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Conception and Fertilization

By:Haider Mohammed

Gametogenesis

FemaleMaleOvumSpermatozoonMaturation process

GametogenesisBoth mitosis and meiosis play a role in gametogensis.Mitosis provide the precursor cells.Meiosis bring about the reduction division that result in gametes.

Cell Cycle

GametogensisSpermatogenesis Oogenesis

Oogenesis

Fully Matured Ovum

Fully matured ovum:Largest cell in the body, 130 micron in diameter Cytoplasm 23 chromosomes (23 x)

Spermatogensis

The process involved in the development of spermatids from the primordial male germ cells and their differentiation into spermatozoa is called spermatogenesis

Stracture of a mature spermatozoonIt has got two parts, a head and a tailThe head:- Condensed nucleus and acrosomal cap.- Acrosome is rich in enzymes.The tail:- Divided into four zones (the neck, the middle piece, the principle piece and the end piece.

Difference between spermatogensis and oogenesisSpermatogensis differ from oogensis in three ways:1- All four product of meiosis develop into sperm while only one of the four become an egg.2- spermatogensis occur throughout adolescence and adulthood.3- sperm are produce continuously without the prolonged interruption like in oogenesis.

OvulationOvulation is a process where by a secondary oocyte is released from ovary following of a mature graafian follicle and become available for conception.In each ovarian cycle, only one secondary oocyte is likely to rupture.It starts at puberty and end menopause.

Ovulation process

Change in the folliclePreovulatory enlargement of the graafian follicle due to accumulation of follicular fluid.Measure about 20 mm in diameter.The cumulus oophorus sprates from rest of granulosa cells.The follicular wall near the ovarian surface becomes thinner.

Change in the oocyteSignificant changes occur just prior to ovulation (few hours).Cytoplasmic volume is increased.Completion of the arrested first meiotic division occur with extrusion of first polar body, each containing haploid number of chromosomes (23 X).

CausesEndocrinal (LH surge, FSH rise)Stretching factors (a passive stretching process of granulosa cells).Contraction of the micromuscles in the theca externa and ovarian stroma due to increase prostaglandin secretion.

EndocrinalLH surge: sustained peak level of estrogen for 24 to 36 hours in the late follicular phase.LH surge occurs from the anterior pituitary. Ovulation approximately occurs 16-24 hours after the LH surge.LH peak persist for about 24 hours.The LH surge stimulates completion of reduction division of the oocyte and intiate luteinization of the granulosa cells, synthesis of progesterone and prostaglandins. FSH rise: preovulatery rise of progesterone facilitate the positive feed back action of estrogen to induce FSH surge lead to increase plasminogen activator. Plasminogen help lysis of the wall of the follicle.

Effect Of OvulationThe follicle is changed into corpus luteum.The ovum is peaked up into the fallopian tube and undergoes either degeneration or further maturation, if fertilization is to occur.Menstruation is unrelated with ovulation.

FertilizationFertilization is a process of fusion of the spermatozoon with the mature ovum.It begins with sperm egg collision and ends with production of mononucleated single cell called the zygot.Objectives: 1- To initiate the embryonic development of the egg. 2- To restore the chromosome number of the species.

Approximation of the gametesFertilizable life span of oocyte is 12 to 24 hours and sperm is 48 to 72 hours.Out of the hundreds in the vagina at single ejaculation, only thousands capacitated spermatozoa enter the uterine tube while only 300-500 reach the ovum.Tubal transport is facilitated by muscular contraction and aspiration action of the uterine tube.It take only few minutes for sperm to reach the fallopian tube.

Contact and Fusion of the gametesCapacitationAcrosomal reactionCortical reactionDevelopment of Zygote

CapacitationThe process of undertaking in the sperm when inside the female genital tract influenced by the secretion of the uterine tube is know as capacitation.Sperm must be in the female genital tract 4-6 hours before they can fertilize an ovum. The sperm undergoes changes in the removal of the glycoprotein coat.

Acrosomal ReactionThe acrosomal layer of the sperm becom reactive and release the enzyme hyaluronidase known as the acrosome reaction.It disperses the corona radiate (outer layer of ovum) allowing access to the zona pellucida.

Cortical reactionThe first sperm that reaches the zona pellucida penetrates it.Penetration of the zona pellucida occur with the aid of several enzymes processed by the sperm which break down the proteins of the zona layer.Upon penetration a chemical reaction know as the cortical reaction occur which makes it impermeable to other sperms.

The plasma membrane of the sperm and oocyte fuse.The ocyte at this stage compelet and second meiotic division and becomes mature, the pronuclei has 23 haploid chromosomes.Male and female pronuclei fuses to form a new nucleus that is a combination of the genetic material from both the sperm and the oocyte diploid cell

Development of zygoteThe male and female gametes each contribute half the complement of the chromosomes to make a total of 46. the new cell is called zygote.The zygote, thus formed, contain both the paternal and maternal genetic materials.Sex of the child is determined by the pattern of the sex chromosome supplied by the spermatozoonIf the spermatozoon contain X chromosome, a female embryo (46,XX) is formed, if it contain a Y chromosome , a male emberyo (46, XX) is formed.

Development of zygote divided into three periods:Pre-embryonic period: first two weeks after fertilization.Embryonic period: 2 8 weeks.Fetal period: 8 Weeks to birth

Pre-embryonic periodThis period include the implantation of the zygote into the endometrium.The embryo takes about 3 days to travel through the fallopian tube to reach the uterus.It takes another 3 days to get implanted, usually in the dorsal wall of the uterus.Thus embryo is about 1 week old when it gets implanted.Zygote undergo mitotic division and cellular replication known as cleavage which result in the formation of smaller cells known as blastomers.

During this period a strong membrane of glycoprotein called zona pellucida surrounds the zygoteThe zygote mainly gets nourishment (glycogen)from the goblet cells of the uterine tubes and later the secretory cells of the uterus.The zygote divided in to :2 cells at day 1,or 30 hours4 at 2 days,8 by 2.5 days,16 by 3 days and called as morula resembling a mulberry 16 to 64 cell stage

Cell bind tightly together in a process known as compactation.Next cavitation occur where by the outermost cells secrete fluid into the morula and a fluid filled cavity or blastocele appears.This result in the formation of the blaster or blastocyst comprising 58 cells.

BlastulationDevelopment of the morula to the blastocyst has occurred by day 4.

BlastocystTrophoblastPlacentaChorionInner cell massFetusAmnionUmbilical cord

Implantation Embedding of the embryo to the wall of uterus.Also called as Nidation.Occurs in the endometrium of the anterior or posterior wall of the body near the fundus on the 6th day which corresponds to the 20th day of regular menstrual cycle.Implantation occur through four stages apposition, adhesion, penetration and invasion.

1- appositionOccurs through pinopod formation. Pinopods are long finger like projections (microvilli) from the endometrial cell surface..This pinopods absorb the endometrial fluid which is secreted by the endometrial gland cells. This fluid, rich in glycogen and mucin provides nutrition to blastocyst intially.

2- AdhesionUnless this fluid is absorbed, adhesion phase cannot occur.Adhesion of blastocyst to the endometrium occurs through the adhesion molecules like integrin, selectin and cadherin (glycoproteins).

3, 4- penetration and invasionOccur through the stromal cells in between the gland and is facilitated by the histolytic action of the blastocyst.With increasing lysis of the stormal cells, the blastocyst is burrowed more and more inside the stratum compactum of the decidua.

The deciduaEndometrium during pregnancy is know as the decidua.Three layer are found.1- the basal layer: lie immediately above the myometrium. Remain unchanged in itself but regenerates the new endomertium during the puerperium2- the functional layer: consist of tortuous glands which are rich in secretions.

It provide a secure anchorage for the placenta and allows it access to nutrition and oxygen, but as soon as the baby is born separation can occur.

3- The compact layer: this layer form the surface of the decidua and is composed of closely packed stroma cells and the neck of the glands.

Germlayers Three layer include:1- The ectoderm: skin and nervous system2- The mesoderm: bones, muscles, heart, blood vessels and some visceral organs.3- The endoderm: mucous membrane and glands.Two cavities appear on either side of embryonic plate (amniotic cavity and yolk sac)

Development of placentaBy the end of 2nd month, trophoblast is character by great no. of secondary and tertiary villi.By four month cytotrophblastic cells and connective tissue disappear, only syncytium and endothelial vessel remain (tertiary villi).In early stage, villi covers the entire surface of chorion.As the pregnancy advances, villi on embryonic pole continues to grow and expand chorion frondosum (bushy chorion).

Villi on an embryonic pole degenerate- chorion leave. The fusion of amnion and chorion occurs to form aminochorionic membrane.Placenta is an organ that connects the developing fetus to the uterine wall to allow.Nutrient uptake.Waste elimination.Gas exchange by the mother blood supply.Hormone production.Formation of a barrier.Function as a fetomaternal organ with 2 components (fetal placenta and maternal placenta).

The placenta barrierSugar, fats and oxygen diffuse from mothers blood to fetus.Urea and CO2 diffuse from fetus to mother.Maternal antibodies actively transported across placenta. Some resistance to disease (passive immunity).Most bacteria are blocked.Many viruses can pass including rubella, chickenpox, sometime HIV.Many drugs and toxins pass including alcohol, heroin and mercury.

Placenta secretion of hormonesProgesterone and HCG (human choroinic gondatropin, the hormone tested for pregnancy): maintain the uterus.Estrogen and CRH (corticotropin releasing hormone): promote labor.