Lesson # 12

Lesson # 12Early Development

OOGENESIS

MITOSIS

OogoniaDiploid

Diploid

Before birth

MEIOSIS I

After puberty

It stops inprophase

MEIOSIS ICompleted

Primaryoocyte

Haploid

Secondaryoocyte

It stops inmetaphase Before ovulation

First polarbody

After ovulationSecondpolarbody

MEIOSIS IICompleted Haploid

Secondaryoocyte

If fertilizationoccurs

MEIOSIS II

Oogenesis

Primary oocytes (diploid) in prophase I of MEIOSIS I.

FSH triggers the start of the ovarian cycle.MEIOSIS I is completed to form one secondary oocyte (haploid) and the first polar body.

Between the third and seventh month of fetal life:

Primary oocytes (diploid) begin MEIOSIS I but it is stopped in prophase I.

During reproductive life: Every month one secondary oocyte begins MEIOSIS II that is stopped in metaphase II. Ovulation occurs, and if the secondary oocyte is fertilized, MEIOSIS II is completed to form the ovum and the second polar body.

At birth:

At puberty:

Oogonia undergo mitosis and produce primary oocytes (diploid).

Primordial follicles

Primary follicles

Secondary follicles

Tertiary follicles

Ovulation

Oogenesis

FSH

One of the primordial follicles matures (once every 28 days) to primary follicle. Meiosis I is completed (haploid secondary oocytes)

Primary oocytes (diploid) joint with nurturing complexes of follicle cells

Corpus albicans

7

Primordial follicles 1

Primary follicle

2

Secondary follicle 3

Tertiary or Mature Graffian follicle

4

The mature follicle contains the secondary oocyte (in metaphase II), which is expelled at the end of the process and then begins the journey through the uterine tubes

5 OvulationCorpus luteum

6

If fertilization does not occur, the corpus degenerates into the corpus albicans

The follicle is transformed in the corpus luteum (yellow body), which for a time produces progesterone that facilitate pregnancy

Once it has been released, the oocyte begins a slow journey to the uterus through the uterine tubes (fallopian tubes)

Ovulation

While within the tube, the oocyte may encounter sperm and become fertilized prior to entering the uterus.

It is the exocytosis of the acrosome, releasing the enzymes needed to penetrate the egg

Hyaluronidase, which digests the hyaluronic acid that binds granulosa cells together.

When a path has been cleared, a sperm binds to the zona pellucida.

Two acrosomal enzymes are released:

Acrosin, a protease similar to trypsin.

Acrosomal reaction1

It prevents the polyspermy (fertilization by two or more sperm).

Cortical reaction2Fertilization membrane

Rejected sperm

Cortical granules

Oocyte Activation

Fertilization and Oocyte Activation

Amphimixis:It is the process by which the male and female pronucleus fuse to form a zygote that contains 46 chromosomes.

The formation of the zygote is the “moment of conception”.

Cleavage:It is a series of cell divisions (mitotic divisions) that produce an ever-increasing number of smaller and smaller daughter cells.The final product of the cleavage is the formation of the morula.

Spermpronucleus

Eggpronucleus

Zygote2-celled stage

DAY 1

1-Cleavage:

Blastomeres It is a solid ball of 16 cells that resemble a mulberry.

The morula lies free in uterine cavity for 4-5 days and divides into a 100 cells or so.

The zona pellucida dis-integrates and releases conceptus: blastocyst.

The Preembryogenic Stage It comprises the first 16 days of development, culminating with the existence of an embryo.

1- Cleavage 2- Implantation3- Embryogenesis

4-celled stage DAY 2

Early morula DAY 3

Advanced MorulaDAY 4

Blastocyst DAY 6

Implanted blastocyst

Fertilization(0 hours)

3- Placentation

2- Implantation DAYS 7-10

It is a series of cell divisions (mitotic divisions) that produce an ever-increasing number of smaller and smaller daughter cells.

Hatching

Cleavage and Blastocyst Formation

Zygote

(DAY 8)

Implantation

Endometrium:

Blastocyst:

(DAY 7)

Blastocoel

TrophoblastInner cell mass

EpitheliumEndometrial gland

The blastocyst attaches to uterine wall 6 days after ovulation, usually on the fundus or the posterior wall of the uterus.

Implantation: It is the process of attachment to uterine wall that begins when blastocyst adheres to the endometrium.

The trophoblast on the attachment side separates into two layers:

The superficial layer in contact with the endometrium. The plasma membranes break down and trophoblastic cells fuse into a multinucleate mass called syncytial trophoblast.

Syncytial trophoblast

The deep layer, close to embryoblast, retains the individual cells divided by membranes and is called cellular trophoblast.

Cellular trophoblast

Inner cell mass (or embryoblast)

Trophoblast:

The trophoblast secrets human chorionic gonadotropin (HCG), which stimulates the corpus luteum to secret estrogen and progesterone (it suppresses menstruation).

Stages of Implantation

Formation of the Amniotic Cavity

It is the arrangement of the inner cell mass into three primary germ layers: ectoderm, mesoderm, and endoderm

Gastrulation and Germ Layer Formation

Endoderm

Mesoderm

Ectoderm

The inner cell mass separates slightly from the trophoblast and creates a narrow space between them: the amniotic cavity

Amniotic cavity

Yolk sac

Once the three primary germ layers are formed, embryogenesis is complete and the individual is considered an embryo. It is about 2 mm long and 16 days old.

Inner cell mass Cellular trophoblast

Syncytial trophoblast

Inner cell mass

Gastrulation:

Gastrulation and Germ Layer Formation

Inner cell mass

The First Trimester

Figure 29–5 Extraembryonic Membranes and Placenta Formation.

Placenta

Amniotic cavity

Umbilical cord

Myometrium

Endometrium