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EMBRYOLOGY

Embryology:  It is the study of the development of an individual before birth (prenatal period). Embryo (G): (en = within; bruein= to swell or to be full); Logos = study Natal = birth; Prenatal = before birth; Postnatal = after birth

• Embryo:  It is the developing individual during the first 2 months or 8 weeks of intrauterine life.

• Fetus:  It is the developing individual from the 3rd month or 9th week of intrauterine life to the time of birth.

• Development before birth is called  prenatal development, and that after birth is called  postnatal development.

• There are three stages in prenatal development. They are  (1) preimplantation, (2) embryonic and (3) fetal periods.

• Gonads:  They are the sex organs that produce  sex cells or  gametes. The  testis  is the male gonad  and the  ovary  is the female gonad. Male gametes are called spermatozoa. Female gametes are called  ova.

• Gametogenesis: It  is  the  process  of  production  of  gametes  in  gonads  or  sex  organs.  In  males  it  is  known  as  spermatogenesis and in females as  oogenesis.

• Fertilization:  It  is  the  process  of  fusion  of  male  and  female  gametes.  It  takes  place  in  the  uterine  tube  of  female  genital  tract.

• Zygote:  It is the single cell that results from fertilization.

• Development:  It is a process where something grows or changes and becomes more advanced.

• Growth:  It is a quantitative change that increases the size.

• Ontogeny:  Complete life cycle of an organism.

• Phylogeny:  Evolutionary history of a group of organisms.

• Differentiation:  It is a qualitative change in structure for an assigned function.

• Organizer:  Any part of the embryo which exerts stimulus on an adjacent part.

• Cell potency:  It is the potential to differentiate into different cell types.


Gonads and Gametes

• Gonads are the paired sex glands that are responsible for the production of  gametes  or  sex cells  that carry out the special function of reproduction. The male sex cells (spermatozoa) are produced in the male gonads  (testes) while the female sex cells (ova) are produced in female gonads (ovaries).

• The formation of spermatozoa in testis is called spermatogenesis, while the formation of ova in the ovary is called  oogenesis. The two are collectively referred to as  gametogenesis.

• The development of a new individual begins at the movement when one male  gamete  (sperm  or spermatozoon) meets and fuses with one female gamete (ovum  or  oocyte). The process of  fusion of male and female gametes is called  fertilization.

• The zygote multiplies and reorganizes to form the miniature new individual called embryo that grows and matures as  fetus  in the mother’s womb and delivered at the end of term of pregnancy.

DEVELOPMENT OF A HUMAN BEING Development is a process where someone or something grows or changes and becomes more advanced. Human development is a continuous process that does not stop at birth. It continues after birth for increase in the size of the body, eruption of teeth, etc.
Development before birth is called  prenatal development, and that after birth is called postnatal development. 
Each period is further subdivided into several stages.

Prenatal Development There are three stages in prenatal development. They are:

1.  Preimplantation/pre-embryonic period
2.  Embryonic period
3.  Fetal period.

Preimplantation/Pre-embryonic Period It extends from fusion of male and female gametes to form single-celled zygote to formation of primitive germ layers of developing organism. It includes 1st and 2nd weeks of intrauterine development. The following morphogenetic events take place during this period.

1.  Fertilization: Fusion of male and female gametes resulting in the formation of zygote.

2.  Cleavage: A series of mitotic divisions of zygote resulting in the formation of morula.

3.  Transportation of cleaving zygote, i.e. morula along the fallopian tube toward the uterus.

4.  Blastocyst: Structural and functional specialization and reorganization of cells (blastomeres) of cleaving zygote that becomes blastocyst.

5.  Implantation: Process of attachment of blastocyst to the uterine endometrium is called implantation.

6.  Specialization of primordial embryonic tissue: It involves specialization  of  blastomeres  to  form  embryonic structures  (embryoblast)  and supportive/nutritive structures  (trophoblast).

7.  Differentiation of embryoblast—to form the primitive two layered (bilaminar) germ disc having ectoderm and endoderm.

8.  Differentiation of trophoblast into  cytotrophoblast  and syncytiotrophoblast.


Embryonic Period It  extends  from  3rd  week  of  intrauterine  life  to  8th  week  of intrauterine  life.  The  following  morphogenetic  events  take place during this period.

1.  Trilaminar germ disc differentiation: Formation of three layered germ disc with  the  appearance  of  mesoderm in between ectoderm and endoderm.

2.  Early organogenesis: Formation of primordia of various organs like lungs, heart, liver, etc.

3.  Formation of extraembryonic supportive organs and membranes: Placenta, umbilical cord, amnion, allantois.

Fetal Period It  extends from 9th week to 9th month. This period includes the following:

1.  Growth of fetus in all dimensions

2.  Specialization of various body structures.


Postnatal Period of Development

It extends from birth of an individual to adulthood. The various stages in postnatal development are as follows:

1.  Neonatal period: It extends from birth to 28 days after birth. These first 4 weeks are critical in the life of the  newborn/neonate  as  various  systems  especially respiratory and cardiovascular have to make adjustments with the external/extrauterine environment.

Neonatology:  The branch of medicine that takes care of neonates is called  neonatology.

Perinatology:  It is  the branch of  medicine that takes care of the  fetus  and  newborn from 28th week of intrauterine life to 6th day of extrauterine life.

2.  Infancy: It extends from 1 month to 1 year and the newborn during this period is called  infant.

3.  Childhood: It extends from 2nd year to 12th year of age and an individual is called a child. It is the period of rapid growth and development. This age is also called pediatric age. Pediatrics  and  pediatrician:  The  medical  branch  that deals with infants and children is called  pediatrics. The specialist who treats them is known as  pediatrician.

4.  Puberty: It extends from 12 years to 16 years. There will be  rapid  physical growth and development of secondary sex characters and it depends on the interaction of sex hormones and growth hormones.

5.  Adolescence:  It  extends  from  17  years  to  20  years. During this period, there will be rapid physical growth and  sexual  maturation.  The  reproductive  ability  is established.

6.  Adulthood: It extends from 21 years to 40 years.

7.  Middle age: It extends from 40 years to 60 years.

8.  Old age: It extends from more than 60 years to death. Ontogeny: Complete life cycle of an organism involving both prenatal and postnatal developments is called ontogeny. It is the expression of blue print of life hidden in genes. It includes progressive changes followed by retrogressive changes. It involves various processes like cell division, differentiation and growth. Phylogeny: Evolutionary/ancestral history of a group of organisms is called phylogeny. It includes developmental changes in various organs (e.g. kidney, heart) and organ systems (e.g. respiratory, skeletal) starting from fishes, amphibians, reptiles, birds and mammals. Ontogeny repeats phylogeny: Life cycle of an organism repeats its  ancestral  history.  This  is  observed  in  the  development  of certain organs viz. heart, lung and kidney.