Spermatogenesis and Spermatogenesis Process

Spermatogenesis

• Spermatogenesis takes approximately 64 days to complete
• During fetal period, Primordial Germ Cells (PGCs) undergo mitosis to form spermatogonia
• At puberty, spermatogonia undergo meiosis to form primary spermatocytes
• Primary spermatocytes divide into secondary spermatocytes (haploid, 2n) which then develop into spermatids
• Spermatids eventually mature into spermatozoa (22+X or 22+Y)

Spermatozoon Structure

• Spermatozoon length: 60-65μm
• Head of spermatozoon consists of condensed nucleus capped by an apical vesicle (acrosome) containing hydrolytic enzymes
• Neck of spermatozoon is short
• Tail of spermatozoon consists of midpiece and long tail
• Midpiece contains large, helical mitochondria which generate energy for swimming
• Long tail contains microtubules that enable swimming

Oogenesis

• During fetal life, PGCs differentiate into oogonia
• By 5th month of pregnancy, oogonia develop into primary oocytes in ovaries
• By birth, approximately 700,000 to 2 million primary oocytes are present
• By puberty, approximately 400,000 primary oocytes remain

Human Oogenesis and Follicular Development

• Follicle consists of oocyte and follicular cells/granulosa cells
• Mature follicle develops from primary oocyte (P.O) to secondary oocyte (S.O)

Ovulation and Menstrual Cycle

• Ovulation is a crucial part of the menstrual cycle that leads to fertilization

Fertilization

• Fertilization is the fusion of the sperm cell nucleus with the egg cell nucleus to produce a zygote (fertilized egg)

Before Fertilization

• Spermatozoa must undergo capacitation to fertilize an egg
• Capacitation involves the destabilization of the acrosomal sperm head membrane
• Capacitation is facilitated by the removal of steroids (e.g. cholesterol) and glycoproteins
• Capacitation increases permeability to Ca2+

Acrosome Reaction

• Release of enzymes (acrosin- & trypsin-like substances) needed to penetrate the zona pellucida

Fertilization Steps

• Acrosome reaction
• Cortical granular reaction/ penetration of the corona radiata
• Penetration of the zona pellucida
• Fusion of the oocyte and sperm cell membranes
• The oocyte resumes meiosis, resulting in a definitive oocyte

Fertilization Results

• Restoration of the diploid number of chromosomes
• Determination of the sex of the new individual
• Initiation of cleavage

Spermatozoa Abnormalities

• Spermatozoa with abnormalities can affect fertility
• Abnormalities include:
  • Small, narrow, or piriform (pear-shaped) heads
  • Double or triple heads
  • Acrosomal defects
  • Double tails
• If < 50% of spermatozoa are normal, fertility decreases
• If abnormal spermatozoa are present in high numbers, infertility increases significantly

Embryonic Development

• Cleavage is the first phase of embryonic development, characterized by a rapid series of mitotic divisions immediately after fertilization.
• The purpose of cleavage is to generate a large number of cells, resulting in an increase in the nucleus-to-cytoplasmic ratio.

Cleavage vs. Normal Mitoses

• Cleavage differs from normal mitoses in two key ways:
  • Blastomeres do not grow in size between successive cell divisions.
  • Cleavage occurs very rapidly.

Blastocyst Formation

• By day 4, the morula, consisting of approximately 30 cells, begins to absorb fluid.
• As the hydrostatic pressure of the fluid increases, a large cavity called the blastocyst cavity (blastocoel) forms.
• The morula develops a fluid-filled cavity and is transformed into a blastocyst.

Implantation

• The blastocyst attaches to the surface of the endometrium.
• Trophoblast cells invade the endometrium.
• Uteroplacental circulation is established.

Implantation

• The blastocyst hatches from the zona pellucida before implanting in the uterine wall
• Human chorionic gonadotropin (HCG) is produced during implantation

2nd Week: Development of the Bilaminar Embryonic Disc

• The embryoblast (inner cell mass) differentiates into the bilaminar embryonic disc, consisting of:
  • Epiblast
  • Hypoblast
• The trophoblast differentiates into:
  • Cytotrophoblast
  • Syncytiotrophoblast, which erodes blood vessels of the endometrium and establishes a haematotrophic circulation
• Cavities formation occurs, including:
  • Amniotic cavity
  • Yolk sac

UteroPlacenta Circulatory System

• On Day 9, isolated spaces called lacunae are formed in the syncytiotrophoblast, which later fill with nutrition secretions from eroded endometrial glands and maternal blood from ruptured maternal capillaries.

Lacunar Network Formation

• By Day 11 and 13, adjacent lacunae fuse to form a lacunar network, allowing maternal vessels to open and maternal blood to flow through.

Embryonic Development

2nd Week: Bilaminar Embryonic Disc

• The embryonic disc consists of two layers: inner cell mass (Epiblast) and Hypoblast.

Formation of Fluid-Filled Sacs

• The Epiblast forms the Amniotic sac, which is fluid-filled.
• The Hypoblast forms the Yolk sac, which is also fluid-filled.

Gastrulation and Embryonic Development

• During the 3rd week, gastrulation occurs, marked by the formation of the primitive streak on the surface of the epiblast.
• The primitive streak is a narrow groove with a slightly bulging side, containing the primitive node at its cephalic end.
• The primitive node is an elevated area surrounding the primitive pit, where cells of the epiblast migrate.

Consequences of Gastrulation

• Cells that invaginate displace the hypoblast, forming the endoderm.
• Cells that come to lie between the epiblast and the newly created endoderm form the mesoderm.
• Remaining cells in the epiblast form the ectoderm.
• The mesoderm spreads laterally and cranially, establishing the fate map.

Neurulation and Embryonic Development

• Neurulation occurs after gastrulation, characterized by craniocaudal folding and lateral folding, transforming the 2D disk into a 3D cylinder.
• By the end of week 8, the embryonic period ends, and all major organ systems have developed, although their function may be minimal.
• The embryo has a distinct human appearance by the end of week 8.

Fate Map Established During Gastrulation

• Cells from the primitive streak contribute to various regions:
  • Cranial region of the node → prechordal plate and notochord
  • Lateral edges of the node → paraxial mesoderm
  • Midstreak region → intermediate mesoderm
  • More caudal part → lateral plate mesoderm
  • Caudal most part → extraembryonic mesoderm

Notochord Formation

• Notochord forms from mesoderm and endoderm via notochordal plate detaching from endoderm
• Definitive notochord differentiates from ectoderm into neuroectoderm, forming neural plate
• Notochord plays a role in vertebrae formation

Other Formations in Trilaminar Embryonic Disc

Cephalic Direction

• Prechordal plate induces neural development
• Oropharyngeal membrane forms the future opening of the oral cavity
• Primitive pit: neurenteric canal temporarily connects amniotic and yolk sac cavities

Caudal End

• Cloacal membrane forms the future opening of the anal and urogenital systems
• Allantois, though rudimentary in humans, may be involved in abnormalities of bladder development

Body Axes

• Three axes are established in the body: anterior-posterior (A-P, also known as craniocaudal), dorso-ventral (D-V), and left-right (L-R)
• The anterior visceral endoderm (AVE) at the cranial end of the hypoblast layer will develop into the head region
• The primitive node and notochord (midline) are involved in determining the left-right (L-R) axis
• SHH and 5-HT are molecules involved in the molecular mechanism of axis establishment

Ectodermal Germ Layer

• Induction of neuroectoderm from overlying ectoderm by notochord and prechordal mesoderm
• Neuroectoderm formation marks the initial event of neurulation

Neurulation

• Formation of neural plate, folds, and groove
• Elevation of lateral edges of neural plate to form neural folds
• Fusion of neural folds to form neural tube
• Communication with amniotic cavity through:
  • Anterior (cranial) neuropore
  • Posterior (caudal) neuropore

Closure of Neural Tube

• Closure of anterior neuropore: around day 25
• Closure of posterior neuropore: around day 28

Future Central Nervous System

• Narrow caudal portion: develops into spinal cord
• Cephalic portion: develops into brain vesicles

Neural Crest Cells

• Located at lateral border of neuroectoderm
• Undergoes epithelial-to-mesenchymal transition
• Fundamentally important and contributes to many organs and tissues
• Sometimes referred to as the fourth germ layer
• Related to one-third of all birth defects
• Related to many cancers, such as melanomas, neuroblastomas, and others

Embryonic Tissues

• Mesoderm: derived from epiblast and extraembryonic tissues
• Mesenchyme: loosely organized embryonic connective tissue, regardless of origin

Ectoderm

• Gives rise to organs and structures that maintain contact with the outside world:
  • Central nervous system
  • Peripheral nervous system
  • Sensory epithelium of the ear, nose, and eye
  • Epidermis, including hair and nails
• Additionally gives rise to:
  • Subcutaneous glands
  • Mammary glands
  • Pituitary gland
  • Enamel of the teeth

Neural Tube Defects (NTDs)

• Caused by failure of neural tube to close
• Effects vary depending on location of lesion:
  • Cranial region: anencephaly (most of the brain fails to form)
  • Cervical region caudally: spina bifida
  • Most common site: lumbosacral region
• Lose neurological function based on level and severity of lesion
• Prevention:
  • 400 μg of folic acid daily
  • 50% to 70% of NTDs can be prevented
  • Take folic acid 3 months prior to conception and throughout pregnancy

Mesodermal Germ Layer

• The mesodermal germ layer divides into four regions: paraxial, lateral plate, parietal, and visceral mesoderm
• Paraxial mesoderm forms close to the midline and thickens
• Lateral plate mesoderm forms laterally
• Parietal mesoderm is continuous with the mesoderm covering the amnion
• Visceral mesoderm is continuous with the mesoderm covering the yolk sac
• These layers line the intraembryonic cavity
• Intermediate mesoderm connects parietal and lateral plate mesoderm

Development of Mesodermal Germ Layer

• Paraxial mesoderm gives rise to future somites
• Intermediate mesoderm develops into future excretory units
• Lateral plate mesoderm differentiates into parietal and visceral mesoderm layers lining the intraembryonic cavity

Somite Differentiation

• Each somite gives rise to three distinct components: sclerotome, myotome, and dermatome
• Sclerotome forms tendon, cartilage, and bone
• Myotome forms segmental muscle
• Dermatome forms the dermis of the back
• Each myotome and dermatome has its own segmental nerve component

Blood and Blood Vessels

• Blood cells and blood vessels arise from mesoderm
• Vasculogenesis: blood vessels form from blood islands
• Angiogenesis: new vessels sprout from existing vessels

Endodermal Germ Layer Derivatives

• The endodermal germ layer gives rise to the primitive gut, which further develops into the gastrointestinal tract and respiratory tract.
• The primitive gut is divided into two parts: the foregut, which forms the oropharyngeal membrane, and the hindgut, which forms the cloacal membrane.

Epithelial Lining and Organs

• The epithelial lining of the primitive gut, allantois, and vitelline duct (intraembryonic) originates from the endoderm.
• The endoderm forms the epithelial lining of several organs, including:
  • Respiratory tract
  • Urinary bladder and urethra
  • Tympanic cavity and auditory tube
• The endoderm also gives rise to the parenchyma of:
  • Thyroid gland
  • Parathyroid glands
  • Liver
  • Pancreas
• Additionally, the endoderm forms the reticular stroma of:
  • Tonsils
  • Thymus