
The
development of
fish
A fish (: fish or fishes) is an aquatic animal, aquatic, Anamniotes, anamniotic, gill-bearing vertebrate animal with swimming fish fin, fins and craniate, a hard skull, but lacking limb (anatomy), limbs with digit (anatomy), digits. Fish can ...
es is unique in some specific aspects compared to the
development of other animals.
Cleavage
Most bony fish eggs are referred to as telolecithal which means that most of the egg cell cytoplasm is yolk. The
yolk
Among animals which produce eggs, the yolk (; also known as the vitellus) is the nutrient-bearing portion of the egg whose primary function is to supply food for the development of the embryo. Some types of egg contain no yolk, for example bec ...
y end of the egg (the
vegetal pole) remains homogenous while the other end (the
animal pole) undergoes cell division.
Cleavage, or initial cell division, can only occur in a region called the blastodisc, a yolk free region located at the animal pole of the egg. The fish
zygote
A zygote (; , ) is a eukaryote, eukaryotic cell (biology), cell formed by a fertilization event between two gametes.
The zygote's genome is a combination of the DNA in each gamete, and contains all of the genetic information of a new individ ...
is meroblastic, meaning the early cell divisions are not complete. This type of
meroblastic cleavage is called discoidal because only the blastodisc becomes the
embryo. In fish, waves of calcium released direct the process of cell division by coordinating the mitotic apparatus with the actin cytoskeleton, propagating cell division along the surface, assists in deepening the cleavage furrow, and finally heals the membrane after separation of blastomeres.
The fate of the first cells, called
blastomeres, is determined by its location. This contrasts with the situation in some other animals, such as mammals, in which each blastomere can develop into any part of the organism. Fish embryos go through a process called mid-blastula transition which is observed around the tenth cell division in some fish species. Once zygotic gene transcription starts, slow cell division begins and cell movements are observable. During this time three cell populations become distinguished. The first population is the yolk syncytial layer. This layer forms when the cells at the vegetal pole of the blastoderm combine with the yolk cell underneath it. Later in development the yolk syncytial layer will be important in directing cell movements of gastrulation. The second cell population is the enveloping layer which is made of superficial cells from the blastoderm that eventually form a single epithelial cell layer. This layer functions in protection by allowing the embryo to develop in a hypotonic solution so the cell will not burst. Finally, the third set of blastomeres are the deep cells. These deep cells are located between the enveloping layer and the yolk syncytial layer and eventually give rise to the embryo proper.
Germ layer formation
Once blastoderm cells have covered almost half of the yolk cell, thickening throughout the margin of deep cells occurs. The thickening is referred to as the germ ring and is made up of a superficial layer, the epiblast which will become
ectoderm
The ectoderm is one of the three primary germ layers formed in early embryonic development. It is the outermost layer, and is superficial to the mesoderm (the middle layer) and endoderm (the innermost layer). It emerges and originates from the o ...
, and an inner layer called the
hypoblast which will become
endoderm and
mesoderm
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.Langman's Medical ...
. As the blastoderm cells undergo
epiboly around the yolk the internalization of cells at the blastoderm margin start to form hypoblast. Presumptive ectoderm or epiblast cells do not internalize but the deep cells (inner layer of cells) do and they become the mesoderm and endoderm. As the hypoblast cells move inward future mesoderm (hypoblast cells) start to move vegetally and proliferate but later in development these cells alter their direction and start moving towards the animal pole. However, endodermal precursors seem to lack a pattern and move randomly over the yolk.
Axis formation
Once the egg has become multicellular and positioned its germ layers with ectoderm on the outside, mesoderm in the middle, and endoderm on the inside body axes have to be determined for proper development. A dorsal-ventral axis has to form and major proteins involved are
BMP and
Wnts. Both proteins are made in the ventral and lateral portions of the developing embryo.
BMP2B induces cells to have ventral and lateral fates while factors such as
chordin can block BMPs to dorsalize the tissue. Wnt8 induces ventral, lateral, and posterior regions of embryonic tissue. Wnt also has inhibitors like noggin to allow for the formation of dorsal tissue. In order to aid in proper development fish have an organizer center called the
Nieuwkoop center. Anterior and posterior axis formation seems to be the result of interplay of
FGFs, Wnt, and
retinoic acid. FGFs, retinoic acid, and Wnts are required to turn on posterior genes.
Neurulation
Neurulation, the formation of the
central nervous system
The central nervous system (CNS) is the part of the nervous system consisting primarily of the brain, spinal cord and retina. The CNS is so named because the brain integrates the received information and coordinates and influences the activity o ...
, is different in fishes than in most other
chordates. Convergence and extension in the
epiblast recruits presumptive neural cells from the epiblast towards the midline where they form a neural keel. A neural keel is a band of neural precursors that develops a slit like lumen to eventually become the neural tube. The
neural tube
In the developing chordate (including vertebrates), the neural tube is the embryonic precursor to the central nervous system, which is made up of the brain and spinal cord. The neural groove gradually deepens as the neural folds become elevated, ...
begins as a solid cord formed from the
ectoderm
The ectoderm is one of the three primary germ layers formed in early embryonic development. It is the outermost layer, and is superficial to the mesoderm (the middle layer) and endoderm (the innermost layer). It emerges and originates from the o ...
. This cord then sinks into the
embryo and becomes hollow, forming the neural tube. This process contrasts with the process in other chordates, which occurs by an infolding of the ectoderm to form a hollow tube.
Throughout the years advances in research have shown that neural formation relies on interactions between extrinsic signaling factors and intrinsic transcription factors. Extrinsic signals involved are BMP, Wnt, and FGF and intrinsic transcription factors like SoxB1 related genes. Secreted proteins such as BMP and its antagonist
Noggin and
chordin act permissively to establish the fate of neural tissue in the dorsal ectoderm and enables the formation of the
neural plate.
Sex determination
Sex determination is variable in fish from environmental factors like temperature to genetic mechanisms. Some fish have XX/XY chromosomes and others have ZZ/ZW. So far one gene in specific,
DMRT1bY, has been described as a sex determining gene. This gene is expressed before gonads develop and differentiate. Mutations in this gene lead to sex reversal from male to female. While this gene plays a major role in sex determination in some fish species other species have variations of this gene as well as some versions of the Sox gene as seen in zebrafish. Many species of fishes are
hermaphrodite
A hermaphrodite () is a sexually reproducing organism that produces both male and female gametes. Animal species in which individuals are either male or female are gonochoric, which is the opposite of hermaphroditic.
The individuals of many ...
s. Some, such as the
painted comber (''Serranus scriba''), are synchronous hermaphrodites. These fish have both
ovaries
The ovary () is a gonad in the female reproductive system that produces ova; when released, an ovum travels through the fallopian tube/oviduct into the uterus. There is an ovary on the left and the right side of the body. The ovaries are endocr ...
and
testes and can produce both
eggs and
sperm
Sperm (: sperm or sperms) is the male reproductive Cell (biology), cell, or gamete, in anisogamous forms of sexual reproduction (forms in which there is a larger, female reproductive cell and a smaller, male one). Animals produce motile sperm ...
at the same time. Others are sequential hermaphrodites. These fishes start life as one sex and undergo a genetically programmed sex change at some point during development. Their
gonad
A gonad, sex gland, or reproductive gland is a Heterocrine gland, mixed gland and sex organ that produces the gametes and sex hormones of an organism. Female reproductive cells are egg cells, and male reproductive cells are sperm. The male gon ...
s have both ovarian and testicular tissues, with one type of tissue predominant while the fish belongs to the corresponding gender.
Notes
References
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{{diversity of fish
Vertebrate developmental biology
Ichthyology
Fish anatomy