Human development starts with the fertilization of an ovum by a sperm.
Development involves changes that transform a single cell (zygote) into a multicellular human.
Changes occur before birth (embryonic and fetal periods) and after birth (infancy, childhood, adolescence, adulthood).
Human Embryology
Deals with the origin and development of a human from fertilization to birth.
Terms Used in Human Prenatal Development
Zygote: The cell formed by the union of an ovum and sperm, marking the beginning of a human being.
Cleavage: Mitotic division of the zygote into daughter cells called blastomeres, which become smaller with each division.
Morula: A ball of 12 to 16 blastomeres formed about three days after fertilization. Resembles a mulberry and is about to enter the uterus.
Blastocyst: Formed when a cavity develops within the morula after it enters the uterus.
Embryo: The cells of the blastocyst that form the inner cell mass (embryoblast). The term is typically used once the bilaminar embryonic disc forms during the second week. The embryonic period lasts until the end of the eighth week, during which initial structures develop and the embryo becomes distinctly human.
Conceptus: Refers to the embryo and its membranes; includes all structures developing from the zygote (embryonic and extraembryonic).
Fetus: The developing human after the embryonic period (from the ninth week to birth), during which systems continue to develop.
Abortion: Birth of an embryo or fetus before it is viable (able to survive outside the uterus). Terminations before 20 weeks are classified as abortions.
Abortus: The product(s) of an abortion, including an embryo or nonviable fetus and its membranes weighing less than 500 gm.
Primordium: The earliest trace or indication of an organ or structure.
Stages of Development
Development is a continuous process from fertilization to adulthood, divided into prenatal and postnatal periods.
Birth is a significant event, but development continues afterward.
Most developmental changes are complete by age 25.
Periods of Fetal Development
Preembryonic Period (Weeks 1-3): From fertilization to formation of the embryonic disk with three germ layers.
Embryonic Period (Weeks 4-8): Rapid growth and differentiation with the formation of major organ systems.
Fetal Period (Weeks 9-40): Continued growth and development of organ systems.
Early Fetal Development (Preembryonic Period)
Fertilization
Approximately 200 million to 500 million spermatozoa are deposited in the vagina during ejaculation.
A large number of spermatozoa are needed to increase the chance of conception, as they must traverse the cervical canal, uterus, and uterine tubes.
Fertilization typically occurs in the ampulla, the widest section of the uterine tubes, near the ovaries.
Spermatozoa undergo capacitation and the acrosome reaction to penetrate the barriers around the secondary oocyte.
Capacitation
A maturation process that enables a freshly ejaculated sperm to fertilize an ovum.
Involves an enzymatic reaction that removes the glycoprotein coating from the spermatozoa and plasma proteins from the seminal fluid.
Glycoproteins are removed from the surface of the acrosome cap.
Occurs in the uterus or uterine tubes and takes about 7 hours.
Acrosome Reaction
Occurs when a capacitated sperm passes through the corona radiata, causing structural changes that result in the fusion of the plasma membranes of the sperm and oocyte.
The acrosome develops perforations, releasing the contents of the acrosomal vesicle.
Lytic (digestive) enzymes are released around the oocyte, dispersing the corona radiata follicular cells and allowing the sperm to contact the zona pellucida.
Enzymes like acrosin produce an opening in the zona pellucida.
The sperm cell membrane and nucleus pass into the ovum.
Progesterone released from the follicle stimulates the acrosome reaction.
Blocks to Polyspermy
Mechanisms ensuring that the fertilized ovum contains only 46 chromosomes.
Fast Block: The electrical resting potential of the oocyte plasma membrane changes from negative to positive, preventing other sperm from entering. This is a brief reaction.
Slow Block (Cortical Reaction): Depolarization causes the ovum to swell, detach remaining sperm, and activate the secondary oocyte to complete the second meiotic division and expel the second polar body.
The ovum matures; its nucleus becomes the female pronucleus.
The sperm head enlarges to form the male pronucleus after entering the ovum cytoplasm.
The male and female pronuclei fuse, and paternal and maternal chromosomes intermingle.
After penetration by a single sperm, the oocyte completes the second meiotic cell division, resulting in a haploid number of chromosomes (22,X) and the second polar body.
Chromosomes arrange to form the female pronucleus.
As the spermatozoon moves closer, the tail detaches, and the nucleus enlarges to form the male pronucleus. The pronuclei fuse, creating a diploid cell known as the zygote.
The zygote contains 22 autosomes and one sex chromosome from each parent (46,XX or 46,XY).
The genetic sex is determined by the father's contribution, with the male parent (XY) providing either an X or a Y chromosome.
Cleavage
Mitotic cell division occurs after fertilization as the zygote travels down the uterine tube, forming two blastomeres.
Cells continue to divide, increasing in number but decreasing in size.
Morula
When the zygote reaches approximately 16 cells (usually on the third day), it is called a morula.
The morula enters the uterus around this time.
The morula consists of an inner cell mass and an outer cell layer.
Individual cells at this stage are called blastomeres.
The outer cell layer forms the trophoblast, which develops into the placenta. The inner cell mass, called the embryoblast, gives rise to the embryo.
Blastocyst
About four days post-fertilization, fluid-filled spaces fuse, forming a large cavity known as the blastocyst cavity.
The morula becomes a blastocyst. The trophoblast forms the wall, and the embryoblast projects into the cavity.
Uterine secretions nourish the blastocyst until implantation.
Results of Fertilization
The diploid number of chromosomes is restored to 23 pairs (46 chromosomes).
The new individual inherits a unique set of genes from its parents.
Sex determination occurs.
Initiation of cleavage stimulates the zygote to begin mitotic cell division.
Implantation
The blastocyst typically implants in the posterior superior wall of the uterus by day 7 after fertilization.
Implantation occurs in the functional layer of the endometrium during the progestational (secretory) phase of the menstrual cycle.
The trophoblast proliferates and differentiates into the cytotrophoblast and syncytiotrophoblast.
The syncytiotrophoblast has finger-like projections that produce enzymes to erode endometrial tissues.
By the end of the seventh day, the blastocyst is superficially implanted.
Formation of the Bilaminar Disc
The syncytiotrophoblast continues to invade the endometrium and becomes embedded.
Spaces in the syncytiotrophoblast, called lacunae, fill with blood from ruptured maternal capillaries and secretions from eroded endometrial glands, nourishing the embryoblast by diffusion.
Lacunae fuse to form a network that becomes the intervillous spaces of the placenta.
Inner Cell Mass Differentiation
The inner cell mass differentiates into two layers:
Hypoblast (Endoderm): Small cuboidal cells.
Epiblast (Ectoderm): A layer of high columnar cells.
These layers form a flattened, circular bilaminar embryonic disc.
The amniotic cavity originates from spaces within the epiblast. As it enlarges, it is covered by a thin layer of epithelial cells.
During the development of the amniotic cavity, trophoblastic cells form the extracoelomic membrane, enclosing the primitive yolk sac, which produces fetal red blood cells.
The chorion comprises the extraembryonic somatic mesoderm, cytotrophoblasts, and syncytiotrophoblasts. It forms the chorionic sac, which contains the embryo, amniotic sac, and yolk sac.
By the end of the second week, the prochordal plate, a thickened area near the cephalic region of the hypoblastic disk, marks the location of the mouth. The chorion is made up of the extra embryonic somatic mesoderm. The cytotrophoblasts, and the syncytiotrophoblasts the chorion forms the chorionic sac, in which the embryo, aminotic and yolk sacs are located. By the end of the second week, a slightly thickened area has formed near the cephalic region of the hypoblastic disk, this area known as prochordal plate, marks the location of the mouth.