The normal development of the canine embryo/fetus is reported by Pieri et al (2015).
| Age (d) | Crown rump length (mm) | |
|---|---|---|
| 21-22 | 7 | |
| 24-25 | 12-14 | |
| 26 | 20 | |
| 28-32 | 22-30 | |
| 35 | 30-50 | |
Pieri N, Souza AF, Casals JB, Roballo K, Ambrósio CE, Martins DS Comparative Development of Embryonic Age by Organogenesis in Domestic Dogs and Cats. Reprod Domest Anim. 2015; 50: 625-635
There are 5 recognised stages of pulmonary development in mammals and there are also transitions from one stage to another, for example pseudoglandular to canalicular and canalicular to saccular.
- embryonic - this is where epithelium buds off the endoderm of the foregut to become the trachea. Branching to form the bronchi corresponds with pulmonary blood vessels. The pulmonary mesenchyme develops.
- pseudoglandular - this is when there is further branching to form a gland like arrangements. The epithelium becomes the bronchi and terminal bronchioles. Type II pneumocytes are visible bydays of gestation. In dogs this occurs from day 35 and 46
- canalicular - this is when the terminal bronchioles narrow and begin to bud the first air spaces. In dogs this occurs from day 48.
- saccular - this is when the airspaces divide by septation to form sacculi or pocket like structures. In dogs this occurs from about day 57 to 60.
- alveolar - this is when secondary septation results in millions of alveoli. In cattle it begins about 224 days gestation or 8 weeks before birth. At this stage, ciliation is prominent. This occurs in the neonatal period.
Sipriani TM, Grandi F, da Silva LC, Maiorka PC, Vannucchi CI. Pulmonary maturation in canine foetuses from early pregnancy to parturition. Reprod Domest Anim. 2009; 44 Suppl 2:137-140
The conceptus in the dog enters the uterus at about day 5 as a blastocyst. They are evenly spaced in the uterus by day 8. There is transuterine migration before they 'settle' in their location. The first placenta formed is the Choriovitteline placenta with trophoectoderm, mesoderm and endoderm (yolk sac). Blood vessels of the yolk sac are present - although they are nonvascular initially but by day 25 are vascularized.
The normal canine placenta is of the zonary type with a placental girdle, marginal hematoma and a chorioallantoic membrane.
The histological appearance is complex and multilayered at the placental girdle (labryinth) and marginal hematoma. Aralla et al (2013) examined placentas from Day 10, 12, 14, 18, 38, 40, 45, 50, 53, 57, 60 and 63 of pregnancy. The fetal placenta has amnion, allantois and chorion with the latter lined by cytotrophoblasts and syncytiotrophoblasts and a connective tissue stroma. On the maternal side there is the endometrial epithelium lining villi, a superficial portion of the uterine glands that form a spongy zone, and then deep uterine glands are adjacent to the myometrium and are dilated. The glands produce 'uterine milk' or histotrophe.
As the interaction of the trophoectoderm (trophoblasts) with the endometrium, trophoblasts enter between the epithelial cells. The endometrial epithelium at the site of attachment become degenerate, undergo cell death and may fuse to form syncytia (symplasm). Some suggest the trophoblasts fuse with the endometrial epithelium (as they do in syndesmochorial placentas of ruminants). An amorphous layer develops between trophoblasts and endothelium. By late in gestation, the placentation is endotheliochorial and syncytiotrophoblasts are around maternal capillaries.
Aralla M, Groppetti D, Caldarini L, Cremonesi F, Arrighi S. Morphological evaluation of the placenta and fetal membranes during canine pregnancy from early implantation to term. Res Vet Sci. 2013; 95: 15-22.
Tesi M, Miragliotta V, Scala L, Aronica E, Lazzarini G, Fanelli D, Rota A, Abramo F. Gross and histological findings in the canine placenta and amnion at term: What's normal, abnormal or pathological? Reprod Domest Anim. 2021; 56: 691-702
Wooding P, Burton G. Comparative Placentation. Springer Berlin Heidelberg, Eds 2008.
Tesi et al (2021) reported on the findings of 72 placentas and 66 amnions of toy or small breed puppies born alive at natural birth or caesarean section. The number of litters were not given.
In the pelvic girdle at macroscopic evaluation, they found necrosis in 52 of 69, vascular ectasia in 12 of 69, clotted blood and fibrin in 2 of 69 placentas. Microscopically, there was necrosis in 58 of 69, mineralisation in 36 of 69, vascular ectasia in 25 of 69, and neutrophils in 19 of 69 placentas.
The marginal hematomas had laminated fibrin in 27 of 69 cases, mineralisation in 8 of 69 cases, neutrophils in 20 of 69 canses and necrotic foci in 3.
The amnion had hypertrophy of the epithelium in 23 of 64, ecema in 20, mineralisation in 18, fibrosis in 15 and rare neutrophils in 8 of 64.
Tesi M, Miragliotta V, Scala L, Aronica E, Lazzarini G, Fanelli D, Rota A, Abramo F. Gross and histological findings in the canine placenta and amnion at term: What's normal, abnormal or pathological? Reprod Domest Anim. 2021; 56: 691-702
There are several ways to age puppies after birth - based on teeth or radiographic features.
Van den Broeck M, Chen Y, Cornillie P. Age estimation of puppies based on the radiographically assessed development of ossification centres in the carpal and metacarpal regions [published online ahead of print, 2023 Jan 24]. Vet Rec. 2023;e2582. doi:10.1002/vetr.2582