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However, for the success in any IVP protocols in goat sound knowledge of physiology of gametogenesis, fertilization and early embryogenesis in vivo is crucial because in vivo information form the basis and guide for any in vitro experiment. Unlike human, laboratory animals, cattle and sheep, fewer studies have been conducted in gametogenesis, fertilization and early embryo development in goat.
Data for sheep and cattle are mostly used as a basis for goat IVP studies. Therefore, the current study is intended to review gametogenesis, sperm oocyte interaction, fertilization and early embryogenetic process in mammals with special reference to goat.
Gametogenesis, fertilization and early embryogenesis or pre-implantation embryo development are crucial periods for normal development of an embryo afterwards.
Until now researches in mammalian gametogenesis, fertilization and early embryogenesis mainly based on the laboratory animals and human Tulsiani et al. Compared with laboratory and farm animals like cattle, sheep and pig, studies in goat are scarce Hafez and Hafez, a; Miyano and Hirao, Therefore, the current paper will briefly review some aspects of gametogenesis, fertilization and early embryogenesis in mammals with special reference to goat.
Genetically and functionally competent gametes are a prerequisite for normal fertilization and early embryo development. The first phase in the sexual reproduction of an organism is gametogenesis, a process of formation of gametes from the germ cells in the testes and ovaries. This process is termed as spermatogenesis in the male and oogenesis in the female. It is the fundamental biological process in both the sexes and the key event of gametogenesis is the halving of the number of chromosomes to produce haploid germ cells sperm and oocyte through meiosis.
Thus, in goat, where the chromosome number of somatic cells is 60, each sperm and each oocyte has only 30 chromosomes. However, until this point spermatogenesis and oogenesis resume their similarity.
After this, in the male, each primary spermatocyte divides meiotically and produces four spermatids, each destined to become a functional sperm. In the female, on the other hand, of the four cells produced from each primary oocyte only one finally becomes a functional oocyte.
A schematic diagram of gametogenesis in mammals has been illustrated in Fig. In mammals, the male and female gametes originate from the embryonic yolk sac. The gametes initially colonize in the primitive gonadal ridge at the early stage of pregnancy by migration through the developing mesentery of the embryo, where germ cells associate with somatic cells; it continues with their multiplication, growth and maturation.
Finally, migrate to the pelvic and inguinal regions to form the ovary or testis, respectively and ends at fertilization. Mammalian spermatogenesis and oogenesis are briefly described below:.
Spermatogenesis: Competent sperm are required for the successful contribution of the paternal genotype to embryo development. The process of spermatogenesis results in the formation of the haploid male gamete required for fertilization of an oocyte. Spermatogenesis is a continual and complex process that involves three major steps: a proliferation- multiplication of spermatogonia by the process of mitosis spermatocytogenesis ; b meiosis spermiogenesis - meiotic divisions whereby the chromosome number is reduced from diploid to haploid and c differentiation- transformation of the round spermatid into the complex structure of the spermatozoon reviewed in Barth and Oko, ; De Kretser et al.
Spermatogenesis in buck and other male mammals begins at puberty through the proliferation of interphase germ cells, continues throughout adult life and takes place inside the seminiferous tubules of the testes. The age of puberty for male goat or buck ranged between 4 and 6 months Jainudeen et al. The walls of the seminiferous tubules contain the differentiating sex cells arranged in a stratified layer, 5 to 8 cells deep.
The outermost layers contain spermatogonia, which have increased in number by spemtocytogenesis. The spemtocytogenesis begins with mitosis of the diploid A spematogonia in the basal compartment of the Sertoli cells. The A spematogonia differentiate into B spermatogonia which enter their final mitotic division prior to their entrance into the pre-leptotene phase of first meiosis meiosis 1. Meiosis occurs in the adluminal compartment of the Sertoli cell and results in the formation of primary spermatocytes.
Condensation of chromosomes occurs during the leptotene phase followed by the zygotene phase. The long pachytene phase involves the crossing over of chromosomal material and is most susceptible to damage.
Meiosis progresses through the diplotene phase, diakinesis, Metaphase I MI and anaphase, finally resulting in secondary spermatocytes 2c, 1n.
Each secondary spermatocyte then enters the second meiotic division meiosis 2 , without the intervention of a resting period, resulting in spermatids with a haploid number of chromosomes and DNA content 1c, 1n , required for fertilization. The round-shaped spermatids proceed through spermiogenesis and metamorphically changed into highly specialized motile cells. Spermiogenesis consists of the Golgi phase, the acrosomal cap phase, the acrosomal phase and the complex maturation stage involves development of the sperm tail reviewed by Barth and Oko, Deviations from the process of normal spermatogenesis result in abnormal morphology or dysfunction of the sperm cell.
The progression from spermatogonia to mature spermatozoa in mammals require approximately 60 to 70 days, with at least three mitotic and two meiotic divisions during spermatogenesis Kupker et al. Finally, the newly formed sperm are released into the lumen of the seminiferous tubules.
These sperm are immotile and still immature in terms of fertilizing capability. Therefore, to acquire motility as well as maturation they need to pass down the epididymis to the ejaculatory duct, which is known as epididymal maturation Elder and Dale, During epididymal maturation, the sperm shed the cytoplasmic droplet, undergo modifications in the protein, carbohydrate and glycoprotein composition of the plasma membrane and acquire a net negative charge Yanagimachi, ; Harrison, Disruptions in the epididymal environment may result in abnormalities of sperm function.
Immediately after ejaculation, the sperm are incapable of fertilizing the oocyte and, therefore, to acquire fertilization potential, they need to undergo functional changes or capacitation , which occur inside the female reproductive tract. Although in comparison with the oocytes, the spermatozoa are very small, however, they are essentially very long and compact cells with a few highly specialized cytoplasmic structures, the flagellum for motility and the acrosome, which is instrumental in sperm-oocyte binding and fusion.
Morphologically and functionally, a goat spermatozoon is composed of four regions: a the head containing nucleus and the acrosome, b the neck containing centrioles, c the middle piece containing mitochondria and d the tail piece or flagellum.
Frozen-thawed goat spermatozoa from proven quality Jermasia buck Jermasia is a synthetic breed developed by the University of Malaya have been shown in Fig. Oogenesis: The maternal contribution to the development of the embryo is determined during formation and maturation of the female gamete, the oocyte. The ability of the oocyte to achieve sperm-oocyte fusion is acquired early in oogenesis or the process of oocyte formation.
The oogenetic products synthesized during oocyte growth must also be sufficient to support embryonic development from fertilization until the activation of the embryonic genome Olszanska and Borgul, Ultimately, the nuclear and ooplasmic maturity of the oocyte influences the success of fertilization and embryo development. In mammals, oogenesis commences during early fetal development, stops at birth and continues during puberty throughout the reproductive life of the female.
After continuation of meiosis, the oogenesis process until completion is very fast. Oogenesis in mammals includes seven steps: a generation of primordial germ cells PGCs , b migration of PGCs to the prospective gonads, c colonization of the gonads by PGCs, d differentiation of PGCs to oogonia, e proliferation of oogonia, f initiation of meiosis and g arrest at the diplotene stage of first meiotic prophase or prophase 1 reviewed in Van den Hurk and Zhao, Oogonia are the early germ cells in the ovary, which increase in number by mitosis.
Oogonial multiplication begins during early fetal development and ends months to years later in the sexually mature adult Picton et al. Once mitosis ceases, the oogonia then grow in size and enter the prophase of the first meiotic division at approximately day 55 of gestation in the ewe McNatty et al. Each oogonium or primary oocyte contains the diploid number of chromosome. The primary oocyte which is transformed from each oogonium is a cell which becomes enclosed in a follicle, known as primordial follicle.
In goat, sheep and cow, large population approximately , of primordial or pre-antral follicles with meiotically incompetent oocytes are present in the ovaries Miyano, ; Miyano and Hirao, ; Zhou and Zhang, Most of them are lost at various stages of development owing to atresia and only a very minority of oocytes becomes available for ovulation.
At birth, all oocytes from growing and dominant follicles are arrested at the diplotene stage of prophase 1 Van den Hurk and Zhao, This dictyate stage is characterized by the enclosure of the chromosomes within the large nucleus, also known as the Germinal Vesicle GV Elder and Dale, The oocytes remain in the arrested state until a few hours before ovulation. Surprisingly, the oocytes may stay at this arrested stage for a longer period of time depending on the species, waiting for the signal to resume growth and subsequent development occurs at puberty.
The age of puberty for female goat or doe is ranged between 5 and 7 months Jainudeen et al. The reason for storing the oocytes in this remarkable frozen meiotic state is unknown Johnson and Everitt, Oocyte growth and development: The growth and development of an oocyte occurs inside an ovarian follicle and oocyte undergoes a progressive series of morphological modifications as it grows and proceeds through the different stages of development Eppig et al.
Although data are lacking for doe, Ariyaratna and Gunawardana indicated from their study that follicular morphology and activity are similar in does and ewes. In the ewe, primordial, primary and secondary follicles, respectively, appear in the fetal ovary at days 75, and McNatty et al. Once a primordial follicle oocyte is activated to grow, it embarks on a complex journey that involves numerous molecular and morphological changes to both the oocyte and the follicle. The modifications are carefully orchestrated and require sensitive communication between the oocyte and surrounding Granulosa Cells or GCs Fair, These structural rearrangements facilitate the increasing energy and nucleic acid synthesis requirements of the developing oocyte and are a prerequisite to the oocyte achieving meiotic competence and embryo developmental potential.
The first sign of morphological change when the oocyte begins to grow is turning of the flat GCs to cuboidal which is known as primary follicle. After completion of the morphological change, the GCs proliferate actively, which cause the follicles to develop and increase in size. Through a series of mitotic division of GCs, unilaminar primary follicles are converted to multilaminar secondary follicles, followed by the antral or tertiary follicles Miyano and Hirao, In the doe, antrum formation began when the GCs are about six cell layers in thickness and the Zona Pellucida ZP is visible at this stage Ariyaratna and Gunawardana, During this growth phase there is a major increase in ooplasmic organelles.
The follicle provides a microenvironment for oocyte growth, development and is responsible for the production of hormones. The walls of mature preovulatory follicles consist of membrana granulosa mural granulosa , theca interna and theca externa.
The GCs are cells of epithelial origin essential for the growth and survival of the oocyte. The CCs surround the oocyte, which nourish the oocyte, are involved in oocyte growth, maturation Buccione et al.
In addition, these cells have also been implicated in the modulation or generation of oocyte maturation inhibitors Tsafriri et al. The CCs in close contact with the oocyte are known as corona radiata. They are in close contact with the oocyte through ooplasmic extensions or processes across the ZP De Loos et al.
The heterologous gap junctions provide the basis for extensive network of intracellular communication among GCs. Oocyte maturation: As mentioned earlier that oocytes are arrested at the diplotene stage of the prophase 1 at birth, they resume meiosis after a long quiescent phase at puberty which involve sequential sub-cellular and molecular transformations by various components of the follicle.
During postnatal life, starting from puberty, ovarian follicles continue to grow, mature and either ovulate or regress. Follicles are recruited continuously until the original store is exhausted. Reinitiation of meiosis in the fully-grown oocyte is the first sign of oocyte maturation, which involves condensation of interphase chromatin, breakdown of nuclear membrane germinal vesicle breakdown: GVBD , spindle formation and chromosome segregation.
In vivo , resumption of meiosis is initiated by a preovulatory LH surge and only occurs in fully grown, meiotically competent oocytes from dominant follicles. Small oocytes in primordial and primary follicles have no ability to resume meiosis. Diameter of mature oocyte in different animals and human is presented in Table 1. During this follicle and oocyte growth phase, oocytes not only acquire competency to resume meiosis, but also acquire ooplasmic maturity, also known as oocyte capacitation, both of which are required to ensure normal fertilization and embryo development Gosden et al.
From in vitro studies, it is found that goat oocytes acquired the ability to initiate meiotic resumption in early antral follicles of 0. In cattle, oocytes originating from follicles larger than 6 mm in diameter yield a significantly higher percentage of blastocyst than the smaller follicles Lonergan et al. To reach the maturity, goat oocytes ooplasm grow from The ZP of a goat oocyte from antral follicle bigger than 2 mm is about 3.
Chapter 9: Multiple choice questions
What generalization can be applied to the pole plasm of Drosophila , the P-granules of C. Two human disorders, Prader-Willi syndrome and Angelman syndrome, occur when a small deletion in a specific region of chromosome 15 is contributed by either the father or mother, respectively. Why does this small deletion not behave as a recessive allele for either syndrome, that is, why is its loss not made up for by the good copy of the region on chromosome 15 contributed by the other parent? In what way, if any, does the chromosomal determination of sex differ in Drosophila and humans? Hermaphrodite is the sexual phenotype of a gain-of-function transformer-1 in both X and XX of C.
Developmental Biology Tutorial Embryonic Induction during Vertebrate Development: Mesoderm Induction in Xenopus The key to the middle kingdom Vertebrate embryos rely extensively upon inductive interactions to diversify the number of different kinds of cells in the embryo. Induction is the process by which one group of cells produces a signal that determines the fate of a second group of cells. This implies both the capacity to produce a signal ligand by the inducing cells and the competence of the responding cells to receive and interpret the signal via a signal transduction pathway. Amphibians are the most extensively studied vertebrates for investigations into embryonic induction. Most contemporary investigations have utilized Xenopus. The two major inductive events during early Xenopus development Slack, , Fig. Mesoderm induction occurs over an extended period of time in the equatorial region of the embryo from about the cell stage to the beginning of gastrulation.
NCBI Bookshelf. Figure 2. Let us look at this life cycle in a bit more detail. First, in most frogs, gametogenesis and fertilization are seasonal events for this animal, because its life depends upon the plants and insects in the pond where it lives and on the temperature of the air and water. A combination of photoperiod hours of daylight and temperature tells the pituitary gland of the female frog that it is spring. If the frog is mature, the pituitary gland secretes hormones that stimulate the ovary to make estrogen.