Lec.3
Fertilization, implantation and early development of embryoFertilization:
Is the fusion of male and female gametes.
In humans, most cells contain 46 chromosomes (diploid). Meiosis generates mature (haploid) eggs or sperm containing 23 chromosomes.
A diploid primary spermatocytes undergoes meiosis to produce 4 haploid mature sperm.
A diploid primary oocytes undergoes meiosis. It produces only one haploid mature ovum (containing one polar body).
In female the 1st meiotic division starts in utero but not completed till puberty producing secondary oocyte and 1st polar body.
Then 2nd meiotic division occur and not completed till fertilization. During the 2nd meiosis, the primary oocyte remains in meiotic arrest. Meiosis will resume once fertilization has occurred to produce mature ovum and 2nd polar body.
In male the spermatogonia divide mitotically at the onset of puberty to produce primary spermatocytes. These undergo a 1st meiotic division to produce the secondary spermatocytes. The diploid secondary spermatocytes further undergo the 2nd meiotic division to produce haploid spermatids.
Female ovum, which contain only X chromosomes while the sperm contain either X or Y chromosomes. Therefore a normal sex combination will be either XX (female) or XY (male).
The ovulated egg is picked up by the fimbria of the fallopian tube and then swept by ciliary action towards the ampulla, where fertilization occurs.
The basic PH of the seminal fluid protects the spermatozoa from the acidity of the vagina. Within minutes after ejaculation, sperm may be found in the cervix and are will move with great speed towards the ampulla, where fertilization of the mature ovulated oocyte usually occurs. To achieve this, the sperm must undergo capacitation.
During this process the inner membrane beneath the acrosome cap becomes primed for fusion with the inner membrane of the ovum. The acrosomal reaction exposes the inner membrane of the sperm. This portion will fuse with the membrane of the ovum. Following penetration of the oocyte, a series of events must occur to form the male pronucleus (decondensation of sperm chromosomes).
The egg must complete its 2nd meiotic division to form the haploid female pronucleus and 2nd polar body is extruded. So fertilization is completed by the fusion of male and female pronuclei and this completed within 20 hours resulting in single cell with diploid genetic constitution called the zygote.
Following fertilization, cleavage occur in form of a rapid successive mitotic divisions that produce the morula stage and then fluid filled cavity is formed and an inner cell mass identified attached eccentrically to the outer layer of flattened cells (trophoblast) the embryo at this stage is called the blastocyst.
Implantation:
The embryo remain in the fallopian tube for 3-4 days. It float freely for up to 72 hours in the uterine cavity. The zona is shed and the blastocyst adhere to the endometrium. Succcessful implantation requires a receptive endometrium that has been appropriately primed with oestrogen and progesterone.A variety of proteolytic enzymes may play a role in separating the endometrial cell and digesting the inter cellular matrix. The cells of blastocyst are differentiated into outer layer mass called trophoblasts give rise to the placenta and inner cell mass give rise to the embryo. The trophoblast differentiated into cytotrophoblast and invasive syncytiotrophpblast.
Following ovulation the corpus lulteum produces estrogen and progesterone to influence the receptivity for implantation leading to decidual changes.
The decidua basalis is the decidual layer directly beneath the site of implantation.
The decidua capsularis is the layer overlying the developing embryo and separating it from the rest of uterine cavity.
The decidua vera (parietalis) is the remaining lining the uterine cavity and it is fused with decidua capsularis by the 4th month. The decidua basalis enter in the formation of the basal plate of the placenta and it is invaded extensively by trophoblastic gait cell. Nitabuch's layer is a zone of fibrinoid degeneration where the trophoblast meets the decidua.
Embryology:
Following successful fertilization, the differentiation of cells into specialized tissues to form interrelated organ system, is known as the embryonic period. It starts with the generation of the embryonic disc during the 2nd week post fertilization (4th weeks after LMP) and ends at the last day of 8th weeks (10th weeks after LMP).
In the 3rd week:
The primitive streak, appears on the dorsal aspect of the germ disc.The mesodermal layer is generated and migrated between the ecto and endodermal layer.
The primary yolk sac grows rapidly, it is an important organ for exchanging metabolites between the mother and the embryo at the time when there is no placenta (full development by day 32 and degeneration by the end of 6th week).
In the 4th week:
The embryo folds into an embryonic cylinder, within which is a craniocaudal, blind ending tube which has 3 segments (forgut, midgut, hindgut).
Primitive heart (cardiac activity is evident by 22 day post fertilization).
Neurolation (development of the nervous system).
Towards the end of the 4th wk, the forgut septates along the mid line into respiratory and digestive primitive elements.
By day 26 the mesonephric duct and mesonephros differentiate and by day 28 the ureteric buds are defined.
By the end of the 4th wk almost all organ systems, albeit immature, can be readily identified.
The external appearance of the embryo change dramatically.