In the preceding unit, you delved into the diverse patterns of cleavage, the cleavage mechanism, and the transformation of a single-layered blastula into a gastrula with three germ layers. Unit 15 focuses on postgastrulation changes, encompassing the rearrangement of embryo cells to establish a definitive body form and the differentiation of organs and organ systems from germ layers, collectively termed morphogenesis.
Upon completion of this unit, you should be able to:
In Unit 14, the transition from a fertilized egg to a blastula was explored. Now, the focus shifts to the rearrangement of cells within the gastrula, leading to the development of adult organs from three primary layers: ectoderm, mesoderm, and endoderm.
A single fertilized egg generates diverse structures, a phenomenon known as differentiation. This process involves cytodifferentiation, histodifferentiation, and the development of organ shapes, all facilitated by morphogenetic processes. These processes include cell division direction and amount, changes in cell shape, cell migration, cell growth, cell death, and alterations in cell membrane and extracellular matrix.
Early embryonic changes and organ rudiment formation involve folding and spreading movements in epithelial cells. These movements are vital for shaping the embryo. Several morphogenetic processes contribute to these transformations:
Palisading: Thickening of epithelium through cell elongation precedes any change, visible in the formation of neural plates and ectodermal structures like the lens, ear, and nasal rudiments.
Evagination and Invagination: Outward folding is termed evagination, while inward folding is invagination. Examples include the formation of neural and laryngo-tracheal tubes.
Groove Formation: Folding along a line results in groove formation, exemplified by the formation of the neural tube and laryngo-tracheal tube.
Inpocketing or Infolding: The formation of lens vesicles or otic vesicles demonstrates inpocketing or infolding, creating pouches from epithelial thickening.
Branched Structures: Folds and pouches undergo modification to form branched structures, influencing the development of various glands.
Cell Shape Changes: Folding or bending of a cell sheet may change the shape of individual cells, influencing the overall shape of the epithelium. This is observed in processes like epiboly during amphibian gastrulation.
Cell Spreading: Cells spread to cover specific areas during development, accompanied by changes in cell shape such as thinning and flattening.
Cell Migration: Mesenchymal cells and primordial germ cells detach from major layers, migrating to new locations and developing into programmed structures.
Selective cell death plays a crucial role in shaping various structures during embryo development. Examples include regions of cell death between developing digits in a chick, contributing to the formation of separate digits.
In summary, Unit 15 explores the intricate morphogenetic processes that shape embryonic development, leading to the formation of diverse organs and tissues.
In the previous subsection, various types of morphogenetic processes were explored, highlighting the mobile nature of embryonic cells. This section delves into the changes in cell shape during morphogenetic processes and the factors guiding cells to their designated locations.
Fibroblasts, the precursors to connective tissue, serve as a model for understanding cell mobility mechanisms. The movement of fibroblasts occurs in two phases:
Adhesion Phase: Cells stretch to the limits of their plasma membrane.
Detachment Phase: The hind part of the cell is pulled forward, propelling the cell by generating thin, fan-shaped regions called lamellae.
To understand how cells move to specific positions, different mechanisms are proposed:
Chemotaxis (a): Directed movement in response to a concentration gradient of a chemical factor. Example: migration of embryonic lymphocytes.
Haptotaxis (b): Directed movement in response to a concentration gradient of an adhesive molecule present in the extracellular matrix.
Galvanotaxis (c): Movement in response to a potential difference between cells. Voltage differences between embryonic regions may influence morphogenesis.
Contact Guidance (d): Physical factors influence cell movement. Cells detect discontinuities in their substratum and migrate accordingly.
The mammalian cells shown alignhg themselves on the grooved surfam
The cytoskeleton, comprising microtubules, microfilaments, and intermediate filaments, plays a crucial role in mediating changes in cell shape and movement during embryonic development.
Microtubules: Hollow cylindrical rods formed by protofilaments. They contribute to the elongation or palisading of epithelial cells.
Microfilaments: Meshwork of actin polymers, involved in the contraction that narrows apical surfaces during cell folding.
Intermediate Filaments: Filaments of intermediate size, with various classes serving functions in different tissues.
Cells adhere to each other or substratum surfaces in their environment. Adhesion is mediated by receptor proteins in the plasma membrane. Extracellular matrix (ECM) molecules, including proteoglycans, collagens, and glycoproteins, form a meshwork in the intercellular space.
Basal Lamina: Dense sheet on the basal surface of epithelial cells formed by certain ECM molecules.
Receptor Proteins: Located in the plasma membrane, mediating adhesion to various ECM molecules.
During vertebrate body development, distinct regions in the three germ layers of the gastrula undergo segregation to form the rudiments of future organs and tissues. This section focuses on the partitioning of the ectoderm and the intricate process of neurulation in frog and chick embryos.
In this section, the early development of organs derived from the mesoderm, positioned between ectoderm and endoderm tissues, will be explored. The mesoderm cells in the neurula stage organize into five distinct regions, each giving rise to specific organs.
Chordamesoderm:
Dorsal Mesoderm (Paraxial Mesoderm):
Intermediate Mesoderm:
Lateral Plate Mesoderm:
Head Mesoderm:
In this subsection, we delve into the intricate process of blood cell development, primarily focusing on erythrocytes or red blood cells (RBCs). The understanding of this process is primarily derived from studies on birds and mammals.
CFU - M, L (Myeloid and Lymphoid Colony Forming Unit):
CFU - S (Somatic Stem Cell) and CFU - L (Lymphoid Stem Cell):
Committed Stem Cells:
BFU - E Cell:
Proerythroblast Stage:
Erythroblast Stage:
Polychromatophilic Stage:
Orthochromatic Stage:
Reticulocyte Stage:
Erythrocyte:
Bone Marrow:
Embryonic Blood Cell Differentiation:
In this section, we shift our focus to the endoderm and its derivatives, specifically examining the development of the gut tube, respiratory apparatus, and primordial germ cells (PGC).
Pharyngeal Region:
Thyroid Gland:
Trachea and Lungs:
Liver, Pancreas, and Gall Bladder:
Mesodermal Involvement:
Primordial Germ Cells (PGCs):
Frog Embryos:
Migration Mechanism:
Birds and Reptiles:
Mammals:
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