Aristotle proposed the modeling principle of any organism as "energia" or "entelecheia" with the ultimate energy called its psyche or soul
Aristotle proposed 3 types of soul operating in the organism: vegetative, animal, and spiritual
History of Developmental Biology as a Science:
Started as an old science, one of the first
Dawn of biology in ancient Greece with Aristotle (384 - 322 BC) being the first to study embryos focused on chick
Aristotle proposed possible organs of an organism including spontaneous generation, hermaphroditism, and bisexual reproduction
Aristotle believed that the semen provides the "force" towards development and the menstrual blood provided the matter
Aristotle proposed the male as the "dynamic element" and the female as the "matter"
Aristotle's influence of religion is very strong
Renaissance of Developmental Biology:
William Harvey (1578 - 1657) proposed that all organisms came from an egg
Harvey's proposal: Omne vivum ex ovum
Spermists claimed that the embryo comes from one sperm only, disputed by physics
Ovists claimed that all generations will come from one egg only, disputed by physics
Theory of preformation arose with ovists and spermists
Leeuwenhoek (1632 - 1723) and Hartsoeker claimed to have seen little men inside individual spermatozoa
Lazaro Spallanzani (1729 - 1799) observed that the female egg develops only in the presence of male semen
Oscar Hertwig (1875) proposed that both sperm and egg contribute to form an embryo
Epigenesist theory proposed by Caspar Friedrich Wolff (1738 - 1794)
Vitalism proposed that a vital force specific to life is the prime mover for the grand unfolding of the organism
Comparative Embryology:
Karl Ernst von Baer (1792 - 1876) discovered eggs in many mammalian species and performed comparative studies
Ernst Haeckl (1834 - 1919) built on von Baer's study and proposed the controversial biogenetic law
Embryo Resemblances proposed by von Baer: all vertebrates pass through the same stages and possess similar structures that will later differentiate
Era of Experimental Embryology:
E.B. Wilson recognized that embryonic development is a manifestation of changes in the cell
T.H. Morgan and Wilson debated on whether the determinants of development came from the nucleus or cytoplasm
August Weismann proposed the germ plasm theory
Wilhelm Roux was the Father of Experimental Developmental Biology and manipulated zygotes or oocytes
Hans Driesch perfected Roux's experiments
Theodore Boveri and Sutton proposed the chromosome theory
Gregor Mendel rediscovered paper on hereditary traits
Studies of Genetics and Unresolved Issues:
Geneticists must provide evidence that genes control early stages of development of structures, not just terminal stages
Birth of Modern Genetics caused a deep divide leading to the establishment of genetics as a science
Unresolved issues include how genes determine hereditary traits, how the nucleus regulates metabolism, and how chromosomes direct different types of cytoplasm
Geneticists separated into those who believed in chromosomes as morphogenetic guides and those who believed in cytoplasm
The Molecular Biology Revolution:
Middle of the 20th century provided means to study the role of genes in development
Powerful techniques like Knock out and knock in genes and PCR were developed
Drosophila sp. and Caenorhabditis elegans were key model systems in molecular biology
Understanding the principles in invertebrates can apply to more advanced organisms
Spermatogenesis:
Histology of the Vertebrate Testes:
Frog testis has tunica albuginea, seminiferous tubules, and spermatocytes
Mouse testis has cyclical arrangement with spermatogonia, primary and secondary spermatocytes
Phases of Spermatogenesis include Spermatocytogenesis, meiosis, and Spermiogenesis
Metaphase and Anaphase differences in meiosis 1 and meiosis 2
Phases of Spermiogenesis:
Golgi phase:
Small PAS positive granules form within a vacuole (proacrosomal vesicle) in the Golgi complex
Centrioles migrate near the cell surface at the side opposite the future acrosome, initiating the formation of the flagellar axoneme
Cap phase:
Acrosomal granule covers the nucleus, becoming the acrosomal cap
Acrosome contains hydrolytic enzymes to digest the corona radiata and the zona pellucida of the ovum
Acrosomal phase:
Anterior pole becomes oriented towards the basement membrane of the seminiferous tubule
Nucleus becomes more elongated facilitated by the manchette
Mitochondria migrate towards the midpiece
Kartagener syndrome:
Characterized by immobile flagella due to a mutation in dynein, a motor protein associated with microtubules
Mutation can lead to immotile sperm and cilia in the respiratory tract and fallopian tube
Maturation phase:
Residual cytoplasm is removed
Golgi phase involves acrosomal vesicles melting the egg membrane and forming the egg of the sperm
Cap phase is relevant in cases of Kartagener syndrome
Acrosomal phase marks the orientation of the anterior pole towards the basement membrane
The Spermatozoon:
Different shapes exist according to species
The duration from spermatogonia to spermatozoa varies
Immature and mature spermatozoa are observed in different species
The cycle and wave arrangement of spermatogenic cells:
Sperm can be continuously delivered as one discharges, another is ready to follow
Sperm delivery is not seasonal and remains fertile as long as healthy
Specific cellular associations are seen in humans with 6 stages
A group of sperm can be readily released at any stage
Each segment is undergoing a cycle
Duration of spermatogenesis (days) in different species:
Mouse: 8.6 days for a cycle, 34.5 days for spermatogenesis
Hamster: 8.7 days for a cycle, 35 days for spermatogenesis
Sprague-Dawley rat: 12.9 days for a cycle, 51.6 days for spermatogenesis
Wistar rat: 13.3 days for a cycle, 53.2 days for spermatogenesis
Man: 16 days for a cycle, 70 to 74 days for spermatogenesis (4.5 cycles)
Supporting Cells - The Sertoli Cells:
Structure:
Sertoli cells do not divide during the reproductive period
They are resistant to infection, X-ray irradiation, and malnutrition
All growing spermatids are associated with Sertoli cells
Sertoli cells are involved in maintaining the blood-testis barrier
Functions:
Support includes protection and nutritional regulation of developing spermatozoa
Phagocytosis of residual cytoplasm
Secretion of various substances including ABP under the influence of testosterone and FSH
Conversion of testosterone to estradiol
Production of Inhibin to suppress FSH release
MDIF promotes regression of Mullerian duct
The Blood-Test Barrier:
Separates the seminiferous tubule into basal and adluminal compartments
Made of tight junctions to prevent paracellular transport
Prevents entry of noxious chemicals and autoimmune responses
Physiological disruption related to spermiation and feedback mechanisms
Gene Transcription During Spermatogenesis:
Transcription at diplotene:
Y chromosome of Drosophila transcribed for sperm viability
Isoform for B tubulin necessary for sperm motility
mRNA for bindin transcribed late in spermatogenesis for binding of sperm and egg
Akap82 transcribed at spermatid stage
mRNA for sperm protamine protein transcribed by spermatid for compacting chromatin
Hormonal Control of Spermatogenesis:
Spermatogenesis at puberty depends on the hypothalamus, pituitary, and gonadal axis to produce testosterone
FSH is crucial for the secretion of ABP and development of the blood-testis barrier
Sperm yield increases with FSH, preventing atresia of Type A spermatogonia
Sexual activity affects FSH levels and inhibin decreases FSH
Oogenesis:
Differs from spermatogenesis in various aspects including product cell motility, size, and genetic composition
Oogenesis involves meiotic arrests and the production of a single functional ovum
Primordial germ cells have different fates leading to the development of oocytes or spermatogonia
Yolk is present in the eggs of all animals except mammals
Mammals do not have yolk in their eggs because they do not produce a significant amount of mRNA
Yolk was discovered in the late 19th Century in the eggs of an amphibian and a fish
Yolk distributes in a specific manner in the oocyte, with ribosomes, mitochondria, platelets, and little cytoplasm found in the animal pole, and a large amount of yolk in the vegetal pole
Animal/vegetal axis is established with yolk platelets in vegetal hemispheres and glycogen granules, lipid inclusions, ER, and ribosomes in the animal pole
Specific RNAs are localized to certain regions of the cytoplasm, credited to the cytoskeleton
Oocytes produce transcripts for themselves and the future zygote, which should be sequestered from others and not translated unless fertilized
Cortical granules found in the actin-rich cortex are important during fertilization to prevent polyspermy and ensure only one sperm fertilizes the egg
Specific localizations are important for development and differentiation, as proven by centrifugation experiments
MPF (Maturation Promoting Factor) drives cells to enter mitosis and is regulated by cyclin-dependent kinase molecules
LH (Luteinizing Hormone) triggers ovulation and is the stimulus for the continuation of meiosis
Ovulation patterns in mammals vary, with eggs ovulated during sexual intercourse in MII (meiosis II) stage
Hormonal regulation of ovulation starts with environmental cues and involves the growth of follicles and initiation of ovulation