Canis familiaris, domestic dog facts at GeoChemBio: life cycle, developmental stages, embryology

Canis lupus familiaris
dog life cycle & development

Unfertilized egg Also ovum, oocyte. One of the main features of the canine oocyte is the darkness of its cytoplasm due to large number of lipid droplets. Oocytes are expelled from their follicles (ovulated) at the Germinal Vesicle (GV) immature stage and resume meiosis in about 24-48 hours after ovulation in the proximal and middle part of uterine tube. They complete their maturation (first metaphase (MI oocytes) and second metaphase (MII oocytes)) during further tubal transport. This is also observed in fox oocytes.
Prenatal Gestation period lasts for about 2 months (~63 days) as defined from ovulation (LH surge is day 0) to parturition. Due to extended viability of dog's gametes the interval from mating to parturition can vary from 57 to 72 days. However, because cleavage between the 2-cell and 16-cell stages occurs more rapidly after fertilization of more mature oocytes the duration of gestation when measured as interval from mating to parturition is similar whether mating occurs before or a few days after oocyte maturation.
- Fertilization Because of mating and ovulation are not tightly coupled as well as unusually prolonged survivability of oocytes and spermatozoa it is difficult to determine exact time when fertilization and beginning of embryo development occur. Penetration of immature oocyte by sperm is rare. In vivo, fertilization occurs in metaphase II oocytes, which are present between 48 and 83 hours after ovulation. At the moment of fertilization, oocytes are present in the middle and distal part of uterine tube.
- Pre-implantation embryo
  - Fertilized egg Zygote. Is about 0.8-1.2 mm in diameter. The fertilized ovum is still surrounded by follicular granulosa cells which start to unbind and disappear only after 5-7 days after ovulation, between 2-pronuclei and 8-cell stages. The potential development rate of zygote is about one cell cycle in 24 hours until 10 days after ovulation.
  - Cleavage Fertilized egg mitotic division starts in the oviduct. Each cleavage yields two blastomeres of about half size of the parent cell.
    - 2-cell embryo At the 2-cell stage, embryo diameter ranges from 1.1 to 1.7 mm.
    - 8-16 cell embryo At this stage embryo diameter varies from 0.188 to 0.2 mm (including zona pellucida) and are still in the distal part of uterine tube after 4.5-12 days after ovulation. 8-cell embryo stage occurs in 8 days after fertilization if insemination took place before oocytes mature and only 3 days following insemination after oocyte maturation. 16-cell embryo stage usually occurs on day 11 after LH surge (day 0) regardless when mating took place.
  - Morula An early embryo that is a compact mass
    of about 16 cells (blastomeres), which
    differentiated into two types of cells,
    outer cells and inner cells. Morulae appear in about 8.5-10 days after ovulation, and the embryos start to enter the uterine horn.
  - Blastocyst A post-morula preimplantation mammalian embryo that develops from a 32-cell stage into a fluid-filled hollow ball of over a hundred cells. A blastocyst has two distinctive tissues. The outer layer of trophoblasts gives rise to extra-embryonic tissues. The inner cell mass gives rise to the embryonic disc and eventual embryo proper. Blastocysts at all stages can be still found in uterine horn.
    - Early blastocyst Embryo size ranges from 0.215 to 0.35 mm.
    - Expanded blastocyst At day 12 after ovulation, the blastocyst reaches 0.5-0.75 mm in diameter.
    - Late blastocyst The embryo reaches ~1.0 mm in diameter, enters uterus, the zona pellucida is thinning and the blastocyst is free-floating in the uterine cavity, growing to 2.3 mm. Before implantation, from 12 to 16-17 days after ovulation, transuterine migration of embryos can occur. This event, also reported in other pluriparus species, such as pig and cat, probably equilibrates the number of fetuses between the two horns when there is a substantial difference. Approximately 16-20 days after ovulation, the blastocyst reaches ~2.5 mm and sheds the zona pellucida.
- Implantation Fixation and implantation begin by 17-19 days. Blastocysts remain unattached as late as days 21-22; invasion of placental trophectoderm occurs as early as day 22 and as late as day 23, and only 1-2 days before heartbeats are detected by sonography.
- Post-implantation
  embryo The period of the embryo begins with implantation of the blastocyst and ends with completion of organogenesis.
  - Gastrula Gastrulation is a process of complicated morphogenetic cell movements that reorganizes a bilayer embryo into one with three layers (outer ectoderm, middle mesoderm, and inner endoderm) and specific orientation (dorsal/ventral; anterior/posterior).
  - Organogenesis The embryo develops in a cephalocaudal sequence: begins with head folds, then neural tube closure, followed by somite formation, appearance of branchial clefts, lens placode, otic placode, cardiac bulge, and the growth of limb buds. By 30^th day of gestation (19 mm embryo) the eyelids and external ear are forming, sensory hairs on the muzzle, chin, and eyebrows develop. Also, at this time, the intestines outgrow their available space in the abdomen and are herniated into the umbilical stalk (umbilical herniation). Five pairs of nipples and distinct forelimb digits are present. After day 30, the gonadal sexual differentiation of the embryo begins. Between 28^th and 35^th ossification begins.
- Fetus The period of the fetus is the time during which characteristic features appear and rapid growth occurs. The period lasts from 35^th day of gestation to parturition. At 35^th day, fetus can be recognized as canine. External features characteristic of fetal stages are development of pigmentation, growth of hairs and claws, closure and fusion of the eyelids, growth of the external ear, elongation of the trunk, and sexual differentiation. By 40 days of gestation, the eyelids are closed and the lids are fused. The umbilical hernia is also eliminated at this stage and claws are formed on all digits. Color markings appear and body hair begins to grow at 45 days of gestation. Specific to the reproductive system, gonadal swellings are prominent in male and female fetuses. By 55 days all deciduous teeth show calcification. The last bones are ossified at 57^th day of gestation.

Post natal

Neonatal period Neonate, whelping - 0-14 days of age. Main activities of puppies during this period are sleeping and nursing until beginning of weaning. Normal weight of a puppy is breed-dependent, generally 500+-150 g for a medium-sized dog. The brain of newborn puppy is myelinated in very few places. Vestibular function is present at birth and is important for positioning during nursing. Muscular coordination is absent. The suckling and rooting reflexes are present soon after parturition. They are strongest during the first 2 weeks before disappearing at approximately 25 days. Unlike homeothermic adults, neonates are poikilothermic (lack an internal temperature regulating mechanism); however, they have well developed behavioral heat-seeking responses. An upright posture in puppies cannot be maintained until 10 to 14 days of age but breed variations exist. Newborn puppies cannot feed and eliminate without external stimulation. Peri- and postnatal losses are relatively high in dogs, with mortality rates ranging from 17% to 30% during the first 8 weeks of life. Stillbirths and deaths within the first week were responsible for the majority of these losses.
Transitional period 14 - 21 days of age. Period of sensory and locomotor development. Opening of the eyes marks the beginning of the period. Eyes usually start opening at about 14 days of age and at 3 weeks of age, all normal puppies have their eyes opened; the cornea appears slightly cloudy for the first 3-4 weeks of life because of increased water content, and the iris is typically bluish-gray because of low pigmentation. An uncoordinated gait begins at 18 to 21 days of age. The external ear canals open between 2 and 3 weeks of age(100% in 4 weeks old puppies). Beginning of startle response to noise marks an end of the period.
Primary socialization
period 3-12 weeks of age. Critical period for ability to form social bonds and to operate in a society (pack). The eruption of first teeth (on average at the age of 19-20 days) marks the beginning of the period. The dog is neurologically mature at 4-6 weeks (different sources) of age with exception of equilibration and adult locomotor abilities such as running and leaping. By day 28^th, coordinated walking and early environmental exploration begins. The period of primary socialization ends around 7-10 weeks of age, which is normal period of weaning. Permanent teeth begin to replace deciduous teeth at about 4 months of age.
Juvenile Secondary socialization period. Independent (in wolves marked by ability to obtain food independently in the wild) and actively growing animal until age of sexual maturity. Permanent teeth fully erupted by 6 months of age. Social behavior and individual personality traits continue to develop until 18 months of age. New modes of behavior such as protective, aggressive, territorial, and cooperative appear as organism undergoes hormonal changes and adapts to environment constantly learning new experiences.

Adult After attainment of sexual maturity. Pubertal estrus occurs variably at 6–14 months in most breeds, with means positively correlated with breed size. Male dogs mature by 10-12 months of age. Life span is highly breed-dependent; well-cared for animals can live for 16 years and more.
Dog Age Conversion Chart

Dog age	Human age
1	15
2	24
3	28
4	32
5	36
6	40
7	44
8	48
9	52
10	56
11	60
12	64
13	68
14	72
15	76
16	80
17	84
18	88
19	92
20	96

Source EMERSON ANIMAL HOSPITAL With increasing age, some dogs develop a neurodegenerative disease that is commonly referred to as canine cognitive dysfunction syndrome (CDS). Diagnosis of CDS can be clinical or based on laboratory tests. The main behavioral changes associated with CDS are disorientation, altered interactions with people or other animals, sleep-wake cycle alterations, house-soiling, and changes in activity level. For comparison: in humans dementia is seen in 1-3% of 65-70 years old, and increases to ~50% of people older over 85 years of age. In dogs, 28% aged 11-12 years showed sign of CDS, compared with 68% of those over 15 years of age.

Illustrating photo credit: http://kotomatrix.ru/