Development Of The Human Zygote Essay Research

Development Of The Human Zygote Essay, Research Paper

Development of the Human Zygote

November 16, 1995

Hundreds of thousands of times a year a single-celled zygote, smaller

than a grain of sand, transforms into an amazingly complex network of cells, a

newborn infant. Through cellular differentiation and growth, this process is

completed with precision time and time again, but very rarely a mistake in the

“blueprint” of growth and development does occur. Following is a description of

how the pathways of this intricate web are followed and the mistakes which

happen when they are not.

The impressive process of differentiation changes a single-cell into a

complicated system of cells as distinct as bold and bone. Although embryonic

development takes approximately nine months, the greatest amount of cellular

differentiation takes place during the first eight weeks of pregnancy. This

period is called embryogenesis.

During the first week after fertilization, which takes place in the

Fallopian tube, the embryo starts to cleave once every twenty-four hours (Fig.

1). Until the eight or sixteen cell stage, the individual cells, or blastomeres,

are thought to have the potential to form any part of the fetus (Leese, Conaghan,

Martin, and Hardy, April 1993). As the blastomeres continue to divide, a solid

ball of cells develops to form the morula (Fig. 1). The accumulation of fluid

inside the morula, transforms it into a hollow sphere called a blastula, which

implants itself into the inner lining of the uterus, the endometrium (Fig. 1).

The inner mass of the blastula will produce the embryo, while the outer layer of

cells will form the trophoblast, which eventually will provide nourishment to

the ovum (Pritchard, MacDonald, and Gant, 1985).

Figure 1:Implantation process and development during

embryogenesis (Pritchard, MacDonald and

Gant, 1985)

During the second week of development, gastrulation, the process by

which the germ layers are formed, begins to occur. The inner cell mass, now

called the embryonic disc, differentiates into a thick plate of ectoderm and an

underlying layer of endoderm. This cellular multiplication in the embryonic

disc marks the beginning of a thickening in the midline that is called the

primitive streak. Cells spread out laterally from the primitive streak between

the ectoderm and the endoderm to form the mesoderm. These three germ layers,

which are the origins of many structures as shown in Table 1, begin to develop.

Table 1: Normal Germ Layer Origin of Structures in Some or all Vertebrates

(Harrison, 1969)

Normal Germ Layer Origin of Structures in Some or All Vertebrates

EctodermMesodermEndoderm Skin epidermis

Hair Feathers Scales Beaks Nails Claws Sebaceous, sweat, and

mammary glands Oral and anal lining tooth enamel Nasal epithelium Lens of

the eye Inner earBrainSpinal cordRetina and other eye partsNerve cells and

gangliaPigment cellsCanal of external earmedulla of the adrenal glandPituitary

gland Dermis of the skinConnective tissueMusclesSkeletal componentsOuter

coverings of the eyeCardiovascular system Heart Blood cells Blood

vesselsKidneys and excretory ductsGonads and reproductive ductsCortex of the

adrenal glandSpleenLining of coelomic cavitiesMesenteries LiverGall

bladderPancreasThyroid glandThymus glandParathyroid glandsPalatine tonsilsMiddle

earEustachian tubeUrinary bladderPrimordial germ cellsLining of all organs of

digestive tract and respiratory tract

During the third week of development, the cephalic (head) and caudal

(tail) end of the embryo become distinguishable. Most of the substance of the

early embryo will enter into the formation of the head. Blood vessels begin to

develop in the mesoderm and a primitive heart may also be observed (Harrison,

1969). Cells rapidly spread away from the primitive streak to eventually form

the neural groove, which will form a tube to the gut. When the neural folds

develop on either side of the groove, the underlying mesoderm forms segmentally

arranged blocks of mesoderm called somite. These give rise to the dermis of the

skin, most skeletal muscles, and precursors of vertebral bodies. the otocyst,

which later becomes the inner ear, and the lens placodes, which later form the

lenses of the adult eyes, are derived from the ectoderm.

The strand of cardiovascular functioning is apparent during the fourth

week. The heart shows early signs of different chambers and begins to pump

blood through the embryo which simultaneously has well developed its kidneys,

thyroid gland, stomach, pancreas, lungs, esophagus, gall bladder, larynx, nd

trachea (Carlson, 1981).

Several new structures are observed, organs continue developing, and

some previously formed structures reorganize during the fifth week of

embryogenesis. The cranial and spinal nerves begin to form a

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