Leech embryogenesis

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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. [1] The teloplasm is known to be a determinant for the specification of the D cell fate. [2] 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, [3] 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 (A, B and C) and 10 teloblasts derived from the D quadrant. [4]

Leech subclass of worms

Leeches are segmented parasitic or predatory worms that belong to the phylum Annelida and comprise the subclass Hirudinea. They are closely related to the oligochaetes, which include the earthworms, and like them have soft, muscular, segmented bodies that can lengthen and contract. Both groups are hermaphrodites and have a clitellum, but leeches typically differ from the oligochaetes in having suckers at both ends and in having external annulations that do not correspond with their internal segmentation. The body is relatively solid, and the spacious body cavity found in other annelids, the coelom, is reduced to small channels.

In embryology, cleavage is the division of cells in the early embryo. The process follows fertilization, with the transfer being triggered by the activation of a cyclin-dependent kinase complex. 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.

In biology, a blastomere is a type of cell produced by cleavage of the zygote after fertilization and is an essential part of blastula formation.

The teloblasts are pairs of five different types (M, N, O, P, and Q) of embryonic stem cells that form segmented columns of cells (germinal band) in the surface of the embryo. [5] The M-derived cells make mesoderm and some small set of neurons, N results in neural tissues and some ventral ectoderm, Q contributes to the dorsal ectoderm and O and P in the leech are equipotent cells (same developmental potential) that produce lateral ectoderm; however the difference between the two of them is that P creates bigger batches of dorsolateral epidermis than O. [2] The sludgeworm Tubifex , unlike the leech, specifies the O and P lineages early in development and therefore, these two cells are not equipotent. [6] Each segment of the body of the leech is generated from one M, O, P cell types and two N and two Q cells types. [2]

Ectoderm

Ectoderm is one of the three primary germ layers in the very early embryo. The other two layers are the mesoderm and endoderm, with the ectoderm as the most exterior layer. It emerges and originates from the outer layer of germ cells. The word ectoderm comes from the Greek ektos meaning "outside", and derma, meaning "skin."

<i>Tubifex</i> genus of annelids

Tubifex is a cosmopolitan genus of tubificid annelids that inhabits the sediments of lakes, rivers and occasionally sewer lines. At least 13 species of Tubifex have been identified, with the exact number not certain, as the species are not easily distinguishable from each other.

The ectoderm and mesoderm of the body trunk are exclusively derived from the teloblast cells in a region called the posterior progress zone. [7] [8] The head of the leech that comes from an unsegmented region, is formed by the first set of micromeres derived from A, B, C and D cells, keeping the bilateral symmetry between the AD and BC cells. [8]

Mesoderm

In all bilaterian animals, the mesoderm is one of the three primary germ layers in the very early embryo. The other two layers are the ectoderm and endoderm, with the mesoderm as the middle layer between them.

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Hemichordate phylum of animals

Hemichordate is a phylum of marine deuterostome animals, generally considered the sister group of the echinoderms. They appear in the Lower or Middle Cambrian and include two main classes: Enteropneusta, and Pterobranchia. A third class, Planctosphaeroidea, is known only from the larva of a single species, Planctosphaera pelagica. The extinct class Graptolithina is closely related to the pterobranchs.

Blastula embryogenesis

The blastula is a hollow sphere of cells, referred to as blastomeres, surrounding an inner fluid-filled cavity called the blastocoele formed during an early stage of embryonic development in animals. Embryo development begins with a sperm fertilizing an egg to become a zygote which undergoes many cleavages to develop into a ball of cells called a morula. Only when the blastocoele is formed does the early embryo become a blastula. The blastula precedes the formation of the gastrula in which the germ layers of the embryo form.

<i>Drosophila</i> embryogenesis Embryogenesis of the fruit fly Drosophila, a popular model system

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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.

Blastocoel

A blastocoel is a fluid-filled cavity that forms in the animal hemisphere of early amphibian and echinoderm embryos, or between the epiblast and hypoblast of avian, reptilian, and mammalian blastoderm-stage embryos.

A germ layer is a primary layer of cells that forms during embryonic development. The three germ layers in vertebrates are particularly pronounced; however, all eumetazoans produce two or three primary germ layers. Some animals, like cnidarians, produce two germ layers making them diploblastic. Other animals such as chordates produce a third layer, between these two layers. making them triploblastic. Germ layers eventually give rise to all of an animal’s tissues and organs through the process of organogenesis.

Organogenesis is the phase of embryonic development that starts at the end of gastrulation and continues until birth. During organogenesis, the three germ layers formed from gastrulation: the ectoderm, endoderm, and mesoderm form the internal organs of the organism.

Embryonic development also embryogenesis is the process by which the embryo forms and develops. In mammals, the term refers chiefly to early stages of prenatal development, whereas the terms fetus and fetal development describe later stages.

In embryology, Carnegie stages are a standardized system of 23 stages used to provide a unified developmental chronology of the vertebrate embryo.

Inner cell mass

In early embryogenesis of most eutherian mammals, the inner cell mass is the mass of cells inside the primordial embryo that will eventually give rise to the definitive structures of the fetus. This structure forms in the earliest steps of development, before implantation into the endometrium of the uterus has occurred. The ICM lies within the blastocoele and is entirely surrounded by the single layer of cells called trophoblast.

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.

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 development The process whose specific outcome is the progression of the eye over time, from its formation to the mature structure. The eye is the organ of sight.

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.

In the field of developmental biology, regional differentiation is the process by which different areas are identified in the development of the early embryo. The process by which the cells become specified differs between organisms.

Fish development

The development of fishes is unique in some specific aspects compared to the development of other animals.

Human embryonic development process of cell division and cellular differentiation of the embryo that occurs during the early stages of development

Human embryonic development, or human embryogenesis, refers to the development and formation of the human embryo. It is characterised by the process of cell division and cellular differentiation of the embryo that occurs during the early stages of development. In biological terms, the development of the human body entails growth from a one-celled zygote to an adult human being. Fertilisation occurs when the sperm cell successfully enters and fuses with an egg cell (ovum). The genetic material of the sperm and egg then combine to form a single cell called a zygote and the germinal stage of development commences. Embryonic development in the human, covers the first eight weeks of development; at the beginning of the ninth week the embryo is termed a fetus. Human embryology is the study of this development during the first eight weeks after fertilisation. The normal period of gestation (pregnancy) is nine months or 38 weeks.

Teloblast large cell in annelid embryos

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.

References

  1. ↑ Fernandez, J.; Olea, N.; Tellez, V.; Matte, C. (1990). "Structure and development of the egg of the glossiphoniid leech Theromyzon rude: reorganization of the fertilized egg during completion of the first meiotic division". Developmental Biology . 137 (1): 142–154. doi:10.1016/0012-1606(90)90015-B. PMID   2295361.
  2. 1 2 3 Weisblat, D. A.; Shankland, M. (1985). "Cell lineage and segmentation in the leech". Philosophical Transactions of the Royal Society B: Biological Sciences . 312 (1153): 39–56. doi:10.1098/rstb.1985.0176. JSTOR   2396301. PMID   2869529.
  3. ↑ Lyons, D. C.; Weisblat, D. A. (2009). "D quadrant specification in the leech Helobdella: actomyosin contractility controls the unequal cleavage of the CD blastomere". Developmental Biology . 334 (1): 46–58. doi:10.1016/j.ydbio.2009.07.007. PMC   3077801  Lock-green.svg. PMID   19607823.
  4. ↑ Sandig, M.; Dohle, W. (1988). "The cleavage pattern in the leech Theromyzon tessulatum (Hirudinea, Glossiphoniidae)". Journal of Morphology . 196 (2): 217–252. doi:10.1002/jmor.1051960210. PMID   3385778.
  5. ↑ Berezovskii, V. K.; Shankland, M. (1996). "Segmental diversification of an identified leech neuron correlates with the segmental domain in which it expresses Lox2, a member of the Hox gene family". Journal of Neurobiology . 29 (3): 319–329. doi:10.1002/(SICI)1097-4695(199603)29:3<319::AID-NEU4>3.0.CO;2-C. PMID   8907161.
  6. ↑ Arai, A.; Nakamoto, A.; Shimizu, T. (2001). "Specification of ectodermal teloblast lineages in embryos of the oligochaete annelid Tubifex: involvement of novel cell-cell interactions". Development . 128 (7): 1211–1219. PMID   11245587.
  7. ↑ Nardelli-Haefliger, D.; Shankland, M. (1993). "Lox10, a member of the NK-2 homeobox gene class, is expressed in a segmental pattern in the endoderm and in the cephalic nervous system of the leech Helobdella". Development . 118 (3): 877–892. PMID   7915671.
  8. 1 2 Shankland, M.; Bruce, A. E. (1998). "Axial patterning in the leech: developmental mechanisms and evolutionary implications". Biological Bulletin . 195 (3): 370–372. doi:10.2307/1543150. JSTOR   1543150. PMID   9924777.
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