- 1. Basic Embryology 3
- 2. Development of the Eye 8
- 3. Development of the Extraocular Muscles 19
- 4. Anatomy of the Eye 22
- 5. Blood Supply 27
- 6. Tear Film Physiology 39
- 7. Aqueous Humor Dynamics 55
- 8. Physiology of Accommodation and Presbyopia 66
- 9. Photochemistry of Vision 75
- 10. Color Vision 86
- 11. Electrophysiological Tests 91
- 12. Ophthalmic Genetics 99
- INTRODUCTION
- FERTILIZED OVUM
- TWO-CELL STAGE
- CLEAVAGE
- MORULA
- FUNCTION OF THE ZONA PELLUCIDA
- BILAMINAR GERM DISK
- EXTRAEMBRYONIC MESODERM
- FORMATION OF CHORION AND AMNION
- PROCHORDAL PLATE
- PRIMITIVE STREAK
- TRILAMINAR GERM DISK
INTRODUCTION
When a sperm meets the ovum, fertilization takes place. This occurs in the ampulla of the uterine tube. One spermatozoon pierces the zona pellucida and enters the ovum. After one spermatozoon has entered the ovum other spermatozoa cannot enter it. The chromosomes of the ovum now assume the shape of nucleus called the female pronucleus. At the same time the head of the spermatozoon separates from the middle piece and tail, and transforms itself into the male pronucleus.
FERTILIZED OVUM
The male and female pronuclei meet but they do not fuse to form one nucleus. Their nuclear membranes disappear and their chromosomes become distinct. Each pronucleus has 23 chromosomes so that the fertilized ovum now has 46 chromosomes in all (Fig. 1.1).
TWO-CELL STAGE
Now, two daughter cells are formed. This is called the two-cell stage of the embryo (Fig. 1.2).
Important points to note at this stage are that:
- the two daughter cells are still surrounded by the zona pellucida.
- each daughter cell is much smaller than the ovum. As subsequent divisions occur the cells become smaller and smaller until they acquire the size of the cells of the body.
CLEAVAGE
The two cells formed undergo a series of divisions. One cell divides first so that we have a “3-cell” stage of the embryo (Fig. 1.3) followed by a “4–cell” stage (Fig. 1.4), a “5–cell” stage, etc.4
This process of subdivision of the ovum into smaller cells is called cleavage.
MORULA
As cleavage proceeds the ovum comes to have 16 cells. It now looks like a mulberry and is called the morula (Fig. 1.5). It is still surrounded by the zona pellucida. If a section is cut across the morula we see that it consists of an inner cell mass that is completely surrounded by an outer layer of cells. The cells of the outer layer will later give rise to a structure called the trophoblast (Fig. 1.6). The inner cell mass gives rise to the embryo proper, whereas the cells of the trophoblast help to provide nutrition to the embryo.
Some fluid now passes into the morula from the uterine cavity, and partially separates the cells of the inner cell mass from those of the trophoblast (Fig. 1.7). As the quantity of fluid increases, the morula acquires the shape of a cyst. The cells of the trophoblast become flattened, and the inner cell mass comes to be attached to the inner side of the trophoblast on one side only (Fig. 1.8). The morula has now become a blastocyst. That side of the blastocyst to which the inner cell mass is attached is called the embryonic or animal pole, while the opposite side is the abembryonic pole.
FUNCTION OF THE ZONA PELLUCIDA
The trophoblast has the property of being able to stick to the uterine epithelium and its cells have the capacity to eat up other cells.5
They can, therefore, invade and burrow into tissues with which they come in contact. As the embryo travels down the uterine tube and the uppermost part of the uterine cavity, it is prevented from “sticking” to the epithelium by the zona pellucida. During this time it receives nutrition, partly from the substances stored within the ovum (e.g. yolk), and partly by diffusion from the uterine secretions. By the time a blastocyst is formed, it is necessary for the embryo to acquire additional sources of nutrition. This is achieved when the blastocyst “sticks” to the uterine endometrium, and gets implanted in it. However, before this can happen, it is necessary for the zona pellucida to disappear. The zona pellucida disappears soon after the morula reaches the uterine lumen. Thus, the function of the zona pellucida is to prevent implantation of the blastocyst at an abnormal site.
BILAMINAR GERM DISK
The blastocyst is a spherical cyst lined by flattened trophoblastic cells, and inside it there is a mass of cells, the inner cell mass, attached eccentrically to the trophoblast.
Some cells of the inner cell mass differentiate (i.e. they become different from others) into flattened cells, that come to line its free surface. These constitute the endoderm, which is thus the first germ layer to be formed.
The remaining cells of the inner cell mass become columnar. These cells form the second germ layer, the ectoderm. The embryo is now in the form of a disk having two layers (Fig. 1.9).
A space appears between the ectoderm (below) and the trophoblast (above). This is the amniotic cavity (Fig. 1.10), filled by amniotic fluid, or liquor amnii. The roof of this cavity is formed by amniogenic cells derived from the trophoblast, while its floor is formed by the ectoderm.
Flattened cells arising from the endoderm (or according to some from trophoblast), spread and line the inside of the blastocystic cavity. In this way, a cavity lined on all sides by cells of endodermal origin is formed. This cavity is called the primary yolk sac (Fig. 1.11).
EXTRAEMBRYONIC MESODERM
The cells of the trophoblast give origin to a mass of cells called the extraembryonic mesoderm (or primary mesoderm). These cells come to lie between the trophoblast and the flattened endodermal cells lining the yolk sac, thus separating them from each other.
These cells also separate the wall of the amniotic cavity from the trophoblast (Fig. 1.12). This mesoderm is called extra-embryonic because it lies outside the embryonic disk. It does not give rise to any tissues of the embryo itself.
Small cavities appear in the extraembryonic mesoderm. Gradually these join together to form larger spaces and, ultimately, one large space is formed. This cavity is called the extraembryonic coelom. With its formation, the extraembryonic mesoderm is split into two layers: (i) the part lining the inside of the trophoblast, and (ii) the outside of the amniotic cavity, is called the parietal or somatopleuric extraembryonic mesoderm. The part lining the outside of the yolk sac is called the visceral or splanchopleuric extraembryonic mesoderm (Fig. 1.13). The extraembryonic coelom does not extend into that part of the extraembryonic mesoderm, which attaches the wall of the amniotic cavity to the trophoblast. The developing embryo, along with the amniotic cavity and the yolk sac, is now suspended in the extraembryonic coelom, and is attached to the wall of the blastocyst (i.e. trophoblast) only by this unsplit part of the extraembryonic mesoderm. This mesoderm forms a structure called the connecting stalk.
FORMATION OF CHORION AND AMNION
At this stage, two very important membranes are formed. One is formed by the parietal extraembryonic mesoderm (on the inside) and the overlying trophoblast (on the outside)—this is called the chorion. The other is the amnion, which is constituted by the amniogenic cells forming the wall of the amniotic cavity (excluding the ectodermal floor). These cells are derived from the trophoblast. The chorion and amnion play an important role in childbirth (parturition).
With the appearance of the extraembryonic mesoderm, and later of the extraembryonic coelom, the yolk sac becomes much smaller than before and is now called the secondary yolk sac. This alteration in size is accompanied by a change in the nature of the lining cells. They are no longer flattened but become cubical (Fig. 1.14).
PROCHORDAL PLATE
At this stage, the embryo proper is a circular disk composed of two layers of cells—the upper layers (towards amniotic cavity) is the ectoderm, the cells of which are columnar, while the lower layer (towards yolk sac) is the endoderm, made up of cubical cells. There is no indication yet of a head or tail end of the embryonic disk.
However, we soon see that, at one circular area near the margin of the disk, the cubical cells of the endoderm become columnar. This area is called the prochordal plate.7
The appearance of the prochordal plate determines the central axis of the embryo (i.e. enables us to divide it into right and left halves), and also enables us to distinguish its head and tail ends (Fig. 1.13).
PRIMITIVE STREAK
Soon after the formation of the prochordal plate some of the ectodermal cells lying along the central axis, near the tail end of the disk, begin to proliferate, and form an elevation that bulges into the amniotic cavity. This elevation is called the primitive streak (Fig. 1.15). The primitive streak is at first a rounded or oval swelling, but with elongation of the embryonic disk it becomes a linear structure lying in the central axis of the disk.
TRILAMINAR GERM DISK
The cells that proliferate in the region of the primitive streak pass sideways, pushing themselves between the ectoderm and endoderm (Fig. 1.16). These cells form the intraembryonic mesoderm (or secondary mesoderm) which is the third germ layer.
The intraembryonic mesoderm spreads throughout the disk except in the region of the prochordal plate. Note that the mesoderm extends cranial to the prochordal plate and here mesoderm from the two sides becomes continuous across the midline (Fig. 1.16). In the region of the prochordal plate, the ectoderm and endoderm remain in contact. In later development, the ectoderm and endoderm mostly persist as a lining epithelium. On the other hand, the bulk of the tissues of the body are formed predominantly from mesoderm. As there is no mesoderm in the prochordal plate, this region remains relatively thin, and later forms the buccopharyngeal membrane. Now, a trilaminar germ disk has formed.