A teloblast is a large cell in the embryos of clitellate annelids which asymmetrically divide to form many smaller cells known as blast cells. These blast cells further proliferate and differentiate to form the segmental tissues of the annelid. Teloblasts are well studied in leeches, though they are also present in the other major class of clitellates: the oligochaetes.
All teloblasts are specified from the D quadrant macromere after the second round of divisions post-fertilization. There are five pairs of teloblasts, one on each side of the embryo. Four of the teloblasts (N, O, P, and Q) give rise to ectodermal tissue and one pair (M) gives rise to mesodermal tissue. The column of blast cells arising out of each teloblast is known as a bandlet. All five bandlets coalesce into one germinal band on each side of the embryo, extending out from the teloblast towards the head (in the rostral direction). The teloblasts are located at the rear of the embryo.
Teloblasts have two separate cytoplasmic domains: the teloplasm and the vitelloplasm. The teloplasm contains the nucleus, ribosomes, mitochondria, and other subcellular organelles. The vitelloplasm contains mostly yolk platelets. Only the teloplasm gets passed onto the daughter stem cells after cell division. [1]
The O and P teloblasts are specified from two separate but identical precursors, which form an equivalence group These two precursor cells are termed O/P cells for their ability to become either O or P teloblasts. Signals from the surrounding cells act to specify which fate the teloblasts and their progeny take on. Interactions with the q bandlet, however transient, can induce the p fate in the adjacent o/p bandlet. [2] The M bandlet has been shown to In some species (i.e. Helobdella triserialis), the provisional epithelium covering the cells plays a role in inducing the O fate. [3] In the absence of cell-cell interactions, the O/P precursors will become O teloblasts. O and P bandlets exhibit very different mitotic patterns (see figure) which are used to identify them in experimental manipulations.
The N and Q teloblasts contribute two blast cells per segment, one making up the anterior half of the segment, the second making up the posterior half of the segment. The O, P, and M lineages contribute one blast cell per segment, but the contributions from each blast cell spans a segmental boundary. These segmental boundaries were discovered by injecting teloblasts with cell lineage tracers after a few blast cells have already been generated. During development, the N and Q bandlets, which eventually have 64 blast cells each, slide past the O,P,and M bandlet, which only have 32 cells. Thus, the segmental boundaries within each bandlet are already specified before all the bandlets come into complete register. [5]
An embryo is an initial stage of development of a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male sperm cell. The resulting fusion of these two cells produces a single-celled zygote that undergoes many cell divisions that produce cells known as blastomeres. The blastomeres are arranged as a solid ball that when reaching a certain size, called a morula, takes in fluid to create a cavity called a blastocoel. The structure is then termed a blastula, or a blastocyst in mammals.
The mesoderm is the middle layer of the three germ layers that develops during gastrulation in the very early development of the embryo of most animals. The outer layer is the ectoderm, and the inner layer is the endoderm.
Gastrulation is the stage in the early embryonic development of most animals, during which the blastula, or in mammals the blastocyst is reorganized into a multilayered structure known as the gastrula. Before gastrulation, the embryo is a continuous epithelial sheet of cells; by the end of gastrulation, the embryo has begun differentiation to establish distinct cell lineages, set up the basic axes of the body, and internalized one or more cell types including the prospective gut.
Segmentation in biology is the division of some animal and plant body plans into a series of repetitive segments. This article focuses on the segmentation of animal body plans, specifically using the examples of the taxa Arthropoda, Chordata, and Annelida. These three groups form segments by using a "growth zone" to direct and define the segments. While all three have a generally segmented body plan and use a growth zone, they use different mechanisms for generating this patterning. Even within these groups, different organisms have different mechanisms for segmenting the body. Segmentation of the body plan is important for allowing free movement and development of certain body parts. It also allows for regeneration in specific individuals.
The somites are a set of bilaterally paired blocks of paraxial mesoderm that form in the embryonic stage of somitogenesis, along the head-to-tail axis in segmented animals. In vertebrates, somites subdivide into the dermatomes, myotomes, sclerotomes and syndetomes that give rise to the vertebrae of the vertebral column, rib cage, part of the occipital bone, skeletal muscle, cartilage, tendons, and skin.
Somitogenesis is the process by which somites form. Somites are bilaterally paired blocks of paraxial mesoderm that form along the anterior-posterior axis of the developing embryo in segmented animals. In vertebrates, somites give rise to skeletal muscle, cartilage, tendons, endothelium, and dermis.
The clitellum is a thickened glandular and non-segmented section of the body wall near the head in earthworms and leeches, that secretes a viscid sac in which eggs are stored. It is located near the anterior end of the body, between the fourteenth and seventeenth segments. The number of the segments to where the clitellum begins and the number of segments that make up the clitellum are important for identifying earthworms. In microdrile earthworms, the clitellum has only one layer, resulting in a smaller quantity of eggs than that of the megadrile earthworms, which have larger multi-layered clitellum that have special cells that secrete albumin into the worms' egg sac.
In embryology, cleavage is the division of cells in the early development of the embryo, following fertilization. The zygotes of many species undergo rapid cell cycles with no significant overall growth, producing a cluster of cells the same size as the original zygote. The different cells derived from cleavage are called blastomeres and form a compact mass called the morula. Cleavage ends with the formation of the blastula, or of the blastocyst in mammals.
The Clitellata are a class of annelid worms, characterized by having a clitellum - the 'collar' that forms a reproductive cocoon during part of their life cycles. The clitellates comprise around 8,000 species. Unlike the class of Polychaeta, they do not have parapodia and their heads are less developed.
The zona limitans intrathalamica (ZLI) is a lineage-restriction compartment and primary developmental boundary in the vertebrate forebrain that serves as a signaling center and a restrictive border between the thalamus and the prethalamus.
An asymmetric cell division produces two daughter cells with different cellular fates. This is in contrast to symmetric cell divisions which give rise to daughter cells of equivalent fates. Notably, stem cells divide asymmetrically to give rise to two distinct daughter cells: one copy of the original stem cell as well as a second daughter programmed to differentiate into a non-stem cell fate.
Mesenchyme is a type of loosely organized animal embryonic connective tissue of undifferentiated cells that give rise to most tissues, such as skin, blood or bone. The interactions between mesenchyme and epithelium help to form nearly every organ in the developing embryo.
An equivalence group is a set of unspecified cells that have the same developmental potential or ability to adopt various fates. Our current understanding suggests that equivalence groups are limited to cells of the same ancestry, also known as sibling cells. Often, cells of an equivalence group adopt different fates from one another.
Eye formation in the human embryo begins at approximately three weeks into embryonic development and continues through the tenth week. Cells from both the mesodermal and the ectodermal tissues contribute to the formation of the eye. Specifically, the eye is derived from the neuroepithelium, surface ectoderm, and the extracellular mesenchyme which consists of both the neural crest and mesoderm.
Within the field of developmental biology, one goal is to understand how a particular cell develops into a final cell type, known as fate determination. Within an embryo, several processes play out at the cellular and tissue level to create an organism. These processes include cell proliferation, differentiation, cellular movement and programmed cell death. Each cell in an embryo receives molecular signals from neighboring cells in the form of proteins, RNAs and even surface interactions. Almost all animals undergo a similar sequence of events during very early development, a conserved process known as embryogenesis. During embryogenesis, cells exist in three germ layers, and undergo gastrulation. While embryogenesis has been studied for more than a century, it was only recently that scientists discovered that a basic set of the same proteins and mRNAs are involved in embryogenesis. Evolutionary conservation is one of the reasons that model systems such as the fly, the mouse, and other organisms are used as models to study embryogenesis and developmental biology. Studying model organisms provides information relevant to other animals, including humans. While studying the different model systems, cells fate was discovered to be determined via multiple ways, two of which are by the combination of transcription factors the cells have and by the cell-cell interaction. Cells’ fate determination mechanisms were categorized into three different types, autonomously specified cells, conditionally specified cells, or syncytial specified cells. Furthermore, the cells’ fate was determined mainly using two types of experiments, cell ablation and transplantation. The results obtained from these experiments, helped in identifying the fate of the examined cells.
Neurogenin-3 (NGN3) is a protein that in humans is encoded by the Neurog3 gene.
Capitella teleta is a small, cosmopolitan, segmented annelid worm. It is a well-studied invertebrate, which has been cultured for use in laboratories for over 30 years. C. teleta is the first marine polychaete to have its genome sequenced.
The annelids, also known as the segmented worms, are a large phylum, with over 22,000 extant species including ragworms, earthworms, and leeches. The species exist in and have adapted to various ecologies – some in marine environments as distinct as tidal zones and hydrothermal vents, others in fresh water, and yet others in moist terrestrial environments.
Leech embryogenesis is the process by which the embryo of the leech forms and develops. The embryonic development of the larva occurs as a series of stages. During stage 1, the first cleavage occurs, which gives rise to an AB and a CD blastomere, and is in the interphase of this cell division when a yolk-free cytoplasm called teloplasm is formed. The teloplasm is known to be a determinant for the specification of the D cell fate. In stage 3, during the second cleavage, an unequal division occurs in the CD blastomere. As a consequence, it creates a large D cell on the left and a smaller C cell to the right. This unequal division process is dependent on actomyosin, and by the end of stage 3 the AB cell divides. On stage 4 of development, the micromeres and teloblast stem cells are formed and subsequently, the D quadrant divides to form the DM and the DNOPQ teloblast precursor cells. By the end stage 6, the zygote contains a set of 25 micromeres, 3 macromeres and 10 teloblasts derived from the D quadrant.
Primordial germ cell (PGC) migration is the process of distribution of primordial germ cells throughout the embryo during embryogenesis.
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