GENERAL EMBRIOLOGY

GAMETOGENESIS

The beginning of life: development begins with fertilization, the process by which the

male gamete, the sperm, and the femalegamete,theoocyte,unitetogiverisetoazygote.-

Two highly specialised sexual haploid cells fuse and give rise to another diploid one

whith an unique dna.

• Where do gametes come from?

Gametes are derived from primordial germ cells (PGCs). These are diploid cells

capable of differenciating. In contrast, somatic cells are the ones found in the

rest of the body and are uncapable of differenciating , although they are diploid

too.

• Origin and migration of PGC

PGC are formed in the epiblast during the second week and that move to the

wall of the yolk sac ( initially intended to feed the embryo). During the fourth

week these cells begin to migrate from the yolk sac toward the developing

gonads, where they arrive by the end of the fifth week. Mitotic divisions

increase their number during their migration and also when they arrive in the

gonad. In preparation for fertilization, germ cells undergo

- Gametogenesis: which includes meiosis, to reduce the number of

chromosomes

- Cytodifferentiation: to complete their maturation.

• Reduction of genetic information --- Meiosis

- Consequences : genetical variability + reduction genetic information

� Teratomas are tumors of disputed origin that often contain a variety of tissues,

such as bone, hair, muscle, gut epithelia, and others. It is thought that these

tumors arise from a pluripotent stem cell that can differentiate into any of the

three germ layers or their derivatives.

• Abnormal divisions : Abnormalities in chromosome number may originate

during meiotic or mitotic divisions

- In meiosis, two members of a pair of homologous chromosomes normally

separate during the first meiotic division so that each daughter cell receives

one member of each pair .Sometimes, however, separation does not occur

and is called nondisjunction.This occurs during either the first or the second

meiotic division of the germ cells. As a result of nondisjunction of the

chromosomes, one cell receives 24 chromosomes, and the other receives

22 instead of the normal 23.

- Mitotic nondisjunctionin an embryonic cell during the earliest cell divisions.

Such conditions produce mosaicism, with some cells having an abnormal

chromosome number and others being normal. Affected individuals may

exhibit few or many of the characteristics of a particular syndrome,

depending on the number of cells involved and their distribution.

SPERMATOGENESIS

Consists on the formation of male gametes / sperm from the PGC.

• 5th week

PGC reach the male gonad which is still in an early stage of life.

• Birth

PCG in the male can be recognized in the sex cords of the testis as large, pale

cells surrounded by supporting cells which are derived from the surface

epithelium of the gland in the same manner as follicular cells, become

sustentacular cells, or Sertoli cells. They keep resting until puberty.

• Puberty

The sex cords develop into the seminiferous tubules due to light. At about the

same time, primordial germ cells give rise to spermatogonial stem cells.

At regular intervals, cells emerge from this stem cell population to form type A

spermatogonia, and their production marks the initiation of spermatogenesis.

Type A Spermatogonia

limited number of mitotic divisions

last cell division

Type B spermatogonia

1 generation goes through mitosis

Primary spermatocytes 46 XY

enter a prolongued prophase (22 days)

completion of meiosis I.

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Secondary spermatocytes.

23 double chromosomes

Meiosis II

Spermatids

23 single chromosomes

� Throughout this series of events, from the time type A cells leave the stem cell

population to formation of spermatids, cytokinesis is incomplete, so that

successive cell generations are joined by cytoplasmic bridges. They are

produced in waves.

• Spermiogenesis ( 64 days)

The spermatids are inmmature sperms that must undergo morphological

changes to become functional. Thesechangesinclude

- Formationoftheacrosome, which covers half of the nuclear surface and

contains enzymes to assist in penetration of the egg and its surrounding

layers during fertilization

- condensation of the nucleus

- formation of neck, middle piece, and tail

- shedding of most of the cytoplasm – residual bodies

• Motion

When fully formed, spermatozoa enter the lumen of seminiferous tubules.

From there, they are pushed toward the epididymis by contractile elements in

the wall of the seminiferous tubules.Although initially only slightly motile,

spermatozoa obtain full motility in the epididymis.

• Abnormal gametes

Abnormal spermatozoa are seen frequently, and up to 10% of all spermatozoa

have observable defects. The head or the tail may be abnormal; spermatozoa

may be giants or dwarfs; and sometimes they are joined.

OOGENESIS

• 5th week

Once primordial germ cells have arrived in the female gonad (ovary) they

differentiate into oogonia 2N2C.

• First months - End 3rd month

These cells experiment mitosis massively

Arranged in clusters surrounded by a layer of flat epitelial ( follicullar cells)

forming clusters.

• Next months

The majority of oogonia continue to divide by mitosis, but some of them arrest

their cell division in prophase of meiosis I and form primary oocytes 2N4C

During the next few months ,oogonia increase rapidly in number

• Fifth month of prenatal development

The total number of germ cells in the ovary reaches its maximum, estimated at

7 million. At this time, cell death begins, and many oogonia as well as primary

oocytes become atretic.

• Seventhmonththemajorityofoogoniahavedegeneratedexcept.Allsurvivingprima

ryoocyteshaveenteredprophase of meiosis I, and most of them are individually

surrounded by a layer of flat epitelial

cells.Aprimaryoocyte,togetherwithitssurroundingflat epithelial cells, is known

as a primordial follicle.

• Near the time of birth

All primary oocytes have started prophase of meiosis I, but instead of

proceeding into metaphase, they enter the diplotene stage, a resting stage.

Primary oocytes remain in prophase and do not finish their first meiotic division

before puberty is reached, apparently because of oocytematurationinhibitor

(OMI)

• During childhood

Most oocytes become atretic; only approximately 400,000 are present by the

beginning of puberty, and fewer than 500 will be ovulated

• Puberty

Each month, 15 to 20 follicles selected from this pool begin to mature, passing

through three stages:

1. As the primordial follicle is stimulated by FSH , surrounding follicular cells

change from flat to cuboidal

andproliferatetoproducestratifiedepitheliumofgranulosa cells –now is called

a primary follicle2N4C

- Granulosa cells rest on a basementmembraneseparatingthemfromthe

ovarythatformthe theca folliculi. As follicles continue to grow, cells of the

theca folliculi organize into an inner layer of secretory cells, the theca

interna, and an outer fibrous capsule, the theca externa.

- Also, granulosa cells and the oocyte secrete a layer of glycoproteins on the

surface of the oocyte, forming the zona pellucida.

- Small, finger-like processes of the follicular cells extend across and provide

blood and oxygen.

2. Fluid-filled spaces appear between granulosa cells. Coalescence of these

spaces forms the antrum, and the follicle is termed a secondary (vesicular,

Graafian) follicle. 2N4C

- Granulosa cells surrounding the oocyte remain intact and form the cumulus

oophorus.

3. Tertiary follicle or Graaf follicle

- With each ovarian cycle, a number of follicles begin to develop, but usually

only one reaches full maturity. The others degenerate and become atretic.

- When the secondary follicle is mature, a surge in luteinizing hormone (LH)

induces the preovulatory growth phase. Meiosis I is completed,

resultinginformationoftwodaughtercellsofunequalsize,eachwith23doublestr

uctured chromosomes. 1N2C

∗ the secondary oocyte, receives most of the cytoplasm

∗ the first polar body, receives practically none. It lies between the

zona pellucida and the cell membrane of the secondary oocyte in

the perivitelline space.

4. The cell then enters meiosis II but arrests in metaphase approximately 3

hours before ovulation. Meiosis II is completed only if the oocyte is

fertilized; otherwise, the cell degenerates approximately 24 hours after

ovulation.

• Abnormalities

- Follicles with 2 oocytes

- Oocytes with 3 nucleus

THE OVARIAN CYCLE

Aproximate duration : 28 days

• 1st phase : Menstruation – gets rid previous layer

• The follicular or proliferative phase (5-14)

At puberty, the female begins to undergo regular monthly cycles. These sexual cycles

are controlled by the hypothalamus. Gonadotropin-releasing hormone (GnRH)

produced by the hypothalamus acts on cells of the anterior pituitary gland, which in

turn secrete gonadotropins.

- The hormone is not necessary to promote development of primordial

follicles to the primary follicle stage, but without it, these primary follicles

die and become atretic.

- Follicular cells then grow and continue meiosis I.

In cooperation, granulosa and thecal cells produce estrogens that:

- cause the uterine endometrium to enter the follicular or proliferative

phase

- cause thinning of the cervical mucus to allow passage of sperm

- stimulate the pituitary gland to secrete LH.

• Mid-cycle ( Around day 14)

At mid-cycle, there is an LH surgeand an increase in FSH caused by an increase

in strogens act in the hypophysis that

- elevates concentrations of maturation-promoting factor, causing oocytes

to complete meiosis I and initiate meiosis II

- Stimulates production of progesterone by follicular stromal cells

(luteinization)

- causes follicular rupture :Meiosis II is also initiated, but the oocyte is

arrested in metaphase approximately 3 hours before ovulation. In the

meantime, the surface of the ovary begins to bulge locally, and at the apex,

an avascular spot, the stigma, appears. The high concentration of LH

increases collagenase activity, resulting in digestion of collagen fibers

surrounding the follicle. Prostaglandin levels also increase in response to

the LH surge and cause local muscular contractions in the ovarian Wall

which extrude the oocyte, which together with its surrounding granulosa

cells from the region of the cumulusoophorus, breaks free (ovulation) and

floats out of the ovary .Some of the cumulus oophorus cells then rearrange

themselves around the zona pellucida to form the corona radiata.

- After ovulation, granulosa cells remaining in the wall of the ruptured

follicle, under the influence of LH, develop a yellowish pigment and change

into lutean cells, which form the corpus luteum and secrete the hormone

progesteronewhich causes the uterine mucosa to enter the progestational

or secretory stage in preparation for implantation of the embryo.

� Ooccyte transport

It is carried into the tube by these sweeping movements of the fimbriae and by

motion of cilia on the epithelial lining. Once in the tube, cumulus cells withdraw

their cytoplasmic processes from the zona pellucida and lose contact with

theoocyte. Once the oocyte is in the uterine tube, it is propelled by cilia with

the rate of transport regulated by the endocrine status during and after

ovulation.

• Secretory or lutheinic phase (14-28)

Progesterone finalices the necessary endometrium preparation in order to the

embryo in case of fecundation. The rise of progesterone in blood causes a rise

in the basal temperatura of the woman´s body.

• No fertilization

If fertilization does not occur, the corpus luteum reaches maximum

developmentapproximately 9 days after ovulation. It can easily be recognized

as a yellowish projection on the surface of the ovary. Subsequently, the corpus

luteum shrinks because of degeneration of lutean cells and forms a mass of

fibrotic scar tissue, the corpus albicans. Simultaneously, progesterone

production decreases, precipitating menstrual bleeding.

• Fertilization

Degeneration of the corpus luteum is prevented by human chorionic

gonadotropin (hCG).The corpus luteum continues to grow and forms the corpus

luteum of pregnancy (corpus luteum graviditatis). Yellowish luteal cells

continue to secrete progesterone until the end of the fourth month;

thereafter,theyregressslowly assecretionofprogesterone byte trophoblastic

component of the placenta becomes adequate for maintenance of pregnancy.

FERTILIZATION

Fertilization,theprocessbywhichmaleandfemalegametesfuse,occursinthe ampullary

region of the uterine tube ( trompa de Falopio).

Spermatozoid are deposited in the vagina during the coitus. Spermatozoids are in a

fluid médium, either semen or sperm, which have a basic pH, while the woman´s

vagina has an acid pH. The semen makes the pH rise and this allows the spermatozoa

travel to the uterine tuve where the ooctyte is. Spermatozoa quickly travel from the

vagina to the utherus and then to the utherine tube thaks to contractions of the

muscular layers from the utherus and the tube .However, spermatozoa are not able to

fertilize the oocyte immediately upon arrival in the female genital tract but must

undergo

- Capacitation: is a period of conditioning in the female reproductive tract

that in the human lasts approximately 7 hourswhich occurs in the uterine

tube, entails epithelial interactions between the sperm and mucosal surface

of the tuve in which glycoprotein coat and seminal plasma proteins are

removed from the plasma membrane that overlies the acrosomal region of

the spermatozoa

- The acrosome reaction :release ofenzymes needed to penetrate the zona

pellucida, including acrosin and trypsin-like substances, which occurs after

binding to the zona pellucida.

1. Phase 1: penetration of the corona radiata

Of the 200 to 300 million spermatozoa deposited in the female genital tract,

only 300 to 500 reach the site of fertilization. Only one of these fertilizes the

egg.

2. Phase 2: penetration of the zona pellucida

Both binding and the acrosome reaction are mediated by the ligand ZP3.

Release of acrosomal enzymes (acrosin) allows sperm to penetrate the zona,

thereby coming in contact with the plasma membrane of the oocyte.

3. Phase 3: fusion of the oocyte and sperm cell membranes

In the human, both the head and tail of the spermatozoon enter the cytoplasm

of the oocyte,but the plasma membrane is left behind on the oocyte surface.

- Cortical and zona reactions.Asaresultofthe releaselysosomal enzymes, the

oocyte membrane becomes impenetrable to other spermatozoa, and the

zona pellucida alters its structure and composition to prevent sperm

binding and penetration. These reactions prevent polyspermy (penetration

of more than one spermatozoon into the oocyte).

- Resumptionofthesecondmeiotic division

∗ the second polar body: receives hardly any cytoplasm

∗ other daughter cell is the definitive oocyte. Its chromosomes (22+X)

arrange themselves in a vesicular nucleus known as the female

pronucleus.

The spermatozoon, meanwhile, moves forward until it lies close to the

female pronucleus. Its nucleus becomes swollen and forms the male

pronucleus , the tail detaches and degenerates. Morphologically, the

male and female pronuclei are indistinguishable.Each pronucleus must

replicate its DNA. If it does not, each cell of the two-cell zygote has only

half of the normal amount of DNA.

• The main results of fertilization

- Restoration of the diploid number of chromosomes, half from the father

and half from the mother.

- Determination of the sex of the new individual. An X-carrying sperm

producesafemale(XX)embryo,andaY-carryingspermproducesamale (XY)

embryo. Hence, the chromosomal sex of the embryo is determined at

fertilization.

- Zygote + Initiation of cleavage.

Uterus at Time of Implantation

The wall of the uterus consists of three layers:

1. endometrium or mucosa lining the inside Wall

2. myometrium, a thick layer of smooth muscle; and

3. perimetrium, the peritoneal covering lining the outside wall

During this menstrual cycle, the uterine endometrium passes through three stages

1. The proliferative phasebeginsattheendofthemenstrualphase,isunder the

influence of estrogen, and parallels growth of the ovarian follicles.

2. The secretory phase begins approximately 2 to 3 days after ovulation in

response to progesterone produced by the corpus luteum.

- If fertilization does not occur, shedding of the endometrium (compact and

spongy layers) marks the beginningofthemenstrualphase.

- Iffertilizationdoesoccur,theendometriumassists in implantation and

contributes to formation of the placenta.

3. At the time of implantation, the mucosa of the uterus is in the secretory

phaseduring which time uterine glands and arteries become coiled and the

tissue becomes succulent. As a result, three distinct layers can be recognized in

the endometrium: a superficial compact layer, an intermediate spongy layer,

and a thin basal layer.Normally, the human blastocyst implants in the

endometrium along the anterior or posterior wall of the body of the uterus,

where it becomes embedded between the openings of the glands . If the

oocyte is not fertilized, venules and sinusoidal spaces gradually become packed

with blood cells, and an extensive diapedesis of blood into the tissue is seen.

4. When the menstrual phase begins, blood escapes from superficial arteries,

and small pieces of stroma and glands break away. During the following 3 or 4

days, the compact and spongy layers are expelled from the uterus, and the

basal layer is the only part of the endometrium that is retained . This layer,

which is supplied by its own arteries, the basal arteries, functions as the

regenerative layer in the rebuilding of glands and arteries in the proliferative

phase