Fertilization, implantation &early embryo development 17-18
Somatic &germ cellsThe human body has 2 types of cells: Somatic cells which are diploid(46chrom- osomes) ,a pair of sex chromosomes which areX&Yand 2 copies ofeach of the autosomal chromosomes. Germ cells are haploid i.e.XorYwith one copy of each uatosome.
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Is the consequences of several complex events that include the final maturation of oocyte and spermatozoa ,transport of the gametes in the female genital tract and following the fusion of the male & female gametes, the assembly of diploid number of chromosomes. .
spermatogenesis
Is the production of mature sperm occur in seminiferous tubules of the testis.the primordial germ cells enter the gonad &devid to form spermatogonia(46) 1ry spermatosyte 2ryspermatocyte spermatozoa spermatid(23) (23)The alkaline PH of the semen & of the cervical secretion protect the sperm & the
sperm moves in the cervical mucus by propulsive effect of the tail aided by the hydrolytic enzymes produced by the acrosomal cap also the uterine contraction will help. Capacitation is the physiologic change that sperms must undergo in the female genital tract before fertilization. Acrosome reaction The acrosome lies over the sperm head as a kind of a modified lysozyme designed to enable the sperm to burrow its way into the oocyte (chemical drill bite)Fertilization
Is the union of male&female pro nuclear element. The ovulated eggs is picked up by the fimbria of the Fallopian tube & then swept by ciliary actions toward the ampulla where fertilization occurs. the ovum temporarily halted for 38 hours & only released by the rising level of P from the corpus lutem (allow time for fertilization)the spermatozoa have the capacity to retain their potency in tube for at least 48hr .Following penetration of the oocyte, the sperm nucleus decondenses to form the male pronucleus, the egg must complete its
2nd meiotic division to form the haploid female pronucleus & a 2nd polar body is extruded. Fertilization is completed by the fusion of male & female pronucles & is completed within 20 hr resulting in single cell with diploid genetic constitution called the zygote.
Cleavage, Morula ,Blastocyst
Following fertilization, cleavage occur in form of a rapid successive mitotic divisions that produce a mulberry like mass (morula). & then a fluid filled cavity is formed & 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 fertilized ovum reaches the endometrial cavity about 3 days after ovulation. The zona is shed & the blastocyst adhere to the endometrium. A variety of proteolytic enzymes may play a role in separating the endometrial cell & digesting the intercellular matrix. Initially, the wall of the blastocyst facing the uterine lumen consist of single cell layer, the thicker opposite wall has two zones, the 1- trophoblast & 2- the inner cell mass (embryoblast ,embryonic disc )Fertilization-first week
By the Fusion of the pronuclei, zygote formed Morula(12-16) blastomere Blastocyst 4-5d a fluid accumulates & increases to separate the cells into 2 parts flattened outer trophoblast & centrally located inner cell mass or embryoblast ;& implantation beginsEnter the uterus
hypoblast
hypoblastThe trophoblast differentiate into inner cytotrophoblast &outer syncytiotrophoblast which erod endometrial blood vessels.
2nd week
As implantation occurs ,changes occur in the inner cell mass that produce the a bilaminar embryonic disc. (epiblst& hypoblast). A small cavity between the inner cell mass & the trophoblst this is the beginning of the amniotic cavity From the site of the hypoblast, the yolk sac formed
Decidual Changes Under the influence of progesterone, the decidua
thickens to a depth of 5-10 mm. The decidua basalis is the decidual layer directly beneath the site of implantation. The decidua capsularis is the layer overlying the developing ovum & seperating it from the rest of the uterine cavity. The decidua vera(parietalis) is the remaining lining of the uterine cavity & its fused with decidua capsularis by the 4th month.The embryonic period
it starts with the generation of the embryonic disc during the 2ndweek post fertilization(4 wks after LMP) & ends at the last day of the 8th wks(10wks after LMP).3rd week
A thick linear band of epiblast, The primitive streak appears caudally on the dorsal aspect of the germ disc . Some of cells proliferate &migrate from the streak to form The mesodermal layer some cells invade the hypoblast to form the embryonic endoderm. The epiblastic cells that remain on the surface called the embryonic ectoderm4th week
Folding the embryo in the median & horizontal plans , converts the trilaminar disc into C-shaped cylinder with formation of head ,tail & incorporation of the yolk sac inside as a primitive gut. Primitive heart , cardiac activity is evident byday22post fertilization. Neurlation (development of nervous systemPri.heart
Embryonic cylinderAt 26-28 day the mesonephric duct, the mesonephrone, ureteric buds are defined. By the end of 4th week the body of the embryo attached to the yolk sac by vitelline duct almost all organ systems, albeit immature can be readily identified.
Changes in the external appearance
During the ensuing 3 weeks the outer aspect of the human embryo changes dramatically .the head start to grow faster,the eyes,ears,nose &mouth recognized. The development of upper limps precedes the lower; the upper limb bud appear at 28 th day By transvaginal US, gestational sac seen within the uterus3- 4w after last menstrual period fetal heart beat seen5-6w.
Further growth &systemic development
The primitive forgut &hidgut are present by end of fourth w.midgut herniates into the base of umbilical cord during 6th w.then undergoes rotation prior to re-entering the abdomen by 12w.Failure to enter results in omphalocele. The swallowing reflex mature gradually ;the fetus swallows amniotic fluid up to 2oml/h at term &failure in this results in increased fluid (polyhydramnios)Alimentary system&energy stores
The large bowel is filled with meconium at term .Defecation in utero, &hence meconium in amniotic fluid, is associated with post-term pregnancy &fetal hypoxia. While body water diminishes,glycogen &fat stores increase graduallyLiver &gallbladder
Primitiveliver appears at about18th day as a diverticulum arise from duodenum. In utero,metabolic function of liver performed by placenta eg unconjugated blirubin from hemoglobin breakdown is actively transported to the mother with small amount conjugated in liver &secreted in the bile (meconium)Physiological jaundice
There is a relative reduction in the necessary enzymes eg glucoronyl transferase . premature infant loss of placental excretion of unconjugated bilibubin reduced conjugationhyperblirubinaemia (jaundice)
Kidney & urinary tract
Nephrogenesis completed by 36 w but maturation of excretory & concentrating ability of the fetal kidney is gradual Productio of fetal urine rises with maturity & it forms much of the amniotic fluid which is protein free ,sugar-free, hypotonic ultra-filtrate plasmaThe fetus requires an effective immune system to resist intrauterine &perinatal infection. IgG origenate mostly from the mother &cross the placenta to provide passive ummunity The fetus form only small amounts of IgM &IgA which donot cross the placenta detection of them in the new born without IgG, indicate fetal infection