EMBRYOLOGY LECTURE 1

CELL DIVISIONS AND GAMETOGENESIS

Lecture plan

Definition of embryology

Cell divisions= mitosis/meiosis

Gametogenesis= (oogenesis/spermatogenesis)

Embryology is the study of the origin and development of an organism. In humans, development starts when an egg (oocyte) from female and a sperm (spermatozoa) from male unite and give rise to a new organism, the zygote in a process is called fertilization.

Cell division= two types of cell divisions

Mitotic division= the cell divides and give rise to two daughter cells with same number of chromosomes as the mother cell

Meiotic division= cell divides and give rise to 4 daughter cells with half number of chromosomes and half DNA material

Mitotic division=fig.1

Before the cell enters mitosis, each chromosome doubles its DNA and forms two chromatids. The chromosomes become long and are widely spread in the cytoplasm

Stages of mitosis

PROPHASE: When mitosis begins, the chromosomes begin to coil, contract and condense

METAPHASE. The chromosomes line up in the equatorial plane

ANAPHASE. Each chromosome undergoes a longitudinal division of the cetromere and give rise to two daughter chromosomes. The daughter chromosomes begins to migrate towards the opposite poles of the cell

TELOPHASE. The daughter chromosomes are at the opposite poles of the cell. The chromosomes uncoil and lenghthen, and the cell cytoplasm divide and gives rise to two daughter cells

DAUGHTER CELL: Each daughter cell has same number of chromosomes as the mother cell.

fig.1

Meiotic divisions=fig.2

The germ cells replicate their DNA before the commencement of meiotic division just like in mitotic division

Purpose= 1, to allow exchange of genetic materials between homologous chromosomes and 2, to reduce to half (23) the number of chromosomes and DNA material

First meiotic division

Purpose = to allow exchange of genetic material

Before the 1st meiotic begin, the germ cell replicates its DNA as in mitotic

There is pairing of homologous chromosomes in a process called synapses which is exact and point to point, except for X-Y combination.

The homologous pair contains 4 chromatids since each chromosome is double structured containing 2 chromatids

There is exchange of chromatids segments between the paired homologous chromosomes. The point of interchange is known as the chiasma. They then split longitudinally, having exchanged blocks of genetic materials, and each chromosome migrate toward opposite poles of the cell.

Each of the two daughter cells has 23 double-structured chromosomes and contains one member of each chromosome pair. The daughter cell has the same amount of DNA as normal cell because each chromosome is still double-structured.

Second meiotic division

Purpose= to reduce to half (23) the number of chromosomes and DNA amount

2nd meiotic division starts immediately after the end of 1st meiotic division and no duplication of DNA material

The 23 double -structure chromosomes divides at the cetromere and each daughter cell receive 23 chromatids. The DNA now is half that of somatic cell

Therefore, a germ cell is formed with haploid number of chromosomes and half amount of DNA

Results

In females, one germ cell gives rise to 4 daughter cells, and each has 22 + 1 X chromosomes. Only one out of the 4 matures into a gamete (tertiary oocyte). The rest 3 become polar

In male, one germ cell give rise to 4 daughter cells and all develop into mature gametes. Two daughter cells contains 22+1 X- chromosomes , and the other two contains 22+1 Y-chromosomes

Things may go wrong during meiotic division= Abnormalities

During the separation of homologous chromosomes in the first meiotic division, each daughter cell is supposed to receive one component of each pair. Sometimes separation does not occur (nondisjuction) and both members of a pair move to one cell. The result is that one cell receives 24 chromosomes and the other 22 chromosomes. Then, when a gamete with 23 chromosomes fuses with a gamete with 24 chromosomes, the result is a cell with 47 chromosomes =trisomy. Eg Downs syndrome is trisomy 21. A gamete with 23 chromosomes fuses with a gamete with 22 chromosomes chromosomes= monosomy

The nondisjuction can also occur during the 2nd meiotic division

fig.2

GAMETOGENESIS= FORMATION OF MALE AND FEMALE GAMETES FROM PRIMORDIAL GERM CELLS

During the process the gametes undergo chromosomal and morphological changes in preparation for fertilization

The normal body (somatic) cell has 46 chromosomes = 23 pairs. One chromosome of each pair from mother and the other from the father.

Primordial germ cells have 46 chromosomes

Germ cells undergo changes in order to form gamete. First, the number of chromosomes is reduced to half (23)= meiotic division. Second, there is alteration in shape. The female germ cell in the ovary becomes larger as its cytoplasm increases. The male germ cell in the testis becomes smaller as it loose a lot of cytoplasm and develops a head, neck, tail.

OOGENESIS= Formation of female gametes= the maturation process from primitive germ cell to mature gamete (tertiary oocyte)=fig.4

Before birth=Prenatal

Primordial germ cells are formed in the epiblast during the second week and appear in the wall of yolk sac at the end of the 3rd week. Fig.3

Primordial germ cells from the wall of yolk sac migrate to the developing ovaries where they arrive in the fifth week5th week and differentiate into oogonia. These cells undergo a number of mitotic divisions and by end 3rd month, they are arranged in clusters and are surrounded by flat epithelial cells.

Majority of oogonia continue with mitotic division while some differentiate into much larger primary oocyst. The primary oocyst replicates their DNA and enters the prophase of the 1st meiotic division, and they are surrounded individually by flat epithelial cells. A primary oocyte, together with its surrounding flat epithelial cells is known as a primordial follicle.

Mitotic division continues and by 5th month of development, the total number of germ cells reaches its maximum estimated at 7 million. Then cells degeneration (atresia) starts, for both oogonia and primary oocyst. By 7th month, majority of oogonia have degenerated and the surviving primary oocytes have entered the prophase of the 1st meiotic division. However, they don’t proceed into the metaphase but, rather they enter into a resting phase during prophase called dictyotene stage.

Fig.3

AT BIRTH AND AFTER= postnatal

At birth, all primary oocytes have finished the prophase of the 1st meiotic division. Primary oocytes remain in prophase and do not finish their first meiotic division before puberty is reached. Their number is estimated at 0.7 to 2 millions. During childhood, majority of the primary oocytes degenerate and at puberty, their number is approximately 40,000.

With the onset of puberty, (1) a number of primordial follicles begin to mature with each ovarian cycle and (2), the primary oocytes complete their first meiotic division. The primary oocyte, still in dictyotene stage increases in size while the surrounding flat epithelial cells, the follicular cells change from flat to cuboidal. The follicle is now called the primary follicle. The primary oocyte is initially in intimate contact with the surrounding follicular cells but later, a layer of acellular material called

zona pellucida develops In between the follicular cells and the primary oocyst.

As growth continues, the follicular cells proliferate, forming a thick cellular layer around the oocyte. In between follicular cells, fluid filled cavities are formed which later coalesce to form one big cavity= follicular antrum. The follicular cells that remain surrounding the oocyte are known as cumulus oophorus. A mature follicle is called tertiary or vesicular follicle. It is surrounded by two layers of connective tissues; an outer fibrous layer called theca externa, and an inner layer called theca interna, which is rich in blood vessels.

With each ovarian cycle, a number of follicles begin to develop but only one reaches maturity while the other degenerate. As soon as the follicle is mature, the primary oocyte resumes its first meiotic division. The results are 2 daughter cells= (i)secondary oocyte, which receive almost all the cytoplasm, and (ii)the polar body, which hardly receive any cytoplasm. The secondary oocyte is shed from the ovary and it will only complete its 2nd meiotic division if it is fertilised, otherwise it degenerates approximately 24 hours after ovulation.

fig.4; OOGENESIS

SPERMATOGENESIS= the maturation process from primordial germ cell to mature male gametes-fig.5

fig.5

The primordial germ cells in males migrate to the sex cords of the testis and they don’t differentiate until puberty unlike in females where differentiation begins in the third month. At birth, the germ cells can be recognised and are surrounded by supportive cells, which later becomes the sunstentacular or sertoli cells.

Shortly before puberty, the sex cord acquires a lumen and becomes the seminiferous tubules. At the same time, the primordial germ cells give rise to spermatogonia. Spermatogonia differentiate into primary

spermatocytes. The primary spermatocytes replicate their DNA and then enter the prophase of the 1st meiotic division. The prophase lasts about 16 days, and then the following phases are completed and two secondary spermatocytes are formed. Each of these cells enter the 2nd meiotic division and results in production of two spermatids.

Spermiogenesis= conversion of spermatid to spermatozoa

The spermatid undergoes series of changes which results in production of the spermatozoa. The changes include; formation of acrosome, condensation of nucleus, formation of neck, body, tail, shedding of most of cytoplasm. It takes approximately 61 days for a spermatogonia to develop into a spermatozoa.

When fully formed, spermatozoa are pushed from seminiferous tubules and are stored in the epididymis where they are capacitated=become motile.

Abnormal spermatozoa are often seen; giant, dwarf, deformed head, tail, sometimes joined. If abnormality affects more than 25% of spermatozoa, fertility is usually impaired.

Events occurring during the first and second maturation divisions. A. Theprimitive female germ cell (primary oocyte) produces only one mature gamete, the matureoocyte. B. The primitive male germ cell (primary spermatocyte) produces four spermatids,all of which develop into spermatozoa.

Reference

Foundations of Embryology, 1st ed., Carlson (1998), New Yolk

An introduction to embryology, B.I balinsky, Sauder College

Text book for medical embryology: Normal and Abnormal development, 1sr ed, F. Fasana (2008), KLB, NRB

Longmans medical embryology, 6th ed, T.W. sadler (1990),

Patterns-Bruce M, McGraw Hill

E resources- http://www.med.unc.edu/embryoimages