BASIC QUALITIES OF LIVING ORGANISMS
The three basic qualities of living organism are:
- Protection: Protection from different environmental conditions like heat, cold, rain, famines, etc. by making provision for food, water, clothing and shelter.
- Growth: It includes both physical (increase in height, weight) and mental (intelligence, social behavior) growth by proper nutrition, customs and practices in the society.
- Propagation of species: Propagation of species by reproduction of new individuals to prevent extinction of species.
Nature facilitates for nurturing these three basic qualities.
REPRODUCTION
- Reproduction is a mechanism to produce new generations continuously.
- The internal sex organs (gonads) produce gametes that differ in each sex.
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 (Fig. 1.1).
Prenatal Development
There are three stages in prenatal development. They are:
- Preimplantation/pre-embryonic period
- Embryonic period
- 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.
- Fertilization: Fusion of male and female gametes resulting in the formation of zygote.
- Cleavage: A series of mitotic divisions of zygote resulting in the formation of morula.
- Transportation of cleaving zygote, i.e. morula along the fallopian tube toward the uterus.
- Blastocyst: Structural and functional specialization and reorganization of cells (blastomeres) of cleaving zygote that becomes blastocyst.
- Implantation: Process of attachment of blastocyst to the uterine endometrium is called implantation.
- Specialization of primordial embryonic tissue: It involves specialization of blastomeres to form embryonic structures (embryoblast) and supportive/nutritive structures (trophoblast).
- Differentiation of embryoblast—to form the primitive two layered (bilaminar) germ disc having ectoderm and endoderm.
- 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.
- Trilaminar germ disc differentiation: Formation of three layered germ disc with the appearance of mesoderm in between ectoderm and endoderm.
- Early organogenesis: Formation of primordia of various organs like lungs, heart, liver, etc.
- 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:
- Growth of fetus in all dimensions
- 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:
- 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.
- Infancy: It extends from 1 month to 1 year and the newborn during this period is called an infant.
- 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.
- 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.
- 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.
- Adulthood: It extends from 21 years to 40 years.
- Middle age: It extends from 40 years to 60 years.
- 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.
In this book, we will study prenatal development only.
EMBRYOLOGY
- It is the science that deals with the processes and regulations in the prenatal growth and development of an organism/individual in the female genital tract. It begins with the fusion of male and female gametes (fertilization) in the fallopian tube up to the birth as a neonate.
- Prenatal development involves repeated division of most of the cells in the body resulting in growth in size, complexity, structural and functional differentiation of body.
- Embryology includes the study of startling integration of various complex molecular, cellular and structural processes that are accountable for the growth and development of a 9-month-old neonate containing 5-7 × 1012 cells from a single-celled zygote. It is also called developmental anatomy.
SUBDIVISIONS OF EMBRYOLOGY
General embryology: It is the study of development during pre-embryonic and embryonic periods (first 8 weeks after fertilization). During this period, the single-celled zygote is converted by cell multiplication, migration and reorganization into a miniature form of an individual with various organs and organ systems of the body.4
Systemic embryology: It is detailed study of formation of primordia and their structural and early functional organization into various organs and systems of the body. It is further subdivided into development of cardiovascular system, digestive system, urinary system, genital system, etc.
Comparative embryology: It is the study of embryos in different species of animals.
Experimental embryology: It is for understanding the effects of certain drugs, environmental changes that are induced (exposure to radiation, stress) on the growth and development of embryos and fetuses of lower animals. The knowledge gained from these experiments can be used for avoiding the harmful effects in the human development. It is a vigorous and promising branch of embryology.
Biochemical and molecular aspects in embryology: Chromosomes, gene sequencing, regulation.
Teratology: This is a branch of embryology that deals with abnormal embryonic and fetal development, i.e. congenital abnormalities or birth defects.
IMPORTANCE OF EMBRYOLOGY IN THE MEDICAL PROFESSION
Normal development: This subject tells us how a single cell (the fertilized ovum, i.e. zygote) develops into a newborn, containing numerous tissues and organs.
Normal adult anatomy: This knowledge helps us to understand many complicated facts of adult anatomy like the location and relations of organs to one another. Examples—location of heart on left side of thoracic cavity, liver on right side of abdominal cavity and its closeness to stomach.
Developmental abnormalities: Embryology helps us understand why some children are born with organs that are abnormal. Appreciation of the factors responsible for abnormal development assists us in preventing, or treating, such abnormalities. Examples—exposure to radiation during pregnancy, use of certain medications during pregnancy or a genetic abnormality that exists in family.
Understanding postnatal and adulthood diseases: The mechanisms (molecular and cellular) taking place during the development of embryo play a key role in the development of a wide range of diseases in adult life. Examples—that can vary from absence of an ear or presence of an extra finger to hypertension, diabetes, depression, cardiovascular and renal diseases. This is known as fetal programming of adult diseases.
Health care strategies for better reproductive outcome: Knowledge of embryology facilitates interpretation of the results of various techniques like fetal ultrasound, amniocentesis, and chorionic villous biopsy. Based on the results, appropriate treatment can be planned. Example—performing surgeries for correction of a defect in the diaphragm prenatally; postnatal correction of a cardiac defect; medical line of management of a diabetic or hypertensive mother.
Therapeutic procedures for infertility/fertility-related problems: If the woman is unable to conceive by natural methods, alternate methods like cloning and in vitro fertilization can be planned. For spacing the pregnancies, various birth control methods (medical and surgical) are available. A basic knowledge of embryology is required for understanding the mechanism of action of these methods.
Stem cell therapy: Cells forming tissues in the embryo are called stem cells. These are undifferentiated cells that can differentiate into specialized cell types. It is an uncommitted cell and depending on the signal it receives, it can develop into many specialized cells. These cells are capable of treating certain diseases in postnatal life.
BASIC PROCESSES IN EMBRYOLOGY
Growth and differentiation are the two basic processes involved in the conversion of a single-celled zygote into a multicellular human newborn.
Growth
It is a quantitative change, i.e. Increase in the bulk. Growth of cells is either by synthesizing new protoplasm in the interphase (G1, S and G2) of cell cycle or reproduction of individual cells of body by mitotic cell divisions. There are four types of growth. They are as follows:
- Multiplicative: This type of growth is the predominant type observed during prenatal period. It is increase in cell number by succession of mitotic divisions without increase in cell size. Example—blastomeres. During prenatal and postnatal development, many cells die by apoptosis (programmed cell death) or they lose the power to grow and divide to form definitive contours of the organs. Examples—the neurons do not divide during postnatal period. The cells of epidermis, intestinal epithelium and blood cells are continuously produced to replenish the cells lost by wear and tear. The liver cells do not divide normally but, if there is loss of two-thirds of liver (removal) they multiply.
- Auxetic: This type of growth is seen in oocytes and certain neurons. The increase in cell size is due to increase in its cytoplasmic content. This alters the nuclear-cytoplasmic 5ratio without alterations in structural genes. If the ratio is altered, it makes the structural genes in nuclear DNA ineffective. This can cause degradation of cytoplasmic proteins. To provide nutrition, there will be cells that surround these larger cells. Example—satellite cells around the larger neurons and follicular cells around oocyte.
- Accretionary: Increased accumulation of intercellular substance resulting in overall growth of structure. This causes increase in length. Example—increase in length of bone and cartilage.
- Appositional: Addition of new layers on previously formed ones. It takes place at the edges, is seen in rigid structures and is responsible for contours. Example—increase in width of bone by addition of lamellae.
Differentiation
It is a qualitative change in structure with an assigned function. Different types of differentiation are as follows:
- Chemodifferentiation: It is an invisible differentiation that takes place at molecular level. The substances producing this type of differentiation are called organizers.
- Histodifferentiation: It takes place at tissue level.
- Organodifferentiation/Organogenesis: This is at organ level and is the basis for organ remodeling.
- Functional differentiation: Hemodynamic changes in blood vessels.
Organizer
Any part of the embryo which exerts a morphogenetic stimulus on an adjacent part or parts. There are three types of organizers:
- Primary organizer: Example—blastopore/primitive streak that induces differentiation of notochord and secondary/intraembryonic mesoderm.
- Secondary organizer: Example—notochord acts as a secondary organizer in stimulating the development of brain and spinal cord.
- Tertiary organizer: Example—neural tube is the tertiary organizer that induces segmentation of paraxial mesoderm into somites.
Stem Cells
- These are undifferentiated cells that are capable of giving rise to more number of cells of same type by replication from which some other kinds of cells arise by differentiation (Fig. 1.2).
- There are two types of stem cells: (1) the embryonic and (2) adult/somatic. Embryonic stem cells are present during embryonic development. Adult stem cells are formed during embryonic development that are tissue-specific and remain so throughout the life of an individual.
- They are the basis for the formation of a tissue and an organ in the body.
- They have the capacity of self-renewal and differentiation.
- Stem cells are classified depending on their potency (cell potency) to differentiate into different cell types.
- Accordingly the cells are named as (Table 1.1):
- Totipotent cells: They can form all the cell types in the embryo in addition to extraembryonic or placental cells. Embryonic cells within the first couple of cell divisions after fertilization are the only cells that are totipotent. Example—zygote, early blastomeres.
- Pluripotent: It can give rise to all of the cell types that make up the body. Embryonic stem cells are considered pluripotent. Example—inner cell mass.
- Multipotent: They can develop into more than one cell type, but are more limited than pluripotent cells. Example—adult stem cells (mesenchymal cells), cord blood stem cells and hematopoietic cells.
- Oligopotent: It can develop into cells of one category only. Example—vascular stem cells that form endothelium and smooth muscle; lymphoid or myeloid stem cells that form blood cells.