate (paraxial MESODERM ) is laid down during gastrulation appearing on eitherside of the midline as the primitive streak regresses.

9 Fate maps demonstrate that 1) the somite cells arisefrom epiblast tissue just caudal to the primitive node, and 2) cells destined to become part of the seg-mental plate enter the primitive streak during the entire process of gastrulation (Fig. 5-4 and Lecture 2).As the paraxial MESODERM begins to condense, whorls of cells called somitomeres first appear(Figure 5-6). Most of the somitomeres will develop into epithelial rosettes, the somites, which can beseen clearly through the covering ectoderm (Figs. 5-3, 5-5). The somitic epithelial cells surround a 5-6. (A) Scanning electron micro-graph and (B) matching sketch of asomitomere. The concentric architectureof these structures is easiest to discern instereophotographs.

10 (Photo courtesy ofDr. Antone Jacobson.)Fig. 5-5. Fate map of the epiblast of a mouse/embryo, showing the zones ofepiblast that ingress through the primitive streak and form the majorstructures of the trilaminar germ disc. This map was deduced on the basis ofcell lineage studies, in which epiblast cells were injected with tracer mol-ecules and their labeled descendants later first form in the cranial end of the embryo and appear progressively more caudalward (Fig. 5-4).As the segmental boundaries are established, cells from one somite will not cross into another. We shallsee that the somites will decondense (undergo epithelial to mesenchymal transformation) soon after theyare formed so that not all somites are present at one time.