Fertilisation to Gastrulation Flashcards
Describe the structure of a spermatozoon and oocyte at the time of fertilisation.
During fertilisation, the genetic material from the haploid sperm cell (spermatozoon) and a haploid secondary oocyte merges into a single diploid nucleus.
The major parts of the sperm are the head and the tail. The flattened head of the sperm contains the nucleus with 23 highly condensed chromosomes, and covering the anterior two thirds of the nucleus is the acrosome, a cap-like vesicle filled with enzymes that help the sperm to penetrate a secondary oocyte to bring about fertilisation. The tail of the sperm contains many mitochondria which provide the energy for locomotion of the sperm.
The oocyte is encased within a zona pellucida (ZP) which is surrounded by corona radiata (CR) which is composed of cumulus cells and ECM. The zona pellucida is a relatively thick, translucent extracellular glycoprotein coat composed of 3-4 different glycoproteins which contain receptors for acrosome-intact sperm.
Broadly outline the events that lead up to fertilisation for both male and female gametes
There are a great series of events that lead up to fertilisation for both male and female gametes. Both the male and female gametes need to develop and acquire the ability to undergo fertilisation.
The formation of gametes in the ovaries is termed oogenesis, and begins in females before they are even born. During early foetal development, primordial (primitive) germ cells migrate from the yolk sac to the ovaries. There, they differentiate within the ovaries into oogonia which are diploid stem cells that divide mitotically to produce millions of germ cells. Before birth, most of these germ cells degenerate in a process known as atresia, however, some develop into larger cells called primary oocytes that enter prophase of meiosis I during foetal development but do not complete that phase until after puberty. Each month after puberty, LH secretion further stimulates the development of several primordial follicles, although only one will typically reach the maturity needed for ovulation. A few primordial follicles start to grow, and eventually develop into mature follicles, which complete meiosis I to produce two haploid cells of unequal size: the smaller being the first polar body and the larger being the secondary oocyte. The secondary oocyte begins meiosis II and then stops in metaphase. Ovulation occurs when the mature follicle ruptures and releases this secondary oocyte.
When semen is deposited in the vagina, the spermatozoa accumulate in the vaginal fornices near the cervix. Depending upon the hormonal status of the woman, the cervical mucous may provide a barrier or allow entry into the uterine cavity. Sperm my travel into the uterus and into the fallopian tube, where a secondary oocyte is located. Sperm that reaches the vicinity of the oocyte within minutes after ejaculation are not capable of fertilising it until several hours later. During this time in the female reproductive tract, the sperm undergo capacitation, a series of functional changes that prepare its plasma membrane to fuse with the oocytes plasma membrane.
For fertilisation to occur, a perm must penetrate the corona radiata and the zona pellucida. Acrosome enzymes and the strong tail movements of the sperm help it to penetrate the cells of the corona radiata and come into contact with the zona pellucida. The ZP has receptors which bind to specific membrane proteins in the sperm head to trigger the acrosomal reaction, the release of the contents of the acrosome, which digest a path through the ZP. The first sperm to penetrate the entire SP and reach the oocyte’s plasma membrane fuses with the oocyte .
The fusion of the sperm with the oocyte causes egg activation. The sperm tail motility ceases and the sperm becomes incorporated into the ooplasm. A wave of depolarisation spreads from the point of sperm-egg fusion, there is a calcium ion influx which causes resumption of meiosis, resulting in the completion of meiosis II and the production of a second polar body. The pronucleus forms around the female chromosome and both the female and male pronuclei enter the S phase of the cell cycle. Fusion of the male and female haploid pronuclei (syngamy) is the true moment of fertilisation. The fertilised ovum (2n) is called a zygote.
What are the early embryonic developments that occur from zygote to implantation and to the formation of the three primary embryonic germ layers (gastrulation)?
The embryonic period begins at fertilisation and continues until the end of the 10th week of gestation.
One fertilisation has occurred and the zygote is formed, rapid mitotic division termed ‘cleavage’ occurs. The first cleavage occurs 30 hours after fertilisation and produces 2 blastomeres. By the fourth day, a solid ball of cells is formed termed the morula.
Further cell divisions occur and a small fluid filled cavity is formed between the cells with an inner cell mass (ICM); this is termed the blastocyst and is present by day five. The outer cell covering is termed the trophoblast and will develop into the foetal portion of the placenta, while most of the inner cell mass will become embryo and umbilical cord.
6-8 days after fertilisation, the blastocyst attaches to the endometrium. A blastocyst ‘hatches’ from the ZN and implants with the inner cell mass side of trophoblast in contact with the endometrium.
On day eight, the trophoblast differentiated into the syntytiotrophoblast and the cytotrophoblast. The inner cell mass differentiates into the bilaminar embryonic disc, which is composed of the hypoblast layer (closest to blastocele) and the epiblast layer (closest to the amniotic cavity). The amniotic cavity is a cavity that develops in the epiblast layer; it continues to develop with a flattened layer of cells (amnion) which eventually encases the developing embryo.
On day nine, the site of implantation scar is covered by fibrin. The syncytiotrophoblast layer develops large spaces called lacunae. A thin membrane from the hypoblast lines the coelomic cavity to form the exocoelomic cavity (primitive yolk sac).
On day 12, the blastocyte is completely embedded in the endometrium. Lacunae now make contact with maternal sinusoids to form the beginning of uteroplacental circulation. Extraembryonic mesoderm develop from the yolk sac and fill the spaces between the trophoblast and amnion and exocoelomic membrane. Spaces soon develop in this layer and they fuse to form a new cavity, the chorionic cavity. The chorion consist of the Extraembryonic mesoderm and two layers of trophoblast. It eventually becomes the principal embryonic part of the placenta.
On day 14, the primitive streak develops on the epidblast, developing from the tail end to the head end. The epiblast cells migrate towards the primitive streak and then beneath it, with some cells displacing the hypoblast to form the endoderm. Some cells come to lie in between the epiblast and the newly formed endoderm, termed the mesoderm. The remaining epiblast cells form the ectoderm. The bilaminar disc becomes a trilaminar disc composed of the ectoderm, mesoderm and endoderm.
Draw the bilaminar disc
Draw the gastrula
